UBC Theses and Dissertations
Local strength and regional bone mineral density profiles of the thoracolumbar endplate Bailey, Christopher Stewart
1.1 Purpose 1. To determine the strength profile of the thoracolumbar endplates using indentation testing. 2. To determine the regional bone mineral density (rBMD) profile of the thoracolumbar endplates using peripheral quantitative computed tomography (pQCT). 3. To compare the thoracolumbar endplate strength profile with the rBMD profile. 1.2 Method Indentation tests using a materials testing machine and axial pQCT were performed on the T9, T12, and L2 endplates of seven fresh-frozen human cadaver spines. A minimum of twenty-five indentations was performed in a rectangular grid (7 columns by 5 rows). A 3mm hemispherical indenter was lowered at 0.2rnrn/s to a depth of 3mm producing endplate failure. Regional BMD was determined using a rectangular grid which over lay the indentation tests so that the density could be determined for the region of bone beneath each indentation test. Regional BMDs were manually determined using pQCT software. Repeated measures, multivariate AVOVA was used to analyse the affect the independent variables (level, endplate) and dependent variables (AP and lateral position) had on the endplate strength and rBMD profiles. 1.3 Results The strongest aspect of the thoracolumbar endplate was in the posterolateral corners. The weakest aspect was found in the centre of the endplate, which was located within the apophyseal ring. The anterior rim was the second strongest aspect of the endplate, especially for T9. The density of the sub-endplate zone decreased from the posterior and anterior aspects towards the centre of the endplate. The density decreased from the centre towards the lateral periphery except in the posterior row, where the posterolateral corners were the densest of the entire endplate. No significant difference existed in the mean strength or density between level or superior/inferior endplate. The strength profile and rBMD profile was influenced by the sagittal alignment of the spine; L2 was relatively stronger and denser in the posterior of the endplate, while T9 was relatively stronger and denser in the anterior of the endplate. This study found only a low to moderate correlation between rBMD and local strength. However, comparison of endplate and rBMD profiles showed similarities. 1.4 Conclusion In conclusion, the thoracolumbar endplate strength profile revealed that the anterior and posterior aspects of the endplate were significantly stronger than the centre. The anterior and posterior aspects of the sub-endplate zone were denser than was the centre. The posterolateral corners were the strongest and densest part, thought to be due to the pedicle insertion. The apophyseal ring was important in explaining the difference in strength between the centre and periphery of the endplate, apart from the increase in strength between the anterocentral aspect of the endplate and the posterior aspect of the endplate, which occurred within the confines of the apophyseal ring. Spinal sagittal contour was shown to influence the endplate strength and rBMD profile. A low to moderate correlation was found between a thoracolumbar endplate strength profile and thoracolumbar endplate rBMD profile. 1.5 Significance This knowledge may assist in preventing intervertebral implant subsidence by influencing implant positioning and design.
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