“We shouldn’t be afraid of the all polyethylene construct. There’s been good evidence since 1984 that it’s a safe construct, ” says Michael Dunbar. Tom Thornhill counters, “Cost is increasingly going to be an issue…and this is going to be a zero sum game. In my practice I use a modular tibial component that is cemented.”
This week’s Orthopaedic Crossfire® debate is “The All Poly Tibia: Cheaper and Better.” For the proposition is Michael J. Dunbar, M.D., F.R.C.S.(C), Ph.D. from Dalhousie University in Nova Scotia; against the proposition is Thomas S. Thornhill, M.D. from Harvard Medical School in Boston. Moderating is Leo A. Whiteside, M.D. of the Missouri Bone and Joint Center in St. Louis.
Dr. Dunbar: “The all poly tibia is a cheaper construct. Why? In order to provide a modular component there is a lot more engineering and machining involved. The engineering is quite complex, and this engineering needs to take place both on the tibial base plate side and on the polyethylene. That raises the question, ‘Why do we need to do that?’ Because issues of fixation and modularity—which have large been solved by engineering—but they weren’t trivial and they can still persist. By these I mean backside wear and the movement associated with modularity.”
“The other issue is that by this engineering that’s required you have less poly per unit of volume. So per bone resection when you use a modular tray you get less poly to run on. When you have sub-critical levels of thin poly on metal-backed trays, any third bodies can become critical wear issues.”
“One of the major claims that is put forward is that the reason we need modularity is that we can go back and do simple revisions. I think that is false. A paper from Mayo looking at the effectiveness of isolated tibial insert exchange in revision TKA [total knee arthroplasty] looked at 56 isolated tibial insert exchanges. I quote: ‘Isolated tibial insert exchange led to a surprisingly high rate of early failure. Tibial insert exchange as an isolated method should therefore be undertaken with caution—even in circumstances for which the modular insert was designed and believed to be of greatest value.’”
“If you think about the reasons we have for failure in TKA, the early failures occur due to infection and instability and the late failures occur for aseptic loosening and polyethylene wear. Neither early nor late reasons for failure can be routinely addressed with an isolated poly exchange. The fact is that all-poly tibial components are not a new concept and there is a lot of data on how they perform—enough to do meta analysis. A 2011 study in Acta Orthopaedica by Tao Cheng had a forest plot demonstrating that there’s no difference between the metal backed and all poly tibias. But, there were significantly more lytic lines in the metal backed group.”
“So what do these lytic lines mean? A 2005 study in Acta Orthopaedica by Hans Hyldahl looked at radiostereometric analysis (RSA) data…two groups of 20 patients—all poly versus metal backed. They found significantly more adverse RSA motion with the metal backed tibial component. Why might this be? It turns out that the rigid metal backed tibial component with a high modulus of elasticity can produce liftoff with asymmetrical tibial load. If you have a keeled construct with a metal back and a high modulus and you get any sort of asymmetric load, then that leads to liftoff. Conversely, on the other side you get subsidence…so the component teeter-totters, which isn’t the case with the all polyethylene component.”
“More concerning is that continuous motion, which is a prognosticator of long term failure, is much higher in the metal backed group than it is in the all poly group.”
“Finally, we’re told that modularity of metal backed tibial components is needed to balance the knee at the time of surgery. I’d suggest that surgeons with a ton of experience—like Dr. Thornhill—do not need this modularity.”
Dr. Thornhill: “If you use an all poly tibia you have to cement it. Unfortunately I don’t have a big argument. I think much of what Michael says is true. Cost is an issue, there are good long term results, and there is better poly. And you don’t have the problem that Mike pointed out of backside wear.”
“All poly tibias are generally done in low demand patients, backside wear is less than it used to be, and there are some facilitated things with modularity. As for backside wear, we now have an improved metal tibial surface with polished cobalt chrome. We have improved poly which is mechanically stronger with better locking mechanisms. We have reduced micromotion, particularly with a better interference fit. As for motion, with crosslinked poly the micromotion almost approaches that of the preassembled unit.”
“There are some interoperative options that are facilitated by this and I agree that doing an isolated poly exchange is not terribly common. But with most systems now we can change conformity. You can go from CR to PS (cruciate retaining to posterior stabilized) and save the tibia, but you have to think about the femur. They now have these ultra-congruent trays, but in my revisions I rarely save the tibial tray. You can increase tibial thickness, which will help you in conformity if you have global instability. But doing that for flexion instability does not work.”
“With regard to tibial bone loss, I think that oftentimes you will see sclerotic bone on the posteromedial side and then porotic bone once you clear the surface off. In this case I think cement is beneficial because it will give a uniform proximal tibial mantle. Then, if you look at the bending moments in an old article (Walker and Reilly, 1979) we see that you do better with offset loads. And I’d have a little issue with the argument of liftoff of a stiff metal backed component because, Michael, you being from the Maritimes know the function of the keel of a boat with or without a centerboard. With cement it shouldn’t lift off unless it bends.”
“As for fixation, if you’re going to use an all poly tibia you must use cement. When you examine the results of cementless tibial fixation things have changed. They now have multiple types of metals that do have better scratch fit and are probably friendlier to ingrowth. And they are now modular so you will get the benefit. We talked about having cemented hips and then hybrid hips, and now mostly cementless hips. Michael, I know you still cement the femur in 95% of yours, but I’d suggest that we will change to cementless tibial designs.”
“In my practice I use a modular tibial component that is cemented. The economics may dictate the change for selected patients because the low demand patient is probably going to need a low demand knee.”
Moderator Whiteside: “Mike?”
Dr. Dunbar: “You summed it up nicely. I think we’re going to be forced to look at the all poly construct because of cost constraints. The concern is that center by center we will just deploy it for all comers. We should become more patient-specific in our approach in identifying those patients who do fine because long term cement will be the weak link on fixation. We’re probably going to be moving away from that based on Nakama’s recent Cochrane review which showed that long term uncemented implants outperform cemented implants. My point is that we shouldn’t be afraid of the all polyethylene construct. There’s been good evidence since 1984 and there continues to be evidence that it’s a safe construct. We shouldn’t apply it to everyone; we also need better tools such as gait analysis, etc., in order to determine who should get these devices.”
Moderator Whiteside: “Tom?”
Dr. Thornhill: “We published an article with a mathematical algorithm showing how to determine how to match a patient’s need with the implant…because cost is going to be an issue. It is going to be a zero sum game. We’re now looking at bundled payments, and when the total cost that is available for reimbursement is affected, we may see a change in the pattern of behavior (giving lower cost implants to people with lower demands).”
Moderator Whiteside: “I’ve heard that for so many years so I maintain some skepticism. Mike, you quoted a study suggesting that revision of the poly component in an isolated circumstance doesn’t work very well. Do you know of any articles in the literature showing that it has worked well?”
Dr. Dunbar: “No.”
Moderator Whiteside: “Well, having written one of those articles…”
Dr. Dunbar: “Right, well we’re trying to make a strong point one way or the other, and trying to pull out some educational points.”
Moderator Whiteside: “I’ll send you a copy.”
Dr. Dunbar: “Sign it, would you?”
Moderator Whiteside: “Tom, do you ever do an isolated revision of the poly?”
Dr. Thornhill: “Yes, and it tends to be in a PS knee that is well aligned, well rotated, and globally loose.”
Moderator Whiteside: “Or worn out?”
Dr. Thornhill: “Yes, as long as I have something that I can get with better locking mechanisms.”
Dr. Dunbar: “I think the analogy is that if your car tire wears asymmetrically and you keep replacing the rubber then you’re better off realigning the tire at the same time. It’s rare to have symmetrical wear, etc.”
Moderator Whiteside: “What about the young, heavy patient? Do you see an advantage to a man in his early 40s—a laborer—with traumatic arthritis who is going to need a knee replacement, and needs to get back to work?”
Dr. Dunbar: “No. I’d suggest that a monoblock, low modulus, advanced porous material that can deform under load would be an advantage in that person.”
Moderator Whiteside: “Tom, same question.”
Dr. Thornhill: “I’d be using a modular one anyway. But there’s one other instance where it happens…extremely rare…but in someone with a well fixed implant I have used an angle bearing insert that could correct some of their malalignment. You can balance the soft tissues. That’s another case where I might leave the tibial component and put in an insert.”
Moderator Whiteside: “Thank you, gentlemen.”
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