Hip Replacement

The fathers of modern hip replacement surgery, the British professor Sir John Charnley and Professor Maurice E. Müller of Bern, are shown in the picture on the left. The picture was taken at the time of a great medical breakthrough—the introduction of "low-friction arthroplasty", the first technique of hip replacement surgery.

Back then, the head of the artificial hip joint was made of metal and rotated about in a plastic (polyethylene) socket. This implant design produced relatively little friction and, thus, relatively little abrasion.

However, we must stress that pain relief was the foremost goal of treatment in those days. The operation itself was so risky that it was performed only when the pain was nearly unbearable.

Quite a few patients were willing to take any risk if there was any hope of making the pain more tolerable. The pain of arthritis can be so intense that it may seem that life is not worth living. Therefore, a number of "heroic" surgeries (including amputation of the entire leg) were performed for the sole purpose of providing relief from the intense pain.

The first hip replacement by Sir Charnley (photo left) was a tremendous improvement as low-friction arthroplasty usually reduced the pain to tolerable levels. In those days, any treatment that allowed the patient to walk a few steps in addition to relieving the pain seemed like a miracle. However, these artificial hip joints were not designed for heavy physical activity, much less sports!

Nonetheless, they generally solved the main problem, namely, the intolerable pain. The preservation of muscles and tendons naturally was not a focus of attention in those days, and full functional recovery of the joint was not expected. Originally, tendons were always detached or cut during the surgery, and the muscles sometimes were ignored completely. Modern minimally invasive hip replacement surgery is completely different (see MicroHip).

What were the main problems associated with these hip replacements? There were mainly two problems: Firstly, dislocation (luxation) was common because the original hip replacements were not very stable. Secondly, they were not very durable. Depending on the patient's activity level, they sometimes wore out and loosened within only a few years. The mechanisms of loosening are varied. However, the two main mechanisms are:

Constant friction from the rotating ball of the artificial joint leads to the abrasion of particles from the relatively soft material of the plastic socket. The abrasion process proceeds relatively slowly at first, but accelerates continuously once the surfaces of the two components no longer fit together perfectly. As the abraded plastic (polyethylene) particles are biologically active, they induce a local inflammatory response (swelling). The inflammation spreads to the adjacent tissues, destroying them with time. By this mechanism, the bone becomes "moth-eaten" and the holes get larger and larger until the artificial joint eventually loosens and the cycle of increasing pain starts again.

The material (methamethacrylate) used for cementation of hip implants is not only dead material, but also is extremely porous and fragile. Moreover, the hardening process requires heating the cement to relatively high temperatures (sometimes more than 80°C), causing an initial death of biological materials. Over the years, the cement cracks and eventually fractures. In addition to the inflammatory response to the polyethylene debris, this mechanism accelerates implant loosening considerably.

Why, then, have researchers like Callaghan et al. 2000 (and Kavanagh, Wroblewski, Older, etc.) reported that the probability of re-operation of a cemented artificial hip joint after 25 years of service is only 77 percent?

The answer is simple: The patients receiving total hip replacements 25 years ago were already relatively old at the time—in most cases, over the age of 70. Because of the intensity of their symptoms, they were not physically active before surgery, which reduced their expectations regarding their level of physical activity after surgery. Thus, weight-bearing and wear-and-tear on the implanted joints tended to be minimal. At the time of the 25-year follow-up study, most of the patients had already been in nursing homes for years, and the question of whether they were able to walk at that time was not even asked. The only criterion evaluated by the researchers was whether surgical retreatment of the hip had been performed. Consequently, most of the older research on cemented implants is completely useless and does not apply to the current state of the art.

The longevity of cemented implant stems and plastic sockets in young and active patients is much lower—in some cases, less than 10 years! Why is that?

The answer is simple: The young generation and "hip" older generations are much more active than in former times. In an article published in the Journal of Bone and Joint Surgery (JBJS) in 1998, the American researcher Tom Schmalzried estimated that the average total hip replacement implanted in the average patient in San Francisco is subjected to approximately 1 million stresses (steps) per year. Four years later, in 2002, the figure estimated by the same group had doubled to 2 million steps per patient per year. This represents a two-fold increase in only four years. For active patients today, the figure can be as high as 4 to 5 million steps per year! Why is this so important?

The original plastic sockets used for total hip replacements were designed for approximately 10 million steps. Thus, if a patient takes 1 million steps per year, one can assume that the total hip replacement will wear out in around 10 years. If the person takes 2 million steps per year, the lifetime of the implant decreases to 5 years and if the number of stresses is higher, it decreases even more. Despite the improvements in plastic materials, plastic sockets simply are not suitable for young and active patients because they are not designed for the stress loads in these active populations.

According to the laws of mechanics, all dead material is subject to fatigue and breaks sooner or later. When this happens, implant loosening occurs.

In cementless total hip replacement systems, new bone forms directly on the implant surface, resulting in biologic fixation. Like all bones, the new bone can adapt to stress loads over the years and regularly regenerate itself. Biologic fixation is not subject to the aging process associated with dead cement materials. Therefore, an X-ray of a cementless implant taken 20 years after surgery can look virtually the same as one obtained shortly after surgery.