Mechanical consequences of rod contouring and residual scoliosis in sublaminar segmental instrumentation.

The mechanical performance of contoured Luque rods in a neuromuscular model of spine deformity was examined to define an upper limit of deformity above which rod stresses would exceed the endurance limit for 316L stainless steel and therefore predict fatigue failure. Bovine constructs varying from 0-120 degrees scoliosis were loaded axially, with strain recordings obtained at the apex of the curve. Relatively low loads produced enough tensile stress to contemplate implant fatigue in all except the nondeformed (0 degrees) construct. Construct stiffness was found to decrease rapidly in spines with greater than 38 degrees deformity. In addition, data on patients who had suffered rod fracture from four different centers were found to compare favorably with experimental observations. We conclude that the vulnerability of Luque rod constructs to implant failure, from a mechanical standpoint, is greater than is generally assumed. Cross-linking of rods was found to increase stiffness. Methods to decrease tensile stresses in the implants and increase stiffness include external immobilization, larger diameter rods, and procedures to enhance correction.