Lateral neck injury assessments in side impact using post mortem human subject tests.

Current neck injury criteria are based on matching upper cervical spine injuries from piglet tests to airbag deployment loads and pairing kinematics from child dummies. These "child-based" scaled data together with adult human cadaver tolerances in axial loading are used to specify neck injury thresholds in axial compression and tension, and flexion and extension moment about the occipital condyles; no thresholds are specified for any other force or moment including lateral bending. The objective of this study was to develop a testing methodology and to determine the lateral bending moment injury threshold under coronal loading. Post mortem human subjects (PMHS) were used. Specimens consisted of whole body and isolated head-neck complexes with intact musculature. Intact specimen positioning included: sitting PMHS upright on a rigid seat, supporting the torso by a plate, maintaining Frankfurt plane horizontal. Isolated head-neck complexes were fixed at T1 with the occiput connected via a custom apparatus to a testing device to induce lateral bending motion. Head angular and linear accelerations and angular velocities were computed using a pyramid nine accelerometer package on the head; specimen-specific physical properties including center of gravity and moments of inertia in the three-dimensions; and equations of equilibrium. These data were used to determine neck loads at the occipital condyles. No specimens sustained injuries, identified by palpation, x-rays, CT, and autopsy. Results from 24 tests indicated that PMHS head-neck complexes can tolerate 75 Nm of coronal moment at low axial load without failure, and this level may be used as an initial estimate of the injury reference value under lateral loading to the human head-neck complex.

Biomechanical properties of human thoracic spine disc segments.

BACKGROUND: The objective was to determine the age-dependent compressive and tensile properties of female and male thoracic spine segments using postmortem human subjects (PMHS). MATERIALS AND METHODS: Forty-eight thoracic disc segments at T4-5, T6-7, T8-9, and T10-11 levels from 12 PMHS T3-T11 spinal columns were divided into groups A and B based on specimen age and loaded in compression and tension. Stiffness and elastic modulus were computed. Stiffness was defined as the slope in the linear region of the force-displacement response. Elastic modulus was defined as the slope of the stress strain curve. Analysis of Variance (ANOVA) was used to determine significant differences (P<0.05) in the disc cross-sectional area, stiffness, and elastic modulus based on gender, spinal level, and group. RESULTS: Specimen ages in group A (28 ± 8 years) were significantly lower than in group B (70 ± 7 years). Male discs had significantly greater area (7.2 ± 2.0 sq cm) than female discs (5.9 ± 1.8 sq cm). Tensile and compressive stiffness values were significantly different between the two age groups, but not between gender and level. Specimens in group A had greater tensile (486 ± 108 N/mm) and compressive (3300 ± 642 N/mm) stiffness values compared to group B specimens (tension: 397 ± 124 N/mm, compression: 2527 ± 734 N/mm). Tensile and compressive elastic modulus values depended upon age group and gender, but not on level. Group A specimens had significantly greater tensile and compressive moduli (2.9 ± 0.8 MPa, 19.5 ± 4.1 MPa) than group B specimens (1.7 ± 0.6 MPa, 10.6 ± 3.4 MPa). Female specimens showed significantly greater tensile and compressive moduli (2.6 ± 1.0 MPa, 16.6 ± 6.4 MPa) than male specimens (2.0 ± 0.7 MPa, 13.7 ± 5.0 MPa). DISCUSSION: Using the two groups to represent "young" and "old" specimens, this study showed that the mechanical response decreases in older specimens, and the decrease is greater in compressive than distractive properties. While the decrease is expected, the relative change between the two modes of loading has not been reported. Another conclusion from the study is that the mechanical properties depend on gender, although not as decisive due to sample size.

Persistent first intersegmental vertebral artery in association with type II odontoid fracture: surgical treatment utilizing a novel C1 posterior arch screw: case report.

OBJECTIVE AND IMPORTANCE: Posterior cervical stabilization for cervical fractures is common, and numerous techniques for fixation have been described. This case describes the novel usage of C1 laminar screws due to a persistent intersegmental artery and congenital fusion of C2-C3. CLINICAL PRESENTATION: A 64-year-old woman presented with loss of consciousness after falling down a flight of stairs. Initial CT scan showed a type II odontoid fracture with significant malalignment, as well as an anomalous congenital fusion of C2-C3 and degenerative spondylolisthesis of C3 to C4. CT angiogram demonstrated bilateral persistent first intersegmental arteries coursing through the C1-C2 neural foramina. INTERVENTION: The patient underwent C2 fracture reduction and posterior C1-C4 fusion. C1 posterior arch screws were placed due to the patient's anomalous vertebral artery location. The construct was anchored caudally by C2 pars interarticularis screws and C4 lateral mass screws. The patient experienced an excellent neurologic and radiographic outcome at 12.5 months. CONCLUSION: Posterior fixation for fractures of the cervical spine is common; however, the use of C1 posterior arch screws for fractures has not previously been described. The presence of a persistent intersegmental course of the vertebral artery, a rare but reported anomaly, should be regarded as a contraindication to placement of C1 lateral mass screws and necessitates careful consideration of the available surgical options.

Biomechanical implications of extending occipitocervical instrumentation to include the subaxial spine.

BACKGROUND: No clear biomechanical data exist regarding where to place the caudal end of a screw-rod occipitocervical instrumentation construct. OBJECTIVE: This study examines whether range of motion (ROM) from the occiput to C2 is altered by subaxial extension of occipitocervical instrumentation constructs. METHODS: Cadaver specimens underwent intact biomechanical testing followed by destabilization via an odontoid osteotomy. Subsequent biomechanical testing was performed of four occipitocervical constructs: occipital plate + C2 pars screws (construct 1), occipital plate + C2 pars screws + C4 lateral mass screws (construct 2), occipital plate + C1-C2 transarticular screws (construct 3), and occipital plate + C1-C2 transarticular screws + C4 lateral mass screws (construct 4). RESULTS: All constructs significantly reduced occiput-C2 ROM in all loading modes compared with the intact cervical spine, with one exception (construct 1, lateral bending). No significant ROM differences were noted when C4 lateral mass screws (construct 4) were added to construct 3. Addition of C4 lateral mass screws (construct 2) to construct 1 decreased the ROM in the flexion mode only. No significant ROM differences were seen between construct 2 and construct 3 in any loading mode. CONCLUSION: The addition of subaxial instrumentation to occipitocervical instrumentation constructs in this study decreased occiput-C2 ROM only when the construct was anchored by C2 pars screws and only in flexion. Screws that cross the C1 to C2 articulation provide stable fixation when combined with an occipital plate, and the addition of subaxial instrumentation to this construct for stabilizing the occipitocervical junction does not significantly decrease ROM.

Trends and variations in cervical spine surgery in the United States: Medicare beneficiaries, 1992 to 2005.

STUDY DESIGN: Retrospective cohort. OBJECTIVE: To describe population-based trends and variations in surgery for degenerative changes of the cervical spine among Medicare beneficiaries, 1992 to 2005. SUMMARY OF BACKGROUND DATA: Degenerative changes of the cervical spine are seen radiographically in over half of the population aged 55 years or greater, and rates of cervical spine surgery have increased over time. Prior studies examined anterior cervical discectomy and fusion procedures in the general population up to 1999, and showed regional variations in care, with the highest rates in the South. The purpose of this study is to explore population-based trends and variations in surgery for degenerative changes of the cervical spine in the elderly. METHODS: From 1992 to 2005, hospital admissions associated with surgery for degenerative changes of the cervical spine were selected from Medicare Part A using ICD9 CM codes. We excluded beneficiaries under 65 years of age, in a capitated health plan, or enrolled for Social Security Disability Income. Diagnosis and type of surgery were defined using ICD9 CM codes. Rates were directly adjusted to age, sex, and race of 2005 Medicare beneficiaries. RESULTS: Of 156,820 qualifying admissions, 52% were men, 88% were white, and 41% were aged 65 to 69 years. The most common primary diagnosis and procedure were cervical spondylosis with myelopathy (36%) and fusion (70%); of the fusions, 58% were anterior. Rates of cervical fusions rose from 1992 to 2005 even after adjustment for age, sex, and race (14.7 to 45 cervical fusions/100,000 beneficiaries). Rates of cervical fusions varied by geographic location, with the highest rates in the Northwest and South Central regions. In 2005, the highest rate of cervical fusions was 140/100,000 beneficiaries in Idaho, compared with 4/100,000 beneficiaries in Washington, DC. CONCLUSION: In the elderly, adjusted rates of cervical spine fusions rose 206% from 1992 to 2005. Marked geographic variation was noted. Future studies should evaluate the efficacy and complications associated with these procedures in the elderly, and better define surgical indications and patient outcome.

Comparison of contemporary occipitocervical instrumentation techniques with and without C1 lateral mass screws.

OBJECTIVE: This study was designed to test the kinematic properties of three occiput-C2 instrumentation constructs with and without supplemental rigid C1 fixation. The results are compared with intact specimens and with constructs incorporating contemporary cabling techniques. METHODS: Five unembalmed human cadaver specimens underwent range of motion (ROM) testing in the intact condition, followed by destabilization with odontoid osteotomy. Destabilized specimens then underwent ROM testing with each of seven occipitocervical instrumentation constructs, all incorporating occipital screws: C1 and C2 sublaminar cables with cable connectors, C2 pars screws +/- C1 lateral mass screws, C2 lamina screws +/- C1 lateral mass screws, and C1-C2 transarticular screws +/- C1 lateral mass screws. RESULTS: All seven constructs demonstrated significantly lower ROM in all loading modes than intact specimens (P < 0.05). With a single exception, the addition of C1 lateral mass screws to the screw-based constructs produced no significant change in ROM in any of the loading modes. Compared with intact specimens, constructs anchored by C1-C2 transarticular screws demonstrated the greatest decrease in ROM, and those anchored by sublaminar cables demonstrated the least decrease in ROM. CONCLUSION: Any of the tested screw-based constructs are likely to provide adequate support for the patient with an unstable craniocervical junction. Therefore, the choice of construct should be based on anatomic considerations. The routine incorporation of C1 lateral mass screws into occipitocervical instrumentation constructs does not seem necessary.

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.

Anatomical, biomechanical, and practical considerations in posterior occipitocervical instrumentation.

BACKGROUND CONTEXT: Patients with cervical myelopathy secondary to craniocervical instability commonly present with spinal cord compression secondary to a combination of static forces and gross instability. Craniocervical arthrodesis is therefore indicated in the treatment of the majority of these conditions. In order to facilitate arthrodesis, techniques for occipitocervical instrumentation have been developed. PURPOSE: To systematically review the anatomy, biomechanics, and practical considerations involved in posterior occipitocervical instrumentation. STUDY DESIGN: Retrospective literature review. PATIENT SAMPLE: Not applicable. OUTCOME MEASURES: Not applicable. METHODS: Retrospective literature review. RESULTS: The anatomic elements of the craniocervical junction include the occipital bone, occipital condyles, atlas (C1), and axis (C2). The occiput-C1 and C1-C2 motion segments possess unique mechanical properties. Occipitocervical instrumentation constructs are comprised of points of fixation and longitudinal elements, each with characteristic strengths and weaknesses. CONCLUSIONS: Analysis of the anatomy, available points of fixation, and the movements to be controlled leads to the choice of a longitudinal element which can control movement by incorporating the strongest points of fixation. By going through this process for each patient, an informed decision may be made regarding the optimal occipitocervical instrumentation construct.

Validation of a clinical finite element model of the human lumbosacral spine.

Very few finite element models on the lumbosacral spine have been reported because of its unique biomechanical characteristics. In addition, most of these lumbosacral spine models have been only validated with rotation at single moment values, ignoring the inherent nonlinear nature of the moment-rotation response of the spine. Because a majority of lumbar spine surgeries are performed between L4 and S1 levels, and the confidence in the stress analysis output depends on the model validation, the objective of the present study was to develop a unique finite element model of the lumbosacral junction. The clinically applicable model was validated throughout the entire nonlinear range. It was developed using computed tomography scans, subjected to flexion and extension, and left and right lateral bending loads, and quantitatively validated with cumulative variance analyses. Validation results for each loading mode and for each motion segment (L4-L5, L5-S1) and bisegment (L4-S1) are presented in the paper.

Single-stage thoracolumbar vertebrectomy with circumferential reconstruction and arthrodesis: surgical technique and results in 15 patients.

OBJECTIVE: Circumferential reconstruction and arthrodesis can be necessary after thoracolumbar vertebrectomy. The authors describe a technique for single-stage thoracolumbar vertebrectomy with circumferential reconstruction and arthrodesis. The surgical results using this technique are reviewed. METHODS: Fifteen patients ranging from 14 to 75 years of age underwent single-stage thoracolumbar vertebrectomy with circumferential reconstruction and arthrodesis. The vertebrectomy was performed through a posterior midline approach. Anterior column reconstruction was performed with expandable or nonexpandable cages. Anterior and posterolateral arthrodeses used autograft. Posterior segmental instrumentation was used in all cases. RESULTS: Fifteen procedures have been performed to date, 4 for tumor and 11 for fracture. The range of treated levels was T4 to L2 (7 thoracic spine and 8 lumbar spine levels). One patient was incomplete preoperatively (Frankel Grade C) and improved to being intact postoperatively (Frankel Grade E), another improved from Frankel Grade C to Frankel Grade D. All other patients were neurologically unchanged postoperatively. Mean operative time was 4.0 hours. Average blood loss was 1100 ml. Average number of levels fused was 5.8 (range 4-9). There were four complications: one delayed transient neurological deficit after deformity correction, one infection, one postoperative myocardial infarction, and one hardware failure. All patients were treated and had a good recovery. CONCLUSION: The authors present a method for thoracolumbar vertebrectomy, circumferential reconstruction, and arthrodesis performed in a single stage, solely via a posterior approach. This is an alternative to anterior (i.e., thoracoabdominal and retroperitoneal) and lateral (i.e., lateral extracavitary) approaches that can be used for circumferential reconstruction and arthrodesis. Potential advantages and pitfalls are discussed.

Cervical ventral epidural pressure response to graded spinal canal compromise and spinal motion.

STUDY DESIGN: A laboratory investigation using a feline model of graded ventral spinal canal compromise was performed. OBJECTIVE: To quantify the effects of graded ventral spinal canal compromise, both in the static condition and in combination with passive spinal motion, on cervical ventral epidural pressure (CVEP). The CVEP effects of laminectomy are also investigated. SUMMARY OF BACKGROUND DATA: Spinal canal compromise, both in the static condition and in combination with passive spinal motion, has been implicated as a cause of spinal cord dysfunction. METHODS: Seventeen cats underwent anterior corpectomy of C3 and placement of a flexible ventral graded compression device incorporating a pressure transducer. Ten animals also underwent laminectomy of C3. The implant was advanced stepwise into the spinal canal. CVEP was measured, at each degree of canal compromise, in the flexed, extended, and neutral positions, as well as during neck movement. RESULTS: CVEP rose as a function of spinal canal compromise. In animals without laminectomy, mean CVEP was higher in the extended position and lower in the flexed position than in the neutral position. Mean CVEP during continuous passive neck movement was found to be higher than mean CVEP in the neutral position. Laminectomy was found to lower CVEP during all conditions examined, although substantial rises in CVEP were still observed in the presence of a residual ventral mass. All reported differences were statistically significant (P < 0.05). CONCLUSIONS: CVEP is elevated by both spinal canal compromise and spinal motion.

Reversal of neurologic deficit in an adult gorilla with severe symptomatic lumbar stenosis.

STUDY DESIGN: A case report is presented. OBJECTIVE: To describe a case of severe symptomatic lumbar stenosis in an adult gorilla successfully treated with decompressive surgery. SUMMARY OF BACKGROUND DATA: Although spinal degenerative disease is thought to be less common in apes than in humans, gorillas and bonobos are known to have higher rates of disease than other African apes. METHODS: A 42-year-old female gorilla presented with a history of progressive lower extremity weakness. She was unable to ambulate normally or climb, pulling herself around with her arms. Her keeper did not believe she was incontinent. Physical examination revealed that ankle plantarflexion and dorsiflexion were absent on the right and weak on the left. Radiographs revealed disc space narrowing with osteophytes at the last mobile segment. Myelogram and computed tomography revealed severe spinal stenosis at the last two mobile segments. Under general anesthesia, a standard laminectomy and bilateral lateral recess decompression were performed. RESULTS: There were no anesthetic or wound complications. The gorilla was noted to have improved ankle strength bilaterally on postoperative day 1. On postoperative day 3, the animal began to walk and climb. At 20 months, the animal is back to her neurologic baseline. CONCLUSIONS: Symptomatic lumbar degenerative disease with neurologic compromise may be treated surgically, with excellent outcome, in the adult gorilla. Though this is an unusual situation, knowledge that this is a potentially reversible condition may benefit other gorillas in captivity.

Retroperitoneal lateral lumbar interbody fusion with titanium threaded fusion cages.

OBJECT: Intertransverse arthrodesis in which instrumentation is placed is associated with an excellent fusion rate; however, treatment of patients with symptomatic nonunion presents a number of difficulties. Revision posterior and traditional anterior procedures are associated with methodological problems. For example, in the latter, manipulation of the major vessels from L-2 to L-4 may be undesirable. The authors describe a method for performing retroperitoneal lumbar interbody fusion (LIF) in which a threaded cage is placed from L-2 through L-5 via a lateral trajectory, and they also detail a novel technique for implanting a cage from L-5 to S-1 via an oblique trajectory. Although they present data obtained over a 2-year period in the study of 15 patients, the focus of this report is primarily on describing the surgical procedure. METHODS: The lateral lumbar spine was exposed via a standard retroperitoneal approach. Using the anterior longitudinal ligament as a landmark, the L2-3 through L4-5 levels were fitted with instrumentation via a true lateral trajectory; the L5-S1 level was fitted with instrumentation via an oblique trajectory. A single cage was placed at each instrumented level. Fifteen symptomatic patients in whom previous lumbar fusion had failed underwent retroperitoneal LIF. Thirty-eight levels were fitted with instrumentation. There have been no instrumentation-related failures, and fusion has occurred at 37 levels during the 2-year postoperative period. CONCLUSIONS: The use of retroperitoneal LIF in which threaded fusion cages are used avoids the technical difficulties associated with repeated posterior procedures. In addition, it allows L2-S1 instrumentation to be placed anteriorly via a single surgical approach. This construct has been shown to be biomechanically sound in animal models, and it appears to be a useful alternative for the management of failed multilevel intertransverse arthrodesis.