gms | German Medical Science

Objective: Aberrant patellar tracking is known to be an important risk factor in the pathogenesis of patellofemoral pain syndrome (PFPS) and patellofemoral osteoarthritis (OA). Although the etiology is still unclear and multifactorial, abnormal tracking of the patella caused by muscular imbalance of the quadriceps muscle is considered to be an important contributing factor. Quadriceps strengthening exercises focusing on the vastus medialis muscle (VM) have been suggested as a therapeutic approach to retain knee extensor strength balance. However, results of these studies are not consistent. Therefore the purpose of this study was to evaluate the influence of the loss of the vastus medialis muscle force on patellar tracking in the intact and anterior cruciate ligament (ACL) deficient knee in vivo.

Methods: Eight skeletally mature New Zealand White Rabbits, weighing 6.0kg ± 0.6 (mean ± SD) were used. The experimental trial consisted of active, concentric (con) and eccentric (ecc) movements of the knee joint, ranging from 30 to 90° of flexion (eleven limbs, n=11). Measurements were first performed with the knee fully intact (intact control); then, the ACL was transected through a medial arthrotomy, and last, the VM was transected to produce a vast muscle force imbalance. For muscle stimulation, a custom made femoral nerve cuff electrode was used. Patellofemoral kinematics (lateral shift, rotation and patellar tilt) were quantified from high speed video (200Hz, optical resolution 0.08mm) which measured the displacement of bone pins imbedded rigidly in the patella relative to bone pins in the distal femur. Statistical analysis was performed using a one way repeated measures ANOVA and Fishers post hoc analysis.

Results and conclusion: VM transection did not alter the patellar tracking, tilting or rotation significantly for any of the force-matched experimental conditions. After ACL transection, patellar tracking occurred more laterally for the concentric and eccentric contractions (p<0.001, mean shift 0.3mm (± 0.48mm), and 0.3mm (±0.51mm), respectively (Figure 1a,b), and caused a significant lateral rotation of the patella (p<0.001, mean rotation 1.8° (±2.2°) and, 2.3° (±1.7°), respectively. No changes in patellar tilt were seen.

In all trials, force-matched contractions were accomplished within 5% of each other. Eccentric contractions resulted in higher force production than concentric, 388 ±83 Ncm, and 252 ±78 Ncm, respectively.

Our results suggest that the isolated loss of vastus medialis muscle force does not affect patellar tracking, rotation or tilting compared to force-matched concentric and eccentric movements in the ACL deficient knee. The findings of this study suggest that muscle imbalance in the ACL-deficient knee does not affect patellar tracking, as has been found previously in the intact, stable knee of rabbits. Therefore, strengthening of the VM in patients with patellar mal-tracking might not restore normal tracking patterns, as has been suggested in the past.