Mapping the structural properties of the lumbosacral vertebral endplates

Study Design. A biomechanical investigation using indentation tests in a hu man cadaveric model to seek variation in the structural properties across t he lower lumbar and sacral endplates. Objectives. To determine 1) if there are regional differences in endplate s trength and 2) whether any differences identified are affected by spinal le vel (lumbar spine vs. sacrum) or endplate (superior vs. inferior). Summary of Background Data. It has been postulated that some regions of the vertebral body may be stronger than others. Conclusive data, either suppor ting or disproving this theory, would be valuable for both spine surgeons a nd implant designers because one mode of failure of interbody implants is s ubsidence into one or both adjacent vertebrae. Methods. Indentation tests were performed at 27 standardized test sites in 62 bony endplates of intact human vertebrae (L3-S1) using a 3-mm-diameter, hemispherical indenter with a test rare of 0.2 mm/sec to a depth of 3 mm. T he failure load and stiffness at each test site were determined using the l oad-displacement curves. Three-way analyses of variance were used to analyz e the resulting data. Results. Both the failure load and stiffness varied significantly across th e endplate surfaces (P < 0.0001), with posterolateral regions being stronge r and stiffer than the central regions. Characteristic distributions were i dentified in the lumbar superior, lumbar inferior, and sacral endplates. Th e failure load distributions were found to differ in l)the superior lumbar and sacral endplates (P = 0.0077), 2) the inferior lumbar and sacral endpla tes IP = 0.0014), and 3) the superior and inferior lumbar endplates (P < 0. 0001). The sacral and inferior lumbar endplates were both found to be stron ger than the superior lumbar endplates (sacrum, P = 0.054; inferior, P = 0. 008) but were not themselves significantly different (P = 0.89), Conclusions. Highly significant regional strength and stiffness variations were identified in The lumbar and sacral endplates. The center of the bone, where implants are currently placed, is the weakest part of the lumbar end plates and is not the strongest region of the sacral endplate.