Beth A. Winkelstein

The Cervical Facet Capsule and Its Role in Whiplash Injury : A Biomechanical Investigation

STUDY DESIGN Cervical facet capsular strains were determined during bending and at failure in the human cadaver. OBJECTIVE To determine the effect of an axial pretorque on facet capsular strains and estimate the risk for subcatastrophic capsular injury during normal bending motions. SUMMARY OF BACKGROUND DATA Epidemiologic and clinical studies have identified the facet capsule as a potential site of injury and prerotation as a risk factor for whiplash injury. Unfortunately, biomechanical data on the cervical facet capsule and its role in whiplash injury are not available. METHODS Cervical spine motion segments were tested in a pure-moment test frame and the full-field strains determined throughout the facet capsule. Motion segments were tested with and without a pretorque in pure bending. The isolated facet was then elongated to failure. Maximum principal strains during bending were compared with failure strains, by paired t test. RESULTS Statistically significant increases in principal capsular strains during flexion-extension loading were observed when a pretorque was applied. All measured strains during bending were significantly less than strains at catastrophic joint failure. The same was true for subcatastrophic ligament failure strains, except in the presence of a pretorque. CONCLUSIONS Pretorque of the head and neck increases facet capsular strains, supporting its role in the whiplash mechanism. Although the facet capsule does not appear to be at risk for gross injury during normal bending motions, a small portion of the population may be at risk for subcatastrophic injury.

Mechanical evidence of cervical facet capsule injury during whiplash: A cadaveric study using combined shear, compression, and extension loading

Study Design. A comparison of cervical facet capsule strain fields in cadaveric motion segments exposed to whiplash-like loads and failure loads. Objectives. To compare the maximum principal strain in the facet capsular ligament under combined shear, bending, and compressive loads with those required to injure the ligament. Summary of Background Data. The cervical facet capsular ligament is thought to be an anatomic site for whiplash injury, although the mechanism of its injury remains unclear. Methods. Motion segments from seven female donors were exposed to quasi-static flexibility tests using posterior shear loads of 135 N applied to the superior vertebra under four compressive axial preloads up to 325 N. The right facet joint was then isolated and failed in posterior shear loading. The Lagrangian strain field in the right facet capsular ligament was calculated from capsular displacements determined by stereophotogrammetry. Statistical analyses examined the effect of axial compression on motion segment flexibility, and compared maximum principal capsular strain between the flexibility and failure tests. Results. Capsular strain increased with applied shear load but did not vary with axial compressive load. The maximum principal strain reached during the flexibility tests was 61% ± 33% of that observed in subcatastrophic failures of the isolated joints. Two specimens reached strains in their flexibility tests that were larger than their corresponding strains at subcatastrophic failure in the failure tests. Conclusions. The cervical facet capsular ligaments may be injured under whiplash-like loads of combined shear, bending, and compression. The results provide a mechanical basis for injury caused by whiplash loading.

Comparative strengths and structural properties of the upper and lower cervical spine in flexion and extension

The purpose of this study is to test the hypothesis that the upper cervical spine is weaker than the lower cervical spine in pure flexion and extension bending, which may explain the propensity for upper cervical spine injuries in airbag deployments. An additional objective is to evaluate the relative strength and flexibility of the upper and lower cervical spine in an effort to better understand injury mechanisms, and to provide quantitative data on bending responses and failure modes. Pure moment flexibility and failure testing was conducted on 52 female spinal segments in a pure-moment test frame. The average moment at failure for the O-C2 segments was 23.7+/-3.4Nm for flexion and 43.3+/-9.3Nm for extension. The ligamentous upper cervical spine was significantly stronger in extension than in flexion (p=0.001). The upper cervical spine was significantly stronger than the lower cervical spine in extension. The relatively high strength of the upper cervical spine in tension and in extension is paradoxical given the large number of upper cervical spine injuries in out-of-position airbag deployments. This discrepancy is most likely due to load sharing by the active musculature.

A novel rodent neck pain model of facet-mediated behavioral hypersensitivity: implications for persistent pain and whiplash injury.

Clinical, epidemiological, and biomechanical studies suggest involvement of cervical facet joint injuries in neck pain. While bony motions can cause injurious tensile facet joint loading, it remains speculative whether such injuries initiate pain. There is currently a paucity of data explicitly investigating the relationship between facet mechanics and pain physiology. A rodent model of tensile facet joint injury has been developed using a customized loading device to apply two separate tensile deformations (low, high; n = 5 each) across the C6/C7 joint, or sham (n = 6) with device attachment only. Microforceps were rigidly coupled to the vertebrae for distraction and joint motions tracked in vivo. Forepaw mechanical allodynia was measured postoperatively for 7 days as an indicator of behavioral sensitivity. Joint strains for high (33.6 +/- 3.1%) were significantly elevated (P < 0.005) over low (11.1 +/- 2.3%). Digitization errors (0.17 +/- 0.20%) in locating bony markers were small compared to measured strains. Allodynia was significantly elevated for high over low and sham for all postoperative days. However, allodynia for low injury was not different than sham. A greater than three-fold increase in total allodynia resulted for high compared to low, corresponding to the three-fold difference in injury strain. Findings demonstrate tensile facet joint loading produces behavioral sensitivity that varies in magnitude according to injury severity. These results suggest that a facet joint tensile strain threshold may exist above which pain symptoms result. Continued investigation into the relationship between injury mechanics and nociceptive physiology will strengthen insight into painful facet injury mechanisms.

Spinal Facet Joint Biomechanics and Mechanotransduction in Normal, Injury and Degenerative Conditions

The facet joint is a crucial anatomic region of the spine owing to its biomechanical role in facilitating articulation of the vertebrae of the spinal column. It is a diarthrodial joint with opposing articular cartilage surfaces that provide a low friction environment and a ligamentous capsule that encloses the joint space. Together with the disc, the bilateral facet joints transfer loads and guide and constrain motions in the spine due to their geometry and mechanical function. Although a great deal of research has focused on defining the biomechanics of the spine and the form and function of the disc, the facet joint has only recently become the focus of experimental, computational and clinical studies. This mechanical behavior ensures the normal health and function of the spine during physiologic loading but can also lead to its dysfunction when the tissues of the facet joint are altered either by injury, degeneration or as a result of surgical modification of the spine. The anatomical, biomechanical and physiological characteristics of the facet joints in the cervical and lumbar spines have become the focus of increased attention recently with the advent of surgical procedures of the spine, such as disc repair and replacement, which may impact facet responses. Accordingly, this review summarizes the relevant anatomy and biomechanics of the facet joint and the individual tissues that comprise it. In order to better understand the physiological implications of tissue loading in all conditions, a review of mechanotransduction pathways in the cartilage, ligament and bone is also presented ranging from the tissue-level scale to cellular modifications. With this context, experimental studies are summarized as they relate to the most common modifications that alter the biomechanics and health of the spine-injury and degeneration. In addition, many computational and finite element models have been developed that enable more-detailed and specific investigations of the facet joint and its tissues than are provided by experimental approaches and also that expand their utility for the field of biomechanics. These are also reviewed to provide a more complete summary of the current knowledge of facet joint mechanics. Overall, the goal of this review is to present a comprehensive review of the breadth and depth of knowledge regarding the mechanical and adaptive responses of the facet joint and its tissues across a variety of relevant size scales.

The Role of Tissue Damage in Whiplash Associated Disorders: Discussion Paper 1

Study Design. Nonsystematic review of cervical spine lesions in whiplash-associated disorders (WAD). Objective. To describe whiplash injury models in terms of basic and clinical science, to summarize what can and cannot be explained by injury models, and to highlight future research areas to better understand the role of tissue damage in WAD. Summary of Background Data. The frequent lack of detectable tissue damage has raised questions about whether tissue damage is necessary for WAD and what role it plays in the clinical context of WAD. Methods. Nonsystematic review. Results. Lesions of various tissues have been documented by numerous investigations conducted in animals, cadavers, healthy volunteers, and patients. Most lesions are undetected by imaging techniques. For zygapophysial (facet) joints, lesions have been predicted by bioengineering studies and validated through animal studies; for zygapophysial joint pain, a valid diagnostic test and a proven treatment are available. Lesions of dorsal root ganglia, discs, ligaments, muscles, and vertebral artery have been documented in biomechanical and autopsy studies, but no valid diagnostic test is available to assess their clinical relevance. The proportion of WAD patients in whom a persistent lesion is the major determinant of ongoing symptoms is unknown. Psychosocial factors, stress reactions, and generalized hyperalgesia have also been shown to predict WAD outcomes. Conclusion. There is evidence supporting a lesion-based model in WAD. Lack of macroscopically identifiable tissue damage does not rule out the presence of painful lesions. The best available evidence concerns zygapophysial joint pain. The clinical relevance of other lesions needs to be addressed by future research.

Transient cervical nerve root compression in the rat induces bilateral forepaw allodynia and spinal glial activation : Mechanical factors in painful neck injuries

Study Design. An in vivo rat model of transient cervical nerve root compression. Objectives. To investigate the potential for cervical nerve root compression to produce behavioral hypersensitivity and examine its dependence on compression. Summary of Background Data. Clinically, nerve root injury has been hypothesized as a potential source of neck pain, particularly because cervical nerve roots are at mechanical risk for injury during neck loading. Lumbar radiculopathy models of nerve root ligation show that mechanical allodynia and spinal glial changes depend on nerve root deformation magnitude. However, no investigation has been performed to examine cervical nerve root compression as a cause of pain. Methods. Two compressive forces (10 and 60 grams force [gf]) were transiently applied to the C7 nerve roots unilaterally using microvascular clips in separate groups (n = 12 each). Sham procedures were also performed in a separate group of rats (n = 12). Bilateral forepaw mechanical allodynia was monitored after surgery for 7 days. On day 7, spinal glial activation was assessed using immunohistochemistry to investigate its dependence on nerve root compressive force, in the context of behavioral hypersensitivity. Results. Bilateral allodynia was observed following injury, which was significantly (P < 0.042) increased over sham and baseline responses. No difference in allodynia was found between the 10 and 60 gf injuries. Astrocytic and microglial activation were observed in the ipsilateral dorsal horn following compression, with only astrocytic activation paralleling allodynia patterns. Conclusions. Results imply a force threshold exists less than 10 gf for persistent pain symptoms following transient cervical nerve root compression. Findings also suggest that spinal glial activation may be related to behavioral sensitivity and may modulate cervical nerve root mediated pain.

The Anatomy and Biomechanics of Acute and Chronic Whiplash Injury

Whiplash injury is the most common motor vehicle injury, yet it is also one of the most poorly understood. Here we examine the evidence supporting an organic basis for acute and chronic whiplash injuries and review the anatomical sites within the neck that are potentially injured during these collisions. For each proposed anatomical site—facet joints, spinal ligaments, intervertebral discs, vertebral arteries, dorsal root ganglia, and neck muscles—we present the clinical evidence supporting that injury site, its relevant anatomy, the mechanism of and tolerance to injury, and the future research needed to determine whether that site is responsible for some whiplash injuries. This article serves as a snapshot of the current state of whiplash biomechanics research and provides a roadmap for future research to better understand and ultimately prevent whiplash injuries.

Repeated injury to the lumbar nerve roots produces enhanced mechanical allodynia and persistent spinal neuroinflammation

Study Design. A lumbar radiculopathy model investigated pain behavioral responses after nerve root reinjury. Objectives. To gain a further understanding of central sensitization and neuroinflammation associated with chronic lumbar radiculopathy after repeated nerve root injury. Summary of Background Data. The pathophysiologic mechanisms associated with chronic radicular pain remain obscure. It has been hypothesized that lumbar root injury produces neuroimmunologic and neurochemical changes, sensitizing the spinal cord and causing pain responses to manifest with greater intensity and longer duration after reinjury. However, this remains untested experimentally. Methods. Male Holtzman rats were divided into two groups: a sham group having only nerve root exposure, and a chromic group in which the nerve root was ligated loosely with chromic gut suture. Animals underwent a second procedure at 42 days. The chromic group was further divided into a reinjury group and a chromic-sham group, in which the lumbar roots were only re-exposed. Bilateral mechanical allodynia was continuously assessed throughout the study. Qualitative assessment of spinal cord glial activation and IL-&bgr; expression was performed. Results. Mechanical allodynia was significantly greater on both the ipsilateral and contralateral sides after reinjury (P < 0.001), and the response did not return to baseline after reinjury, as it did with the initial injury. There were also persistent spinal astrocytic and microglial activation and interleukin-1&bgr; expression. Conclusions. The bilateral responses support central modulation of radicular pain after nerve root injury. An exaggerated and more prolonged response bilaterally after reinjury suggests central sensitization after initial injury. Neuroinflammatory activation in the spinal cord further supports the hypothesis that central neuroinflammation plays an important role in chronic radicular pain.

The role of mechanical deformation in lumbar radiculopathy: An in vivo model

Study Design. In vivo strain techniques were used in an animal radiculopathy model. Objective. To quantify the severity of compressive nerve root injury and characterize its effect on resultant mechanical allodynia in a lumbar radiculopathy model. Summary of Background Data. Clinical and experimental work indicate many factors contributing to radicular pain mechanisms, including mechanical injury. Although it has been suggested that the degree of mechanical injury to the nerve root affects the nature of the pain response, no study has quantified local in vivo injury biomechanics, nor have such measures been linked with the resulting magnitude of mechanical allodynia or other clinical symptoms. Methods. Male Holtzman rats were divided into a sham group with only nerve root exposure or a ligation group in which the nerve root was tightly ligated with a single silk suture. Using image analysis, nerve root radial strains were calculated at the time of injury and after surgery. The animals were grouped according to ligation strain for analysis. Mechanical allodynia was continuously assessed throughout the study. Results. Compressive strains in the nerve root ranged from 7.8% to 61% (mean, 30.8% ± 14.5%). Animals undergoing larger ligation strains exhibited heightened mechanical allodynia after injury. This was significant using a 12-g von Frey filament (P = 0.05). After surgery, the nerve roots displayed tissue swelling, which was relatively uniform in the low-strain group and less so in the high-strain group. Conclusions. For the first time, in vivo biomechanical analysis of tissue deformations was used to investigate the role of mechanics in radicular pain. Overall mechanical allodynia was greater for more severe nerve root injuries (greater strains) in an animal model, suggesting that mechanical deformation plays an important role in the pain mechanism. Continued work is underway to understand the complex interplay between mechanics and the physiology of radicular pain.