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The goal of this study was to compute the dynamic neck loads during simulated high-speed bilateral facet dislocation and investigate the injury mechanism.Ten osteoligamentous functional spinal units (C3/4, n = 4; C5/6, n = 3; C7/T1, n = 3) were prepared with muscle force replication, motion tracking flags, and a 3.3-kg mass rigidly attached to the upper vertebra. Frontal impacts of increasing severity were applied to the lower vertebra until dislocation was achieved. Inverse dynamics was used to calculate the dynamic neck loads during dislocation. Average peak impact acceleration required to cause dislocation ranged between 7.6 and 11.6 g. This resulted in dynamic neck loads applied at average peak rates of 906 Nm/s for flexion moment, 8017 N/ for anterior shear, and 8100 N/s for axial compression. To determine the temporal event patterns, the average occurrence times of the load and motion peaks were statistically compared (P0.05).Among average peak loads, axial compression of 233.6 N was first to occur followed by anterior shear force of 73.1 N and flexion moment of 30.7 Nm. Among average peak motions, axial separation of 5.3 mm was first to occur followed by flexion rotation of 63.1 degrees and anterior shear of 21.5 mm. Subsequently, average peak posterior shear force of 110.3 N was observed as the upper facet became locked in the intervertebral foramina. Average peak axial compression of 6.6 mm occurred significantly later than all preceding events.During bilateral facet dislocation, the main loads included flexion moment and forces of axial compression and anterior shear. These loads caused flexion rotation, facet separation, and anterior translation of the upper facet relative to the lower. The present data help elucidate the injury mechanism of cervical facet dislocation.

Thoracolumbar burst fracture is one of the most common and most studied injuries of the spine. Radiography and quantitative discomanometry have previously demonstrated that intervertebral discs adjacent to a burst fracture were disrupted; however, there have been conflicting data on the properties of the next-adjacent discs. Also, no data exists on the localization of injury within each disc. The objective of this study is to use an in vitro biomechanical design to examine the flexibility differences of intervertebral discs adjacent and next-adjacent to burst fracture vertebrae. Ten cadaveric thoracolumbar (T11–L3) spines with L1 burst fracture included adjacent (T12–L1, L1–L2) and next-adjacent (T11–T12, L2–L3) intervertebral discs. The bending flexibilities (μm/N) of each disc under 50 N of compression were determined at 25 points: the center point, along with the combinations of three radii (1 cm, 2 cm, 3 cm) and eight angles. The overall flexibility of each disc and the presence of any regional differences were then evaluated. Across all radii, the T11μT12 disc was statistically less flexible than the other three discs (p < 0.01). The difference between the flexibilities of the average anterior region and the average posterior region was significant at many 2-cm and all 3-cm radii; this difference was greater in the T12–L1 disc than in any of the other three discs. Thus, the upper next-adjacent intervertebral disc (T11–T12) was not as susceptible to mechanical disruption as were the other three discs. The anterior region of each disc may also have a higher propensity for mechanical disruption than the posterior region, especially at larger radii.

The goal of the present study was to determine the dynamic sagittal flexibility coefficients, including coupling coefficients, throughout the human cervical spine using rear impacts. A biofidelic whole cervical spine model (n=6) with muscle force replication and surrogate head was rear impacted at 5 g peak horizontal accelerations of the T1 vertebra within a bench-top mini-sled. The dynamic main and coupling sagittal flexibility coefficients were calculated at each spinal level, head/C1 to C7/T1. The average flexibility coefficients were statistically compared (p

STUDY DESIGN An in vitro human cadaveric biomechanical study. OBJECTIVES To quantify effects on operated and other levels, including adjacent levels, due to CHARITE disc implantations versus simulated fusions, using follower load and the new hybrid test method in flexion-extension and bilateral torsion. SUMMARY OF BACKGROUND DATA Spinal fusion has been associated with long-term accelerated degeneration at adjacent levels. As opposed to the fusion, artificial discs are designed to preserve motion and diminish the adjacent-level effects. METHODS Five fresh human cadaveric lumbar specimens (T12-S1) underwent multidirectional testing in flexion-extension and bilateral torsion with 400 N follower load. Intact specimen total ranges of motion were determined with +/-10 Nm unconstrained pure moments. The intact range of motion was used as input for the hybrid tests of 5 constructs: 1) CHARITE disc at L5-S1; 2) fusion at L5-S1; 3) CHARITE discs at L4-L5 and L5-S1; 4) CHARITE disc at L4-L5 and fusion at L5-S1; and 5) 2-level fusion at L4-L5-S1. Using repeated-measures single factor analysis of variance and Bonferroni statistical tests (P < 0.05), intervertebral motion redistribution of each construct was compared with the intact. RESULTS In flexion-extension, 1-level CHARITE disc preserved motion at the operated and other levels, while 2-level CHARITE showed some amount of other-level effects. In contrast, 1- and 2-level fusions increased other-level motions (average, 21.0% and 61.9%, respectively). In torsion, both 1- and 2-level discs preserved motions at all levels. The 2-level simulated fusion increased motions at proximal levels (22.9%), while the 1-level fusion produced no significant changes. CONCLUSIONS In general, CHARITE discs preserved operated- and other-level motions. Fusion simulations affected motion redistribution at other levels, including adjacent levels.

Elongation-induced vertebral artery (VA) injury has been hypothesized to occur during nonphysiological coupled head motions during automobile impacts. Although previous work has investigated VA elongation during head-turned and head-forward rear impacts, no studies have performed similar investigations for frontal or side impacts.The present study quantified dynamic VA elongations during simulated frontal and side automotive collisions, and compared these data with corresponding physiological limits.In vitro biomechanical study of dynamic VA elongation during simulated impacts.A biofidelic whole cervical spine model with muscle force replication and surrogate head underwent simulated frontal impacts (n=6) of 4, 6, 8, and 10 g or left side impacts (n=6) of 3.5, 5, 6.5, and 8 g.Average (SD) maximum physiological VA elongation was 7.1 (3.2) mm, measured during intact flexibility testing. Average peak dynamic elongation of right VA during left side impact, up to 17.4 (2.6) mm, was significantly greater (p.05) than physiological beginning at 6.5 g, whereas the highest average peak VA elongation during frontal impact was 2.5 (2.4) mm, which did not exceed the physiological limit. Side impact, as compared with frontal impact, caused earlier occurrence of average peak VA elongation, 113.8 (13.5) ms versus 155.0 (46.2) ms, and higher average peak VA elongation rate, 608.8 (99.0) mm/s versus 130.0 (62.9) mm/s.Elongation-induced VA injury is more likely to occur during side impact as compared with frontal impact.

Background Traditionally, spinal degeneration and injury have been associated with abnormal intervertebral motion; thus, treatment for lowback pain has centered on prevention of motion through spinal fusion. Although the rate of successful spinal fusions is improving, complications such as adjacent-level syndrome emphasize the need to develop alternatives for treating spinal degeneration. In an effort to improve the clinical outcomes associated with such treatment, we hypothesized that spinal stabilization and a consequent reduction in symptoms is achievable without the harsh restrictions to spinal motion imposed by fusion. This idea was based on the principle of the neutral zone and the neutral zone hypothesis of back pain. Development Performance requirements for a novel device were determined through a series of biomechanical experiments. From these data, the Stabilimax NZ was developed to provide stabilization to a degenerated or surgically destabilized spine while maintaining the maximum possible total range of motion. Applied Spine Technologies Inc has tested 70 bilateral assemblies of the final design of the Stabilimax NZ, and all exceeded the biomechanical, static, fatigue, wear, and histological requirements necessary to initiate clinical investigation. Discussion The Stabilimax NZ device has been systematically designed and tested under protocols developed by Applied Spine Technologies in conjunction with Panjabi, Patwardhan, and Goel. The device decreased the neutral zone in destabilized spines while maintaining substantial range of motion. Clinical Relevance Development testing has been submitted to the US Food and Drug Administration and permission obtained to initiate an investigational device exemption trial to clinically investigate the efficacy of the Stabilimax NZ device.

To quantify the dynamic loads and intervertebral motions throughout the cervical spine during simulated rear impacts.Using a biofidelic whole cervical spine model with muscle force replication and surrogate head and bench-top mini-sled, impacts were simulated at 3.5, 5, 6.5, and 8 g horizontal accelerations of the T1 vertebra. Inverse dynamics was used to calculate the dynamic cervical spine loads at the centers of mass of the head and vertebrae (C1-T1). The average peak loads and intervertebral motions were statistically compared (P0.05) throughout the cervical spine.Load and motion peaks generally increased with increasing impact acceleration. The average extension moment peaks at the lower cervical spine, reaching 40.7 Nm at C7-T1, significantly exceeded the moment peaks at the upper and middle cervical spine. The highest average axial tension peak of 276.9 N was observed at the head, significantly greater than at C4 through T1. The average axial compression peaks, reaching 223.2 N at C5, were significantly greater at C4 through T1, as compared to head-C1. The highest average posterior shear force peak of 269.5 N was observed at T1.During whiplash, the cervical spine is subjected to not only bending moments, but also axial and shear forces. These combined loads caused both intervertebral rotations and translations.

STUDY DESIGN Determination of alar, transverse, and apical ligament strains during simulated head-turned rear impact. OBJECTIVES To quantify the alar, transverse, and apical ligament strains during head-turned rear impacts of increasing severity, to compare peak strains with baseline values, and to investigate injury mechanisms. SUMMARY OF BACKGROUND DATA Clinical and epidemiologic studies have documented upper cervical spine ligament injury due to severe whiplash trauma. There are no previous biomechanical studies investigating injury mechanisms during head-turned rear impacts. METHODS Whole cervical spine specimens (C0-T1) with surrogate head and muscle force replication were used to simulate head-turned rear impacts of 3.5, 5, 6.5, and 8 g horizontal accelerations of the T1 vertebra. The peak ligament strains during impact were compared (P < 0.05) to baseline values, obtained during a noninjurious 2 g acceleration. RESULTS The highest right and left alar ligament average peak strains were 41.1% and 40.8%, respectively. The highest transverse and apical ligament average strain peaks were 17% and 21.3%, respectively. There were no significant increases in the average peak ligament strains at any impact acceleration compared with baseline. CONCLUSIONS The alar, transverse, and apical ligaments are not at risk for injury due to head-turned rear impacts up to 8 g. The upper cervical spine symptomatology reported by whiplash patients may, therefore, be explained by other factors, including severe whiplash trauma in excess of 8 g peak acceleration and/or other impact types, e.g., offset, rollover, and multiple collisions.

Previous methods to determine spinal ligament deformation have included either custom-designed transducers or computational methods using rigid body transformation of kinematic data. Goals of the present study were to describe a computational methodology to determine dynamic deformations of an arbitrarily oriented ligament in a spine specimen and its associated errors.Calculation of ligament deformation in a spinal segment with vertebral motion tracking flags utilized digital stereophotography, lateral neutral posture radiograph, and detailed quantitative anatomy to develop geometrical relationships between flag markers and ligament attachment points. A custom jig, consisting of two flags each with four markers, was constructed to quantify errors associated with computed ligament deformation, flag marker translation, and flag rotation.Average error in ligament deformation was dependent upon motion direction and ranged between 0.03 mm (SD 0.45 mm) and 0.28 mm (SD 0.18 mm). Average error for flag marker translation ranged between 0.02 mm (SD 0.14 mm) and 0.11 mm (SD 0.39 mm), and for flag rotation ranged between -0.06 degrees (SD 0.17 degrees ) and 0.07 degrees (SD 0.12 degrees ).Accuracy of the present technique was equivalent to or greater than that of previous methods. The present technique utilized relatively cost-effective digital stereophotography, and may be used to calculate strain in ligaments not readily accessible for transducer application. The methodology has wide-spread applicability for analyses of dynamic or static spinal or other ligament strains, and may be used to determine spinal canal and intervertebral foramen narrowing and area reduction.

Elongation-induced vertebral artery injury has been hypothesized to occur during non-physiological coupled axial rotation and extension of head. No studies have quantified dynamic vertebral artery elongation during head-turned rear impacts. Therefore, we evaluated effect of rotated head posture vs. forward head posture at the time of impact on dynamic vertebral artery elongation during simulated rear impacts.A whole cervical spine model with surrogate head and muscle force replication underwent either simulated head-turned (n = 6) or head-forward (n = 6) rear impacts of 3.5, 5, 6.5 and 8 g. Continuous dynamic vertebral artery elongation was recorded using custom transducer and compared to physiological values obtained during intact flexibility testing.Average (SD) peak dynamic vertebral artery elongation of up to 30.5 (2.6) mm during head-turned rear-impact significantly exceeded (P0.05) the physiological beginning at 5 g. Highest peak elongation of 5.8 (2.1) mm during head-forward rear impact did not exceed physiological limit. Head-turned rear impact caused earlier occurrence of average peak vertebral artery elongation, 84.5 (4.2) ms vs. 161.0 (43.8) ms, and higher average peak vertebral artery elongation rate, 1336.7 (74.5) mm/s vs. 211.5 (97.4) mm/s, as compared to head-forward rear impact.Elongation-induced vertebral artery injury is more likely to occur in those with rotated head posture at the time of rear impact, as compared to head-forward.

To assess the biomechanical stability relative to screw length and K-wire augmentation in scaphoid fracture fixation using a flexibility testing protocol and cadaver scaphoids whose soft tissue attachments remained undisturbed. Our hypothesis was 2-fold: increasing screw length and augmenting fixation with a K-wire would improve fracture fragment stability, individually and in combination.Flexion and extension loading applied through wrist tendons was performed on 10 cadaveric wrists after volar wedge scaphoid osteotomy and internal fixation. Each wrist participated in 3 experimental groups: short screw, long screw, and long screw augmented with a K-wire transfixing the distal pole to the capitate. Interfragmentary displacements were measured.Analysis of variance showed significantly less fracture fragment motion with longer screws than with short screws in 4 of the 6 displacement axes. The flexion/extension axis rotations for the short, long, and augmented long-screw groups were 8.2 degrees +/- 4.8 degrees, 3.9 degrees +/- 1.6 degrees, and 1.8 degrees +/- 1.3 degrees, respectively. Although K-wire augmentation reduced displacement of the fracture fragments it did not decrease interfragmentary motion significantly when compared with the long-screw group.Under physiologically applied loading of cadaveric wrists with unstable scaphoid waist fractures the long screw provided significantly greater stability than the short screw. Although K-wire augmentation in the long-screw group did improve stability the improvements were not significant. Based in part on the biomechanical data from this study it is our recommendation that the optimally placed screw for scaphoid fracture fixation stability is a long screw positioned down the central axis of the scaphoid deep into subchondral bone.

Head-turned whole cervical spine model was stabilized with muscle force replication and subjected to simulated rear impacts of increasing severity. Multiplanar flexibility testing evaluated any resulting injury.To identify and quantify cervical spine soft tissue injury and injury threshold acceleration for head-turned rear impact, and to compare these data with previously published head-forward rear and frontal impact results.Epidemiologically and clinically, head-turned rear impact is associated with increased injury severity and symptom duration, as compared to forward facing. To our knowledge, no biomechanical data exist to explain this finding.Six human cervical spine specimens (C0-T1) with head-turned and muscle force replication were rear impacted at 3.5, 5, 6.5, and 8 g, and flexibility tests were performed before and after each impact. Soft tissue injury was defined as a significant increase (P0.05) in intervertebral flexibility above baseline. Injury threshold was the lowest T1 horizontal peak acceleration that caused the injury.The injury threshold acceleration was 5 g with injury occurring in extension or axial rotation at C3-C4 through C7-T1, excluding C6-C7. Following 8 g, 3-plane injury occurred in extension and axial rotation at C5-C6, while 2-plane injury occurred at C7-T1.Head-turned rear impact caused significantly greater injury at C0-C1 and C5-C6, as compared to head-forward rear and frontal impacts, and resulted in multiplanar injuries at C5-C6 and C7-T1.

Study design Finite element model of L3-S1 segment and confirmatory cadaveric testing were used to investigate the biomechanical effects of a mobile core type artificial disc (Charite artificial disc; DePuy Spine, Raynham, MA) on the lumbar spine. Objective To determine the effects of the Charite artificial disc across the implanted and adjacent segments. Summary of background data Biomechanical studies of artificial discs that quantify parameters, like the load sharing and stresses, are sparse in the literature, especially for mobile-type core artificial disc designs. In addition, there is no standard protocol for studying the adjacent segmental effects of such implants. Methods Human osteo-ligamentous spines (L1-S1) were tested before and after L5-S1 Charite artificial disc placement. The data were used to validate further an intact 3-dimensional (3-D) nonlinear L3-S1 finite element model. The model was subjected to 400-N axial compression and 10.6 Nm of flexion/extension pure moments (load control) or pure moments that produced the overall rotation of the L3-S1 Charite model equal to the intact case (hybrid approach). Resultant motion, load, and stress parameters were analyzed at the experimental and adjacent levels. Results Finite element model validation was achieved only with the load-controlled experiments. The hybrid approach, believed to be more clinically relevant, revealed that Charite artificial disc leads to motion increases in flexion (19%) and extension (44%) at the L5-S1 level. At the instrumented level, the decrease in the facet loads was less than at the adjacent levels; the corresponding decrease being 26% at L3-L4, 25% at L4-L5, and 13.4% at L5-S1 when compared to the intact. Intradiscal pressure changes in the L4-L5 and L3-L4 segments were minimal. Shear stresses at the Charite artificial disc-L5 endplate interface were higher than those at S1 interface. However, in the load control mode, the increase in facet loads in extension was approximately 14%, as compared to the intact case. Conclusions The hybrid testing protocol is advocated because it better reproduces clinical observations in terms of motion following surgery, using pure moments. Using this approach, we found that the Charite artificial disc placement slightly increases motion at the implanted level, with a resultant increase in facet loading when compared to the adjacent segments, while the motions and loads decrease at the adjacent levels. However, in the load control mode that we believe is not that clinically relevant, there was a large increase in motion and a corresponding increase in facet loads, as compared to the intact.

Intervertebral Neck Injury Criterion (IV-NIC) is based on the hypothesis that dynamic three-dimensional intervertebral motion beyond physiological limits may cause multiplanar injury of cervical spine soft tissues. Goals of this study, using a biofidelic whole human cervical spine model with muscle force replication and surrogate head in simulated side impacts, were to correlate IV-NIC with multiplanar injury and determine IV-NIC injury threshold for each intervertebral level.Using a bench-top apparatus, side impacts were simulated at 3.5, 5, 6.5, and 8 g horizontal accelerations of the T1 vertebra. Pre- and post-impact flexibility testing in three-motion planes measured the soft tissue injury, i.e., significant increase (p0.05) in neutral zone (NZ) or range of motion (RoM) at any intervertebral level, above corresponding physiological limit.IV-NIC in left lateral bending correlated well with total lateral bending RoM (R = 0.61, P0.001) and NZ (R = 0.55, P0.001). Additionally, the same IV-NIC correlated well with left axial rotation RoM (R = 0.50, P0.001). IV-NIC injury thresholds (95% confidence limits) varied among intervertebral levels and ranged between 1.5 (0.6-2.4) at C3-C4 and 4.0 (2.4-5.7) at C7-T1. IV-NIC injury threshold times were attained beginning at 84.5 ms following impact.Present results suggest that IV-NIC is an effective tool for determining multiplanar soft tissue neck injuries by identifying the intervertebral level, mode, time, and severity of injury.

Background. Radiofrequency treatment is increasingly used to treat shoulder instability. Patients are asked to restrain their physical activities after this treatment but there is no precise information concerning the necessity for the restrain. Methods. There were two groups of ten specimens each. Treatment group specimens were stretched to subfailure injury, treated by a radiofrequency probe, and then cyclically loaded. Control group specimens were stretched to the same subfailure injury, Sham treated, and then cyclically loaded. Between each step of the experiment in both groups there was a relaxation test to examine the ligament viscoelastic properties. At the end, each ligament was stretched to failure and the load–elongation curve obtained. Findings. Relaxation forces decreased after the subfailure injury in both groups (average 76% and 81% of intact state, in treatment and control groups, respectively). In the treatment group, the relaxation forces first increased after the radiofrequency treatment (average 99% of intact state), and then decreased after the cyclic loading (average 50% of intact state). The treated ligaments failed at lower loads and smaller deformations than the controls. Interpretation. The radiofrequency treatment restored viscoelastic properties of the injured ligaments, but cyclic loading degraded these. Protection of the treated ligament is advised during the immediate post-operative period. 2004 Published by Elsevier Ltd.

Although spine fusion is a versatile and effective technique in the treatment of spinal disorders, increased stresses on adjacent unfused levels lead to symptomatic adjacent level degeneration in many patients. The goal of nonfusion devices in spine surgery is to ablate or unload painful structures while preserving segmental motion. The intended performance of nonfusion devices such as disc replacement, nucleus pulposus replacement, and posterior stabilization devices can be understood from the biomechanics of the functional spinal unit in health and disease and the interplay between the motion segment and the device. Implant design issues can also markedly affect performance.

Automotive collision simulations have been performed using either incremental or single trauma. In single trauma, a single impact is performed, while in incremental trauma, a series of impacts of increasing severity are executed. Equivalency of incremental and single trauma for soft tissue injury severity due to the final impact has not been established. Thus, the purpose of the present study was to investigate whether incremental and single trauma produced similar cervical spine subfailure injury severity due to simulated frontal impacts.Porcine cervical spine specimens (C2-T1) of the incremental trauma group were subjected to five frontal impacts (2, 3.5, 5, 6.5, 8 g), while single trauma specimens were subjected to a single impact (8 g). Flexibility tests were performed on specimens while intact and following each impact. Intact and post 8 g flexibility parameters were compared within incremental and single trauma groups and between groups.No significant differences (P0.05) were found between incremental and single trauma groups when either intact or post 8 g flexibility parameters were compared. Significant increases in flexibility parameters from intact to post 8 g were observed in both groups, indicating soft tissue injury.Incremental and single trauma produced equivalent subfailure cervical spine injury in simulated impacts, for the experimental conditions studied. This study may facilitate greater use of the incremental trauma protocol in future experimental designs.

STUDY DESIGN Facet joint kinematics and capsular ligament strains were evaluated during simulated whiplash of whole cervical spine specimens with muscle force replication. OBJECTIVES To describe facet joint kinematics, including facet joint compression and facet joint sliding, and quantify peak capsular ligament strain during simulated whiplash. SUMMARY OF BACKGROUND DATA Clinical studies have implicated the facet joint as a source of chronic neck pain in whiplash patients. Prior in vivo and in vitro biomechanical studies have evaluated facet joint compression and excessive capsular ligament strain as potential injury mechanisms. No study has comprehensively evaluated facet joint compression, facet joint sliding, and capsular ligament strain at all cervical levels during multiple whiplash simulation accelerations. METHODS The whole cervical spine specimens with muscle force replication model and a bench-top trauma sled were used in an incremental trauma protocol to simulate whiplash of increasing severity. Peak facet joint compression (displacement of the upper facet surface towards the lower facet surface), facet joint sliding (displacement of the upper facet surface along the lower facet surface), and capsular ligament strains were calculated and compared to the physiologic limits determined during intact flexibility testing. RESULTS Peak facet joint compression was greatest at C4-C5, reaching a maximum of 2.6 mm during the 5 g simulation. Increases over physiologic limits (P < 0.05) were initially observed during the 3.5 g simulation. In general, peak facet joint sliding and capsular ligament strains were largest in the lower cervical spine and increased with impact acceleration. Capsular ligament strain reached a maximum of 39.9% at C6-C7 during the 8 g simulation. CONCLUSIONS Facet joint components may be at risk for injury due to facet joint compression during rear-impact accelerations of 3.5 g and above. Capsular ligaments are at risk for injury at higher accelerations.

Clinical instability is an important cause of low back pain. Although there is some controversy concerning its definition, it is most widely believed that the loss of normal pattern of spinal motion causes pain and/or neurologic dysfunction. The stabilizing system of the spine may be divided into three subsystems: (1) the spinal column; (2) the spinal muscles; and (3) the neural control unit. A large number of biomechanical studies of the spinal column have provided insight into the role of the various components of the spinal column in providing spinal stability. The neutral zone was found to be a more sensitive parameter than the range of motion in documenting the effects of mechanical destabilization of the spine caused by injury and restabilization of the spine by osteophyle formation, fusion or muscle stabilization. Clinical studies indicate that the application of an external fixator to the painful segment of the spine can significantly reduce the pain. Results of an in vitro simulation of the study found that it was most probably the decrease in the neutral zone, which was responsible for pain reduction. A hypothesis relating the neutral zone to pain has been presented. The spinal muscles provide significant stability to the spine as shown by both in vitro experiments and mathematical models. Concerning the role of neuromuscular control system, increased body sway has been found in patients with low back pain, indicating a less efficient muscle control system with decreased ability to provide the needed spinal stability.  2003 Elsevier Science Ltd. All rights reserved.

STUDY DESIGN: This study biomechanically evaluated three fixation devices for stability with posterior two- and three-column injuries. OBJECTIVES: To find an effective means of posteriorly stabilizing injuries at the cervicothoracic junction. SUMMARY OF BACKGROUND DATA: The cervicothoracic spine is complex anatomically and has been a difficult challenge in approach and stabilization of traumatic and degenerative disorders. METHODS: Twenty-one human cadaveric spines (C3-T3) were loaded in flexion, extension, lateral bending, and axial torsion. A posterior two-column injury was created at C7-T1. One of three posterior fixation systems was applied (two rod-screw systems, one plate-screw system, all with screws at C5, C6 and T1, T2). The spines were tested again. A three-column injury was created by transecting the remaining anterior structures; the spines were tested a final time. RESULTS: In flexion-extension, there were no significant differences in stiffness between intact and instrumented two-column injury specimens for all systems; the instrumented three-column injury was significantly (P < 0.05) less stiff than intact specimens in extension. Ranges of motion and neutral zones decreased from intact to instrumented two-column injuries and increased from intact to three-column constructs. In lateral bending and axial rotation, all systems were stiffer than intact spines for both injuries; ranges of motion and neutral zones were reduced for both injuries compared with intact specimens. CONCLUSION: All three systems stabilize the cervicothoracic junction with a posterior two-column injury in flexion-extension, lateral bending, and axial rotation; none was adequate for a three-column injury, particularly in extension. A three-column injury at this level would warrant supplemental anterior fixation.

Am menschlichen Wirbelsaulenmodell wurde die multidirektionale Stabilitat kurzstreckiger ventraler, dorsaler und kombinierter Spondylodesen (L1 bis L3) nach Korpektomie L2 und Wirbelkorperersatz mit einem Karbonfaserimplantat untersucht. 8 menschliche Wirbelsaulenpraparate (T12 bis L4) wurden fur die biomechanische Testung aufgearbeitet. Reine Momente (2,5; 5, und 7,5 Nm) in Flexion/Extension, in axialer Rotation und in Seitneigung wurden in einer Flexibilitatsmaschine auf die Wirbelsaulenpraparate aufgetragen. Die Relativbewegungen der einzelnen Wirbelkorper wurden mit dem OPTOTRAC Bewegungsmesssystem aufgezeichnet. Eine ventrale, 2 dorsale Pedikelschraubensysteme und 2 kombinierte Stabilisierungen wurden getestet. Moment-Weg-Diagramme, Bewegungsumfang (ROM) und neutrale Zone (NZ) wurden ausgewertet. Wahrend die dorsale und die kombinierte Instrumentierung in allen Bewegungsebenen eine sehr hohe Stabilitat zeigten, erwies sich die ventrale Stabilisierung Instrumentierung unter Berucksichtigung der eingesetzten Implantate als unbefriedigend.

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. An in vitro biomechanical study of the stabilizing effect of pedicle screw instrumentation on experimental thoracolumbar burst fractures. Objectives. To evaluate the effects of different adjustments applied by the pedicle screw fixation device on the stability of the spine–device construct. Summary of Background Data. Pedicle screw devices are widely used to accomplish spinal reduction and provide stability to an injured spine. In previous biomechanical studies the stability of the spine–device constructs has been examined for many devices. However, no study has quantitatively assessed the associations between the device adjustments and the stability of the construct. Methods. Five-vertebrae human cadaveric specimens with burst fracture at L1 vertebra were studied. Pedicle screw fixation device was attached to the T12 and L2 vertebrae. Five device adjustments (pure compression, pure distraction, pure extension, a combination distraction–extension, and neutral posture) were studied. Multidirectional flexibility test was performed when intact, after burst fracture, and after each device adjustment to document spinal stability. Results. The construct stability had a complex association to the device adjustment. For example, the maximum flexion and extension stabilities were achieved by pure compression and distraction–extension combination adjustments, respectively. Pure distraction and pure extension adjustments decreased the construct stability. Conclusions. The device adjustments affected the spinal construct stability differently in different directions. Although pure compression provided the most stability in most directions, the combined distraction–extension adjustment may be more suitable considering the neural decompression also.

The short segmental pedicle screw device is widely used for the decompression of neural elements and reduction of normal anatomy. Many biomechanical studies concerning proper decompression are available. However, no study has determined the optimal device adjustment for reduction of the burst fracture to the normal anatomy. In this study, cadaveric thoracolumbar spine specimens (T11-L3) with L1 burst fractures were studied. A pedicle screw device was attached to the pedicles of the T12 and L2 vertebrae. Spinal postural changes were determined due to a set of eight clinically relevant adjustments of the device. The adjustments were combinations of axial translation (distraction/compression) and extension. The adjustments caused varying changes in spinal posture. The sequence of applying the translation and extension had no effect on the spinal posture changes. The adjustment combining 5 mm distraction with 6 degrees extension brought the burst fracture closest to the intact state, compared to all other adjustments. With this adjustment, on average the spine became 0.9 mm compressed and 2.0 degrees lordotic, compared to the intact. The results of the study show that the device adjustments of axial translation and sagittal angulation can be applied in any sequence, with the same results. The combination of 5 mm distraction with 6 degrees extension was the device adjustment that produced the closest anatomical reduction.

Objective. To study alterations in the ligament mechanical characteristics due to a subfailure stretch delivered at high speed. Design. An in vitro study of rabbit anterior cruciate ligaments. Background. Although ligamentous sprains occur more frequently than the complete failures, only a few biomechanical studies have investigated the effects of such injuries. Purpose of the study was to document changes in elastic, failure and viscoelastic properties of a ligament that had been subjected to a high-speed subfailure stretch. Methods. Thirteen paired fresh adult rabbit femur-anterior cruciate ligament-tibia preparations were used. One ligament of each pair (control) was subjected to two relaxation tests and then stretched until failure. The other ligament (experimental) was subjected sequentially to relaxation test, subfailure stretch (80% of the failure deformation of the control), relaxation test, and then stretched until failure. Load–deformation curve until failure was characterized by nine parameters, which included failure force, deformation and energy absorbed, and deformations measured at various load values, and stiffness. Relaxation curve was parameterized by five relaxation forces measured at 10, 30, 50, 130, and 180 s. Results. Due to subfailure stretch, there were no changes in failure force and stiffness, while deformations increased, and energy absorbed and relaxation forces decreased. Conclusions. The 80% subfailure stretch delivered at high speed increased the deformations in load–deformation tests, and decreased the forces in relaxation tests. Relevance Incomplete injuries of the ligaments are more prevalent than the complete injuries. The incomplete injury is a subfailure stretch delivered at high speed. The present study provides the results of simulating the incomplete injury using an in vitro ligament model.

Study design Biomechanics of posterolateral spinal fusion were studied in an in vivo rabbit model. Objectives To determine the extent of stabilization produced by posterolateral lumbar fusion and to test the hypothesis that motions are not completely eliminated after successful fusion. Summary of background data Previous human cadaveric studies, clinical studies, and animal studies have attempted to characterize the biomechanics of posterolateral fusion. Such studies have been limited by either methods of fusion modeling or methods of stability testing. No previous study has examined biologic fusion with a physiologic biomechanical testing technique. Methods Ten adult New Zealand white rabbits underwent L5-L6 intertransverse process fusion using autogenous iliac crest bone graft. Rabbits were killed 5 weeks after surgery. Only one time point was studied. This time point was chosen because previous pull-apart studies have shown plateauing of rabbit fusion mass strength and stiffness around this time. Spines were then harvested and evaluated with manual palpation and an established flexibility testing protocol. Resulting data were compared with previously acquired, nonoperative spine flexibility data. Results Two animals were excluded because of complications. Of those that were fused (n = 5), biomechanical testing revealed significant decreases in flexion (81%), extension (61%), and right and left lateral bending (67% and 83%, respectively) (P Conclusions These findings define the amount of motion reduction that can be expected with posterolateral fusions in the rabbit model at 5 weeks. These results suggest that motion was significantly decreased but was not eliminated.

Question: What should the radiographic criteria be for a successful arthrodesis for lumbar interbody fusion cages? The definition of successful arthrodesis following anterior lumbar fusion is controversial. The comparison of different surgical arthrodesis techniques, interbody prostheses, and bone g

Study Design. An established rabbit intertransverse process lumbar fusion model was used to evaluate osteo genic protein (OP)-1 as a potential graft substitute. Objectives. To determine whether OP-1 is effective in producing intertransverse process lumbar fusion in a rab bit model. of Background Data. Autogenous iliac crest bone is the gold standard in grafting material for inducing intertransverse process fusion. However, bone graft sub stitutes are being considered as supplementary or alter native means to achieve such fusion with less morbidity Relatively little research has been undertaken to investi gate the efficacy of OP-1 in this role. Methods, Single-level intertransverse process lumba fusions were performed at L5-L6 of 31 New Zealand White rabbits. These were divided into three study groups: autograft, carrier alone, and carrier with OP-1 The animals were killed 5 weeks after surgery. Resultan fusion masses were evaluated by manual palpation, radi ography, biomechanical multidirectional flexibility test ing, and histology. Results. Seven rabbits (23%) were excluded because of complications. Of the remaining 24 rabbits, 5 (63%) o the 8 in the autograft group had fusion detected by man ual palpation, none (0%) of the 8 in the carrier-alone group had fusion, and all 8 (100%) in the OP-1 group had fusion. Radiographs were 55% sensitive and 92% specific for determining fusion. Biomechanical testing results cor related well with those of manual palpation. Histologi cally, autograft specimens were predominantly fibrocar tilage, OP-1 specimens were predominantly maturing bone, and carrier-alone specimens did not show signifi cant bone formation. Conclusions. OP-1 was found to reliably induce solic intertransverse process fusion in a rabbit model at 5 weeks.

Study Design. An in vitro biomechanical study of lumbar spine segments. Objective. To study the characteristics of the stiffness test protocol. Summary of Background Data. In an in vitro study using a flexibility protocol, forces are applied and motions are measured; no center of rotation needs to be specified. In a study using a stiffness protocol, the forces are measured and the motions are applied. This does require the center of rotation to be specified. Many biomechanical studies of the spine are available, but there is lack of clarity concerning which of these two test protocols is appropriate to achieve a certain study goal. Methods. Five-vertebrae lumbar spine specimens with burst fractures in the middle vertebrae (L1) were used. Specially designed apparatus applied flexion and extension rotations around five centers of rotations located on anteroposterior line through the middle of L1. Maximum moment of 4 Nm was applied. Results. The authors found load-displacement curves, ranges of motion, and neutral zones obtained at the five centers of rotations to be markedly different. The center of rotation located at the posterior longitudinal ligament produced large range of motion and neutral zones in comparison to the centers of rotation located at the anterior longitudinal ligament and the spinous process tip (P < 0.01). Conclusions. The stiffness protocol requires that a center of rotation be specified. Shown here is the significant variability in the load-displacement curves, depending on the choice of the location of the center of rotation. Certain center of rotation locations may block the natural motions of the spine and may result in tissue damage.

It is important to know the condition of neural spaces during the nonoperative treatment of thoracolumbar burst fractures. The goals of the current study were to identify the correlation between the degree of deformity of a fractured vertebra and the encroachment of neural spaces, and to determine how the encroachment and the deformity can be improved by the extension posture simulating the postural reduction. Experimental burst fractures were produced in L1 vertebrae of nine human thoracolumbar spine segments (T11-L3) with neural spaces lined with tiny steel balls. Lateral radiographs were taken in neutral and extended posture before and after the trauma. Anterior vertebral height, posterior vertebral height, vertebral height ratio, vertebral kyphotic angle, posterior vertebral body angle, and the cross diagonal angle were the geometric parameters used to describe the vertebral deformity. The canal diameter and superior and inferior intervertebral foramen areas were defined as the neural spaces. All parameters were measured on the scanned images of radiographs, as seen on the computer screen. Among the vertebral body parameters, the posterior vertebral height, posterior vertebral body angle, and cross diagonal angle showed significantly higher correlations with the canal encroachment. The extended posture did not improve the canal and intervertebral foramen encroachments. The kyphotic deformity (vertebral kyphotic angle and anterior vertebral height) was improved but the deformity of the vertebral posterior wall (posterior vertebral height and posterior vertebral body angle) was not improved because of the extended posture.

Study Design. This human cadaveric study evaluated disc degeneration of the lumbar spine using magnetic resonance imaging and quantitative discomanometry. Objective. To determine if a correlation exists between magnetic resonance imaging and quantitative discomanometry in determining disc degeneration of the lumbar spine. Summary of Background Data. Several studies analyzing disc degeneration of the lumbar spine have compared magnetic resonance imaging with discography and discomanometry. The reported results are conflicting. No studies exist that compare magnetic resonance imaging and quantitative discomanometry in assessing the disc degeneration of the lumbar spine. Methods. Three fresh human cadaveric thoracolumbar spine specimens (two T11-S1 and one L1-S1) that included a total of 19 discs were used. Spines were scanned with magnetic resonance imaging, and the scans were read by a neuroradiologist. Using the quantitative discomanometry technique, discs were injected with normal saline, and pressure-volume curves were collected and quantified with six parameters: intrinsic pressure, leakage pressure, initial slope, slope from 0.0 to 0.1 mL, maximum pressure, and volume at maximum pressure. Data analysis was performed using Spearman's Rank Correlation (Rho) statistic. Results. Based on the results from 19 discs, an overall good correlation between magnetic resonance imaging scores and the six quantitative discomanometry parameters was demonstrated. With exception of the volume at maximum pressure, correlation coefficients ranged between 0.61 to 0.78 with a P < 0.05. Conclusions. Magnetic resonance imaging scores and quantitative discomanometry parameters correlated well in the assessment of disc degeneration of the lumbar spine. Quantitative discomanometry may be an important technique for evaluating early disc degeneration, especially tears of the anular fibers, which may be missed on magnetic resonance imaging.

Study Design. The intervertebral disc, in a sheep model, was used to assess the effect of directly repairing three different anular incisions on the subsequent healing strength of the intervertebral disc. Objectives. To assess whether directly repairing an anular defect, made at the time of lumbar discectomy, could influence the healing rate and strength of the anulus fibrosus. Methods. Twenty-four sheep underwent a retroperitoneal approach to five lumbar disc levels, An anular incision, followed by partial discectomy was done at each exposed level. Anular incisions used in this study consisted of 1) a straight transverse slit, 2) a cruciate incision, and 3) a window or box excision. Healing strength was measured at three time intervals: 2 weeks, 4 weeks, and 6 weeks. Each anular incision type was performed on 30 lumbar discs, 10 discs in each time interval. Five discs in each time interval underwent direct repair, and five discs were left unrepaired to heal as controls. The sheep were killed at 2, 4, and 6 weeks after surgery. The lumbar spines were removed en bloc, and the intervertebral discs were subjected to pressure-volume testing to assess the anular strength of repaired versus unrepaired disc injuries at each time interval. Results. Statistical analysis was performed to evaluate the effects of healing time, incision technique, and repair on the pressure-volume characteristics of the involved discs. Pressure-volume testing showed trends of stronger healing for repaired discs, but at no time interval was any significant difference found between repaired and nonrepaired anular strength. Of the nonrepaired discs, the box incision was only 40 to 50% as strong as the slit or cruciate incised discs during early healing. Conclusion. Direct repair of anular incisions in the lumbar spine does not significantly alter the healing strength of the intervertebral disc after lumbar discectomy.

Study Design. The role of bone graft force in stabilizing an instrumented cervical spine was evaluated for onelevel and three-level corpectomy models using in vitro experiments. Objectives. To investigate the role of bone graft force in enhancing stability of anterior cervical plate, and to study effects of fatigue loading. Summary of Background Data. The anterior cervical plate system is used widely in stabilizing the cervical spine after spinal corpectomy and grafting. Many factors such as applied screw torque, screw pullout force, plate strength, plate geometry, and type of bone graft have been studied. However, the role of bone graft in stabilizing the anterior plate system has not been explored. Methods. Two models (one-level and three-level) incorporating corpectomy, strut graft, and anterior plate were constructed from eight human spine specimens (C2‐T1). The flexibility of an intact specimen and two constructs with graft forces of 0 N and 100 N was determined. A flexibility test, simulating physiologic loads, consisted of pure moments of flexion, extension, lateral bending, and axial torques up to 1 Nm. For each moment, range of motion and neutral zone were determined. The stability potential index was defined as the decrease in motion caused by instrumentation, as compared with intact motion. A larger stability potential index indicates a more stable spinal construct. Repeated measures analysis of variance was used to determine the significant changes. Results. In both models, bone graft force increased during extension, decreased during flexion, and showed minor changes during axial torsion and lateral bending. Higher bone graft force increased stability potential index‐neutral zone and stability potential index‐range of motion in the three-level model in all directions, but only in flexion‐ extension in the one-level model. Fatigue loading decreased bone graft force to a greater extent in the three-level model. Conclusions. In the corpectomy-graft-anterior-plate model, graft force decreased in flexion and increased in extension. Higher graft force increased and fatigue decreased stability of the spinal construct in the three-level 2000;25:1649 ‐1654 Anterior cervical plate instrumentation is an effective treatment to stabilize the cervical spine after a multilevel corpectomy for degenerative, traumatic, and neoplastic cervical spine. The anterior cervical plate prevents the strut graft from extrusion and provides immobility and alignment to the instrumented cervical spine. Anterior cervical plate systems offer the cervical spine stability and rigidity, which is needed for solid bony fusion 2,4,11 and early arthrodesis. 9 Clinical studies indicate that the success rate is approximately 90% for the two-level cervical corpectomy instrumentation, but only 50% to 70% for the three-level corpectomy in

STUDY DESIGN An in vitro biomechanical study of experimental burst fractures and a pedicle screw device. OBJECTIVES To investigate the effects that adjustments of a pedicle screw device have on the neural spaces of the burst fracture. SUMMARY OF BACKGROUND DATA Decompression of the neural spaces is important for the recovery of neural function after a burst fracture. No studies, experimental or clinical, are available that have attempted to relate the pedicle screw device adjustments to the decompression of the neural spaces. METHODS Burst fractures were produced at L1 vertebra in nine human lumbar spine specimens. Pedicle screw devices were attached to T12 and L2. Eight device adjustments, consisting of pure translations (distraction or compression), pure extension, and combinations of translation and extension were applied. The dimensional changes in the canal and the superior and inferior intervertebral foramens were measured. Functions of restoration and improvement were determined for the adjustments to evaluate the effects of each adjustment and to determine the optimal adjustment. Analysis of variance was used to find statistically significant differences, with significance set at P values less than 0.05. RESULTS Significant differences were observed in the results of the eight adjustments. The most effective adjustments were the combination of 5-mm distraction with 6 degrees extension or 10-mm distraction alone. The worst adjustment was 5 mm of compression. CONCLUSIONS Restoring compromised neural spaces in a patient with burst fracture is necessary. The choice of a device adjustment was found to be an important factor in the decompression of the neural spaces after the burst fracture. Combined distraction with extension and distraction alone were found to decompress the canal andintervertebral foramens maximally in a burst fracture.

Physiologic motions of the human, sheep, and calf lumbar spines have been well characterized. The size, cost, and ease of care all make the rabbit an attractive alternative choice for an animal lumbar spine model. However, comparisons of normal biomechanical characteristics of the rabbit lumbar spine have not been made to the spines of larger species. The purpose of this study was to establish baseline physiologic kinematic data for the rabbit lumbar spine. Ten skeletally mature New Zealand white rabbit osteoligamentous spines were obtained. L4-L7 spine segments were harvested and mounted. Multi-directional flexibility testing was performed by applying pure moments up to 0.27 Nm. Resulting rotations were measured using an Optotrak system. Data were analyzed for each intervertebral level in the three planes of rotation. The three levels tested had roughly similar range of motion (ROM). The mean (SD) angular ROMs in flexion for L4-L5, L5-L6, L6-L7 were 12.10 degrees (2.59 degrees), 12.38 degrees (2.70 degrees), and 15.17 degrees (3.22 degrees), respectively. The ROMs in extension were 5.86 degrees (1.21 degrees), 5.58 degrees (1.48 degrees), and 6.13 degrees (2.03 degrees). Lateral bending and axial rotation were roughly symmetric due to the symmetric nature of the spine. For right lateral bending, the ROMs were 8.25 degrees (2.44 degrees), 4.96 degrees (1.70 degrees ), and 4.25 degrees (1.20 degrees). For left axial rotation, the ROMs were 1.23 degrees (1.16 degrees), 0.35 degrees (0.61 degrees), 0.87 degrees (0.64 degrees ). Neutral zone (NZ) was on average 60% (29%) of ROM for the motions studied. The physiologic ROM of the New Zealand white rabbit lumbar spine was found to be similar between the rabbit and human. This relatively conserved physiologic flexibility supports the use of the rabbit as a model of the lumbar spine for kinematic studies. However, the overall NZ was found to be a greater percentage of ROM in the rabbit than the corresponding percentage in the human (60% as compared to 25%). This suggested that the rabbit lumbar spine has a greater laxity than that of the human.

Study design The internal architecture of cervical spine pedicles was investigated by thin sectioning and digitization of radiographic images. Objectives To provide quantitative information on the internal dimensions and cortical shell thicknesses of the middle and lower cervical pedicles. Summary of background data Although there have been a number of studies presenting data on the external dimensions of the cervical pedicle, little is known regarding its internal architecture and cortical shell thickness along the pedicle axis. Methods Twenty-five human cervical vertebrae (C3-C7) were secured to a thin-sectioning machine to produce three 0.7-mm-thick pedicle slices along its axis. Plain radiographs of the pedicle slices were scanned and digitized to facilitate measurement of the internal dimensions. Computer software was specifically developed to determine the external dimensions (i.e., pedicle height and width) and the internal dimensions (i.e., cortical shell thicknesses of the superior, inferior, lateral, and medial walls and the cancellous core height and width) of cervical pedicles. Results Superior and inferior wall cortical thicknesses of pedicle thin slices were similar, whereas the lateral wall cortical thickness was significantly smaller than the medial wall thickness. The medial cortical shell (average value range: 1.2-2.0 mm) was measured to be 1.4 to 3.6 times as thick as the lateral cortical shell (average value range: 0.4-1.1 mm). When medial and lateral cortical thicknesses were normalized for external dimensions, the combined cortical shell thickness was thinnest at C7 (average value range: 18. 6-25.6% of the external width), and this result was statistically significant when compared with other vertebral levels. Conclusions The cervical pedicle is a complex, three-dimensional structure exhibiting extensive variability in internal morphology. Characteristics of the cervical pedicle at different spinal levels must be noted before transpedicular screw fixation.

Transpedicular screw fixation has recently been shown to be successful in stabilizing the middle and lower cervical spine. Controversy exists, however, over its efficacy, due to the smaller size of cervical pedicles and the proximity of significant neurovascular structures to both lateral and medial cortical walls. To aid the spinal surgeon in the insertion of pedicle screws, a number of studies have been performed to quantify the gross dimensions and angulations of the cervical pedicle. Notwithstanding these quantitative studies, there has been a conspicuous absence of research reporting the qualitative characteristics of the cervical pedicle. The purpose of our study was to provide comparative graphical data that would systematically document the anatomic variability in cervical pedicle morphology. Such information should better elucidate the complexity of the pedicle as a three-dimensional structure and provide the spinal surgeon with a more complete understanding of cervical pedicle architecture. Twenty-six human cervical vertebrae (C3–C7) from six fresh-frozen spines were secured to a thin sectioning apparatus to produce three 0.7-mm- thick pedicle slices along its axis. Radiographs taken of these pedicle slices were scanned, digitized, and traced to facilitate visual comparison. The pedicle slices were found to exhibit substantial variability in composition and shape, not only between individual spines and vertebral levels, but also within the pedicle axis. However, the lateral cortex was consistently found to be thinner than the medial cortex in all samples. These physical findings must be noted by surgeons attempting transpedicular screw fixation in the cervical spine.

The effect of an anabolic steroid (nandrolone decanoate, 20 mg/kg) and a corticosteroid (methylprednisolone acetate, 25 mg/kg) on healing muscle injured with a drop-mass technique in a reproducible muscle contusion injury model in the rat was studied. Healing was determined by measuring active contractile tension in each muscle and histologic analysis. At day 2, the corticosteroid group showed significant improvement in both twitch and tetanic strength relative to the controls. At day 7, this effect was reversed and the corticosteroid muscles were significantly weaker than the control muscles, but there was still no significant effect seen in the anabolic steroid group. At day 14, the corticosteroid muscles were totally degenerated, with disorganized muscle fiber architecture. The anabolic steroid muscles were significantly stronger in twitch, and a similar trend was seen in tetanus relative to control muscles. The results indicate that in an animal model corticosteroids may be beneficial in the short term, but they cause irreversible damage to healing muscle in the long term, including disordered fiber structure and a marked diminution in force-generating capacity. Anabolic steroids may aid in the healing of muscle contusion injury to speed the recovery of force-generating capacity. Although anabolic steroids are considered renegade drugs, they may have an ethical clinical application to aid healing in severe muscle contusion injury, and their use in the treatment of muscle injuries warrants further research.

To study changes in viscoelasticity of a ligament due to an incomplete or a subfailure injury.An in vitro study of anterior cruciate ligament preparations.The viscoelastic properties are an inherent part of the physical characteristics of a ligament. An injury to a ligament alters both its elastic and viscous properties. Although the effects of several parameters on the mechanical properties of a ligament have been studied, there is no information in the literature concerning the effect of an incomplete or subfailure injury on its viscoelastic behavior.Ten pairs of rabbit femur-anterior cruciate ligament-tibia specimens were used. A standardized relaxation (Relax) test was adopted to quantify the viscoelastic behavior, before and after a subfailure injury. One member of the pair was subjected to three sequential tests: Relax 1; Relax 2; and stretch to failure. The other member of the pair was subjected to other three tests: Relax 3; 80% subfailure injury, i.e. stretch of 80% of failure deformation; and Relax 4.We found that the relaxation test by itself (Relax 1 vs Relax 2), did not affect the viscoelasticity of the ligament. On the other hand, the 80% subfailure injury (Relax 3 vs Relax 4) affected the ligament viscoelastic behavior. The force was decreased by about 50% at time zero (10.46 vs 4.79 N, p = 0.014), and at 180 s (8.14 vs 4.11 N, p = 0.018). Fitting a three-element linear viscoelastic solid model to our data, we found the serial spring stiffness to decrease by about 50% (p = 0.01), the parallel spring remained unchanged, and there was a tendency for the dashpot coefficient to decrease (by 57%, p = 0.09).The 80% subfailure injury decreased the initial stiffness of the ligament, and tended to decrease its viscoelastic property.Subfailure or incomplete injuries of ligaments are more common than the complete injuries. The present study describes the effects, on both the elastic and viscous properties, of a ligament subjected to a subfailure injury.

Whiplash injury to the cervical spine is poorly understood. Symptoms often do not correlate to the clinical findings. It has been hypothesized that the long-term clinical symptoms associated with whiplash have their basis in mechanical derangement of the cervical spine caused at the time of trauma. Before such a hypothesis can be proven, one needs to document and quantify the soft tissue injuries of the cervical spine in whiplash. The purpose of the study was to quantify the mechanical changes that occur in the cervical spine specimen as a result of experimental whiplash trauma. Utilizing a whiplash trauma model, injuries to human cadaveric cervical spine specimens (C0 – T1 or C0 – C7) were produced by increasingly severe traumas. The flexibility tests determined the motion changes at each intervertebral level in response to 1.0 Nm pure flexion-extension moment. Parameters of range of motion (ROM) and neutral zone (NZ) were determined before and after each trauma. Significant flexibility increases first occurred in the lower cervical spine after 4.5–g rear-end (anteriorly directed) acceleration of the T1 vertebra. At this acceleration magnitude, extension ROM and NZ at C5 – C6 increased (P < 0.05) by 98% and 160% respectively. There was also a tendency (P < 0.1) for the extension NZ at C0 – C1 and C6 – C7 levels to increase after the 6.5-g acceleration by 52% and 241% respectively. There were no such tendencies for the ROM parameter. We have identified the threshold and sites of whiplash injury to the cervical spine. This information should help the clinician make more precise diagnoses in the case of whiplash trauma patients.

Biomechanical models have been used for the understanding of the basic normal function and dysfunction of the cervical spine and for testing implants and devices. Biomechanical models can be broadly categorized into four groups: 1) Physical models, made of nonanatomic material (e.g., plastic blocks), are often used for the testing of spinal instrumentation when only the device is to be evaluated. 2) In vitro models consisting of a cadaveric spine specimen are useful in providing basic understanding of the functioning of the spine. Human specimens are more suitable for these models than are animal specimens whenever anatomy, size (for instrumentation), and kinematics are important. Animal specimens are less costly, easier to obtain, and often have less variability but should be used with care because of the absence of anatomic fidelity with the human. 3) In vivo animal models provide the means to model living phenomena, such as fusion, development of disc degeneration, instability, and adaptive responses in segments adjacent to spinal instrumentation. Choosing the appropriate animal is important. The appropriate animal should have spinal loading, kinematics, kinetics, vertebral size, and healing-fusion rates as similar to those in humans as possible. For better interpretation of in vivo animal experimental results, in vitro biomechanical study using the same animal cadaveric specimen is useful but has not been used routinely. 4) Computer models are developed from mathematical equations that incorporate geometry and physical characteristics of the human spine and may be advantageously used for problems that are difficult to model by other means. Examples are the changes in disc and vertebral stresses in response to graded transection of facet joints and the study of changes in endplate loading caused by disc degeneration. Because these models are purely mathematical, their validation is essential. Validation is best achieved by first incorporating high-quality geometry and physical characteristics of the human spine and then comparing the model predictions with experimental observations. Sometimes an enthusiastic researcher may use a computer model beyond its validation boundary, making the model's predictions unreliable. In general, it is important to remember that a biomechanical model, similar to any other model, represents only a certain aspect of the real living human being. The aspect chosen for representation should be selected with great care. The model should be designed to answer specifically the question asked. Its predictions are valid only within the boundaries of assumptions and limitations that it incorporates.

Injuries of the thoracolumbar spine are serious, disabling, and costly to society. These injuries vary from mild ligament tears to severe bony fractures. Increased range of motion (ROM) and neutral zone (NZ) have been suggested as indicators of the resulting clinical instability. The purpose of the present study was to investigate the relative sensitivities and merits of the ROM and NZ in relation to spinal injuries of the thoracolumbar junction. A graded spinal trauma experiment was designed, in which the threshold of injury and injury progression were examined. Ten thoracolumbar human spine specimens (T11–L1) were traumatized using a high-speed incremental trauma model. The ROM and NZ, which indicate altered mechanical properties, were determined for three physiological motions: flexion/extension (FE), lateral bending (LB), and axial rotation (AR). The injury threshold was found to be 84 J (or 84 Nm) by examining both ROM and NZ for all motion types (P < 0.05), but the NZ was more sensitive. At the injury threshold, the NZ showed an overall average increase of 566% above that of the intact, while the equivalent increase in the ROM was only 94%. The NZ was also a more sensitive parameter documenting the progression of the injury beyond the injury threshold. After the maximum trauma of 137 J, the NZs for the three motions (FE, LB, and AR) increased by 700%, 1700%, and 3000% above their respective intact values. Corresponding increases in the ROM were much smaller: 115%, 184%, and 425% respectively. Direct extrapolation of the in vitro experimental findings to the clinical situation, as always, should be done with care. Our findings, however, suggest that the ROM, as measured from functional radiographs of a traumatized patient, may underestimate the true injury to the spinal column.

OBJECTIVE: To propose a different hypothesis of whiplash injury mechanism based on a series of experimental studies summarized in this communication. DESIGN: A series of biomechanical studies simulating whiplash trauma using isolated human cadaveric spine specimens. BACKGROUND: Whiplash injuries are on the rise as reported in several recent studies, due primarily to the increased traffic density. Although the symptoms associated with whiplash have been described, our understanding of the injury mechanism remains poor. The prevailing view of neck hyper-extension causing the injury has not been supported by recent experimental studies. METHODS: Eight fresh human cadaveric cervical spine specimens were prepared and traumatized to varying degrees under controlled conditions using a bench-top model of whiplash trauma. Before and after each trauma, the specimen was studied by functional radiography and flexibility test to document changes in the anatomic alignment and biomechanical properties at each level indicating injuries sustained. At the end of all testing, CT-scans, MRI and cryomicrotome images were obtained. During each trauma, relative motions of all intervertebral joints were recorded with a high speed movie camera. Elongations of the vertebral artery and several capsular ligaments were also monitored during the trauma using specially designed transducers. RESULTS: The hyper-extension hypothesis of injury mechanism was not supported by these studies. We found a distinct bi-phasic kinematic response of the cervical spine to whiplash trauma. In the first phase, the spine formed an S-shaped curve with flexion at the upper levels and hyper-extension at the lower levels. In the second phase, all levels of the cervical spine were extended, and the head reached its maximum extension. The occurrence of anterior injuries in the lower levels in the first phase was confirmed by functional radiography, flexibility tests and imaging modalities. The largest dynamic elongation of the capsular ligaments was observed at C6-C7 level during the initial S-shaped phase of whiplash. Similarly, the maximum elongation of the vertebral artery occurred during the S-shape phase of whiplash. CONCLUSION: We propose, based upon our experimental findings, that the lower cervical spine is injured in hyperextension when the spine forms an S-shaped curve. Further, this occurs in the first whiplash phase before the neck is fully extended. At higher trauma accelerations, there is a tendency for the injuries to occur at the upper levels of the cervical spine. Our findings provide truer understanding of whiplash trauma and may help in improving the diagnosis, treatment, and prevention of these injuries.

The BAK spinal fusion system has been applied to laparoscopic anterior lumbar interbody fusion. The system, consisting of a pair of cylindrical implants with threads, placed symmetrically about the sagittal plane, functions by tensioning the annulus fibrosis. Cylindrical plugs of increasing size are inserted prior to the implant placement. As the procedure may affect spinal posture and disc height, we measured changes due to incremental plug insertion using human cadaveric spine specimens (L5–S1, n = 4). Multi-directional flexibility of the construct was also measured as a function of plug size. The disc height change was found to increase initially and then to level off at 13-mm diameter plugs. In the sagittal plane, the intervertebral posture first shifted towards kyphotic then came back to the initial lordotic posture with plugs of bigger size. However, changes in disc height and spine posture were not statistically significant. Comparing the neutral zone (NZ) flexibility after inserting the plugs to the intact values, neither the flexion/extension nor the axial rotation NZ showed any singificant change. In lateral bending, the NZ decreased after the insertion of 13-mm plugs (p < 0.05). Insertion of plugs of increasing size from 9 mm to 12 mm decreased the range of motion (ROM) in all directions (p < 0.05). Insertion of 13-mm and 14-mm plugs decreased the flexion/ extension and lateral bending ROM, but not the axial rotation ROM, probably indicating some injury to the annulus fibers.

Objective. To study biomechanics and failure characteristics of the alar and transverse ligaments of the upper cervical spine at two different extension rates. Design. In vitro biomechanical study using human and alar and transverse ligament specimens. Background. Previous studies of these ligaments have been at relatively slow extension rates, even though the real life trauma occurs at significantly higher extension rates. Methods. Fresh human cadaveric alar and transverse ligaments were subjected to slow (0.1 mm/s) and then fast (920 mm/s) extension rates. Force and elongation curves were recorded during the testing, and several biomechanical parameters were computed and compared. First the specimen was stretched at the slow extension rate up to 70 N for the alar ligament and 140 N for the transverse ligament. Then the specimens were stretched until failure at the fast extension rate. Rasults. Avarage initial lengths of the alar and transverse ligaments were 11.2 and 18.0 mm, respectively. At 70 N, the average alar ligament strain decreased from 16.4 to 0.3%, the stiffness increased from 80 to 2316 N/mm and the energy absorbed decreased from 47.4 to 1.3 Nrom, as the extension rate increased from the slow to the fast. At failure, the average strain, force and energy absorbed at the fast extension rate were respectively 3.1%, 367 N and 123 Nmm. For the transverse ligament at 140 N, the strain decreased from 12.5 to 0.6%, the stiffness increased from 141 to 1472 N/mm, and the energy absorbed decreased from 96.1 to 7.6 Nmm as the extension rate went from slow to the fast. At failure, strain, force and energy absorbed for the transverse ligament at the fast extension rate were, respectively, 2.3% and 436 N and 101 Nmm respectively. Conclusion. Within the physiological limits, the strain and energy absorbed decreased to less than one tenth, while the stiffness increased to greater than ten times as the extension rate increased, for both the alar and transverse ligaments. When failed at the faster rate, the alar ligament, although weaker of the two, absorbed greater energy to failure because of its higher failure strain. Relevance Injuries to the upper cervical spine have extremely serious clinical consequences because of the anatomic location. Alar and transverse ligaments provide most of the stabilit in this region. This is the first study to determine the mechanical properties of these ligaments at an extension rate that is closer to real life trauma.

STUDY DESIGN The multidirectional stability potential of anterior, posterior, and combined instrumentations applied at L1-L3 was studied after L2 corpectomy and replacement with a carbon-fiber implant. OBJECTIVES To evaluate the biomechanical characteristics of short-segment anterior, posterior, and combined instrumentations in lumbar spine tumor vertebral body replacement surgery. SUMMARY OF BACKGROUND DATA The biomechanical properties of many different spinal instrumentations have been studied in various spinal injury models. Only a few studies, however, investigate the stabilization methods in spinal tumor vertebral body replacement surgery. METHODS Eight fresh frozen human cadaveric thoracolumbar spine specimens (T12-L4) were prepared for biomechanical testing. Pure moments (2.5 Nm, 5 Nm, and 7.5 Nm) of flexion-extension, left-right axial torsion, and left-right lateral bending were applied to the top vertebra in a flexibility machine, and the motions of the L1 vertebra with respect to L3 were recorded with an optoelectronic motion measurement system after reconditioning. The L2 vertebral body was resected and replaced by a carbon-fiber cage. Different fixation methods were applied to the L1 and L3 vertebrae. One anterior, two posterior, and two combined instrumentations were tested. Load-displacement curves were recorded and neutral zone and range of motion parameters were determined. RESULTS The anterior instrumentation provided less potential stability than the posterior and combined instrumentations in all motion directions. The anterior instrumentation, after vertebral body replacement, showed greater motion than the intact spine, especially in axial torsion (range of motion, 10.3 degrees vs 5.5 degrees; neutral zone, 2.9 degrees vs. 0.7 degrees; P < 0.05). Posterior instrumentation provided greater rigidity than the anterior instrumentation, especially in flexion-extension (range of motion, 2.1 degrees vs. 12.6 degrees; neutral zone, 0.6 degrees vs. 6.1 degrees; P < 0.05). The combined instrumentation provided superior rigidity in all directions compared with all other instrumentations. CONCLUSIONS Posterior and combined instrumentations provided greater rigidity than anterior instrumentation. Anterior instrumentation should not be used alone in vertebral body replacement.

Study design Whiplash injuries were studied in an experiment using whole cervical spine specimen. Objectives To develop a whiplash trauma model that uses a whole cervical spine specimen, and to show the feasibility and unique features of such a model. Summary of background data Whiplash trauma has been simulated in biomechanical experiments using volunteers, whole body cadavers, animals, anthropometric dummies, and mathematic models. These experiments require large facilities, are expensive, and provide limited information about cervical spine injuries. Methods An alternate approach, in which a bench-top sled accelerating apparatus is used to produce whiplash trauma, has been developed to study such trauma in whole cervical spine specimens. Several transducers were developed to monitor soft tissue injuries during the trauma. The model also provides quantification of injuries to the cervical spine. Results To assess the feasibility and usefulness of the model, a specimen was traumatized, and the following parameters were monitored during the trauma: linear acceleration of the sled, linear and angular acceleration of the head surrogate, displacements of the head surrogate, loads at T1 and C1 vertebrae, and linear deformations of capsular ligaments and vertebral artery. Conclusions This model, which incorporates a fresh cadaveric whole human cervical spine specimen, can simulate whiplash trauma effectively and is useful in providing a comprehensive set of clinically relevant information during the trauma. This model gives insight into the complex events and interactions that cause the injuries that occur during whiplash trauma.

Study design A bench-top trauma sled was used to apply four intensities of whiplash trauma to human cadaveric cervical spine specimens and to measure resulting intervertebral rotations using high-speed cinematography. Objectives To determine the cervical spine levels most prone to injury from whiplash trauma and to hypothesize a mechanism for such injury. Summary of background data Whiplash injuries traditionally have been ascribed to hyperextension of the head, but other mechanisms such as hypertranslation also have been suggested. Methods Six occiput to T1 (or C7) fresh cadaveric human spines were studied. Physiologic flexion and extension motions were recorded with an Optotrak motion analysis system by loading up to 1.0 Nm. Specimens then were secured in a trauma sled, and a surrogate head was attached. Flags fixed to the head and individual vertebrae were monitored with high-speed cinematography (500 frames/sec). Data were collected for 12 traumas in four classes defined by the maximum sled acceleration. The trauma classes were 2.5 g, 4.5 g, 6.5 g, and 8.5 g. Significance was defined at P Results In the whiplash traumas, the peak intervertebral rotations of C6-C7 and C7-T1 significantly exceeded the maximum physiologic extension for all trauma classes studied. The maximum extension of these lower levels occurred significantly before full neck extension. In fact, the upper cervical levels were consistently in flexion at the time of maximum lower level extension. Conclusions In whiplash, the neck forms an S-shaped curvature, with lower level hyperextension and upper level flexion. This was identified as the injury stage for the lower cervical levels. A subsequent C-shaped curvature with extension of the entire cervical spine produced less lower level extension.

STUDY DESIGN: This study examined the coactivation of trunk flexor and extensor muscles in healthy individuals. The experimental electromyographic data and the theoretical calculations were analyzed in the context of mechanical stability of the lumbar spine. OBJECTIVES: To test a set of hypotheses pertaining to healthy individuals: 1) that the trunk flexor-extensor muscle coactivation is present around a neutral spine posture, 2) that the coactivation is increased when the subject carries a load; and 3) that the coactivation provides the needed mechanical stability to the lumbar spine. SUMMARY OF BACKGROUND DATA: Theoretically, antagonistic trunk muscle coactivation is necessary to provide mechanical stability to the human lumbar spine around its neutral posture. No experimental evidence exists, however, to support this hypothesis. METHODS: Ten individuals executed slow trunk flexion-extension tasks, while six muscles on the right side were monitored with surface electromyography: external oblique, internal oblique, rectus abdominis, multifidus, lumbar erector spinae, and thoracic erector spinae. Simple, but realistic, calculations of spine stability also were performed and compared with experimental results. RESULTS: Average antagonistic flexor-extensor muscle coactivation levels around the neutral spine posture as detected with electromyography were 1.7 +/- 0.8% of maximum voluntary contraction for no external load trials and 2.9 +/- 1.4% of maximum voluntary contraction for the trials with added 32-kg mass to the torso. The inverted pendulum model based on static moment equilibrium criteria predicted no antagonistic coactivation. The same model based on the mechanical stability criteria predicted 1.0% of maximum voluntary contraction coactivation of flexors and extensors with zero load and 3.1% of maximum voluntary contraction with a 32-kg mass. The stability model also was run with zero passive spine stiffness to simulate an injury. Under such conditions, the model predicted 3.4% and 5.5% of maximum voluntary contraction of antagonistic muscle coactivation for no extra load and the added 32 kg, respectively. CONCLUSIONS: This study demonstrated that antagonistic trunk flexor-extensor muscle coactivation was present around the neutral spine posture in healthy individuals. This coactivation increased with added mass to the torso. Using a biomechanical model, the coactivation was explained entirely on the basis of the need for the neuromuscular system to provide the mechanical stability to the lumbar spine.

Study design Ten fresh, cadaveric, two-vertebrae, functional spinal units were used to study the pathoanatomy, intervertebral foraminal area, and flexibility changes after posterior and transforaminal decompression. Objectives To determine the feasibility of an endoscopic transforaminal approach as an alternative to conventional approaches, to establish the adequacy of transforaminal decompression without destabilizing the spine, and to study the structural changes in the spine after decompressions. Summary of the background data Posterior decompression entails major dissection and excision of bone and ligaments to access the spinal canal. Posterior decompression may be complicated by acute or chronic spinal instability, and the adequacy of lateral decompression is highly subjective. Methods The functional spinal units were mounted in quick-setting epoxy blocks. Pre- and postoperative computed tomography scans were taken to study changes in the foraminal area. Pre- and postoperative flexibility and anatomic studies were performed to compare the results. Results A 45.5% increase in the intervertebral foraminal area was possible, there was no flexibility change, and minimal anatomic damage to the spine was noted after transforaminal decompression. A 34.2% increase in the intervertebral foraminal area and a significant increase in extension and axial rotation flexibility were noted after the posterior decompression. Conclusion Transforaminal decompression produced a significantly larger increase in the intervertebral foraminal area than posterior decompression, without increasing the range of motion or neutral zone in any direction. Because there was no violation of the anatomic integrity of the spine in the transforaminal approach, the risk of surgically induced instability was minimized. Endoscopic transforaminal decompression is a feasible alternative to current approaches.