Search

STUDY DESIGN.: In vitro biomechanical study. OBJECTIVE.: To characterize cervical total disc replacement (TDR) kinematics above two-level fusion, and to determine the effect of fusion alignment on TDR response. SUMMARY OF BACKGROUND DATA.: Cervical TDR may be a promising alternative for a symptomatic adjacent level after prior multilevel cervical fusion. However, little is known about the TDR kinematics in this setting. METHODS.: Eight human cadaveric cervical spines (C2-T1, age: 59 ± 8.6 years) were tested intact, after simulated two-level fusion (C4-C6) in lordotic alignment and then in straight alignment, and after C3-C4 TDR above the C4-C6 fusion in lordotic and straight alignments. Fusion was simulated using an external fixator apparatus, allowing easy adjustment of C4-C6 fusion alignment, and restoration to intact state upon disassembly. Specimens were tested in flexion-extension using hybrid testing protocols. RESULTS.: The external fixator device significantly reduced range of motion (ROM) at C4-C6 to 2.0 ± 0.6°, a reduction of 89 ± 3.0% (P < 0.05). Removal of the fusion construct restored the motion response of the spinal segments to their intact state. The C3-C4 TDR resulted in less motion as compared to the intact segment when the disc prosthesis was implanted either as a stand-alone procedure or above a two-level fusion. The decrease in motion of C3-C4 TDR was significant for both lordotic and straight fusions across C4-C6 (P < 0.05). Flexion and extension moments needed to bring the cervical spine to similar C2 motion endpoints significantly increased for the TDR above a two-level fusion compared to TDR alone (P < 0.05). Lordotic fusion required significantly greater flexion moment, whereas straight fusion required significantly greater extension moment (P < 0.05). CONCLUSION.: TDR placed adjacent to a two-level fusion is subjected to a more challenging biomechanical environment as compared to a stand-alone TDR. An artificial disc used in such a clinical scenario should be able to accommodate the increased moment loads without causing impingement of its endplates or undue wear during the expected life of the prosthesis. [ABSTRACT FROM AUTHOR]

Background: Previous clinical trials have assessed the percentage of participants who utilized further health care after a period of conservative care for low back pain, however no chiropractic clinical trial has determined the total amount of care during this time and any differences based on assigned treatment group. The objective of this clinical trial follow-up was to assess if there was a difference in the total number of office visits for low back pain over one year after a four week clinical trial of either a form of physical therapy (Exercise Program) or a form of chiropractic care (Flexion Distraction) for chronic low back pain.Methods: In this randomized clinical trial follow up study, 195 participants were followed for one year after a four-week period of either a form of chiropractic care (FD) or a form of physical therapy (EP). Weekly structured telephone interview questions regarded visitation of various health care practitioners and the practice of self-care for low back pain.Results: Participants in the physical therapy group demonstrated on average significantly more visits to any health care provider and to a general practitioner during the year after trial care (p<0.05). No group differences were noted in the number of visits to a chiropractor or physical therapist. Self-care was initiated by nearly every participant in both groups.Conclusions: During a one-year follow-up, participants previously randomized to physical therapy attended significantly more health care visits than those participants who received chiropractic care. © 2006 Cambron et al., licensee BioMed Central Ltd. [ABSTRACT FROM AUTHOR]

Competing Interests: Disclosure: The Disclosure of Potential Conflicts of Interest form is provided with the online version of the article (http://links.lww.com/JBJS/I171).

Abstract: Orthopaedic surgeons in training and in their careers can experience a lack of confidence and imposter syndrome. Confidence is built early through continuous improvement, accomplishments, support, and reinforcement. Although it is normal to lack confidence at times, the goal is to recognize this issue, work on visualizing success, and know when to seek help. Mentors can help mentees to build confidence and to normalize thoughts of insecurity and imposter syndrome. It is critical to develop and to maintain resilience, grit, emotional intelligence, courage, and vulnerability during training and throughout one's entire orthopaedic career. Leaders in the field must be aware of these phenomena, be able to talk about such issues, have methods to combat the harmful effects of imposter syndrome, and create a safe, supportive environment conducive to learning and working. Leading well builds not only confidence in oneself but also self-confidence in others. Leaders who are able to build the confidence of individuals will enhance team dynamics, wellness, and overall productivity as well as individual and organizational success., Competing Interests: Disclosure: The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article ( http://links.lww.com/JBJS/H418 )., (Copyright © 2023 by The Journal of Bone and Joint Surgery, Incorporated.)

Background: The need for posterior longitudinal ligament (PLL) resection during cervical total disc arthroplasty (TDA) has been debated. The purpose of this laboratory study was to investigate the effect of PLL resection on cervical kinematics after TDA., Methods: Eight cadaveric cervical spine specimens were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) to moments of ±1.5 Nm. After testing the intact condition, anterior C5-C6 cervical discectomy was performed followed by PLL resection and implantation of a compressible, 6-degrees-of-freedom disc prosthesis (M6-C, Spinal Kinetics Inc, Sunnyvale, California). Next, a second prosthesis was implanted at C6-C7 with PLL intact. Finally, the C6-C7 PLL was resected while the disc prosthesis remained in place. Segmental range of motion (ROM) and stiffness in the high flexibility zone around the neutral posture were analyzed using repeated measures ANOVA., Results: At C5-C6, following TDA and PLL resection, FE, LB, and AR ROMs decreased significantly. Anterior and posterior disc height, segmental lordosis, and flexion stiffness increased significantly. At C6-C7, TDA with the PLL intact resulted in a significant increase in anterior disc height and segmental lordosis with no change in posterior disc height. FE, LB, and AR ROMs all decreased significantly, while flexion stiffness increased significantly compared to intact. PLL resection at C6-C7 did not result in a notable change compared to TDA with PLL intact. At the same level, flexion stiffness decreased following PLL resection compared to TDA with a value closer to intact. Two-level TDA (C5-C7) with PLL resection did not result in a loss of segmental stability., Conclusion: PLL resection did not significantly affect motion segment kinematics following cervical TDA using a prosthesis with inherent stiffness. Motion segment stiffness loss after PLL resection can be compensated for by a TDA design that can provide resistance to angular motion., Competing Interests: Disclosures and COI: This study was supported by funds from the Department of Veterans Affairs, Washington, DC, and Spinal Kinetics Inc, Sunnyvale, CA.

Purpose: In this article, we summarize our work on understanding the influence of cervical sagittal malalignment on the mechanics of the cervical spine., Methods: Biomechanical studies were performed using an ex vivo laboratory model to study the kinematic and kinetic response of human cervical spine specimens in the setting of cervical sagittal imbalance. The model allowed controlled variations of C2-C7 Sagittal Vertical Alignment (C2-C7 SVA) and T1-Slope so that clinically relevant sagittally malaligned profiles could be prescribed, while maintaining horizontal gaze, and their biomechanical consequences studied., Results: Our results demonstrated that increasing C2-C7 SVA caused flexion of lower cervical (C2-C7) segments and hyperextension of suboccipital (C0-C1-C2) segments to maintain horizontal gaze. An increase in C2-C7 SVA increased the lower cervical neural foraminal areas. Conversely, increasing T1-slope predominantly influenced subaxial cervical lordosis and, as a result, decreased cervical neural foraminal areas. Therefore, we believe patients with increased upper thoracic kyphosis and radicular symptoms may respond with increased forward head posture (FHP) as a compensatory mechanism to increase their lower cervical neural foraminal area and alleviate nerve root compression as well as reduce the burden on posterior muscles and soft and bony structures of the cervical spine. Increasing FHP (i.e., increased C2-C7 SVA) was associated with shortening of the cervical flexors and occipital extensors and lengthening of the cervical extensors and occipital flexors, which corresponds to C2-C7 flexion and C0-C2 extension. The greatest shortening occurred in the suboccipital muscles, suggesting considerable load bearing of these muscles during chronic FHP. Regardless, there was no evidence of nerve compression within the suboccipital triangle. Finally, cervical sagittal imbalance may play a role in exacerbating adjacent segment pathomechanics after multilevel cervical fusion and should be considered during surgical planning., Conclusions: The results of our biomechanical studies have improved our understanding of the impact of cervical sagittal malalignment on pathomechanics of the cervical spine. We believe this improved understanding will assist in clinical decision-making.

Objective: To test the following hypotheses: (i) anterior cervical discetomy and fusion (ACDF) using stand-alone interbody spacers will significantly reduce the range of motion from intact spine; and (ii) the use of a static or a rotational-dynamic plate will significantly augment the stability of stand-alone interbody spacers, with similar beneficial effect when compared to each other., Methods: Eleven human cadaveric subaxial cervical spines (age: 48.2 ± 5.4 years) were tested under the following sequence: (i) intact spine; (ii) ACDF at C 4 -C 5 using a stand-alone interbody spacer; (iii) ACDF at C 5 -C 6 and insertion of an interbody spacer (two-level construct); and (iv) randomized placement of either a two-level locking static plate or a rotational-dynamic plate., Results: Insertion of stand-alone cage at C 4 -C 5 and C 5 -C 6 caused a significant decrease in the range of motion compared to intact spine (P < 0.05). Placement of both the locking and the rotational dynamic plate further reduced the range of motion at C 4 -C 5 and C 5 -C 6 compared to stand-alone cage (P < 0.01). No significant differences in range of motion restriction at either C 4 -C 5 or C 5 -C 6 were found when the two plating systems were compared (P > 0.05)., Conclusions: Cervical stand-alone interbody spacers caused significant restriction in the range of motion. Both plates significantly augmented the stability of stand-alone interbody spacers, with similar stabilizing effect., (© 2017 Chinese Orthopaedic Association and John Wiley & Sons Australia, Ltd.)

Background: Forward head posture (FHP) may be associated with neck pain and poor health-related quality of life. Literature describes only qualitative muscle length changes associated with FHP., Objective: The purpose of this study was to quantify how muscle-tendon unit lengths are altered when human cadaveric specimens are placed in alignments representing different severities of FHP., Design: This biomechanical study used 13 fresh-frozen cadaveric cervical spine specimens (Occiput-T1, 54±15 y)., Methods: Specimens' postural changes simulating increasing FHP severity while maintaining horizontal gaze were assessed. Specimen-specific anatomic models derived from computed tomography-based anatomic data were combined with postural data and specimen-specific anatomy of muscle attachment points to estimate the muscle length changes associated with FHP., Results: Forward head posture was associated with flexion of the mid-lower cervical spine and extension of the upper cervical (sub-occipital) spine. Muscles that insert on the cervical spine and function as flexors (termed "cervical flexors") as well as muscles that insert on the cranium and function as extensors ("occipital extensors") shortened in FHP when compared to neutral posture. In contrast, muscles that insert on the cervical spine and function as extensors ("cervical extensors") as well as muscles that insert on the cranium and function as flexors ("occipital flexors") lengthened. The greatest shortening was seen in the major and minor rectus capitis posterior muscles. These muscles cross the Occiput-C2 segments, which exhibited extension to maintain horizontal gaze. The greatest lengthening was seen in posterior muscles crossing the C4-C6 segments, which exhibited the most flexion., Limitations: This cadaver study did not incorporate the biomechanical influence of active musculature., Conclusions: This study offers a novel way to quantify postural alignment and muscle length changes associated with FHP. Model predictions are consistent with qualitative descriptions in the literature., (© 2017 American Physical Therapy Association)

Study Design: A biomechanical study using human spine specimens., Objective: The aim of this study was to assess whether the presence of cervical sagittal imbalance is an independent risk factor for increasing the mechanical burden on discs adjacent to cervical multilevel fusions., Summary of Background Data: The horizontal offset distance between the C2 plumbline and C7 vertebral body (C2-C7 Sagittal Vertical Axis (SVA)) or the angle made with vertical by a line connecting the C2 and C7 vertebral bodies (C2-C7 tilt angle) are used as radiographic measures to assess cervical sagittal balance. There is level III clinical evidence that sagittal imbalance caused by kyphotic fusions or global spinal sagittal malalignment may increase the risk of adjacent segment pathology., Methods: Thirteen human cadaveric cervical spines (Occiput-T1; age: 50.6 years; range: 21-67) were tested first in the native intact state and then after instrumentation across C4-C6 to simulate in situ two-level fusion. Specimens were tested using a previously validated experimental model that allowed measurement of spinal response to prescribed imbalance. The effects of fusion on segmental angular alignments and intradiscal pressures in the C3-C4 and C6-C7 discs, above and below the fusion, were evaluated at different magnitudes of C2-C7 tilt angle (or C2-C7 SVA)., Results: When compared with the pre-fusion state, in situ fusion across C4-C6 segments required increased flexion angulation and resulted in increased intradiscal pressure at the C6-C7 disc below the fusion in order to accommodate the same increase in C2-C7 tilt angle or C2-C7 SVA (P < 0.05). The adjacent segment mechanical burden due to fusion became greater with increasing C2-C7 tilt angle or SVA., Conclusion: Cervical sagittal imbalance arising from regional and/or global spinal sagittal malalignment may play a role in exacerbating adjacent segment pathomechanics after multilevel fusion and should be considered during surgical planning., Level of Evidence: N/A.

Study Design: A biomechanical study using human spine specimens., Objective: To study postural compensations in lordosis angles that are necessary to maintain horizontal gaze in the presence of forward head posture and increasing T1 sagittal tilt., Summary of Background Data: Forward head posture relative to the shoulders, assessed radiographically using the horizontal offset distance between the C2 and C7 vertebral bodies (C2-C7 [sagittal vertical alignment] SVA), is a measure of global cervical imbalance. This may result from kyphotic alignment of cervical segments, muscle imbalance, as well as malalignment of thoracolumbar spine., Methods: Ten cadaveric cervical spines (occiput-T1) were tested. The T1 vertebra was anchored to a tilting and translating base. The occiput was free to move vertically but its angular orientation was constrained to ensure horizontal gaze regardless of sagittal imbalance. A 5-kg mass was attached to the occiput to mimic head weight. Forward head posture magnitude and T1 tilt were varied and motions of individual vertebrae were measured to calculate C2-C7 SVA and lordosis across C0-C2 and C2-C7., Results: Increasing C2-C7 SVA caused flexion of lower cervical (C2-C7) segments and hyperextension of suboccipital (C0-C1-C2) segments to maintain horizontal gaze. Increasing kyphotic T1 tilt primarily increased lordosis across the C2-C7 segments. Regression models were developed to predict the compensatory C0-C2 and C2-C7 angulation needed to maintain horizontal gaze given values of C2-C7 SVA and T1 tilt., Conclusion: This study established predictive relationships between radiographical measures of forward head posture, T1 tilt, and postural compensations in the cervical lordosis angles needed to maintain horizontal gaze. The laboratory model predicted that normalization of C2-C7 SVA will reduce suboccipital (C0-C2) hyperextension, whereas T1 tilt reduction will reduce the hyperextension in the C2-C7 segments. The predictive relationships may help in planning corrective strategy in patients experiencing neck pain, which may be attributed to sagittal malalignment., Level of Evidence: N/A.

Study Design: Human cadaveric biomechanical study., Objective: To determine the fixation strength of laterally directed, cortical pedicle screws under physiological loads., Summary of Background Data: Lateral trajectory cortical pedicle screws have been described as a means of obtaining improved fixation while minimizing soft-tissue dissection during lumbar instrumentation. Biomechanical data have demonstrated equivalent strength in a quasi-static model; however, no biomechanical information is available comparing the fixation of cortical with traditional pedicle screws under cyclic physiological loads., Methods: Seventeen vertebral levels (T11-L5) underwent quantitative computed tomography. On 1 side, a laterally directed, cortical pedicle screw was inserted with a traditional, medially directed pedicle screw placed on the contralateral side. With the specimen constrained in a testing apparatus, each screw underwent cyclic craniocaudal toggling under incrementally increasing physiological loads until 2 mm of head displacement occurred. Next, uniaxial pullout of each toggled screw was performed. The number of craniocaudal toggle cycles and load (N) required to achieve pedicle screw movement as well as axial pullout resistance (N) were compared between the 2 techniques., Results: The mean trabecular bone mineral density of the specimens was 202 K2HPO4 mg/cm. Cortical pedicle screws demonstrated significantly improved resistance to toggle testing, requiring 184 cycles to reach 2 mm of displacement compared with 102 cycles for the traditional pedicle screws (P=0.002). The force necessary to displace the screws was also significantly greater for the cortical versus the traditional screws (398 N vs. 300 N, P=0.004). There was no statistical difference in axial pullout strength between the previously toggled cortical and traditional pedicle screws (1722 N vs. 1741 N, P=0.837)., Conclusion: Laterally directed cortical pedicle screws have superior resistance to craniocaudal toggling compared with traditional pedicle screws., Level of Evidence: N/A.

Study Design: We quantified the segmental biomechanics of a cervical total disc replacement (TDR) before and after progressive posterior decompression. We hypothesized that posterior decompressive procedures would not significantly increase range of motion (ROM) at the index TDR level., Objective: To quantify the kinematics of a cervical total disc replacement (TDR) before and after posterior cervical decompression., Summary of Background Data: A reported yet unaddressed issue is the potential for the development of same-segment disease after implantation of a cervical TDR and the implications of same-segment posterior decompression on TDR mechanics., Methods: Eight human cadaveric cervical spines C3-C7 were tested in flexion-extension, lateral bending, and axial rotation while intact, after C5-C6 TDR, C5-C6 unilateral foraminotomy, C5-C6 bilateral foraminotomies, and after C5 laminectomy in combination with the bilateral foraminotomies. Moment versus angular motion curves were obtained for each testing step, and the load-displacement data were analyzed to determine the range of angular motion for each step., Results: Unilateral foraminotomy did not result in a statistically significant increase in flexion-extension ROM, and did not increase the ROM to a degree greater than normal. Although bilateral foraminotomies did increase flexion-extension ROM, motion remained within a physiological range. A full laminectomy added to the bilateral foraminotomies significantly increased ROM and was also associated with distortion of the load-displacement curves., Conclusion: With respect to segmental biomechanics as demonstrated, we think that for same-segment disease, a unilateral foraminotomy can be performed safely. However, the impact of in vivo conditions was not accounted for in this model, and it is possible that cyclical loading and other physiological stresses on such a construct may affect the behavior and lifespan of the implant in a way that cannot be predicted by a biomechanical study. Bilateral foraminotomies would require close observation and additional clinical follow-up, whereas complete laminectomy combined with bilateral foraminotomies should be avoided after TDR given the significant changes in kinematics. In addition, future disc replacement designs may need to account for changes after posterior decompression for same-segment disease., Level of Evidence: N/A.

Study Design: In vitro biomechanical study., Objective: To test the hypotheses: (1) an anchored spacer device would decrease motion similarly to a plate-spacer construct, and (2) the anchored spacer would achieve a similar reduction in motion when placed adjacent to a previously fused segment., Summary of Background Data: An anchored spacer device has been shown to perform similar to the plate-spacer construct in previous biomechanical evaluation. The prevalence of adjacent segment disease after fusion is well established in the literature.There is currently no evidence supporting the use of an anchored interbody spacer device adjacent to a previous fusion., Methods: Eight human cervical spines (age: 45.1 ± 13.1 yr) were tested in moment control (±1.5 Nm) in flexion-extension, lateral bending, and axial rotation without preload. Flexion-extension was then retested under 150-N preload. Spines were tested intact and after anterior cervical discectomy and fusion (ACDF) at C4-C5 and C6-C7 with either a plate-spacer or anchored spacer construct (randomized). The specimens were tested finally with an ACDF at the floating C5-C6 segment using the anchored spacer device adjacent to the previous fusions., Results: Both the plate-spacer and anchored spacer significantly reduced motion from the intact spine in flexion-extension, lateral bending, and axial rotation (P < 0.005). There was no statistically significant difference between the 2 fusion constructs in their abilities to reduce motions (P = 1.0). ACDF using the anchored spacer at the floating C5-C6 level (in between the plate-spacer and anchored spacer constructs) resulted in significant motion reductions in all modes of testing (P < 0.05). These motion reductions did not significantly differ from those of a single-level anchored-spacer construct or a single-level plated ACDF., Conclusion: The anchored spacer provided significant motion reductions, similar to a plated ACDF, when used as a single-level fusion construct or placed adjacent to a previously plated segment., Level of Evidence: N/A.

Study Design: Biomechanical human cadaveric study., Objective: We hypothesized that increasing compressive preload will reduce the segmental instability after nucleotomy, posterior ligament resection, and decompressive surgery., Summary of Background Data: The human spine experiences significant compressive preloads in vivo due to spinal musculature and gravity. Although the effect of destabilization procedures on spinal motion has been studied, the effect of compressive preload on the motion response of destabilized, multisegment lumbar spines has not been reported., Methods: Eight human cadaveric spines (L1-sacrum, 51.4 ± 14.1 yr) were tested intact, after L4-L5 nucleotomy, after interspinous and supraspinous ligaments transection, and after midline decompression (bilateral laminotomy, partial medial facetectomy, and foraminotomy). Specimens were loaded in flexion (8 Nm) and extension (6 Nm) under 0-N, 200-N, and 400-N compressive follower preload. L4-L5 range of motion (ROM) and flexion stiffness in the high-flexibility zone were analyzed using repeated-measures analysis of variance and multiple comparisons with the Bonferroni correction., Results: With a fixed set of loading conditions, a progressive increase in segmental ROM along with expansion of the high-flexibility zone (decrease of flexion stiffness) was noted with serial destabilizations. Application of increasing compressive preload did not substantially change segmental ROM, but did significantly increase the segmental stiffness in the high-flexibility zone. In the most destabilized condition, 400-N preload did not return the segmental stiffness to intact levels., Conclusion: Anatomical alterations representing degenerative and iatrogenic instabilities are associated with significant increases in segmental ROM and decreased segmental stiffness. Although application of compressive preload, mimicking the effect of increased axial muscular activity, significantly increased the segmental stiffness, it was not restored to intact levels; thereby suggesting that core strengthening alone may not compensate for the loss of structural stability associated with midline surgical decompression. This suggests that there may be a role for surgical implants or interventions that specifically increase flexion stiffness and limit flexion ROM to counteract the iatrogenic instability resulting from surgical decompression., Level of Evidence: N/A.

Introduction: Anterior cervical decompression and fusion is a well-established procedure for treatment of degenerative disc disease and cervical trauma including flexion-distraction injuries. Low-profile interbody devices incorporating fixation have been introduced to avoid potential issues associated with dissection and traditional instrumentation. While these devices have been assessed in traditional models, they have not been evaluated in the setting of traumatic spine injury. This study investigated the ability of these devices to stabilize the subaxial cervical spine in the presence of flexion-distraction injuries of increasing severity., Methods: Thirteen human cadaveric subaxial cervical spines (C3-C7) were tested at C5-C6 in flexion-extension, lateral bending and axial rotation in the load-control mode under ±1.5 Nm moments. Six spines were tested with locked screw configuration and seven with variable angle screw configuration. After testing the range of motion (ROM) with implanted device, progressive posterior destabilization was performed in 3 stages at C5-C6., Results: The anchored spacer device with locked screw configuration significantly reduced C5-C6 flexion-extension (FE) motion from 14.8 ± 4.2 to 3.9 ± 1.8°, lateral bending (LB) from 10.3 ± 2.0 to 1.6 ± 0.8, and axial rotation (AR) from 11.0 ± 2.4 to 2.5 ± 0.8 compared with intact under (p < 0.01). The anchored spacer device with variable angle screw configuration also significantly reduced C5-C6 FE motion from 10.7 ± 1.7 to 5.5 ± 2.5°, LB from 8.3 ± 1.4 to 2.7 ± 1.0, and AR from 8.8 ± 2.7 to 4.6 ± 1.3 compared with intact (p < 0.01). The ROM of the C5-C6 segment with locked screw configuration and grade-3 F-D injury was significantly reduced from intact, with residual motions of 5.1 ± 2.1 in FE, 2.0 ± 1.1 in LB, and 3.3 ± 1.4 in AR. Conversely, the ROM of the C5-C6 segment with variable-angle screw configuration and grade-3 F-D injury was not significantly reduced from intact, with residual motions of 8.7 ± 4.5 in FE, 5.0 ± 1.6 in LB, and 9.5 ± 4.6 in AR., Conclusions: The locked screw spacer showed significantly reduced motion compared with the intact spine even in the setting of progressive flexion-distraction injury. The variable angle screw spacer did not sufficiently stabilize flexion-distraction injuries. The resulting motion for both constructs was higher than that reported in previous studies using traditional plating. Locked screw spacers may be utilized with additional external immobilization while variable angle screw spacers should not be used in patients with flexion-distraction injuries.

Study Design: A biomechanical cadaveric study of lumbar spine segments., Objective: To compare the immediate stability provided by parallel-shaped and anatomically shaped carbon fiber interbody fusion I/F cages in posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) constructs with posterior pedicle screw instrumentation., Summary of Background Data: Few biomechanical data are available on the anatomically shaped cages in PLIF and TLIF constructs., Methods: Twenty human lumbar segments were tested in flexion-extension (FE) (8 N m flexion, 6 N m extension), lateral bending (LB) (± 6 N m), and torsional loading (± 5 N m). Each segment was tested in the intact state and after insertion of interbody cages in one of 3 constructs: PLIF with 2 parallel-shaped or anatomically shaped cages and TLIF with 1 anatomically shaped cage. All cages received supplementary pedicle screw fixation. The range-of-motion (ROM) values after cage insertion and posterior fixation were compared with the intact specimen values using analysis of variance and multiple comparisons with Bonferroni correction., Results: All constructs significantly reduced segmental motion relative to intact (P < 0.001). The motion reductions in FE, LB, and axial rotation were 85 ± 15%, 83 ± 18%, and 67 ± 6.8% for the PLIF construct using parallel cages, 79 ± 5.5%, 87 ± 10%, and 66 ± 20% for PLIF using anatomically shaped cages, and 90 ± 6.8%, 87 ± 12%, and 77 ± 22% for TLIF with an anatomically shaped cage. In FE and LB, the reductions in the ROM caused between the 3 constructs were equivalent (P > 0.05). In axial rotation, the TLIF cage provided significantly greater limitation in the ROM compared with the parallel-shaped PLIF cage (P = 0.01)., Conclusions: The parallel-shaped and anatomically shaped I/F cages provided similar stability in a PLIF construct. The greater stability of the TLIF construct was likely due to a more anterior placement of the TLIF cage and preservation of the contralateral facet joint.

Study Design: A biomechanical study using human lumbar spines., Objective: To test the hypotheses that with increasing implant height (1) the range of motion (ROM) of the implanted segment will decrease, (2) the segmental lordosis will increase, and (3) the size of the neural foramens will increase., Summary of Background Data: Little is known about the effects of the implant height on the segmental motion and foraminal size at the implanted level., Methods: Seven human lumbar spines (age, 54.4+/-11.4 years; L1-sacrum) were tested intact, and after discectomy at L4-L5 and sequential insertion of ProDisc-L implants (Synthes Spine, Paoli, PA) of increasing heights (10, 12, and 14 mm). The specimens were tested in flexion (8 Nm) and extension (-6 Nm) with a 400 N follower preload as well as in lateral bending (+/-6 Nm) and axial rotation (+/-5 Nm) without preload. Three-dimensional motions were measured at L4-L5. Foraminal sizes at L4-L5 were measured in the specimen's neutral posture under a 400 N preload for the intact spine and after each implantation using finely graded cylindrical probes. Segmental lordosis was measured in the specimen's neutral posture under a 400 N preload by analyzing digital fluoroscopic images. Effects of implant height on the kinematics, foraminal size, and segmental lordosis were assessed using paired comparisons with Bonferroni correction., Results: Increasing implant height from 10 mm to 14 mm caused a significant decrease (P<0.05) in segmental ROM by up to 37%+/-21% in flexion/extension, 33%+/-18% in lateral bending, and 29%+/-28% in axial rotation. Increasing implant height also produced a significant increase in segmental lordosis (P<0.05): from 9.7 degrees+/-2.9 degrees at 10 mm, to 16.1 degrees+/-5.1 degrees at 14 mm. The increase in foraminal size, while significant, was only 4.6%+/-3.2% when comparing 10 mm to 14 mm implants., Conclusion: These results suggest that a smaller implant height should be selected to optimize the ROM of the implanted segment and maintain sagittal balance.

Study Design: Biomechanical study using human cadaveric cervical spines., Objective: To evaluate the construct stability of 3 different segmental occipitoatlantoaxial (C0-C1-C2) stabilization techniques., Summary of Background Data: Different C0-C1-C2 stabilization techniques are used for unstable conditions in the upper cervical spine, all with different degrees of risk to the vertebral artery. Techniques with similar stability but less risk to the vertebral artery may be advantageous., Methods: Six human cadaveric cervical spines (C0-C5) (age: 74 +/- 5.0 years) were used. After testing the intact spines, instability was created by transecting the transverse and alar ligaments. The spines were instrumented from the occiput to C2 using 3 different techniques which varied in their attachment to C2. All spines had 6 screws placed into the occiput along with lateral mass screws at C1. The 3 variations used in attachment to C2 were (1) C2 crossing laminar screws, (2) C2 pedicle screws, and (3) C1-C2 transarticular screws. The C1 lateral mass screws were removed before placement of the C1-C2 transarticular screws. Range of motion across C0-C2 was measured for each construct. The data were analyzed using repeated measures ANOVA. The following post hoc comparisons were made: (1) intact spine versus each of the 3 techniques, (2) laminar screw technique versus the pedicle screw technique, and (3) laminar screw technique versus the transarticular screw technique. The level of significance was alpha = 0.01 (after Bonferroni correction for 5 comparisons)., Results: All 3 stabilization techniques significantly decreased range of motion across C0-C2 compared to the intact spine (P < 0.01). There was no statistical difference among the 3 stabilization methods in flexion/extension and axial rotation. In lateral bending, the technique using C2 crossing laminar screws demonstrated a trend toward increased range of motion compared to the other 2 techniques. CT scans in both axial and sagittal views demonstrated greater proximity to the vertebral artery in the pedicle and transarticular screw techniques compared to the crossing laminar screw technique., Conclusion: Occipitoatlantoaxial stabilization techniques using C2 crossing laminar screws, C2 pedicles screws, and C1-C2 transarticular screws offer similar biomechanical stability. Using the C2 crossing laminar screw technique may offer an advantage over the other techniques due to the reduction of the risk to the vertebral artery during C2 screw placement.

Study Design: An in vitro three-dimensional (3D) flexibility test of human C3-C7 cervical spine specimens., Objective: To test the hypothesis that anterior cervical fusion with a wedged graft and a locked plate can effectively stabilize the cervical spine after complete anterior and posterior segmental ligamentous release., Summary of Background Data: Distraction-flexion Stage 3 injuries of the lower cervical spine (bilateral facet dislocations) are usually reduced under awake cranial traction. When the magnetic resonance imaging reveals a traumatic disc prolapse, anterior cervical discectomy and fusion (ACDF) is usually recommended. Most authors advise combining ACDF with posterior instrumentation to address the insufficiency of the posterior elements. However, there is clinical evidence that ACDF with a locked plate alone suffices for the treatment of these injuries, especially in young patients. Still, there are no biomechanical studies on the effect of a locked plate on the complete anterior and posterior ligamentous-deficient young cervical spine under physiologic preload., Methods: Eight fresh frozen human lower cervical spines (C3-C7) from young donors (age, 44.5 years; range, 21-63 years) were used. A 3D flexibility test was conducted using a moment of 0.8 Nm without preload. Flexion-extension was additionally tested using a moment of 1.5 Nm under 0 and 150 N follower preload. Spines were tested first intact, then after complete C5-C6 discectomy with posterior longitudinal ligament resection and ACDF with a wedged bone graft and a rigid locked plate, and finally after complete release of the supraspinous, interspinous, and intertransverse ligaments; the facet capsules; and ligamentum flavum. RESULTS.: When tested under 0.8 Nm moment without preload, complete posterior and anterior ligamentous release did not significantly increase the ROM of the ACDF construct in flexion-extension (P > 0.025), lateral bending (P > 0.025), and axial rotation (P > 0.025). When tested under 1.5 Nm moment with or without a compressive preload, the complete posterior and anterior ligamentous release did not significantly affect the ROM of the ACDF construct (P > 0.01). The application of preload significantly reduced the motion at the C5-C6 ACDF construct with ligamentous disruption in comparison with the motion in the absence of a preload (P < 0.01)., Conclusion: Anterior cervical fusion with a wedged graft and a rigid constrained (locked) plate can effectively stabilize the nonosteoporotic cervical spine after complete posterior element injury when excessive ROM is prevented (for example, by the use of postoperative external immobilization). Even when the construct is subjected to higher moments, adequate stability can be achieved when physiologic preload is present. Osteoporosis and lack of sufficient preload due to poor neuromuscular control may affect long-term screw stability, and additional external immobilization may be needed until fusion matures.

Background: Reherniation within the first year following subtotal lumbar discectomy is a rare but noteworthy event. We performed a retrospective, case-controlled study to evaluate the clinical outcomes after early recurrent lumbar disc reherniation., Methods: The records of 1320 patients who had undergone primary subtotal lumbar discectomy were analyzed retrospectively by an independent reviewer. Patients with documented reherniation within twelve months were evaluated with regard to the location of the reherniation, the neurologic status, the rate of reoperation, and the subjective outcome. Patients were evaluated on the basis of a physical examination and a review of medical records. Disc morphology, anular competence, and the presence of free fragments were categorized with use of a modified five-part Carragee classification system. The mean duration of follow-up for this group was 52.6 months. Clinical outcomes were assessed with use of the Oswestry score and the modified criteria of McNab. Twenty-nine historical control patients who had undergone uncomplicated subtotal lumbar discectomy were selected., Results: We identified fourteen recurrent lumbar disc herniations within one year after the index procedure. All fourteen patients had radicular pain and weakness prior to, and complete relief of radiculopathy after, the index procedure. All reherniations occurred at the same level as the index procedure, but eight occurred in a different direction than the original herniation. All patients underwent reexploration and discectomy, and two underwent single-level posterolateral arthrodesis. Two patients underwent a third procedure. The average Oswestry score at the time of the latest follow-up was 6.4 for the recurrent herniation group, compared with 6.9 for the controls. The outcomes according to the modified McNab criteria were not significantly different between the groups, with the numbers available. The mean duration of follow-up after the second discectomy was 52.6 months., Conclusions: The rate of early reherniation after subtotal lumbar discectomy is low (1%). It is important to consider the possibility of iatrogenic instability during surgery on the lumbar spine for the treatment of reherniation. Patients who undergo reoperation because of early recurrent lumbar disc herniation can have clinical outcomes comparable with those of patients undergoing an uncomplicated subtotal lumbar discectomy.

Study Design: In vitro biomechanical study., Objective: To quantify the effects of uncinatectomy on cervical motion after total disc replacement (TDR)., Summary of Background Data: The effect of uncinatectomy on TDR motion is unknown. Partial uncinatectomy may be required to decompress the foramen; however, the residual uncinates can potentially limit TDR motion and serve as a source of progressive spondylosis. Complete resection of the uncinates may decrease this risk yet endanger destabilizing the segment., Methods: Seven human cervical spines (C3-C7) (age, 63.4 +/- 6.9 years) were tested first intact and then after implantation of a metal-on-polyethylene ball-and-socket semiconstrained prosthesis at C5-C6. Following this, gradually increased uncinatectomy was performed in the following order: 1) right partial-posteromedial (two thirds), 2) right complete, and 3) bilateral complete resection. Specimens were tested in flexion-extension, lateral bending, and axial rotation (+/-1.5 Nm). Flexion-extension was tested under 150 N follower preload., Results: TDR without uncinatectomy increased C5-C6 flexion-extension range of motion from 8.4 degrees +/- 3.5 degrees to 11.6 degrees +/- 3.4 degrees, but statistical significance was not reached (P > 0.05). Lateral bending decreased from 6.2 degrees +/- 2.2 degrees to 3.1 degrees +/- 1.4 degrees, with a trend for statistical significance (P = 0.07). Axial rotation decreased from 5.5 degrees +/- 2.4 degrees to 4.3 degrees +/- 1.4 degrees after the implantation (P > 0.05). Both right partial and right complete uncinatectomy resulted in nearly symmetrical restoration of lateral bending to intact values and significantly increased flexion-extension compared with intact (P < or = 0.05); however, axial rotation still did not differ from intact (P > 0.05). Complete bilateral resection also restored lateral bending to intact values (7.3 degrees +/- 2.7 degrees, P > 0.05); however, it resulted in significant increase in range of motion in flexion-extension (14.1 degrees +/- 3.0 degrees, P < or = 0.05) and axial rotation (8.7 degrees +/- 2.4 degrees, P < or = 0.05)., Conclusion: Unilateral complete or even partial uncinatectomy can normalize lateral bending after TDR. Bilateral complete uncinatectomy is not necessary to restore lateral bending and may result in significantly increased range of motion in flexion-extension and axial rotation compared with intact values.

The possible occurrence of metabolic acidosis in patients with intestinal ileus is not well recognized. We describe a patient with acute alcohol-induced pancreatitis and a large transverse colon ileus in which plasma bicarbonate dropped rapidly in the absence of an increase in the plasma anion gap. The urinary anion gap and ammonium excretion were consistent with an appropriate renal response to metabolic acidosis and against the possibility of respiratory alkalosis. The cause of the falling plasma bicarbonate was ascribed to intestinal bicarbonate sequestration owing to the enhancement of chloride-bicarbonate exchange in a dilated paralyzed colon.

Background: A model was developed to predict changes in pelvic volume associated with increasing pubic diastasis in unstable pelvic fractures., Methods: Intact and postfracture pelvic volumes were calculated in 10 cadavers using computerized axial tomography (CT). The true pelvis was assumed to be either a sphere, a cylinder, or a hemi-elliptical sphere. Using the appropriate equations for calculating the volume of each of these shapes, pelvic volume was predicted and then compared with the measured values., Results: The observed volume changes associated with increasing pubic diastasis were much smaller than previously reported. The mean difference between the measured and predicted volume was 20.0 +/- 9.9% for the sphere, 10.7 +/- 6.5% for the cylinder, and 4.5 +/- 5.9% for the hemi-elliptical sphere. The differences between these means were statistically significant (p < 0.001)., Conclusions: This data suggests that the hemi-elliptical sphere best describes the geometric shape of the true pelvis and better predicts quantitative changes in pelvic volume relative to an increasing pubic diastasis as the radius has little effect on the change in volume. Due to the small changes in volume observed with increasing diastasis, factors other than the absolute change in volume must account for the clinically observed effects of emergent pelvic stabilization.

Objective: Flexion distraction is a commonly used form of chiropractic care with chiropractor utilization rates of 58%. However, no previous randomized clinical trial has assessed the effectiveness of this form of care. The objective of this investigation was to compare the pain and disability during the year after active care based on treatment group allocation (Flexion Distraction versus Exercise Program)., Study Design: Randomized clinical trial, follow-up., Subjects: Two hundred and thirty-five (235) subjects who were previously randomized to either chiropractic care (flexion distraction) or physical therapy (exercise program) within a clinical trial., Outcome Measures: Subjects were followed for 1 year via mailed questionnaires to assess levels of pain (Visual Analog Scale) and dysfunction (Roland Morris)., Results: Study subjects had a decrease in pain and disability after intervention regardless of which group they attended (p < 0.002), however, during the year after care, subjects who received chiropractic care (flexion distraction therapy) had significantly lower pain scores than subjects who received physical therapy (exercise program) (p = 0.02)., Conclusions: In this first trial on flexion distraction care, flexion distraction was found to be more effective in reducing pain for 1 year when compared to a form of physical therapy.

Many clinical trials on chiropractic management of low back pain have neglected to include specific forms of care. This study compared two well-defined treatment protocols. The objective was to compare the outcome of flexion-distraction (FD) procedures performed by chiropractors with an active trunk exercise protocol (ATEP) performed by physical therapists. A randomized clinical trial study design was used. Subjects, 18 years of age and older, with a primary complaint of low back pain (>3 months) were recruited. A 100 mm visual analogue scale (VAS) for perceived pain, the Roland Morris (RM) Questionnaire for low back function, and the SF-36 for overall health status served as primary outcome measures. Subjects were randomly allocated to receive either FD or ATEP. The FD intervention consisted of the application of flexion and traction applied to specific regions in the low back, with the aid of a specially designed manipulation table. The ATEP intervention included stabilizing and flexibility exercises, the use of modalities, and cardiovascular training. A total of 235 subjects met the inclusion/exclusion criteria and signed the informed consent. Of these, 123 were randomly allocated to FD and 112 to the ATEP. Study patients perceived significantly less pain and better function after intervention, regardless of which group they were allocated to (P<0.01). Subjects randomly allocated to the flexion-distraction group had significantly greater relief from pain than those allocated to the exercise program (P=0.01). Subgroup analysis indicated that subjects categorized as chronic, with moderate to severe symptoms, improved most with the flexion-distraction protocol. Subjects categorized with recurrent pain and moderate to severe symptoms improved most with the exercise program. Patients with radiculopathy did significantly better with FD. There were no significant differences between groups on the Roland Morris and SF-36 outcome measures. Overall, flexion-distraction provided more pain relief than active exercise; however, these results varied based on stratification of patients with and without radiculopathy and with and without recurrent symptoms. The subgroup analysis provides a possible explanation for contrasting results among randomized clinical trials of chronic low back pain treatments and these results also provide guidance for future work in the treatment of chronic low back pain.

Background Context: In patients with osteoporosis, changes in spinal alignment after a vertebral compression fracture (VCF) are believed to increase the risk of fracture of the adjacent vertebrae. The alterations in spinal biomechanics as a result of osteoporotic VCF and the effects of deformity correction on the loads in the adjacent vertebral bodies are not fully understood., Purpose: To measure 1) the effect of thoracic VCFs on kyphosis (geometric alignment) and the shift of the physiologic compressive load path (loading alignment), 2) the effect of fracture reduction by balloon (bone tamp) inflation in restoring normal geometric and loading alignment and 3) the effect of spinal extension alone on fracture reduction and restoration of normal geometric and loading alignment., Study Design/setting: A biomechanical study using six fresh human thoracic specimens, each consisting of three adjacent vertebrae with all soft tissues and bony structures intact., Methods: In order to reliably create fracture, cancellous bone in the middle vertebral body was disrupted by inflation of bone tamps. After removal of the bone tamps, the specimen was compressed using bilateral loading cables until a fracture was observed with anterior vertebral body height loss of >/=25%. Fracture reduction was performed under a compressive preload of 250 N first under the application of extension moments, and then using inflatable bone tamps. The vertebral body heights, kyphotic deformity of the fractured vertebra and adjacent segments and location of compressive load (cable) path in the fractured and adjacent vertebral bodies were measured on video-fluoroscopic images., Results: The VCF caused anterior wall height loss of 37+/-15%, middle-height loss of 34+/-16%, segmental kyphosis increase of 14+/-7.0 degrees and vertebral kyphosis increase of 13+/-5.5 degrees (p<.05). The compressive load path shifted anteriorly by about 20% of anteroposterior end plate width in the fractured and adjacent vertebrae (p=.008). Bone tamp inflation restored the anterior wall height to 91+/-8.9%, middle-height to 91+/-14% and segmental kyphosis to within 5.6+/-5.9 degrees of prefracture values. The compressive load path returned posteriorly relative to the postfracture location in all three vertebrae (p=.004): the load path remained anterior to the prefracture location by about 9% to 11% of the anteroposterior end plate width. With application of extension moment (6.3+/-2.2 Nm) until segmental kyphosis and compressive load path were fully restored, anterior vertebral body heights were improved to 85+/-8.6% of prefracture values. However, the middle vertebral body height was not restored and vertebral kyphotic deformity remained significantly larger than the prefracture values (p<.05)., Conclusions: The anterior shift of the compressive load path in vertebral bodies adjacent to VCF can induce additional flexion moments on these vertebrae. This eccentric loading may contribute to the increased risk of new fractures in osteoporotic vertebrae adjacent to an uncorrected VCF deformity. Bone tamp inflation under a physiologic preload significantly reduced the VCF deformity (anterior and middle vertebral body heights, segmental and vertebral kyphosis) and returned the compressive load path posteriorly, approaching the prefracture alignment. Application of extension moments also was effective in restoring the prefracture geometric and loading alignment of adjacent segments, but the middle height of the fractured vertebra and vertebral kyphotic deformity were not restored with spinal extension alone.

Study Design: The authors conducted an in vitro biomechanical flexibility study of T2-S1 specimens in flexion-extension under compressive follower preloads of physiological magnitudes., Objectives: The objectives of this study were to test the hypotheses that 1) the thoracolumbar spine will support compressive preloads of in vivo magnitudes and 2) allow physiological mobility under flexion-extension moments if the preload is applied along an optimized follower load path that approximates the kypholordotic curve of the thoracolumbar spine., Summary of Background Data: In the absence of muscle forces, the ligamentous thoracolumbar spine specimens cannot support the compressive loads expected in vivo. As a result, the flexibility of the thoracolumbar spine in flexion-extension has not been studied in vitro under physiological compressive preloads., Methods: Seven human thoracolumbar spines (T2-sacrum) were subjected to flexion and extension moments (up to 8 and 6 Nm, respectively) under compressive preloads from 0 to 800 N applied along an optimized follower preload path. The experimental technique applied the compressive preload such that: 1) it minimized the internal shear forces and bending moments resulting from the preload application, 2) made the internal force resultant compressive, and 3) caused the preload path to approximate the tangent to the curve of the thoracolumbar spine. The range of motion was measured in the T2-sacrum, T2-T11, T11-L1, and L1-sacrum regions., Results: All thoracolumbar specimens supported the compressive follower preload up to 800 N without damage or instability. At 800 N preload, the total flexion-extension range of motion of the T2-sacrum region decreased by 22%, from a mean of 73 degrees to 57 degrees (P < 0.05). The range of motion of the T2-T11 and L1-sacrum regions decreased from the baseline value by 23% and 30%, respectively, at a preload of 800 N. The sagittal mobility of the thoracolumbar junction (T11-L1) was not affected by the preload. The follower preload did not significantly affect the proportion of the total T2-sacrum flexion-extension range of motion contributed by the T2-T11 and L1-sacrum regions of the thoracolumbar spine., Conclusions: The optimized follower preload vector minimizes the effects of artifact moment and shear force on the range of motion of the thoracolumbar spine in flexion-extension. This model allows the entire thoracolumbar spine to be investigated under physiological loading for different clinical applications.

Study Design: A biomechanical study of lumbar threaded interbody cage construct under varying compressive preloads of similar magnitudes to those experienced in vivo during daily activities., Objectives: To test the hypothesis that supplemental translaminar facet screws would enhance the stability (ability to reduce segmental angular motion) of threaded interbody cages in flexion-extension during activities in which the spine is subjected to low compressive preloads, and therefore the stand-alone interbody cage construct is least stable., Summary of Background Data: Controversy exists over whether threaded anteriorly placed interbody cages can be routinely used as "stand-alone" devices or whether they require supplemental posterior stabilization to achieve successful fusion. Biomechanical studies suggest that under conditions of low preloads, the motion segment treated with stand-alone cages might be less stable, particularly in extension. METHODS.: Eight human lumbar spine specimens (from L1 to sacrum) were tested intact, after insertion of 2 threaded cylindrical cages (BAK) at L5-S1 and after supplemental translaminar facet screw fixation. They were subjected to flexion and extension moments under progressively increasing magnitude of externally applied compressive follower preload from 0 to 1200 N. The range of angular motion in flexion-extension at L5-S1 was analyzed to assess the effect of translaminar facet screws on the stability of the cage construct for different compressive preloads., Results: In flexion, over 0 to 400 N preload, the supplemental translaminar facet screw fixation reduced the L5-S1 angular motion relative to intact by 71% to 74% as compared to 40% to 44% for the cages alone. This difference was statistically significant (P < 0.05). In extension at 0 N preload, the cages allowed more angular motion than the intact segment, whereas with translaminar facet screw fixation, the motion was reduced to the level of the intact segment. At 400 N preload, supplemental TLFS fixation significantly increased the stability of the cages, reducing the extension angular motion by 60% of intact (P = 0.04). Supplemental translaminar facet screw fixation did not significantly increase the stability provided by the cages in flexion or extension at the 1200 N preload magnitude., Conclusions: In vivo during activities of daily living, interbody cage constructs are subject to varying compressive preloads due to external loads generated by paraspinal musculature, and our results suggest that the stability created by the cage (reduction in segmental angular motion) is not constant. The cage construct is likely to be least stable in extension during activities that impart low compressive preloads to the lumbar spine. Supplemental translaminar facet screw fixation will enhance stability of the motion segment treated with threaded cages, particularly during conditions of low compressive preloads, the very condition in which the cage alone is least effective in providing stability.

Background: Insertion of an anterior lumbar interbody fusion cage has been shown to reduce motion in a human spine segment in all loading directions except extension. The "stand-alone" cages depend on compressive preload produced by anular pretensioning and muscle forces for initial stabilization. However, the effect that the in vivo compressive preload generated during activities of daily living has on the construct is not fully understood. This study tested the hypothesis that the ability of the cages to reduce the segmental motions in flexion and extension is significantly affected by the magnitude of the externally applied compressive preload., Methods: Fourteen specimens from human lumbar spines were tested intact and after insertion of two threaded cylindrical cages at level L5-Sl. They were subjected to flexion and extension moments under progressively increasing magnitudes of externally applied compressive follower preload from 0 to 1200 N. The range of motion at level L5-S1 after cage insertion was compared with the value achieved in the intact specimens at each compressive preload magnitude., Results: The cages significantly reduced the L5-S1 flexion motion at all preloads (p < 0.05). They decreased flexion motion by 29% to 43% of that of the intact specimens for low preloads (0 to 400 N) and by 69% to 79% of that of the intact specimens under preloads of 800 to 1200 N. In extension, in the absence of an externally applied preload, the cages permitted 24% more motion than the intact segment (p < 0.05). In contrast, they reduced the extension motion at preloads from 200 to 1200 N. Under preloads of 800 to 1200 N, the reduction in extension motion after cage placement was 42% to 48% of that of the intact segment (p < 0.05). The reduction of motion in both flexion and extension after cage placement was significantly greater at preloads of 800 to 1200 N compared with the motion reductions at preloads of < or =400 N (p < 0.05)., Conclusions: In contrast to the observed extension instability under anular tension preload only, the two-cage construct exerted a stabilizing effect on the motion segment (a reduction in segmental motion) in flexion as well as extension under externally applied compressive preloads of physiologic magnitudes. The external compressive preload significantly affected the stabilization provided by the cages. The cages provided substantially more stabilization, both in flexion and in extension, at larger preloads than at smaller preloads., Clinical Relevance: The study suggests that the segment treated with an anterior lumbar interbody fusion cage is relatively less stable under conditions of low external compressive preload. The magnitude of preload required to achieve stabilization with stand-alone cages may be only partially achieved by anular pretensioning. Since the magnitude of the preload across the disc space due to muscle activity can vary with activities of daily living, supplemental stabilization of the cage construct may provide a more predictably stable environment for lumbar spine fusion.

Traditional experimental methods are unable to study the kinematics of whole lumbar spine specimens under physiologic compressive preloads because the spine without active musculature buckles under just 120 N of vertical load. However, the lumbar spine can support a compressive load of physiologic magnitude (up to 1200 N) without collapsing if the load is applied along a follower load path. This study tested the hypothesis that the load-displacement response of the lumbar spine in flexion-extension is affected by the magnitude of the follower preload and the follower preload path. Twenty-one fresh human cadaveric lumbar spines were tested in flexion-extension under increasing compressive follower preload applied along two distinctly different optimized preload paths. The first (neutral) preload path was considered optimum if the specimen underwent the least angular change in its lordosis when the full range of preload (0-1200 N) was applied in its neutral posture. The second (flexed) preload path was optimized for an intermediate specimen posture between neutral and full flexion. A twofold increase in flexion stiffness occurred around the neutral posture as the preload was increased from 0 to 1200 N. The preload magnitude (400 N and larger) significantly affected the range of motion (ROM), with a 25% decrease at 1200 N preload applied along the neutral path. When the preload was applied along a path optimized for an intermediate forward-flexed posture, only a 15% decrease in ROM occurred at 1200 N. The results demonstrate that whole lumbar spine specimens can be subjected to compressive follower preloads of in vivo magnitudes while allowing physiologic mobility under flexion-extension moments. The optimized follower preload provides a method to simulate the resultant vector of the muscles that allow the spine to support physiologic compressive loads induced during flexion-extension activities.

A retrospective independent radiographic and chart review was undertaken for 17 patients who underwent a unique anterior salvage procedure for iatrogenic and progressive postoperative spondylolisthesis. This one-stage anterior transabdominal discectomy, reduction, stabilization, and arthrodesis was first performed in 1979. Of the 17 patients, all complained of leg pain, 14 of back pain, 11 had neurogenic claudication, and 2 were bedridden preoperatively because of their pain. Of the 17 patients, 7 had no neurologic deficits, 2 had cauda equina syndrome, and the remaining 8 had motor root deficits. The average number of posterior operations before our salvage procedure was 1.8, with a range of 1 to 3. Eight patients had an average of 1.6 attempts at posterior arthrodesis, with a range of 1 to 3 procedures. Two patients had a grade I spondylolisthesis, 11 a grade II, and 4 a grade III. Follow-up was available for 16 patients from 2 years and 3 months to 11 years and 5 months after the index operation (mean, 6 years and 5 months). One patient with severe cardiovascular disease died perioperatively. This anterior procedure was able to restore spinal stability and decompress the neural elements in 13 of 16 patients. Eleven obtained a solid arthrodesis. Three patients required further spinal surgery: two posterior fusions for symptomatic nonunions and one posterior foraminotomy for persistent foraminal stenosis. No patient deteriorated neurologically, the two with cauda equina syndrome recovered, and all but one patient with motor root deficits recovered fully. At latest follow-up, there were six excellent, seven good, and three fair results. There were no poor results. Although technically difficult and troubled by complications, the relative historical merits and principles of this unique anterior salvage procedure probably warrant further consideration, especially in light of evolving anterior surgical technologies.

Study Design: An experimental approach was used to test human cadaveric cervical spine specimens., Objective: To assess the response of the cervical spine to a compressive follower load applied along a path that approximates the tangent to the curve of the cervical spine., Summary of Background Data: The compressive load on the human cervical spine is estimated to range from 120 to 1200 N during activities of daily living. Ex vivo experiments show it buckles at approximately 10 N. Differences between the estimated in vivo loads and the ex vivo load-carrying capacity have not been satisfactorily explained., Methods: A new experimental technique was developed for applying a compressive follower load of physiologic magnitudes up to 250 N. The experimental technique applied loads that minimized the internal shear forces and bending moments, loading the specimen in nearly pure compression., Results: A compressive vertical load applied in the neutral and forward-flexed postures caused large changes in cervical lordosis at small load magnitudes. The specimen collapsed in extension or flexion at a load of less than 40 N. In sharp contrast, the cervical spine supported a load of up to 250 N without damage or instability in both the sagittal and frontal planes when the load path was tangential to the spinal curve. The cervical spine was significantly less flexible under a compressive follower load compared with the hypermobility demonstrated under a compressive vertical load (P < 0.05)., Conclusion: The load-carrying capacity of the ligamentous cervical spine sharply increased under a compressive follower load. This experiment explains how a whole cervical spine can be lordotic and yet withstand the large compressive loads estimated in vivo without damage or instability.

Anterior discectomy and fusion with an interbody bone graft and anterior plate is a common procedure in cervical spine surgical management. However, the graft may be shielded from some mechanical loading by the plate. Mechanical testing was performed on six cadaveric calf spines that were subjected to a simulated anterior cervical discectomy and fusion with an interbody bone graft alone and with an anterior plate to determine the amount of load sharing between the graft and plate. The load-displacement data were used to compute the amount of load sharing between the graft and the plate as a continuous function of the applied axial compression load. Although the percent load transmitted through the graft decreased (53 to 41%) as the axial load increased (45 to 90 N), the magnitude of load transmitted through the graft increased (24 to 37 N), with corresponding intervertebral strains <6%. In a single-level procedure, an anterior cervical plate serves as a load-sharing device rather than a load-shielding device, enabling graft consolidation as observed in clinical studies.

Study Design: The Alpine goat model for multilevel anterior cervical discectomy and fusion was used to analyze the use of an intervertebral fusion device to promote an arthrodesis after anterior cervical discectomy. Comparisons were drawn with biomechanical, histologic, and radiographic data., Objectives: To analyze the use of an intervertebral fusion device, with and without a bone graft substitute, to promote an arthrodesis anterior cervical discectomy., Summary of Background Data: In previous studies, the goat cervical spine has proven to be an excellent model for examining the healing of fusions using bone grafts, instrumentation, or bone substitutes., Methods: Three-level anterior cervical dissectomies were performed on 21 mature Alpine goats. Three treatment groups of seven goats each were used. Group I used a standard titanium cervical BAK device filled with autogenous bone graft. Group II used a hydroxyapatite-coated BAK device filled with autogenous bone graft. Group III used a BAK device filled with recombinant human bone morphogenetic protein-2., Results: Radiographically, no cages became displaced. Lucencies were seen around 3 of the 21 cages in Group 1, 4 cages in Group II, and none in Group III. Fluorochrome analysis revealed that the recombinant human bone morphogenetic protein-2-filled cages had an accelerated rate of bone growth around and through each cage-vertebral body interface at 3 weeks. A successful arthrodesis was also more likely with a recombinant human bone morphogenetic protein-2-filled cage (95%) than the hydroxyapatite-coated (62%) or the standard (48%) cage. Biomechanical stiffness testing did not reveal any statistically significant differences between the three groups. There was a tendency for successfully arthrodesed interspaces to be stiffer than those that were not., Conclusions: The use of a threaded intervertebral fusion cage, with or without hydroxyapatite coating, filled with autogenous bone graft provides a fusion rate that is slightly better than those previously reported using autogenous interbody bone grafts with or without plate stabilization. Recombinant human bone morphogenetic protein-2-filled cages resulted in a much higher arthrodesis rate and accelerated bone formation compared with either autogenous bone-filled BAK devices, or autogenous interbody bone grafts with or without plate stabilization.

Study Design: Cadaveric human and bovine lumbar spine models simulating the acute postoperative period were used to compare the biomechanical properties of two designs of intervertebral body threaded fusion cages. The instrumented spines were compared with intact spines and with spines with resected posterior elements, representing a revision case., Objective: To determine the relative biomechanical performance of these competing devices., Summary of Background Data: These cages are currently under clinical investigation, and basic biomechanical data are needed., Methods: Insertion torques and maximum pushout loads were measured for each cage. Intact spines, posteriorly instrumented spines (posterior lumbar interbody fusion), and spines with resected posterior elements were loaded in axial compression, flexion and extension bending, and axial torsion. Stiffness comparisons were made between the different configurations., Results: Insertion torques and pushout loads were similar for the cages. Both cages significantly increased stiffnesses above those of the intact spines and the resected spines. The BAK-instrumented spines were more stiff in axial compression, while the Threaded Interbody Fusion Device spines were more stiff in extension., Conclusions: This study revealed the two cages to have similar biomechanical characteristics immediately after posterior insertion and warrant further clinical studies.

Anterior instrumentation in the treatment of thoracolumbar fractures has progressed significantly during the past 2 decades. These fixation systems have evolved to meet the anatomic, biomechanical, and imaging challenges associated with internal fixation of the thoracolumbar spine. The evolution of these devices will be reviewed, and from this, the indications and surgical techniques necessary for the safe and effective use of the device will be discussed. This study also reports the authors' initial clinical experience using the Z plate anterior thoracolumbar plating system in the treatment of thoracolumbar burst fractures. The study consists of 12 consecutive adult patients who underwent a 1-stage anterolateral decompressive and stabilization procedure for burst fractures from T9-L3. The indications for surgery included neurologic deficits, deformity, progressive kyphosis, and late pain. Ten of the 12 patients maintained their postoperative sagittal alignment or a significant portion of their kyphosis reduction. Two patients with severe kyphotic deformities greater than 50 degrees lost 10 degrees and 20 degrees of their reduction at last followup. All 3 patients with neurologic deficits recovered. There were no neurologic or perioperative complications. Eleven of the 12 patients obtained a good or excellent functional outcome. Anterior arthrodesis using instrumentation stabilization after a 1-stage anterolateral decompression and reduction procedure can yield successful clinical results in the treatment of thoracolumbar burst fractures.

We evaluated the results for fifteen patients in whom symptomatic osteoarthrosis of the atlanto-axial joints had been treated with an atlanto-axial or occipitocervical arthrodesis between 1973 and 1990. Thirteen patients had long-term follow-up (average duration, seven years and two months; range, four years and two months to nineteen years and two months). The two remaining patients had died: one, four days postoperatively, from a cardiopulmonary arrest, and the other, one year and eight months postoperatively, from complications related to bladder cancer. Preoperatively, all fifteen patients reported pain in the occipitocervical region that increased with any attempt at rotation of the neck and was unresponsive to immobilization with a collar and to analgesics. The average duration of the symptoms before the arthrodesis was three years. One patient had acute quadriparesis. All patients had radiographic evidence of oesteoarthrosis involving the lateral atlanto-axial articulations. Four patients had atlanto-axial instability with an average of five millimeters (range, three to ten millimeters) of motion at the anterior atlanto-odontoid interval. Six patients had an associated spontaneous subaxial fusion, which was secondary to osteoarthrosis in five; three of the five also had atlanto-axial instability. Fourteen patients were managed with a posterior arthrodesis and one, with an anterior transoral arthrodesis. The procedures were performed to relieve pain, to stabilize the atlanto-axial joints, and to restore neurological function. Of the fourteen patients who were followed, thirteen had a solid fusion and one had a stable pseudarthrosis. The patient who had quadriparesis recovered. At the latest follow-up evaluation, thirteen patients had an excellent result and one had a fair result as determined with use of a modification of the criteria of Robinson et al. There were no poor results. Atlanto-axial arthrodesis can effectively relieve occipitocervical pain and correct atlanto-axial instability secondary to osteoarthrosis.

Cervical spine infections arise from a variety of etiologies including postsurgical, iatrogenic, and hematogenous routes. Clinical history, physical examinations, and diagnostic studies all play an integral role in the diagnosis and treatment of these infections. Successful treatment depends on a proper and timely diagnosis, understanding the etiology, and defining the extent to which the infectious process involves the spinal supporting and neurologic elements. Surgical treatment is required when there is abscess formation, instability, progressive kyphosis secondary to vertebral body collapse, or canal compromise with neurologic deficits.

The emergent care of an unstable pelvic ring disruption in the patient who is hemodynamically unstable includes rapid application of military antishock trousers or an external fixator in an attempt to control bleeding and hemodynamically stabilize the patient. However, use of the military antishock trousers is limited in that it restricts access to the abdomen and lower extremities. The external fixator is limited at many institutions by the need to apply it in the operating room. Two experimental devices have been developed to provide emergent pelvic fracture reduction and temporary stabilization in the trauma room, while maintaining access to the abdomen and lower extremities. This study compared the efficacy of pelvic fracture reduction and stabilization in a cadaveric model using an external fixator with the efficacy of 2 experimental devices, the pelvic stabilizer and the pelvic c-clamp. Based on their ability to restore pelvic volume and reduce pubic diastasis and their application time, the 3 devices performed similarly. Complications in applying each device were noted but were of less clinical significance for the external fixator. Surgeon practice on cadavera before clinical use will help ensure safe application of either experimental device in the trauma room.

Objective: Determine if laparotomy further destabilizes an unstable pelvic injury and increases pelvic volume, and if reduction and stabilization restores pelvic volume and prevents volume changes secondary to laparotomy., Design: Cadaveric pelvic fracture model., Materials and Methods: Unilateral open-book pelvic ring injuries were created in five fresh cadaveric specimens by directly disrupting the pubic symphysis, left sacroliac joint, and sacrospinous and sacrotuberous ligaments. Pelvic volume was determined using computerized axial tomography for the intact pelvis, disrupted pelvis with both a laparotomy incision opened and closed, and disrupted pelvis stabilized and reduced using an external fixator with the laparotomy incision opened., Measurements and Main Results: The average volume increase in the entire pelvis (from the top of the iliac crests to the bottom of the ischial tuberosities) between a nonstabilized injury with the abdomen closed and then subsequently opened was 15 +/- 5% (423 cc). The average increase in entire pelvic volume between a stabilized and reduced pelvis and nonstabilized pelvis, both with the abdomen open, was 26 +/- 5% (692 cc). The public diastasis increased from 3.9 to 9.3 cm in a nonstabilized pelvis with the abdomen closed and then subsequently opened. Application of a single-pin anterior-frame external fixator reduced the pubic diastasis anatomically and reduced the average entire and true (from the pelvic brim to the ischeal tuberosities) pelvic volumes to within 3 +/- 4 and 8 +/- 6% of the initial volume, respectively., Conclusions: We believe that the abdominal wall provides stability to an unstable pelvic ring injury via a tension band effect on the iliac wings. Our results demonstrate that a laparotomy further destabilized an open-book pelvic injury and subsequently increased pelvic volume and pubic diastasis. This could potentially increase blood loss from the pelvic injury and delay the tamponade effect of reduction and stabilization. A single-pin external fixator prevents the destabilizing effect of the laparotomy and effectively reduces pelvic volume. These data support reduction and temporary stabilization of unstable pelvic injuries before or concomitantly with laparotomy.

Objective: To present two patients with transient paraplegia from intercostal nerve blocks after transthoracic discectomy., Methods: The preoperative, intraoperative, and postoperative courses of two patients who underwent transthoracic discectomies were reviewed. A literature review of complications from thoracic disc excisions and intercostal nerve blocks also was conducted., Results: Two patients experienced transient paraplegia from intercostal nerve blockade after thoracotomy for thoracic discectomies. Although transient paraplegia (or regional anesthesia) after intercostal nerve blocks has been reported and is consistent with the clinical course seen in our two patients, it has not been reported after thoracotomy for decompression of the spinal cord., Conclusion: Because of the inherent risk of neurologic injury after thoracic discectomy, the authors discourage the use of intercostal nerve blocks for relieving postoperative pain after transthoracic discectomy.