Biomechanical factors influencing disruption of the intervertebral disc

Abstract

Purpose: To investigate, quantify and model the influence of three biomechanical factors on the severity of mechanically induced nuclear disruption in healthy bovine caudal intervertebral discs. The biomechanical parameters investigated were flexion, hydration and rate of compressive loading. Methodology: A preliminary study was conducted with an artificially severed annular wall to investigate the cohesive nature of the isolated nucleus and its tendency to form clefts when loaded. A second, more clinically relevant model using whole bovine discs was then conducted to investigate whether significant clefts could be induced in healthy discs by controlling the above biomechanical factors. A finite element model of the bovine caudal disc was also constructed to predict the complex mechanical conditions that exist within the disc. Results: High degrees of flexion and hydration are significant risk factors in nuclear disruption of discs having an annular division (P<0.001 & P<0.003 respectively), while the rate of loading showed no significant effect (P=0.37). The intact disc study also showed that flexion and hydration are significant risk factors (P<0.009), while the rate of loading showed no significant effect (P>0.3). The finite element model predicted a high concentrations of strain energy density within the disc in the peripheral region of the interface between the soft tissue and the bony endplates. This is consistent with clinical observations of annular rim lesions. Conclusions: The results of the divided wall model and the undivided wall model, both showed the same the level of significance for flexion and hydration, as risk factors for disruption of the nucleus. While flexion is well known to be a primary risk factor, traditionally the level of hydration has been attributed less importance. The results of the present research suggest that hydration level may be of equal importance with flexion. Therefore it can be concluded that any rigorous study examining the mechanisms of intervertebral disc prolapse should consider the influence of potentially unknown variables such as the level of degeneration and the hydration state of the disc.