Structural Efficacy of a Novel Porous Tantalum Implant for Osteonecrosis Grafting

Introduction: Vascularized fibular grafting is a promising treatment for femoral head osteonecrosis (ON), the rationale in part being to support the subchondral plate during structural reconstitution of the underlying (weakened) cancellous bone. Unfortunately, donor site morbidity is appreciable, including weakness, discomfort, and sensory abnormalities. An implant made from porous tantalum (Hedrocel trabecular metal, Implex Corp., Allendale, NJ) has been developed to substitute for a structural fibular graft. Earlier, 3-D finite element analysis suggested that such a porous tantalum graft would reduce maximum stress in a necrotic femoral head to a similar extent as does a natural fibular graft. However, the computer model assumed perfect graft incorporation, and did not include a curettaged lesion as is done clinically. The purpose of this study was to directly measure structural efficacy of porous tantalum ON grafts, using a physical model of ON grafting that did not include perfect graft incorporation, and did include a curettaged lesion, thus representing the early postoperative period. Methods: Five porous tantalum grafts (10mm diameter) were straingaged 20mm away from the tip of the graft, after milling out a shallow trough to accommodate the straingage and the wires. The straingage, when the graft was inserted into the femur, faced downward (Figure 1). This straingage location was in the area of maximum compressive stress in a finite element model. Composite surrogate femurs (Pacific Research Laboratories, Vashon, WA) were utilized, for which the “northern hemisphere” of the original femoral heads were replaced with custom surrogates that matched the stiffness of human femoral heads. These femoral heads had a curettaged necrotic defect created by hollowing out a 28.6 mm diameter core of cancellous bone analog, to the depth of the cortical shell (Figure 1). The cores for the grafts were drilled along the midplane of the femur, at a 50° angle from the lateral diaphysis. Subchondral plate deflection ( ), used to index the supportive effect of the grafts, was measured with an LVDT (Model GHSA 750-050, Macro Sensors, Pennsauken, NJ). The LVDT was mounted at the undersurface of the femoral head; its tip contacted the subchondral plate adjacent to the graft contact site (Figure 1).