In vitro characterization of the relationship between the Q-angle and the lateral component of the quadriceps force

Although the Q-angle is routinely measured, the relationship between the Q-angle and the lateral component of the quadriceps force acting on the patella is unknown. Five cadaver knees were flexed on a knee simulator with a normal Q-angle, and flexed after increasing and decreasing the Q-angle by shifting the quadriceps origin laterally and medially, respectively. The motion of the femur, tibia and patella was tracked from 20 to 90° of flexion using electromagnetic sensors. The motion of landmarks used to quantify the Qangle was tracked to determine the ʹdynamic Q-angleʹ during flexion. The lateral component of the force applied by the actuator secured to the quadriceps tendon was also quantified throughout flexion. Increasing the initial Q-angle significantly (p <0.05) increased the dynamic Q-angle and the lateral force exerted through the quadriceps tendon throughout flexion. Decreasing the initial Q-angle significantly decreased the dynamic Q-angle at 90° of flexion and significantly decreased the lateral force exerted through the quadriceps tendon from 20 to 40° of flexion. Even though the dynamic Q-angle changes during flexion, an abnormally large initial Q-angle can be an indicator of an abnormally large lateral force acting on the patella during flexion.