Trousdale v. MacDonald: The Cemented All Poly Tibia in the Active <60 Patient
OTW Staff • Fri, June 16th, 2017
This week’s Orthopaedic Crossfire® debate was part of the 17th Annual Current Concepts in Joint Replacement® (CCJR), Spring meeting, which took place in Las Vegas this past May. This week’s topic is “The Cemented All Poly Tibia in the Active <60 Patient.” For the proposition is Robert T. Trousdale, M.D., Mayo Clinic, Rochester, Minnesota. Opposing is Steven J. MacDonald, M.D., F.R.C.S.(C), University of Western Ontario, London, Ontario, Canada. Moderating is Thomas S. Thornhill, M.D., Harvard Medical School, Boston, Massachusetts.
Dr. Trousdale: A few facts about all poly tibias or monoblock tibial components. First, the best long term results with fixed bearing knees to date are those with monoblock tibial components. Second, many modular fixed bearing designs have been relatively disastrous and are one reason we spend so much time talking about peri-articular osteolysis.
What are the pros of an all poly tibia? First, you can resect less tibia for the same poly thickness and for someone who’s young that’s a big advantage. Further, there’s less osteolysis, better long-term survivorship than some modular designs, and better loading of the proximal tibia. DEXA studies and bone density studies have looked at monoblock all poly tibias versus stiff cobalt chrome trays and there may be better stress distribution and loading of the bone with all poly tibias. And, of course, they are a lot less expensive than modular designs.
The major negative is that you can’t do an isolated polyethylene exchange with a monoblock component.
I would argue that late polyethylene exchange only applies to a small number of our patients that need a revision surgery, and I question whether it’s worth it, given all the negatives of modularity when only a small percentage of our patients will benefit from its use.
Some data from New York City (Rodriguez, CORR). The PFC design, 7 year survivorship, comparing the survival of metal backed tibias versus all poly tibias. Only 4% of the all polys had osteolysis, whereas 25% of the metal backed modular total knees had osteolysis.
Weber (JOA), looked at a modular tibial tray versus a monoblock tibia tray, and saw a 2.2 times higher revision rate with the modular tray and 3.4 times more radiolucency about the implant. And osteolysis was 17 times higher around the implant. So, in every outcome measure, the monoblock tibia wins out over the modular tibia.
At Mayo from 1985 to 2005, we did over 16,000 primary total knees and the all poly tibias had a significantly lower risk of revision versus the modular components. And the risks associated with the all poly tibial components were not affected by age, sex or body mass index [BMI]. It’s my belief that an all poly tibia is better for the younger patient.
Across all age groups, except for patients greater than 85 years of age, the all polys had a better survivorship compared to metal backed tibias, and those greater than 85 they were the same.
If you look at body mass index across all BMI groups, except the heavy-set patients, the all polys have better survivorship. In the heavy-set patients they were the same.
I think we can conclude that “all polyethylene” tibial components have significantly improved implant survival rates compared to the metal backed tibias despite age and weight issues. There are significantly reduced rates and risk of postoperative infection compared to metal backed tibias. And there are significantly reduced rates of tibial component fracture and loosening compared to the metal backed tibias.
Do we need modularity? Of course we do. We need it in our revision patients. We need it in our complex primary patients—those with large deformity, the real heavy-set patients. I think there is probably a benefit in putting a small stem on the modular tibial trays, so you need it in those patients. And in those patients with major ligamentous problems, you need a modular tibial tray to help facilitate a stable, well balanced knee.
So I conclude by saying well-designed monoblock all poly tibial components are entering their fifth decade of use. To date they’ve provided very durable fixation and I continue to use all poly monoblock components for the majority of my knee replacements. It’s my belief that the benefits of modularity do not outweigh their negatives in the majority of our patients that need total knee replacement.
The debate isn’t just all poly versus a modular tray. The debate is supposed to be about what we do in the active, less than 60-year-old patient.
How are we doing in that patient population? Not very well.
The Australian Registry shows age is a strong negative predictor to successful outcome. Patients in that younger category—less than 55—have a 15% revision rate at 14 years. But it’s not just in Australia. The U.K. Registry shows the exact same pattern.
Is an all poly tibia really the answer?
In the Australian registry, all poly, in general, did pretty well. They had a higher revision rate than modular trays, about half a percent, and there’s a bias in most registries to use all poly in the older population. In essence, it’s biasing towards all poly a little bit. But if you then apply these numbers on a U.S. basis or global scale, you are going to have a much higher cumulative revision rate overall with current generation modular trays. You need to be able to distinguish between 30 years ago to today.
Another registry, the Swedish Registry looks, once again, at 27,000 procedures. They actually concluded that the all poly tibia components were at least as good as or maybe superior. They give a relative risk reduction of 0.75 for all poly, so that would back up what Rob is saying.
But then we need to look at the issue of age because once again, the younger population that we’re debating had a two-fold relative risk of revision.
The U.K. Registry is very helpful. They’ve looked at CR and PS, and you can see the average age of those patients is 69-70; and the average age of their all poly is 74. Is that relevant? Of course, it’s relevant because we have to then put that on a scope of the failure rates per age. So, if only we had the data adjusted for age.
So, let’s really debate what we’re talking about: the young, active patient. If you’re a 55-year-old male, what’s your revision at 7 years? Whether you get a mobile or fixed PS doesn’t seem to make much difference. But if you get an all poly tray, you have a significantly increased risk of that tray failing. Maybe not at Mayo Clinic, but everywhere else in the U.S.
What about the older patient population. Absolutely not. The all poly tray performs well in that lower demand patient. Why is it important to correct for age? Because if you look at the U.K. National Joint Registry and you look at all comers, all poly is performing well. We’re not debating whether you should ever use an all poly, it’s whether you should use it in the young, active patient. And I think the data is pretty compelling that the failure rate is higher.
So, why the poor results of an all poly in the younger patient population? An all poly has higher stresses and less uniform stress distribution and some increased micromotion. And the thought there is that in the higher-demand active patient that might lead to the increased revision rate.
Lastly, I wanted to point out because I heard recently the thought that an all poly tray has a lower risk of infection. Let’s think about that. Is that true?
The Swedish Registry shows a relative risk reduction of 0.63, so you think, “That’s kind of a compelling argument. Maybe the risk is lower, although it doesn’t make a lot of sense.” Be very aware of data interpretation, because they didn’t say there was a lower incidence of infection. They said there’s a lower revision risk for infection. Is that distinguishable? What it means is if someone presents to you with an infected total knee, what do you do if you have a modular tray? You do an I&D, you do a polyethylene exchange and that counts as a revision. If that same patient with an all poly tibia comes, you’re still going to do an I&D and wash it out, but you can’t, of course, exchange the polyethylene. It doesn’t count as a revision because the polyethylene is not exchanged.
Interesting to know in the Mayo Clinic database if that’s the same issue there as well with infection, because it’s not counted if you don’t change the component, it’s still the same incidence of infection however.
Do I think there’s a role for all poly in 2016? I do. There’s substantial cost savings to our health care environments. There’s close to equivalent results even with current generations. And I would strongly consider it in the older, lower demand patients. But in the young, active patient, where we see higher failure rates, I think it is the wrong application.
Moderator Thornhill: Rob, I’ll give you one minute to rebut because he basically said everything you said is wrong.
Dr. Trousdale: Steve is pretty good at torturing the data out of registries to prove his point. Data’s data.
In our registry for the infection issue, reoperation with debridement, that counts as a revision whether you change the polyethylene or not.
The other problem with the registries is they count that relative flat-on-flat design that did relatively poorly. If you throw those out, I think most registries will probably tilt in favor of the all poly tibia if you throw out a poorly designed all poly tibia.
As to the data on the Australian Registry, the reoperation rate in younger patients is pretty close at 5.6% versus 5.1%—really not much difference. Then there’s patient bias issues, which we have in our shop as well, with who you put an all poly tibia in and who you don’t. You can argue the sicker patient may be getting all poly at the higher risk of infection. Despite that fact the all polys have a lower infection risk.
It’s a matter of torturing the data and I think if you torture it close enough, the data is pretty close and in our shop the all poly seems to win across all ages including young, active patients.
Moderator Thornhill: Steve, the thing of looking at these total condylar knees, these are people who are either dead or they’re in nursing homes and doing next to nothing. If you get rid of screw osteolysis, back-side wear and some of the things that we’ve done, when would you ever use an all poly tibia in somebody under 60?
Dr. MacDonald: I wouldn’t. I think that Rob is sometimes correct, usually not, so it’s not really torturing the data if you actually just report it, so it’s pretty clear when they break it into the age aliquots that the younger patients are having a higher failure rate.
Moderator Thornhill: How often in people under 60, Rob, do you use all poly tibias?
Dr. Trousdale: I use them in the majority of people under 60. The people I don’t are younger patients that are doing ultra-high torque activities to the knees—professional golfers, golfing every day, hitting the golf club 1,000 times, the professional skier, high-end squash or tennis player. Maybe the benefit of a rotating platform—I say maybe because the mid-term data doesn’t support that—but the long-term data will support the improved sort of wear characteristics of that design. But the majority of people under 60, if they’re not ultra-high impact, I’m doing a monoblock tibial tray.
Moderator Thornhill: So you said something and I always thought…I remember years ago they were looking at loading of the proximal tibia… it’s best if you use a metal backed component OR an all poly greater than 10mm because of the loading. But you said it’s actually better…
Dr. Trousdale: Stress distribution—that was data from the early ’80s. Stress distribution is better with a metal-backed tray, there’s no question about that. Loading of the tibia is a different issue. That may be better with a more flexible tray than a very stiff, say, thick cobalt-chrome tray.
Moderator Thornhill: Okay, gentlemen, great job. Appreciate it.
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