A mechanical basis for whiplash injury: The cervical facet joint, spinal motion segment, and combined loading

Whiplash injuries impose a tremendous cost on society. While the cervical facet joint has been suggested as a source of neck pain in whiplash injury, the mechanical foundation of its role in low-velocity collisions remains unknown. Studies indicate poorer clinical outcome for pretwist of the head and neck, yet mechanical evidence supporting this hypothesis is lacking. No experimental study has assessed the mechanical role of the facet joint and its capsular ligament in whiplash injury. This study aims to determine the injury risk for this joint during vertebral motions experienced during whiplash. Estimates of facet capsule loading due to neck muscle contractions evaluate the additional risk for injury to this joint. Cadaveric motion segments underwent pure moment testing in neutral configurations and following an axial pretorque. Full-field maximum principal strains in the capsule were determined during bending and at tensile failure. Flexibility testing was performed to describe joint motions for generalized three-dimensional loading. Matrix models were developed to evaluate which modeling approaches have the strongest predictiveability for vertebral motions during combined loading. Quantitative histology determined that an average of 22.4% of the facet capsule is covered by muscle insertions, estimating the risk for muscle-mediated facet capsule injury. Maximum principal strains in the facet capsule were almost doubled during bending in the presence of a pretorque. Using strain analysis, the facet capsule is not at risk for catastrophic injury during whiplash bending motions. However, in the presence of an axial pretorque, approximately 1% of vehicle occupants are at risk for subcatastrophic injury and therefore pain development. Muscle fiber insertions offer an additional load path to the capsule in the presence of muscle contractions, suggesting that greater than 1% of the population may be at risk for pain development during whiplash. Flexibility model predictions best predicted vertebral rotations for a matrix containing both primary and coupled terms and using incremental nonlinear representations of vertebral responses. This study provides mechanical data validating the cervical facet joint's role in whiplash injury and supports suggestions that axial pretorque may affect clinical outcomes in whiplash injury.