Where Are We Now? Queen and colleagues performed a randomized trial to determine whether fixed- or mobile-bearing total ankle arthroplasty (TAA) results in better clinical and biomechanical improvement at 1-year followup. Similar to the results of previous studies [2, 3, 5, 8], the authors found no substantial differences between the 15 patients with fixed-bearing implants and the 18 patients with mobile-bearing implants. However, analysis of TAA survivorship based on high-quality national arthroplasty registries show less-favorable results than those following total hip or knee replacements, including a high revision rate (10% at 5 years, of which 40% is from aseptic loosening) [4]. The majority of TAAs included in national arthroplasty registries are mobile-bearing implants, which theoretically offer biomechanical advantages, but are also associated with polyethylene concerns [7, 9]. Fixed-bearing implants may be more reliable and improve TAA survivorship. Prior research reports a slight superiority favoring fixed-bearing prostheses over mobile-bearing designs in terms of clinical outcomes [3], risk of revision [7], and joint biomechanics [5]. Where Do We Need To Go? Although no important differences were found, the current study confirms the necessity of objectively comparing fixed- and mobile-bearing TAA designs, and emphasizes the importance of using both clinical and biomechanical criteria for this comparison. Even so, several unanswered questions remain: (1) To what extent can we tolerate a difference between implant designs in order to determine the superiority of either the mobile or the fixed-mobile concept? (2) What is the best way to assess clinical outcomes? (3) What are the minimal requirements for a comprehensive and sensitive biomechanical assessment? Biomechanical assessment offers the potential to detect factors that can help predict outcomes. However, comprehensive clinical assessment is a priority because we need to know the treatment's efficacy from the patient's perspective in terms of functional performance. Regarding biomechanical comparisons, although the authors of the current study performed a comprehensive gait assessment evaluating temporal distance, kinematics, and kinetics data, the biomechanical differences between the fixed- and mobile-bearing TAAs may be subtle and perhaps require foot-and-ankle-specific gait analysis focusing on longer walking distances than currently available in gait laboratories and daily living conditions [1, 6]. How Do We Get There? In order to compare fixed- versus mobile-bearing implants, researchers generally examine implants from different manufacturers. Only the Salto® ankle system (Integra Life Sciences, Plainsboro, NJ, USA) has both fixed-bearing (Salto Talaris®) and mobile-bearing (Salto®) options. Comparing fixed- and mobile-bearing implants from different manufacturers is a challenge because there are several possible biases resulting from the differences in the design of the prostheses, including the anatomic or nonanatomic shape of the articulating surfaces, the amount of mediolateral talar resurfacing, and the degree of congruence between the polyethylene and the metal. This could be overcome by including a larger number of patients and several fixed- and mobile-bearing prosthesis designs from the data in national arthroplasty registries—just one more reason to support their systematic and high-quality implementation. Beside a high number of patients, registry data are objective, offer long-term followup, and can provide relatively precise estimates of revision risk in different patient populations. Unfortunately, registries do not allow us to evaluate biomechanical parameters, and of course, they do not gather gait-analysis data. As demonstrated in the present study, where only 40 of 185 potentially eligible patients could be included, developing a large prospective and randomized database that compares gait analysis after the implantation of a fixed- or mobile-bearing TAA is a major challenge. However, in order to recruit enough patients for a sufficiently powered randomized trial, a multicenter design will be needed. I note, though, that since implant problems typically occur between 5 to 10 years after surgery, studying mid- and long-term results, including clinical and biomechanical assessment, may be even more important than randomization. Finally, gait analysis usually requires specialized facilities, even if modern, wearable systems have been validated to assess the patient during activities of daily living [6]. Defining a set of important gait parameters and validating them as the gold standard for TAA-assessment would be an important research goal because it would favor a more systematic use of gait analysis in the assessment of TAAs and facilitate comparison of the results, even between different studies. Of course, these assessments need to be performed in combination with clinical evaluation using validated and comprehensive outcome tools.
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Open Access
Oct 1, 2020
Chelsea S Mathews + 5
