Malchau Debates Engh Over Highly Cross-Linked Poly

“In the mid-term, at least, ” says Henrik Malchau, “highly cross-linked polyethylene is doing well. “Wait a minute, ” counters C. Anderson Engh. “Even your own recent data emphasizes that the outcome is not clear, even after a nine year experience.”

This week’s Orthopaedic Crossfire® debate is, “Cross-Linked Poly 30 Year Hip: We Can All Go Home Now.” For the proposition was Henrik Malchau, M.D. from Harvard Medical School. Against the proposition was C. Anderson Engh, Jr., M.D. of Anderson Orthopedic Research Institute; moderating was Thomas S. Thornhill, M.D. of Harvard Medical School. 

Dr. Malchau: “We can all go home…cross-linked poly will solve the problem. I don’t have data that confirms 30 year outcomes, but I have an outcome that is superior to the disastrous outcome of some metal-on-metal (MOM) total hips and let the patient move around in silence in contrast to ceramic-on-ceramic (COC).”

“Registries: from Australia, they have 173, 591 total hips with primary diagnosis of osteoarthritis. MOM has the highest revision rate; ceramic on conventional poly is number two, and metal on conventional poly is number three, then ceramic on highly cross-linked poly (HCLP), followed by ceramic-on-ceramic, then metal on HCLP, finally ceramicised metal on HCLP. So HCLP is doing good at ten years.”

“The first study was at Mass General (MGH)…a multicenter study looking into conventional head sizes. Another study looked at larger head sizes (>32mm). The Longevity/Durasul has been used in more than two million patients, and the clinical outcome is excellent.”

“There are three analysis methods—confusing. You can do linear wear rates (postop versus the most recent); you can do group regression where you use all the data; or you can do individual regression where you have a minimum of three points required. I still don’t know which is better. MGH had 241 hips…we have not revised any components due to wear; we have no indication of osteolysis, and we have done CTs on a number without identifying osteolysis. There is no wear…at ten years, same story.”

“The multicenter study: we have 278 hips with close to 4, 000 film comparisons where we have very high quality criteria. You have a flat curve. None of the three methods show any difference between the head sizes or any increased head penetration…no wear detected and no increase in femoral head penetration in the late versus the early period.”

“Finally, the large head study…three centers with 486 patients. We found a significant difference in median linear wear. The steady state linear wear and individual regression analysis didn’t show any difference. So the low femoral head penetration rate we previously reported is still—up to 12 years—showing very good results. The large head shows a possible increase in penetration rate, but significantly reduced wear rates.”

“I know what my opponent is going to tell me…because we did explant analysis and we found that implants that have been in a patient then put on the shelf for a number of months or years—something happened there—maybe cyclic load or absorbed lipids. So we looked at fresh explants and implants and we found some oxidation, but very low still…and I doubt that it would really affect long-term outcome.”

“There are more concerns about the sequential, irradiated material that have a lot of free radicals. In an example of a four year X3 retrieval…the oxidation levels are up to 1, which is white banding, and there might be clinical problems associated to this. Therefore, vitamin E where you add an antioxidant in your poly, blocking your free radicals might be better than the first generation highly melted.”

“So based on multicenter radiographic studies, and the nearly 175, 000 Australian patients mid-term results are extremely encouraging. Why wouldn’t it last another 15 years? Thank you.”

Dr. Engh: “The literature on cross-linked polyethylene looks good…and I use it daily. All of the reported wear rates [.05, .003, .088, .04, .031, .03 mm/year] are with 28mm heads…not many of us are still using 28mm femoral heads.”

“To make a reliable prediction for a 30 year hip there should be no unexplained occurrences, no lingering questions, and we should be using them in appropriate patients. First of all, oxidation. We know that it leads to mechanical failure by way of embrittlement; it may lead to increased wear because of decreased cross-linking.”

“In an article from Dartmouth, 22% of cases had measurable oxidation on retrievals that was at similar levels to gamma inert retrievals at the same time; the oxidation appeared to be correlated with the time in vivo. Another article looked at retrievals and measured oxidation immediately upon retrieval and after storage in air. There was minimal oxidation in these explants at removal, but when the explanted polyethylene was put on the shelf oxidation levels increased.”

“Looking at an article from Boston, there was no difference comparing the 28mm and the 32mm, but there appeared to be a slight trend for the 32mm to be greater. In another article—by Lachiewicz—they looked at 36mm and 40mm heads and actually saw greater volumetric wear. They advised caution using larger femoral heads in younger, active patients. So they combined theirs: 534 hips, two techniques (a dual radiographic technique looking at the one year film and the follow-up film and then a group regression).”

“In the dual radiographic technique the large heads had significantly increased linear and volumetric penetration. But when they analyzed it differently they had no significant difference. They concluded that the differing statistical analysis makes interpretation difficult.”

“Component position: An award winning article indicated that as many as 50% of hips that are put in are outside of the target range, potentially leading to edge loading and cracks at the edge of the polyethylene.”

“Patients who are focused on function are not too concerned about longevity, and are pushing the limits of total hip. Lastly, taper corrosion. We’ve seen a bit of that with MOM, and I believe it is reappearing with metal on polyethylene.”

“Lastly, I want to point out an article from Henrik in 2011 on the introduction of innovations. The case studied was that of heavily cross-linked polyethylene. He indicated that the clinical experience to date, although it generally supported the use, recent data emphasizes that the outcome is not clear, even after nine years. Thank you.”

Moderator Thornhill: “That’s harsh, Andy. Let’s pick on Seth! I have patients who want the 30 year knee based upon laboratory testing. Can you extrapolate it out to 30 years?”

Dr. Malchau: “Hard to predict, and I don’t know how many of our hips will still be working in 30 years. But I don’t think that the current literature on explants contradicts a good long-term result. Despite the fact that we found oxidation in the sequential irradiated polys, Crossfire and X3, we still have to get the clinical failures. The paper you quoted, Andy, from Dartmouth—there’s really no control there at all of how those explants are stored. Most of the oxidation in that paper is probably happening after explantation.”

Moderator Thornhill: “Hips are failing with dislocation, with periprosthetic fracture, etc. What do you tell a patient?”

Dr. Engh: “I think this poly is good—I use it all the time. What I’m trying to do is avoid the 36/38/40mm heads.”

Moderator Thornhill: “Yes, but how long is my hip going to last, Dr. Engh?”

Dr. Engh: “You’re going to take yours with you.”

Moderator Thornhill: “We have a bioethics course here at noon. Anyway, so what is your preferred head size?”

Dr. Engh: “32mm”

Moderator Thornhill: “Henrik?”

Dr. Malchau: “32mm”

Moderator Thornhill: “Henrik, do you still use metal-on-metal?”

Dr. Malchau: “I do.”

Moderator Thornhill: “What percentage of your practice is resurfacing?”

Dr. Malchau: “Around 5%. My only problem with resurfacing is that I did two or three females and they come back now with a contralateral hip and ask me to do that, but I won’t do it. But the big males they’re doing great.”

Moderator Thornhill: “So minimal thickness of poly in a hip, in the dome and in the periphery…”

Dr. Malchau: “Dogma says that it needs to be 6mm, but I don’t believe that’s needed with the modern highly cross-linked polys. I think we can go thinner, which means we can have more conservative bone resection on the acetabular side. I would go 36mm in elderly patients with dislocation risks, but in a younger patient I would stay at 32mm.”

Moderator Thornhill: “What’s the minimal thickness of polyethylene that you would accept at the dome and at the periphery?”

Dr. Malchau: “At the dome, four; at the periphery, two.”

Moderator Thornhill: “Two at the periphery, even though in something that’s a little vertical the edge wear…”

Dr. Malchau: “Even the edge loading, that’s fine.”

Moderator Thornhill: “You have a robust revision practice as well.”

Dr. Malchau: “I get all the patients from Brigham, yes.”

Moderator Thornhill: “Andy, same question.”

Dr. Engh: “I would say 4mm and 4mm. The rim…that’s the area that’s exposed to the joint fluid…the area that if it’s going to oxidize and if the oxidation is going to be important then we’ll see it there. It needs to be a good modern design; if that 4mm is captured within the metal and supported it’s OK, but a lot of these designs are getting that 4mm by lateralizing the poly 2mm and 4mm. I’m cautious about using the first polyethylene that I can use with a larger head size, in other words that 36mm and a 52/54mm because they have to get the edge by lateralizing that polyethylene…that puts load.”

Dr. Malchau: “You mean the poly needs to be supported by metal or cement? You need to support it; you can’t build up the rim. It’s going to fail.”

Moderator Thornhill: “When you’re talking about your minimal thickness you’re talking about something that’s got low wear, good mechanical properties, and is oxidatively stable.”

Dr. Malchau: “The Swedish randomized trial showed a blip from five to seven [years], but when they got follow up at 10 years that blip disappeared. What we did to get that answer was to initiate a multicenter U.S. study where we couldn’t reproduce that finding. The Swedes have since addressed it and there is no increase anymore.”

Moderator Thornhill: “I’m trying to find areas of disagreement.”

Dr. Malchau: “We did have 500 patients in the large head multicenter study and we have a slight increase in linear wear rates. Of course that will translate to a higher volumetric wear rate, but it’s still unclear what is the best way to calculate volumetric wear. So there are indications in one of the analyses we did that 36mm is associated to a slightly increased linear wear rate. But still, there are a number of statistical issues…we have three statisticians fighting for a year over which way to go. And I still don’t know. Two or three methods show no significant difference.”

Moderator Thornhill: “Andy, last word.”

Dr. Engh: “I think we agree on more than we disagree. There are concerns, and we still need to look at the explants.”

Moderator Thornhill: “Henrik, a question from the audience. ‘Will vitamin E be the solution to the melt/annealing property and maintaining oxidative resistance without losing mechanical properties?’”

Dr. Malchau: “No doubt that vitamin E in the lab tests has a mechanical toughness that’s the best we have seen so far. Will the vitamin E stay in the poly? We’re trying to do anything to chase it out—boil it in lipid solvents—we can’t get it out. But we need to collect data; so far it looks good, but it’s early.”

Moderator Thornhill: “Thank you.”

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