Evaluation of Early Migration of Medially Stabilized Arthroplasty as a Predictor of Long Time Survivorship (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 knee implants is 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. Kinematics of replaced knees is 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 hip joints which are simple ball-and socket joints, the kinematics of knee joints is more complex. The kinematics are a combination of a rolling and gliding motion of the femoral condyles and rotation of the tibia. 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. All of these implants have satisfying survivorship, but unicondylar arthroplasty which retains both cruciate ligaments has the best clinical results with survivorship at 15 years 93%. In total knee replacement (TKR) the Anterior Cruciate Ligament (ACL) and possibly the Posterior Cruciate Ligament (PCL) are sacrified. When sacrificing the one or both of the cruciate ligaments, natural knee kinematics are affected. The function of the cruciate ligaments can be mimicked by different designs of the tibial insert.

The most used knee implant in Norway, the NexGen Cruciate-retaining (CR) implant design, does not retain the ACL and does not mimic natural knee kinematics. The tibial insert of the medial pivot implant design (GMK Sphere) has a constrained medial ball in socket joint and at the same time allows lateral anterioposterior movement. This new design was developed by a group of dedicated researchers. The design intention is to resemble the function of the cruciate ligaments and at the same time allow for lateral anterioposterior movement (rollback).