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Cup position may not influence articular surface replacement failure
Even articular surface replacements implanted with cup inclinations within the safe zone of 55° failed.
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Failure of articular surface replacements is linked with bearing and taper wear, not cup position, according to a study presented by AVF “Tony” Nargol, FRCS, at the British Orthopaedic Association and the Irish Orthopaedic Association Combined Meeting 2011.
“With the [articular surface replacement] ASR total hip replacement (Depuy, Warsaw, Ind.), we discovered that the taper was wearing out,” Nargol told Orthopedics Today. “But that same taper wear was seen with other types of metal-on-metal hip replacements. While the ASR was used in 94,000 patients, potentially, there are another million patients with other metal-on-metal devices who could be at risk. The problem could be a lot larger than most people realize.”
The investigators initiated a prospective study of 505 cases. Mean follow-up was 52 months. They looked at Kaplan Meier survivorship rates at up to 7 years. They discovered a 25% failure rate, which increased to 50% with an added stem.
Failure in the safe zone
The team aimed for targets of 40° to 45° before the manufacturer set a target of less than 55° for the cup inclination safe zone, according to Nargol.
“What strikes you is that peak failures are not at the higher cup angles, but actually in the middle of 45° to 55°,” Nargol said.
The team found that 75 of studied cases had inclinations less than 55°.
“From 2007 on, we have realized cup position is critical and our mean has come down to 40°,” Nargol said.
Nargol and his team compared their findings to those from other centers. One center had a mean of 49°, another 50°.
“What this demonstrates is, whatever position you implant the ASR they can fail,” Nargol said. “We have a 50% failure at 7 years with the majority in the safe zone. Failure has no relationship to cup position. The national joint registry last year had a 12% failure. We are waiting to see what happens. My guess is it will be double and nearly triple that rate next year.”
In a follow-up interview, Nargol noted that the 2011 failure rate released by the National Joint Registry (NJR) in the United Kingdom was 29% at 6 years.
Unexplained jump
At 6 years, Nargol noted that the researchers witnessed an acceleration in ASR failures.
“If you look at the graph of all failures of metal-on-metal hips in the UK NJR, at 6 years there is a jump in the failures,” Nargol told Orthopedics Today. “There is a sharp rise at 6 years in the NJR that cannot be explained by the ASR xl failures. We think this is the taper problem starting to be shown in the NJR. So it is a worrying year or two ahead. We think eventually the ASR failure rate will get up to 50% in the next year and 75% in the next 2 or 3 years, while mechanical taper failure will increase with metal-on-metal total hip replacements. We have now been proven correct about our warning to the world about the ASR, and we warn about the rising failures of tapers.” – by Renee Blisard
Reference:
- Gandhi JN, Sidaginamale RP, Mereddy PK, Langton D, et al. Catastrophic failure of the articular surface replacement (ASR) after 5 years. Presented at the British Orthopaedic Association and the Irish Orthopaedic Association Combined Meeting 2011. Sept. 13-19. Dublin.
- AVF “Tony” Nargol, FRCS, can be reached at the Joint Replacement Unit, University Hospital of North Tees, Hardwick, Stockton-on-Tees; 01642 617617; email: a.nargol@hotmail.co.uk.
- Disclosure: Nargol has no relevant financial disclosures.