“Saying that modular metal backed tibial trays are somehow the ‘gold standard’ for modern TKA just isn’t true, ” says David Lewallen. “Wait, ” says Tom Thornhill. “Most all-poly tibia results are in low demand patients. And, backside wear is now substantially better than it was during the period of time that David showed us.”
This week’s Orthopaedic Crossfire® debate is “The All-Poly Tibia: Cheaper and Better.” For the proposition is David G. Lewallen, M.D. at Mayo Clinic in Rochester, Minnesota; against the proposition is Thomas S. Thornhill, M.D. from Harvard Medical School. Moderating is Cecil H. Rorabeck, M.D., F.R.C.S.(C) from the University of Western Ontario.
Dr. Lewallen: “There are no ideas more dangerous than the things we think we know for sure, and then a few years later realize these things weren’t so valid. Saying that modular metal backed tibial trays are somehow the ‘gold standard’ for modern TKA [total knee arthroplasty] just isn’t true. The biggest challenge in revision surgery this past decade has been related to particulate debris, and it’s not a coincidence that this exploded at the same time the industry converted to modular designs that allowed for backside wear, metal debris, and small particulates from those third body particles.”
“I’m not saying that backside wear is the only issue, but I think it was one of the issues that was a driver for the early osteolytic cases. And it’s not like we didn’t see poly wear years ago. But we didn’t see the kind of lysis around well fixed implants that has become so familiar to us…that we accept without question.”
“Modularity has been proposed as a great advantage because then we can change the liner. But if you examine the literature you see that it’s usually a bad idea. Why? Because many times the things that led to early poly wear were not corrected with a simple poly exchange (malrotation, flexion-extension gap problems). This is in contrast to the occasional late poly change in a patient at 12-15 years. In those very few cases it may provide some small advantage. But abrasion on the top of titanium trays is a problem, and you see it in more than one design. It means extrusion of the inserts…painful.”
“The literature on backside wear and locking mechanism shows problems. In a study of Dr. Ranawat’s cases (Rodriguez, J.A., CORR, 2001) of modular monoblock versus all-poly tibial components, there was a big difference at seven years (75% with metal backed modular versus 96% with all poly monoblock). In another series (Weber, A.B., Journal of Arthroplasty, 2002) they found a 2.5 times revision rate, higher radiolucencies, and balloon osteolysis was 17 times more common in the modular devices. What are we doing?”
“We reviewed 10, 000 patients and 14, 524 primary TKAs at our institution and examined implant design issues on the tibia. We found that across designs polyethylene tibias outperformed metal backed modular designs. This was true even after correction for age and gender. Initially it looked like the CRs (cruciate retaining) were better than the PS (posterior stabilized), but when we removed one problem design the other designs showed equivalent performance of CR and PS.”
“The etiology of osteolysis is multifactorial…but modular designs are part of the problem. Perhaps crosslinked poly and vitamin E poly may be ways to solve the problem, but there are things we still don’t know. The knee is not a hip, and there’s a different pattern of abrasion, as well as delamination and pitting. As far as expense, recent redesigns of modular implants are an improvement. And if you choose a chrome/cobalt rotating platform design, then you have an issue of stress shielding. If you don’t do revision surgery you don’t care about this; but if you do, this matters.”
“You should consider the use of cemented all-poly designs for some, if not all, of your patients.”
Dr. Thornhill: “I have a two-handed stranglehold on a loser because I think there are many things that David says that are true. In terms of cost, an all-poly tibia is only going to get more important; they also have good long term results. Our polys are now more wear resistant, have better mechanical strength, and they are more oxidatively resistant; they also don’t have the problems with backside wear.”
“However, most all-poly tibia results are in low demand patients. And, backside wear is now substantially better than it was during the period of time that David showed us. From that time period until now those problems—which were actually issues of poly and locking mechanism—have improved.”
“In most cases we no longer use screws and thus avoid the issues of screw osteolysis, fretting, and portals for the bioactive particles to get into the tibial interface and cause loosening. Also, we’ve improved the metal tibial surface and reduced micromotion by having a better locking mechanism and a better interference fit. When the issues David discussed were occurring, I actually moved away from fixed bearing to where almost 80% of my cases were rotating platforms. I did this because I thought that was the easiest way to get away from backside wear. While I still use some rotating platforms, my percentage has almost flipped back to all fixed bearing.”
“There has been a switch from titanium to a cobalt chrome tray. The poly not only has better mechanical properties, but it has a better force fit, a beefed-up bumper mechanism. Now, I agree with what David said about there being problems with this, but it does facilitate some interoperative and revision options…with the caveat that if there are other problems, isolated component exchange is not very good.”
“Now that we have tibial femoral bearing mechanisms with the inserts—CR, PS, ultra-congruent—you can change your congruity within the CR and the PS profile. Sometimes on the PS side you do have to be careful of what femoral component you are using. And I must admit that each time I revise a knee I invariably put a tibial insert in that’s about two millimeters thicker just because of the loosening that I’ve caused by entering.”
“Tibial bone loss and weakness are important when we talk about kinematic alignment and allowing varus. There can be a disparity of bone and sclerosis; in such a case, cement creates a uniform proximal tibial mantle. You can do that with an all-poly tibia and with a metal back. In an old article by Walker and Reilly (1979) they discuss the lower bending stresses in a metal-backed tray in that situation. Think for a moment: if you are now tolerating varus then that may come into play more in a kinematic alignment knee. The bending moment in the poly tray may in fact be beneficial.”
“As for the clearing of cement, if you use a modular system you can make sure that you’ve cleared it—particularly if you’re using a tibial insert as a trial when the cement is curing. Regarding fixation options, we will get to cementless components, which will hurt the all-poly tibia. So at present I use a modular tibial component and some all-poly components in elderly, low demand patients. But the economics may dictate a change in selected patients.”
Moderator Rorabeck: “David, is this an all or nothing thing with you?”
Dr. Lewallen: “Currently, I think probably around 30-40% of my patients get an all-poly tibia. I use modular implants in complex primaries where stems or augments are needed because that is a requirement. I also use a modular design in the very obese patient—it is technically easier than putting in a monoblock. I will use a modular implant with some very young patients, hedging my bet for the second or third decade (for exchange of the insert). I also have access to a monoblock metal-backed implant that I’ll use as an alternative to the all-poly sometimes.”
Moderator Rorabeck: “Is there any minimum thickness of an all-poly tray that you would recommend?”
Dr. Lewallen: “I haven’t avoided using the narrowest of the available implants. They’re much thicker than the stated size because for labeling purposes…but they’re right around 10mm/”
Moderator Rorabeck: “Tom, do you use any all-poly tibias in your practice?”
Dr. Thornhill: “Yes, in the elderly, low demand patient who has reasonable bone.”
Moderator Rorabeck: “An elderly person with reasonable deformity and reasonable range of motion?”
Dr. Thornhill: “Yes, and with reasonable tibial bone.”
Moderator Rorabeck: “What about reasonable thickness?”
Dr. Thornhill: “The studies say that once you get above 8-10mm of poly it behaves almost like the stiffness of a metal backed tibia, not only for the bending moments but for the Von Mises stresses in the proximal tibial bone.”
Moderator Rorabeck: “David, of the 10, 000 cases that you were doing, presumably the content the oxidation of the tibial inserts varied?”
Dr. Lewallen: “Yes.”
Moderator Rorabeck: “Did that have any impact?”
Dr. Lewallen: “We weren’t able to look at that variable because some of the manufacturers actually changed the poly three or four times during the study period—so we weren’t sufficiently powered to look at that issue.”
Moderator Rorabeck: “So Tom, if you were going to do one today, what kind of poly would you use on an all-poly tibia?”
Dr. Thornhill: “I would use a moderately crosslinked poly with an antioxidant in it. And I don’t think it’s like we were discussing with the mouse calvarial model that they’re less immunoreactive. I just think that it’s oxidatively resistant. I think you can do that with any number of antioxidants, including vitamin E. I’m not sure about the role of that in the hip.”
Moderator Rorabeck: “Would it be fair to say that if you were going to do this that you’d be mainly doing CR type knees?”
Dr. Lewallen: “I do PS exclusively. It does raise the point of vitamin E. I think this is attractive when it comes to a post because it maintains a little better strength than the irradiated product. But for surgeons who are under pressure about cost, it’s certainly a way to lower your profile.”
Moderator Rorabeck: “Thank you both.”
Please visit www.CCJR.com to register for the 2014 CCJR Winter Meeting December 10 – 13, 2014 in Orlando, Florida.
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