Abstract
Introduction The structure of the disc is both complex and inhomogeneous, and it functions as a successful load-bearing organ by virtue of the integration of its various structural regions. These same features also render it impossible to assess the failure strength of the disc from isolated tissue samples which at best can only yield material properties.
Methods This study investigated the intrinsic failure strength of the intact bovine caudal disc under a simple mode of internal hydrostatic pressure. Using a hydraulic actuator, coloured hydrogel was injected under monitored pressure into the nucleus through a hollow screw insert which passed longitudinally through one of the attached vertebrae.
Results Failure did not involve vertebra/endplate structures. Rather, failure of the disc annulus was indicated by the simultaneous manifestation of a sudden loss of gel pressure, a flood of gel coloration appearing in the outer annulus and audible fibrous tearing. A mean hydrostatic failure pressure of 18±3 MPa was observed which was approximated as a thick-wall hoop stress of 45±7 MPa.
Discussion The experiment provides a measurement of the intrinsic strength of the disc using a method of internal hydrostatic loading which avoids any disruption of the complex architecture of the annular wall. Although the disc is subjected to a much more complex pattern of loading than is achieved using simple hydrostatic pressurization, this mode provides a useful tool for investigating alterations in intrinsic disc strength associated with prior loading history or degeneration.
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
