Addition of lateral bending range of motion measurement to standard sagittal measurement to improve diagnosis sensitivity of ligamentous injury in the human lower cervical spine.

PURPOSE: This study examined the cervical spine range of motion (ROM) resulting from whiplash-type hyperextension and hyperflexion type ligamentous injuries, and sought to improve the accuracy of specific diagnosis of these injuries. METHODS: The study was accomplished by measurement of ROM throughout axial rotation, lateral bending, and flexion and extension, using a validated finite element model of the cervical spine that was modified to simulate hyperextension and/or hyperflexion injuries. RESULTS: It was found that the kinematic difference between hyperextension and hyperflexion injuries was minimal throughout the combined flexion and extension ROM measurement that is commonly used for clinical diagnosis of cervical ligamentous injury. However, the two injuries demonstrated substantially different ROM under axial rotation and lateral bending. CONCLUSIONS: It is recommended that other bending axes beyond flexion and extension are incorporated into clinical diagnosis of cervical ligamentous injury.

The effects of ligamentous injury in the human lower cervical spine.

Damage is often sustained by the anterior longitudinal ligament (ALL) and ligamentum flavum (LF) in the cervical spine subsequent to whiplash or other cervical trauma. These ligaments afford substantial cervical stability when healthy, but the ability of the ALL and LF to stabilize the spine when injured is not as conclusively studied. In order to address this issue, the current study excised ALL and LF tissues from cadaveric spines and experimentally simulated whiplash-type damage to the isolated ligaments. Stiffnesses and toe region lengths were measured for both the uninjured and damaged states. These ligamentous mechanical properties were then inputted into a previously-validated finite element (FE) model of the cervical spine and the kinematic effects of various clinically relevant combinations of ligamentous injury were predicted. The data indicated three and five-fold increases in toe region length for the LF and ALL injury variants, respectively. These toe length distensions resulted in FE predictions of supra-physiologic ranges of motion, and these motions were comparable to spines with no ligamentous support. Finally, a set of cadaveric cervical spine ligament-sectioning experiments confirmed the FE predictions and supported the finding that partial injury to the relevant ligaments produces equivalent cervical kinematic signatures to spines that have completely compromised ALL and LF tissues.

Finite element modeling of kinematic and load transmission alterations due to cervical intervertebral disc replacement.

STUDY DESIGN: A parametric finite element investigation of the cervical spine. OBJECTIVE: To determine what effect, if any, cervical disc replacement has on kinematics, facet contact parameters, and anterior column loading. SUMMARY OF BACKGROUND DATA: Anterior cervical discectomy and fusion has been a standard treatment for certain spinal degenerative disorders, but evidence suggests that fusion contributes to adjacent-segment degeneration. Motion-sparing disc replacement implants are believed to reduce adjacent-segment degeneration by preserving kinematics at the treated level. Such implants have been shown to maintain the mobility of the intact spine, but the effects on load transfer between the anterior and posterior elements remain poorly understood. METHODS: To investigate the effects of disc replacement on load transfer in the lower cervical spine, a finite element model was generated using cadaver-based computed tomography imagery. Mesh resolution was varied to establish model convergence, and cadaveric testing was undertaken to validate model predictions. The validated model was altered to include disc replacement prosthesis at the C4/C5 level. The effect of disc-replacement on range of motion, anteroposterior load distribution, contact forces in the facets, as well as the distribution of contact pressure on the facets were examined. Three sizes of implants were examined. RESULTS: Model predictions indicate that the properly sized implant retains the mobility, load sharing, and contact force magnitude and distribution of the intact case. Mobility, load sharing, nuclear pressures, and contact pressures at the adjacent motion segments were not strongly affected by the presence of the properly sized implant, indicating that disc replacement may not be a significant cause of postoperative adjacent-level degeneration. Implant size affected certain mechanical parameters, such as anteroposterior load sharing, and did not affect compliance or range of motion. CONCLUSION: The results of this work support the continued use of motion sparing implants in the lower cervical spine. Load sharing data indicate that implant size may be an important factor that merits further study; although, the deleterious effects of improper size selection may be less significant than those of fusion.

Correlation of mechanical properties within the equine third metacarpal with trabecular bending and multi-density micro-computed tomography data.

Computed tomography (CT) data can be employed with respect to determining mechanical properties and has been used to predict parameters such as elastic modulus, yield strength, and ultimate strength of intact bone. Micro-computed tomography (muCT) possesses the resolution capable of detecting apparent bone density in extremely local regions and can characterize the trabecular structure. It has been asserted that this micro-structure is susceptible to micro-buckling and bending, which has a controversial role in predicting the global mechanical properties of bone. The current study measured the mechanical properties of relatively high apparent density bone from the equine distal third metacarpal. The mechanical properties were correlated with trabecular morphology parameters and apparent densities of localized regions obtained with muCT. These data were used to test two hypotheses: (1) accounting for trabecular bending using trabecular morphology parameters would provide better global mechanical property predictions than using only apparent density, and (2) regions of low apparent density dominate the overall mechanical behavior and provide greater correlation to the measured mechanical properties than regions of high apparent density. The data indicated that accounting for trabecular bending with morphological parameters resulted in stronger correlations to mechanical properties than correlations that relied only on apparent density (r2= 0.91 versus r2= 0.81). Low apparent density regions were more strongly correlated with mechanical properties than high apparent density regions (r2= 0.85 versus r2= 0.77), demonstrating the importance of selecting appropriate regions when attempting to predict mechanical properties from CT data.