Introduction

The first medially stabilized (MS) total knee arthroplasty (TKA) was developed in the early 1990s to improve tibiofemoral stability [1]. Based on experience with preceding designs and observations of the native knee, the developers sought to achieve a stable articulation with a fully conforming medial ball-in-socket articulation and ligament balancing technique [1]. Axial rotation about the long-axis was permissible via a less-constrained lateral condyle [1]. After its introduction, magnetic resonance imaging studies confirmed the observed medial stability in the native knee with axial tibial rotation exhibited by translation on the lateral side [[2], [3], [4]]. Subsequently the MS concept was shown to reproduce a similar kinematic profile to the native knee [5] and promising patient outcomes were reported [1,[6], [7], [8]].

Despite its theoretical potential, wider acceptance of the MS knee concept has been limited. Inconsistent outcomes for devices with the MS concept [[9], [10], [11], [12]] and the success of traditional designs are likely to have contributed to this. Nevertheless in 2022, the annual report of the Australian Joint Replacement Registry reported use of the concept had grown in the recent decade to 10% of all primary TKAs in Australia [13]. With growing interest in the concept and with the introduction of new MS knee implants, there is a need to assess the evidence for its reliability and any vulnerabilities regarding patient selection or its wider adoption.

Using complications data and patient-reported outcome measures (PROMs), the aim of this study was to assess the clinical performance of a second-generation MS knee design when introduced to general use by multiple surgeons who had not been involved in developing the device and did not previously use a MS knee. To the authors’ knowledge this is the only large cohort, prospective, multicenter outcomes study reported in the literature evaluating the introduction of a second-generation MS knee design to treat a normal demographic of TKA patients, independent of designer surgeons and surgeons with prior experience of using the concept. It was hypothesized that high levels of function and patient satisfaction would be observed when assessing the 2-year outcomes of this second-generation MS design.