Mechanical evaluation of cadaver retrieval specimens of cementless bone-ingrown total hip arthroplasty femoral components

Abstract

Early retrieval prior to gross failure of implants can provide valuable information for critical issues in total joint arthroplasty. To evaluate fixation of the femoral component in total hip arthroplasty, two femoral specimens—one Mallory-Head (Biomet, Warsaw, IN) specimen and one AML (DePuy, Warsaw, IN) specimen—were retrieved after 2–8 years of successful use in active patients. Radiographs were made and evaluated for trabecular apposition to the porous-coated areas of the stem, then torsional and axial load tests were performed for each specimen to determine micromotion and displacement at the bone-implant interface. Both implants had radiographic signs of bone ingrowth. No permanent rotational displacement was found in either specimen during torsional load testing, but rotational and axial micromotion were found in both. These findings indicate excellent fixation of implant to bone, and no slippage at the bone-implant interface. The Mallory-Head implant had much greater elastic displacement than the AML, and histological examination showed cancellous bone ingrowth into the porous-coated portion of the Mallory-Head stem. The AML implant, which withstood much higher torsional loads, was found upon histological evaluation to have dense cortical-cancellous bone ingrowth. Strength of attachment of the metal implant to bone was good in both specimens, and neither had slippage at this interface. Differences in mechanical behavior can be attributed to the type of bone supporting the implant. The Mallory-Head implant had a severely worn titanium femoral head, so the joint was full of particulate metal debris. Particle migration appeared to be especially well controlled by the closed-pore type of porous coating of the Mallory-Head stem. Since the AML implant had a cobalt-chromium femoral head, and consequently had no detectable metallic wear debris, a comparison of the barrier effect of the two types of porous coating could not be made. The beaded porous surface of the AML femoral component also seemed to be a barrier to polyethylene particle migration. Osteolysis was not found around either implant, and neither implant appeared to be clinically affected by particulate debris despite long-term service.