Methods and results

The Q-angle represents the angle between the vector of action of the quadriceps and patellar tendon. The normal angle is 14^+/−3° in males and 17^+/−3° in females. An increased Q-angle is associated with an increased risk of patellar instability, although there is disagreement on its reliability and validity. It can be affected by the anatomical points used to record the measurement, the position of the limb and whether the quadriceps are relaxed or contracted. TT-TG is ascertained by axial CT scanning, with a value exceeding 20mm associated with patellar instability.

Q-angles were measured in patients presenting to the patella clinic who had previously undergone Lyon protocol CT scanning for patellar instability. Patients were positioned supine with both feet in neutral rotation taped to a specially designed wooden board (the same position used for CT scanning). The anatomical landmarks were the anterior superior iliac spine, the centre of the patella and the centre of the tibial tuberosity. Both knees were measured with the quadriceps relaxed (relaxed Q-angle) and contracted (contracted Q-angle).

Thirty-four knees were measured, 24 pathological and 10 non-pathological. Pearson moment correlation demonstrated a significant correlation between relaxed Q-angle and TT-TG in all knees (R=-0.377; p=0.028). In pathological knees, contracted Q-angle also demonstrated a significant correlation with TT-TG but to a lesser extent than relaxed Q-angle (R=-0.428; p=0.037, R=-0.578; p=0.003 respectively). Linear regression analysis demonstrated relaxed Q-angle as a significant predictor of TT-TG distance in pathological knees. Contracted Q-angle was not significant.