RESUMEN
The resistance of patella against lateral displacement [i e. the stability], was studied under a range of conditions in vitro at a range of knee flexion angles. Muscle forces were applied in physiological directions along the separate quadriceps muscles. Normal muscle actions with constant tension showed constant patellar stability up to sixty degrees knee flexion, and then a significant increase at ninety degrees. A pathological knee was simulated by relaxing vastus medialis, and lateral stability then dropped by 47% Isolated retinacular release in the pathological knee did not improve stability significantly towards normal, while 6mm tibial tubercle medialisation restored stability by 52% towards normal, except neat full extension. Combined release plus tubercle medialisation restored stability 59% towards normal in knee flexion and 23% up to ten degrees flexion