Editor’s Spotlight/Take 5: Polyethylene Wear Increases in Liners Articulating With Scratched Oxidized Zirconium Femoral Heads

What is the bearing surface of choice in THA? The answer to this deceptively simple question continues to elude surgeons, more than a half-century into the era of modern THA.

The “right” bearing should be wear resistant, biocompatible, and unlikely to fracture. And as we’ve learned from the many articular surfaces that have fallen short—think Teflon, carbon-fiber-reinforced polyethylene, early ceramics, and metal-on-metal (MoM)—we get no points for achieving one of those goals at the expense of any other. Finally, we need to be open to the idea that there may be more than one “right” bearing, and that we may learn that a choice might be both effectively and ethically made based on age, anticipated activity level, or other factors. If we can identify patients who may do equally well with several bearing options, then cost can enter the equation.

This issue of Clinical Orthopaedics and Related Research^® contains the latest Hip Society Proceedings, and bearing surfaces are well covered herein. An award-winning paper found a metal-on-crosslinked polyethylene articulation looking strong at 15 years [3], with fewer revisions than conventional polyethylene, and—at least thus far—no late bearing fractures, which had been an earlier concern. And while the main question about the latest MoM designs has been answered (they have a high risk of premature revision), this month’s CORR^® takes up some important second-order concerns, including the use of genotyping to better identify those patients at risk for reactions to metal species [6], and the relationship between the indications for revision of MoM THAs and the risk of complications following those revisions, including an increased risk of death among patients undergoing revision for infection [9].

But perhaps the most-surprising findings came from a study about a bearing couple that does not get as much attention as some of the others: Ceramic-on-polyethylene (CoP) [1]. This study of one particular ceramic (oxidized zirconium) found that prosthetic hip dislocations and reduction maneuvers cause surface damage on these femoral heads, and those scratched ceramic surfaces then potentiate increased polyethylene wear. I found this study convincing, as it married retrieved-implant analysis to robust laboratory validation tests in a wear simulator, and because it compared scratching to other kinds of surface damage. And importantly, the authors evaluated the question in the setting of contemporary crosslinked polyethylene, so the findings are not just convincing, but also clinically relevant.

I note that the laboratory approaches used here can be applied broadly to improve patient care across our specialty, far beyond arthroplasty surgery. For that reason, both the study and the interview with its corresponding author, Timothy M. Wright PhD from the Hospital for Special Surgery in New York, should be of compelling interest to all of us, regardless of whether we perform THA surgery.

I emphasize that this study evaluated only one particular type of ceramic (oxidized zirconium), which may behave differently from others. Oxidized zirconium is manufactured as a thin ceramic coating on top of a metal femoral head; here, it was used against a crosslinked polyethylene counterfacing surface. For these reasons, the bearing studied may wear differently from other CoP bearing couples, and differently from ceramic-on-ceramic (CoC) THA articulations. Even so, oxidized zirconium on crosslinked polyethylene certainly has its enthusiasts for exactly this application [5], and it remains in wide use. In fact, this bearing surface was created in an attempt to offset another well-known shortcoming of ceramic THA bearings: Fracture. By transforming a thin layer at the surface of a metallic head to a stable ceramic, the designers hoped to provide the bearing-surface advantages of a ceramic with the ductility (and fracture resistance) of a metal femoral head.

We know that ceramic bearings have well-known tribological advantages in terms of wear over cobalt chrome in the laboratory, but neither ceramic-on-crosslinked polyethylene bearing couples [12, 14] nor oxidized zirconium on crosslinked polyethylene bearings [4, 10] seem to reduce wear or improve survivorship when compared to metal-against-crosslinked polyethylene. That being so, and now in view of the very-concerning findings by Dr. Wright’s group, it seems reasonable to ask some hard questions about oxidized zirconium femoral heads in THA.

Please join me in the Take-5 interview that follows as Dr. Wright fields some of those tough questions, and shares his viewpoints about the power of retrieval studies in orthopaedic surgery, based on a career in the field.

Take Five Interview with Timothy M. Wright, PhD, corresponding author of “Polyethylene Wear Increases in Liners Articulating With Scratched Oxidized Zirconium Femoral Heads”

Seth S. Leopold MD: Congratulations on this powerful study. Before we get into the details, let’s take a step back and think about the nonarthroplasty surgeon reading our interview. What do you see as the major avenues of research outside of arthroplasty where implant retrieval and analysis still can answer important, clinically relevant questions?

Timothy M. Wright PhD: To be sure, in orthopaedic surgery the emphasis in retrieval-based studies has been on arthroplasty, primarily because of the propensity for bearing surfaces to wear, but also because the loads are sufficiently large enough to cause component fracture and, as we’ve seen recently, to cause unacceptable fretting and corrosion at modular junctions. However, as more and more synthetic scaffolds are introduced in efforts to engineer musculoskeletal tissues, especially in weight bearing applications like cartilage, meniscal, and intervertebral disk repair, we’ll need to be focused on how these scaffolds perform, and retrieval analysis can pay an important role.

Dr. Leopold: What will it take to achieve those goals?

Dr. Wright: We’ve begun to think about how we’ll approach these more-complex problems. Some of the required tools will be different than the ones we use in retrieval analysis of arthroplasty components. Perhaps the most-important steps will be in the retrieval itself, and the subsequent cleaning and sampling that will be required. And, of course, the integration of histology and the search for molecular and cellular markers in surrounding tissue will play a much bigger role, since the device itself is intended to “engineer” changes in surrounding tissues.

Dr. Leopold: To what degree might your findings in this study generalize to ceramics other than oxidized zirconium that are in common use against crosslinked polyethylene surfaces, in terms of the risk of you raised?

Dr. Wright: I don’t think that you can generalize our findings beyond the oxidized ceramic that we examined, because the particular type of surface damage—mainly cracking and upsetting of the ceramicized bearing layer leading to large scratches—has not been observed with solid ceramic heads. While it is true that solid heads will experience metal transfer from the edge of the cup liner during a dislocation episode, other than slightly raising the surface roughness, the metal transfer is not as abundant a polyethylene wear generator as the scratches found on oxidized zirconium. We had your concern when we planned the experiment. That’s why we ran solid ceramic heads with metal transfer in our hip simulator, which showed no appreciable wear for those specimens.

Dr. Leopold: What did you learn here that may inform thought about CoC articulations, which, as you know, are implanted in individuals who may be subjecting them to the highest demands over the longest periods of time: Our young, active patients [2, 13]?

Dr. Wright: Because we only concerned ourselves with CoP bearings in our study, it’s difficult to draw any conclusions about CoC articulations. It’s worth emphasizing, however, that oxidized zirconium bearing against polyethylene has provided excellent wear behavior in general. Nonetheless, we felt it was necessary, given the damage that we were observing in oxidized zirconium heads revised for dislocation, to explore the ramifications of the damage to subsequent wear. Surgeons can now use our data to consider revision surgery or watchful waiting in cases of dislocation of these types of heads.

Dr. Leopold: Given the lack of any compelling evidence that ceramic-on-crosslinked polyethylene is more wear-resistant than metal-on-polyethylene (or that CoP reduces the risk of osteolysis or wear-related revisions) [12, 14], and that the same goes for oxidized zirconium on crosslinked polyethylene [4, 10], would it be reasonable to use your study to justify avoiding ceramic-on-poly bearing couples for routine primary THA?

Dr. Wright: I don’t think you can use the results of our study to raise concern about CoP bearings in general. We were looking at a specific type of damage on a specific type of ceramic bearing. And even then, unless the heads had substantial damage, the wear rate against highly crosslinked polyethylene was quite low. Your larger question is still the correct one, namely, are CoP bearings advantageous? At this point it appears to be a tradeoff between higher cost and a small, but not zero, chance of catastrophic failure on the one hand, and evidence that ceramic heads reduce the chance of mechanically assisted crevice corrosion [7, 8] and subsequent adverse local tissue reactions [11] on the other hand.

Timothy M. Wright PhD