A method to simulate in vivo cervical spine kinematics using in vitro compressive preload

Reduction in segmental flexibility because of disc degeneration is accompanied by higher changes in facet loads than changes in disc pressure: a poroelastic C5-C6 finite element investigation

BACKGROUND CONTEXT

Nerve fiber growth inside the degenerative intervertebral discs and facets is thought to be a source of pain, although there may be several other pathological and clinical reasons for the neck pain. It, however, remains difficult to decipher how much disc and facet joints contribute to overall degenerative segmental responses. Although the biomechanical effects of disc degeneration (DD) on segmental flexibility and posterior facets have been reported in the lumbar spine, a clear understanding of the pathways of degenerative progression is still lacking in the cervical spine.

PURPOSE

To test the hypothesis that after an occurrence of degenerative disease in a cervical disc, changes in the facet loads will be higher than changes in the disc pressure.

STUDY DESIGN

To understand the biomechanical relationships between segmental flexibility, disc pressure, and facet loads when the C5-C6 disc degenerates.

METHODS

A poroelastic, three-dimensional finite element (FE) model of a normal C5-C6 segment was developed and validated. Two degenerated disc models (moderate and severe) were built from the normal disc model. Biomechanical responses of the three FE models (normal, moderate, and severe) were further studied under diurnal compression (at the end of the daytime activity period) and moment loads (at the end of 5 seconds) in terms of disc height loss, angular motions, disc pressure, and facet loads (average of right and left facets).

RESULTS

Disc deformation under compression and segmental rotational motions under moment loads for the normal disc model agreed well with the corresponding in vivo studies. A decrease in segmental flexibility because of DD is accompanied by a decrease in disc pressure and an increase in facet loads. Biomechanical effects of degenerative disc changes are least in flexion. Segmental flexibility changes are higher in extension, whereas changes in disc pressure and facet loads are higher in lateral bending and axial rotation, respectively.

CONCLUSIONS

The results of the present study confirmed the hypothesis of higher changes in facet loads than in disc pressure, suggesting posterior facets are more affected than discs because of a decrease in degenerative segmental flexibility. Therefore, a degenerated disc may increase the risk of overloading the posterior facet joints. It should be clearly noted that only after degeneration simulation in the disc, we recorded the biomechanical responses of the facets and disc. Therefore, our hypothesis does not suggest that facet joint osteoarthritis may occur before degeneration in the disc. Future cervical spine-based experiments are warranted to verify the conclusions presented in this study.

Biomechanical analysis of the range of motion after placement of a two-level cervical ProDisc-C versus hybrid construct

STUDY DESIGN

The study design was that of an in vitro human cadaveric biomechanical analysis.

OBJECTIVES

The objective of this study was the biomechanical analysis of the range of motion (ROM) of a 2-level intact spine control versus total, then operative- and adjacent-segment ROM after (1) 2-level ProDisc-C placement (PP), (2) anterior cervical discectomy and fusions (ACDFs), and (3) hybrid constructs of both. Follower load and multidirectional testing were performed in each instance.

SUMMARY OF BACKGROUND DATA

With in vivo cervical arthroplasties gaining in popularity, limited biomechanical data are available, which highlight changes in the adjacent-level biomechanics after multilevel procedures.

METHODS

Biomechanical testing for ROM was performed using 7 cadaveric C4-T1 spine specimens. Moments up to 2 Nm with a 100 N follower load were applied in flexion/extension (F/E), right and left lateral bending (LB), and right and left axial rotation (AR). Specimens were tested in the intact state and then with a combination of ProDisc-C arthroplasty and ACDF at C5-C6 and C6-C7.

RESULTS

In the 2-level PP group, the increase in ROM in F/E, LB, and AR of C4-T1 occurs due to an increased ROM at the operative level. The ROM of the level adjacent to the operative levels showed no significant change, except at C4-C5 in LB. For the latter level, the ROM of C4-C5 in each direction showed increases for each parameter. In the 2-level fusion C5-C6 and C6-C7 fusion (FF) group, the ROM in F/E, LB, and AR of C4-T1 was decreased because of a decrease in ROM primarily at the fused levels, and the ROM of adjacent levels was increased. In the ProDisc-C/Fusion (PF) and Fusion/ProDisc-C (FP) groups undergoing placements of a 1-level ProDisc-C/1-level fusion with cage and plate, both groups showed no significant ROM change of C4-T1 when compared with the control and no significant change at adjacent levels, with the exception of C4-C5 in LB.

CONCLUSION

(1) Two-level ACDFs decrease whereas 2-level PPs increase the entire C4-T1 ROM. (2) ACDF/ProDisc-C hybrid operations do not alter the C4-T1 ROM. (3) For the ACDF/ProDisc-C hybrid operative groups, the combined ROM of the operative levels showed no significant difference when compared with that of the intact spine. (4) Regarding adjacent-level ROM, a 2-level ACDF increases ROM, but 2-level ProDisc-C and hybrid ACDF/PPs do not show significant change except for LB at C4-C5. (5) When the segmental distribution of C4-T1 ROM is plotted as the percentage of total motion, it demonstrates that for PF and FP groups, the combined ROM of the C5-C6 and C6-C7 operative levels are similar to that of the intact spine in EF and LB. For the PP group, the combined ROM of the operative levels increased, whereas the combined ROM for the FF group is decreased. The decrease or increase of the adjacent C4-C5 or C7-T1 level ROM compensates for the operative levels.

A dynamic method for in vitro multisegment spine testing

Robotics recently spread to spine biomechanical research. The aim of the present work is to describe and validate a new method for in vitro studying of a multisegmental spinal specimen under dynamic conditions. This method relies on the use of a simulator with six degrees of freedom (to impose movements in all directions), an optoelectric apparatus (for collecting kinematics data) and an original system for attaching kinematic markers, allowing their precise removal and replacement under different examination conditions. The accuracy of measurements as well as their reproducibility under static and dynamic conditions is reported here in the study of a human lumbar spinal specimen (L1-sacrum). The method appears to be reliable and reproducible, and should therefore enable future studies of variations in mobility between healthy and pathological spines, to better understand the influence of different implants on spinal kinematics.

A new biofidelic sagittal plane surrogate neck for head-first impacts

OBJECTIVE

To evaluate a prototype sagittal plane surrogate neck model designed to provide a biofidelic response to head-first impacts with a straightened cervical posture.

METHODS

Published biomechanical studies were used in the design to define the range of motion (ROM) and stiffness in both flexion-extension rotation and axial compression. The neck was tested in a series of head-first impacts on a drop tower to investigate the temporal aspects of the kinetic axial force response for the head and neck. A separate series of flexion-extension tests was conducted in a spinal motion simulator to assess its ROM and bending stiffness.

RESULTS

In impacts with a 104 N axial preload, the surrogate head and neck displayed a bimodal response to force development in agreement with published studies of cadaveric head-first impacts. In bending without an axial preload, the neck had an ROM and bending stiffness representative of cadaveric human spines and it included a large neutral zone, but with the incremental addition of axial preload these metrics were somewhat reduced.

CONCLUSIONS

The model appears suitable for studying the scenario of sagittal plane, aligned column impacts. Further design refinements are required to provide biofidelity in both sagittal bending and head-first impacts using a single level of axial preload. This would be necessary to study impact scenarios where considerable sagittal plane neck rotation occurs at impact. The model has identified some key concepts that must be considered for continued design and improvement of a dedicated dummy neck for head-first impacts.

Motion changes in adjacent segments due to moderate and severe degeneration in C5-C6 disc: a poroelastic C3-T1 finite element model study

STUDY DESIGN

Biomechanics of normal vertebral segments adjacent to a degenerated segment in the cervical spine.

OBJECTIVE

To test the hypothesis of higher motion changes in the segment immediately inferior to a degenerated segment.

SUMMARY OF BACKGROUND DATA

Past research has shown how disc degeneration (DD) affects adjacent segments; however, these studies are conducted only on the lumbar spine or the experimental protocols used are characterized by the presence of degeneration in adjacent segments. The question arises as to how much of the degenerative effect in a particular segment is internal to degeneration at that segment and how much is caused by degeneration at adjacent segments. It would be clinically relevant to analyze biomechanical changes in adjacent segments in the cervical spine by considering DD at only one segment, where adjacent segments remain normal.

METHODS

A poroelastic, 3-dimensional finite element model of a normal C3-T1 segment was validated and then used for the degenerative study. Two additional C3-T1 models were developed with moderate and severe degenerative C5-C6 disc grades. Disc geometry and tissue material properties were modified to simulate C5-C6 DD. Intersegmental rotational motions (C4-C5, C5-C6, and C6-C7) for the 3 C3-T1 models were computed under moment loads.

RESULTS

With progressive C5-C6 DD, motion decreased at that segment. At adjacent segments, higher motion changes were observed mainly in flexion/extension. Inferior C6-C7 motion changes were higher than superior C4-C5 motion changes. The inferior C6-C7 motion was affected even when C5-C6 DD was moderate, and it was further affected by severe C5-C6 DD. The superior C4-C7 motion, however, was mostly affected by severe C5-C6 DD.

CONCLUSION

The hypothesis of higher motion changes in the normal C6-C7 segment immediately inferior to a degenerated C5-C6 segment was found to be true. Future experiments on multisegmental cervical spines are recommended to verify the current data.

Local and global subaxial cervical spine biomechanics after single-level fusion or cervical arthroplasty

An experimental in vitro biomechanical study was conducted on human cadaveric spines to evaluate the motion segment (C4-C5) and global subaxial cervical spine motion after placement of a cervical arthroplasty device (Altia TDI,Amedica, Salt Lake City, UT) as compared to both the intact spine and a single-level fusion. Six specimens (C2-C7) were tested in flexion/extension, lateral bending, and axial rotation under a +/- 1.5 Nm moment with a 100 N axial follower load. Following the intact spine was tested; the cervical arthroplasty device was implanted at C4-C5 and tested. Then, a fusion using lateral mass fixation and an anterior plate was simulated and tested. Stiffness and range of motion (ROM) data were calculated. The ROM of the C4-C5 motion segment with the arthroplasty device was similar to that of the intact spine in flexion/extension and slightly less in lateral bending and rotation, while the fusion construct allowed significantly less motion in all directions. The fusion construct caused broader effects of increasing motion in the remaining segments of the subaxial cervical spine, whereas the TDI did not alter the adjacent and remote motion segments. The fusion construct was also far stiffer in all motion planes than the intact motion segment and the TDI, while the artificial disc treated level was slightly stiffer than the intact segment. The Altia TDI allows for a magnitude of motion similar to that of the intact spine at the treated and adjacent levels in the in vitro setting.

Anterior cervical discectomy and fusion with a locked plate and wedged graft effectively stabilizes flexion-distraction stage-3 injury in the lower cervical spine: a biomechanical study

STUDY DESIGN

An in vitro three-dimensional (3D) flexibility test of human C3-C7 cervical spine specimens.

OBJECTIVE

To test the hypothesis that anterior cervical fusion with a wedged graft and a locked plate can effectively stabilize the cervical spine after complete anterior and posterior segmental ligamentous release.

SUMMARY OF BACKGROUND DATA

Distraction-flexion Stage 3 injuries of the lower cervical spine (bilateral facet dislocations) are usually reduced under awake cranial traction. When the magnetic resonance imaging reveals a traumatic disc prolapse, anterior cervical discectomy and fusion (ACDF) is usually recommended. Most authors advise combining ACDF with posterior instrumentation to address the insufficiency of the posterior elements. However, there is clinical evidence that ACDF with a locked plate alone suffices for the treatment of these injuries, especially in young patients. Still, there are no biomechanical studies on the effect of a locked plate on the complete anterior and posterior ligamentous-deficient young cervical spine under physiologic preload.

METHODS

Eight fresh frozen human lower cervical spines (C3-C7) from young donors (age, 44.5 years; range, 21-63 years) were used. A 3D flexibility test was conducted using a moment of 0.8 Nm without preload. Flexion-extension was additionally tested using a moment of 1.5 Nm under 0 and 150 N follower preload. Spines were tested first intact, then after complete C5-C6 discectomy with posterior longitudinal ligament resection and ACDF with a wedged bone graft and a rigid locked plate, and finally after complete release of the supraspinous, interspinous, and intertransverse ligaments; the facet capsules; and ligamentum flavum. RESULTS.: When tested under 0.8 Nm moment without preload, complete posterior and anterior ligamentous release did not significantly increase the ROM of the ACDF construct in flexion-extension (P > 0.025), lateral bending (P > 0.025), and axial rotation (P > 0.025). When tested under 1.5 Nm moment with or without a compressive preload, the complete posterior and anterior ligamentous release did not significantly affect the ROM of the ACDF construct (P > 0.01). The application of preload significantly reduced the motion at the C5-C6 ACDF construct with ligamentous disruption in comparison with the motion in the absence of a preload (P < 0.01).

CONCLUSION

Anterior cervical fusion with a wedged graft and a rigid constrained (locked) plate can effectively stabilize the nonosteoporotic cervical spine after complete posterior element injury when excessive ROM is prevented (for example, by the use of postoperative external immobilization). Even when the construct is subjected to higher moments, adequate stability can be achieved when physiologic preload is present. Osteoporosis and lack of sufficient preload due to poor neuromuscular control may affect long-term screw stability, and additional external immobilization may be needed until fusion matures.

Anulus fibrosus tension inhibits degenerative structural changes in lamellar collagen

Mechanical stress is one of the risk factors believed to influence intervertebral disc degeneration. Animal models have shown that certain regimes of compressive loading can induce a cascade of biological effects that ultimately results in cellular and structural changes in the disc. It has been proposed that both cell-mediated breakdown of collagen and the compromised stability of collagen with loss of anular tension could result in degradation of lamellae in the anulus fibrosus (AF). To determine whether this may be important in the AF, we subjected entire rings of de-cellularized AF tissue to MMP-1 digestion with or without tension. Biomechanical testing found trends of decreasing strength and stiffness when tissues were digested without tension compared with those with tension. To determine the physiologic significance of tissue level tension in the AF, we used an established in vivo murine model to apply a disc compression insult known to cause degeneration. Afterward, that motion segment was placed in fixed-angle bending to impose tissue level tension on part of the AF and compression on the contralateral side. We found that the AF on the convex side of bending retained a healthy lamellar appearance, while the AF on the concave side resembled tissues that had undergone degeneration by loading alone. Varying the time of onset and duration of bending revealed that even a brief duration applied immediately after cessation of compression was beneficial to AF structure on the convex side of bending. Our results suggest that both cell-mediated events and cell-independent mechanisms may contribute to the protective effect of tissue level tension in the AF.

Measurement of vertebral kinematics using noninvasive image matching method-validation and application

STUDY DESIGN

In vitro and in vivo laboratory study.

OBJECTIVE

To validate a dual fluoroscopic image matching technique for measurement of in vivo spine kinematics.

SUMMARY OF BACKGROUND DATA

Accurate knowledge of the spinal structural functions is critical to understand the biomechanical factors that affect spinal pathology. Many studies have investigated vertebral motion both in vitro and in vivo. However, determination of in vivo motion of the vertebrae under physiologic loading conditions remains a challenge in biomedical engineering because of the limitations of current technology and the complicated anatomy of the spine.

METHODS

In in vitro validation, an ovine spine was moved to a known distance in a known speed by an MTS machine. The dual fluoroscopic system was used to capture the spine motion and reproduce the moving distance and speed. In in vivo validation, a living subject moved the spine in various positions under weightbearing. The fluoroscopes were used to reproduce the in vivo spine positions 5 times. The standard deviations in translation and orientation of the 5 measurements were used to evaluate the repeatability of technique.

RESULTS

The translation positions of the ovine spine could be determined with a mean accuracy less than 0.40 mm for the image matching technique using magnetic resonance image-based vertebral models. The spine speed could be reproduced within an accuracy of 0.2 mm/s. The repeatability of the method in reproducing in vivo human spine 6DOF kinematics was less than 0.3 mm in translation and less than 0.7 degrees in orientation.

CONCLUSION

The image matching technique was accurate and repeatable for noninvasive measurement of spine vertebral motion. The technique could be a useful tool for determination of vertebral positions and orientations before and after surgical treatment of spinal pathology to evaluate and improve the efficacy of the various surgical methods in restoring normal spine function.

Normative segment-specific axial and coronal angulation corridors of subaxial cervical column in axial rotation

STUDY DESIGN

In contrast to clinical studies wherein loading magnitudes are indeterminate, experiments permit controlled and quantifiable moment applications, record kinematics in multiple planes, and allow derivation of moment-angulation corridors. Axial and coronal moment-angulation corridors were determined at every level of the subaxial cervical spine, expressed as logarithmic functions, and level-specificity of range of motion and neutral zones were evaluated.

HYPOTHESIS

segmental primary axial and coupled coronal motions do not vary by level.

SUMMARY OF BACKGROUND DATA

Although it is known that cervical spine responses are coupled, segment-specific corridors of axial and coronal kinematics under axial twisting moments from healthy normal spines are not reported.

METHODS

Ten human cadaver columns (23-44 years, mean: 34 +/- 6.8) were fixed at the ends and targets were inserted to each vertebra to obtain kinematics in axial and coronal planes. The columns were subjected to pure axial twisting moments. Range of motion and neutral zone for primary-axial and coupled-coronal rotation components were determined at each spinal level. Data were analyzed using factorial analysis of variance. Moment-rotation angulations were expressed using logarithmic functions, and mean +/-1 standard deviation corridors were derived at each level for both components.

RESULTS

Moment-angulations responses were nonlinear. Each segmental curve for both components was well represented by a logarithmic function (r2 > 0.95). Factorial analysis of variance indicated that the biomechanical metrics are spinal level-specific (P < 0.05).

CONCLUSION

Axial and coronal angulations of cervical spinal columns show statistically different level-specific responses. The presentation of moment-angulation corridors for both metrics forms a dataset for the normal population. These segment-specific nonlinear corridors may help clinicians assess dysfunction or instability. These data will assist mathematical models of the spine in improved validation and lead to efficacious design of stabilizing systems.

Range of motion change after cervical arthroplasty with ProDisc-C and prestige artificial discs compared with anterior cervical discectomy and fusion

OBJECT

Range of motion (ROM) changes were evaluated at the surgically treated and adjacent segments in cadaveric specimens treated with two different cervical artificial discs compared with those measured in intact spine and fusion models.

METHODS

Eighteen cadaveric human cervical spines were tested in the intact state for the different modes of motion (extension, flexion, lateral bending, and axial rotation) up to 2 Nm. Three groups of specimens (fitted with either the ProDisc-C or Prestige II cervical artificial disc or submitted to anterior cervical discectomy and fusion [ACDF]) were tested after implantation at C6-7 level. The ROM values were measured at treated and adjacent segments, and these values were then compared with those measured in the intact spine.

RESULTS

At the surgically treated segment, the ROM increased after arthroplasty compared with the intact spine in extension (54% in the ProDisc-C group, 47% in the Prestige group) and in flexion (27% in the ProDisc-C group, 10% in the Prestige group). In bending and rotation, the postarthroplasty ROMs were greater than those of the intact spine (10% in the ProDisc-C group and 55% in the Prestige group in bending, 17% in the ProDisc-C group and 50% in the Prestige group in rotation). At the adjacent levels the ROMs decreased in all specimens treated with either artificial disc in all modes of motion (< 10%) except for extension at the inferior the level (29% decrease for ProDisc-C implant, 12% decrease for Prestige disc). The ROM for all motion modes in the ACDF-treated spine decreased at the treated level (range 18-44%) but increased at the adjacent levels (range 3-20%).

CONCLUSIONS

Both ProDisc-C and Prestige artificial discs were associated with increased ROM at the surgically treated segment compared with the intact spine with or without significance for all modes of testing. In addition, adjacent-level ROM decreased in all modes of motion except extension in specimens fitted with both artificial discs.

Strength of the cervical spine in compression and bending

STUDY DESIGN

Cadaveric motion segment experiment.

OBJECTIVES

To compare the strength in bending and compression of the human cervical spine and to investigate which structures resist bending the most.

SUMMARY OF BACKGROUND DATA

The strength of the cervical spine when subjected to physiologically reasonable complex loading is unknown, as is the role of individual structures in resisting bending.

METHODS

A total of 22 human cervical motion segments, 64 to 89 years of age, were subjected to complex loading in bending and compression. Resistance to flexion and to extension was measured in consecutive tests. Sagittal-plane movements were recorded at 50 Hz using an optical two-dimensional "MacReflex" system. Experiments were repeated 1) after surgical removal of the spinous process, 2) after removal of both apophyseal joints, and 3) after the disc-vertebral body unit had been compressed to failure. Results were analyzed using t tests, analysis of variance, and linear regression. Results were compared with published data for the lumbar spine.

RESULTS

The elastic limit in flexion was reached at 8.5 degrees (SD, 1.7 degrees ) with a bending moment of 6.7 Nm (SD, 1.7 Nm). In extension, values were 9.5 degrees (SD, 1.6 degrees ) and 8.4 Nm (3.5 Nm), respectively. Spinous processes (and associated ligaments) provided 48% (SD, 17%) of the resistance to flexion. Apophyseal joints provided 47% (SD, 16%) of the resistance to extension. In compression, the disc-vertebral body units reached the elastic limit at 1.23 kN (SD, 0.46 Nm) and their ultimate compressive strength was 2.40 kN (SD, 0.96 kN). Strength was greater in male specimens, depended on spinal level and tended to decrease with age.

CONCLUSIONS

The cervical spine has approximately 20% of the bending strength of the lumbar spine but 45% of its compressive strength. This suggests that the neck is relatively vulnerable in bending.

Level-dependent coronal and axial moment-rotation corridors of degeneration-free cervical spines in lateral flexion

BACKGROUND

Aging, trauma, or degeneration can affect intervertebral kinematics. While in vivo studies can determine motions, moments are not easily quantified. Previous in vitro studies on the cervical spine have largely used specimens from older individuals with varying levels of degeneration and have shown that moment-rotation responses under lateral bending do not vary significantly by spinal level. The objective of the present in vitro biomechanical study was, therefore, to determine the coronal and axial moment-rotation responses of degeneration-free, normal, intact human cadaveric cervicothoracic spinal columns under the lateral bending mode.

METHODS

Nine human cadaveric cervical columns from C2 to T1 were fixed at both ends. The donors had ranged from twenty-three to forty-four years old (mean, thirty-four years) at the time of death. Retroreflective targets were inserted into each vertebra to obtain rotational kinematics in the coronal and axial planes. The specimens were subjected to pure lateral bending moment with use of established techniques. The range-of-motion and neutral zone metrics for the coronal and axial rotation components were determined at each level of the spinal column and were evaluated statistically.

RESULTS

Statistical analysis indicated that the two metrics were level-dependent (p < 0.05). Coronal motions were significantly greater (p < 0.05) than axial motions. Moment-rotation responses were nonlinear for both coronal and axial rotation components under lateral bending moments. Each segmental curve for both rotation components was well represented by a logarithmic function (R(2) > 0.95).

CONCLUSIONS

Range-of-motion metrics compared favorably with those of in vivo investigations. Coronal and axial motions of degeneration-free cervical spinal columns under lateral bending showed substantially different level-dependent responses. The presentation of moment-rotation corridors for both metrics forms a normative dataset for the degeneration-free cervical spines.

Biomechanical study of anterior cervical corpectomy and step-cut grafting with bioabsorbable screws fixation in cadaveric cervical spine model

STUDY DESIGN

An in vitro biomechanical study.

OBJECTIVE

To determine the initial stability of a novel construct in a 1-level cadaveric cervical spine model by comparing it with a conventional method.

SUMMARY OF BACKGROUND DATA

Lots of endeavors have been made to enhance fusion rates and reduce complications in the anterior cervical spine procedure.

METHODS

There were 12 fresh human cadaveric cervical spines (C3-C7) randomly divided into 2 groups: group 1, 1-level corpectomy of C5 and step-cut grafting with bioabsorbable screw fixation (SCAS); and group 2, 1-level corpectomy of C5 and strut grafting with anterior plate fixation (SP). For each specimen, the intact underwent a flexibility test first, followed by the instrumented construct. Rotational angles of the C4-C6 segment were measured to study the immediate stability of anterior cervical corpectomy and SCAS, compared with the intact and anterior cervical corpectomy and SP.

RESULTS

Both anterior cervical corpectomy with SCAS and with SP significantly (P < 0.01) decreased the motions of C4-C6 in all 6 degrees of freedom after instrumentation. Compared with anterior cervical corpectomy and SP, anterior cervical corpectomy and SCAS had higher stability (P < 0.05) in extension, and comparable stability (P > 0.05) in flexion and axial rotation, but lower stability (P <or= 0.05) in lateral bending.

CONCLUSION

Anterior cervical corpectomy and SCAS, a novel method of anterior cervical spine decompression and reconstruction, was introduced. The in vitro biomechanical study showed that anterior cervical corpectomy and SCAS had sufficient immediate stability except for the lateral bending, compared with anterior cervical corpectomy and SP, in a 1-level cadaveric cervical spine model. However, an animal experimental in vivo evaluation of anterior cervical corpectomy and SCAS still has to be performed.

Multiplanar cervical spine injury due to head-turned rear impact

STUDY DESIGN

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.

OBJECTIVES

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.

SUMMARY OF BACKGROUND DATA

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.

METHODS

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 (P < 0.05) in intervertebral flexibility above baseline. Injury threshold was the lowest T1 horizontal peak acceleration that caused the injury.

RESULTS

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.

CONCLUSIONS

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.

Intervertebral motion after incremental damage to the posterior structures of the cervical spine

STUDY DESIGN

Compare intervertebral motion after incremental damage to posterior cervical structures in whole cadavers to motion in asymptomatic subjects.

OBJECTIVE

Determine if damage to the posterior structures of the cervical spine can be detected by quantitative analysis of flexion-extension radiographs.

SUMMARY OF BACKGROUND DATA

Simulated damage to the posterior structures of the cervical spine can change intervertebral motion, if intervertebral motion before damage is known. It is not known if intervertebral motion measured from flexion-extension radiographs can be used to detect damage to the posterior structures if motion before damage is not known.

METHODS

Incremental injury to posterior ligaments and facet joints was simulated in 12 whole cadavers. Intervertebral motion was measured from flexion-extension images using validated and clinically applicable software. Measurements were compared to previously published measurements for asymptomatic subjects.

RESULTS

Extensive damage could be simulated in all the cervical spines without intervertebral motion exceeding the 95% confidence limits for asymptomatic subjects. After sectioning all posterior ligaments, destroying both facet joints, and then sectioning the posterior longitudinal ligaments, intervertebral motion exceeded the 95% confidence intervals in 69% of the cadavers. Intervertebral shear decreased with incremental damage to posterior structures.

CONCLUSIONS

Radiographic assessment of the cervical spine may not be sufficient to exclude even extensive damage to the posterior structures of the cervical spine.

Influence of Preload Magnitudes and Orientation Angles on the Cervical Biomechanics

OBJECTIVE

Although a number of in vivo, in vitro, and finite element studies have attempted to delineate the natural biomechanics, injury mechanisms, and surgical techniques of the cervical spine, none has explored the influence of various preload magnitudes and orientations on the biomechanical responses.

METHODS

A nonlinear three-dimensional finite element model of the lower cervical spine (C5-C6) was used for this study. The model was tested under four preload magnitudes and three orientations. For every preload, magnitude, and orientation, pure moments of 1.8 Nm were applied to the superior surface of the moving vertebra (C5) in flexion, extension, lateral bending, and torsion. The resulting rotational motions were obtained and compared against literature data.

RESULTS

The predicted biomechanical responses under the same loading directions varied, depending on the preload magnitudes and orientations. With flexion and extension, increasing the preload magnitudes and varying the C5-C6 orientation in the sagittal plane changed the rotational motions by 1% and 18%, respectively. Under normal orientation and with increasing preload magnitudes, flexion and extension increased, whereas lateral bending and torsion decreased. These changes were found to be influenced by several spinal components: posterior facets, passive ligaments, and stiffening of the intervertebral disc. The predicted responses under the direction of loading varied significantly, depending on the preload magnitudes and orientations. Under fixed preload magnitudes and varying the three types of orientations, rotational motions were not affected under flexion but changed under extension, lateral bending, and axial rotations. Under normal orientation and increasing preload magnitudes, biomechanical responses under flexion and extension increased, whereas lateral bending and torsion decreased. Changes in the predicted responses were found to be influenced by several spinal components: posterior facets, passive ligaments, and stiffening of the intervertebral disc.

CONCLUSION

The findings of the current study were important for the further understanding of the cervical biomechanics during in vitro testing.

Axial load—dependent cervical spinal alterations during simulated upright posture: a comparison of healthy controls and patients with cervical degenerative disease

Object. The objectives of this study were to simulate the upright loading condition in the cervical spine by applying a new compression device during supine posture and to assess intervertebral angles and cross-sectional areas of the spinal cord and dural tube before and during axial compression.

Methods. A magnetic resonance (MR) imaging-compatible device was developed to create axial compression with the patient in the supine position. Lateral radiographs were obtained in upright and supine positions with an axial load of 0% (supine) and by applying a cervical compression device at 7, 10, and 13% of body weight (BW) in 18 control individuals and seven symptomatic patients with cervical degenerative disc disease (DDD). Additionally, cervical MR images acquired in 17 controls and 12 patients were compared before and during an axial load of 8.4% BW in terms of anteroposterior diameter and cross-sectional area of the dural sac.

The supine intervertebral angles with loads of 0, 7, 10, and 13% of the individuals' BW relative to upright posture were −8.1 ± 1.3, −2.3 ± 1.4, 1.3 ± 1.9, and 2.8 ± 2°, respectively. Subsequent axial force was interpolated as 8.9% of BW to simulate upright cervical spine alignment. Under an axial loading similar to that created by the upright posture, the dural sac narrowed at the C5–6 interspace in asymptomatic individuals and at the C6–7 interspace in patients with cervical DDD.

Conclusions. This cervical compression device may be a useful tool to simulate upright cervical spinal alignment. The results of this study help in understanding the pathophysiology of symptoms related to cervical degenerative disorders in upright posture.

Do sagittal plane anatomical variations (angulation) of the cervical facets and C2 odontoid affect the geometrical configuration of the cervical lordosis?

Anthropometric and statistical evaluation of measurements from digitization of 252 lateral cervical radiographs were used to investigate any correlation between radiographic measurements of cervical lordosis with sagittal plane facet angulation, articular pillar height, and inclination of the C2 odontoid with respect to the body of C2. Some researchers have hypothesized that facet and odontoid architecture variations can cause a reduction in cervical lordosis. To evaluate this hypothesis, the posterior aspect of the C2 dens, vertebral body corners, and superior and inferior facet surfaces of C2-C7 were digitized on 252 lateral cervical X-rays to calculate global angle, segmental angles, dens angle, facet angles, and facet height. No correlation between facet angle, articular pillar height, and cervical curve was found. Similarly, no correlation between the sagittal angle of the dens and any angle of cervical curvature was identified. There was correlation between the global ARA C2-C7 angle and the Cobb angles at C1-C7 (r = 0.71) and C2-C7 (r = 0.82). There was correlation between the global inclination of the atlas vertebral angle (APL) and the Cobb angle at C1-C7 (r = 0.66), Cobb angle at C2-C7 (r = 0.39), ARA C2-C7 (r = 0.42), and anterior translation of C2 compared to C7 (r = -0.46). Because no correlation between cervical facet and odontoid architecture and any segmental or global angle of cervical lordosis was found, conservative and surgical rehabilitative techniques aimed at the reduction of sagittal cervical deformities do not need to account for a patient's architecture of the cervical facets nor odontoid.

Kinematics of the subaxial cervical spine in rotation in vivo three-dimensional analysis

STUDY DESIGN

Three-dimensional intervertebral motions of the subaxial cervical spine during head rotation were investigated in healthy volunteers using three-dimensional magnetic resonance imaging (MRI).

OBJECTIVES

To document intervertebral coupled motions of the subaxial cervical spine during rotation.

SUMMARY OF BACKGROUND DATA

In vivo three-dimensional kinematics of the subaxial cervical spine in rotation have not previously been well described, since they are too complicated to follow using conventional radiography or computed tomography techniques.

METHODS

Ten healthy volunteers underwent three-dimensional MRI of the cervical spine in 11 positions with 15 degrees increments during head rotation using a 1.0-T imager. Relative motions of the subaxial cervical spine were calculated by automatically superimposing a segmented three-dimensional MRI of the vertebra in the neutral position over images of each position using volume registration. Three-dimensional motions of adjacent vertebrae were represented with 6 df (6 degrees of freedoms) by Euler angles and translations on the coordinate system defined by Panjabi, then visualized in animations using surface bone models.

RESULTS

Mean axial rotation of the subaxial cervical spine in maximum head rotation (69.5 degrees ) was 2.2 degrees at C2-C3, 4.5 degrees at C3-C4, 4.6 degrees at C4-C5, 4.0 degrees at C5-C6, 1.6 degrees at C6-C7, and 1.5 degrees at C7-T1. Coupled lateral bending with axial rotation was observed in the same direction as axial rotation at all levels (C2-C3, 3.6 degrees ; C3-C4, 5.4 degrees; C4-C5, 5.0 degrees ; C5-C6, 5.3 degrees ; C6-C7, 4.9 degrees ; C7-T1, 1.2 degrees ). Coupled extension with axial rotation occurred in the middle cervical region (C2-C3, 1.4 degrees ; C3-C4, 2.3 degrees ; C4-C5, 1.5 degrees ), while in the lower cervical region, flexion was coupled with axial rotation (C5-C6, 0.9 degrees ; C6-C7, 2.4 degrees ; C7-T1, 3.0 degrees ).

CONCLUSIONS

We investigated intervertebral motions of the subaxial cervical spine during head rotation using a three-dimensional imaging system, and obtained the first accurate depictions of in vivo coupled motion. These findings will be helpful as the basis for understanding abnormal conditions.

Modeling of the sagittal cervical spine as a method to discriminate hypolordosis: results of elliptical and circular modeling in 72 asymptomatic subjects, 52 acute neck pain subjects, and 70 chronic neck pain subjects

STUDY DESIGN

Computer analysis of digitized vertebral body corners on lateral cervical radiographs.

OBJECTIVES

Using elliptical and circular modeling, the geometric shape of the path of the posterior bodies of C2-C7 was sought in normal, acute pain, and chronic pain subjects. To determine the least squares error per point for paths of geometric shapes, minor axis to major axis elliptical ratios (b/a), Cobb angles, sagittal balance of C2 above C7, and posterior tangent segmental and global angles.

SUMMARY OF BACKGROUND DATA

When restricted to cervical lordotic configurations, normal, acute pain, and chronic pain subjects have not been compared for similarities or differences of these parameters. Conventional Cobb angles provide only a comparison of the endplates of the distal vertebrae, while geometric modeling provides the shape of the entire sagittal curves, the orientation of the spine, and segmental angles.

METHODS

Radiographs of 72 normal subjects, 52 acute neck pain subjects, and 70 chronic neck pain subjects were digitized. For normal subjects, the inclusion criteria were no kyphotic cervical segments, no cranial-cervical symptoms, and less than +/- 10 mm horizontal displacement of C2 above C7. In pain subjects, inclusion criteria were no kyphotic cervical segments and less than 25 mm of horizontal displacement of C2 above C7. Measurements included segmental angles, global angles of lordosis (C1-C7 and C2-C7), height-to-length ratios, anterior weight bearing, and from modeling, circular center, and radius of curvature.

RESULTS

In the normal group, a family of ellipses was found to closely approximate the posterior body margins of C2-C7 with a least squares error of less than 1 mm per vertebral body point. The only ellipse/circle found to include T1, with a least squares error of less than 1 mm, was a circle. Compared with the normal group, the pain group's mean radiographic angles were reduced and the radius of curvature was larger. For normal, acute, and chronic pain groups, the mean angles between posterior tangents on C2-C7 were 34.5 degrees, 28.6 degrees, and 22.0 degrees, C2-C7 Cobb angles were 26.8 degrees, 16.5 degrees, and 12.7 degrees, and radius of curvature were r = 132.8 mm, r = 179 mm, and r = 245.4 mm, respectively.

CONCLUSIONS

The mean cervical lordosis for all groups could be closely modeled with a circle. Pain groups had hypolordosis and larger radiuses of curvature compared with the normal group. Circular modeling may be a valuable tool in the discrimination between normal lordosis and hypolordosis in normal and pain subjects.

In vitro biomechanics of cervical disc arthroplasty with the ProDisc-C total disc implant

An in vitro biomechanical study was conducted to compare the effects of disc arthroplasty and anterior cervical fusion on cervical spine biomechanics in a multilevel human cadaveric model. Three spine conditions were studied: harvested, single-level cervical disc arthroplasty, and single-level fusion. A programmable testing apparatus was used that replicated physiological flexion/extension, lateral bending, and axial rotation. Measurements included vertebral motion, applied load, and bending moments. Relative rotations at the superior, treated, and inferior motion segment units (MSUs) were normalized with respect to the overall rotation of those three MSUs and compared using a one-way analysis of variance with Student–Newman–Keuls test (p < 0.05). Simulated fusion decreased motion across the treated site relative to the harvested and disc arthroplasty conditions. The reduced motion at the treated site was compensated at the adjacent segments by an increase in motion. For all modes of testing, use of an artificial disc prosthesis did not alter the motion patterns at either the instrumented level or adjacent segments compared with the harvested condition, except in extension.

An improved biomechanical testing protocol for evaluating spinal arthroplasty and motion preservation devices in a multilevel human cadaveric cervical model

Object

An experimental study was performed to determine the biomechanical end-mounting configurations that replicate in vivo physiological motion of the cervical spine in a multiple-level human cadaveric model. The vertebral motion response for the modified testing protocol was compared to in vivo motion data and traditional pure-moment testing methods.

Methods

Biomechanical tests were performed on fresh human cadaveric cervical spines (C2–T1) mounted in a programmable testing apparatus. Three different end-mounting conditions were studied: pinned–pinned, pinned–fixed, and translational/pinned–fixed. The motion response of the individual segmental vertebral rotations was statistically compared using one-way analysis of variance and Student-Newman-Keuls tests (p < 0.05 unless otherwise stated) to determine differences in the motion responses for different testing methods.

Conclusions

A translational/pinned–fixed mounting configuration induced a bending-moment distribution across the cervical spine, resulting in a motion response that closely matched the in vivo case. In contrast, application of pure-moment loading did not reproduce the physiological response and is less suitable for studying disc arthroplasty and nonfusion devices.

Biomechanical testing of an artificial cervical joint and an anterior cervical plate

An in vitro biomechanical study was conducted to determine the effects of fusion and nonfusion anterior cervical instrumentation on cervical spine biomechanics in a multilevel human cadaveric model. Three spine conditions were studied: harvested, single-level artificial cervical joint, and single-level graft with anterior cervical plate. A programmable testing apparatus was used that replicated physiologic flexion/extension and lateral bending. Measurements included vertebral motion, applied load, and bending moments. Relative rotations at the superior, implanted, and inferior motion segment units (MSUs) were normalized with respect to the overall rotation of those three MSUs and compared using a one-way analysis of variance (P < 0.05). Application of an anterior cervical plate decreased motion across the fusion site relative to the harvested and artificial joint spine conditions. The reduced motion was compensated for by an increase in motion at the adjacent segments. Use of an artificial cervical joint did not alter the motion patterns at either the instrumented level or the adjacent segments compared with the harvested condition for all modes of testing.