Researchers Take a Critical Look at Rotating-Platform TKA

Researchers from the Medical University of South Carolina and the Dartmouth-Hitchcock Medical Center noticed an interesting failure pattern specific to rotating-platform total knee arthroplasty (TKA) and decided to explore further.

Vincent D. Pellegrini, Jr., M.D., professor and vice chair of Research and Education Affairs at Dartmouth and senior author on a recent study titled, “Functional Flexion Instability After Rotating-Platform Total Knee Arthroplasty,” which appears in the June 15, 2020 online edition of The Journal of Bone and Joint Surgery, explained to OTW how this issue came up.

“It so happens that there are geographic ‘pockets’ in the U.S. where enthusiastic surgeons used a lot of rotating-platform TKA devices. This set the stage to explore and define the “at risk” loading conditions that might predispose a knee to insert instability and subluxation, and to quantify tolerances for flexion-extension gap asymmetry and laxity in order to prevent these adverse events.”

“Roughly 30 years ago DePuy started making the low contact stress TKA, which was predicated on high surface contact areas between femur and tibia and designed to reduce contact stresses and related wear in the polyethylene insert. One version featured a single piece of polyethylene with a central post on the undersurface and was dubbed a ‘rotating platform knee.’ This evolved into the rotating platform option of the Sigma Press-Fit Condylar device, which more recently persists as an option for the Attune knee, which comes in both fixed and rotating platform polyethylene versions,” Pellegrini explained.

“Patients often report a sensation of instability and laxity in the knee during deep knee flexion or descending an incline—especially going down a set of stairs, pivoting, or crossing their legs. These people would uncross their legs to stand up and their knees were locking…or when they were getting up, the knee would shift or give way. Over time, other knee replacements result in wear and perhaps a minor sense of instability but are not typically accompanied by such dramatic mechanical symptoms.”

Having revised several of those problematic knees, Dr. Pellegrini found that when he converted them to a fixed bearing prosthesis, the problem was typically solved. “I observed that the flexion gap with the knee at 90 degrees was asymmetric and not balanced medial to lateral. In all those that I revised, the lateral side was more lax, so it seemed that the issue was not so much balancing the extension gap to the flexion gap, but rather balancing the medial and lateral sides of the flexion gap.”

“This is because, for example, in single leg stance or when you sit, the medial compartment of the knee is compressed and the lateral compartment may be slightly gapped open. This situation, where the resulting lateral gap is slightly larger than the medial side, is not uncommon after TKA in a varus knee; we balance gaps, but it’s an imperfect art and patients can end up a bit more lax laterally than medially.”

“For this study, we performed TKA with an rotating platform implant in six fresh-frozen cadaveric limbs using a gap balancing technique, followed by sequential femoral component revision with variable-thickness polyethylene inserts to systematically represent 5 flexion-extension mismatch and asymmetry conditions.”

“Each configuration was subjected to mechanical loading at 0°, 30°, and 60°. Rotational displacement of the insert on the tibial baseplate, lateral compartment separation, and insert concavity depth were measured with use of a digital caliper.” Yield torque was used as a surrogate for ease of insert rotation on the tibial baseplate and escape of the femoral component from the poly insert.

“We suspected that laxity on the lateral side allowed the poly to rotate because the larger gap allowed the femoral condyle to ‘escape’ from the concavity of the insert. When you have a fixed bearing knee, the femur cannot ‘escape’ because of subtle insert rotation under the condyle and the margin for error for the femur to stay aligned with the tibia is the height of the post. In the rotating-platform TKA design, the depth of the insert concavity, which in some cases is only a few millimeters, defines the amount of gap asymmetry that is tolerated before insert rotation out from under the femoral condyle can occur.”

“This results in a very narrow margin for error and a rather unforgiving technique for a successful TKA operation. The depth of concavity of the poly defines how much lateral opening of that gap you can tolerate before the plastic escapes from underneath the femur.”

It is a demanding technique, says Dr. Pellegrini…and the ultimate question is: “Can this technique translate easily and broadly to a community orthopedic practice?”