Abstract
Introduction Vertebral deformity, disc disorganisation, and change to vertebral bone architecture are morphological features that are associated with degeneration of the spine and with back pain. Observations from our earlier studies found that the BV/TV is always a maximum in the inferior third of the vertebral body (VB), and minimum in the central third. Animal model studies have reported that the strain in loaded vertebra is a minimum in the central third of the vertebra. There have been no studies investigating the direct affect of VB removal on functional spinal unit (FSU) stiffness, strain magnitude and the distribution of pressure immediately adjacent to the sectioned VB. There were a number of aims for this study. The first aim was to determine whether the strain varies between supero-inferior locations on the VB. The second aim was to determine if strain symmetry was present across the normal VB. The third aim was to determine whether transverse sectioning of the VB alters the stiffness, strains and pressure distributions of the functional spinal unit (FSU) and VB.
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 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. The inferior VB was then sectioned transversely at 1/3 of its height and placed under preload for one hour for re-equilibration and re-tested. This procedure was repeated at 2/3 of VB height and immediately adjacent to the endplate. Outcome measures were FSU stiffness, axial strain, peak pressure and average pressure. Data was statistically analysed using repeated measures ANOVA or paired t-tests.
Results The results of the first aim found no significant difference in strains within the right lateral or left lateral (P > 0.134) columns of strain gauges. However, for the anterior column of strain gauges, the superior strain was 30% higher than the inferior strain (P = 0.047). The results of the second aim found no significant differences between laterally opposing strain gauges (P > 0.139). For the third aim, transverse sectioning of the VB over three levels produced no significant differences for FSU stiffness (P = 0.275), strains for any strain gauge (P > 0.087), or peak and average pressures (P > 0.076).
Discussion This complex pilot study has shown that overall, axial cortical strain in a normal, ovine FSU did not vary with VB supero-inferior location laterally, but did vary anteriorly. Strains were symmetrical between laterally opposing VB locations at each of three levels, and was not affected by transverse sectioning of the VB at three levels. The finding that anterior column strains differ, may relate to changes in load distribution governed by VB surface second moment of area differences (laterally compared to anteroposteriorly), and the absence of a disc inferiorly. 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
