Abstract
Introduction The influence of annular tears on the biomechanical inter-relationship between the disc and vertebral body (VB) has a potentially important role in the mechanism of subsequent biological changes in disc and bone. It is postulated that changes in the disc may result in increased or abnormal spinal segment motion, modified load distribution across the spinal joint and altered cancellous bone architecture. There have been no studies investigating the direct effect of disc injury on functional spinal unit (FSU) stiffness and the distribution of pressure immediately adjacent to the disc inferior endplate. The aim of this study was to determine whether minor and severe radial tear injuries to the disc alters FSU stiffness and VB surface pressure distribution.
Methods Six ovine FSUs were used in this study. The posterior elements were removed leaving the isolated disc in each FSU. The inferior VB was transversely cut immediately inferior to the endplate and the neutral axis of bending (NAB) identified and marked. FSUs were equilibrated in a saline bath at room temperature for four hours under a constant preload of approximately 0.25 MPa prior to testing. After equilibration, FSUs were transferred to a saline bath in a materials testing machine (Instron 8511, Instron, High Wycombe, UK) and a real-time pressure sensor (I-Scan 5076, Tekscan Inc., MA, USA) placed under the inferior VB.
While maintaining the preload, FSUs were loaded in axial compression at 0.1 Hz through the NAB to 1 MPa in a saline bath for 5 sinusoidal cycles. Once tested, a radial tear was introduced via scalpel injury into the left postero-lateral region of the annulus and tested after one hour of re-equilibration. A final, more severe injury, in the form of removal of a 5 mm x 2 mm window of annulus at the same location was performed and tested after re-equilibration.
Outcome measures were FSU stiffness, peak pressure, average pressure, contact area, and centroid of force location. Data was statistically analysed using repeated measures ANOVA or paired t-tests.
Results No significant differences in stiffness was found as a result of disc injury (P = 0.857), nor for peak and average pressure, contact area and centroid location (P > 0.179).
Discussion These results may not be surprising given that the disc has been shown to be remarkably resilient under axial compression, even with a severe annular or nuclear injury. Further insight will be revealed when other modes of loading are performed in both ovine and human discs for the main study planned to be undertaken in the near future.
The abstracts were prepared by Professor Bruce McPhee. Correspondence should be addressed to him at Orthopaedics Division, The University of Queensland, Clinical Sciences Building, Royal Brisbane & Women’s Hospital, Herston, Qld, Australia
