Experimental Methods to Inform Diagnostic Approaches for Painful TMJ Osteoarthritis.

Temporomandibular joint (TMJ) osteoarthritis (OA) is a degenerative disease of the joint that can produce persistent orofacial pain as well as functional and structural changes to its bone, cartilage, and ligaments. Despite advances in the clinical utility and reliability of the Diagnostic Criteria for Temporomandibular Disorders, clinical tools inadequately predict which patients will develop chronic TMJ pain and degeneration, limiting clinical management. The challenges of managing and treating TMJ OA are due, in part, to a limited understanding of the mechanisms contributing to the development and maintenance of TMJ pain. OA is initiated by multiple factors, including injury, aging, abnormal joint mechanics, and atypical joint shape, which can produce microtrauma, remodeling of joint tissues, and synovial inflammation. TMJ microtrauma and remodeling can increase expression of cytokines, chemokines, and catabolic factors that damage synovial tissues and can activate free nerve endings in the joint. Although studies have separately investigated inflammation-driven orofacial pain, acute activity of the trigeminal nerve, or TMJ tissue degeneration and/or damage, the temporal mechanistic factors leading to chronic TMJ pain are undefined. Limited understanding of the interaction between degeneration, intra-articular chemical factors, and pain has further restricted the development of targeted, disease-modifying drugs to help patients avoid long-term pain and invasive procedures, like TMJ replacement. A range of animal models captures features of intra-articular inflammation, joint overloading, and tissue damage. Although those models traditionally measure peripheral sensitivity as a surrogate for pain, recent studies recognize the brain's role in integrating, modulating, and interpreting nociceptive inputs in the TMJ, particularly in light of psychosocial influences on TMJ pain. The articular and neural contributors to TMJ pain, imaging modalities with clinical potential to identify TMJ OA early, and future directions for clinical management of TMJ OA are reviewed in the context of evidence in the field.

Intra-articular nerve growth factor regulates development, but not maintenance, of injury-induced facet joint pain & spinal neuronal hypersensitivity.

OBJECTIVE: The objective of the current study is to define whether intra-articular nerve growth factor (NGF), an inflammatory mediator that contributes to osteoarthritic pain, is necessary and sufficient for the development or maintenance of injury-induced facet joint pain and its concomitant spinal neuronal hyperexcitability. METHOD: Male Holtzman rats underwent painful cervical facet joint distraction (FJD) or sham procedures. Mechanical hyperalgesia was assessed in the forepaws, and NGF expression was quantified in the C6/C7 facet joint. An anti-NGF antibody was administered intra-articularly in additional rats immediately or 1 day following facet distraction or sham procedures to block intra-articular NGF and test its contribution to initiation and/or maintenance of facet joint pain and spinal neuronal hyperexcitability. NGF was injected into the bilateral C6/C7 facet joints in separate rats to determine if NGF alone is sufficient to induce these behavioral and neuronal responses. RESULTS: NGF expression increases in the cervical facet joint in association with behavioral sensitivity after that joint's mechanical injury. Intra-articular application of anti-NGF immediately after a joint distraction prevents the development of both injury-induced pain and hyperexcitability of spinal neurons. Yet, intra-articular anti-NGF applied after pain has developed does not attenuate either behavioral or neuronal hyperexcitability. Intra-articular NGF administered to the facet in naïve rats also induces behavioral hypersensitivity and spinal neuronal hyperexcitability. CONCLUSION: Findings demonstrate that NGF in the facet joint contributes to the development of injury-induced joint pain. Localized blocking of NGF signaling in the joint may provide potential treatment for joint pain.

Upregulation of GLT-1 by treatment with ceftriaxone alleviates radicular pain by reducing spinal astrocyte activation and neuronal hyperexcitability.

Cervical nerve root injury commonly leads to radicular pain. Normal sensation relies on regulation of extracellular glutamate in the spinal cord by glutamate transporters. The goal of this study was to define the temporal response of spinal glutamate transporters (glial glutamate transporter 1 [GLT-1], glutamate-aspartate transporter [GLAST], and excitatory amino acid carrier 1) following nerve root compressions that do or do not produce sensitivity in the rat and to evaluate the role of glutamate uptake in radicular pain by using ceftriaxone to upregulate GLT-1. Compression was applied to the C7 nerve root. Spinal glutamate transporter expression was evaluated at days 1 and 7. In a separate study, rats underwent a painful root compression and were treated with ceftriaxone or the vehicle saline. Glial glutamate transporter expression, astrocytic activation (glial fibrillary acidic protein [GFAP]), and neuronal excitability were assessed at day 7. Both studies measured behavioral sensitivity for 7 days after injury. Spinal GLT-1 significantly decreased (P < 0.04) and spinal GLAST significantly increased (P = 0.036) at day 7 after a root injury that also produced sensitivity to both mechanical and thermal stimuli. Within 1 day after ceftriaxone treatment (day 2), mechanical allodynia began to decrease; both mechanical allodynia and thermal hyperalgesia were attenuated (P < 0.006) by day 7. Ceftriaxone also reduced (P < 0.024) spinal GFAP and GLAST expression, and neuronal hyperexcitability in the spinal dorsal horn, restoring the proportion of spinal neurons classified as wide dynamic range to that of normal. These findings suggest that nerve root-mediated pain is maintained jointly by spinal astrocytic reactivity and neuronal hyperexcitability and that these spinal modifications are associated with reduced glutamate uptake by GLT-1.

Three-dimensional kinematic stress magnetic resonance image analysis shows promise for detecting altered anatomical relationships of tissues in the cervical spine associated with painful radiculopathy.

For some patients with radiculopathy a source of nerve root compression cannot be identified despite positive electromyography (EMG) evidence. This discrepancy hampers the effective clinical management for these individuals. Although it has been well-established that tissues in the cervical spine move in a three-dimensional (3D) manner, the 3D motions of the neural elements and their relationship to the bones surrounding them are largely unknown even for asymptomatic normal subjects. We hypothesize that abnormal mechanical loading of cervical nerve roots during pain-provoking head positioning may be responsible for radicular pain in those cases in which there is no evidence of nerve root compression on conventional cervical magnetic resonance imaging (MRI) with the neck in the neutral position. This biomechanical imaging proof-of-concept study focused on quantitatively defining the architectural relationships between the neural and bony structures in the cervical spine using measurements derived from 3D MR images acquired in neutral and pain-provoking neck positions for subjects: (1) with radicular symptoms and evidence of root compression by conventional MRI and positive EMG, (2) with radicular symptoms and no evidence of root compression by MRI but positive EMG, and (3) asymptomatic age-matched controls. Function and pain scores were measured, along with neck range of motion, for all subjects. MR imaging was performed in both a neutral position and a pain-provoking position. Anatomical architectural data derived from analysis of the 3D MR images were compared between symptomatic and asymptomatic groups, and the symptomatic groups with and without imaging evidence of root compression. Several differences in the architectural relationships between the bone and neural tissues were identified between the asymptomatic and symptomatic groups. In addition, changes in architectural relationships were also detected between the symptomatic groups with and without imaging evidence of nerve root compression. As demonstrated in the data and a case study the 3D stress MR imaging approach provides utility to identify biomechanical relationships between hard and soft tissues that are otherwise undetected by standard clinical imaging methods. This technique offers a promising approach to detect the source of radiculopathy to inform clinical management for this pathology.

Transient nerve root compression load and duration differentially mediate behavioral sensitivity and associated spinal astrocyte activation and mGLuR5 expression.

Injury to the cervical nerve roots is a common source of neck pain. Animal models of nerve root compression have previously established the role of compression magnitude and duration in nerve root-mediated pain and spinal inflammation; yet, the response of the spinal glutamatergic system to transient nerve root compression and its relationship to compression mechanics have not been studied. The glutamate receptor, mGluR5, has a central role in pain, and its expression by neurons and astrocytes in the spinal cord may be pivotal for neuronal-glial signaling. This study quantified spinal GFAP and mGluR5 expression following nerve root compressions of different magnitudes and durations in the rat. Compression to the C7 nerve root was applied for a duration that was either above (10 min) or below (3 min) the critical duration for mediating afferent discharge rates during compression. To also test for the effect of the magnitude of the compression load, either a 10 gf or a 60 gf was applied to the nerve root for each duration. Mechanical allodynia was assessed, and the C7 spinal cord was harvested on day 7 for immunofluorescent analysis. Double labeling was used to localize the expression of mGluR5 on astrocytes (GFAP) and neurons (MAP2). Seven days after injury, 10 min of compression produced significantly greater behavioral sensitivity (P<0.001) and spinal GFAP expression (P=0.002) than 3 min of compression, regardless of the compression magnitude. Nerve root compression at 60 gf produced a significant increase (P<0.001) in spinal mGluR5 for both of the durations studied. There was no difference in the distribution of mGluR5 between astrocytes and neurons following nerve root compression of any type. The glutamatergic and glial systems are differentially modulated by the mechanics of nerve root compression despite the known contribution of glia to pain through glutamatergic signaling.

Activating transcription factor 4, a mediator of the integrated stress response, is increased in the dorsal root ganglia following painful facet joint distraction.

Chronic neck pain is one of the most common musculoskeletal disorders in the US. Although biomechanical and clinical studies have implicated the facet joint as a primary source of neck pain, specific cellular mechanisms still remain speculative. The purpose of this study was to investigate whether a mediator (activating transcription factor; 4ATF4) of the integrated stress response (ISR) is involved in facet-mediated pain. Holtzman rats underwent C6/C7 facet joint loading that produces either painful (n=16) or nonpainful (n=8) responses. A sham group (n=9) was also included as surgical controls. Behavioral sensitivity was measured and the C6 dorsal root ganglia (DRGs) were harvested on day 7 to evaluate the total and neuronal ATF4 expression. In separate groups, an intra-articular ketorolac injection was administered either immediately (D0 ketorolac) or 1 day (D1 ketorolac) after painful facet joint loading. Allodynia was measured at days 1 and 7 after injury to assess the effects on behavioral responses. ATF4 and BiP (an indicator of ISR activation) were separately quantified at day 7. Facet joint loading sufficient to elicit behavioral hypersensitivity produced a threefold increase in total and neuronal ATF4 expression in the DRG. After ketorolac treatment at the time of injury, ATF4 expression was significantly (P<0.01) reduced despite not producing any attenuation of behavioral responses. Interestingly, ketorolac treatment at day 1 significantly (P<0.001) alleviated behavioral sensitivity at day 7, but did not modify ATF4 expression. BiP expression was unchanged after either intervention time. Results suggest that ATF4-dependent activation of the ISR does not directly contribute to persistent pain, but it may sensitize neurons responsible for pain initiation. These behavioral and immunohistochemical findings imply that facet-mediated pain may be sustained through other pathways of the ISR.

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-beta 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-1beta 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.

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.

Nerve injury proximal or distal to the DRG induces similar spinal glial activation and selective cytokine expression but differential behavioral responses to pharmacologic treatment.

The specific mechanisms by which nervous system injury becomes a chronic pain state remain undetermined. Historically, it has been believed that injuries proximal or distal to the dorsal root ganglion (DRG) produce distinct pathologies that manifest in different severity of symptoms. This study investigated the role of injury site relative to the DRG in (1) eliciting behavioral responses, (2) inducing spinal neuroimmune activation, and (3) responding to pharmacologic interventions. Rats received either an L5 spinal nerve transection distal to the DRG or an L5 nerve root injury proximal to the DRG. Comparative studies assessed behavioral nociceptive responses, spinal cytokine mRNA and protein expression, and glial activation after injury. In separate studies, intrathecal pharmacologic interventions by using selective cytokine antagonists (interleukin-1 [IL-1] receptor antagonist and soluble tumor necrosis factor [TNF] receptor) and a global immunosuppressant (leflunomide) were performed to determine their relative effectiveness in these injury paradigms. Behavioral responses assessed by mechanical allodynia and thermal hyperalgesia were almost identical in the two models of persistent pain, suggesting that behavioral testing may not be a sensitive measure of injury. Spinal IL-1beta, IL-6, IL-10, and TNF mRNA and IL-6 protein were significantly elevated in both injuries. The overall magnitude of expression and temporal patterns were similar in both models of injury. The degree of microglial and astrocytic activation in the L5 spinal cord was also similar for both injuries. In contrast, the pharmacologic treatments were more effective in alleviating mechanical allodynia for peripheral nerve injury than nerve root injury, suggesting that nerve root injury elicits a more robust, centrally mediated response than peripheral nerve injury. Overall, these data implicate alternate nociceptive mechanisms in these anatomically different injuries that are not distinguished by behavioral testing or the neuroimmune markers used in this study.

Quantification of neural tissue injury in a rat radiculopathy model: comparison of local deformation, behavioral outcomes, and spinal cytokine mRNA for two surgeons.

Clinical and experimental work indicate that a variety of factors contribute to radicular pain mechanisms, including mechanical injury. While it has been qualitatively suggested that the magnitude of nerve root mechanical injury affects the nature of the pain response, no study has quantified the local in vivo injury biomechanics in these models. Therefore, it was the purpose of this study to develop and implement an in vivo method to quantify compressive nerve root injury strain severity and characterize its effect on the resulting responses in an existing lumbar radiculopathy rat model. Male Holtzman rats were divided into a sham group with only nerve root exposure or a ligation group with the nerve root tightly ligated using silk suture. Using image analysis, nerve root radial strains were calculated at the time of injury for two surgeons. Mechanical allodynia was continuously assessed throughout the study and spinal cord cytokine mRNA levels were assayed on postoperative day 7. The degree of intersurgeon variability for imposing a ligation injury in this model was also assessed. Mean compressive injury strains in the nerve root were 32.8+/-14.2% and were not different for the two experimenters. Animals undergoing more severe ligation strains exhibited significantly heightened allodynia following injury and greater upregulation of the inflammatory cytokines IL-1alpha/beta, IL-6, and IL-10. Results indicate a direct correlation of local nerve root injury severity with the ensuing physiologic responses associated with nociception.

An anatomical investigation of the human cervical facet capsule, quantifying muscle insertion area.

Facet capsule injury has been hypothesised as a mechanism for neck pain. While qualitative studies have demonstrated the proximity of neck muscles to the cervical facet capsule, the magnitude of their forces remains unknown owing to a lack of quantitative muscle geometry. In this study, histological techniques were employed to quantify muscle insertions on the human cervical facet capsule. Computerised image analysis of slides stained with Masson's trichrome was performed to characterise the geometry of the cervical facet capsule and determine the total insertion area of muscle fibres into the facet capsule for the C4-C5 and C5-C6 joints. Muscle insertions were found to cover 22.4+/-9.6% of the capsule area for these cervical levels, corresponding to a mean muscle insertion area of 47.6+/-21.8 mm2. The magnitude of loading to the cervical facet capsule due to eccentric muscle contraction is estimated to be as high as 51 N. When taken in conjunction with the forces acting on the capsular ligament due to vertebral motions, these forces can be as high as 66 N. In that regard, these anatomical data provide quantitative evidence of substantial muscle insertions into the cervical facet capsular ligament and provide a possible mechanism for injury to this ligament and the facet joint as a whole.

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.

Experimental and computational characterization of three-dimensional cervical spine flexibility.

Cervical spine behavior for generalized loading is often characterized using a full three-dimensional flexibility matrix. While experimental studies have been aimed at determining cervical motion segment behavior, their accuracy and utility have been limited both experimentally and analytically. For example, the nondiagonal terms, describing coupled motions, of the matrices have often been omitted. Flexibility terms have been primarily represented as constants despite the known nonlinear stiffening response of the spine. Moreover, there is presently no study validating the flexibility approach for predicting vertebral motions; nor have the effects of approximations and simplifications to the matrix representations been quantified. Yet, the flexibility matrix currently forms the basis for all multibody dynamics models of cervical spine motion. Therefore, the purpose of this study is to fully quantify the flexibility relationships for cervical motion segments, examine the diagonal and nondiagonal components of the flexibility matrix, and determine the extent to which multivariable relationships improve cervical spine motion prediction. To that end, using unembalmed human cervical spine motion segments, a full battery of flexibility tests were performed for a neutral orientation and also following an axial pretorque. Primary and coupled matrix components were described using linear and piecewise nonlinear incremental constants. An additional approach utilized multivariable incremental relationships to describe matrix terms. Measured motions were predicted using structural flexibility methods and were evaluated using RMS error of the difference between the predicted and measured responses. Results of this study provide a full set of flexibility relationships describing primary and coupled motions for C3-C4 and C5-C6 motion segment levels. Analysis of these data indicates that a flexibility matrix using incremental responses describing primary and coupled motions offers improved predictions over using linear methods (p<0.01). However, there is no significant improvement using more generalized nonlinear terms represented by the multivariable functional approach (p<0.2). Based on these findings, it is suggested that a multivariable approach for flexibility is more demanding experimentally and analytically while not offering improved motion prediction.

Human Cervical Motion Segment Flexibility and Facet Capsular Ligament Strain under Combined Posterior Shear, Extension and Axial Compression.

The cervical facet capsular ligaments are thought to be an important anatomical site of whiplash injury, although the mechanism by which these structures may be injured during whiplash remains unclear. The purpose of this study was to quantify the intervertebral flexibility and maximum principal strain in the facet capsular ligament under combined shear, bending and compressive loads similar to those which occur during whiplash loading. Two motion segments (C3-4 and C5-6) from seven female donors (50 +/- 10 years) were exposed to quasi-static posterior shear loads of 135 N applied to the superior vertebra on four occasions while under compressive axial preloads of 0 N, 45 N, 197 N and 325 N. Vertebral body motions and the full Lagrangian strain field in the right facet capsular ligament were measured using stereophotogrammetry. After flexibility testing, the right facet joint of each motion segment was isolated and failed in posterior shear. Differences in the kinematic response of the vertebrae and maximum principal strain in the capsular ligaments under the four axial preloads were tested using repeated-measures ANOVA's for each load step. Although significant differences were observed at two axial load levels in the kinematic sequence (197 N and 325 N), neither the regressed flexibility nor the maximum principal strain in the facet capsular ligament varied significantly with axial compression (p > 0.14). Maximum principal strain during the flexibility tests reached 61 +/- 33 percent of the maximum principal strain observed in sub-catastrophic failures of the isolated joints. Two of the thirteen specimens reached strains in their flexibility tests which were larger than their corresponding strains at sub-catastrophic failure in the failure tests. These results suggest that the cervical facet capsular ligaments may be injured under combined shear, bending and compression load levels that occur in rear-end impacts.

The biomechanics of cervical spine injury and implications for injury prevention.

Most catastrophic cervical spinal injuries occur as a result of head impacts in which the head stops and the neck is forced to stop the moving torso. In these situations the neck is sufficiently fragile that injuries have been reported at velocities as low as 3.1 m/s with only a fraction of the mass of the torso loading the cervical spine. Cervical spinal injury occurs in less than 20 ms following head impact, explaining the absence of a relationship between clinically reported head motions and the cervical spinal injury mechanism. In contrast, the forces acting on the spine at the time of injury are able to explain the injury mechanism and form a rational basis for classification of vertebral fractures and dislocations. Fortunately, most head impacts do not result in cervical spine injuries. Analysis of the biomechanical and clinical literature shows that the flexibility of the cervical spine frequently allows the head and neck to flex or extend out of the path of the torso and escape injury. Accordingly, constraints which restrict the motion of the neck can increase the risk for cervical spine injury. These observations serve as a foundation on which injury prevention strategies, including coaching, helmets, and padding, may be evaluated.

Epidemiology, classification, mechanism, and tolerance of human cervical spine injuries.

A review of published research is presented to examine human cervical spine injury epidemiology, classification, mechanism, and tolerance. Synthesis of the literature identifies several areas of cervical spine injury biomechanics in which the current understanding is greater than that suggested by individual investigations. Specifically, epidemiologic studies show an age dependent variation in the location of cervical spine injury. A classification scheme is developed on the basis of published work, in which the classes are defined by the resultant force acting at the site of injury. Further, for compression injuries it appears that a compression force tolerance criterion exists, and that eccentricity of the compressive force can be used to predict the type of cervical injury produced. However, to date, prediction of location of injury within the cervical spine has not been attempted. In particular, a compressive tolerance criterion is suggested between 2.75 and 3.44 kN for the adult cervical spine. In contrast, tolerance criteria for cervical injuries in other forms of loading are less well characterized. Review of the literature on spinal cord injury biomechanics and pediatric cervical spine injury reinforces the need for continued investigation in these areas.

Modification of the cortical impact model to produce axonal injury in the rat cerebral cortex.

Diffuse axonal injury (DAI) is a form of brain injury that is characterized by morphologic changes to axons throughout the brain and brainstem. Previous biomechanical studies have shown that primary axonal dysfunction, ranging from minor electrophysiologic disturbances to immediate axotomy, can be related to the rate and level of axonal deformation. Some existing rodent head injury models display varying degrees of axonal injury in the forebrain and brainstem, but the extent of axonal damage in the forebrain has been limited to the contused hemisphere. This study examined whether opening the dura mater over the contralateral hemisphere could direct mechanical deformation across the sagittal midline and produce levels of strain sufficient to cause a more widespread, bilateral forebrain axonal injury following cortical impact. Intracranial deformation patterns produced by this modified cortical impact technique were examined using surrogate skull-brain models. Modeling results revealed that the presence of a contralateral craniotomy significantly reduced surrogate tissue herniation through the foramen magnum, allowed surrogate tissue movement across the sagittal midline, and resulted in an appreciable increase in the shear strain in the contralateral cortex during the impact. To evaluate the injury pattern produced using this novel technique, rat brains were subjected to rigid indentor impact injury of their left somatosensory motor cortex (1.5 mm indentation, 4.5-4.9 m/sec velocity, and 22 msec dwell time) and examined after a 2-7 day survival period. Neurofilament immunohistochemistry revealed numerous axonal retraction balls in the subcortical white matter and overlying deep cortical layers in the right hemisphere beneath the contralateral craniotomy. Retraction balls were not seen at these positions in normals, sham controls, or animals that received cortical impact without contralateral craniotomy and dural opening. The results from these physical modeling and animal experiments indicate that opening of the contralateral dura mater permits translation of sufficient mechanical deformation across the midline to produce a more widespread pattern of axonal injury in the forebrain, a pattern that is distinct from those produced by existing fluid percussion and cortical impact techniques.