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Background: Anterior column realignment is an attractive minimally invasive treatment for sagittal imbalance. Expandable spacers offer controlled tensioning of the anterior longitudinal ligament (ALL) during release, which could optimize correction and anterior column stability. This study investigated the biomechanical and radiographic effects of single-level anterior-to-psoas lumbar interbody fusion (ATP-LIF) with expandable spacers and sequential ALL release., Methods: In vitro range of motion tests were performed on 7 fresh-frozen cadaveric spines (L2-L5) with a ±7.5 Nm load applied in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). After testing intact spines, single-level (L3-L4) ATP-LIFs were performed and supplemented with posterior screws, rods, and integrated lateral screws and tested after (1) no ALL release (ATP-LIF); (2) resection of 1/3 the ALL's width (1/3 ALL release); (3) resection of 2/3 the ALL's width (2/3 ALL release); and (4) complete ALL resection (3/3 ALL release). Following each partial ALL release, rods were removed, and spacers were expanded until the torque limit was reached. Rods were then reapplied, and lateral radiographs were taken to analyze changes in intervertebral angle (IVA), foraminal height, foraminal area, and posterior and anterior disc height (PDH and ADH)., Results: In ATP-LIF constructs, range of motion decreased in FE (18% intact), LB (14% intact), and AR (30% intact), while IVA, PDH, ADH, foraminal height, and foraminal area increased. PDH and ADH increased linearly with sequential ALL release and spacer expansion, while LB and AR remained stable. FE increased slightly (+15%-16% intact, <1°) following 2/3 ALL release but remained stable afterward. IVA increased exponentially with sequential ALL release, gaining 8.8° ± 3.2° with complete release., Conclusions: The present study found improved biomechanics and radiographic parameters following ATP-LIF with intact ALL, minimal biomechanical differences between partial and complete ALL release, and greater correction and height restoration with complete release. Future clinical testing is necessary to determine the impact of this finding on patient outcomes., Clinical Relevance: Controlled tensioning of the ALL before and after ligament release allows for potential optimization between restoring sagittal balance and maximizing construct stability in a minimally invasive approach., Competing Interests: Declaration of Conflicting Interests : J.P.H. receives consulting fees from Globus Medical and received support for travel for this study. J.P.M., J.M.M., and B.S.B. are paid employees of and also receive stock options from Globus Medical. J.W. was a full-time co-op at the time of this study from Drexel University and was paid by Globus Medical for this role., (This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2024 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.)

Objective This study assessed whether robotic-assisted navigation (RAN) spine surgery outcomes, including operative time and pedicle screw accuracy, continue to improve with extended experience beyond 200 cases. Methods This is a retrospective review of 60 patients who underwent lumbosacral transforaminal interbody fusion using RAN. Patients were segmented into three groups of 20 consecutive cases each. The first group represented a surgical performance baseline leading up to the investigating surgeon's 200th RAN case. The subsequent two groups were selected beyond the 200th case with an average of 15 cases between groups. Pedicle screw accuracy and intraoperative outcomes were assessed. Statistical results were significant if p<0.05. Results Measures of surgical efficiency significantly improved beyond the investigating surgeon's 200th RAN case. As case number increased, the following parameters significantly decreased: registration time (group 1: 16.9±6.5, group 2: 12.9±3.0, group 3: 8.7±1.6 minutes; p<0.05), screw insertion time (group 1: 14.9±3.5, group 2: 10.9±2.0, group 3: 8.4±2.7 minutes; p<0.05), and total operative time significantly decreased from group 1 (175.9±58.2 minutes) to group 2 (135.8±23.9 minutes) (p=0.013) with a non-significant decrease to group 3 (121.5±32.3 minutes). Accuracy (Grade = A) significantly increased across groups (group 1: 87%, group 2: 94%, group 3: 98%; p=0.024). Group 1 had the highest misplacement rate of 3.7% (4/108 screws). The overall misplacement rate was 1.4% (4/290 screws) (Grade C-E). There was a higher rate of lateral screw misplacement compared to medial misplacement. Conclusion Even with a small number of initial cases, RAN spine surgery can consistently be performed with high accuracy and acceptable intraoperative outcomes. However, this study demonstrated refined outcomes with extended robotic experience., Competing Interests: Human subjects: Consent was obtained or waived by all participants in this study. Animal subjects: All authors have confirmed that this study did not involve animal subjects or tissue. Conflicts of interest: In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: Isabella Lucyk and Brandon Bucklen are salaried employees (with stock options) of Globus Medical, Inc (GMI). ., (Copyright © 2024, Benech et al.)

Objectives: To assess the biomechanical differences between linked and unlinked constructs in young and osteoporotic cadavers in addition to osteoporotic sawbones., Methods: Intraarticular distal femur fractures with comminuted metaphyseal regions were created in three young matched pair cadavers, three osteoporotic matched pair cadavers, and six osteoporotic sawbones. Precontoured distal femur locking plates were placed in addition to a standardized retrograde nail, with unitized constructs having one 4.5 mm locking screw placed distally through the nail. Nonunitized constructs had seven 4.5 mm locking screws placed through the plate around the nail, with one 5 mm distal interlock placed through the nail alone. Cadaveric specimens were subjected to axial fatigue loads between 150 and 1500 N (R Ratio = 10) with 1 Hx frequency for 10,000 cycles. Sawbones were axially loaded at 50% of the ultimate load for fatigue testing to achieve runout, with testing performed with 30 and 300 N (R Ratio = 10) loads with 1 Hz frequency for 10,000 cycles., Results: In young cadavers, there was no difference in the mean cyclic displacement of the unitized constructs (1.51 ± 0.62mm) compared to the non-unitized constructs (1.34 ± 0.47mm) (Figure 4A), (p = 0.722). In osteoporotic cadavers, there was no difference in the mean cyclic displacement of the unitized constructs (2.46 ± 0.47mm) compared to the non-unitized constructs (2.91 ± 1.49mm) (p =0.639). There was statistically no significant difference in cyclic displacement between the unitized and non-unitized groups in osteoporotic sawbones(p = 0.181)., Conclusions: Linked constructs did not demonstrate increased axial stiffness or decreased cyclical displacement in comparison to unlinked constructs in young cadaveric specimens, osteoporotic cadaveric specimens, or osteoporotic sawbones., Competing Interests: The authors report no conflict of interest., (Copyright © 2024 Wolters Kluwer Health, Inc. All rights reserved.)

Study Design: Biomechanical cadaveric study., Objectives: Multi-rod constructs maximize posterior fixation, but most use a single pedicle screw (PS) anchor point to support multiple rods. Robotic navigation allows for insertion of PS and cortical screw (CS) within the same pedicle, providing 4 points of bony fixation per vertebra. Recent studies demonstrated radiographic feasibility for dual-screw constructs for posterior lumbar spinal fixation; however, biomechanical characterization of this technique is lacking., Methods: Fourteen cadaveric lumbar specimens (L1-L5) were divided into 2 groups (n = 7): PS, and PS + CS. VCF was simulated at L3. Bilateral posterior screws were placed from L2-L4. Load control (±7.5Nm) testing performed in flexion-extension (FE), lateral bending (LB), axial rotation (AR) to measure ROM of: (1) intact; (2) 2-rod construct; (3) 4-rod construct. Static compression testing of 4-rod construct performed at 5 mm/min to measure failure load, axial stiffness., Results: Four-rod construct was more rigid than 2-rod in FE ( P < .001), LB ( P < .001), AR ( P < .001). Screw technique had no significant effect on FE ( P = .516), LB ( P = .477), or AR ( P = .452). PS + CS 4-rod construct was significantly more stable than PS group ( P = .032). Stiffness of PS + CS group (445.8 ± 79.3 N/mm) was significantly greater ( P = .019) than PS (317.8 ± 79.8 N/mm). Similarly, failure load of PS + CS group (1824.9 ± 352.2 N) was significantly greater ( P = .001) than PS (913.4 ± 309.8 N)., Conclusions: Dual-screw, 4-rod construct may be more stable than traditional rod-to-rod connectors, especially in axial rotation. Axial stiffness and ultimate strength of 4-rod, dual-screw construct were significantly greater than rod-to-rod. In this study, 4-rod construct was found to have potential biomechanical benefits of increased strength, stiffness, stability., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Introduction: The use of a robotic system for the placement of pedicle screws in spine surgeries is well documented in the literature. However, there is only a single report in the United States describing the use of a robotic system to place two screws in osseous fixation pathways (OFPs) commonly used in the treatment of pelvic and acetabular fractures in a simulated bone model. The purpose of this study was to demonstrate the use of a robotic system to place screws in multiple, clinically relevant OFPs in a cadaveric model and to quantitatively measure accuracy of screw placement relative to the preoperative plan., Methods: A single cadaveric specimen was obtained for the purpose of this study. All surrounding soft tissues were left intact. Screws were placed in OFPs, namely iliosacral (IS), trans-sacral (TS), Lateral Compression-II (LC-II), antegrade anterior column (AC) and antegrade posterior column (PC) of the right hemipelvis using standard, fluoroscopically assisted percutaneous or mini-open technique. Following the placement of screws into the right hemipelvis using standard techniques, screws were planned and placed in the same OFPs of the contralateral hemipelvis using the commercially available ExcelsiusGPS ® robotic system (Globus Medical Inc., Audubon, PA). After robotic-assisted screw placement, a post-procedure CT scan was obtained to evaluate actual screw placement against the pre-procedure plan. A custom-made image analysis program was devised to measure screw tip/tail offset and angular offset on axial and sagittal planes., Results: For different OFPs, the mean tip offset, tail offset and angular offsets were 1.6 ± 0.9 mm (Range 0.0-3.6 mm), 1.4 ± 0.4 mm (Range 0.3-2.5 mm) and 1.1 ± 0.4° (Range 0.5-2.1), respectively., Conclusion: In this feasibility study, surgeons were able to place screws into the clinically relevant fracture pathways of the pelvis using ExcelsiusGPS ® for robotic-assisted surgery. The measured accuracy was encouraging; however, further investigation is needed to demonstrate that robotic-assisted surgery can be used to successfully place the screws in the bony corridors of the pelvis to treat traumatic pelvic injuries., (© 2024. The Author(s), under exclusive licence to Springer-Verlag France SAS, part of Springer Nature.)

Background and Objectives: Instrumented spinal fusion constructs sometimes fail because of fatigue loading, frequently necessitating open revision surgery. Favorable outcomes after percutaneous juxtapedicular cement salvage (perc-cement salvage) of failing instrumentation have been described; however, this approach is not widely known among spine surgeons , and its biomechanical properties have not been evaluated. We report our institutional experience with perc-cement salvage and investigate the relative biomechanical strength of this technique as compared with 3 other common open revision techniques., Methods: A retrospective chart review of patients who underwent perc-cement salvage was conducted. Biomechanical characterization of revision techniques was performed in a cadaveric model of critical pedicle screw failure. Three revision cohorts involved removal and replacement of hardware: (1) screw upsizing, (2) vertebroplasty, and (3) fenestrated screw with cement augmentation. These were compared with a cohort with perc-cement salvage performed using a juxtapedicular trajectory with the failed primary screw remaining engaged in the vertebral body., Results: Ten patients underwent perc-cement salvage from 2018 to 2022 to address screw haloing and/or endplate fracture threatening construct integrity. Pain palliation was reported by 8/10 patients. Open revision surgery was required in 4/10 patients, an average of 8.9 months after the salvage procedure (range 6.2-14.7 months). Only one revision was due to progressive hardware dislodgement. The remainder avoided open revision surgery through an average of 1.9 years of follow-up. In the cadaveric study, there were no significant differences in pedicle screw pullout strength among any of the revision cohorts., Conclusion: Perc-cement salvage of failing instrumentation is reasonably efficacious. The technique is biomechanically noninferior to other revision strategies that require open surgery for removal and replacement of hardware. Open revision surgery may be avoided by perc-cement salvage in select cases., (Copyright © Congress of Neurological Surgeons 2023. All rights reserved.)

Background and Objectives: Despite frequent use, stereotactic head frames require manual coordinate calculations and manual frame settings that are associated with human error. This study examines freestanding robot-assisted navigation (RAN) as a means to reduce the drawbacks of traditional cranial stereotaxy and improve targeting accuracy., Methods: Seven cadaveric human torsos with heads were tested with 8 anatomic coordinates selected for lead placement mirrored in each hemisphere. Right and left hemispheres of the brain were randomly assigned to either the traditional stereotactic arc-based (ARC) group or the RAN group. Both target accuracy and trajectory accuracy were measured. Procedural time and the radiation required for registration were also measured., Results: The accuracy of the RAN group was significantly greater than that of the ARC group in both target (1.2 ± 0.5 mm vs 1.7 ± 1.2 mm, P = .005) and trajectory (0.9 ± 0.6 mm vs 1.3 ± 0.9 mm, P = .004) measurements. Total procedural time was also significantly faster for the RAN group than for the ARC group (44.6 ± 7.7 minutes vs 86.0 ± 12.5 minutes, P < .001). The RAN group had significantly reduced time per electrode placement (2.9 ± 0.9 minutes vs 5.8 ± 2.0 minutes, P < .001) and significantly reduced radiation during registration (1.9 ± 1.1 mGy vs 76.2 ± 5.0 mGy, P < .001) compared with the ARC group., Conclusion: In this cadaveric study, cranial leads were placed faster and with greater accuracy using RAN than those placed with conventional stereotactic arc-based technique. RAN also required significantly less radiation to register the specimen's coordinate system to the planned trajectories. Clinical testing should be performed to further investigate RAN for stereotactic cranial surgery., (Copyright © 2023 The Author(s). Published by Wolters Kluwer Health, Inc on behalf of Congress of Neurological Surgeons.)

Study Design: A retrospective chart review., Objective: The aim of this study was to evaluate the screw accuracy of thoracic pedicle screws placed with a robot-guided navigation system., Summary of Background Data: Thoracic pedicles are smaller in diameter than lumbar pedicles, making pedicle screw placement difficult. Misplaced pedicle screws may present complications including decreased construct stability, and increased risks of neurological deficits and blood vessel perforation. There is a dearth of knowledge on thoracic pedicle screw accuracy placed with a robot., Materials and Methods: A retrospective analysis of the robot-assisted placement of thoracic pedicle screws was performed. Preoperative and postoperative computed tomography (CT) scans of the implanted thoracic screws were collected to assess screw placement accuracy, pedicle breadth, and placement deviations. A CT-based Gertzbein and Robbins System was used to classify pedicle screw accuracy in 2 mm increments. A custom image overlay software was used to determine the deviations between the preoperatively planned trajectory of pedicle screws and final placement at screw entry (tail), and tip in addition to the angular deviation., Results: Seventy-five thoracic pedicle screws were implanted by navigated robotic guidance in 17 patients, only 1.3% (1/75) were repositioned intraoperatively. Average patient age and body mass index were 57.5 years and 25.9 kg/m 2 , respectively, with 52.9% female patients. Surgery diagnoses were degenerative disk disease (47.1%) and adjacent segment disease (17.6%). There were zero complications, with no returns to the operating room. According to the CT-based Gertzbein and Robbins pedicle screw breach classification system, 93.3% (70/75) screws were grade A or B, 6.6% (5/75) were grade C, and 0% were grade D or E. The average deviation from the preoperative plan to actual final placement was 1.8±1.3 mm for the screw tip, 1.6±0.9 mm for the tail, and 2.1±1.5 degrees of angulation., Conclusions: The current investigation found a 93.3% accuracy of pedicle screw placement in the thoracic spine. Navigated robot assistance is a useful system for placing screws in the smaller pedicles of the thoracic spine., Level of Evidence: Level III-retrospective nonexperimental study., Competing Interests: A.B.V. has received royalties, teaching and research support from Globus Medical Inc. N.T. and B.S.B. have been employed and granted stock options by Globus Medical Inc. The remaining authors declare no conflict of interest., (Copyright © 2023 The Author(s). Published by Wolters Kluwer Health, Inc.)

Study Design: In vitro biomechanical study investigating the coupled motions of the whole normative human thoracic spine (TS) and lumbar spine (LS) with rib cage., Objective: To quantify the region-specific coupled motion patterns and magnitudes of the TS, thoracolumbar junction (TLJ), and LS simultaneously., Background: Studying spinal coupled motions is important in understanding the development of complex spinal deformities and providing data for validating computational models. However, coupled motion patterns reported in vitro are controversial, and no quantitative data on region-specific coupled motions of the whole human TS and LS are available., Methods: Pure, unconstrained bending moments of 8 Nm were applied to seven fresh-frozen human cadaveric TS and LS specimens (mean age: 70.3 ± 11.3 years) with rib cages to elicit flexion-extension (FE), lateral bending (LB), and axial rotation (AR). During each primary motion, region-specific rotational range of motion (ROM) data were captured., Results: No statistically significant, consistent coupled motion patterns were observed during primary FE. During primary LB, there was significant ( p  < 0.05) ipsilateral AR in the TS and a general pattern of contralateral coupled AR in the TLJ and LS. There was also a tendency for the TS to extend and the LS to flex. During primary AR, significant coupled LB was ipsilateral in the TS and contralateral in both the TLJ and LS. Significant coupled flexion in the LS was also observed. Coupled LB and AR ROMs were not significantly different between the TS and LS or from one another., Conclusions: The findings support evidence of consistent coupled motion patterns of the TS and LS during LB and AR. These novel data may serve as reference for computational model validations and future in vitro studies investigating spinal deformities and implants., Competing Interests: The authors disclose that this study was performed at Globus Medical, Inc. (GMI), using its six‐degrees‐of‐freedom motion simulator. MRO and SB have no disclosures to report. Cadaveric specimens and related materials were provided by GMI, at which JM and BSB are salaried employees with stock options., (© 2023 The Authors. JOR Spine published by Wiley Periodicals LLC on behalf of Orthopaedic Research Society.)

Purpose: To evaluate the motion-preserving properties of vertebral body tethering with varying cord/screw constructs and cord thicknesses in cadaveric thoracolumbar spines., Methods: In vitro flexibility tests were performed on six fresh-frozen human cadaveric spines (T1-L5) (2 M, 4F) with a median age of 63 (59-to-80). An ± 8 Nm load was applied to determine range of motion (ROM) in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) in the thoracic and lumbar spine. Specimens were tested with screws (T5-L4) and without cords. Single (4.0 mm and 5.0 mm) and double (4.0 mm) cord constructs were sequentially tensioned to 100 N and tested: (1) Single 4.0 mm and (2) 5.0 mm cords (T5-T12); (3) Double 4.0 mm cords (T5-12); (4) Single 4.0 mm and (5) 5.0 mm cord (T12-L4); (6) Double 4.0 mm cords (T12-L4)., Results: In the thoracic spine (T5-T12), 4.0-5.0 mm single-cord constructs showed slight reductions in FE and 27-33% reductions in LB compared to intact, while double-cord constructs showed reductions of 24% and 40%, respectively. In the lumbar spine (T12-L4), double-cord constructs had greater reductions in FE (24%), LB (74%), and AR (25%) compared to intact, while single-cord constructs exhibited reductions of 2-4%, 68-69%, and 19-20%, respectively., Conclusions: The present biomechanical study found similar motion for 4.0-5.0 mm single-cord constructs and the least motion for double-cord constructs in the thoracic and lumbar spine suggesting that larger diameter 5.0 mm cords may be a more promising motion-preserving option, due to their increased durability compared to smaller cords. Future clinical studies are necessary to determine the impact of these findings on patient outcomes., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Study Design: Cadaveric biomechanics study., Purpose: This study investigated the effects of unilateral sacroiliac joint (SIJ) fixation for fusion with/without L5-S1 fixation on contralateral SIJ range of motion (ROM)., Overview of Literature: SIJ fusion raises concerns that unilateral SIJ stabilization for fusion may increase contralateral SIJ mobility, leading to accelerated SIJ degeneration. Also, prior lumbosacral fixation may lead to accelerated SIJ degeneration, due to adjacent level effects. SIJ fixation biomechanics have been evaluated, showing a reduced-ROM, but SIJ fixation effects on contralateral nonfixated SIJ remain unknown., Methods: Seven human lumbopelvic spines were used, each affixed to six-degrees-of-freedom testing apparatus; 8.5-Nm pure unconstrained bending moments applied in flexion-extension, lateral bending, and axial rotation. The ROM of left and right SIJ was measured using a motion analysis system. Each specimen tested as (1) intact, (2) injury (left), (3) L5-S1 fixation, (4) unilateral stabilization (left), (5) unilateral stabilization+L5-S1 fixation, (6) bilateral stabilization, and (7) bilateral stabilization+L5-S1 fixation. Both left-sided iliosacral and posterior ligaments were cut for injury condition to model SIJ instability before surgery., Results: There were no statistical differences between fixated and contralateral nonfixated SIJ ROM following unilateral stabilization with/without L5-S1 fixation for all loading directions (p>0.930). Injured condition and L5-S1 fixation provided the largest motion increases across both joints; no significant differences were recorded between SIJs in any loading direction (p>0.850). Unilateral and bilateral stabilization with/without L5-S1 fixation reduced ROM compared with the injured condition for both SIJs, with bilateral stabilization providing maximum stability., Conclusions: In the cadaveric model, unilateral SIJ stabilization with/without lumbosacral fixation did not lead to significant contralateral SIJ hypermobility; long-term changes and in vivo response may differ.

Objective: Posterior cervical fusion is a common surgical treatment for patients with myeloradiculopathy or regional deformity. Several studies have found increased stresses at the cervicothoracic junction (CTJ) and significantly higher revision surgery rates in multilevel cervical constructs that terminate at C7. The purpose of this study was to investigate the biomechanical effects of selecting C7 versus T1 versus T2 as the lowest instrumented vertebra (LIV) in multisegmental posterior cervicothoracic fusion procedures., Methods: Seven fresh-frozen cadaveric cervicothoracic spines (C2-L1) with ribs intact were tested. After analysis of the intact specimens, posterior rods and lateral mass screws were sequentially added to create the following constructs: C3-7 fixation, C3-T1 fixation, and C3-T2 fixation. In vitro flexibility tests were performed to determine the range of motion (ROM) of each group in flexion-extension (FE), lateral bending (LB), and axial rotation (AR), and to measure intradiscal pressure of the distal adjacent level (DAL)., Results: In FE, selecting C7 as the LIV instead of crossing the CTJ resulted in the greatest increase in ROM (2.54°) and pressure (29.57 pound-force per square inch [psi]) at the DAL in the construct relative to the intact specimen. In LB, selecting T1 as the LIV resulted in the greatest increase in motion (0.78°) and the lowest increase in pressure (3.51 psi) at the DAL relative to intact spines. In AR, selecting T2 as the LIV resulted in the greatest increase in motion (0.20°) at the DAL, while selecting T1 as the LIV resulted in the greatest increase in pressure (8.28 psi) in constructs relative to intact specimens. Although these trends did not reach statistical significance, the observed differences were most apparent in FE, where crossing the CTJ resulted in less motion and lower intradiscal pressures at the DAL., Conclusions: The present biomechanical cadaveric study demonstrated that a cervical posterior fixation construct with its LIV crossing the CTJ produces less stress in its distal adjacent discs compared with constructs with C7 as the LIV. Future clinical testing is necessary to determine the impact of this finding on patient outcomes.

Objective: The two most common revision options available for the management of loose pedicle screws are larger-diameter screws and cement augmentation into the vertebral body for secondary fixation. An alternative revision method is impaction grafting (pedicoplasty) of the failed pedicle screw track. This technique uses the impaction of corticocancellous bone into the pedicle and vertebral body through a series of custom funnels to reconstitute a new pedicle wall and a neomedullary canal. The goal of this study was to compare the biomechanics of screws inserted after pedicoplasty (impaction grafting) of a pedicle defect to those of an upsized screw and a cement-augmented screw., Methods: For this biomechanical cadaveric study the investigators used 10 vertebral bodies (L1-5) that were free of metastatic disease or primary bone disease. Following initial screw insertion, each screw was subjected to a pullout force that was applied axially along the screw trajectory at 5 mm per minute until failure. Each specimen was instrumented with a pedicoplasty revision using the original screw diameter, and on the contralateral side either a fenestrated screw with cement augmentation or a screw upsized by 1 mm was inserted in a randomized fashion. These revisions were then pulled out using the previously mentioned methods., Results: Initial screw pullout values for the paired upsized screw and pedicoplasty were 717 ± 511 N and 774 ± 414 N, respectively (p = 0.747) (n = 14). Revised pullout values for the paired upsized screw and pedicoplasty were 775 ± 461 N and 762 ± 320 N, respectively (p = 0.932). Initial pullout values for the paired cement augmentation and pedicoplasty were 792 ± 434 N and 880 ± 558 N, respectively (p = 0.649). Revised pullout values for the paired cement augmentation and pedicoplasty were 1159 ± 300 N and 687 ± 213 N, respectively (p < 0.001)., Conclusions: Pedicle defects are difficult to manage. Reconstitution of the pedicle and creation of a neomedullary canal appears to be possible through the use of pedicoplasty. Biomechanically, screws that have been used in pedicoplasty have equivalent pullout strength to an upsized screw, and have greater insertional torques than those with the same diameter that have not been used in pedicoplasty, yet they are not superior to cement augmentation. This study suggests that although cement augmentation appears to have superior pullout force, the novel pedicoplasty technique offers promise as a viable biological revision option for the management of failed pedicle screws compared with the option of standard upsized screws in a cadaveric model. These findings will ultimately need to be further assessed in a clinical setting.

Background: Traditional minimally invasive fluoroscopy-based techniques for pedicle screw placement utilize guidance, which may require fluoroscopic shots. Computerized tomography (CT) navigation results in more accurate screw placement. Robotic surgery seeks to establish access and trajectory with greater accuracy., Objective: This study evaluated the screw placement accuracy of a robotic platform., Methods: Demographic data, preoperative/postoperative CT scans, and complication rates of 127 patients who underwent lumbosacral pedicle screw placement with minimally invasive navigated robotic guidance using preoperative CT were analyzed., Results: On the GRS scale, 97.9% (711/726) of screws were graded A or B, 1.7% (12/726) of screws graded C, 0.4% (3/726) of screws graded D, and 0% graded E. Average offset from preoperative plan to final screw placement was 1.9 ± 1.5 mm from tip, 2.2 ± 1.4 mm from tail and 2.9 ± 2.3° of angulation., Conclusions: Robotic-assisted surgery utilizing preoperative CT workflow with intraoperative fluoroscopy-based registration improves pedicle screw placement accuracy within a patient's pedicles., (© 2022. The Author(s).)

Objectives: To evaluate the effect of a traditional "center-center" end point for distal tibia nailing in comparison with a lateral-of-center end point on fracture malalignment in a cadaver model., Methods: Nine matched pairs of human cadaveric lower-extremity specimens were used to model the effect of nail end point on fracture alignment in extra-articular distal tibia fractures. After simulation of the fracture through a standardized osteotomy, 1 member of each pair was fixed with an intramedullary nail using a "center-center" end point, whereas a lateral-of-center end point was used for the other member of the pair. Specimens were stripped of soft tissue, and digital calipers were used to measure fracture translation and gap medially, laterally, anteriorly, and posteriorly. Coronal plane angulation at each fracture was measured on the final mortise image., Results: The average coronal angulation was 7.0 degrees of valgus (with a SD of 4.1) in central-end point specimens versus 0.2 degrees of valgus (SD = 1.5) in lateral-end point specimens ( P < 0.001). Lateral-end point specimens also demonstrated significantly less fracture gap medially (mean 0.2 vs. 3.1 mm for central-end point specimens, P < 0.001), anteriorly (mean 0.1 vs. 1.3 mm, P = 0.003), and posteriorly (mean 0.3 vs. 2.2 mm, P = 0.003). Lateral-end point specimens also showed less lateral translation (mean 0.6 vs. 1.6 mm, P = 0.006)., Conclusions: Lateral-of-center nail end points may help surgeons restore native alignment in extra-articular distal tibia fractures and avoid valgus malalignment., Competing Interests: The authors report no conflict of interest., (Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.)

Introduction Interbody spacers are necessary for achieving disc height restoration when surgical intervention is used for the treatment of severe degenerative disc disease. Minimally invasive lateral lumbar interbody fusion (MIS LLIF) is a popular surgical approach that historically uses large static interbody spacers through a lateral approach. However, static spacers have been associated with iatrogenic distraction and excessive impaction forces, which may increase the risk of subsidence and loss of lordosis, compromising stability. Expandable interbody spacers with or without adjustable lordosis may help address these concerns by maximizing segmental lordosis and aiding in sagittal balance correction. This study describes the clinical and radiographic outcomes of patients treated with expandable interbody spacers with or without adjustable lordosis, for MIS LLIF. Materials and methods This is retrospective, single-surgeon Institutional Review Board-exempt chart review was of 103 consecutive patients who had undergone MIS LLIF at one to two contiguous level(s) utilizing expandable interbody spacers with or without adjustable lordosis (66/103 patients had adjustable lordosis spacers). Collection of clinical and radiographic functional outcomes occurred at preoperative and postoperative time points through 24 months. Results One-hundred and three consecutive patients were evaluated-average age, 58.2 ± 12.1 years; 42.1% (45/107) were female. There were 78.6% (81/103) one-level cases and 21.4% (22/103) two-level cases for a total of 125 levels; 44.8% (56/125) were performed at L4-5 and 34.4% (43/125) at L3-4. The average estimated blood loss was 24.6 ± 12.3cc. Mean operative time was 61.0 ± 19.1 min, and mean fluoroscopic time was 28.2 ± 14.6 sec. Visual Analog Scale (VAS) back and leg pain scores decreased significantly by an average of 7.1 ± 1.0 points at 24 months ( p <0.001). Oswestry Disability Index (ODI) scores significantly decreased by a mean of 67.4 ± 8.9 points at 24 months ( p <0.001). Lumbar lordosis significantly improved by a mean of 3.1 ± 8.8° at 24 months ( p =0.001). Anterior, middle, and posterior disc height significantly increased at 24 months by averages of 4.7 ± 3.1, 4.0 ± 3.0, and 2.1 ± 2.2mm, respectively ( p <0.001). Neuroforaminal height had significantly increased at 24 months by a mean of 3.0 ± 3.6mm ( p <0.001). Segmental lordosis significantly improved by 3.7 ± 2.9° at 24 months ( p <0.001). There were 51 patients with abnormal preoperative Pelvic Incidence-Lumbar Lordosis (PI-LL) measurements that significantly improved by 9.1 ± 4.9° ( p< 0.001) and 52 patients with normal preoperative PI-LL measurements that improved by 0.2 ± 4.6° ( p= 0.748) at 24 months. One-hundred percent fusion occurred at all levels, and no findings of radiolucency were observed. One case of subsidence (1/125, 0.8%) was reported at 24 months. No implanted-related complications were reported, with 0% pseudoarthrosis and no secondary surgery required at the operative levels. Conclusion Indirect decompression and sagittal correction were achieved and maintained through a 24-month follow-up. Functional clinical outcomes significantly improved based on decreased VAS pain and ODI scores at 24 months. This study resulted in positive clinical and radiographic outcomes for patients who underwent MIS LLIF with expandable interbody spacers with or without adjustable lordosis., Competing Interests: The authors have declared financial relationships, which are detailed in the next section., (Copyright © 2021, Li et al.)

Background Context: Osseointegration is a pivotal process in achieving a rigid fusion and ultimately a successful clinical outcome following interbody fusion surgery. Advancements in 3D printing technology permit commonly used titanium interbody spacers to be designed with unique architectures, such as a highly interconnected and specific porous structure that mimics the architecture of trabecular bone. Interbody implants with a microscale surface roughness and biomimetic porosity may improve bony ongrowth and ingrowth compared to traditional materials., Purpose: The purpose of this study was to compare the osseointegration of lumbar interbody fusion devices composed of surgical-grade polyetheretherketone (PEEK), titanium-alloy (TAV), and 3D-printed porous, biomimetic TAV (3DP) using an in vivo ovine model., Study Design: In Vivo Preclinical Animal Study METHODS: Eighteen sheep underwent two-level lateral lumbar interbody fusion randomized with either 3DP, PEEK, or TAV interbody spacers (n=6 levels for each spacer per time point). Postoperative time points were 6 and 12 weeks. Microcomputed tomography and histomorphometry were used to quantify bone volume (BV) within the spacers (ingrowth) and the surface bone apposition ratio (BAR) (ongrowth), respectively., Results: The 3DP-treatment group demonstrated significantly higher BV than the PEEK and TAV groups at 6 weeks (77.3±44.1 mm 3 , 116.9±43.0 mm 3 , and 108.7±15.2 mm 3 , respectively) (p<.05). At 12 weeks, there were no BV differences between groups (p>.05). BV increased in all groups from the 6- to 12-week time points (p<.05). At both time points, the 3DP-treated group (6w: 23.6±10.9%; 12w: 36.5±10.9%) had significantly greater BAR than the PEEK (6w: 8.6±2.1%; 12w: 14.0±5.0%) and TAV (6w: 6.0±5.7%; 12w: 4.1±3.3%) groups (p<.05)., Conclusions: 3DP interbody spacers facilitated greater total bony ingrowth at 6 weeks, and greater bony ongrowth postoperatively at both 6 and 12 weeks, in comparison to solid PEEK and TAV implants., Clinical Significance: Based on these findings, the 3DP spacers may be a reasonable alternative to traditional PEEK and TAV spacers in various clinical applications of interbody fusion., (Copyright © 2021. Published by Elsevier Inc.)

Objective: Traditional iliac (TI) screws require extensive dissection, involve offset-connectors, and have prominent screw heads that may cause patient discomfort. S2 alar-iliac (S2AI) screws require less dissection, do not need offset connectors, and are less prominent. However, the biomechanical consequences of S2AI screws crossing the alar-iliac joint is unknown. The present study investigates the fixation strength of a modified iliac (MI) screw, which has a more medial entry point and reduced screw prominence, but does not cross the alar-iliac joint., Methods: Eighteen sacropelvic spines were divided into 3 groups (n = 6): TI, S2AI, and MI. Each specimen was fixed unilaterally with S1 pedicle screws and pelvic fixation according to its group. Screws were loaded at ±10 Nm at 3Hz for 1000 cycles. Motion of each screw and rod strain above and below the S1 screw was measured., Results: Toggle of the S1 screw was lowest for the TI group, followed by the MI and S2AI groups, but there were no significant differences (P = 0.421). Toggle of the iliac screw relative to the pelvis was also lowest for the TI group, followed by the MI group, and was greatest for the S2AI group, without significant differences (P = 0.179). Rod strain was similar across all groups., Conclusions: No statistically significant differences were found between the TI, S2AI, and MI techniques with regard to screw toggle or rod strain. Advantages of the MI screw include its lower profile and a medialized starting point eliminating the need for offset-connectors., (Published by Elsevier Inc.)

Background: There is evidence that interfragmentary fracture gap size may affect the compression achievable with a modern headless compression screw (HCS). This mechanical study compared the compression achieved by 3 commercial HCS systems through various fracture gaps: CAPTIVATE Headless (Globus Medical, Inc, Audubon, Pennsylvania), Synthes (DePuy Synthes, Westchester, Pennsylvania), and Acumed Acutrak 2 (Acumed LLC, Hillsboro, Oregon). Methods: Screws were inserted into a custom test fixture composed of polyurethane synthetic bone foam fragments, separated by a layer of easily compressible polyurethane foam simulating a fracture gap. Compression was measured after final insertion and countersinking. The effect of the interfragmentary fracture gap size on the compression generated was also investigated. Results: The CAPTIVATE Headless 3.0 mm screw (70.1 ± 5.7 N) and the Synthes 3.0 mm screw (64.9 ± 7.3 N) achieved similar compressive forces after final countersink. Similar comparisons were found for the CAPTIVATE Headless 2.5 mm and Synthes 2.4 mm screws, and the CAPTIVATE Headless 4.0 mm and Acutrak 2 Standard screws. The final compression of the CAPTIVATE Headless 2.5 mm and Synthes 2.4 mm screws was not significantly affected when the fracture gap was doubled from 2 to 4 mm, but was reduced significantly by 95.9% with the Acutrak 2 Micro screw. Conclusion: When comparing like-sized screws, the CAPTIVATE, Synthes, and Acutrak 2 HCS systems demonstrated similar potential compressive forces. However, compared with the CAPTIVATE Headless and Synthes HCS systems, which are inserted with a compression sleeve that is not gap distance-dependent, the Acutrak 2 HCS system demonstrated less compression when the simulated fracture gap size was increased to 4 mm.

Study Design: In vitro biomechanical study., Objective: Investigate effects of sacroiliac joint (SIJ) fusion and iliac fixation on distal rod strain in thoracolumbar fusions., Summary of Background Data: Instrument failure is a multifactorial, challenging problem frequently encountered by spinal surgeons. Increased rod strain may lead to instrumentation failure and rod fracture., Methods: Seven fresh frozen human cadaveric specimens (T9-pelvis) used. Six operative constructs tested to investigate changes in rod strain at L5-S1 and S1-Ilium rods, posterior pedicle screws/rods from T10-S1 (PS), PS + bilateral iliac screw fixation, PS + unilateral iliac screw fixation (UIS), PS+UIS+3 unilateral SIJ screws, PS + 3 unilateral SIJ screws, and PS +6 bilateral SIJ screws. Uniaxial strain gauges were used to measure surface strain of rods during flexion-extension., Results: In flexion-extension, bilateral iliac screws added significant strain to L5-S1 compared with long fusion constructs ending at S1 (PS) (P < 0.05). Unilateral iliac fixation exhibited highest strain to L5-S1 ipsilateral rod, was significantly higher compared with bilateral iliac fixation and PS construct. Unilateral and bilateral SIJ fusion did not significantly change L5-S1 rod strain compared with PS. When measuring S1-Ilium rod strain, unilateral pelvic fixation had highest reported rod strain, approached significance compared with bilateral iliac screws (P = 0.054). Addition of contralateral SIJ fusion did not affect rod strain at S1-ilium on side with unilateral fixation., Conclusion: Results showed additional fixation below S1 to pelvis added significant rod strain. Unilateral pelvic screws had highest rod strain; SIJ fusion did not affect rod strain. Findings can help guide surgeons when associated risk of rod failure is a consideration.Level of Evidence: N/A., (Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.)

Background: A well-known problematic sequela of chest wall resections is development of scoliosis. Despite the seriousness and frequency of scoliosis following chest well resection, the etiology and biomechanical information needed to understand this progression aren't well-known., Methods: Range of motion of six specimen (C7-L2) was captured using a custom-built six degrees-of-freedom machine in each of three physiological rotation axes. Left posterior ribs were sequentially resected 7cm from the rib head, starting at the 5 th rib and continuing until the 10 th rib. Injured specimen were instrumented with unilateral anterior rod fixation and then with additional unilateral posterior fixation, each starting at T4 and then extended distally as ribs were resected. Relative motion between the constructs' proximal and distal ends was measured in all three axes for the intact, injured, unilateral anterior, and unilateral anterior with unilateral posterior constructs., Findings: Raw motion of the injured specimen increased in a stepwise manner as ribs were resected. Averaged across all injury sizes, the unilateral anterior construct significantly reduced motion by 47.0±13.4% in lateral bending (P=.001). The combined anterior-posterior construct significantly reduced motion by 57.6±15.9% in flexion/extension (P<.001), 70.3±12.2% in lateral bending (P<.001), and 51.1±14.5% in axial rotation (P<.001). Combined anterior-posterior fixation was significantly more stable than anterior-only fixation in flexion/extension (P=.002)., Interpretation: Regardless of injury size, posterior rib resection did not create significant immediate instability of the thoracic spine. Concurrent spinal stabilization was shown to maintain thoracic spine stability. Combined anterior-posterior fixation proved to be significantly more rigid than an anterior-only construct., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Background: Integrated lateral lumbar interbody fusion (LLIF) devices have been shown to successfully stabilize the spine and avoid complications related to posterior fixation. However, LLIF has increased subsidence risk in osteoporotic patients. Cement augmentation through cannulated pedicle screws enhances pedicle fixation and cage-endplate interface yet involves a posterior approach. Lateral application of cement with integrated LLIF fixation has been introduced and requires characterization. The present study set out to evaluate kinematic and load-to-failure properties of a novel cement augmentation technique with an integrated LLIF device, alone and with unilateral pedicle fixation, compared with bilateral pedicle screws and nonintegrated LLIF (BPS + S)., Methods: Twelve specimens (L3-S1) underwent discectomy at L4-L5. Specimens were separated into 3 groups: (1) BPS + S; (2) polymethyl methacrylate (PMMA) augmentation, integrated LLIF, and unilateral pedicle screws (PMMA + UPS + iS); and (3) PMMA and integrated LLIF (PMMA + iSA) without posterior fixation. Flexion-extension, lateral bending, and axial rotation were applied. A compressive load was applied to L4-L5 segments until failure. An analysis was performed ( P < .05)., Results: Operative constructs significantly reduced motion relative to intact specimens in all motion planes ( P < .05). BPS + S provided the most stability, reducing motion by 71.6%-86.4%, followed by PMMA + UPS + iS (68.1%-79.4%) and PMMA + iSA (62.9%-81.9%); no significant differences were found ( P > .05). PMMA + UPS + iS provided the greatest resistance to failure (2290 N), followed by PMMA + iSA (1970 N) and BPS + S (1390 N); no significant differences were observed ( P > .05)., Conclusions: Cement augmentation of vertebral endplates via the lateral approach with integrated LLIF moderately improved cage-endplate strength compared to BPS + S in an osteoporotic model; unilateral pedicle fixation further improved failure load. Reconstruction before and after application of unilateral pedicle screws and rods was biomechanically equivalent to anteroposterior reconstruction. Overall, initial results suggest that integrated LLIF with cement augmentation may be a viable alternative in the presence of osteoporosis., (This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2021 ISASS.)

Study Design: In silico finite element study., Objective: The aim of this study was to evaluate the effect of six construct factors on apical rod strain in an in silico pedicle subtraction osteotomy (PSO) model: traditional inline and alternative Ames-Deviren-Gupta (ADG) multi-rod techniques, number of accessory rods (three- vs. four-rod), rod material (cobalt-chrome [CoCr] or stainless steel [SS] vs. titanium [Ti]), rod diameter (5.5 vs. 6.35 mm), and use of cross-connectors (CC), or anterior column support (ACS)., Summary of Background Data: Rod fracture following lumbar PSO is frequently reported. Clinicians may modulate reconstructs with multiple rods, rod position, rod material and diameter, and with CC or ACS to reduce mechanical demand or rod contouring. A comprehensive evaluation of these features on rod strain is lacking., Methods: A finite element model (T12-S1) with intervertebral discs and ligaments was created and validated with cadaveric motion data. Apical rod strain of primary and accessory rods was collected for 96 constructs across all six construct factors, and normalized to the Ti two-rod control., Results: Regardless of construct features, CoCr and SS material reduced strain across all rods by 49.1% and 38.1%, respectively; increasing rod diameter from 5.5 mm to 6.35 mm rods reduced strain by 32.0%. Use of CC or lumbosacral ACS minimally affected apical rod strain (<2% difference from constructs without CC or ACS). Compared to the ADG technique, traditional inline reconstruction reduced primary rod strain by 32.2%; however, ADG primary rod required 14.2° less rod contouring. The inline technique produced asymmetrical loading between left and right rods, only when three rods were used., Conclusion: The number of rods and position of accessory rods affected strain distribution on posterior fixation. Increasing rod diameter and using CoCr rods was most effective in reducing rod strain. Neither CC nor lumbosacral ACS affected apical rod strain., Level of Evidence: N/A.

Study Design: In silico finite element study., Objective: The aim of this study was to evaluate effects of six construct factors on rod and screw strain at the lumbosacral junction in an in silico pedicle subtraction osteotomy (PSO) model: traditional inline and alternative Ames-Deviren-Gupta (ADG) multi-rod techniques, number of accessory rods (three-rod vs. four-rod), rod material (cobalt-chrome [CoCr] or stainless steel [SS] vs. titanium [Ti]), rod diameter (5.5 vs. 6.35 mm), and use of cross-connectors (CC), or anterior column support (ACS)., Summary of Background Data: Implant failure and pseudoarthrosis at the lumbosacral junction following PSO are frequently reported. Clinicians may modulate reconstructs with multiple rods, rod position, rod material, and diameter, and with CC or ACS to reduce mechanical demand. An evaluation of these features' effects on rod and screw strains is lacking., Methods: A finite element model (T12-S1) with intervertebral discs and ligaments was created and validated with cadaveric motion data. Lumbosacral rod and screw strain data were collected for 96 constructs across all six construct factors and normalized to the Ti 2-Rod control., Results: The inline technique resulted in 12.5% to 51.3% more rod strain and decreased screw strain (88.3% to 95%) compared to ADG at the lumbosacral junction. An asymmetrical strain distribution was observed in the three-rod inline technique in comparison to four-rod, which was more evenly distributed. Regardless of construct features, rod strain was significantly decreased by rod material (CoCr > SS > Ti), and increasing rod diameter from 5.5 mm to 6.35 mm reduced strain by 9.9% to 22.1%. ACS resulted in significant reduction of rod (37.8%-59.8%) and screw strains (23.2%-65.8%)., Conclusion: Increasing rod diameter, using CoCr rods, and ACS were the most effective methods in reducing rod strain at the lumbosacral junction. The inline technique decreased screw strain and increased rod strain compared to ADG., Level of Evidence: N/A.

Background  Bridge plating for distal radius fractures is indicated for complex fractures with comminution, extensive articular involvement, and/or cases requiring immediate weight bearing. Bridge plate fixation of distal radius fractures is a well-documented treatment method; however, failures have been reported with repetitive loading through the bridged distal radius fracture. Plate design is implicated as a cause of plate fracture in select clinical studies but few mechanical tests comparing bridge plate designs have been reported. This study sought to determine the impact of plate design on bridge plates intended to allow for immediate weight-bearing. Methods  Axial static ( n  = 3) and dynamic testing ( n  = 3) was performed on three distraction plates designs: bridge plate 1 (BP1) with central holes, bridge plate 2 (BP2) without central holes, and locking compression plate (BP3). Plates were loaded in axial compression with a simulated 10-mm fracture gap. Results  Significant static load differences were noted between all groups. Static load to failure for BP1, BP3, and BP2 were 240 ± 5 N, 398 ± 9 N, and 420 ± 3 N, respectively ( p  < 0.05). BP1 was the only plate series that failed during dynamic testing; all other plates achieved 100,000 cycles. Failure mode was a fracture occurring through the central screw hole of BP1. Finite element analysis demonstrated the effects of central screw holes on stress, strain, and plastic deformation under loading. Conclusion  Unused screw holes are the mechanical weak points; plates designed without these central screw holes are expected to survive greater load values. The threshold for clinical importance will need to be determined by future studies., Competing Interests: Conflict of Interest The authors would like to disclose that this study was performed at Globus Medical Inc., where G. M. H., J. A. H., W. W., and B. S. B. are employees. A. M. I. is a paid consultant for Globus Medical Inc. B. S. B. reports personal fees from Globus Medical, Inc., during the conduct of the study. J. A. H. reports personal fees from Globus Medical, Inc., during the conduct of the study. G. M. H. reports personal fees from Globus Medical, Inc., during the conduct of the study. A. I. reports personal fees from Globus Medical, Inc., during the conduct of the study. W. W. reports personal fees from Globus Medical, Inc., during the conduct of the study. A. M. I. is a consultant for and receive royalties and consulting fees from a company involved in the manufacture of a device examined in this study. G. M. H., J. A. H., and W. W. are salaried employees of a company involved in the manufacture of a device examined in this study. All plates and screws were either purchased or provided by the manufacturer of a device examined in this study., (Thieme. All rights reserved.)

Background: Traditional pedicle screws are the gold standard for lumbar spine fixation; however, cortical screws along the midline cortical bone trajectory may be advantageous when lumbar decompression is required. While biomechanic investigation of both techniques exists, cortical screw performance in a multi-level lumbar laminectomy and fusion model is unknown. Furthermore, longer-term viability of cortical screws following cyclic fatigue has not been investigated., Methods: Fourteen human specimens (L1-S1) were divided into cortical and pedicle screw treatment groups. Motion was captured for the following conditions: intact, bilateral posterior fixation (L3-L5), fixation with laminectomy at L3-L5, fixation with laminectomy and transforaminal lumbar interbody fusion at L3-L5 both prior to, and following, simulated in vivo fatigue. Following fatigue, screw pullout force was collected and "effective shear stress" [pullout force/screw surface area] (N/mm 2 ) was calculated; comparisons and correlations were performed., Findings: In flexion-extension and lateral bending, all operative constructs significantly reduced motion compared to intact (P < 0.05), regardless of pedicle or cortical screws; only posterior fixation with and without laminectomy significantly reduced motion in axial rotation (P < 0.05). Pedicle screws significantly increased average pullout strength (944.2 N vs. 690.2 N, P < 0.05), but not the "effective shear stress" (1.01 N/mm 2 vs. 1.1 N/mm 2 , P > 0.05)., Interpretation: In a posterior laminectomy and fusion model, cortical screws provided equivalent stability to pedicle screw fixation, yet had significantly lower screw pullout force. No differences in "effective shear stress" warrant further investigation of the effect of screw length/diameter in the aforementioned screw trajectories., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Study Design: Cadaveric study., Objective: To evaluate accuracy, radiation exposure, and surgical time of a new robotic-assisted navigation (RAN) platform compared with freehand techniques in conventional open and percutaneous procedures., Methods: Ten board-certified surgeons inserted 16 pedicle screws at T10-L5 (n = 40 per technique) in 10 human cadaveric torsos. Pedicle screws were inserted with (1) conventional MIS technique (L2-L5, patient left pedicles), (2) MIS RAN (L2-L5, patient right pedicles), (3) conventional open technique (T10-L1, patient left pedicles), and (4) open RAN (T10-L1, patient right pedicles). Output included (1) operative time, (2) number of fluoroscopic images, and (3) screw accuracy., Results: In the MIS group, compared with the freehand technique, RAN allowed for use of larger screws (diameter: 6.6 ± 0.6 mm vs 6.3 ± 0.5 mm; length: 50.3 ± 4.1 mm vs 46.9 ± 3.5 mm), decreased the number of breaches >2 mm (0 vs 7), fewer fluoroscopic images (0 ± 0 vs 108.3 ± 30.9), and surgical procedure time per screw (3.6 ± 0.4 minutes vs 7.6 ± 2.0 minutes) (all P < .05). Similarly, in the open group, RAN allowed for use of longer screws (46.1 ± 4.1 mm vs 44.0 ± 3.8 mm), decreased the number of breaches >2 mm (0 vs 13), fewer fluoroscopic images (0 ± 0 vs 24.1 ± 25.8) (all P < .05), but increased total surgical procedure time (41.4 ± 8.8 minutes vs 24.7 ± 7.0 minutes, P = .000) while maintaining screw insertion time (3.31.4 minutes vs 3.1 ± 1.0 minutes, P = .650)., Conclusion: RAN significantly improved accuracy and decreased radiation exposure in comparison to freehand techniques in both conventional open and percutaneous surgical procedures in cadavers. RAN significantly increased setup time compared with both conventional procedures.

Background: Adult spinal deformity surgery requires use of long thoracolumbar instrumentation, which is associated with risk of postoperative proximal junctional kyphosis (PJK). Tethering has been used in spinal surgery but not around the spinous process (SP) in the context of preventing PJK., Methods: Researchers applied a nondestructive hybrid loading protocol to 7 T8-L2 cadaveric specimens in flexion-extension, lateral bending, and axial rotation (AR). A rigid construct (pedicle screws and rods) and 1- and 2-level SP constructs were tested, as was a hand-tie technique. SP tethering (SPT) constructs use clamps on both sides of the SP; SPT helix constructs use 1 clamp and wrap around the SP., Results: All tether constructs showed greater motion at the instrumented level and less motion at adjacent levels compared to rigid constructs. In AR, 1- and 2-level SPT constructs restricted first instrumented level motion to a greater extent when compared with other tether constructs ( P ≤ .05). Passing the band through the T10 SP did not produce significant biomechanical differences compared to passing it through the T9-T10 interspinous ligament ( P > .05). Hand-tied constructs demonstrated more motion compared to tensioned constructs ( P > .05). Intradiscal pressure results corroborated motion data., Conclusions: SPT at the proximal end of a rigid construct produced more favorable biomechanical outcomes at instrumented and adjacent levels than were seen with a completely rigid construct. Clinical research is needed to determine whether these methods reduce the risk of PJK among patients., Level of Evidence: 3., Clinical Relevance: This work sheds light on the biomechanical stability of proximal tethering constructs in an effort to enhance the surgeon's ability to reduce rates of proximal junctional kyphosis and failure in thoracolumbar spinal fusion surgery., (This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2020 ISASS.)

Study Design: Finite-element analysis., Objectives: Intraoperative contouring of rods is a common procedure for spine surgeons to match the native curvature of the spine, but it may lead to premature weakening of the rod. This study investigated the effect of different bending methods on rod fatigue performance. Rod failure in the cervical spine is of clinical concern, particularly when spanning the cervicothoracic region and when considering corrective osteotomies for deformity correction and global spinal alignment., Methods: Finite-element models were developed to simulate rod bending (3.5 mm D, 40 mm L) to achieve a 23° angle with 3 different bending methods: French single, multiple bending, and in situ bending. Simulations were conducted in 4 steps: rod bending, rod spring back, residual stress relaxation, and F1717 mechanical test simulation., Results: French single bending resulted in the highest residual stress concentrations for both titanium (TiAlV) and cobalt chrome (CoCr) at 783 MPa and 507 MPa, respectively. During F1717 test simulation, the French single bent rod had its highest tensile stress in the middle, with 917 MPa and 623 MPa, respectively, for TiAlV and CoCr, compared to in situ (580 MPa and 586 MPa for TiAlV and CoCr) and the French multiple bent rod (765 MPa and 619 MPa for TiAlV and CoCr). The computational model found that CoCr rods made the construct least prone to deformation., Conclusions: French single bend with TiAlV rods put the construct at highest risk of failure. CoCr rods led to minimal physical changes in microstructure while showing evidence of flattening.

Study Design: Cadaveric study., Summary of Background Data: Pedicle screw fixation is an established means of stabilizing the thoracic and lumbar spine. However, there are associated complications including pedicle breach which can result in neurological injury, durotomy, vascular injury, and suboptimal fixation., Objective: The aim of this study is to determine whether use of a navigated robotic platform results in fewer pedicle breaches and the underlying reasons for any difference in pedicle breach rates., Materials and Methods: Ten board-certified neuro- and orthopedic spine surgeons inserted 80 percutaneous lumbar screws in 10 unembalmed human cadavers. Forty screws were inserted using conventional fluoroscopic guidance and 40 were inserted using a navigated robotic platform. None of the participating surgeons had any prior experience with navigated robotic spine surgery. At the end of the study each screw was assessed with a computed tomography scan, plain radiographs and visual inspection to determine the presence or absence of pedicle breaches., Results: Forty percent (40%) of screws inserted using conventional fluoroscopic guidance breached compared with 2.5% of screws inserted with robot assistance (P=0.00005). Lateral breaches accounted for 88.2% (15/17) of all breaches. Detailed analysis revealed that the starting point of screws that breached laterally were significantly more lateral than that of the contralateral accurate screw (P=0.016). Pedicle screw diameter, length, and angulation in the transverse plane did not differ significantly between accurate screws and those that breached (P>0.05)., Conclusions: The use of a navigated robotic platform in the present study resulted in significantly fewer pedicle breaches. This was achieved through correct starting point selection with subsequent safe pedicle screw insertion.

Background Context: The use of zero-profile devices and the need for posterior fixation in conjunction with a cervical hybrid decompression model have yet to be investigated., Purpose: To compare the biomechanics of zero-profile and fixed profile cervical hybrid constructs composed of anterior cervical discectomy and fusion (ACDF) and anterior cervical corpectomy and fusion (ACCF). Fixed profile devices included anterior plating, whereas zero-profile devices included integrated screws., Study Design: In vitro cadaveric biomechanical study., Methods: Twelve fresh-frozen cadaveric spines (C2-C7) were divided into two groups of equal bone mineral density, fixed profile versus zero profile (n=6). Groups were instrumented from C3-C6 with either (1) an expandable ACCF device and a static ACDF spacer with an anterior plate (Hybrid-AP) or (2) a zero-profile ACCF spacer with adjacent zero-profile ACDF spacer (Hybrid-Z). Motion was captured for the (1) intact condition, (2) a hybrid model with lateral mass screws (LMS), (3) a hybrid model without LMS, and (4) a hybrid model without LMS following simulated repetitive loading (fatigue)., Results: Hybrid-AP with LMS reduced motion in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) by 77%, 88%, and 82%, respectively, compared with intact. Likewise, Hybrid-Z with LMS exhibited the greatest reduction in motion relative to intact in FE, LB, and AR by 90%, 95%, and 66%, respectively. Following simulated in vivo fatiguing, an increase in motion was observed for both groups in all planes, particularly during Hybrid-Z postfatigue condition where motion increased relative to intact by 29%. Overall, biomechanical equivalency was observed between Hybrid-AP and Hybrid-Z groups (p>.05). Three (50%) of the Hybrid-Z group specimens exhibited signs of implant migration from the inferior endplate during testing., Conclusions: Fixed profile systems using an anterior plate for supplemental fixation is biomechanically more favorable to maintain stability and prevent dislodgement. Dislodgement of 50% of the Hybrid-Z group without LMS emphasizes the necessity for posterior fixation in a zero-profile cervical hybrid decompression model., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Background: Although the rib cage provides substantial stability to the thoracic spine, few biomechanical studies have incorporated it into their testing model, and no studies have determined the influence of the rib cage on adjacent segment motion of long fusion constructs. The present biomechanical study aimed to determine the mechanical contribution of the intact rib cage during the testing of instrumented specimens., Methods: A cyclic loading (CL) protocol with instrumentation (T4-L2 pedicle screw-rod fixation) was conducted on five thoracic spines (C7-L2) with intact rib cages. Range of motion (±5 Nm pure moment) in flexion-extension, lateral bending, and axial rotation was captured for intact ribs, partial ribs, and no ribs conditions. Comparisons at the supra-adjacent (T2-T3), adjacent (T3-T4), first instrumented (T4-T5), and second instrumented (T5-T6) levels were made between conditions (P ≤ 0.05)., Findings: A trend of increased motion at the adjacent level was seen for partial ribs and no ribs in all 3 bending modes. This trend was also observed at the supra-adjacent level for both conditions. No significant changes in motion compared to the intact ribs condition were seen at the first and second instrumented levels (P > 0.05)., Interpretation: The segment adjacent to long fusion constructs, which may appear more grossly unstable when tested in the disarticulated spine, is reinforced by the rib cage. In order to avoid overestimating adjacent level motion, when testing the effectiveness of surgical techniques of the thoracic spine, inclusion of the rib cage may be warranted to better reflect clinical circumstances., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Study Design: In vitro cadaveric biomechanical study., Objective: To compare the biomechanics of integrated anchor and blade versus traditional screw fixation techniques for interbody fusion., Methods: Fifteen cadaveric cervical spines were divided into 3 equal groups (n = 5). Each spine was tested: intact, after discectomy (simulating an injury model), interbody spacer alone (S), integrated interbody spacer (iSA), and integrated spacer with lateral mass screw and rod fixation (LMS+iS). Each treatment group included integrated spacers with either screw, anchor, or blade integrated spacers. Constructs were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) under pure moments (±1.5 N m)., Results: Across all 3 planes, the following range of motion trend was observed: Injured > Intact > S > iSA > LMS+iS. In FE and LB, integrated anchor and blade significantly decreased motion compared with intact and injured conditions, before and after supplemental posterior fixation ( P < .05). Comparing tested devices revealed biomechanical equivalence between screw, anchor, and blade fixation methods in all loading modes ( P > .05)., Conclusion: All integrated interbody devices reduced intact and injured motion; lateral mass screws and rods further stabilized the single motion segment. Comparing screw, anchor, or bladed integrated anterior cervical discectomy and fusion spacers revealed no significant differences., Competing Interests: Declaration of Conflicting Interests: The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: The authors would like to disclose that the study was performed at Globus Medical, Inc (GMI), using its 6-degrees-of-freedom motion simulator. Surgeons PMA, IC (Globus Medical—royalties, Nuvasive—royalties, Stryker Spine—consultant, SpinalCyte—stock, Spine Innovations—stock, Cytonics—stock, Spine Wave—royalties, SpineCraft—consultant), CZ, and BK have no conflict of interests related to the devices examined in this study; none were compensated for their efforts in the present study. Cadaveric specimens and related materials were provided by GMI, at which JAH, MM, and BSB are employees.

Background: Sacroiliac joint hypermobility or aberrant mechanics may be a source of pain. The purpose of this study was to assess sacroiliac joint range of motion after simulated adjacent lumbosacral instrumented fusion, with or without sacroiliac joint fusion, with lateral sacroiliac screws., Methods: In this in vitro biomechanical study, seven cadaveric specimens were tested on a six-degrees-of-freedom machine under load control. Left posterior sacroiliac joint ligaments were severed to maximize joint range of motion. Influence of lumbosacral instrumentation on sacroiliac joint motion, with or without fixation, was studied., Findings: During flexion-extension in the setting of posterior sacroiliac joint injury and L5-S1 fixation, sacroiliac joint range of motion increased to 195% of intact. After fixation with lateral sacroiliac screws, average range of motion reduced to 144% of intact motion. Sacroiliac joint screws thus partially stabilized the joint and reduced motion. Use of 6 bilateral sacroiliac joint screws with L5-S1 screw and rod fixation in lateral bending and axial rotation yielded the greatest reduction in range of motion. Without lumbosacral fixation, baseline motion of the sacroiliac joint was reduced, and sacroiliac joint screw alone, using either 2, 3, or 6 screws, was able to restore motion at or below the level of an intact joint., Interpretation: Sacroiliac joint ligament injury with existing lumbosacral fixation doubled sacroiliac joint range of motion, but thereafter, fixation with lateral sacroiliac screws decreased range of motion of the injured sacroiliac joint. Screw configuration played a minor role, but generally, 6 sacroiliac joint screws had the greatest motion reduction., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Objective: The sacroiliac joint (SIJ) is a known source of low-back pain. Randomized clinical trials support sacroiliac fusion over conservative management for SIJ dysfunction. Clinical studies suggest that SIJ degeneration occurs in the setting of lumbosacral fusions. However, there are few biomechanical studies to provide a good understanding of the effect of lumbosacral fusion on the SIJ. In the present study, researchers performed a biomechanical investigation to discern the effect of pelvic versus SIJ fixation on the SIJ in lumbosacral fusion., Methods: Seven fresh-frozen human cadaveric specimens were used. There was one intact specimen and six operative constructs: 1) posterior pedicle screws and rods from T10 to S1 (PS); 2) PS + bilateral iliac screw fixation (BIS); 3) PS + unilateral iliac screw fixation (UIS); 4) PS + UIS + 3 contralateral unilateral SIJ screws (UIS + 3SIJ); 5) PS + 3 unilateral SIJ screws (3SIJ); and 6) PS + 6 bilateral SIJ screws (6SIJ). A custom-built 6 degrees-of-freedom apparatus was used to simulate three bending modes: flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Range of motion (ROM) was recorded at L5-S1 and the SIJ., Results: All six operative constructs had significantly reduced ROM at L5-S1 in all three bending modes compared to that of the intact specimen (p < 0.05). In the FE mode, the BIS construct had a significant reduction in L5-S1 ROM as compared to the other five constructs (p < 0.05). SIJ ROM was greatest in the FE mode compared to LB and AR. Although the FE mode did not show any statistically significant differences in SIJ ROM across the constructs, there were appreciable differences. The PS construct had the highest SIJ ROM. The BIS construct reduced bilateral SIJ ROM by 44% in comparison to the PS construct. The BIS and 6SIJ constructs showed reductions in SIJ ROM nearly equal to those of the PS construct. UIS and 3SIJ showed an appreciable reduction in unfused SIJ ROM compared to PS., Conclusions: This investigation demonstrated the effects of various fusion constructs using pelvic and sacroiliac fixation in lumbosacral fusion. This study adds biomechanical evidence of adjacent segment stress in the SIJ in fusion constructs extending to S1. Unilateral pelvic fixation, or SIJ fusion, led to an appreciable but nonsignificant reduction in the ROM of the unfused contralateral SIJ. Bilateral pelvic fixation showed the greatest significant reduction of movement at L5-S1 and was equivalent to bilateral sacroiliac fusion in reducing SIJ motion.

Objective: Posterior reduction with pedicle screws is often used for stabilization of unstable spondylolisthesis to directly reduce misalignment or protect against micromotion while fusion of the affected level occurs. Optimal treatment of spondylolisthesis combines consistent reduction with a reduced risk of construct failure. The authors compared the reduction achieved with a novel anterior integrated spacer with a built-in reduction mechanism (ISR) to the reduction achieved with pedicle screws alone, or in combination with an anterior lumbar interbody fusion (ALIF) spacer, in a cadaveric grade I spondylolisthesis model., Methods: Grade I slip was modeled in 6 cadaveric L5-S1 segments by creation of a partial nucleotomy and facetectomy and application of dynamic cyclic loading. Following the creation of spondylolisthesis, reduction was performed under increasing axial loads, simulating muscle trunk forces between 50 and 157.5 lbs, in the following order: bilateral pedicle screws (BPS), BPS with an anterior spacer (BPS+S), and ISR. Percent reduction and reduction failure load-the axial load at which successful reduction (≥ 50% correction) was not achieved-were recorded along with the failure mechanism. Corrections were evaluated using lateral fluoroscopic images., Results: The average loads at which BPS and BPS+S failed were 92.5 ± 6.1 and 94.2 ± 13.9 lbs, respectively. The ISR construct failed at a statistically higher load of 140.0 ± 27.1 lbs. Reduction at the largest axial load (157.5 lbs) by the ISR device was tested in 67% (4 of 6) of the specimens, was successful in 33% (2 of 6), and achieved 68.3 ± 37.4% of the available reduction. For the BPS and BPS+S constructs, the largest axial load was 105.0 lbs, with average reductions of 21.3 ± 0.0% (1 of 6) and 32.4 ± 5.7% (3 of 6) respectively., Conclusions: While both posterior and anterior reduction devices maintained reduction under gravimetric loading, the reduction capacity of the novel anterior ISR device was more effective at greater loads than traditional pedicle screw techniques. Full correction was achieved with pedicle screws, with or without ALIF, but under significantly lower axial loads. The anterior ISR may prove useful when higher reduction forces are required; however, additional clinical studies will be needed to evaluate the effectiveness of anterior devices with built-in reduction mechanisms.

Study Design: In vitro cadaveric study., Purpose: To compare biomechanical performance, trial and implant insertion, and disc distraction during implant placement, when two interbody devices, an in situ continuously expandable spacer (CES) and a traditional static spacer (SS), were used for transforaminal lumbar interbody fusion., Overview of Literature: Severe degenerative disc diseases necessitate surgical management via large spacers to increase the disc space for implants. Next-generation interbody devices that expand in situ minimize insertion forces, optimize fit between vertebral endplates, and limit nerve root retraction. However, the literature lacks characterization of insertion forces as well as details on other parameters of expandable and static spacers., Methods: Ten cadaveric segments (L5-S1) were divided into two groups (n=5) and implanted with either CES or SS. Each specimen experienced unconstrained pure moment of ±6 Nm in flexion-extension, lateral bending, and axial rotation to assess the contribution of CES and SS implants in biomechanical performance. Radiographic analysis was performed during trial and implant insertion to measure distraction during spacer insertion at the posterior, central, and anterior disc regions. Pressure sensors measured the force of trial and implant insertion., Results: Biomechanical analysis showed no significant differences between CES and SS in all planes of motion. A total 2.6±0.9 strikes were required for expandable spacer trials insertion and 2.6±0.5 strikes for CES insertion. A total of 8.4±3.8 strikes were required to insert SS trials and 4.2±1.6 strikes for SS insertion. The total force per surgery was 330 N for CES and 635 N for SS. Fluoroscopic analysis revealed a significant reduction in distraction during implant insertion at the posterior and anterior disc regions (CES, 0.58 and 0.14 mm; SS, 1.04 and 0.78 mm, respectively)., Conclusions: Results from the three study arms reveal the potential use of expandable spacers. During implant insertion, CESs provided similar stability, required less insertion force, and significantly reduced over-distraction of the annulus compared with SS.

Background: Cervical vertebral body fractures generally are treated through an anterior-posterior approach. Cervical pedicle screws offer an alternative to circumferential fixation. This biomechanical study quantifies whether cervical pedicle screws alone can restore the stability of a three-column vertebral body fracture, making standard 360° reconstruction unnecessary., Methods: Range of motion (2.0 Nm) in flexion-extension, lateral bending, and axial rotation was tested on 10 cadaveric specimens (five/group) at C2-T1 with a spine kinematics simulator. Specimens were tested for flexibility of intact when a fatigue protocol with instrumentation was used to evaluate construct longevity. For a C4-6 fracture, spines were instrumented with 360° reconstruction (corpectomy spacer + plate + lateral mass screws) (Group 1) or cervical pedicle screw reconstruction (C3 and C7 only) (Group 2)., Findings: Results are expressed as percentage of intact (100%). In Group 1, 360° reconstruction resulted in decreased motion during flexion-extension, lateral bending, and axial rotation, to 21.5%, 14.1%, and 48.6%, respectively, following 18,000 cycles of flexion-extension testing. In Group 2, cervical pedicle screw reconstruction led to reduced motion after cyclic flexion-extension testing, to 38.4%, 12.3%, and 51.1% during flexion-extension, lateral bending, and axial rotation, respectively., Interpretation: The 360° stabilization procedure provided the greatest initial stability. Cervical pedicle screw reconstruction resulted in less change in motion following cyclic loading with less variation from specimen to specimen, possibly caused by loosening of the shorter lateral mass screws. Cervical pedicle screw stabilization may be a viable alternative to 360° reconstruction for restoring multilevel vertebral body fracture., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Study Design: In vitro biomechanical investigation., Purpose: To compare the biomechanics of integrated three-screw and four-screw anterior interbody spacer devices and traditional techniques for treatment of degenerative disc disease., Overview of Literature: Biomechanical literature describes investigations of operative techniques and integrated devices with four dual-stacked, diverging interbody screws; four alternating, converging screws through a polyether-ether-ketone (PEEK) spacer; and four converging screws threaded within the PEEK spacer. Conflicting reports on the stability of stand-alone devices and the influence of device design on biomechanics warrant investigation., Methods: Fourteen cadaveric lumbar spines were divided randomly into two equal groups (n=7). Each spine was tested intact, after discectomy (injured), and with PEEK interbody spacer alone (S), anterior lumbar plate and spacer (AP+S), bilateral pedicle screws and spacer (BPS+S), circumferential fixation with spacer and anterior lumbar plate supplemented with BPS, and three-screw (SA3s) or four-screw (SA4s) integrated spacers. Constructs were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Researchers performed one-way analysis of variance and independent t -testing ( p ≤0.05)., Results: Instrumented constructs showed significantly decreased motion compared with intact except the spacer-alone construct in FE and AR ( p ≤0.05). SA3s showed significantly decreased range of motion (ROM) compared with AP+S in LB ( p ≤0.05) and comparable ROM in FE and AR. The three-screw design increased stability in FE and LB with no significant differences between integrated spacers or between integrated spacers and BPS+S in all loading modes., Conclusions: Integrated spacers provided fixation statistically equivalent to traditional techniques. Comparison of three-screw and four-screw integrated anterior lumbar interbody fusion spacers revealed no significant differences, but the longer, larger-diameter interbody spacer with three-screw design increased stabilization in FE and LB; the diverging four-screw design showed marginal improvement during AR., Competing Interests: Conflict of Interest: Travel fare and materials for the study were provided by Globus Medical, Inc. C.A.K. did not receive funds related to his participation in this study. C.A.K. has nothing to disclose. J.A.H., M.M.H., A.M., B.S.B., and S.K. are paid employees of Globus Medical, Inc.

Purpose: To investigate biomechanical properties of posterior transpedicular-transdiscal (TPTD) oblique lumbar screw fixation whereby the screw traverses the inferior pedicle across the posterior disc space into the super-adjacent body and lateral trapezoidal interbody spacer., Methods: Eight fresh-frozen osteoligamentous human cadaveric spines (L1-S1) were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR), with pure bending moment set at 7.5 Nm. Surgical constructs included (1) intact spine; (2) bilateral pedicle screw (BPS) fixation at L3-L4; (3) TPTD screw fixation at L3-L4; (4) lateral L3-L4 discectomy; (5) TPTD screw fixation with lateral interbody spacer (TPTD+S); and (6) BPS fixation with lateral interbody spacer (BPS+S). Peak range of motion (ROM) at L3-L4 was normalized to intact for statistical analysis., Results: In FE and LB, all posterior fixation with or without interbody spacers significantly reduced motion compared with intact and discectomy. BPS and BPS+S provided increased fixation in all planes of motion; significantly reducing FE and LB motion relative to TPTD (p = 0.005, p = 0.002 and p = 0.020, p = 0.004, respectively). In AR, only BPS significantly reduced normalized ROM to intact (p = 0.034); BPS+S provided greater fixation compared with TPTD+S (p = 0.005)., Conclusions: Investigators found less stiffness with TPTD screw fixation than with BPS regardless of immediate stabilization with lateral discectomy and spacer. Clinical use should be decided by required biomechanical performance, difficulty of installation, and extent of paraspinal tissue disruption.

Purpose: Pedicle subtraction osteotomy (PSO) is performed to treat rigid, sagittal spinal deformities, but high rates of implant failure are reported. Anterior lumbar interbody fusion has been proposed to reduce this risk, but biomechanical investigation is lacking. The goal of this study was to quantify the (1) destabilizing effects of a lumbar osteotomy and (2) contribution of anterior lumbar interbody fusion (ALIF) at the lumbosacral junction as recommended in literature., Methods: Fourteen fresh human thoracolumbosacral spines (T12-S1) were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Bilateral pedicle screws/rods (BPS) were inserted at T12-S1, cross connectors (CC) at T12-L1 and L5-S1, and anterior interbody spacers (S) at L4-5 and L5-S1. In one group, PSO was performed in seven specimens at L3. All specimens were sequentially tested in (1) Intact; (2) BPS; (3) BPS + CC; (4) BPS + S; and (5) BPS + S + CC; a second group of seven spines were tested in the same sequence without PSO. Mixed-model ANOVA with repeated measures was performed (p ≤ 0.05)., Results: At the osteotomy site (L2-L4), in FE, BPS, BPS + CC, BPS + S, BPS + CC + S reduced motion to 11.2, 12.9, 10.9, and 11.4%, respectively, with significance only found in BPS and BPS + S construction (p ≤ 0.05). All constructs significantly reduced motion across L2-L4 in the absence of PSO, across all loading modes (p ≤ 0.05). PSO significantly destabilized L2-L4 axial rotational stability, regardless of operative construction (p ≤ 0.05). Across L4-S1 and L2-S1, all instrumented constructs significantly reduced motion, in both PSO- and non-PSO groups, during all loading modes (p ≤ 0.05)., Conclusions: These findings suggest anterior interbody fusion minimally immobilizes motion segments, and interbody devices may primarily act to maintain disc height. Additionally, lumbar osteotomy destabilizes axial rotational stability at the osteotomy site, potentially further increasing mechanical demand on posterior instrumentation. Clinical studies are needed to assess the impact of this treatment strategy.

Study Design: In vitro cadaveric biomechanical study., Objective: To assess revision pullout strength of novel anchored screws (AS) versus conventional larger diameter traditional pedicle screws (TPS) in an osteoporotic model., Summary of Background Data: Pedicle screws are the most ubiquitous method of treating spinal pathologies requiring lumbar fusion. Although these screws are effective in providing 3-column stabilization of the spine, revision surgeries are occasionally necessary, particularly for geriatric and osteoporotic populations. Innovative technologies should be tested to ensure continued improvement in revision techniques., Methods: For 4 specimens at L2-L5 (T-score=-3.6±0.54), 6.5-mm-diameter TPS were inserted into left and right pedicles and were pulled out; revision screws were then inserted. Polyether-ether-ketone anchors, designed to expand around a 6.5-mm screw, were inserted into all left pedicles. On the contralateral side, 7.5-mm-diameter TPS were inserted at L2-L3, and 8.5-mm-diameter TPS at L4-L5. Pullout testing was performed at 10 mm/min. The maximum pullout strength and insertion forces were recorded., Results: The initial average pullout force (6.5-mm screw) was 837 N (±329 N) and 642 N (±318 N) in L2-L3 and L4-L5 left pedicles, and 705 N (±451 N) and 779 N (±378 N) in L2-L3 and L4-L5 right pedicles, respectively. Comparison of revision pullout forces versus initial pullout forces revealed the following: 87% and 63% for AS in L2-L3 and L4-L5 left pedicles, respectively; 56% for 7.5-mm and 93% for 8.5-mm TPS in L2-L3 and L4-L5 right pedicles, respectively., Conclusions: Anchor sleeves with 6.5-mm-diameter pedicle screws provided markedly higher resistance to screw pullout than 7.5-mm-diameter revision screws and fixation statistically equivalent to 8.5-mm-diameter screws, possibly because of medial-lateral expansion within the vertebral space and/or convex filling of the pedicle. AS results had the lowest SD, indicating minimal variability in bone-screw purchase.

Background: Lateral lumbar interbody fusion is powerful for correcting degenerative conditions, yet sagittal correction remains limited by anterior longitudinal ligament tethering. Although lordosis has been restored via ligament release, biomechanical consequences remain unknown. Investigators examined radiographic and biomechanical of ligament release for restoration of lumbar lordosis., Methods: Six fresh-frozen human cadaveric spines (L3-S1) were tested: (Miller et al., 1988) intact; (Battie et al., 1995) 8mm spacer with intact anterior longitudinal ligament; (Cho et al., 2013) 8mm spacer without intact ligament following ligament resection; (Galbusera et al., 2013) 13mm lateral lumbar interbody fusion; (Goldstein et al., 2001) integrated 13mm spacer. Focal lordosis and range of motion were assessed by applying pure moments in flexion-extension, lateral bending, and axial rotation., Findings: Cadaveric radiographs showed significant improvement in lordosis correction following ligament resection (P<0.05). The 8mm spacer with ligament construct provided greatest stability relative to intact (P>0.05) but did little to restore lordosis. Ligament release significantly destabilized the spine relative to intact in all modes and 8mm with ligament in lateral bending and axial rotation (P<0.05). Integrated lateral lumbar interbody fusion following ligament resection did not significantly differ from intact or from 8mm with ligament in all testing modes (P>0.05)., Interpretation: Lordosis corrected by lateral lumbar interbody fusion can be improved by anterior longitudinal ligament resection, but significant construct instability and potential implant migration/dislodgment may result. This study shows that an added integrated lateral fixation system can significantly improve construct stability. Long-term multicenter studies are needed., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Purpose: Lateral lumbar disc prosthesis (LLDP) is an innovative device used to restore motion in select patients through a lateral retroperitoneal approach. No in vitro biomechanical studies have been published. Further, the potential for in toto circumferential joint restoration when use of this anterior disc is combined with facet replacement remains unqualified but signifies a potentially interesting clinical direction., Methods: Researchers conducted a biomechanical feasibility study of an LLDP designed to investigate parameters of disc sizing used with bilateral facet joint replacement in a cadaveric model. Tested constructs at L4-L5 included (1) intact, (2) LLDP, (3) LLDP + wide discectomy, (4) LLDP + bilateral facetectomy, and (5) LLDP + bilateral facet joint replacement (BFJR). Investigators tested instrumented constructs (2-5) with an LLDP at compact-fit and lax-fit heights and used raw data to perform statistical analysis by repeated measures analysis of variance (ANOVA), along with Student-Newman-Keuls post hoc analysis (p ≤ 0.05)., Results: Increased height of the LLDP resulted in significantly less motion compared with intact. Widening the discectomy while using lax-fit sizing led to motion similar to intact in flexion-extension. As expected, motion was greater with lax-fit height than with compact-fit height in all loading modes and constructs, as is noted with a widened discectomy. The L4-L5 center of rotation was maintained regardless of placement of the LLDP., Conclusions: After bilateral facetectomy, reconstruction of the three-joint complex achieved by combining the LLDP with BFJR may provide a viable alternative to current clinical treatment regimens.

Study Design: Researchers created a proper type II dens fracture (DF) and quantified a novel current posterior fixation technique with spacers at C1-C2. A clinical case study supplements this biomechanical analysis., Purpose: Researchers explored their hypothesis that spacers combined with posterior instrumentation (PI) reduce range of motion significantly, possibly leading to better fusion outcomes., Overview of Literature: Literature shows that the atlantoaxial joint is unique in allowing segmental rotary motion, enabling head turning. With no intervertebral discs at these joints, multiple ligaments bind the axis to the skull base and to the atlas; an intact odontoid (dens) enhances stability. The most common traumatic injury at these strong ligaments is a type II odontoid fracture., Methods: Each of seven specimens (C0-C3) was tested on a custom-built six-degrees-of-freedom spine simulator with constructs of intact state, type II DF, C1-C2 PI, PI with joint capsulotomy (PIJC), PI with spacers (PIS) at C1-C2, and spacers alone (SA). A bending moment of 2.0 Nm (1.5°/sec) was applied in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). One-way analysis of variance with repeated measures was performed., Results: DF increased motion to 320%, 429%, and 120% versus intact (FE, LB, and AR, respectively). PI significantly reduced motion to 41%, 21%, and 8%. PIJC showed negligible changes from PI. PIS reduced motion to 16%, 14%, and 3%. SA decreased motion to 64%, 24%, and 54%. Reduced motion facilitated solid fusion in an 89-year-old female patient within 1 year., Conclusions: Type II odontoid fractures can lead to acute or chronic instability. Current fixation techniques use C1-C2 PI or an anterior dens screw. Addition of spacers alongside PI led to increased biomechanical rigidity over intact motion and may offer an alternative to established surgical fixation techniques., Competing Interests: Conflict of Interest: Authors M.M.H., N.K., S.S.Y., and B.S.B. are salaried employees of Globus Medical Inc. Authors RTD, LB, and JMD have no relationships to disclose. All biomechanical testing was performed at Globus Medical Inc.

Study Design: In vitro cadaveric biomechanical study., Objective: To assess effects of 4-rod reconstruction, rod material, and anterior column support on motion and surface rod strain in a pedicle subtraction osteotomy model., Summary of Background Data: Pedicle subtraction osteotomy (PSO) can correct significant sagittal deformity of the lumbar spine; however, revision rates are high. To reduce rod strain and the incidence of rod fracture, clinical use of multi-rod construction, cobalt chrome (CoCr) alloy rods, and interbody spacers adjacent to PSO has been proposed. Investigating both motion and rod strain is necessary to determine the biomechanical efficacy of these techniques., Methods: Five specimens (T12-S1) underwent PSO at L3 with pedicle screw stabilization at L1-S1. Pedicle subtraction was adjusted to achieve a final lordosis of 70°. Flexion-extension (FE), lateral bending, and axial rotation were applied. Linear strain gauges measured surface rod strain during FE on primary and accessory rods at PSO level. Testing evaluated (1) accessory rods (4-Rod) added at PSO level versus primary rods (2-Rod); (2) Ti versus CoCr rods; and (3) lateral interbody spacers (S) inserted adjacent to PSO. One-way and three-way analysis of variance was performed (P ≤ 0.05)., Results: All constructs significantly reduced FE and lateral bending motion relative to intact (P < 0.001). The main effect of accessory rods in reducing FE motion was significant (P = 0.021). Accessory and CoCr rods reduced relative surface strain on the primary rod, irrespective of construct (P < 0.001). CoCr 4-Rod + S provided the greatest reduction in strain (76% decrease; P = 0.003)., Conclusion: Accessory and CoCr rods provided greatest reduction in motion and rod strain at PSO level. Interbody devices minimally affected motion-induced strain and might act primarily to maintain disc height. Clinicians must assess whether surface strain and motion reduction minimize the incidence of rod fracture., Level of Evidence: N/A.

Pedicle subtraction osteotomy (PSO) can provide major sagittal correction for adult spinal deformity, sometimes achieving correction greater than 30°. Surgeons have reported failure rates up to 30% within 2 years, increasing to 46% after 4 to 5 years (SDC Figure 1, http://links.lww.com/BRS/B87). Revision following failure is difficult on the patient. PSO has achieved positive outcomes, but when used to correct posture in older patients with spinal deformity and fixed sagittal imbalance, rod breakage often leads to multiple surgical revisions. These complications must be avoided.

Background Context: Conservative treatment for adolescent idiopathic scoliosis is often unsuccessful and requires surgical intervention. Theoretically, anterior fusionless surgery can achieve correction as the patient grows to skeletal maturity., Purpose: The objective of the present study was to determine differences in range of motion (ROM) between multiple anterior tether constructs and tensioning techniques. Coronal plane Cobb angles were evaluated., Study Design/setting: This is a cadaveric biomechanical study., Methods: Cadaveric spines underwent biomechanical testing to investigate two factors relevant to anterior tether reconstruction: (1) effect of fixation at the T4, superior, and T12, inferior, levels (S-I), as opposed to fixation at all T4-T12 continuous levels (Cont.); and (2) tensioning of the tether sequentially (SEQ T) or only at terminal points (T). Reconstructions were conducted at Cont., and ROM and coronal plane Cobb angles were measured. Rigid rods (R) were used as control for the tether. Funding for the present study was provided by Globus Medical, Inc., and three of five authors are employees of Globus Medical, Inc., Results: Normalized lateral bending ROM for intact was 100(±33)%. The S-I R construct reduced motion to 39(±8)%. Tethering at terminal points resulted in ROM for S-I T and S-I No T of 61(±21)% and 70(±17)%, respectively. Screws placed at every level resulted in motion of 28(±9)% for the Cont. R construct, and a stepwise increase in motion to 44(±15)%, 47(±18)%, and 71(±19)%, respectively, for Cont. SEQ T, Cont. T, and Cont. No T. These relative trends were the same in all loading modes. Average change in overall coronal plane Cobb angle from intact was 4.6(±3.2)° and 9.9(±5.5)° for Cont. T and Cont. SEQ T constructs, respectively., Conclusions: Tensioned tether constructs allowed greater ROM than rigid constructs, and no significant difference in ROM was noted between tensioning techniques. Sequential tensioning can produce greater correction with no biomechanical advantage., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Background Context: Traditional reconstruction for burst fractures involves columnar support with posterior fixation at one or two levels cephalad/caudad; however, some surgeons choose to only stabilize the vertebral column., Purpose: The aim was to distinguish biomechanical differences in stability between a burst fracture stabilized through a lateral approach using corpectomy spacers of different end plate sizes with and without integrated screws and with and without posterior fixation., Study Design/setting: This was an in vitro biomechanical study assessing thoracolumbar burst fracture stabilization., Methods: Six human spines (T11-L3) were tested on a six-degrees-of-freedom simulator enabling unconstrained range of motion (ROM) at ±6 N·m in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) after a simulated burst fracture at L1. Expandable corpectomy spacers with/without integrated screws (Fi/F; FORTIFY Integrated/FORTIFY; Globus Medical, Inc., Audubon, PA, USA) were tested with different end plate sizes (21×23 mm, 22×40-50 mm). Posterior instrumentation (PI) via bilateral pedicle screws and rods was used one level above and one level below the burst fracture. Lateral plate (LP) fixation was tested. Devices were tested in the following order: intact; Fi21×23; Fi21×23+PI; F21×23+PI+LP; F21×23+LP; F22×40-50+LP; F22×40-50+PI+LP; Fi22×40-50+PI; Fi22×40-50., Results: In FE and AR, constructs without PI showed no significant difference (p<.05) in stability compared with intact. In LB, F22×40-50+LP showed a significant increase in stability relative to intact, but no other construct without PI reached significance. In FE and LB, circumferential constructs were significantly more stable than intact. In AR, no construct showed significant differences in motion when compared with the intact condition., Conclusions: Constructs without posterior fixation were the least stable of all tested constructs. Circumferential fixation provided greater stability in FE and LB than lateral fixation and intact. Axial rotation showed no significant differences in any construct compared with the intact state., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Study Design: Survey of spine surgeons and biomechanical comparison of screw pullout forces., Objective: To investigate what may be a suboptimal practice regularly occurring in spine surgery., Summary of Background Data: In order for a tap to function in its intended manner, the pitch of the tap should be the same as the pitch of the screw. Undertapping has been shown to increase the pullout force of pedicle screws compared with line-to-line tapping. However, given the way current commercial lumbar pedicle screw systems are designed, undertapping may result in a tap being used that has a different pitch from that of the screw (incongruent pitch)., Methods: A survey asked participants questions to estimate the proportion of cases each participant performed in the prior year using various hole preparation techniques. Participant responses were interpreted in the context of manufacturing specifications of specific instrumentation systems. Screw pullout forces were compared between undertapping with incongruent pitch and undertapping with congruent pitch using 0.16 g/cm polyurethane foam block and 6.5-mm screws., Results: Of the 3679 cases in which participants reported tapping, participants reported line-to-line tapping in 209 cases (5%), undertapping with incongruent pitch in 1156 cases (32%), and undertapping with congruent pitch in 2314 cases (63%). The mean pullout force for undertapping with incongruent pitch was 56 N (8%) less than the mean pullout force for undertapping with congruent pitch. This is equivalent to 13 lb., Conclusion: This study estimates that for about 1 out of every 3 surgical cases with tapping of lumbar pedicle screws in the United States, hole preparation is being performed by undertapping with incongruent pitch. This study also shows that undertapping with incongruent pitch results in a decrease in pullout force by 8% compared with undertapping with congruent pitch. Steps should be taken to correct this suboptimal practice., Level of Evidence: 3.