**6. Midflexion stability**

For a successful TKR, the wide range of movement is not the only objective to achieve, but also joint stability plays a crucial role. During most activities of daily living, the knee is loaded not only in full extension but also in midflexion, and therefore, stability throughout flexion should be considered an important outcome measure. As the PCL is thought to serve as a secondary stabilizer of the knee during varus and valgus stresses, it is likely that the presence of this ligament has some effects on midflexion stability.

In a cadaveric study of 1999, Mihalko observed "…that a major result of posterior cruciate ligament sacrifice is the creation of a larger flexion gap. This result provides insight into relative joint line changes that can occur after posterior cruciate ligament sacrifice…," suggesting… "the need for greater attention to flexion stability when sacrificing the posterior cruciate ligament" [18]. In a 2008 study, Tsuneizumi et al. similarly concluded that "…the PCL kept the knee stable against distal traction force in the flexion position, and sacrifice of this ligament caused joint laxity in different ranges. The increases in the flexion gap after resection of the PCL varied among individuals" [19].

In 2013, Hino et al. focused on stability through the range of movement pinpointing the exact degrees that make differences between CR and PS TKRs stability: "Specifically, CR knees had significantly less laxity in the flexion range of 10 to 30° than PS knees…" demonstrating "…a significant decrease in joint laxity at 120° of flexion for CR-TKRs and in contrast PS TKRs had an increase in joint laxity between 10 and 20° of flexion post-operatively. Overall, CR knees demonstrated less joint laxity than PS knees throughout the whole range of movement." They concluded that "CR knees have less post-operative laxity, especially in deep flexion…" and "...this may be associated with the lower flexion range that could be seen in CR knees" [20].
