“In essence, Highlycrosslinked Polyethylene (HCLP) for total knees at this time is probably a bad idea.” argues Dr. Engh. Not so, counters Dr. Hofmann. “We have nine years of clinical data and haven’t seen any clinical problems with HCLP. As we said a few years ago, it was a wine before its time…I think it’s time to drink the wine.”
In an Orthopaedic Crossfire® debate titled, “Using Crosslinked Polymers in TKA: Surely Tough Enough, ” Dr. Gerard A. Engh of the Anderson Orthopaedic Institute argues that conventional PE (polyethylene) has its merits, while Dr. Aaron A. Hofmann of the University of Utah disagrees pointing out that HCLP has better long term wear. Both sides let the free radicals fly in this spirited Crossfire debate. Who comes out on top? You are the judge.
Dr. Hofmann: “We’re going to be discussing smooth-running knees. The subject is Crosslinked Polyethylene (PE) in TKA…and I thought, ‘Surely Engh is tough enough to listen to this talk and I’m sure he will agree with me at the end of this debate. So even Seth Greenwald said years ago that what we have is good enough…and if you agree then you can continue to use standard polyethylene, but if you don’t think what we have is good enough you should consider changing to highly crosslinked.”
“Every major manufacturer has made mistakes in trying to come up with new polyethylene…I would say that highly crosslinked polyethylene (HCLP)—just like in the hip—is not new, it’s been around since 2001 and I’ve used it consistently since then.”
“Gamma radiation is a good thing for PE wear. Unfortunately it causes free radicals, which can break down, oxygenate and cause oxidative degradation over time. If you look at the standard sterilization doses used versus wear, the more you radiate PE the better the wear. Unfortunately you still have these free radicals and we don’t know when these PEs were born. I can drink my Bud Light and know exactly when it was made, but I can’t look at my PE package and know how old it is and if it’s degradated—even in the package.”
“Every major manufacturer has developed HCLP or moderately CLP for knees. X3 is available and if you look at the manufacturer’s data you see a remarkable decrease in wear. DePuy/JNJ shows moderately CLP…again, retaining mechanical integrity with oxidative stability. A manufacturer shows a marked decrease in wear characteristics with a newer PE. But you’ve got to get rid of the free radicals and that is done by most manufacturers by heating to stabilize those free radicals so they can’t oxidate and combine with oxygen.”
“If you look at some of the lab data, done by Muratoglu [Orhun Muratoglu, Ph.D.] looking at aged Durasul (HCLP), you don’t see the subsurface cracks and delamination that you see in our standard PE or standard PE that was delaminated. He did another test looking at adverse wear, for example if you had a very high, very tight PCL, and what would happen if you got edge loading. Standard PE, even at 500, 000 cycles does not tolerate that. You get some damage with HCLP.”
“A favorite subject of Jerry Engh is backside wear. This has been looked at by Muratoglu as well for Prolong, which is not as crosslinked as Durasul. If you reheat the deformations around the screw holes, the amount of backside wear is almost tenfold more with conventional than with HCLP. There are clinical papers available…two year minimum follow-up showing no problems with CLP [Minoda et al.]. Another study…out of Roy Bloebaum’s lab in Utah looked at retrieved implants with a minimum two year follow-up showing that these things aren’t falling apart early on.”
“In our own clinical data published last year looking at our first 100 starting in February 2001 when this material became available, it really didn’t show any fractures, revisions, osteolysis, or problems with the PE.”
“When we looked at our standard versus our HCLP there was a marked difference…17% incidence of some lucencies under the tray with a standard PE and almost none with the HCLP. A patient that had one of each, had radiolucencies (on the right) under the screw. Another implant—just retrieved—patient that had after seven years a slightly loose knee was unhappy with that…gave me a chance to look at this PE as we put a thicker one in. We saw a bit of surface damage on the articular side. On the back side you can still read the fine print—the serial number and how thick the PE is, so there is almost zero backside wear.”
“In conclusion we have nine year clinical results and haven’t seen any clinical problems with it. As we said a few years ago it was a wine before its time…I think it’s time to drink the wine.”
Dr. Engh: “I think we all realize that we had a problem in the ‘80s and ‘90s with accelerated PE wear that was caused by gamma irradiation in air, but this was not a material problem. Then there was carbon fiber reinforcement, heat pressing, changing the molecular structure of the material. All of these failed horribly. The solution came by changing the method of sterilization to either non-gamma or gamma in inert.”
“The largest retrieval study is from Dartmouth. They had over 1, 600 implants; 32 of these were sterilized by ethylene oxide…non gamma, no delamination, minimal penetration rates. These are six implants that were in situ for more than 15 years and they showed negligible wear. Out of our own institution…retrieved AMK [Anatomic Modular Knee] implants, low dose gamma in barrier, linear wear rates of .02 mm per year. This would take 50 years to wear one millimeter.”
“In a small number of retrieved implants the wear rates for cubic millimeters is quite low. If we look at wear of conventionally sterilized inserts versus acetabular cups, the wear rates of the former is less than a quarter in terms of linear wear of acetabular cups and less than half in terms of volumetric wear. But the problem with HCLP is not only that we don’t have a wear problem as long as we don’t sterilize with gamma in air, but we’ve reduced the tensile strength of the material, and this can cause fatigue problems. We get smaller wear particles that are more bioreactive…and I would argue that there aren’t any clinical studies to date proving that there is reduced wear.”
“If we look at retrieved inserts….again from our institution…gamma irradiated versus gas plasma with low dose gamma irradiation. When we get out after five years those that have been irradiated are showing considerably greater pitting on both the top side and on the back side of the implants compared to non irradiated implants. With increasing radiation doses we decrease the toughness; in more constrained devices such as posterior stabilized implants we open the problem of potential implant fracture.”
“Looking at HCLP and reduced strength when we increase the dose of radiation, we decrease the material strength. Also, the wear debris is smaller and more bioreactive. The wear debris from HCLP is less than a micron in diameter, and these particles have a more robust inflammatory response.”
“If we look at retrieval wear scores…HCL versus low dose (sterilized in) inert (conditions), there is no difference in the percent of pre- or post-melt surface damage. Other studies show the same thing…HCL material, no statistical difference in terms of wear scores compared to conventional material. We have FDA reports of implant fractures using this HCL material. Tibial PE fractures: pegs wearing or breaking off the back side of these implants as reasons for revision. Or if we look at osteolysis-there are already cases reported of osteolysis with very short term follow-up reports.”
“In essence, HCLP for total knees at this time is probably a bad idea.”
Moderator Duncan: “So Aaron, it seems that from Dr. Engh’s standpoint, if we’re careful with sterilization there’s no reason to change, particularly when you take into account the potential downside of reduced fracture toughness of HCLP.”
Dr. Hofmann: “Maybe it’s performing well in his hands…it hasn’t performed well in mine, nor in our own implant retrievals, nor in our clinical follow-up of our patients with conventional versus standard.”
Dr. Engh: “I have not retrieved an implant in the last nine years that was not sterilized by gamma radiation in air for either wear or osteolysis. It’s so dramatically different because I had enormous problems when it was sterilized with gamma in air and if it had a long shelf life. There’s another problem, and we can never really compare what Aaron is telling us with HCLP because after they highly cross link poly they quench the free radicals, but for every one of you in the audience, when you use conventional poly that has low dose gamma and is placed in a barrier, for some reason manufacturers don’t quench the free radicals. Why—if it’s important to do that in the highly crosslinked material—is it not important to do that in the conventional material? And that’s why we won’t be able to compare these two because oxidation may happen after we put it in the patient.”
Moderator Duncan: “What about the future of the HCLP in which Vitamin E has been used in the manufacturing process?”
Dr. Hofmann: “HCLP is available with every manufacturer and I think we’re gong to be seeing that with every implantation. The only confusing thing is guys like Jerry who say that conventional PE is still a good thing, and so that’s why some surgeons have continued to use it. You asked why don’t the manufacturers squelch all the free radicals in all the PE…it’s because people still want conventional. And it’s certainly driven by economics. You get conventional PE being priced at one price and HCLP as the state of the art and it’s $100-$200 more…that drives what surgeons use.”
Dr. Engh: “At my institution it’s more like $1, 000 more for the HCLP. I wish a manufacturer was here to tell me why that’s so important. Now we use Vitamin E to quench it…we use other methods. Why, if it’s so important in the HCLP isn’t it in the conventional—and most of us today are putting in the conventional? We’ve been using conventional PE in barrier a lot in our institution and the wear rates are quite low even with that. But I don’t have 10-20 year results to find out if oxidation in vivo is a problem.”
Moderator Duncan: “Thank you, gentlemen.”
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