Biomechanical factors influencing nuclear disruption of the intervertebral disc

A disc model with full anular division was used to investigate how different biomechanical parameters influence the severity of nuclear disruption during compressive loading. To quantify the manner in which flexion, hydration, and loading rate contribute to the breakdown in the intrinsic cohesive structure of the nucleus pulposus. The risk of disc herniation is known to increase when the disc is loaded in flexed positions. However, there is a lack of experimental data showing how a combination of flexion with different loading rates and hydration levels affects the extent of nuclear disruption. A reproducible state of full hydration was established for isolated bovine caudal discs. A period of static preloading at an applied stress of 1 MPa was used to obtain a consistent state of partial hydration. Then 96 discs were subjected to a full-thickness division of the anulus fibrosus and compressed while hydration level, degree of flexion, and rate of loading were varied systematically. A full spectrum of nuclear damage was observed in the tests, ranging from no detectable disruption to sudden sequestration of the entire nucleus. These results were quantified, and a general correlation was established between the severity of disruption and the different loading parameters. The degree of flexion and the level of hydration were shown to play an important role in influencing the tendency of the nucleus to break loose and extrude through a preexisting anular division. Interestingly, the rate of loading appeared to have only a minor effect on the severity of damage induced in discs that incorporated a full depth anular division.