Inching Closer to the End of the Debate Between Fixed Versus Mobile-Bearing Total Ankle Arthroplasty

The study by Assal et al. presents an interesting perspective on fixed versus mobile-bearing prostheses in total ankle arthroplasty (TAA) because the study utilizes similarly designed prostheses from the same manufacturer, with all surgical procedures performed by 1 of 2 surgeons.

The Salto (integra) prosthesis was designed by French surgeons in 1997, showing good initial results with short- and mid-term follow-up studies1,2. This prosthesis has a 3-component design with a mobile-bearing spacer. The Salto Talaris (integra) was introduced to the U.S. market in 2006 and to the European market in 2012. There were minor design changes between the Salto and the Salto Talaris, mainly to accommodate a fixed bearing on the tibial component of the latter prosthesis in order to allow 510(k) approval by the Food and Drug Administration in the U.S.

Over the years, there have been multiple studies discussing the short- and mid-term outcomes of either the Salto or the Salto Talaris, with prostheses showing acceptable if not excellent results3–5.

The debate regarding fixed versus mobile-bearing prostheses has been ongoing for many years. The concept of a mobile-bearing prosthesis was introduced with the theoretical advantage of having better-conforming articular surfaces and lower contact stress, with the dissipation of stress over a more consistent area of the joint. Another theoretical advantage was the self-correction and positioning of the polyethylene spacer, reducing wear and stress on the spacer itself. However, the majority of studies have not confirmed these theoretical advantages and in fact have suggested that the use of a mobile-bearing prosthesis leads to a higher rate of complications in the first year postoperatively6.

In a 2014 study retrospectively comparing the use of fixed versus mobile-bearing prostheses with a minimum follow-up of 2 years, Gaudot et al. reported that mobile-bearing prostheses had higher rates of radiolucent lines (40%) and cyst formation (24%) compared with fixed-bearing prostheses (12% and 3%, respectively)7.

One strength of the study by Assal et al. was that it included 2 relatively large cohorts, with 131 fixed-bearing TAAs and 171 mobile-bearing TAAs. The mobile-bearing procedures were performed in the earlier half of the study period. As a result, 1 theoretical concern was regarding the surgical experience of the surgeons over time; however, the surgeons had been performing mobile-bearing TAAs since 2000, with the study period beginning in 2004, so they were adequately experienced with the procedure by the outset of the study. The authors also reported that the complication rate in the mobile-bearing group was stable throughout the study period and did not decrease with time.

The most important conclusion of this study was that the rate of revision was 3 times higher among mobile-bearing compared with fixed-bearing prostheses. Most concerning was that most of the reported failures of mobile-bearing TAAs were because of cyst formation and gutter impingement due to heterotopic bone formation, with these prostheses showing higher rates of heterotopic bone formation in the medial and lateral gutters.

There were several questions that went unanswered in the study by Assal et al and that should be addressed in future studies.

What caused the increased rate of lysis, cysts, hypertrophic bone, and failure among mobile-bearing TAAs? One possible explanation is the smaller spacer utilized in the mobile-bearing implant, which increases the load per square inch of the polyethylene insert, contributing to greater wear.

Was there a higher rate of backside wear between the polyethylene insert and the tibial baseplate among mobile-bearing TAAs? This effect could cause an excess of problematic polyethylene debris.

Was there potential for edge-loading between the polyethylene spacer and the tibial component if the overall alignment was not reasonably accurate?

Along with these questions, the greatest shortcoming of the study was that it was not a prospective, randomized trial, but instead was essentially a comparison of the results of 2 longitudinal studies. However, 1 strength was that the study utilized 2 prostheses with similar designs from the same manufacturer, which will be of substantial value if the trial can continue to longer-term follow-ups.

The debate regarding fixed versus mobile-bearing prostheses will continue, but the study by Assal et al. provided an interesting window into the potential shortcomings of mobile-bearing TAAs. I congratulate the authors on an excellent paper.