Brian D. Stemper

Gender dependent cervical spine segmental kinematics during whiplash

Clinical and epidemiological studies have frequently reported that female occupants sustain whiplash injuries more often than males. The current study was based on the hypothesis that segmental level-by-level cervical intervertebral motions in females are greater than in males during rear impact. The hypothesis was tested by subjecting 10 intact human cadaver head-neck complexes (five males, five females) to rear impact loading. Intervertebral kinematics were analyzed as a function of spinal level at the time of maximum cervical S-curve, which occurred during the loading phase. Segmental angles were significantly greater (p<0.05) in female specimens at C2-C3, C4-C5, C5-C6, and C6-C7 levels. Because greater angulations are associated with stretch in the innervated components of the cervical spinal column, these findings may offer a biomechanical explanation for the higher incidence of whiplash-related complaints in female patients secondary to rear impact acceleration.

Primary Blast Traumatic Brain Injury in the Rat: Relating Diffusion Tensor Imaging and Behavior

The incidence of traumatic brain injury (TBI) among military personnel is at its highest point in U.S. history. Experimental animal models of blast have provided a wealth of insight into blast injury. The mechanisms of neurotrauma caused by blast, however, are still under debate. Specifically, it is unclear whether the blast shockwave in the absence of head motion is sufficient to induce brain trauma. In this study, the consequences of blast injury were investigated in a rat model of primary blast TBI. Animals were exposed to blast shockwaves with peak reflected overpressures of either 100 or 450 kPa (39 and 110 kPa incident pressure, respectively) and subsequently underwent a battery of behavioral tests. Diffusion tensor imaging (DTI), a promising method to detect blast injury in humans, was performed on fixed brains to detect and visualize the spatial dependence of blast injury. Blast TBI caused significant deficits in memory function as evidenced by the Morris Water Maze, but limited emotional deficits as evidenced by the Open Field Test and Elevated Plus Maze. Fractional anisotropy, a metric derived from DTI, revealed significant brain abnormalities in blast-exposed animals. A significant relationship between memory deficits and brain microstructure was evident in the hippocampus, consistent with its role in memory function. The results provide fundamental insight into the neurological consequences of blast TBI, including the evolution of injury during the sub-acute phase and the spatially dependent pattern of injury. The relationship between memory dysfunction and microstructural brain abnormalities may provide insight into the persistent cognitive difficulties experienced by soldiers exposed to blast neurotrauma and may be important to guide therapeutic and rehabilitative efforts.

Validation of a head-neck computer model for whiplash simulation.

A head-neck computer model was comprehensively validated over a range of rear-impact velocities using experiments conducted by the same group of authors in the same laboratory. Validations were based on mean ±1 standard deviation response curves, i.e. corridors. Global head-neck angle, segmental angle and local facet joint regional kinematic responses from the model fell within experimental corridors. This was true for all impact velocities (1.3, 1.8 and 2.6 ms−1). The non-physiological S-curvature lasted approximately 100 ms. The present, comprehensively validated model can be used to conduct parametric studies and investigate the effects of factors such as active sequential and parallel muscle contractions, thoracic ramping and local tissue strain responses, as a function of cervical level, joint region and impact velocity in whiplash injury assessment.

Anatomical gender differences in cervical vertebrae of size-matched volunteers.

Study Design. Clinical literature consistently identifies women as more susceptible to trauma-related neck pain, commonly resulting from soft tissue cervical spine injury. Structural gender differences may explain altered response to dynamic loading in women leading to increased soft tissue distortion and greater injury susceptibility. Objective. Identify anatomic gender differences in cervical spinal geometry that contribute to decreased column stability in women. Summary of Background Data. Previous studies investigating male and female vertebral and vertebral body geometry demonstrated female vertebral dimensions were smaller. However, populations were not size matched and parameters related to biomechanical stability were not reported. Methods. Computed tomography scans of the cervical spine were obtained from size-matched young healthy volunteers. Geometrical dimensions were obtained at the C4 level and analysis of variance determined significant gender differences. Results. Two volunteer subsets were size matched based on sitting height and head circumference. All geometrical measures were greater in men for both subsets. Vertebral width and disc-facet depth were significantly greater in men. Additionally, segmental support area, combining interfacet width and disc-facet depth, was greater in men, indicating more stable intervertebral coupling. Conclusion. Present results of decreased linear and areal cervical dimensions leading to decreased column stability may partially explain increased traumatic injury rates in women.

Quantitative anatomy of subaxial cervical lateral mass: an analysis of safe screw lengths for Roy-Camille and magerl techniques.

Study Design. Determination of lateral mass screw lengths with Roy-Camille and Magerl techniques of screw insertion using computerized tomography in 98 young, asymptomatic North American volunteers. Objective. To provide reliable and normative data on safe screw lengths using the Roy-Camille and Magerl techniques of lateral mass fixation in the subaxial cervical spine. Summary of Background Data. Lateral mass screw lengths have been studied in the past using differing subject and measurement characteristics and small sample sizes. Results demonstrated considerable variation in screw length and influencing factors. Inappropriate screw lengths can result in neurovascular injury during screw insertion, facet joint damage, or inadequate fixation. Methods. Bicortical screw lengths were bilaterally measured at each spinal level from C3–C7 in 98 young volunteers using computed tomography reconstructions through the lateral masses obtained in the plane of the screw in Roy-Camille and Magerl techniques. Results. With both techniques, trajectories were longest at C4–C6, shorter at C3, and shortest at C7. Screw lengths were greater in males when compared with females at all levels. Average Magerl screw lengths were approximately 2.6 mm longer at C3–C6 levels, and approximately 1.3 mm longer at the C7 level when compared with Roy-Camille technique. There was minimal correlation between screw lengths and anthropometric measurements including stature, body weight, and neck length. Conclusion. Significant variations exist at each subaxial level with either technique. We recommend the surgeon determine screw lengths for fixation at each level using preoperative sagittal oblique computed tomography scans, which provide the most accurate technique of preoperative templating for screw length.

Bone mineral density of human female cervical and lumbar spines from quantitative computed tomography

Study Design. This study determined bone mineral density (BMD) of cervical, thoracic, and lumbar vertebrae in healthy asymptomatic human subjects. Objectives. To test the hypothesis that BMD of neck vertebrae (C2–C7) is equivalent to BMD of lumbar vertebrae (L2–L4). Summary of Background Data. BMD of lumbar vertebrae is correlated to their strength. Although numerous studies exist quantifying BMD of the human lumbar spine, such information for the cervical spine is extremely limited. In addition, BMD correlations are not established between the two regions of the spinal column. Methods. Adult healthy human female volunteers with ages ranging from 18 to 40 years underwent quantitative computed tomography (CT) scanning of the neck and back. All BMD data were statistically analyzed using paired nonrepeating measures ANOVA techniques. Significance was assigned at a P < 0.05. Linear regression analyses were used to compare BMD as a function of level and region; ±95% confidence intervals were determined. Results. When data were grouped by cervical (C2–C7), thoracic (T1), and lumbar (L2–L4) spines, mean BMD was 260.8 ± 42.5, 206.9 ± 33.5, and 179.7 ± 23.4 mg/mL. Average BMD of cervical vertebrae was higher than (P < 0.0001) thoracic and lumbar spines. Correlations between BMD and level indicated the lowest r value for T1 (0.42); in general, the association was the strongest in the lumbar spine (r = 0.89–0.95). The cervical spine also responded with good correlations among cervical vertebrae (r ranging from 0.66 to 0.87). Conclusions. The present study failed to support the hypothesis that BMD of lumbar spine vertebrae is equivalent to its cranial counterparts. The lack of differences in BMD among the three lumbar vertebral bodies confirms the appropriateness of using L2, L3, or L4 in clinical or biomechanical situations. However, significant differences were found among different regions of the vertebral column, with the cervical spine demonstrating higher trabecular densities than the thoracic and lumbar spines. In addition, the present study found statistically significant variations in densities even among neck vertebrae.

Prognosis after whiplash injury: where to from here? Discussion paper 4

Study Design. Nonsystematic review and discussion of prognosis after whiplash injury. Objective. To summarize the research and identify a research agenda for improving prognostic models after whiplash injury. Summary of Background Data. With up to 50% of individuals failing to fully recover after whiplash injury, the capacity to determine a precise estimate of prognosis will be important. Systematic reviews note inconsistencies and shortcomings of research in this area. Methods. A nonsystematic review and discussion. Results. Most prognostic whiplash studies are phase 1 (exploratory) studies with few confirmatory or validation studies yet available. It is recognized that whiplash is a heterogeneous condition and clinicians require prognostic indicators for clinical use. Although the evidence is not sufficiently strong to make firm recommendations, there are some prognostic factors that have shown consistency across studies and could be considered as preliminary flags or guides to gauge patients potentially at risk of poor recovery. These include pain and/or disability levels, neck range of movement, cold and mechanical hyperalgesia and psychological factors of recovery beliefs/expectations, post-traumatic stress symptoms, depression, and pain catastrophizing. It is not known whether these factors can be modified or whether modification will improve outcomes, thus they should not be considered directives for management. Research priorities identified to develop improved predictive models include confirmation and validation of factors identified in phase 1 studies; investigation of the interaction between variables; investigation of the predictive value of changes in variables over time; the inclusion of validated outcomes including measures of pain and disability as well as perceived recovery and psychological outcomes. Conclusion. The current evidence is not sufficiently robust to be able to confidently predict outcome after whiplash injury. A preliminary set of consistent factors has been proposed to assist clinicians in identifying individuals at risk of poor recovery. Directions for the development of improved prognostic models are discussed.

Computerized Tomographic Morphometric Analysis of Subaxial Cervical Spine Pedicles in Young Asymptomatic Volunteers

BACKGROUND Although cervical spine pedicle screws have been shown to provide excellent fixation, widespread acceptance of their use is limited because of the risk of injury to the spinal cord, nerve roots, and vertebral arteries. The risks of pedicle screw insertion in the cervical spine can be mitigated by a three-dimensional appreciation of pedicle anatomy. Normative data on three-dimensional subaxial pedicle geometry from a large, young, and asymptomatic North American population are lacking. The purpose of the present study was to determine three-dimensional subaxial pedicle geometry in a large group of young volunteers and to determine level and sex-specific morphologic differences. METHODS Helical computerized tomography scans were made from the third cervical to the seventh cervical vertebra in ninety-eight volunteers (sixty-three men and thirty-five women) with an average age of twenty-five years. Pedicle width, height, length, and transverse and sagittal angulations were measured bilaterally. Pedicle screw insertion positions were quantified in terms of mediolateral and superoinferior offsets relative to readily identifiable landmarks. RESULTS The mean pedicle width and height at all subaxial levels were sufficient to accommodate 3.5-mm screws in 98% of the volunteers. Pedicle width and height dimensions of <4.0 mm were rare (observed in association with only 1.7% of the pedicles), with 82% occurring in women and 72% occurring unilaterally. Screw insertion positions generally moved medially and superiorly at caudal levels. Transverse angulation was approximately 45 degrees at the third to fifth cervical levels and was less at more caudal levels. Sagittal angulation changed from a cranial orientation at superior levels to a caudal orientation at inferior levels. Mediolateral and superoinferior insertion positions and sagittal angulations were significantly dependent (p < 0.05) on sex and spinal level. Transverse angulation was significantly dependent (p < 0.05) on spinal level. CONCLUSIONS Pedicle screw insertion points and orientation are significantly different (p < 0.05) at most subaxial cervical levels and between men and women. Preoperative imaging studies should be carefully templated for pedicle size in all patients on a level-specific basis. Although the prevalence was low, women were more likely to have pedicle width and height dimensions of <4.0 mm.

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.

Reflex muscle contraction in the unaware occupant in whiplash injury.

Study Design. Computer modeling and parametric analysis were used to determine the effect of reflex contraction of the neck muscles in the unaware occupant in whiplash. Objective. To delineate effects of reflex contraction on spinal segmental kinematics during the retraction phase. Summary of Background Data. The ability of reflex neck muscle contraction to mitigate whiplash injury in the unaware occupant remains unclear. Analyzing relative timing between electromyographic and head-neck kinematics, previous investigators theorized that muscle contraction alters spinal kinematics, decreasing injury likelihood. Other investigators suggested that injury occurs during the initial (retraction) phase of head-neck kinematics, before significant muscle force generation. Methods. Computer modeling was used to determine reflex contraction effects on segmental angulations, implementing parametric analysis techniques to vary reflex delay and impact severity. Results. Shorter reflex delays had a greater effect on segmental angulations later in the event and at lower impact severities. However, the magnitude of this effect, particularly at higher impact severities and during maximum cervical S-curvature (factors implicated in the whiplash injury mechanism) was minimal, altering segmental angulations by a maximum of 19%. Conclusions. Because reflex contraction did not substantially alter spinal kinematics, muscle contraction likely does not initiate in sufficient time to mitigate whiplash injuries that may occur during the retraction phase.