Facet joint kinematics and injury mechanisms during simulated whiplash.

STUDY DESIGN Facet joint kinematics and capsular ligament strains were evaluated during simulated whiplash of whole cervical spine specimens with muscle force replication. OBJECTIVES To describe facet joint kinematics, including facet joint compression and facet joint sliding, and quantify peak capsular ligament strain during simulated whiplash. SUMMARY OF BACKGROUND DATA Clinical studies have implicated the facet joint as a source of chronic neck pain in whiplash patients. Prior in vivo and in vitro biomechanical studies have evaluated facet joint compression and excessive capsular ligament strain as potential injury mechanisms. No study has comprehensively evaluated facet joint compression, facet joint sliding, and capsular ligament strain at all cervical levels during multiple whiplash simulation accelerations. METHODS The whole cervical spine specimens with muscle force replication model and a bench-top trauma sled were used in an incremental trauma protocol to simulate whiplash of increasing severity. Peak facet joint compression (displacement of the upper facet surface towards the lower facet surface), facet joint sliding (displacement of the upper facet surface along the lower facet surface), and capsular ligament strains were calculated and compared to the physiologic limits determined during intact flexibility testing. RESULTS Peak facet joint compression was greatest at C4-C5, reaching a maximum of 2.6 mm during the 5 g simulation. Increases over physiologic limits (P < 0.05) were initially observed during the 3.5 g simulation. In general, peak facet joint sliding and capsular ligament strains were largest in the lower cervical spine and increased with impact acceleration. Capsular ligament strain reached a maximum of 39.9% at C6-C7 during the 8 g simulation. CONCLUSIONS Facet joint components may be at risk for injury due to facet joint compression during rear-impact accelerations of 3.5 g and above. Capsular ligaments are at risk for injury at higher accelerations.