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.