Total hip replacement is one of the most frequently performed, universally successful surgical procedures ever developed in the history of medicine. The procedure is not only expected to dramatically improve the quality of one’s life, but also to last an extremely long time and hopefully for the duration of an individual’s lifespan. To understand the complexity of the emerging concerns of metal on metal as a bearing surface, one must first understand what represents a total hip replacement.
A total hip replacement, quite simply will reconstruct the cup of the pelvis and the ball of the femur with artificial components. In its basics, the components replacing the cup and the ball must adhere to the underlying bone of the pelvis and the upper femur. This may be accomplished in a variety of methods, the most acceptable being the use of surgical “cement” to anchor the components in place and/or the use of implant coatings manufactured to allow bone to grow directly to the components thus locking them biologically to the underlying bone.
Breaking this down further, the cup can be replaced with a one-piece component or modular components consisting of an anchoring shell and then an insert placed into the shell. On the femoral side, the components can consist of a ball and stem, with the stem placed into the marrow of the femur and anchored into place with or without cement. Most often, the ball and the stem are modular and the ball can be varied in size, material and mechanical properties. The ball can further be replaced by placing a cap on top of the ball of the femur that has been surgically altered to accept the new cap, with or without cement, similar to the above applications. The fixation of the shell of the cup and the stem of the femur are predictable in their anticipated adherence to the underlying bone. The use of surgical cement as an anchoring substance has vastly published results demonstrating the longevity of both the cup of the pelvis and the femoral replacement, either cap or stem. Perhaps to be over-simplistic, the adherence of the cup and the stem or the cap has been universally successful with or without cement and their immediate loosening or failure has more or less been eradicated.
That being said, what are we concerned about as far as the major cause of failure with hip arthroplasty? The number one failure occurs as a result of the bearing surface which represents the articulation of the ball with the cup. The bearing surfaces currently available are metal on polyethylene, ceramic on polyethylene, ceramic on ceramic, ceramic on metal and metal on metal. On the cup side, the bearing surface would be attached to the underlying shell either in the factory or at the time of surgery or a non-modular cemented polyethylene cup or metal cup. On the femur side, whether it is metal or ceramic bearing, the ball is tapped onto the stem of the femur or a non modular situation where the cap is pressed directly onto the bone, with or without cement.
Now that we understand the variety of components, metal on polyethylene and ceramic on polyethylene are still the most widely used bearing surfaces. Data regarding their performance and functional outcomes is extensive. Both of these bearing surfaces carry excellent long-term results, with polyethylene wear as the number one cause of failure and not the ceramic or metal. With new polyethylenes available, there has been significantly reduced incidence of failure and incorrect positioning of the cup most likely the cause of failure which creates increased stress on the polyethylene.
Now let’s shift to the hard on hard (metal or ceramic) bearing surfaces. These hard on hard bearing surfaces do not have enough clinical data to make absolute conclusions as to their predictable longevity at this time. We know that ceramic on ceramic almost never shows any significant wear. The problem with ceramic on ceramic is that there is a reported incidence of noise being generated from the bearing surface referred to as squeaking. This is not an immediate problem by and large, but rather occurs over time and is now reported in over 20% of cases in some series. Ceramic on metal, unfortunately, does not have extensive data and, therefore, no conclusions can be made regarding its ultimate wear predictability.
Finally, we come to the emerging problem of metal on metal as a bearing surface. Metal on metal is not a new bearing surface as it has been around for over 50 years. The reason why no solid conclusions can be made is simply because of the variability in the manufacturing techniques and materials of both the cup and the ball, the surgical techniques to position the cup and the ball have changed over time, and the number necessary to generate the ultimate success or failure of these components is virtually impossible to determine. So what is alarming? What is alarming is this; we are now seeing an upward trend in failure of metal on metal components that is occurring at eight years and beyond. The first thing that is well-documented is that metal on metal creates a metallic debris situation of not only small particles that accumulate regionally around the hip joint, but also systemically in organs throughout the body. The increase in the particles is reported as high as 75% and more above normal. Equally alarming is the fact that metal ions are released into the system and also are absorbed not only locally, but potentially in all body organs. The problem with metal ion debris is that chromium and cobalt are known carcinogens. Again, because of the extreme difficulty in amassing numbers to come up with logical conclusions, there is no proof that metal ion debris will cause increased cancer. However, it is universally accepted that this is a concern. So since this is not a proven entity, we will ignore cancer for now,
What we can’t ignore is the fact that these components are, in fact, debonding from the bone at an alarming rate. The debonding is not on the femoral side by and large, but rather on the cup side. The largest orthopedic companies in the world have spent considerable monies marketing these products and have been forced to withdraw them from the orthopaedic marketplace. In particular, Zimmer has already announced major recalls and even though there is no class action suit, this most likely is soon to happen, DePuy (Johnson & Johnson) has announced a recall of one of its metal shells at the end of this past year because of failure and Smith & Nephew’s metal product, even though not recalled and extremely popular outside the United States, is now being closely reevaluated. The number one surgeon in England and the biggest proponent of this Birmingham Hip System have discussed with America’s number one metal on metal expert and proponent the fact that major concerns are now arising in England regarding not only loosening, but some type of immunologic process that has disastrous consequences.
Over 15 years ago, I was asked to be one of the investigators for metal on metal components and declined. I have significant concerns regarding the articular surface and the longevity of various products.
I have personally re-operated on patients with failed metal on metal components with loose cups, and at the time of surgery noted a milky-like substance that was so alarming that I felt we were dealing with an infection. After discussions with several extremely knowledgeable metal on metal proponents, it appears that this situation is, in fact, not new and has lead the number one proponent of metal on metal as a bearing surface in the United States to recommend that larger metal balls in combination with metal cups no longer be used. In addition, the same physician has recommended that if a metal on metal bearing surface is to be used; a small ball should be used with a stem and not a cap placed on the femur, commonly referred to as a resurfacing procedure. As you may have guessed, this topic is nothing short of explosive in the orthopedic community.
Throughout the country, and particularly in Southern Florida, metal on metal and resurfacing procedures have been widely touted as the procedure of choice for young people. The reason for this is the preservation of more bone on the femur side, so that if and when another operation is needed, more bone will be available to reconstruct the femur. This, of course, implies that by using this device and using this explanation, someone is anticipating failure of these components. Another curious fact is that the resurfacing procedures are not recommended in women of childbearing ages. If this procedure is so good that it is recommended for young people, why are young females in childbearing years being advised not to use the components? It appears that there is, indeed, a concern that the debris generated from these components may have serious consequences on a fetus.
So why have I never used metal on metal bearing surfaces, and in particular, the resurfacing arthroplasty? The answer never sounded right. It still does not sound right. There is something wrong with this situation that from this point on is only going to get worse. Please remember, not everybody with a metal on metal bearing surface is going to have a failure, some of metal on metal’s biggest proponents have said emphatically that this is not an issue if the components are placed properly, but rather, it is more of a technically related issue, which simply implies that they are not being manufactured correctly or are not being placed in the proper positions. While the manufacturing issue is completely out of my realm, the technical issues are pretty straightforward to me. No one, no matter how good they think they are, can be perfect every single time. If these components and this bearing surface represent an unforgiving operation, then again I quite simply am not going to do this, and will not subject any patient to something that I do not think is appropriate.
After all is said and done, I have the distinct ability to look back over 20 years in my private practice in South Florida and can honestly tell you that the failure rate of our total hips are less than 1%.
Given the odds, with the success rate in the area of 98-99%, why on earth am I going to change to a bearing surface with all the potential disasters that I have listed above?