Fice 1 INVESTIGATION OF FACET JOINT RESPONSE UNDER REAR IMPACT CONDITIONS USING FE MODEL OF THE CERVICAL SPINE

Whiplash injury resulting from rear impact is a significant issue in terms of societal cost, and the resulting pain and reduction in quality of life. The facet joints in the cervical spine have been identified as a source of pain in whiplash injuries; however, the responses of these joints are difficult to measure in vivo or in vitro. In this study, a detailed explicit FE model of the cervical spine was used to investigate facet joint response under rear impact loading conditions. The model represents a mid-size male with detailed vertebrae, discs, ligaments and Hilltype active muscles. This model was previously validated extensively at the segment level and validated for frontal impact scenarios. In this study, the cervical spine model was validated against rear impact volunteer and cadaver tests (13 volunteers exposed to 28 rear impacts at speeds of 5 to 7kph; 26 cadavers exposed to rear impacts at speeds of 5 to 15.5kph) using experimental acceleration, displacement and rotation traces of the T1. Capsular ligament (CL) strains were measured in the model and compared to values presented in the literature to identify pain or sub-catastrophic failure. Simulation of 4, 7, and 10g rear impacts showed good agreement with the experimental data. The predicted CL strains were below or near the approximate threshold for pain and sub-catastrophic damage (35% strain), and exceeded this value for a 12g rear impact case. This study included muscle activation, and provides a link between published strain limits for facet joint capsules evaluated in controlled lab conditions and strains predicted under rear impact loadings.