Total joint replacement is an efficacious treatment for osteoarthritis of hips and knees. Both total knee replacement (TKR) and total hip replacements (THR) have excellent implant survivorship. However, patient satisfaction is lower in TKR than THR. A possible cause of the discrepancy is the unnatural knee kinematics after TKR. Various implant designs have been developed to solve the problem. However, most of the designs are based on experimental data and not on in vivo kinematics. In this study, we will analyze the in vivo kinematics of the Global Medacta Knee Sphere (GMK Sphere) implant and compare it with a well documented design and implant (NexGen Cruciate Retaining (CR), Zimmer Biomet). We assume our study will contribute to the development of more satisfying knee implants.

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.

The tibial insert of the medially stabilized implant design (Medacta International, GMK Sphere) has a constrained medial "ball in socket" joint and at the same time allows lateral anterioposterior movement (roll back). This new design was developed by a group of dedicated researchers. The design intention is to resemble the native kinematics of the knee by mimicing the function of the cruciate ligaments and at the same time allow for lateral anterioposterior movement (rollback). Theoretically, knee implant designs, which retain the function of cruciate ligaments are predicted to have the closest kinematics to normal knees compared to other designs and to have the potential to achieve the highest patient satisfaction.

Analytical Method:

Radiostereometric Analysis (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. When combining RSA with fluoroscopy, we get Fluoroscopic Roentgen Stereophotogrammetric Analysis (FRSA), a method with high accuracy and precision. Few implants have been analysed kinematically in vivo. We plan to investigate the knees during in vivo motion and weight-bearing using FRSA.

Purpose of the study:

The aim of this study is to analyse the in vivo kinematics of a medially stabilized knee arthroplasty implant (GMK Sphere) and to compare it with a well known design (Nexgen CR, Zimmer Biomet) by using FRSA.

By using flat panel fluoroscopy, we will document and compare the kinematics of the two implants in all 6 degrees of freedom (DOF).

26 patients will be randomized into two groups, one will receive the NexGen CR prosthesis, the other group the GMK Sphere.

Inclusion Criteria:

1. Patients with knee osteoarthritis

Exclusion Criteria:

1. Preoperative severe deformity (Femoro-tibial angle < 175°or > 190° measured on a full-length leg image at weight bearing)
2. Preoperative flexion contracture more than 15°
3. Preoperative limited range of motion under anesthetics (less than 110°)
4. Less than 50 or more than 75 years of age at the time of surgery
5. Use of walking aids because of other musculoskeletal and neuromuscular problems
6. Preoperative diagnosis other than osteoarthritis and avascular necrosis (e.g. rheumatoid arthritis, tumors)
7. Revision arthroplasty
8. Obesity with BMI>35
9. Impaired collateral ligaments
10. Postoperative KOOS score less than 80
11. Malposition of femoral and tibial implants (Internally rotated or more han 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)
12. Limb surgery within 3 months before the analysis
13. Postoperative revision surgery due to deep wound infection

Non-blind