Abstract
Introduction The influence of annular tears on the biomechanical inter-relationship between the disc and vertebral body has a potentially important role in the mechanism of subsequent biological changes in disc and bone. The disc is a complex structure, exhibiting visco-elastic behaviour that is highly dependent on its condition and fluid content. Studies have shown that the stiffness of the disc is altered by its water content in human, ovine and bovine discs. It has also been shown that disc stiffness or modulus can be preserved if the level of water in the disc is kept constant. The importance of maintaining a reproducible state of stress in the disc during sequential testing of the same specimen is crucial to ensuring consistency of results and minimising systematic experimental errors. The aims of this study were to assess the reliability of sequential testing of the same specimen, and to determine whether stiffness, strains and pressure distribution can be restored to pre-testing levels under a uniform hydration loading environment.
Methods Six ovine FSUs with isolated discs were used in this study. Eight, 1-mm strain gauge rosettes were then bonded to the inferior VB of each FSU at lateral and anterior positions and three heights. FSUs were equilibrated for four hours in a saline bath at room temperature in a materials testing machine. A real-time pressure sensor was placed under the VB. FSUs were tested in axial compression at 0.1 Hz to 1 MPa for 5 sinusoidal cycles. Once tested, the FSU was placed under 0.25 MPa preload for one hour in the water bath for re-equilibration and tested again. Pilot studies by this group have shown that one hour is sufficient to return the disc to its original equilibrium state in a bath after testing, with no associated change in stiffness. This sequence was repeated four times to produce a total of five tests on each FSU. Outcome measures were FSU stiffness, axial strain, peak pressure, average pressure and contact area. Data was statistically analysed using intra-class correlation coefficients (ICC), and repeated measures ANOVA or paired t-tests.
Results The ICC for the five repeated stiffness measures was 0.24 (i.e 24% of the variation in the results was due to between-specimen tests with 76% of the variation due to within-specimen tests). Repeated measures ANOVA found no significant effect on stiffness due to repeating the test five times (P = 0.445). The ICC for the eight axial strains ranged from 0.8 to 0.99. There were no significant differences within any of the eight axial strains over the five repeats (P > 0.287). ICCs, and P values (in brackets) from repeated measures ANOVA, were 0.91 (0.179) for peak pressure, 0.85 (0.44) for average pressure and 0.99 (0.077) for contact area.
Discussion The largest systematic variation was seen for stiffness and this may be due to the tissue changes over the 9 hours of testing. Axial strains showed good to excellent agreement over the five repeated tests as did all pressure parameters. We conclude that the method of allowing one hour for re-equilibration in ovine discs produces a reproducible state of stress in the disc and minimises experimental errors.
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
