Commentary & Perspective

Commentary & Perspective on
"Comparison of Polyethylene Wear Associated with Cobalt-Chromium and Zirconia Heads After Total Hip Replacement"
by Young-Hoo Kim, MD
and on
"Clinical Performance of Durasul Highly Cross-Linked Polyethylene Acetabular Liner for Total Hip Arthroplasty at Five Years"
by Lawrence D. Dorr, MD, et al.

Commentary & Perspective by
Stephen Li, PhD*,
Medical Device Testing and Innovations, Sarasota, Florida

Until the late 1990s, the articulation of the vast majority of total hip replacements was comprised of cobalt-chromium femoral heads and acetabular liners of ultra-high molecular weight polyethylene irradiated at dosages between 25 and 40 kGy. However, the need to reduce the polyethylene wear rate has spawned the introduction of alternative bearing surfaces and modifications in the processing of ultra-high molecular weight polyethylene.

These two papers deal with two such approaches to reduce the polyethylene wear rate in total hip replacements. These papers illustrate the potential for improved performance in total hip replacements as well as the difficulties in documenting these improvements.

The first approach, reported by Kim, compared zirconia ceramic femoral heads to cobalt-chromium heads, both of which articulated with ultra-high molecular weight polyethylene acetabular liners that had been irradiated with between 2.4 and 4 Mrad, which was within the traditional 25 to 40 kGy range. The paper reports on the polyethylene wear rates of the total hip prosthesis in fifty-two patients (average age, forty-two years) who had undergone bilateral hip replacements and received a different femoral head material on each side. After an average of seven years of follow-up, it was found that the average polyethylene wear rate was 0.08mm/yr for the zirconia heads and 0.17 mm/yr for the cobalt-chromium heads.

In the second paper, Dorr et al. reported that, in a series of sixty-six patients in which hips with cobalt-chromium heads articulated against either traditionally irradiated polyethylene or highly cross-linked polyethylene, the wear rates of the traditionally irradiated polyethylene at one and five years was 0.15 mm/yr and 0.04 mm/yr, respectively. The wear rates for the highly cross-linked polyethylene liners were 0.074 mm/yr and 0.011 mm/yr at the same time points.

Unfortunately, although both papers reported reduced wear rates in comparison to those of traditional materials, it is difficult to actually compare the results. The two papers differ in their methods of measuring the wear rates and reporting the wear results as well as in their patient populations, randomization of patients, acetabular shell and femoral stem designs, and surgeons—a partial list of factors that can influence the wear rate besides the femoral head material of the type of polyethylene. A direct comparison of the wear rates reported in these papers is therefore of very limited value.

It is critical to note that in the paper by Kim, only the overall wear rates were provided. It is not known, for instance, if the wear rate on the side with zirconia heads always exhibited a lower wear rate than the side with cobalt-chromium heads for each patient. If there were patients in whom the side with zirconia did not have lower wear than the side with cobalt-chromium, then the benefit of a zirconia head is much less obvious. The paper does not explain why previous publications do not demonstrate this benefit of zirconia, nor does it provide any information regarding the normal wear variation between the two sides of a bilateral hip replacement and how this difference influences the final result.

In the paper by Dorr, the wear rate for polyethylene irradiated with a conventional dose and highly cross-linked polyethylene were remarkably similar to the seven-year wear rates reported by Kim. However, that is where the similarity ends. The polyethylene groups in the Dorr paper exhibited decreasing wear rates. Both groups had wear rates that were less than 0.1 mm/yr at the five-year time point. This is an important observation as it has been pointed out that when the polyethylene wear rate is less than 0.1 mm/yr, there is generally no osteolysis for ten years. If this guideline holds true for this group of patients, it will take more than ten years for the highly cross-linked polyethylene products to demonstrate a reduction in osteolysis.

It is pertinent to consider failure modes other than polyethylene wear that were not mentioned by either author. It has been reported that the highly cross-linked polyethylene products have a reduced resistance to fracture along with their improved wear properties^1-3. What, then, will be the fate of the highly cross-linked liners when high stresses are present, such as in instances of impingement?

These two papers report on the promise of reduced polyethylene wear in total hip replacements. However, caution should be exercised when comparing the results between studies, as factors other than materials may play important, but not as obvious, roles.

*The author did not receive grants or outside funding in support of his research or preparation of this manuscript. He did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.

References

1. Bradford L, Baker DA, Graham J, Chawan A, Ries MD, Pruitt LA. Wear and surface cracking in early retrieved highly cross-linked polyethylene acetabular liners. J Bone Joint Surg Am. 2004;86:1271-82.
2. Walsh HA, Furman BD, Li S. The effects of cross linking on the fracture and fatigue properties of UHMWPE acetabular cups. Transactions of the Society for Biomaterials. 2001, p 592.
3. Crowninshield RD, Maloney WJ, Wentz DH, Humphrey SM, Blanchard CR. Biomechanics of large femoral heads: what they do and don't do. Clin Orthop Relat Res. 2004;429:102-7.

Copyright © 2005 by the The Journal of Bone and Joint Surgery, Inc.