This week’s Orthopaedic Crossfire® debate was part of the 19th Annual Current Concepts in Joint Replacement® (CCJR®), Spring meeting, which took place in Las Vegas. This week’s topic is “The Cemented All Poly Tibia: Regardless of Age & BMI” For is David G. Lewallen, M.D. – Mayo Clinic, Rochester, Minnesota. Opposing is Aaron A. Hofmann, M.D. – Hofmann Arthritis Institute, Salt Lake City, Utah. Moderating is William J. Maloney III, M.D. – Stanford Hospital & Clinics, Stanford, California.
Dr. Lewallen: My task is to explain why I suspect most people in the audience aren’t doing enough all poly tibias. I put it to you that the concept that the Gold Standard for modern knee replacement is modular metal backed tibial trays will soon be on a pile of discarded thoughts.
Why is that? The biggest challenge of revision knee in the past two decades has been polyethylene wear, osteolysis and bone loss. We thought we knew why this has happened. We thought it came from the top side wear of the tibial component. However, we never saw lysis around implants despite lots of poly wear on the top side before the 1990s.
What’s going on?
The tarnished Gold Standard is based on modular components with implants that are moving around. They’re all rotating platforms. They just weren’t made for that. They all move to some degree, and some move more than others.
Articles from our institution tell you that isolated poly liner exchange is a bad idea (Babis et al., JBJS-Am 2002 and JBJS-Am 2001). One of the main arguments for having this is so surgeons can have the flexibility of swapping the poly liner at a later time. There are rare instances in the patient with wear at 10 or 12 years with a great knee until then where that’s a good operation. All those liner swap-outs at 1, 2, 3, 5 years are generally a bad idea because they are not fixing the underlying problem that’s resulting in the revision.
The unintended consequences of modular implants are: locking mechanism instability; backside abrasion and third body wear. It’s the metal particles, not because they’re so much metal, but because they drive down the size of the particulate. And it’s particle size that is everything for the biologic response, the debris, the wear and the lysis.
There are articles about locking mechanism failure and backside wear. The literature is replete with reports of problems related to this technology and yet we stick with it.
One of the early reports (Rodriguez, JA, Clin Orthop 2001) really called attention to the difference between all poly and modular implants. Same design and clear survival differences between the all poly monoblock (96%) version and a metal-backed modular (75%) version at 7 years.
We did a review at our institution (Maradit-Kremers et al., JBJS-Am 2014)—10,000 patients; 14,500 primary knees; 9-years mean follow-up; 18 different implant designs. Modular metal-backed and all poly versions. I can tell you, all-poly tibias did better than the metal-backed modular versions, even after we corrected for age and gender. Younger patients still did better.
A meta-analysis (Voss, et al., JOA 2016): 28 articles, 95,000 knees, all-poly tibias had a lower revision rate and fewer adverse events than the modular versions. Across the world’s literature.
A knee is not a hip, Cross-linked poly is not going to do it for us. Maybe rotating platforms will help, but the jury is still out. Right now the data clearly shows that all-poly implants do better than modular metal-backed.
