In Vivo Stability of an Uncemented Total Knee Arthroplasty With Medially Stabilized Design

Worldwide the number of patients requiring treatment for osteoarthritis is increasing (Kurtz et al. 2007). Especially the young population will increase within the group of patients for arthroplasty (Kurtz et. al. 2009). Learmonth describes hip arthroplasty as the "operation of the century" because patients are highly satisfied with pain relief and function after the procedure (Learmonth et al, 2007). Knee arthroplasties have in recent years also shown promising results and have surpassed hip arthroplasty in frequency in western countries. However, patient satisfaction is not as high (Dieppe et al. 2012, Carr et al, 2013). Reported problems are insufficient function (Hawker et al. 2013) and persistent pain (Wylde et al. 2012). On the other hand, knee arthroplasties are increasingly implanted in younger and more active patients (Rabi et al. 2012, Ibrahim et al. 2010) who require high function and quality of life. Improvement of knee implants is an urgent issue in the field of orthopedic research.

Knee Kinematics and implant designs

A possible cause of lower function of replaced knees is the unnatural postoperative knee kinematics. Kinematics of replaced knees is closely related to their function. For example, replaced knees with excellent flexion angles have kinematic similarities to normal knees (Watanabe et al. 2013) and malalignments of implants can cause postoperative pain (Bell et al. 2014).

Compared to hip joints, which are simple ball-and socket joints, the kinematics of knee joints is more complex. It is a combination of rolling and gliding motion of femoral condyles and rotation of tibia (Smith et al. 2003). Based on the kinematics of the normal knee joint, various attempts have been made on the design of knee implants to reconstruct normal kinematics after replacement surgery. In healthy knees the contributing anatomical structures for knee kinematics are two cruciate ligaments (anterior cruciate ligament; ACL, and posterior cruciate ligament; PCL) inside the joint (Figure. 1). However, in knees with osteoarthritis, ACL may be degenerated by inflammation and often not deserve retaining. Therefore, the majority of total knee implant designs sacrifices ACL. Instead, various attempts have been made on implant design to develop knee implants with increased stability.

One of the designs is PCL-retaining (CR) knee with extended posterior femoral radius in flexion to provide greater femoral/tibial contact area in high flexion. This implant design allows for PCL retention and theoretically enhances antero-posterior rollback. CR knees are widely used for many decades and various implants with this design are available. Although they have favorable clinical results in the point of survivorship (Chalidis et al. 2011), CR knees do not reproduce femoral rollback during flexion in vivo (Watanabe et al. 2013).