Bassam A. Masri, M.D., FRCSC
Associate Professor and Head,
Division of Lower Limb Reconstruction and Oncology
University of British Columbia
Vancouver, BC

Despite the success of total hip replacement, failure still complicates a significant number of cases. About 10% of hip replacement operations are revision procedures, and this proportion is expected to increase with the aging population, and the ever-decreasing age at which these operations are done.  

In the late 1980's debates raged as to which fixation strategy: cementless or cemented was best to prevent aseptic loosening, felt to be the worst long-term problem after hip replacement. Indeed, in 1987, Jones and Hungerford published an article with the title, "cement disease", in support of a move to cementless fixation for the prevention of loosening. Despite many advances in implant design and fixation technique, there is no evidence in the literature than any of today's technologies are any better than the tried and true cemented Charnley or Exeter total hip replacement.

In the mid 1990's, it became apparent that cement disease is a gross misnomer, and there was nothing evil about cement. The culprit was polyethylene in particular, and particulate debris in general. It became apparent that polyethylene wear led to the generation of particulate debris, which then stimulated macrophages to release cytokines which stimulated osteoclasts and led to what we now know as the true enemy of hip replacements, namely "osteolysis". Because of this problem, the search began for alternate bearing surfaces to reduce polyethylene wear and particulate debris burden.

In the late 1990's highly cross-linked polyethylenes were introduced as the perpetual motion machines of hip replacement. In some hip simulator testing they did not show much wear at up to 20 million cycles! With these impressive results, this technology has been embraced, to the best of my knowledge, faster than any other advance in total hip arthroplasty. But where are the clinical results? The answer is that there are hardly any. Grobbelaar, Oonishi and Wrobleski reported on a few non-consecutive cases of highly cross-linked polyethylene that were manufactured using techniques that are no longer used today. Their results were encouraging; however the numbers were small. Furthermore, there are concerns regarding the fatigue properties of highly cross-linked polyethylenes, but to the best of my knowledge, there are no published reports of early catastrophic failures. Despite these unknowns, it is anecdotally reported that 70% of US hospitals no longer stock regular polyethylene and have switched completely to cross-linked polyethylenes.

If the results of these improved polyethylenes are so good, why don't we all switch to highly cross-linked polyethylenes and make revision total hip arthroplasty a thing of the past? I wish that things were this simple. For starters, the natural history of "new and improved" polyethylenes over the past 20-30 years has been checkered to say the least. Just think back to the carbon-fiber reinforced polyethylene and to hylamer. Despite early enthusiasm, these experiments failed miserably.

This brings us back to the main topic of this article, the use of standard polyethylenes in total hip arthroplasty. Are they so bad that they need to be abandoned? I don't think so. Since the mid 1990's, we have learned about the dangers of polyethylene oxidation, fusion defects, etc. Manufacturing processes have improved to reduce the numbers of asperities within the polyethylene. Furthermore, gamma radiation sterilization in air has been abandoned, in favor of sterilization in an inert gas, and vacuum packaging to minimize the risk of oxidation until after implantation, where oxidation can occur to a lower extent. A byproduct of this change in sterilization technique is that so-called old polyethylenes are now cross-linked, but not to the same degree as the newer highly cross-linked polyethylenes. If we subscribe to the concept of cross-linking to reduce wear, then the newer generation standard polyethylenes will no doubt wear less than the polyethylenes of the mid 1990's. Because they are not irradiated to the same extent as the highly cross-linked poly-ethylenes, they retain their mechanical properties, and catastrophic failure under normal circumstances should not even be a theoretical concern. Thus, will the current generation of "old polyethylene" behave as badly as its predecessors from the mid 1990's? I don't think so, but this is only one man's opinion.

What about cost? The highly cross-linked polyethylenes are significantly more expensive, often sold at twice the price of standard polyethylenes. Based on the previous discussion on better sterilization techniques, and the promise of better performance, can we justify the cost of the highly cross-linked polyethylenes for the older patient? Not only can we not justify the increased cost, but also the potential gamble that we take when we introduce new technology in a patient who would be served very well with the existing technology. For the elderly patient, standard polyethylenes will therefore serve the patient for his or her entire life, and in my opinion, there is no role for highly cross-linked polyethylenes except in the situation when very large femoral heads are used. These should be used with caution, because of the reduced mechanical properties.

There is, indeed, a role for highly cross-linked polyethylenes, but the recipients of this product should be subjected to scientific scrutiny, to monitor them and to make sure that any impending failures are addressed early. This is yet another example of why national registries are absolutely essential so that any trends towards adverse effects are picked up early. We should all practice evidence-based orthopaedics, follow our patients carefully, and enter them in the Canadian Joint Registry. In closing, should we be throwing away standard polyethylenes? Absolutely not!