The Strength Profile of the Thoracolumbar Endplate Reflects the Sagittal Contours of the Spine

Study Design. Human cadaveric biomechanical study of indentation tests on the thoracolumbar vertebral endplates. Objective. To map the strength profile of the thoracolumbar endplates using indentation testing, and to document any changes in this profile with vertebral level. Summary of Background Data. The strength profile of the lumbosacral endplate was described previously. Based on these data, we hypothesized that the periphery of the endplate would be stronger than the center and that the strength profile would vary with the sagittal contour and level of the spine. Methods. Indentation testing was performed on the T9, T12, and L2 endplates of fresh-frozen human cadaver spines, using a materials testing machine. A 3-mm hemispherical indenter was lowered at 0.2 mm/s to a depth of 3 mm to produce local endplate failure. A minimum of 25 indentations were performed in a rectangular grid (rows: lateral, left to right; columns: A-P, anterior to posterior). Three-way analysis of variance was used to address changing strength profile patterns. Results. There were highly significant variations of indentation strength across the endplates in both the lateral and anterior to posterior directions. Each row of indentations was significantly stronger than the rows anterior to it (P < 0.04), except for the most anterior row. The most lateral columns were stronger than the central columns (P < 0.05). The ratio of the mean strength of the posterior row compared to that of the anterior row was significantly different across levels (P = 0.026). Conclusion. The periphery of the thoracolumbar endplate was stronger than the center. The difference in posterior to anterior endplate strength ratio between vertebral levels suggests a relative strength increase in the anterior aspect of the endplate with rostral ascent into the thoracic spine.