The effect of a stiff spinal implant and its loosening on bone mineral content in canines.

A canine spine surgery model was used to study bone mineral loss induced in vertebrae bypassed by stiff implants. Loosening of implants was measured, and a determination was made as to the effect of loosening on the degree of bone mineral loss. Sixteen dogs were implanted with a rigid screw-rod construct and randomized to one of four groups depending on the motion segments immobilized. No fusions were performed. Bone mineral was studied by DEXA at baseline, and postoperatively at 3, 6, and 9 months. Loosening measurements were made at the implant-implant interface and the implant-bone interface at 9 months postoperatively. Results show that bone mineral loss occurred uniformly at 3 months postoperatively in vertebrae spanned by the implants, with the average bone mineral density at 3 months representing 81% of baseline. When the implant remained secure, bone mineral loss persisted at 6 and 9 months but did not worsen. When the implants loosened, there was recovery of bone mineral to the baseline level by 9 months postoperative.