Measurement and analyses of the effects of adjacent end plate curvatures on vertebral stresses.

BACKGROUND

Vertebral end plates of the lumbosacral spine have various degrees of concavity and convexity. It is believed that the shape of the end plates alters the distribution of loads transferred along the spine, between the vertebrae. Animal models have been regularly used in the design and development of vertebral disc implants and cages; to date, very little information is known about the animal vertebral end plate curvature.

PURPOSE

The purpose was to measure and analyze the end plate curvature in the cadaver human male-female, chimpanzee, and canine lumbar vertebral bones.

STUDY DESIGN/SETTING: Nondestructive and nontouching scanning method was designed to obtain curvature in anterior-posterior and medial-lateral directions in the cadaver bones. Statistical analysis was performed on the data collected, and this data was then used to create a biomechanical model to evaluate the load transmission.

METHOD

Measurements in anterior-posterior and medial-lateral directions were performed on human, canine, and chimpanzee cadaver lumbar bones to obtain accurate data for the end plate curvatures. Six sets of measurements (on human male-female L4 lower to S1 upper end plates) were performed. A parametric vertebral motion segment model (with and without posterior elements) that includes the experimental curvature information was developed. The characteristic kidney-shaped cross-sectional model was created using a parametric equation. This model was used to perform finite element analyses investigating the effects of the location of maximum curvature on the stress distributions.

RESULTS

The measurements for different species showed that the canine and chimpanzees, the quadrupeds, have entirely different curvature of their upper end plates compared with those in humans, the bipeds. Also, the curvatures of the human S1 upper end plates are significantly different from the rest of the vertebrae. This is a very useful piece of information in the comparison of these species. The stress distribution varied as the location of the maximum curvature shifted from the center to a more posterior position. The stresses in the vertebral core were found to decrease, with the shell taking more loads.

CONCLUSIONS

This provides essential information for rehabilitation and surgical techniques, including designs for various interbody devices such as fusion cages, bone grafts, and disc prosthesis.