In vitro kinematics of fixed versus mobile bearing in unicondylar knee arthroplasty

When performing unicondylar knee arthroplasty (UKA), the surgeon can choose between two fundamentally different designs: a mobile-bearing (MB) inlay with high conformity, or a low-conformity, fixed bearing (FB) inlay. There is an ongoing debate in the orthopaedic community about which design is superior. To date, there have been no comparative biomechanical studies regarding each system's effects on the quadriceps force and the medial contact pressure. The purpose of this study was to investigate these alterations in vitro before and after UKA with two prosthesis systems, representing the MB and FB designs. FB and MB unicondylar knee prosthesis designs were tested in sequence under isokinetic extension in an in vitro simulator. In each case, the required quadriceps extension force was determined before and after implantation of a medial UKA. Furthermore, the tibiofemoral contact pressures were evaluated for both prosthesis designs. The quadriceps force maximum was achieved at 106A degrees and 104A degrees of flexion with the FB and MB designs, respectively. Implantation of the FB UKA resulted in a significant increase in the necessary maximum quadriceps force (p = 0.006). In addition, implantation of the MB UKA resulted in a significantly higher extension force (p = 0.03). The difference between the two groups was statistically significant in deep flexion (p = 0.03), with higher forces in MB UKA. The MB design showed significantly increased quadriceps extension force compared with the FB inlay in deep flexion. Although the FB design showed higher maximum peak pressures concentrated on a smaller area, the pressure introduction in deep flexion was lower, compared to MB inserts.