Stress distribution changes in bovine vertebrae just below the endplate after sustained loading

To describe the pattern of stress distribution in the vertebral body just behind the endplate, and to document its changes due to sustained loading.

Twelve fresh bovine coccygeal motion segments were dissected and tested. Each specimen was axially loaded with a sustained compressive force of 50% of its estimated compressive strength. Before loading, after 1.5 h and after 3 h of loading, the distribution of the axial pressure under the bottom vertebra (i.e., just below its top endplate) was recorded at three force levels (25%, 37.5% and 50% of the estimated compressive strength), using pressure-sensitive film.

Stress distribution over the endplate was found to be fairly uniform. At low compression forces, the stress was the highest centrally. With increased compression and after sustained compression the uniformity improved through a significant redistribution of stress to the periphery. No stress peaks were found to occur after sustained loading.

Stress peaks after sustained loading cannot explain the occurrence of endplate fractures in sustained cyclic compression in non-degenerated discs. Competing explanations, such as creep, and fatigue failure, would appear more likely candidates.

It has been hypothesised that compression induced fractures of the lumbar vertebral endplate constitute an important etiological factor for low back pain. Competing theories exist on the fracture mechanism in sustained loading and these would have different implications with respect to prevention. The present study evaluated one of these theories.