Reliability of the quadriceps angle measurement.

The quadriceps angle (Q-angle) is used to determine patellofemoral alignment. Although this measurement has been used to evaluate and treat patellofemoral joint pathology, few studies have examined its reliability. This study evaluated the interobserver and intraobserver reliability of the Q-angle measurement. To investigate the interobserver reliability of the Q-angle, 25 individuals of varying levels of training served as observers and participants as each measured the other 24 participants. To investigate the intraobserver reliability of the Q-angle, 3 of the observers measured 13 of the participants an additional 2 times. Additionally, clinically derived Q-angle measurements were compared with radiographically derived measurements. The reliability analysis was performed using intraclass correlation coefficients. For interobserver measurements, the intraclass correlation coefficients ranged from 0.17-0.29 for the four variables evaluated (right and left, extension and flexion). For intraobserver measurements, the intraclass correlation coefficients ranged from 0.14-0.37. The average intraclass correlation coefficient between the clinically and radiographically derived measurements ranged from 0.13-0.32. This study demonstrates poor interobserver and intraobserver reliability of Q-angle measurement and poor correlation between clinically and radiographically derived Q-angles.