The decision in late August by orthopaedic device manufacturer DePuy to recall one of its leading artificial hip replacement products sent shockwaves through the global medical sector.
The Articulating Surface Replacement (ASR) hip was recalled after figures showed higher than expected failure rates compared with similar products. The ASR system has been implanted into 93,000 patients worldwide, 8,000 of whom live in the UK. Many of these patients are now expected to see orthopaedic surgeons to assess their artificial joints to see if they need replacing.
ASR is part of a class of large-diameter, monoblock hip resurfacing and replacement devices often selected by surgeons for younger patients because it was thought the stable design would reduce the chances of dislocation after surgery. The device features a metal-on-metal ball-and-joint socket which was thought to be more effective and hard-wearing for patients than older style metal-on-polymer systems where the polymer tended to wear away, causing the joint to fail. But concerns over the performance of ASR started to arise when patients who had received the hip system began reporting groin pain.
That prompted a team of experts at a North East university to start investigating the discredited hip joints. “I got an email out of the blue back in late 2007 from an orthopaedic surgeon, Dave Langton, who was doing some research into these problems,” says Dr Tom Joyce, a reader in biotribology at the school of mechanical and systems engineering at Newcastle University. “He was finding that he was getting some patients coming back to him not long after their operation complaining of pain. The common factors were they all had the DePuy ASR hip and, interestingly, they were all women. So we started investigating.”
Expert in tribology
Joyce, a chartered engineer and fellow of the IMechE, is a respected authority in tribology, having worked for many years in industry before taking up his biotribology position at Newcastle University. His work with Langton took several directions, starting with the surgeon taking blood from patients who were suffering pain and sending it away for analysis. The findings revealed much higher cobalt and chrome levels in the blood than would be expected. The ASR hips were made of cobalt, chrome and molybdenum. So the pair suspected that the metal ball on the end of the femur was rubbing on the metal head of the ASR system, which is fitted into the pelvis, and so releasing tiny metal fragments into the body.
“My gut feeling from the start was that this was a wear problem rather than a corrosion problem, because the combination of cobalt and chrome has been used in people for years and it hasn’t really corroded significantly,” says Joyce. “Conceptually, anything that goes in the body is going to show some corrosion – but only a small amount.”
Langton then supplied Joyce with ASR hips that had been removed from patients who were suffering pain. Joyce began by looking at the roughness of the articulating surfaces, which were designed to operate under fluid film lubrication, to keep the two surfaces separate, using synovial fluid which is produced naturally in the body.
“To achieve that fluid film lubrication there would need to be a mirror finish on both components,” says Joyce. “So I took these devices and measured them to see what the surface roughness actually was. These had roughened – they were a lot rougher than they should have been which indicated essentially that they weren’t operating under fluid film lubrication, but they were actually in the boundary lubrication regime. So it meant that we had metal-on-metal contact, the metal had ground away on itself.”
Funding was obtained from Joint Action, the charity of the British Orthopaedic Association, to allow the purchase of a high-precision coordinate measurement machine which was used to give a more accurate calculation of the exact damage that was being done. “What we found was some disturbingly high wear volumes arising from the cups and the heads,” says Joyce.
Joyce believes there were two possible reasons as to why the metal was coming into contact. One was that, unless the hip was aligned perfectly, the ball was coming out of the cup, and literally banging back into place, starting the wear process. The other was that, because of the position of the cup within the body, the articulating surfaces were not getting enough fluid over them, meaning that the fluid that was being produced was being compressed and squeezed out of the joint.
This thinking was informed by the fact that the problems seemed to be occurring mainly in women. A series of weight-bearing X-rays were carried out, helping Joyce and Langton to develop a theory on this, relating to the size and position of the artificial cup. The research showed that, in general, the problems were occurring with smaller cups, positioned in the body with higher degrees of inclination and anteversion.
“When we looked at the pelvis of women, because they are child bearing, it meant those angles were worse. And also women tend to be smaller than men. So that explained to us why these failures were occurring in women,” he says. “There’s a lot of data coming out of researchers from Australia to back up our opinion. We’ve argued all along that this is not specifically a gender issue; it’s to do with the sizes of cup and its angles, which are more likely to be found in women.”
DePuy has now announced a worldwide recall of the ASR hip, but Joyce is continuing with his work and is currently evaluating more failed devices, including some from the US. All of the findings from his research activities have been passed on to the company. Joyce will also be speaking at an IMechE event at Birdcage Walk on 14 December about the implications of the DePuy situation for the orthopaedic industry.