Large sizes of vertebral body replacement do not reduce the contact pressure on adjacent vertebral bodies per se

Large implants for vertebral body replacement (VBR) have a large footprint, and are normally supported by stronger bone at the rim of the vertebral body. But they also necessitate a greater corpectomy defect in the vertebral body concerned. In order to study the effect of implant size on contact pressure on the adjacent vertebral bodies and thus the risk of implant subsidence, an osseoligamentous finite element model of the lumbar spine was employed. The VBR was inserted at the level of L4 and additionally stabilized by posterior spinal instrumentation. Flat and curved vertebral endplates, small and large corpectomy defects, different implant positions and axial preloads as well as normal and osteoporotic vertebral bodies were simulated. Contact pressures in the vertebral body are increased for a curved vertebral endplate in comparison with a flat one, they are increased when an additional implant preload was assumed, and they are usually decreased for an osteoporotic vertebra when compared to a non-osteoporotic one. In some cases the average contact pressures were higher for the small-sized VBR, in others for the large-sized one. Our results reveal that from the mechanical point of view, a small-sized VBR is not generally disadvantageous.