Changes in patellofemoral joint contact pressures caused by vastus medialis muscle weakness.
BACKGROUND
Patellofemoral joint pain is a common knee disorder, but its underlying causes remain unknown. One proposed mechanism is an imbalance in force in the knee extensor muscles. Specifically, the vastus medialis and vastus lateralis are thought to play a crucial role in proper patellar tracking, and weakness in vastus medialis is thought to lead to a lateral shift in the patella causing increased contact pressures and pain. The purpose of this study was to create an animal model of vastus medialis weakness and to test the effect of this weakness on patellofemoral contact pressures.


METHODS
Experiments were performed using New Zealand white rabbits (mass 4.9-7.7 kg, n=12). Loading of the patellofemoral joint was produced by femoral nerve stimulation of the knee extensor muscles. Knee extensor imbalance was produced by vastus medialis ablation. Fuji pressure sensitive film was used to record contact area, shape and pressures for maximal and sub-maximal, matched-force contractions at knee angles of 30°, 60°, and 90°.


RESULTS
Patellofemoral peak pressures, average pressures, contact areas and contact shapes were the same across all loading conditions for matched-force contractions before and after elimination of vastus medialis.


CONCLUSIONS
We conclude that vastus medialis weakness does not cause changes in patellofemoral contact pressures. Since the muscular and knee joint geometry in rabbits and humans is similar, we question the idea of vastus medialis weakness as a cause of patellar mal-tracking and patellofemoral joint pain.
Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada.
Andrew
A
Sawatsky
Doug
D
Bourne
Monika
M
Horisberger
Azim
A
Jinha
Walter
W
Herzog
D000818:Animals; D001696:Biomechanical Phenomena; D004195:Disease Models, Animal; D005267:Femoral Nerve / Q000503:physiopathology; D006801:Humans; D007717:Knee / Q000503:physiopathology; D007719:Knee Joint / Q000503:physiopathology; D035002:Lower Extremity / Q000503:physiopathology*; D008953:Models, Anatomic; D009119:Muscle Contraction; D018908:Muscle Weakness / Q000503:physiopathology*; D018482:Muscle, Skeletal / Q000503:physiopathology; D010146:Pain; D057071:Patellofemoral Joint / Q000503:physiopathology*; D011312:Pressure; D052097:Quadriceps Muscle / Q000503:physiopathology; D011817:Rabbits