Stability of Contemporary Knee Arthroplasty

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).

Small alterations in implant design can influence the survival of implants. The final design of the GMK Sphere was introduced in 2012. New implants should be monitored and assessed in small, controlled trials with high precision measuring methods.

Analytical Method

RSA has been used in orthopedic research fields since 1970s. The original application of this method was for the evaluation of implant migration (i.e. fixation) and polyethylene wear of artificial joints using static X-ray pictures. Clinically relevant association between early migration of tibial implants detected by RSA and late revision for loosening has been reported. Also, attempts to measure in vivo polyethylene wear have been reported using RSA.

Purpose of this study

The aim of this study is to analyse the in vivo performance (e.g. stability) of a medially stabilized knee arthroplasty implant (Medacta International, GMK Sphere) using static RSA method over a period of 5 years. 2-year results have already been analysed and are inconclusive in regards to migration as the implant is apparently stable, but shows greater than anticipated movement. Mid-term follow-up (5-year) is therefore essential to evaluate migration of this implant. We therefore wish to obtain 5-year follow-ups of these patients.

Additionally, the investigators will analyse the wear in the ball and socket (medial) side of the tibiofemoral articulation.

30 patients will go through stability and wear testing with static RSA.

This study will contribute to the safety for patients with this new implant by providing basic knowledge of this knee arthroplasty and promoting further development of knee implant designs.

Inclusion criteria

• Patients with knee osteoarthritis

Exclusion criteria

• Preoperative severe deformity (Femoro-tibial angle < 175°or > 190°measured on a full-length leg image at weight bearing)
• Preoperative flexion contracture more than 15°
• Preoperative limited range of motion under anesthetics (less than 110°)
• Less than 50 or more than 75 years of age at the time of surgery
• Use of walking aids because of other musculoskeletal and neuromuscular problems
• Preoperative diagnosis other than osteoarthritis and avascular necrosis (e.g. rheumatoid arthritis, tumors)
• Revision arthroplasty
• Obesity with BMI >35
• Impaired collateral ligaments
• Malposition of femoral and tibial implants (Internally rotated or more than 10° externally rotated implants will be excluded. The rotation of femoral implant is measured on postoperative CT images in reference to surgical epicondylar line. The rotation of tibial implant is determined according to Berger's measurement)
• Postoperative revision surgery due to deep wound infection