The effects of bone density and disc degeneration on the structural property distributions in the lower lumbar vertebral endplates

Abstract

In this study, we hypothesized that vertebral bone density and disc degeneration would affect the structural property distributions of the lower lumbar vertebral endplates (L3–L5). The results may have implications for improving interbody implant designs to better resist subsidence. A 3 mm diameter hemispherical indenter was used to perform indentation tests at 0.2 mm/s to a depth of 3 mm at 27 standardized locations in 55 bony endplates of intact human lumbar vertebrae (L3–L5). The resulting load–displacement curves were used to extract the failure load and stiffness of each test site. Bone density was measured using lateral DEXA scans. Disc condition was determined using a four-point grading scale. Three-way analyses of variance were used to analyze the relationships between the data. The overall failure load decreased with bone mineral density (BMD) in the superior ( p<0.0001) and inferior ( p=0.011) lumbar endplates. In both endplates, the posterolateral regions were significantly stronger than more central regions. With increasing BMD, this difference became more pronounced in the superior endplates only ( p=0.005). Increased disc degeneration was associated with an overall failure load decrease in the inferior lumbar endplates ( p=0.002). The strength in the central regions of the superior endplates was reduced with increasing degeneration, but this was not observed peripherally ( p=0.001). Stiffness magnitude or distribution was not significantly affected by BMD or disc degeneration. The locations of the strongest regions of the endplate did not change with either bone density or disc degeneration. This implies that implant shapes designed using the basic structural property maps for the L3–L5 endplates are appropriate for use in patients with a wide range of pathologies, even though overall failure loads are generally lower in patients with reduced bone density and greater degrees of disc degeneration.