The purpose of this study was to quantify the intervertebral flexibility and maximum principal strain in the facet capsular ligament under combined shear, bending and compressive loads similar to those which occur during whiplash loading. Two motion segments (C3-4 and C5-6) from seven female donors (50 +/- 10 years) were exposed to quasi-static posterior shear loads of 135 N applied to the superior vertebra on four occasions while under compressive axial preloads of 0 N, 45 N, 197 N and 325 N. Vertebral body motions and the full Lagrangian strain field in the right facet capsular ligament were measured using stereophotogrammetry. After flexibility testing, the right facet joint of each motion segment was isolated and failed in posterior shear. Although significant differences were observed at two axial load levels in the kinematic sequence (197 N and 325 N), neither the regressed flexibility nor the maximum principal strain in the facet capsular ligament varied significantly with axial compression. Maximum principal strain during the flexibility tests reached 61 +/- 33 percent of the maximum principal strain observed in sub-catastrophic failures of the isolated joints. Two of the thirteen specimens reached strains in their flexibility tests which were larger than their corresponding strains at sub-catastrophic failure in the failure tests. These results suggest that the cervical facet capsular ligaments may be injured under combined shear, bending and compression load levels that occur in rear-end impacts.
Abstract