The implications of stress patterns in the vertebral body under axial support of an artificial implant.

Clinical studies show an association between changed load patterns both in the disc and its adjacent vertebral body, with painful degenerated discs. This suggests that failure to restore the normal loading pattern on implantation of a disc replacement could be a cause of lower clinical success rate. In the present study the variations of load patterns in the vertebra after disc implantation was studied using a simplified finite element models of natural and artificial discs. The effect of implant size and presence of voids at the implant-bony endplate interface were studied, for the worst case scenario of no bone remodelling. An altered stress pattern was observed in the vertebrae of implanted segments. The use of smaller size implants and presence of voids at the interface caused localized stress concentration in the endplate and adjacent cancellous bone. The study results support the hypothesis that current implants fail to restore normal loading patterns in the vertebral body, and the localized high stress regions could be the source of pain, and the cause of low success rate of TDRs.