Monday, May 7, 2012

The Exact Problem with the ASR—Why is it Failing?

While DePuy Orthopedics originally stated they were phasing out the ASR artificial hip implant due to slow sales, months later they warned doctors that the ASR implant seems to have a higher than normal failure rate which occurred soon after surgery in many patients. These actions occurred after more than two years of negative reports about the ASR implant which led to questions as to why the company didn’t halt sales earlier. While some experts believe the ASR implant contained a basic design flaw which made proper implantation difficult, DePuy disputed such claims while agreeing to phase out the ASR across the globe by the end of 2010. The ASR had shown a higher than normal failure rate in several studies, most particularly in small-statured patients (typically women) or those with weakened bones. The ASR is one of several models of metal-on-metal hip implant sold by DePuy.

In some recipients the implant ball and cup can rub against one another with such force that tiny shavings of cobalt and chromium ions are released into the surrounding muscle and tissue as well as the bloodstream. This metal debris can lead to extreme levels of inflammation in the joint, infection and much more serious health issues as well, and at a minimum can require revision surgery much sooner than the expected fifteen years artificial hip implants are meant to last. Although the FDA released the ASR implant for use in conventional hip replacement surgeries in 2005, they failed to require DePuy to put the device through clinical trials. Starting in 2008 the FDA has received over 300 complaints from recipients of the ASR implant. DePuy has attempted to lay at least a portion of the blame for ASR failure squarely on the surgeons who implant the devices stating that in order for the ASR cup to function as intended, the angle it is positioned in the hip must be near-perfect. Several orthopedic surgeons, however, believe the design of the ASR is at fault as the cup is significantly shallower than other similar devices. There are four primary reasons experts believe the ASR cup has failed to perform as expected:

The Angle of the Cup

One of the perceived design flaws of the ASR lies in its relatively low angle tolerances which can lead to revision surgery at great expense and pain. The coverage angle of the ASR is significantly less than comparable implants. The Conserve Plus has a 174? outside and 170? inside coverage angle which is more than the 164? inside of the Birmingham model and considerably more than the 148? of the ASR. When the coverage angle of the metal cup is less than 170? there will be much less leeway both in terms of a safe inclination angle as well as the use of small component sizes.

In other words, the devices which offer a wider coverage angle have a larger margin for error and less edge loading risk even when the cup is not perfectly positioned at the recommended 40-45?. The coverage angle of these devices is design-specific and in the original metal-plastic joints, the coverage angle was 180?. The metal-on-metal devices lessened the coverage angle in an attempt to prevent the two metal parts from damaging one another during movement. The coverage of the ASR ball decreases with a cup with a higher abduction, a smaller size or a smaller angle of the implant and a smaller coverage results in less socket surface to cover the ball leading to more wear.

Thin Cup

The ASR implants are relatively thin at the rim—3.5mm in an ASR 50mm cup as opposed to 5.25 mm in the comparable Pinnacle cup. This reduction in thickness of the ASR cup leads to the potential of the deformation in the hard bone or seizing of the bearing which inhibits bone ingrowth. The ASR hip cup revision surgeries have mentioned loosening in the cup with surgeons noting that the cup in many instances simply “popped right out,” and there was little evidence of expected bony ingrowth even though the cup appeared to be in the proper position.

The ASR’s one-piece construction also lacks holes for viewing the hip socket through the cup during installation, making it difficult for the surgeon to properly confirm that the cup is fully seated in the socket during the surgery. Improperly seated cups can loosen early on following the surgery, allowing the cup to rotate into a less-than-optimal position which leads to excessive wear. The thin cup of the ASR is also much more easily deformed with any impact, leading to the necessity of subsequent revision surgeries.

Distance between Ball and Cup is Too Narrow

While the original intent of the ASR hip implant design was to allow greater motion of the ball within the socket, the DePuy ASR cup was so extremely shallow it resulted in a greater concentration of force over the limited contact area between the ball and socket. The basic premise of fitting the ball within the cut states that the tighter the fit, the less room for imperfect motion; the more perfectly the surfaces are matched and the less space between the ball and cup the less wear would occur on the components. What was not taken into account was that such a “perfect” fit, with little distance between the ball and cup would cause the metal components to seize, leaving no room for motion at all.

As the DePuy ASR is designed the slightest deformation during implantation can lead to complete loss of space and metal-on-metal wear, and since cups are likely to deform when hammered into the bone during surgery, this is a significant issue. If the cup is placed in an exact 45 degree position during surgery, the low-wear theory might hold up however few surgeons possess such exacting skill. Two scientists in the UK evaluated a large number of ASR implants which had been removed during revision surgeries. In a large number of these the measured clearances were less than 70 micrometers with some being less than 50 micrometers. This extremely narrow clearance was a DePuy manufacturing design and makes the device much more likely to fail from surface wear.

HA coating

Finally, the DePuy ASR implements a thin film covering which is composed of layers of materials designed to promote bone growth—in this instance the covering material is hydroxyapatite which is a natural component of the human bone. This film is meant to release a growth stimulant which helps grow new bone, and as the bone forms, the HA coating acts as a filler in the spaces which surround the ASR implant, in theory securing it to the patient’s bone. While DePuy’s efforts were admirable, the hydroxyapatite coating has failed because the films were too thick and unstable, causing them to break away from the implant. A study of the DePuy ASR HA-coated cups found granules had caused abrasive wear producing severe problems with few warning signs. Since HA is much harder than cobalt, chromium and titanium, it is a given that these particles will only increase wear in the device. A team of MIT chemical engineers has recently developed a new coating which is thought to help the joint implants more properly adhere to the patient’s bones and prevent early failures. The thickness of the film and the amount of growth factor released are controlled using a layer-by-layer assembly process.

All in all, the ASR simply possessed too many design flaws not to fail, causing a great number of revision surgeries which are both painful and expensive.

Author: Sullo & Sullo Attorneys

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