A model to study the disc degeneration process.

Initiation and propagation of different types of events that lead to disc degeneration and the effect of the degenerating process on the disc mechanical performance is difficult to study by experimental methods. This study aimed to develop and use a finite element model of a motion segment without posterior elements to study the disc degeneration process. The model was used to study the development of anular tears, nuclear clefts, and endplate fractures and subsequent propagation of these degenerative processes due to compressive and bending loads. The analyses showed that the failure always started at the end plates indicating that they are the weak link in the body-disc-body unit. The analyses also showed that anular injuries are unlikely to be produced by pure compressive loads. The model predicted that it would require a larger extension moment as compared to flexion moment to initiate and propagate failure in a motion segment, which leads to the conclusion that the motion segment is stiffer in extension. The model also suggested that the presence of discrete peripheral tears in the anulus fibrosus may have a role in the formation of concentric anular tears and in accelerating the degenerating process of the disc.