Various prostheses have been developed to achieve better clinical outcomes for total knee arthroplasty (TKA). Obtaining a greater range of motion (ROM) [1], adequate stability in daily activities [2], and durability are essential factors [3]. The design of prostheses for Asian populations should consider the morphological differences in knee geometry between races to optimize function, because Asian knees have different morphological features from Caucasian knees [4].

A high-flexion posterior-stabilized (PS) total knee prosthesis, named ACTIYAS (Kyocera, Kyoto, Japan; Fig. 1) has been developed for Asian populations, and has been applied clinically since November 2010. The component design was based on computed tomography images of Japanese osteoarthritic knees, with the aim of accommodating most size variations in Asian patients. Recently, Yanagisawa et al. [5] reported that the femoral component of this implant could provide more comprehensive coverage of the dimensional variation found in the Japanese population, as well as in female-specific versions of the prosthesis.

The femoral component is made of zirconia ceramics, which exhibit low friction and high durability when used in conjunction with a polyethylene insert, as shown by knee simulation tests [6]. Ceramic materials generally provide a more polished, more hydrophilic, and harder surface compared with cobalt-chrome alloys [7], and zirconia ceramics are softer than alumina ceramics with lower fracture risk. Ceramic materials have the additional advantages of allowing the avoidance of metal allergies or hypersensitivity, which are generally caused by cobalt-chrome alloy femoral components [8], while also allowing for clear magnetic resonance images to detect postoperative lesions with fewer artifacts compared with those for metal components [9]. Zirconia ceramics were selected for the femoral component based on these theoretical advantages.

The in vivo knee kinematics of this novel PS total knee prosthesis for Asian patients was previously compared with a popular high-flexion PS prosthesis during standing, lunging, and kneeling activities [10]. Knees with the new prosthesis showed slightly greater implant flexion angles than those with the conventional prosthesis, although the range of motion was not significantly different. The femoral roll-back pattern of the new prosthesis was different from that of a conventional knee, reflecting the post-cam design. Other clinical studies have revealed stable kinematics and clinical benefits of this prosthesis [[11], [12], [13], [14], [15]]. However, midterm clinical outcomes with this prosthesis have not been previously reported. Therefore, the purpose of this study was to investigate mid-term clinical results at a minimum 5-year follow-up of this high–flexion PS total knee prosthesis in Asian populations.