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 cadav eric motion segments exposed to whiplash-like loads and failure loads. Objectives. To compare the maximum principal strain in the facet capsular l igament under combined shear, bending, and compressive loads with those req uired 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-sta tic flexibility tests using posterior shear loads of 135 N applied to the s uperior vertebra under four compressive axial preloads up to 325 N. The rig ht facet joint was then isolated and failed in posterior shear loading. The Lagrangian strain field in the right facet capsular ligament was calculate d from capsular displacements determined by stereophotogrammetry. Statistic al analyses examined the effect of axial compression on motion segment flex ibility, and compared maximum principal capsular strain between the flexibi lity 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 t he flexibility tests was 61% +/- 33% of that observed in subcatastrophic fa ilures of the isolated joints. Two specimens reached strains in their flexi bility tests that were larger than their corresponding strains at subcatast rophic failure in the failure tests. Conclusions. The cervical facet capsular ligaments may be inured under whip lash-like loads of combined shear, bending, and compression. The results pr ovide a mechanical basis basis for injury caused by whiplash loading.