THE EFFECTS OF IMPLANT STIFFNESS ON THE BYPASSED BONE-MINERAL DENSITY AND FACET FUSION STIFFNESS OF THE CANINE SPINE

Study Design. Effects of spinal implant stiffness and removal/retention on bypassed bone mineral density and column/fusion stiffness were studied in dogs. Methods. After facet fusion and bicortical peripedicle screw placement, one group of eight dogs received 6.35 mm and another 4.76 mm rod instrumentation at L3-L5. At 12 weeks, four in each group had implants removed. Bone mineral density was analyzed by dual energy x-ray absorptiometry at 1 to 24 weeks. Axial compressive stiffness of the L3-L5 construct, spinal column, fused facets, and instrumentation were measured. Percent load through the vertebral column was predicted. Results. Five observations were made for this canine model. First, stiffer implants resulted in more bypassed bone mineral loss at 6 and 12 weeks, plateauing and not different at 24 weeks. Second, after implant removal, a significant and similar rebound in bone mineral density occurred. Third, 4.76 mm rod instrumentation (initially 71% load through column) resulted in stiffer posterior fusions and vertebral columns than 6.35 mm rod instrumentation (initially 57% load through column). Fourth, marked stiffening of the anterior-middle columns (apparently disks) occurred. Fifth, percent load borne by the vertebral column increased with time. Conclusions. There appears to be a range of percent load through the vertebral column that creates optimum fusion/column stiffening while limiting bone stress shielding effects. The 6.35 mm rod constructs were predicted to allow greater than 70% axial load through the adult human thoracic/lumbar spine, implying biologic responses similar to 4.76 mm rods in dogs.