Retropatellar contact characteristics before and after total knee arthroplasty
Introduction
A general problem in total knee arthroplasty, should the patella surface be replaced or not, has been discussed for quite some time. Even though the causes for patellar problems are not conclusively determined, high retropatellar pressures in an unfavorable position appear to be a main contributory factor [2], [4], [5]. Potential solutions are an external rotation of the femoral component, an implantation of a patellar replacement in different positions and a lateral release of the patella in combination with strengthening exercises of the vastus medialis muscle.
Lee et al. [11] investigated the influence of femoral rotation with Fuji film and demonstrated that an internal rotation of the femur caused an increased lateral contact stress of the patella, an external rotation increased the medial contact stress. Rhoads et al. [13] found a medialization of the patella between 20 and 80° of knee flexion with a 10° internal rotation of the femoral component. In flexion angles beyond 80° this effect vanished. Anouchi et al. [1] found comparable values for 5° of rotation. McLain et al. [12] demonstrated that patellar resurfacing reduced the contact area. Hsu et al. [6] revealed that patellar designs with improved conformity increased the contact area. Hsu et al. [7] and Jiang et al. [9] showed that the contact area decreased between 15 and 90° in comparison to a healthy knee joint.
Wackerhagen et al. [16] evaluated the influence of a splitting of the retinaculum and saw a significant decrease of the local force in the lateral and proximal quadrant that depended on the flexion angle, the direction of muscle force and the prosthetic design. A good congruence between patella and femur resulted in a marked reduction of joint contact forces. Takahashi et al. [15] demonstrated with Fuji film that a lateral retinaculum splitting does not cause major changes. Hsu et al. [8] revealed that a reduction of the contact force with retinaculum splitting could only be achieved in more extreme flexion angles. Between 30 and 75° they even found an increased contact force. The contact area moved to the lateral and proximal part of the patella.
The review of literature did not show an investigation which compared the situation with and without prosthesis or an evaluation of the effects of femoral rotation, muscle force direction and placement of the patellar resurfacing in a single prosthetic design. The present investigation will try to fill this gap.
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Material and methods
Six cadaveric specimens with a mean age of 71.6 years (range 60–86 years) were investigated before and after implantation of a knee prosthesis. The cadavers were embalmed in a modified Thiel fixation that largely preserves bone and soft tissue characteristics and does not cause functional deficits.
A prosthesis model Genesis I (Smith & Nephew, Schenefeld, Germany) and the ‘onlay’ version of the patellar resurfacing in size ‘small’ and ‘medium’ were used. We used the size ‘large’ cemented
Results
The results before prosthesis implantation (Fig. 2) demonstrated signs of contact less frequently in 45° and 120° of flexion as compared to 60° and 90°. In the inferior quadrants imprints were rarely seen. A comparison of the muscle pull directions indicates that with a laterally dominant muscle force the superior medial and central medial quadrant often had contact. With a medial force the lateral central quadrant had the most frequent contact. An external rotation of the femur caused large
Discussion
Contrary to the situation without prosthesis, contact was measured also at 45° and 120° of flexion after implantation of the prosthesis. In accordance with Benjamin et al. [3] no marked difference between various flexion angles was found. Generally, a slight proximal shift of the contact areas was found after implantation. With respect to the goal to achieve a centralized patellar guidance the present results appear favorable indicating a most frequent contact in the central middle and superior
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