In a previous article we wrote on bone deterioration, we discussed how the bones of your hip are constructed and how a hip implant strives to mimic the qualities so that you can maintain a normal range of movement and a fairly active life.
In this article, we'd like to show you how the ASR XL Acetabular System was constructed - and why it fell so short of what hip implant patients needed to give them back their previous quality of life.
What a Hip Implant is Up Against
Any hip implant is going head-to-head against the natural construction of your body, which is no easy feat. What we think of as our hip is actually two bones: the acetabulum (also called the hip socket) and the femoral head, which is the rounded top of your femur (the main bone that runs the length of your thigh).
As you can see, the two bones of your hip are actually attached to each other securely by two ligaments. The shorter ligament at the center of the femur head is the transverse acetabular ligament, while the longer ligament that runs from the ilium down to the femur itself is the iliofemoral ligament.
These ligaments, along with the cartilage surrounding the hip joint, are the reason your hip doesn't dislocate on a regular basis. The iliofemoral ligament in particular is extraordinarily strong - in fact, the strongest ligament in the human body - and when you are standing or sitting, this ligament flexes or releases to allow a range of movement without letting the hip move out of the socket.
For its part, the transverse acetabular ligament and the surrounding cartilage make up the acetabular labrum, whose purpose is to deepen the hip socket so that the head of the femur can't slip out. The deeper the hip socket, the more secure the femur bone becomes and the less likely your hip will dislocate.
When your natural hip is removed to make way for a hip implant, neither of those ligaments is in place, and the cushion of cartilage is removed to make way for an artificial replacement. Removing the labrum means that your hip is 92% more likely to suffer contact stresses and 40% more likely to allow the femur and acetabulum to touch.
As you can see, your hip is a sophisticated and well-constructed mechanism, and it is extremely difficult to create a man-made replacement for it. Let's take a look at some of the best attempts available in the form of conventional hip replacement and total hip replacement systems, including the ASR Acetabular System.
Conventional Hip Replacement (also called Conventional Hip Arthroplasty)
In a conventional hip replacement, the acetabulum is resurfaced with a new socket, replacing the ring of cartilage as pictured above with an artificial cup made of plastic, ceramic, or metal. The head of the femur is then removed and replaced with a long stem capped with a ball meant to mimic the femur head, as shown below:
In the conventional hip replacement, the plastic cup above is meant to act as the bearing surface, which means that it is the location where the ball and socket contact each other. The liner cushions the point of contact, but it also makes the socket more shallow, which can mean less range of movement and a higher likelihood of dislocation.
The creators of the total hip replacement system hoped to eliminate the problems associated with conventional hip replacement by boldly doing away with a piece of the original design.
Total Hip Replacement
The socket on a conventional hip replacement was technically composed of two pieces: the acetabular cup and the plastic liner, the latter fitting snugly into the former. In a total hip replacement, the design simply eliminated the plastic liner, fitting the femoral head directly into the acetabular cup.
You can see the difference in the image below. The top implant [Fig. 3] is a conventional hip replacement system, with an acetabular cup, a poly (plastic) liner, and a femoral head. The lower implant [Fig. 4] is a total hip replacement system, with only an acetabular cup and femoral head.
This design allowed the femoral head to be much larger, which made it more stable and less likely to dislocate. It also meant, however, that there was no cushion between the two hard surfaces of the acetabular cup and the femoral head.
Many total hip replacement systems are extremely successful, and many surgeons prefer them over the two-piece conventional hip replacement systems, particularly for younger patients who are more active and more likely to dislocate their hip implant by testing the limits of its range of movement.
However, the one-piece system made meticulous design engineering absolutely essential to avoid friction between the acetabular cup and the femoral head.
Which is where DePuy's ASR Acetabular System went wrong.
ASR XL Acetabular Total Hip Replacement System
The ASR XL Acetabular System is a total hip replacement system that uses the model in Fig. 4 above. It uses a traditional femoral ball and stem, and has a 1-piece metal bearing socket placed in the acetabulum. The "ASR" in the title is simply DePuy's trademark designation for this class of single-piece sockets, while the "XL" refers to the fact that DePuy made their implant available in larger sizes for patients whose bodies could accommodate them.
As we've already explained, much of the cushioning and connecting tissue that would normally keep your hip bones properly in place have been removed to accommodate a hip implant. Much of the cartilage is missing, which studies have shown decreases the amount of synovial fluid available to lubricate the joint. The synovial fluid will have an even more difficult time lubricating the joint if the implant has been poorly designed, restricting the flow of fluid between the pieces of the hip implant.
Poor lubrication can cause patients a great deal of pain, but it can also cause friction between the femoral head and socket, encouraging particles of the implant to detach and enter the bloodstream. If the friction is sufficient, the accumulation of metal ions in the bloodstream and the surrounding tissue can cause metallosis, heavy metal poisoning, metal sensitivity, bone deterioration and tissue damage.
The evidence is mounting to show that the DePuy ASR XL Acetabular system was so poorly designed that the friction between ball and socket is far beyond normal, and in fact may be causing all of the problems above in an inordinate percentage of patients.
Furthermore, several doctors who have analyzed the case say that even without the medical consequences to patients' overall health, the ASR XL Acetabular System is still failing at catastrophic rates purely because the design is not made to wear well over time. DePuy has acknowledged a failure rate of 13%, which is astronomically higher than the usual failure rate for hip implants overall at .5-3%.
A hip implant is the only option for many people who have suffered extreme trauma or have arthritis, osteoporosis, or other debilitating diseases. It is unconscionable that any company allowed an insufficiently tested hip implant to get to market and be placed in thousands of people.DePuy compounded this error with their decision to keep the ASR Acetabular System on the market for three years after it had received notification of high revision rates in Australia.
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