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.

Human Cervical Motion Segment Flexibility and Facet Capsular Ligament Strain under Combined Posterior Shear, Extension and Axial Compression.

The cervical facet capsular ligaments are thought to be an important anatomical site of whiplash injury, although the mechanism by which these structures may be injured during whiplash remains unclear. 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. Differences in the kinematic response of the vertebrae and maximum principal strain in the capsular ligaments under the four axial preloads were tested using repeated-measures ANOVA's for each load step. 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 (p > 0.14). 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.