The study, published today in the online journal BMJ Open, looked at revision rates at one hospital trust for the Depuy Pinnacle hip implant.

Highlighting the increasing number of hips that were manufactured outside the stated specification - with more than a third of hips manufactured to the wrong size – the research team said this may be to blame for the high failure rate of this implant.

They looked specifically at the long term performance of the 36 mm Pinnacle metal on metal hip—the most commonly implanted metal hip in the world—in a bid to uncover the risk factors associated with early failure and the need for further surgery.

In all the 434 cases examined, metal fragments had leaked from the implant into the bloodstream, while abundant metal staining of tissues visible to the naked eye (metallosis) had occurred in around one in five (19%).

Pinnacle hip the most commonly implanted in the world

Tom Joyce, one of the authors on the paper and Professor of Orthopaedic Engineering at Newcastle University, said:

“In this latest study we’ve found problems not just after implantation but also at the manufacturing stage and it appears some of the components have not been made to the correct sizes.

“If the two parts of the implant don’t match properly then the implant could be more likely to fail.

“The Pinnacle hip is the most commonly implanted metal on metal hip in the world and it is estimated that around 180,000 people might be walking around with one of these artificial hips inside them.

“It’s important that patients are closely monitored so that appropriate action can be taken.

“This research follows our work in 2008 where we highlighted the risks associated with the ASR metal-on-metal hip implants after we found evidence of wear in devices that had been removed from patients and detected metal fragments in the patients’ bloodstream.”

What the study found

The use of metal on metal hips has plummeted over the past five years, but thousands remain in place. A better understanding of the factors associated with a higher risk of failure would not only help those patients fitted with them, but could also inform the design of future products, say the researchers.

They reviewed the progress of 434 patients (243 women and 191 men) fitted with 489 metal on metal total hip replacements at one hospital trust in northern England, and monitored for an average of 7.5 years after the procedure.

In all, 71 metal hips required surgical removal and replacement, adding up to a revision rate of 16.4%, which the researchers describe as “unacceptably high.”

A metal on metal hip consists of a metal ‘ball,’ which acts as the top of the thigh bone (femoral head). This fits inside a metal liner, which acts as the replacement socket.

Total replacement of both (bilateral) hip joints and smaller thickness liners were risk factors for failure.

But implantation from 2006 onwards also carried a significantly higher risk of revision (replacement of a failed artificial joint with a new implant), possibly because of the increasing tendency from this date to manufacture implants outside of their intended specification, say the researchers.

Before 2006, only five out of 43 hips (12%) failed to meet the manufacturer’s product specification. But after 2006 more than a third (36%; 43 out of 118) failed to comply.

Furthermore, in over 40% of cases examined the taper surface was defective. This describes the inside of the femoral head that connects with the femoral stem, or large portion of the upper thigh bone—the load-bearing part of the implant. This defect was significantly associated with excessive metal particle release.

Metal hips implanted into women were also more likely to fail, but the researchers caution that twice as many women as men had bilateral hip replacements, and when the findings were analysed according to sex and liner thickness, smaller thickness liners had the greater impact.

Data from the National Joint Registry for England and Wales for 2014 indicate that 11,781 metal on metal Pinnacle hips have been implanted, prompting the researchers to calculate that 180,000 people around the world are now walking around with these hips. These patients might be at risk of early revision surgery, they suggest.

Research: Retrospective study of the performance of the Pinnacle metal on metal (MoM) total hip replacement: a single centre investigation in combination with the findings of a national retrieval centre. DOI 10.1136/bmjopen-2015-007847
Journal: BMJ Open