“Modular platforms are more expensive, unproven, and you have a rigid implant along with bone stress shielding, ” argues David Lewallen. But Tom Thornhill counters, “You have fewer options with an all poly tibia, including the fact that you can’t go without cement. And if you cemented an all poly tibia you can sometimes miss the fact that there is cement left towards the back.”
This week’s Orthopaedic Crossfire® debate was part of the 31st Annual CCJR – Winter meeting, which took place in Orlando this past December. This week’s topic is “The All Poly Tibia in Patients <60: An Affordable Care Act Alternative.” For the proposition is David G. Lewallen, M.D. of Mayo Clinic in Rochester, Minnesota. Thomas S. Thornhill, M.D. of Harvard Medical School is in opposition. Moderating is Michael J. Dunbar, M.D., F.R.C.S. (C), Ph.D. from Dalhousie University.
Dr. Lewallen: “There are no ideas more dangerous in medicine than the things we think we know for sure. For example, that suction irrigation tubes are optimal for the two stage treatment of infected total joints or that bone loss around failed total joints is due to cement disease. And on and on.”
“So what about modular, metal backed tibial trays? My biggest challenge has been huge bone loss from a manmade iatrogenic plague related to implant design. It involves particulate poly with cement debris, facilitated by metal particulate, facilitating activation of macrophages and histiocytes and significant bone loss.”
“There are many types of poly wear and deformation…and I’m not saying that the only reason they fail is because of modular trays. But it’s an important part of the equation and it’s been underestimated. We thought that the tibial articular surface was the main focus for problems, but if that was really the problem then where were all of these cases in the early part of my career when we had implants that weren’t optimal in design or materials and that had lots of wear?”
“There are isolated cases that have done well, where the poly is worn out at 10-14 years and you change the poly and the patient improves. But these early poly exchanges usually end in disaster.”
“The other problem with modular trays is locking mechanism instability on the interface that can lead to debris, backside wear and osteolysis. It’s not a single design problem.”
“And regarding what we tell patients about survivorship, we quote old data that’s from metal backed, monoblock implants…and then we put in a modular device. If you actually graph the results then the modular implants don’t do as well. In his own series, Dr. Ranawat has shown a big difference between all poly and metal backed implants (96% versus 75%). A report by Weber (Journal of Arthroplasty, 2002) showed a several-fold increase in revision, lucencies, and osteolysis with modular implants.”
“We at Mayo just published a review of tibial component designs that involved 14, 524 primary knees. We had 865 revisions, and we found that across all designs, all poly tibias outperformed their metal backed alternatives. Interestingly, the cruciate retaining (CR) knees seemed to do better than the posterior stabilized (PS) knees…until we took out one implant that was overrepresented in the series. After that, the difference between the CR and PS knees disappeared.”
“The etiology of osteolysis is multifactorial, but modular, titanium baseplates are part of the problem. So why not solve it with crosslinked poly? A knee is not a hip. We have issues with the manner in which wear occurs, and with delamination and pitting. There are also concerns about the performance of the material in terms of the connection and the post. They are also more expensive.”
“Rotating platforms were another design effort to solve this backside issue. But there are tradeoffs with anything we change. These platforms are more expensive, unproven, and you have a rigid implant along with bone stress shielding.”
Dr. Thornhill: “We published a study last year using quality adjusted life-years and incremental cost effectiveness ratios. It basically said that if you want to offer a 20% decrease in long term failure at a 50% increased cost you’re only going to have an advantage if you use it in young people.”
“There are long term results of all-poly tibias, and we do have better poly in terms of wear, mechanical strength, and oxidative resistance. I use moderately crosslinked poly in the knee and then there is no backside wear.”
“Chit Ranawat (and others) have results showing that all poly tibias are found in low demand patients. Backside wear is reduced; it clearly was a problem—a poly problem and a component design problem.”
“To reduce backside wear we now have an improved metal tibial surface, better poly, and reduced micromotion with a better locking mechanism and better interference fit. And this is in lots of different implant systems. Also, we moved from a titanium to a highly polished cobalt-chrome tray (maintaining the same thickness). We have a wider profile so there is a better force fit; there is an increased locking mechanism because that was the failure leading to significant backside wear and motion.”
“Backside wear and motion has decreased by about 90% with modularity. Now, if you just do a poly exchange you’re often going to miss the problem you’re looking for. But there are intraoperative options. You could go to slightly more constraint or move to a more congruent liner. But if you switch from a CR to a PS then you must change other things in terms of flexion/extension balances. If I’m opening the knee for almost any reason then I generally destabilize the knee a bit and I put in a poly that is somewhat thicker. In an all-poly I don’t have that option.”
“Tibial bone loss is a problem. In a knee with a sclerotic rim that’s been drilled, when you curette everything that’s there and prepare to cement, you have a discontinuous mantle. Cement creates a uniform proximal tibial mantle. But if you have an all poly tibia then the high bending stresses on the poly are mollified by having a metal tray with lower bending stresses.”
“When I put in my trial and try to cement it I am very careful to prevent cement in the back; if you use an all-poly tibia you can sometimes miss that. And the last thing is fixation options. I used to debate against uncemented tibias. There is no uncemented option here, however, and we are increasingly moving to uncemented tibias. In an all-poly tibia you must have cement.”
“I use a modular tibial component with a newer poly, moderately crosslinked. The economics may dictate this change in selected patients. But I don’t think these are the patients—less than 60.”
Moderator Dunbar: “David, Tom made a good point about the concept of intraoperative options. How do you feel about losing that option?”
Dr. Lewallen: “You must decide at some point in the procedure in terms of the width of the poly, and with current trials it’s easy to do that. Some people are insecure about the notion of not being able to swap out the insert until they put the first stitch in the extensor mechanism, but it’s easy to make that decision about five minutes earlier.”
Moderator Dunbar: “Do you ever find yourself in a situation where you have a tibial baseplate in and you’re trialed with a poly and gone on to a different poly?”
Dr. Lewallen: “If you have trial components then there is slightly more stability to the knee than with cemented components. You can factor that into your decision if you can’t decide which poly width to use.”
Moderator Dunbar: “Dave made the point that the data we’re quoting about metal backed tracks are actually about monoblocks. Tom, how do you feel about that?”
Dr. Thornhill: “The mid 1990s were a dark period because the gamma in air polyethylene decreased the mechanical strength and increased backside wear. The tighter the knee the more balanced it is. Our trials are a bit tighter, partly because of the friction…most of them are delrin and they are somewhat stickier.”
Moderator Dunbar: “Dave, the all poly—meaning the lack of an interface between the modularity—or is it about the modulus?”
Dr. Lewallen: “It’s hard to separate the two. But, we’ve now seen enough retrievals of these radial abrasions on the trays, and it’s the metal particulate that helps facilitate small particles of polyethylene. It’s not the total amount of wear that is so astronomical in some of these metal backed trays, but the metal particulate allows very small particulate poly to be generated…and hence the huge lesions.”
Moderator Dunbar: “So would you prefer an all poly or a monoblock with a low modulus baseplate?”
Dr. Lewallen: “I actually use a monoblock porous ingrowth implant, which you can also cement. It eliminates backside motion, so that’s a good option in a young patient if you want to go to a cementless implant.”
Moderator Dunbar: “Do you think these prostheses deform under load?”
Dr. Lewallen: “Having some deformation in the proximal tibial isn’t a bad thing in order to maintain bone quality. There are data showing better preservation of bone on quantitative computed tomography around such implants.”
Dr. Thornhill: “You need to load the bone, and once you get above roughly 10mm of poly you increase the stiffness. But when there is a big disparity between the medial and lateral sides then you’re better off with metal backing.”
Moderator Dunbar: “So maybe we’re oversimplifying it. Dave, when would you not use an all poly?”
Dr. Lewallen: “Some knees have major bone deficiency at the time of primary surgery. If you need a stem or a metal augment then that’s when the tradeoff is worth it.”
Moderator Dunbar: “Tom, who is the best patient for an all poly?”
Dr. Thornhill: “An elderly, low demand patient.”
Moderator Dunbar: “Thank you, gentlemen.”
Please visit www.CCJR.com to register for the 2015 CCJR Spring Meeting, December 9 – 12 in Orlando.
