Metal-on-metal hybrid hip resurfacing arthroplasty has been performed more frequently in recent years1,2. Its resurgence is attributed to a better understanding of metallurgy and to stricter tolerances with regard to the design of the metal-on-metal articulation. One currently used device is the Cormet hip resurfacing system (Corin Group, Cirencester, Gloucestershire, United Kingdom), a metal-on-metal implant with a cemented femoral component and a press-fit uncemented acetabular component. The cobalt-chromium acetabular component is coated with plasma-sprayed unalloyed titanium and overlaid with hydroxyapatite3.
In this report, we present the cases of two patients in whom hip resurfacing arthroplasty with this implant failed because of loosening of the acetabular shell secondary to debonding of the surface coating. To our knowledge, there have been no previously reported cases of such failure.
Both patients were informed that data concerning the cases would be submitted for publication, and they consented.
Case 1. A fifty-six-year-old woman presented in April 2003 with painful end-stage osteoarthritis of the left hip, for which nonoperative measures to obtain pain relief had failed. The patient underwent a left hip resurfacing arthroplasty in May 2003 with use of the Cormet resurfacing system. This procedure, performed by the senior author (M.A.J.), was one of 1,183 hip arthroplasties (102 of which were performed by the senior author) in a U.S. Food and Drug Administration Investigational Device Exemption study of that system4. The preoperative Harris hip score was 51. Initially, the patient did well, with excellent pain relief and function. The Harris hip score was 92 points at the postoperative visits at eleven and twenty-six months, and the postoperative radiographs that were made at the twenty-six month follow-up visit showed excellent component positioning and an absence of radiolucent lines (Fig. 1-A). Thirty-four months postoperatively, the patient had sudden onset of left hip pain and a grinding sensation after working out on an exercise bicycle. Radiographs showed a probable loosening of the acetabular component (Fig. 1-B). The patient was advised to return in two weeks for a repeat radiograph, at which time there was obvious loosening with gross displacement of the acetabular component (Fig. 1-C). A workup for infection was negative.
The patient then underwent a revision left hip replacement procedure with removal of the components and insertion of an uncemented conventional hip replacement. During surgery, it was noted that the titanium surface texturing had debonded from the back side of the acetabular shell and was adherent to the pelvis (Fig. 1-D). Curettes were used to remove most of the embedded coating, followed by standard reaming of the acetabulum. The reamers did not appear to be damaged by the process. It was not possible to determine if the hydroxyapatite was still attached to the surface of the porous coating. The anterior column of the pelvis had been eroded somewhat, and the posterior column had an irregular surface, necessitating enlargement of the acetabulum and placement of screws to secure the new cup. It was because of this enlargement and the requirement that screws be used to supplement cup fixation that the decision was made to implant an uncemented conventional hip replacement implant rather than another resurfacing component.
Because the revising surgeon was part of the Food and Drug Administration study group, the failed cup was first returned to the manufacturer (Corin) for analysis. The manufacturer determined that the components had been properly inserted and positioned. The failed cup was then sent to the Department of Mechanical Engineering at Bath University, United Kingdom, for independent testing. The examiners concluded that 80% of the surface coating had debonded from the cup. They could not establish a definitive cause for the failure and, specifically, could not attribute it to "the raw material, the machining process, or the coating process."5
Case 2. A sixty-four-year-old female ballet dancer underwent a right hip resurfacing arthroplasty with the Cormet resurfacing system in 2004. Her medical history was notable for a left total hip arthroplasty that had been performed in 2005. The patient presented to our institution in December 2006 with a six-week history of painless clicking and grinding of the right hip. The workup for infection was negative. Radiographs showed loosening of the acetabular component, and the patient underwent revision surgery with conversion to a conventional total hip prosthesis in April 2007. Intraoperatively, the surface coating was noted to have debonded from the prosthesis and was firmly adherent to the acetabular bone. It was not possible to assess the status of the hydroxyapatite. Because the revising surgeon was not part of the Food and Drug Administration study group and was not aware of the requirement to return the failed cup to the manufacturer for testing, the cup was discarded. The resurfacing arthroplasty was revised to a conventional total hip arthroplasty. At the time of the most recent follow-up, the patient had no complaints of pain or instability, and the components were stable radiographically.