The importance of the third knee compartment on outcome following total knee arthroplasty

Abstract

The third compartment, the patellofemoral joint, appears to have been neglected somehow when discussing optimal knee kinematics in total knee arthroplasty (TKA). How much impact will good function of the patellofemoral joint have on the clinical outcome after TKA? There is an extensive amount of discussion on whether or not the patella should be resurfaced in TKA. Some surgeons resurface the patella routinely, some never and some selectively, depending on the shape of the patella. A meta-analysis of all randomized studies until 2009 showed no difference between resurfacing or not of the patella in terms of postsurgical rate of anterior knee pain, knee pain score and quality of life [10]. The overall incidence of anterior knee pain after TKA without patella resurfacing ranged between 12 and 87 % [14, 20]. Anterior knee pain has been shown to impair patient’s quality of life. The literature review about anterior knee pain after TKA published in the current issue shows that there is more than one reason for patellofemoral pain [21]. Variables, which may directly influence patellofemoral loading, appear to show an impact on clinical and functional outcome. In conclusion, correct kinematic seems to be important. Biomechanical studies have proven that patella kinematic during knee flexion is very complex because of the combination of rotation, flexion and mediolateral shift. Considering the amount of patellofemoral forces of more than 3 times body weight, minor changes of patella tracking may cause significant impact on the loading condition [16]. In vivo fluoroscopy data were used, and patellofemoral force was calculated using a 3D mathematical model. Interestingly, patellofemoral contact did not differ between normal, cruciate retaining and posterior stabilized total knee arthroplasty. The data were in contrast to the biomechanical findings reported by others. It has been shown that patellofemoral pressure increases up to 1.5–2.5 times after TKA in comparison with the natural knee [13]. Studies have also reported about the impact of the component design on femoropatella pressure [4]. Of course, it depends on the components shape of both the patella and the trochlea groove. Six different patella designs are most commonly used in TKA: the dome, offset dome, modified dome, anatomic mobile bearing, anatomical and cylindrical type of patella [15]. The design of the patella component significantly influences patella tracking also. The dome-shaped patella shows a more internal and external rotation for instance, compared with a more anatomical component [7]. Anatomical studies have shown that the morphology of the femur, tibia and patella varies tremendously. How much of the anatomy do we restore when performing TKA? Do we require a more patient-specific trochlea design? There seems to be a direct relation between the shape of the trochlea and patella kinematic and pain [9]. The articular cartilage depth, the length of the trochlea groove and the height of the patella significantly influence patella tracking. It is well known that femoral component malrotation will affect the femoropatella joint [5, 11]. Increased internal rotation will cause overstuffing of the lateral facet. R. Becker (&) Department of Orthopaedics and Traumatology, Hochstrasse 26, 14770 Brandenburg/Havel, Germany e-mail: roland_becker@yahoo.de