PATHOGENESIS/PATHOPHYSIOLOGY

BIOMECHANICAL ANALYSIS AND CLINICAL STUDY OF A POROUS TANTALUM IMPLANT FOR INTERVENING IN FEMORAL HEAD AVN

¹Robert Poggie, ¹Robert Cohen, ²Michael Christie,³Anneliese Heiner, ³Thomas Brown
1Implex Corp., Allendale, NJ ²Vanderbilt U., Nashville, TN 3U. of Iowa, Iowa City, IA

Introduction: This paper provides the mechanical characteristics of a porous metal implant for intervening in AVN, reviews the clinical outcome for a patient that received a custom implant, and reviews the study protocol of a recently initiated clinical study of the implant and core decompression.

Materials & Methods: The porous tantalum trabecular metal (Hedrocel, Implex Corp.) is 80% porous with a modulus of elasticity similar to cancellous bone. The implant is 10 mm in diameter, is offered in 70 - 130 mm lengths in 5 mm increments, and possesses threads for engagement of the lateral cortex. Flexural rigidity of the implant and human fibula was measured in 4-point bending. Fatigue strength testing per ISO 7206-4 and ISO 7206-8 was performed, and the efficacy of the rod in supporting the subchondral plate was performed with a femoral head defect model, and via a 3-D axisymmetric FEA model. A custom implantation was performed before initiation of the clinical study, and has been followed for two years.

Results: Flexural rigidity of the porous tantalum implant and the human fibula were found to be nearly equivalent. Finite element analysis showed that the porous tantalum implant and fibular graft provide similar mechanical support to the subchondral plate. Mechanical testing in the defect model showed the porous tantalum to decrease subchondral plate deflection by an average of 29%, which was comparable to that found for a non-implantable graft surrogate (27%). After 5 million cycles of fatigue loading in the same model (330 - 2300 N, 2.5 Hz), change in subchondral plate deflection and subsidence were minimal. The pre-op and follow-up radiographs for the patient receiving a custom implant in March of 1998 indicate that the implant and necrotic lesion are stable. The US FDA approved a comprehensive IDE study of the porous tantalum implant in April of 2000. The study is a randomized trial, with the comparative patient groups receiving either the porous tantalum implant or core decompression. Clinical outcome measures include the Harris Hip Score, SF-12 Quality of Life, radiographic analysis, and adverse events. The study is approved for recruitment of 522 patients with Stage I or II AVN; 174 patients are required for statistical significance.

Conclusions: The results of the biomechanical analysis, prior pre-clinical testing, the encouraging clinical outcome of the custom implant, and preliminary positive results for the IDE clinical study indicate that this porous tantalum implant may be an effective means for intervening in femoral head AVN.