Mechanical evidence of cervical facet capsule injury during whiplash: a cadaveric study using combined shear, compression, and extension loading.

STUDY DESIGN A comparison of cervical facet capsule strain fields in cadaveric motion segments exposed to whiplash-like loads and failure loads. OBJECTIVES To compare the maximum principal strain in the facet capsular ligament under combined shear, bending, and compressive loads with those required to injure the ligament. SUMMARY OF BACKGROUND DATA The cervical facet capsular ligament is thought to be an anatomic site for whiplash injury, although the mechanism of its injury remains unclear. METHODS Motion segments from seven female donors were exposed to quasi-static flexibility tests using posterior shear loads of 135 N applied to the superior vertebra under four compressive axial preloads up to 325 N. The right facet joint was then isolated and failed in posterior shear loading. The Lagrangian strain field in the right facet capsular ligament was calculated from capsular displacements determined by stereophotogrammetry. Statistical analyses examined the effect of axial compression on motion segment flexibility, and compared maximum principal capsular strain between the flexibility and failure tests. RESULTS Capsular strain increased with applied shear load but did not vary with axial compressive load. The maximum principal strain reached during the flexibility tests was 61% +/- 33% of that observed in subcatastrophic failures of the isolated joints. Two specimens reached strains in their flexibility tests that were larger than their corresponding strains at subcatastrophic failure in the failure tests. CONCLUSIONS The cervical facet capsular ligaments may be injured under whiplash-like loads of combined shear, bending, and compression. The results provide a mechanical basis for injury caused by whiplash loading.