Biomechanics of Total Disc Replacements

People: Noah Bonnheim

Total disc replacement is an alternative to spinal fusion as a surgical treatment for degenerative disc disease and other spinal pathologies. Although there are about 500,000 spinal fusion surgeries performed each year in the U.S.—at a cost of about $50B—there is substantial evidence suggesting that lumbar fusion can accelerate degeneration at adjacent spinal levels. This complication, and variable long-term clinical outcomes, has motivated interest in total disc replacements. This class of implants is designed to preserve relative motion between adjacent vertebral bodies; it is thought that motion preservation will result in more normal physiologic levels of stress in the bone compared with a fusion surgery (which prevents motion and thus increases stress). However, little is known about the mechanical environment within a vertebral body supporting a total disc replacement implant, in part because previous analyses have not accounted for the microstructural complexity of human vertebral bone. Addressing this issue, our research effort involves micro-computed tomography based finite element analysis on human lumbar vertebrae virtually implanted with total disc replacements, thereby capturing the geometric complexity of the trabecular microstructure and thin cortical shell.

 So far, we’ve learned that total disc replacement can affect stress in the bone tissue deep in the vertebral body, not just adjacent to the implant. Relative to the expected physiologic distribution of stress in the bone (as measured by loading the vertebral body via a disc-like material), the tissue-level stresses appear to be more sensitive to implant size than implant material. Finally, for implants of typical clinical size, impingement of the implant induced while bending can substantially increase stress in local regions of the vertebra and may therefore be one factor contributing to subsidence in vivo.