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MINI: Identifying fracture is important for understanding vertebral mechanics. Isolated cadaveric thoracolumbar vertebrae were compressed, and surface strains were measured using digital image correlation. Fracture locations from video analysis were qualitatively similar to the locations of high compressive strains and local damage occurred before the maximum force was reached. STUDY DESIGN Ex vivo compression experiments on isolated cadaveric vertebrae. OBJECTIVE To qualitatively compare the fracture locations identified in video analysis with the locations of high compressive strain measured with digital image correlation (DIC) on vertebral bodies and to evaluate the timing of local damage to the cortical shell relative to the global yield force. SUMMARY OF BACKGROUND DATA In previous ex vivo experiments, cortical bone fracture has been identified using various methods including acoustic emission sensors, strain gages, video analysis, or force signals. These methods are, however, limited in their ability to detect the location and timing of fracture. We propose use of DIC, a noncontact optical technique that measures surface displacement, to quantify variables related to damage. METHODS Isolated thoracolumbar human cadaveric vertebral bodies (n = 6) were tested in compression to failure at a quasi-static rate, and the force applied was measured using a load cell. The surface displacement and strain were measured using DIC. Video analysis was performed to identify fractures. RESULTS The location of fractures identified in the video corresponded well with the locations of high compressive strain on the bone. Before reaching the global yield force, more than 10% of the DIC measurements reached a minimum principal strain of 1.0%, a previously reported threshold for cortical bone damage. CONCLUSION DIC measurements provide an objective measure that can be used to identify the location and timing of fractures during ex vivo vertebral experiments. This is important for understanding fracture mechanics and for validating vertebral computational models that incorporate failure. LEVEL OF EVIDENCE N /A.

MINI: Identifying fracture is important for understanding vertebral mechanics. Isolated cadaveric thoracolumbar vertebrae were compressed, and surface strains were measured using digital image correlation. Fracture locations from video analysis were qualitatively similar to the locations of high compressive strains and local damage occurred before the maximum force was reached.Ex vivo compression experiments on isolated cadaveric vertebrae.To qualitatively compare the fracture locations identified in video analysis with the locations of high compressive strain measured with digital image correlation (DIC) on vertebral bodies and to evaluate the timing of local damage to the cortical shell relative to the global yield force.In previous ex vivo experiments, cortical bone fracture has been identified using various methods including acoustic emission sensors, strain gages, video analysis, or force signals. These methods are, however, limited in their ability to detect the location and timing of fracture. We propose use of DIC, a noncontact optical technique that measures surface displacement, to quantify variables related to damage.Isolated thoracolumbar human cadaveric vertebral bodies (n = 6) were tested in compression to failure at a quasi-static rate, and the force applied was measured using a load cell. The surface displacement and strain were measured using DIC. Video analysis was performed to identify fractures.The location of fractures identified in the video corresponded well with the locations of high compressive strain on the bone. Before reaching the global yield force, more than 10% of the DIC measurements reached a minimum principal strain of 1.0%, a previously reported threshold for cortical bone damage.DIC measurements provide an objective measure that can be used to identify the location and timing of fractures during ex vivo vertebral experiments. This is important for understanding fracture mechanics and for validating vertebral computational models that incorporate failure.N /A.

Study Design. A biomechanical human cadaveric study. Objective. To determine the percentage of shear force supported by posterior lumbar spinal devices of varying stiffnesses under anterior shear loading in a degenerative spondylolisthesis model. Summary of Background Data. Clinical studies have demonstrated beneficial results of posterior arthrodesis for the treatment of degenerative spinal conditions with instability. Novel spinal implants are designed to correct and maintain spinal alignment, share load with the spine, and minimize adjacent level stresses. The optimal stiffness of these spinal systems is unknown. To our knowledge, low-stiffness posterior instrumentation has not been tested under an anterior shear force, a highly relevant force to be neutralized in the clinical case of degenerative spondylolisthesis. Methods. The effects of implant stiffness and specimen condition on implant load and intervertebral motion were assessed in a biomechanical study. Fifteen human cadaveric lumbar functional spinal units were tested under a static 300 N axial compression force and a cyclic anterior shear force (5–250 N). Implants (high-stiffness [HSI]: o 5.5-mm titanium, medium-stiffness [MSI]: o 6.35 × 7.2-mm oblong PEEK, and low-stiffness [LSI]: o 5.5-mm round PEEK) instrumented with strain gauges were used to calculate loads and were tested in each of 3 specimen conditions simulating degenerative changes: intact, facet instability, and disc instability. Intervertebral motions were measured with a motion capture system. Results. As predicted, implants supported a significantly greater shear force as the specimen was progressively destabilized. Mean implant loads as a percent of the applied shear force in order of increasing specimen destabilization for the HSI were 43%, 67%, and 76%; mean implant loads for the MSI were 32%, 56%, and 77%; and mean implant loads for the LSI were 18%, 35%, and 50%. Anterior translations increased with decreasing implant stiffness and increasing specimen destabilization. Conclusion. Implant shear stiffness significantly affected the load sharing between the implant and the natural spine in anterior shear ex vivo. Low-stiffness implants transferred significantly greater loads to the spine. This study supports the importance of load-sharing behavior when designing new implants.

STUDY DESIGN: Quantitative in vivo ultrasound imaging study of spinal cord and dura morphology after acute experimental spinal cord injury (SCI) and decompression in a pig model. OBJECTIVE: To study the morphological changes of the spinal cord and dura immediately after surgical decompression for acute SCI. SUMMARY OF BACKGROUND DATA: Surgical decompression for traumatic SCI is currently a topic of debate. After decompression, relief of bony impingement on the thecal sac and spinal cord can be confirmed intraoperatively. However, postoperative imaging often reveals that the cord has swollen to fill the subarachnoid space. Little is known about the extent and timing of this morphological response. METHODS: Yucatan miniature pigs received sham surgery (N = 1) or a moderate (N = 6, 20 g, 2.3 m/s) or high (N = 6, 20 g, 4.7 m/s) severity weight-drop SCI followed by 8 hours of sustained compression (100 g) and 6 hours of postdecompression monitoring. Sagittal-plane ultrasound images were used to quantify spinal cord, dura, and subarachnoid space dimensions preinjury and once per hour after decompression. RESULTS: Animals with a moderate SCI exhibited a residual cord deformation of up to 0.64 mm within 10 minutes of decompression, which tended to resolve during 6 hours because of tissue relaxation and swelling. For animals with high-severity SCIs, cord swelling was immediate and resulted in occlusion of the subarachnoid space within 10 minutes to 5 hours, whereas this occurred for only half of the moderate injury group. CONCLUSION: Decompression of an acute SCI may result in residual cord deformation followed by gradual swelling or immediate swelling leading to subarachnoid occlusion. The response is dependent on initial injury severity. These observations may partly explain the lack of benefit of decompression in some patients and suggest a need to reduce cord swelling to optimize the clinical outcome after acute SCI.

Vertebral body fractures are the most common type of osteoporotic fractures.1,2 These fractures represent a growing public health concern because the incidence is projected to increase as the population ages.3 Vertebral fractures are associated with substantial increases in back pain, functional limitation, disability, and mortality.4,5 Falls are a major contributing factor to the occurrence of osteoporotic fractures6; 41% of vertebral fractures in older men are associated with falls from standing height or lower.7 The most common vertebral fracture site for older men is the lumbosacral spine, which represents 56% of all cases.7 Falls from standing height or lower also account for between 8% and 26% of spinal cord injuries,7,8 and more than two-thirds of these injuries occur in older adults.8 The risk of vertebral fracture in a backward fall depends, by definition, on a “factor of safety.” This factor of safety is defined as the ratio of the force that will typically result in failure under a loading configuration similar to that applied in the fall (i.e., tolerance) divided by the peak-applied force. Previous studies have measured the force applied to the buttocks in subinjurious backward falls in human subjects.9 The compressive force required to cause fracture of lumbar vertebrae has been determined by testing isolated cadaver spine segments.10–17 However, the peak forces occurring at the lumbosacral spine during a backward fall are not known, and this is a barrier to the development of injury prevention strategies. An intervention that has been suggested to reduce the incidence of fall-related fractures is the use of compliant or low-stiffness flooring.9,18,19 In a previous experimental study conducted by a subset of this study’s authors, human subjects fell from standing onto floors of various stiffnesses.9 Impact forces at the buttocks decreased with decreases in floor stiffness; however, the effect on spine forces was not addressed. To determine the effect of floor stiffness on spine forces experimentally, direct measurement of spinal component loads would be necessary; however, these are difficult to measure even during benign activities in vivo.10,11 Numerical models have been used to study the loads generated in the human spine during various activities,12–15 and this method has also been used to study spinal forces during trauma.16–18 Some of the previous models have been complex, have simulated multiple degrees of freedom at each joint, and have incorporated anatomic details such as the rib cage, lumbar muscles, viscera, facet joints, and intervertebral discs.12,14–16 Axial spine forces are related to fracture risk19–23; therefore, they are of primary interest in this study. It was our objective to develop an appropriate model to allow examination of the effect of floor stiffness on axial loading at various levels of the spine during a fall. We had access to a unique set of data to incorporate into our model. These human subject-specific data from our previous experimental study provided a level of verification for the present computational model.9 No previous mathematical modelling or experimental studies have examined spine forces resulting from a backward fall onto the buttocks. Accordingly, the specific objectives in this study were (1) to develop and verify a lumped-parameter model to predict intervertebral forces at several levels of the lumbosacral spine during backward falls from standing, (2) to evaluate the effect of floor stiffness on these forces, and (3) to examine how these compare with the axial force associated with fracture of the spine.

STUDY DESIGN: Cross-sectional study. OBJECTIVE: To determine the effect of age and sex on the three-dimensional kinematics of the cervical spine. SUMMARY OF BACKGROUND DATA: Spine kinematics information has important implications for biomechanical model development, anthropomorphic test device development, injury prevention, surgical treatment, and safety equipment design. There is a paucity of data of this type available for children, and it is unknown whether cervical spine kinematics of the pediatric population is different than that of adults. The helical axis of motion (HAM) of the spine provides unique information about the quantity and quality (coupling etc.) of the measured motion. METHODS: Ninety subjects were recruited and divided into 6 groups based on sex and age (young children aged 4-10 years, older children aged 11-17 years, adults aged 25+ years). Subjects actively moved their head in axial rotation, lateral bending, and flexion/extension. An optoelectronic motion analysis system recorded the position of infrared markers placed on the first thoracic vertebrae (T1) and on tight-fitting headgear worn by the subjects. HAM parameters were calculated for the head motion with respect to T1. RESULTS: HAM location in axial rotation and flexion/extension was more anterior in young females compared to adult females. Young females had a more anterior HAM location in flexion/extension compared to young males, indicating an effect of sex. For females, the HAM locations of adults were superior to those of children in flexion/extension and lateral bending whereas in males the HAM locations of adults were inferior to those of children. Age-related differences in HAM orientation were also observed in axial rotation and lateral bending. CONCLUSION.: Cervical spine kinematics vary with age and sex. The variation in spine mechanics based on age and sex found in the present study may indicate general trends that would grow stronger in even younger children (age Language: en

Study design An ex vivo biomechanical study using porcine spinal segments. Objective To produce a biomechanical model of both spondylolysis and spondylolisthesis using an accelerated cyclic loading model with intermittent impulse loads. Summary of background data Only a few models of spondylolisthesis appropriate for biomechanical testing have been presented previously. Past modeling attempts have largely required nonphysiologic gross fracture of the pars before testing and have resulted in nonphysiologic endplate fracture. In these tests no clinically relevant spondylolisthesis was seen at the end of testing. A reproducible, clinically relevant model of both spondylolysis and spondylolisthesis would allow study of these disease processes, and facilitate the development and evaluation of advanced spinal implants optimized specifically for these pathologies. Methods Five porcine lumbar functional spinal units were tested (2 L4-L5, 3 L6-S1) after small notches had been created in the pars and after the disc had specific collagen fibers in the anterior anulus sectioned. Specimens were loaded with a constant cranial-caudal compressive force of 300 N and the application of cyclic anterior shear loads between 300 and 600 N with intermittent impulse loads to 1500 N until pars fracture occurred. Elevated cyclic loading then continued between 500 and 800 N. Results All specimens displayed bilateral pars fracture with the fractures passing through the points of notching and no damage to endplates or facet joints. Clinically-relevant Grade II spondylolisthesis was achieved in all 5 specimens. The mean slip at the conclusion of testing was 33%. Conclusion Cyclic shear loading with intermittent impulse loads can reliably create fracture in the pars interarticularis in ex vivo porcine spine segments. Subsequent cyclic anterior motion of the superior vertebra results in clinically-relevant spondylolysis and spondylolisthesis.

STUDY DESIGN Nucleus pressure was measured within porcine intervertebral discs (IVDs) with a novel in-fiber Bragg grating (FBG) sensor (0.4 mm diameter) and a strain gauge (SG) sensor (2.45 mm). OBJECTIVE To validate the accuracy of a new FBG pressure sensor designed for minimally invasive measurements of nucleus pressure. SUMMARY OF BACKGROUND DATA Although its clinical utility is controversial, it is possible that the predictive accuracy of discography can be improved with IVD pressure measurements. These measurements are typically obtained using needle-mounted SG sensors inserted into the nucleus. However, by virtue of their size, SG sensors alter disc mechanics, injure anulus fibers, and can potentially initiate or accelerate degenerative changes thereby limiting their utility particularly clinically. METHODS Six functional spinal units were loaded in compression from 0 N to 500 N and back to 0 N; nucleus pressure was measured using the FBG and SG sensors at various locations along anterior and anterolateral axes. RESULTS On average maximum IVD pressures measured using the FBG and SG sensors were within 9.39% of each other. However, differences between maximum measured pressures from the FBG and SG sensors were larger (22.2%) when the SG sensor interfered with vertebral endplates (P < 0.05). The insertion of the FBG sensor did not result in visible damage to the anulus, whereas insertion of the SG sensor resulted in large perforations in the anulus through which nucleus material was visible. CONCLUSION The new FBG sensor is smaller and less invasive than any previously reported disc pressure sensor and gave results consistent with previous disc pressure studies and the SG sensor. There is significant potential to use this sensor during discography while avoiding the controversy associated with disc injury as a result of sensor insertion.

Study Design. An in vitro flexibility study of C2-T1 specimens under compressive preload. Objective. To determine three-dimensional flexibility test moments needed to obtain spinal kinematics representative of the in vivo spine studies. Summary of Background Data. Most previous three-dimensional in vitro cervical spine studies have used equal moments in all three planes to evaluate spinal flexibilities. Recent advances have made it possible to apply physiologic compressive preload. It is unclear what moments should be applied to these preloaded spine segments to simulate in vivo kinematics. Methods. Six fresh human cadaveric cervical spine specimens (C2-T1) were used. The preload (100 N) was applied by flexible cables, which passed through guides attached to each vertebra. Flexibility tests of flexion-extension and bilateral axial torsion and lateral bending were performed. Two protocols were compared, 1:1:1 with equal pure moments of 1 Nm for each direction and 2:4:2 with pure moments of 2 Nm for flexion-extension and lateral bending and 4 Nm for axial torsion. Ranges of motion were calculated from the flexibility tests. Results. The 2:4:2 protocol resulted in significantly better agreement with in vivo data than did the 1:1:1 protocol, in flexion-extension, the 2 Nm value was within 17% of the average in vivo value, in axial torsion, the 4 Nm value was within 22% of the in vivo average. In lateral bending, the 2 Nm value was within 15% of the in vivo average. Conclusions. To obtain human in vivo-like kinematics using 100 N preioad, the 2:4:2 protocol is to be recommended.

STUDY DESIGN In vivo human volunteer study of the intervertebral postural changes and muscle activity levels while tensing the neck muscles. OBJECTIVE To determine if actively tensing the neck muscles changes the posture of the cervical spine and, because axial impact neck injury often occurs while inverted, whether these changes exist both upright and upside down. SUMMARY OF BACKGROUND DATA Rollover accidents are dynamic and complex events in which head contacts with the vehicle interior can cause catastrophic neck injuries. Computational modeling has suggested that active neck muscles may increase the risk of cervical spine fracture in a rollover crash. Cadaver testing has also demonstrated that overall neck alignment and curvature are key to understanding and preventing catastrophic neck injuries. Although muscle activity and neck posture affects the resulting injury, there are currently no in vivo data describing how tensing the neck muscles influences intervertebral posture. METHODS Eleven human subjects (6 females, 5 males) actively tensed their neck muscles while seated upright and inverted. Vertebral alignment was measured using fluoroscopy and muscle activity was recorded using surface and indwelling electrodes in 8 neck muscles. RESULTS On average, tensed muscles increased cervical spine curvature and anterior motion of the cervical vertebrae relative to the torso. These changes, which were magnified by inversion, indicate that cervical intervertebral posture differs considerably between the relaxed and tensed states. CONCLUSION Active muscle contraction can change the vertebral alignment in upright and inverted postures. This change in posture may alter the load path and injury mechanics during an axial head impact and may help explain the disparity between the neck injuries observed in real-world rollover accidents and ex vivo cadaver experiments. LEVEL OF EVIDENCE N/A.

STUDY DESIGN A biomechanical human cadaveric study. OBJECTIVE To create a biomechanical model of low-grade degenerative lumbar spondylolisthesis (DLS), defined by anterior listhesis, for future testing of spinal instrumentation. SUMMARY OF BACKGROUND DATA Current spinal implants are used to treat a multitude of conditions that range from herniated discs to degenerative diseases. The optimal stiffness of these instrumentation systems for each specific spinal condition is unknown. Ex vivo models representing degenerative spinal conditions are scarce in the literature. A model of DLS for implant testing will enhance our understanding of implant-spine behavior for specific populations of patients. METHODS Four incremental surgical destabilizations were performed on 8 lumbar functional spinal units. The facet complex and intervertebral disc were targeted to represent the tissue changes associated with DLS. After each destabilization, the specimen was tested with: (1) applied shear force (-50 to 250 N) with a constant axial compression force (300 N) and (2) applied pure moments in flexion-extension, lateral bending and axial rotation (±5 Nm). Relative motion between the 2 vertebrae was tracked with a motion capture system. The effect of specimen condition on intervertebral motion was assessed for shear and flexibility testing. RESULTS Shear translation increased, specimen stiffness decreased and range of motion increased with specimen destabilization (P < 0.0002). A mean anterior translation of 3.1 mm (SD 1.1 mm) was achieved only after destabilization of both the facet complex and disc. Of the 5 specimen conditions, 3 were required to achieve grade 1 DLS: (1) intact, (3) a 4-mm facet gap, and (5) a combined nucleus and annulus injury. CONCLUSION Destabilization of both the facet complex and disc was required to achieve anterior listhesis of 3.1 mm consistent with a grade 1 DLS under an applied shear force of 250 N. Sufficient listhesis was measured without radical specimen resection. Important anatomical structures for supporting spinal instrumentation were preserved such that this model can be used in future to characterize behavior of novel instrumentation prior to clinical trials.

An in vitro biomechanical investigation in the human lumbar spine focuses on the functional significance of vertebral bone density and intervertebral disc degenerations.To determine that interrelationship between vertebral bone density and intervertebral disc degeneration, their effect on normal spine motion, and their significance in the biotechnical performance of interbody fixation techniques.A relationship between vertebral bone density and intervertebral disc degeneration has been suggested, but a definitive relationship has not been established. The effect of vertebral bone density and intervertebral disc degeneration on interbody stabilization remains unknown despite the rapidly increasing use of this surgical method for patients with chronic low back pain.The vertebral bone density and intervertebral disc degeneration of 72 functional spinal units were determined using dual energy x-ray absorptiometry scans and macroscopic grading, respectively. A three-dimensional flexibility test was performed on 24 functional spinal units in the intact and stabilised conditions. The compressive behavior of the bone-implant interface was evaluated in 48 functional spinal units.The vertebral bone density in moderately degenerated disc was significantly lower than at all other levels of intervertebral disc degeneration. Increasing intervertebral disc degeneration resulted in more axial rotation and less lateral bending. In flexion-extension and lateral bending, better vertebral bone resulted in significantly better stabilization. This trend was observed also in axial compression in which higher failure loads were observed with greater bone densities.The authors conclude a significant relationship exists between bone density and disc degeneration, bone density is a highly important factor in the performance of interbody stabilization, and disc degeneration, is of moderate importance in signal motion.

Study design In vivo large animal (pig) model study of cerebrospinal fluid (CSF) pressures after acute experimental spinal cord injury (SCI). Objective To determine how the CSF pressure (CSFP) and CSF pulse pressure amplitude (CSFPPA) cranial and caudal to the injury site change after an acute SCI with subsequent thecal occlusion and decompression. Summary of background data Lowering intrathecal pressure via CSF drainage is currently instituted to prevent ischemia-induced SCI during thoracoabdominal aortic aneurysm surgery and was recently investigated as a potential intervention for acute traumatic SCI. However, in SCI patients, persistent extradural compression commonly occludes the subarachnoid space. This may generate a CSFP differential across the injury site, which cannot be appreciated with lumbar catheter pressure measurements. Methods Anesthetized pigs were subjected to an acute contusive SCI at T11 and 8 hours of sustained compression (n = 12), or sham surgery (n = 2). CSFP was measured cranial and caudal to the injury site, using miniature pressure transducers, during compression and for 6 hours after decompression. Results The cranial-caudal CSFP differential increased (mean, 0.39 mm Hg/h), predominantly due to increased cranial pressure. On decompression, cranial CSFP decreased (mean, -1.16 mm Hg) and caudal CSFP increased (mean, 0.65 mm Hg). The CSFP differential did not change significantly after decompression. Cranial CSFPPA was greater than caudal CSFPPA, but this differential did not change during compression. On decompression, the caudal CSFPPA increased in some but not all animals. Conclusion Although extradural compression exists at the site of injury, lumbar CSFP may not accurately indicate CSFP cranial to the injury. Decompression may provide immediate, though perhaps partial, resolution of the pressure differential. CSFPPA was not a consistent indicator of decompression in this animal model. These findings may have implications for the design of future clinical protocols in which CSFP is monitored after acute SCI.

A biomechanical study using ex vivo bovine spinal cord and dura, and a synthetic surrogate spinal cord with bovine dura.To investigate the effect of cerebrospinal fluid (CSF) on spinal cord deformation characteristics and to evaluate the biofidelity of a new surrogate spinal cord using an ex vivo bovine model of the burst fracture process.Spinal cord injury is associated with significant personal, economic and social costs. The role of CSF during the injury event and its effect on the spinal cord deformation and neurologic injury is not well understood. Such knowledge could inform preventative strategies and clinical interventions and aid the development and validation of experimental and computational models.The transverse impact of a propelled bone fragment analogue with bovine and surrogate cord models was recorded with high speed video and the images analyzed to determine deformation trajectories. Each cord specimen was tested in 3 states: with dura and CSF, with dura only, and without dura. The effect of these states on deformation magnitude, duration, and energy loss parameters was assessed. RESULTS.: The estimated spinal cord deformation was significantly reduced, although not eliminated, in the presence of CSF when compared to the bare state. The duration of deformation was generally increased in the presence of CSF, though this difference was not statistically significant. This may indicate a reduction in the cord-fragment interaction force for a given impulse. The dura was found to have no significant effect on deformation parameters for the bovine spinal cord. The deformation of the surrogate cord gave similar trends for the different states in comparison to the bovine cord, but was significantly less than the bovine spinal cord for all conditions.The results indicate that the protective mechanism of CSF may not eliminate cord deformationunder the high energy transverse impact characteristic of a burst fracture. However, CSF may contribute to a lessening of cord deformation and applied force.

STUDY DESIGN An in vitro cadaveric study comparing cage-vertebra interface strengths for 3 different screw-cement configurations. OBJECTIVES To determine the effects of cement augmentation of pedicle screws on cage-vertebra interface failure properties for 2 interbody device shapes (elliptical or cloverleaf); and to compare between pedicle and anterior vertebral body screws with cement augmentation. SUMMARY OF BACKGROUND DATA Pedicle or anterior screw fixation is commonly used with interbody device fixation. Cement has recently been shown to augment screw fixation in the osteoporotic spine by improving the screw-bone interface strength. The effect of cement augmentation of pedicle or anterior screws on cage-vertebra interface properties has not been previously studied or compared. METHODS An elliptical or a cloverleaf-shaped indentor covering 40% of the endplate was axially compressed against the superior endplate of 48 thoracolumbar vertebrae. Each vertebra had polymethylmethacrylate cement augmentation of 1) anterior screws, 2) pedicle screws, or 3) pedicle screws without cement. Compressive load was applied through a mechanism that allowed unconstrained rotation of the indentors. RESULTS Cement augmentation of pedicle screws resulted in significantly higher failure loads (54%) and failure strength (69%) for both shaped indentors when compared with uncemented pedicle screws. There was no significant difference in failure load and failure strength between pedicle and anterior screws with cement augmentation. Indentor shape was not a significant factor on failure load or failure strength. CONCLUSIONS Cage-vertebra interface properties were improved when cement was used to augment vertebral and pedicle screws. Cement augmentation of pedicle or anterior screws may reduce interbody device subsidence.

An in vitro biomechanical study to quantify 3-dimensional kinematics of the lumbar spine following facet arthroplasty.To compare the multidirectional flexibility properties and helical axis of motion of the Total Facet Arthroplasty System (TFAS) (Archus Orthopedics, Redmond, WA) to the intact condition and to posterior pedicle screw fixation.Facet arthroplasty in the lumbar spine is a new concept in the field of spinal surgery. The kinematic behavior of any complete facet arthroplasty device in the lumbar spine has not been reported previously.Flexibility tests were conducted on 13 cadaveric specimens in an intact and injury model, and after stabilization with the TFAS and posterior pedicle screw fixation at the L4-L5 level. A pure moment of +/-10 Nm with a compressive follower preload of 600 N was applied to the specimen in flexion-extension, axial rotation, and lateral bending. Range of motion (ROM), neutral zone, and helical axis of motion were calculated for the L4-L5 segment.ROM with the TFAS was 81% of intact in flexion (P = 0.035), 68% in extension (P = 0.079), 88% in lateral bending (P = 0.042), and 128% in axial rotation (P = 0.013). The only significant change in neutral zone with TFAS compared to the intact was an increase in axial rotation (P = 0.011). The only significant difference in helical axis of motion location or orientation between the TFAS and intact condition was an anterior shift of the helical axis of motion in axial rotation (P = 0.013).The TFAS allowed considerable motion in all directions tested, with ROM being less than the intact in flexion and lateral bending, and greater than the intact in axial rotation. The helical axis of motion with the TFAS was not different from intact in flexion-extension and lateral bending, but it was shifted anteriorly in axial rotation. The kinematics of the TFAS were more similar to the intact spine than were the kinematics of the posterior fixation when applied to a destabilized lumbar spine.

STUDY DESIGN Posterior thoracolumbar spine implants retrieved as part of routine clinical practice over a 2-year period were analyzed to identify wear and corrosion. OBJECTIVE Engineering analyses of retrieved posterior instrumentation for indications of performance and failure and correlation of this information with clinical factors. SUMMARY OF BACKGROUND DATA Recent studies have reported spinal instrumentation particulate wear debris and have noted the importance of design considerations at implant connector interfaces. METHODS A total of 57 implants were analyzed from patients (39 female, 18 male) whose average age at implantation was 43.9 years (range, 13.7-77.4 years). Time of implantation ranged from 2 months to 13.5 years. The top 3 implantation diagnoses were radiculopathy (33%), scoliosis (30%), and back pain (25%). Metallurgical analyses were performed to characterize the wear and/or corrosion, and fractures of the implants. RESULTS Wear was present in 75%, corrosion in 39%, and fractures in 7% of the retrieved implants. Wear and/or corrosion was more prevalent, with respect to the total number of implants retrieved, in implants that had been in service at least 1 year. There was no evidence of corrosion in any of the Ti implants, whereas corrosion was present (with wear) in 58% of the stainless steel (SS) implants. Wear and corrosion were more frequently observed in long rods than in short rods. Implantation times were longer for SS implants than for Ti implants. CONCLUSIONS Retrieved rods exhibited corrosion, wear, and fracture, with wear and corrosion mainly located at the interfaces with hooks, screws, or cross-connectors. The mechanisms causing this material loss in situ, as well as what local or systemic responses it may stimulate are of clinical significance and should be studied further.

STUDY DESIGN An in vitro biomechanical study. OBJECTIVES To develop and evaluate a new in vitro whole cervical spine model that provides to the specimen, in vivo-like mechanical characteristics. SUMMARY OF BACKGROUND DATA In vitro studies of kinematics, kinetics, and trauma using isolated spine specimens (head-T1 vertebra) have usually applied upward force to the head, resulting in tensile spine forces, contrary to the physiological compressive forces present in vivo. Further, the in vitro load-displacement curves have never been compared with the corresponding in vivo data. METHODS A novel muscle force replication (MFR) system is presented. It consists of a set of compressive forces applied to the various vertebrae and occiput of a whole cervical spine specimen. Two protocols, with and without MFR, were evaluated using standardized flexibility testing. Ranges of motion (ROM) and load-displacement curves were documented, and contrasted with similar in vivo data. RESULTS Results for the MFR were found to be similar to the in vivo measurements, with respect to the intersegmental and whole neck motions as well as the load-displacement curves, thus validating the MFR approach. CONCLUSIONS The new model advances the in vitro testing, which uses whole cervical spine specimens.

Load sharing in stabilized spinal segments was evaluated using sequential injury and stabilization with a posterior instrumentation system under an in vitro flexibility protocol.To analyze the partitioning of applied loads between anatomic and implanted structures of lumbar functional spinal units stabilized with a posterior instrumentation system. To identify surgical indications for which the risk of fixator breakage in vivo is high.Relatively few groups have experimentally measured the in vitro and in vivo forces and/or moments supported by posterior instrumentation systems, and no analysis, of the load sharing in these systems has been performed. This information will provide novel insight into implant fatigue life, and the degree to which the spinal anatomy is shielded from the applied load and will allow the verification of mathematical models for new injury scenarios.Specimen kinematics were determined using an optoelectronic tracking system. Intradiscal pressure and the forces and moments supported by the implants were measured using, respectively, a needle-mounted pressure sensor and strain gauges mounted on the spinal implants.A large majority of the applied moments were supported by an equal and opposite force pair between the intervertebral disc and fixator rods in flexion and extension and an equal and opposite force pair between the left and right fixator rods in lateral bending. Torsional moments were shared approximately equally between the posterior elements, intervertebral disc, an equal and opposite shear force pair in the transverse plane between the right and left fixators and internal fixator moments.When posterior instrumentation devices are used to stabilize severe anterior column injuries, they are at risk of fracture secondary to reversed bending moments.