What does the Q-angle measure?
The bigger the Q-angle, the greater the force trying to pull the patella to the outer side. Normally, when the knee is straight (extension) the kneecap sits at the shallow section of the trochlea. As the knee flexes ('bends'), it is pulled into the deeper part of the trochlea at about 30 degrees of flexion, then runs centrally in the trochlear groove during the rest of bending. If the Q-angle is high, the soft tissue restraints and the trochlear groove are not always sufficient to keep the patella where it should be.
Re-evaluating the functional implications of the Q-angle and its relationship to in-vivo patellofemoral kinematics. Freedman BR, Brindle TJ and Sheehan FT. Clin Biomech (Bristol, Avon). 2014 Dec; 29(10): 1139–1145.
Q-angle in patellofemoral pain: relationship with dynamic knee valgus, hip abductor torque, pain and function. Almeida GPL, de Moura Campos Carvalho e Silva AP França FJR, Magalhães MO, Burke TN and Marquesa AP. Rev Bras Ortop. 2016 Mar-Apr; 51(2): 181–186..
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