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, patients' satisfaction is lower in TKR than THR. A possible cause of the discrepancy is the unnatural knee kinematics after TKR. Various implants designs have been developed to solve the problem. The most common fixation mode is cemented TKR with good survival up to 15 years. However, newer series in younger patients also have shown lasting survival with uncemented implants (Nilsson et al 2006, Prudhon et al. 2017). Among various different designs, medially stabilized total knee, which are designed to reproduce natural knee kinematics with medial ball-in-socked design, is a promising implant (Australian registry report 2018). Dynamically the medial pivot knee performs more naturally (Bragnazoli et al, 2019) compared to other designs. Most data for this design is available only for the cemented version. Up to now there is no safety study performed that confirms the stability over time for this implant with uncemented fixation.

In this study, we will therefore analyze the in vivo stability of an uncemented knee implant with medially stabilized design. Our study will contribute to the understanding of fixation and lead to safety to the patient.

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).

Another design is a medially stabilized knee, which has "ball-in-socket" medial femoro-tibial articulation to maintain anterior-posterior stability. The implants with this design concept are gaining popularity in clinical practice and their performances is under crude evaluation recently. Both cruciate ligaments are sacrificed in this arthroplasty and the characteristic geometry of femoral implant and polyethylene insert plays a key role in its stability. The implants on the market have been slightly altered in their design composition because of some inferior results. ,Bragnazoli et al (2019) showed that these implants have closer kinematics to normal knees. This might have the potential to achieve higher patient's satisfaction, lower polyethylene wear rate, and less migration in vivo compared to CR knees.

Analytical method of knee prosthesis

In this study we will employ radiostereometric analysis (RSA) method. The positions of implants are determined using tantalum markers inserted in the bones and implant models. Since 1970s, this method has been used in many orthopedic research fields and we already have performed many studies using RSA for the evaluations of fixation and wear of artificial joints (Øhrn et al. 2018, Petursson et al 2017,). The advantage of RSA method is its high accuracy of 0.1-0.2 mm for translations and 0.3 degrees for rotations (Garling et al. 2005) and less manual procedures in the analysis. Moreover, clinically relevant association between early migration of tibial implants detected by RSA and late revision for loosening has been reported (Pijls et al.2012, Molte et al 2016).

In an ongoing RSA study with cemented implants it is shown that migration analysis is feasible.

Recently low dose CT-based micromotion analysis (CTMA) appears to be a feasible tool for motion analysis of implants. Erikson has shown in a phantom study that motion analysis is feasible as it offers similar precision levels as the gold standard RSA (Eriksson et al 2019). Early results of clinical data are promising for implants in the shoulder (Broden et al. 2020) and in the hip (Otten et al 2017).

Fixation of Total Knee Replacement

Cemented fixation is the most widespread standard around the world and still regarded as the golden standard (Nugent et al 2019). It provides lasting fixation up to 15 - 20 years. Meanwhile it has been raised the question whether uncemented fixation could improve long time outcome in the demanding young patient (Nilsson et al. 2006, Wojtowicz et al 2019). Hybrid fixation has proven superior survival over cemented fixation in selected series (Petursson et al. 2015). Prudholm et al. (2017) showed 94% survival with uncemented implants with HA surface after 11 years. The theoretical advantages of cementless TKA are bone stock preservation, cement debris protection and the potential to achieve biologic fixation of the implant to the bone. Uncemented fixation is however, greatly depending on the implant surface and cannot be extrapolated from one implant surface to the other. Therefore a thorough clinical evaluation is needed for new surfaces.

Bone density

Good bone stock is essential for lasting fixation in uncemented TKA. Li et al (2000) found a relevant correlation between migration and preoperative bone density. Initial bone stock was restored 2 years after surgery. The initial bone seemed to be related to local activities at the interface, which may be surface dependent (Li et al. 2001). Also Andersen et al (2017) found a clear relation between migration of the tibia implant and bone stock.

The bone stock will be evaluated intraoperative by the surgeon by palpation and inspection.

Purpose of this study

The primary aim of this study is to analyze the in vivo stability over 2 years of a new uncemented medially stabilized knee arthroplasty design using static RSA. The data will be compared to known limits of safe migration (Pijls, Valstar et al. 2012) up to 2 years and with outcomes of the previously mentioned study in which the cemented version of the medially stabilized design was used (Øhrn F-D 2021). Secondary we want to validate CT-based micromotion analysis (CTMA) by comparing it to RSA.

Inclusion Criteria:

Patients referred to Oslo University Hospital, Ullevål for knee replacement surgery will be included to this study for a total of 30 subjects.

Exclusion Criteria:

1. 75 and more years of age at the time of surgery
2. Use of walking aids because of other muscoloskeletal and neuromuscular problems
3. Preoperative diagnosis other than osteoarthritis and avascular necrosis (e.g. rheumatoid arthritis, tumors)
4. Obesity with BMI>35
5. Lateral collateral ligament deficient knee
6. Reduced bone quality in the proximal tibia before surgery
7. Insufficient language to answer questionnaires in Norwegian