Comparison of in Vivo Kinematics of Different Implants in ACL-sacrificing vs. Medially Stabilized Arthroplasty (Implant Design Study)

Worldwide the number of patients requiring treatment for osteoarthritis is increasing due to increasing obesity, an ageing population and a high demanding younger population. Learmonth describes hip arthroplasty as the "operation of the century" because patients are highly satisfied with pain relief and function after the procedure. 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. Reported problems are insufficient function and persistent pain. On the other hand, knee arthroplasties are increasingly implanted in younger and more active patients who require high function and quality of life. Improvement of results after knee arthroplasty is therefore an urgent issue in the field of orthopaedic research.

Knee kinematics and implant designs:

A possible cause of lower function of replaced knees is the unnatural postoperative knee kinematics. The kinematics of replaced knees are closely related to their function. Studies show that replaced knees with excellent flexion angles have kinematic similarities to normal knees and malalignments of implants can cause postoperative pain.

Compared to the hip joint which is a simple ball-and-socket joint, the kinematics of knee is more complex. Native knee kinematics are a combination of a rolling and gliding motion of the femoral condyles and rotation of the tibia, where roll back of the lateral femoral condyle is larger than medially. Roll back is important for high degrees of flexion. Native knee kinematics are not fully understood, but the anterior cruciate ligament (ACL) seems to be an integral part, as knees without a functioning ACL show decreased roll back. 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, but in knee osteoarthritis the ACL and posterior cruciate ligament (PCL) is often deficient as they are damaged by inflammation. Therefore, there are three concepts regarding the retaining of these ligaments in knee replacement surgery. 1: Sacrifice both the ACL and PCL, 2: Retain the PCL (cruciate retaining (CR) design), 3: Retain both the ACL and PCL (bicruciate-reataining design).

The function of the cruciate ligaments can be mimicked by different designs of the tibial insert. The CR design has been widely used in the northern countries, especially the Zimmer Biometri NexGen CR implant in Norway. There are migration data for the NexGen implant with high precision measurements available in the literature. However, the NexGen CR implant does not retain the ACL and does not mimic natural knee kinematics.