July 18, 2012 — Metal-on-metal hip implants fail because of loosening, metal reactions, pseudotumors, and seizing components. According to new research published in the journal Orthopedics has found that the hip implants tend to fail within two years. Furthermore, the most common reason for failure was a loosening of the acetabular cup. The two types of hip implants that were responsible for most of the failures were the ASR and the Zimmer Durom cups.
From a total of 80 patients undergoing revision surgery for problems with metal-on-metal hip implants, the researchers found the following complications and rates:
- Aseptic acetabular loosening — 56.25%
- Infection — 12.5%
- Metal hypersensitivity — 6.25%
- Failed resurfacing — 6.25%
- Fracture — 5%
- Loose stem — 3.75%
- Seizing of device components — 1.25%
- Cup malposition — 1.25%
- Femoral stem fracture — 1.25%
The majority of patients who required revision surgery had a loosening of the acetabular hip cup, a complication that occurs when the round head of the femur falls out of the hip socket. This complication can cause severe pain and decreased ability to walk. Of the 45 cups that loosened, nearly 80% were Durom acetabular cups, and nearly 10% were the DePuy ASR cup. The remaining 10% were Magnum, Biomet, Pinnacle, or Conserve hip implants.
The researchers also found that metal-on-metal hip implants tended to fail within 2-3 years after the implant. Nearly 80% of patients had revision surgery within two years, and 92.5% of the patients had revision surgery within three years.
Furthermore, 13% of these patients had local tissue reactions. This side effect is likely caused by the effect of metal particles. When the metal-on-metal parts grind together, small particles of chromium debris accumulate in soft tissues around the hip implant. The chromium oxidizes, corrodes, and releases charged cobalt ions. Cobalt is known to be genotoxic, meaning that it can damage cellular DNA. Over time, this can lead to metal poisoning (metallosis), soft tissue reactions, necrotic (dead) tissue, bone loss, muscle damage, severe pain, swelling, inflammation, and the formation of pseudotumors.
The results of the study emphasize the fact that researchers are still trying to understand the safety and risks associated with metal-on-metal hip implants.
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