Abstract
Introduction
Quadriceps weakness, which is often reported following total knee arthroplasty (TKA), affects patients' abilities to perform activities of daily living [1]. Implant design features, particularly of the patella-femoral joint, influence the mechanical advantage of the extensor mechanism. This study quantifies the changes in extensor mechanism moment arms due to different patellar resurfacing options during TKA.
Methods
Posterior-stabilized TKR surgery was performed on seven cadaveric knees which were subsequently mounted in the Kansas Knee Simulator (KKS) [2]. A dynamic physiological squat was simulated between 5° and 80° knee flexion at 50% body weight while knee kinematics, including the lines of action of the rectus femoris (RF) muscle and patellar tendon (PT), were recorded using an optical tracking system. The simulation was performed after three patella treatment options: 1) leaving the native patella Unresurfaced, 2) resurfaced with a medialized Dome patella, and 3) resurfaced with a medialized Anatomic patella which included a conforming lateral facet. Moment arms from the tibio-femoral helical axis to the line of action of the PT and the RF were calculated for each patella condition.
Results
The quadriceps moment arm for the Anatomic patella is smaller than the Dome during extension (Fig. 1A). Past 55° the Anatomic moment arm becomes larger than the dome. Patellar tendon moment arm for Anatomic is bigger than the dome in extension (0–50°) but smaller in flexion (50–80°) (Fig. 1B). The overall shape of the Unresurfaced patella moment arm through flexion, for both the patellar tendon and the quadriceps, was more similar to the Anatomic than the Dome although the difference in magnitude was not consistent between the six knees.
Discussion
The orientations of both the RF and PT lines of action, which were used to determine the moment arm, were correlated with patellofemoral kinematics. A more extended position of the patella resulted in an increase of the PT while decreased the RF moment arm. This explains the difference between the Anatomic and dome moment arms for both PT and RF since the Anatomic patella was more extended between 0–60° knee flexion (Fig. 1C). The similarity in the PT and RF moment arms shape between the Anatomic and the Unresurfaced resurfaced was due to the similarity in their conforming geometry. The less conforming geometry of the Dome patella made it less constrained and allowed the forces applied by the RF and PT to have a greater influence on patellofemoral kinematics and moment arms than Anatomic and Unresurfaced patellae.
The small changes in PT and RF moment arm observed in this study can result in large effect on muscle loads that are required to perform more strenuous activities. Multiple methodologies have been reported in literature to calculate moment arm. Future work will examine the effects of different methodologies on moment arm calculations as well as validation of results by examining the change in quadriceps moment arm required to perform certain activities.
