Sunday, March 4, 2012

How the ASR XL Acetabular System Causes Bone Deterioration

When DePuy issued their hip recall, they offered to pay for the revision surgery needed to replace the faulty hip with a new implant. They appear to believe this is the full extent of their responsibility in repairing the damage they've done to thousands of people who had a hip replacement implanted in their bodies.

It's not enough. Because the damage the ASR XL Acetabular system caused doesn't end when the hip is removed. It's only just beginning.

How Your Hips Work

If this were an after-school special, we'd show you a few clips at this point: Elvis shaking his hips on stage back in the day, or a ballet dancer extending her leg over her head. The after-school special would be missing the point, however - your hips are essential to nearly every movement you make. You use them every time you stand, sit, crouch, or lean over. You use them getting into and out of cars, climbing up the porch steps to your front door, even just standing there.

A hip is a little bit like oxygen. You don't notice how essential it is until it's gone.

There's a reason you barely notice your hip's contribution to your everyday movements: your body has constructed it more or less perfectly. The hip bone is a portion of your pelvis, and it has a rounded cup-shape in its center into which the top of your femur bone fits, called the acetabulum.

Hip Anatomy-Hip Bones

The top of your femur fits smoothly into the acetabulum and rotates when you move, cushioned by synovial fluid that naturally lubricates the hip joint. When you get a hip implant, the idea is to mimic this natural construction of your hip joint as closely as possible, including the range of rotation, the lubrication, and the way the ball (the head) and socket (the acetabulum) fit together.

How a Hip Implant Works

There are two basic ways of affixing the hip implant to the rest of your skeleton. In both, the top of your femur is sawn off and replaced with an artificial new head, while the hip bone itself is shaved down to accommodate a man-made socket. Essentially, the acetabulum is carved out to the shape of the new socket.

The difference lies in how the new femur head and socket are attached to the surrounding bone. In older patients, the most common tactic is to use a bone cement to attach them. In younger patients, however, the surrounding bone is still versatile and capable of regrowth. Since cement has some serious drawbacks, including the risk that it will shrink or crack over time and the fact that it is not especially secure when it comes to withstanding forceful movements like jumping, another procedure has been developed for younger patients.

The cement-free procedure involves creating an environment that encourages the hip bone to grow onto the implant and hold it into place. To do this, the ASR XL Acetabular system included a cup with a pitted exterior, giving the bone lots of small holes to grow into and secure the implant further.

DePuy Pitted Surface

The small craters and pits give the surrounding bone places to hold onto as it grows around and into the surface of the implant. When this procedure works well, patients can often look forward to extended use of the hip implant for as much as 20 years.

This is more or less what DePuy promised the surgeons who recommended the ASR XL Acetabular System to their patients. 20 years turned out to be a gross overestimate.

How the ASR XL Acetabular System Failed

The ASR XL Acetabular System was a cement-free hip implant. Its design inserted a long stem into the femur bone for increased stability, and featured a socket that had that moon-crated surface which would encourage the hip bone to grow onto it and secure the implant in place.

DePuy X-Ray

ASR XL Acetabular

Unfortunately, the design was deeply flawed. The head and socket did not fit together properly, causing too much friction. There was no room for lubrication to seep into the space between the head and the socket, which meant the patients were in constant pain as the two parts of the hip implant ground together. Even worse, the friction was causing pieces of the implant - tiny metal ions - to rub off and seep into the bloodstream, poisoning the surrounding tissue and bone.

Under circumstances like that, it's no wonder the surrounding bone couldn't grow onto the implant to secure it in place. The bone was under an onslaught of abuse from increased friction and pressure as well as metallosis. Since there wasn't sufficient lubrication around the hip implant, the metal couldn't circulate freely throughout the body - not that this would have been much of an improvement, since the materials the ASR XL Acetabular System are made of include cobalt and chromium.

Since the metal ions couldn't circulate, they stayed put in the hip joint, causing massive bone deterioration, metallosis, metal poisoning, tissue damage and necrosis.

Why You Can't Just Take the Hip Implant Out

The obvious solution to a hip implant that is slowly poisoning you from the inside is to remove it. However, DePuy seems to think the problem ends there - which it doesn't.

As we've explained, you need healthy, living bone to make a hip implant succeed. Once the surgeon removes the ASR XL Acetabular System, there isn't much in the way of healthy bone left

There are two reasons for this: one is that the hip implant has been slowly poisoning the surrounding bone and discouraging growth. The second reason is much more simple: the hip bone simply hasn't had time to recover from the abuse of the initial surgery. The surgeon shaved away part of the acetabulum to make room for the implant, which is traumatic for the bone. It takes time to recover.

Normally, the hip bone would have 15 or 20 years to recover

With the DePuy hip recall, many patients' hip bones have had as few as two or three years, which simply isn't enough time. When a new device is put in, odds are very good that their bodies will reject the hip implant. That area has suffered a great deal of trauma and the body hasn't had time to forget it - so it will attack, and reject, any new foreign material.

Even a new hip implant - one that works the way it should.

Every hip implant causes more bone trauma and increases the chances that the next 510 will be rejected. Patients with working implants can expect to get a good 15 to 20 years out of their hips before they confront the fact that a new implant may only last half as long.

Those who received the ASR XL Acetabular System? They have to confront it now - along with the myriad side effects of the implant's poor engineering.

Replacing the hip implant isn't enough. We believe DePuy should pay full compensation for problems they've caused.

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Author: Andrew Sullo
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