Contribution of disc degeneration to osteophyte formation in the cervical spine: a biomechanical investigation.

Cervical spine disorders such as spondylotic radiculopathy and myelopathy are often related to osteophyte formation. Bone remodeling experimental-analytical studies have correlated biomechanical responses such as stress and strain energy density to the formation of bony outgrowth. Using these responses of the spinal components, the present study was conducted to investigate the basis for the occurrence of disc-related pathological conditions. An anatomically accurate and validated intact finite element model of the C4-C5-C6 cervical spine was used to simulate progressive disc degeneration at the C5-C6 level. Slight degeneration included an alteration of material properties of the nucleus pulposus representing the dehydration process. Moderate degeneration included an alteration of fiber content and material properties of the anulus fibrosus representing the disintegrated nature of the anulus in addition to dehydrated nucleus. Severe degeneration included decrease in the intervertebral disc height with dehydrated nucleus and disintegrated anulus. The intact and three degenerated models were exercised under compression, and the overall force-displacement response, local segmental stiffness, anulus fiber strain, disc bulge, anulus stress, load shared by the disc and facet joints, pressure in the disc, facet and uncovertebral joints, and strain energy density and stress in the vertebral cortex were determined. The overall stiffness (C4-C6) increased with the severity of degeneration. The segmental stiffness at the degenerated level (C5-C6) increased with the severity of degeneration. Intervertebral disc bulge and anulus stress and strain decreased at the degenerated level. The strain energy density and stress in vertebral cortex increased adjacent to the degenerated disc. Specifically, the anterior region of the cortex responded with a higher increase in these responses. The increased strain energy density and stress in the vertebral cortex over time may induce the remodeling process according to Wolff's law, leading to the formation of osteophytes.

A critical appraisal of the reporting of the National Acute Spinal Cord Injury Studies (II and III) of methylprednisolone in acute spinal cord injury.

From the beginning, the reporting of the results of National Acute Spinal Cord Injury Studies (NASCIS) II and III has been incomplete, leaving clinicians in the spinal cord injury (SCI) community to use or avoid using methylprednisolone in acute SCI on the basis of faith rather than a publicly developed scientific consensus. NASCIS II was initially reported by National Institutes of Health announcements, National Institutes of Health facsimiles to emergency room physicians, and the news media. The subsequent report in the New England Journal of Medicine implied that there was a positive result in the primary efficacy analysis for the entire 487 patient sample. However, this analysis was in fact negative, and the positive result was found only in a secondary analysis of the subgroup of patients who received treatment within 8 hours. In addition, that subgroup apparently had only 62 patients taking methylprednisolone and 67 receiving placebo. The NASCIS II and III reports embody specific choices of statistical methods that have strongly shaped the reporting of results but have not been adequately challenged or or even explained. These studies show statistical artifacts that call their results into question. In NASCIS II, the placebo group treated before 8 hours did poorly, not only when compared with the methylprednisolone group treated before 8 hours but even when compared with the placebo group treated after 8 hours. Thus, the positive result may have been caused by a weakness in the control group rather than any strength of methylprednisolone. In NASCIS III, a randomization imbalance occurred that allocated a disproportionate number of patients with no motor deficit (and therefore no chance for recovery) to the lower dose control group. When this imbalance is controlled for, much of the superiority of the higher dose group seems to disappear. The NASCIS group's decision to admit persons with minor SCIs with minimal or no motor deficit not only enables statistical artifacts it complicates the interpretation of results from the population actually sampled. Perhaps one half of the NASCIS III sample may have had at most a minor deficit. Thus, we do not know whether the results of these studies reflect the severely injured population to which they have been applied. The numbers, tables, and figures in the published reports are scant and are inconsistently defined, making it impossible even for professional statisticians to duplicate the analyses, to guess the effect of changes in assumptions, or to supply the missing parts of the picture. Nonetheless, even 9 years after NASCIS II, the primary data have not been made public. The reporting of the NASCIS studies has fallen far short of the guidelines of the ICH/FDA and of the Evidence-based Medicine Group. Despite the lucrative "off label" markets for methylprednisolone in SCI, no Food and Drug Association indication has been obtained. There has been no public process of validation. These shortcomings have denied physicians the chance to use confidently a drug that many were enthusiastic about and has left them in an intolerably ambiguous position in their therapeutic choices, in their legal exposure, and in their ability to perform further research to help their patients.

Directed and enhanced neurite growth with pulsed magnetic field stimulation.

Pulsed magnetic field (PMF) stimulation was applied to mammalian neurons in vitro to influence axonal growth and to determine whether induced current would direct and enhance neurite growth in the direction of the current. Two coils were constructed from individual sheets of copper folded into a square coil. Each coil was placed in a separate water-jacketed incubator. One was energized by a waveform generator driving a power amplifier, the other was not energized. Whole dorsal root ganglia (DRG) explant cultures from 15-day Sprague-Dawley rat embryos were established in supplemented media plus nerve growth factor (NGF) at concentrations of 0-100 ng/mL on a collagen-laminin substrate. Dishes were placed at the center of the top and bottom of both coils, so that the DRG were adjacent to the current flowing in the coil. After an initial 12 h allowing DRG attachment to the substrate floor, one coil was energized for 18 h, followed by a postexposure period of 18 h. Total incubation time was 48 h for all DRG cultures. At termination, DRG were histochemically stained for visualization and quantitative analysis of neurite outgrowth. Direction and length of neurite outgrowth were recorded with respect to direction of the current. PMF exposed DRG exhibited asymmetrical growth parallel to the current direction with concomitant enhancement of neurite length. DRG cultures not PMF exposed had a characteristic radial pattern of neurite outgrowth. These results suggest that PMF may offer a noninvasive mechanism to direct and promote nerve regeneration.

Biomechanical study of pediatric human cervical spine: a finite element approach.

Although considerable effort has been made to understand the biomechanical behavior of the adult cervical spine, relatively little information is available on the response of the pediatric cervical spine to external forces. Since significant anatomical differences exist between the adult and pediatric cervical spines, distinct biomechanical responses are expected. The present study quantified the biomechanical responses of human pediatric spines by incorporating their unique developmental anatomical features. One-, three-, and six-year-old cervical spines were simulated using the finite element modeling technique, and their responses computed and compared with the adult spine response. The effects of pure overall structural scaling of the adult spine, local component developmental anatomy variations that occur to the actual pediatric spines, and structural scaling combined with local component anatomy variations on the responses of the pediatric spines were studied. Age- and component-related developmental anatomical features included variations in the ossification centers, cartilages, growth plates, vertebral centrum, facet joints, and annular fibers and nucleus pulposus of the intervertebral discs. The flexibility responses of the models were determined under pure compression, pure flexion, pure extension, and varying degrees of combined compression-flexion and compression-extension. The pediatric spine responses obtained with the pure overall (only geometric) scaling of the adult spine indicated that the flexibilities consistently increase in a uniform manner from six- to one-year-old spines under all loading cases. In contrast, incorporation of local anatomic changes specific to the pediatric spines of the three age groups (maintaining the same adult size) not only resulted in considerable increases in flexibilities, but the responses also varied as a function of the age of the pediatric spine and type of external loading. When the geometric scaling effects were added to these spines, the increases in flexibilities were slightly higher; however, the pattern of the responses remained the same as found in the previous approach. These results indicate that inclusion of developmental anatomical changes characteristic of the pediatric spines has more of a predominant effect on biomechanical responses than extrapolating responses of the adult spine based on pure overall geometric scaling.

A box coil for the stimulation of biological tissue and cells in vitro and in vivo by pulsed magnetic fields.

An alternative coil system to the Helmoholtz coil-pair is described for the stimulation of biological tissue and cells: a relatively large box coil made of copper or aluminum sheet stock. The design is based on the principal determinant of the induced electric field, namely, the magnetic vector potential (A), in the equation, [formula: see text]. The second term in the equation is needed when boundaries of the conducting medium are in close proximity to the region of interest, such as in a culture dish. An electric surface charge builds up on the boundaries to generate an electric field which cancels [formula: see text] at the surface. The effectiveness of the new coil is demonstrated in a study of the outgrowth enhancement of axons from rat embryonic dorsal root ganglia.

Economics of managed care in spinal cord injury.

OBJECTIVE: To determine and describe trends in economic variables related to the care of individuals with spinal cord injury (SCI) and significant changes in these trends coincident with major developments in medical care cost control. DATA SOURCES: Data from the National Spinal Cord Injury Statistical Center (NSCISC) database were used to review the economic trends in SCI management from 1973 to 1998 and their relation to managed care and other health care cost-containment measures. A panel of SCI health care specialists was interviewed to determine the appropriate data variables to be reviewed. The Shepherd Center Care Health Management Program, Atlanta, GA, is presented as an example of a fiscally successful managed care program for patients with SCI. DATA EXTRACTION: Data from the NSCISC database for the years studied were extracted and converted to a form suitable for analysis by means of the statistical software SAS. DATA SYNTHESIS: Statistical techniques included multiple regression analysis, logistic regression analysis, and model selection methods. CONCLUSIONS: Trends in economic variables, in the care of individuals with SCI show changes coincident with the introduction of Diagnostic Related Groups (DRGs) and managed care as models for provider reimbursement. Significant changes occurred in acute care charges, rehabilitation charges, length of stay, rehospitalization 1 year postinjury, time from injury to admission to a Model System, and discharges to a nursing home.

Biomechanical effect of anterior cervical spine fusion on adjacent segments.

The biomechanical effects of superior (C4-C5) and inferior (C5-C6) level fusions with different graft materials on the adjacent unaltered components were quantified using an anatomically accurate and experimentally validated C4-C5-C6 finite element model. Smith-Robinson and Bailey-Badgley fusion procedures were analyzed with five different types of inter-body fusion materials with varying stiffnesses. Intact and surgically altered finite element models were subjected to physiologic compression, flexion, extension and lateral bending. The external axial and angular stiffness, and the internal unaltered intervertebral disc (C5-C6 for the superior and C4-C5 for inferior fusion) and C5 vertebral body stresses were determined. The superior level fusion resulted in the highest increase in external response in lateral bending for all implant materials in both surgical procedures. In contrast, the inferior level fusion produced a higher increase in the C4-C5 disc and C5 vertebral body stresses in compression than the superior level fusion in both surgical procedures. The increased internal stress responses reflecting the changes in the load-sharing following inferior level fusion may explain clinical observations such as enhanced degeneration subsequent to surgery. Because of the inclusion of three levels in the present multi-segment finite element model, it was possible to determine these responses in the unaltered adjacent components of the cervical spine.

Finite element modeling of the cervical spine: role of intervertebral disc under axial and eccentric loads.

An anatomically accurate, three-dimensional, nonlinear finite element model of the human cervical spine was developed using computed tomography images and cryomicrotome sections. The detailed model included the cortical bone, cancellous core, endplate, lamina, pedicle, transverse processes and spinous processes of the vertebrae; the annulus fibrosus and nucleus pulposus of the intervertebral discs; the uncovertebral joints; the articular cartilage, the synovial fluid and synovial membrane of the facet joints; and the anterior and posterior longitudinal ligaments, interspinous ligaments, capsular ligaments and ligamentum flavum. The finite element model was validated with experimental results: force-displacement and localized strain responses of the vertebral body and lateral masses under pure compression, and varying eccentric anterior-compression and posterior-compression loading modes. This experimentally validated finite element model was used to study the biomechanics of the cervical spine intervertebral disc by quantifying the internal axial and shear forces resisted by the ventral, middle, and dorsal regions of the disc under the above axial and eccentric loading modes. Results indicated that higher axial forces (compared to shear forces) were transmitted through different regions of the disc under all loading modes. While the ventral region of the disc resisted higher variations in axial force, the dorsal region transmitted higher shear forces under all loading modes. These findings may offer an insight to better understand the biomechanical role of the human cervical spine intervertebral disc.

Dynamic analysis of penetrating trauma.

BACKGROUND: Whereas considerable literature exists on the wounding mechanics of high velocity projectiles in the military domain, there is a paucity of such data from projectiles routinely encountered in the civilian population in the United States. This study was undertaken to develop a methodology and to determine the dynamics of penetrating trauma secondary to low velocity projectiles (200-300 m/sec). To demonstrate the feasibility of the methodology and the experimental protocol, two markedly different projectiles were chosen in the study. METHODS: Two projectiles were discharged into a human tissue simulant; one projectile was smooth and the other was of the expansion type. High-speed video photographic analysis and synchronized trigger techniques were used to describe the path of the projectile during its travel within the simulant. The temporal transient and residual profiles demonstrating the "wound involvement" were computed. RESULTS: Results indicated a stark contrast between the two cases. There was a ratio of approximately three-to-one in the maximum wound involvement due to penetration. Transient wave oscillations during penetration and perforation of the projectile from the tissue simulant demonstrated significant differences in amplitudes and time durations. In addition, the residual wound involvement profiles indicated differences in the injury potential. CONCLUSIONS: This study has provided an experimental methodology to delineate the temporal dynamic behavior of penetrating projectiles. To fully quantify and differentiate the dynamic differences in the temporal behaviors of the numerous available projectiles (with various combinations in design, type of equipment, and discharge), further research in this area is clearly necessary. The present protocol lends itself to be used to systematically analyze all these behaviors. Quantified data may assist clinical personnel in the management of penetrating trauma.

Human head-neck biomechanics under axial tension.

A significant majority of cervical spine biomechanics studies has applied the external loading in the form of compressive force vectors. In contrast, there is a paucity of data on the tensile loading of the neck structure. These data are important as the human neck not only resists compression but also has to withstand distraction due to factors such as the anatomical characteristics and loading asymmetry. Furthermore, evidence exists implicating tensile stresses to be a mechanism of cervical spinal cord injury. Recent advancements in vehicular restraint systems such as air bags may induce tension to the neck in adverse circumstances. Consequently, this study was designed to develop experimental methodologies to determine the biomechanics of the human cervical spinal structures under distractive forces. A part-to-whole approach was used in the study. Four experimental models from 15 unembalmed human cadavers were used to demonstrate the feasibility of the methodology. Structures included isolated cervical spinal cords, intervertebral disc units, skull to T3 preparations, and intact unembalmed human cadavers. Axial tensile forces were applied, and the failure load and distraction were recorded. Stiffness and energy absorbing characteristics were computed. Maximum forces for the spinal cord specimens were the lowest (278 N +/- 90). The forces increased for the intervertebral disc (569 N +/- 54). skull to T3 (1555 N +/- 459), and intact human cadaver (3373 N +/- 464) preparations, indicating the load-carrying capacities when additional components are included to the experimental model. The experimental methodologies outlined in the present study provide a basis for further investigation into the mechanism of injury and the clinical applicability of biomechanical parameters.

Instrumented artificial spinal cord for human cervical pressure measurement.

Spinal cord injuries continue to generate large individual and societal costs. The study of spinal cord injury has been undertaken from the perspective of animal studies to understand cord functioning, and from the use of cadaver material to understand ligamentous column failure. The present study was conducted to develop a tool to link results from both these methods of research. An instrumented artificial spinal cord was designed, constructed, and evaluated under different testing scenarios. Properties of the in vivo animal cord were obtained using the dorsal impact method and reproduced in a collagen-encased gelatin physical model. The cord was instrumented in seven places using thin, non-invasive piezo-electric pressure sensors. The instrumented artificial cord was then evaluated in the canal of a human cadaver head-neck column under dynamic loading conditions. A C5 compression fracture correlated to high local pressure changes. These results demonstrate the feasibility of using this new tool to understand the mechanisms of spinal cord injury.

Rotational stability of a spinal pedicle screw/rod system.

Although the geometry of spinal instrumentation constructs may significantly affect efficacy, the variation in biomechanical data may not assist the clinician in an appropriate selection. The purpose of the present study was to quantify the effects of transverse fixators on rotational strength of a common pedicle-screw-with-rods system. Pedicle screws were mounted in blocks of polymethyl-methacrylate at angles to reproduce the configuration of placement in the human lumbar spine. Twenty cycles of +/- 12 N-m axial rotation moment was applied, and the steady-state response was used in the analysis. Configurations tested included both medial and lateral placement of longitudinal rods as well as the addition of one or two transverse rods. Up to a 20% difference in stiffness was noted between medial and lateral placement of longitudinal rods when no transverse rods were mounted. A maximum difference in flexibility of 6% was noted between the use of one and two transverse rods. For medially placed rods, a single transverse connector will add significant rotational stiffness even for shorter rod lengths; for laterally placed longitudinal rods, only the longer rod lengths need a transverse connector.

Fusion rate and biomechanical stiffness of hydroxylapatite versus autogenous bone grafts for anterior discectomy. An in vivo animal study.

STUDY DESIGN: The fusion rate and biomechanical stiffness were evaluated for 56 goat spinal units from 14 animals that had anterior discectomies and grafting procedures completed using hydroxylapatite and autogenous bone and survived for 6, 12, and 24 week healing times. OBJECTIVES: Harvested spinal units underwent radiographic imaging to assess fusion, biomechanical testing in axial compression, flexion, extension, lateral bending, and axial rotation to assess strength, and histological analysis. The above results were compared for the two procedures and the different healing times. SUMMARY OF BACKGROUND DATA: Because of some of the complications associated with the use of autogenous iliac crest bone graft in spine fusions, there has been considerable interest in the use of calcium phosphate ceramics as a possible substitute for a grafting material. One of the attractive features of calcium phosphate ceramics is the resulting strong bond that is formed with the host bone unlike other inert compounds. METHODS: Surgeries were done at four sites on each animal with two in the cervical spine and two in the lumbar spine. Radiography was done during the survival time and postsacrifice. Biomechanical testing was done on the day of sacrifice under physiological loads. Both hard tissue sections and decalcified sections were histologically evaluated. RESULTS: A 55% fusion rate for bone preparations and a 50% fusion rate for the hydroxylapatite (HA) units was found for the 12 and 24 week preparations. The HA preparations were better at maintaining disc space height. The biomechanical analysis revealed significantly higher stiffness values for fused preparations than for nonfused samples under extension, lateral bending, and axial rotation. Fused units demonstrated no statistical difference in biomechanical stiffness between HA versus autogenous bone units for any mode of loading. CONCLUSIONS: Our results indicate that these dense, nonresorbable hydroxylapatite blocks perform as well as autogenous bone for anterior spinal fusions in this animal model. The use of this hydroxylapatite material in anterior spine fusions may have some clinical validity.

Continuous motion analysis of the head-neck complex under impact.

The objective of the present study was to analyze the localized kinematic biodynamics of the human head-neck complex under impact loading. Unembalmed human cadaveric head-neck complexes were subjected to axial compressive forces delivered using an electrohydraulic testing device. The head-neck complex was aligned along the stiffest-axis; musculature was simulated using preloaded springs and cables; and retroreflective targets were inserted into the vertebral body, the facet joint articulation, and the spinous process at every level of the cervical column. At dynamic loading rates (1.8-5.1 m/min), mid to lower cervical spine injuries consistently occurred in these preparations. Continuous motion analysis of the components (vertebral body, intervertebral disk, facet joint, and the spinous process) at all levels of the cervical spine showed the temporal order of the transfer of the external load. Injuries documented by computed tomography and cryomicrotomy techniques correlated with the kinematics of the structure. The application of dynamic loading to the head-neck complex coupled with high-speed, continuous-motion analysis of the intervertebral components of the entire cervical column makes possible the definition of the temporal kinematic mechanics that are fundamental to the understanding of the biodynamics of cervical spine trauma. Using these procedures, we have correlated the kinematics with the onset and pattern of neck injury secondary to impact forces.

Failure of synthes anterior cervical fixation device by fracture of Morscher screws: a biomechanical study.

Anterior cervical fixation using the Synthes system has become increasingly popular. Two screw types for anchoring the plates include a "solid" titanium expansion screw and a plasma-sprayed fenestrated expansion screw that permits bony ingrowth. These screws were compared clinically and in the laboratory. In our first 20 cases using Synthes plates secured by Morscher fenestrated screws, 3 failures were observed, unilaterally in 1 patient and bilaterally in 2 others. In the unilateral screw failure, the contralateral screw was "solid" and did not fail. In the mechanical studies, screws were secured in the Synthes plate and embedded into methylmethacrylate and subjected to a sinusoidal bending moment to the mid-shaft of the screw. Load deflection data and cycles to failure were recorded. Fenestrated screws were found to demonstrate nearly twice as much deformation at failure and tolerated significantly fewer cycles to failure than did "solid" screws (p < 0.05). Because benefits of bony ingrowth into the screw are not well identified, the risks of fenestrated screw failure should preclude their routine use.

Biomechanical evaluation of Caspar cervical screws: comparative stability under cyclical loading.

Anterior cervical instrumentation is used as an adjunct to bone fusion; however, definitive biomechanical data to support some applications and techniques are lacking. In the absence of supportive experimental data, posterior cortical penetration has been recommended with the Caspar system. Previously, we compared the axial pull-out strength of Caspar screws with and without posterior cortical penetration. This study compares the stability of unicortical versus bicortical screw penetration groups under cyclical loading simulating physiological flexion-extension. Caspar screws were placed in human cadaveric vertebrae with or without posterior cortical purchase. Each screw was separately tested, simulating flexion-extension to 200 cycles. Deformation time data allowed a direct comparison of screw "wobble" with and without posterior cortical purchase. The mean deformation differences between subcortical and bicortical groups were statistically significant and increased over time within both groups. Enhanced stability was noted with bicortical purchase throughout most of the examined range, becoming more pronounced over longer periods of cyclical loading. Significant (P < 0.05) increases in deformation over time were noted for both groups, suggesting potentially significant deterioration at the screw-bone interface, despite bicortical purchase. Such deterioration with repeated flexion-extension loading may be of concern in the use of Caspar plates in the presence of multicolumn instability.

Effects of anterior vertebral grafting on the traumatized lumbar spine after pedicle screw-plate fixation.

This study was conducted to determine the effects of corpectomy and anterior strut grafting on the biomechanics of traumatized lumbar spine after pedicle screw-plate fixation. Eight lumbar spines were loaded until fracture (initial cycle) and then reloaded to the same deformation (injury cycle). After transpedicular fixation, spines were again loaded (fixation cycle). Partial corpectomy of the fractured body and anterior strut grafting were accomplished; the spine reloaded (strut cycle). Spine angles were measured and biomechanical strength and kinematic parameters analyzed. Load-deformation relationships were similar for fixation and strut cycles until maximum load; at failure, loads were higher for the former (P < 0.05), however. Alignment was improved by stabilization or stabilization plus anterior grafting (P < 0.05). Vertebral height was best maintained by grafting as an adjunct to pedicle fixation (P < 0.05). Kinematics were largely unaffected by grafting, except for reduced motion at the posterior vertebral targets between the fixated levels (P < 0.05). The strength of the fixated spine is relatively unchanged by corpectomy and anterior grafting; alignment may be improved in the latter group.

Is chronic spinal cord injury associated with increased risk of venous thromboembolism?

To determine the incidence of symptomatic thromboembolism in patients with chronic spinal cord injury, a retrospective review of patients followed in a Veteran's Affairs Spinal Cord Injury Unit was conducted. Followed for a mean of 13.7 years after injury, 287 patients were reviewed. Forty events were identified, an incidence of 10 percent. Thirty-three (83 percent) occurred in the first 6 months following injury. The remainder occurred at 1, 1.5, 7, 9, 10, 12, and 14 years after injury, an incidence of 0.17 percent per year. The incidence of clinically significant thromboembolism in spinal cord injury decreases dramatically after the first 6 months to a level similar to that in the general population (0.18 percent). Possible explanations for this include: 1) immobilization by itself may not be a risk factor for thromboembolism; 2) physiologic adaptations in the chronic state may protect against thromboembolism; and, 3) thromboembolism occurs, but remains subclinical in most patients.