Reliability of Goniometry-Based Q-Angle

Objective To establish the stability reliability, precision, and minimum value for detecting real differences for quadriceps angle (Q-angle) measurements based on standardized protocols and surface goniometry. Design An intratester reliability study. Setting University research laboratory. Participants Fifty-two healthy, young, relatively lean adults (25 men and 27 women) from the university community with no history of knee injury. Methods Q-angle was assessed with surface goniometry on 2 separate occasions separated by 48 hours. Subjects assumed a supine position with: (1) extended hips and knees, (2) neutral hip rotational position, (3) neutral foot position, and (4) isometrically contracted quadriceps femoris muscles. The axis of a manual extendable-arm goniometer was placed over the center of the right patella with the proximal arm situated over the anterior-superior iliac spine and the distal arm over the center of the tibial tuberosity. Main Outcome Measures Stability reliability was calculated with use of intraclass correlation (ICC, 2-way random model) and precision was calculated by standard error of measurement (SEM). The 95% limits of agreement also were calculated to estimate the minimum detectable difference in Q-angles. The lowest acceptable ICC was set at ≥0.70 for stability reliability. Results The following ICC (SEM) values were found: all subjects=0.88 (1.0°), men=0.77 (1.0°), and women=0.85 (1.0°). The 95% limits of agreement were 3° for the same 3 groups. Conclusions The surface goniometry protocol described herein appeared to be reliable for relatively lean young men and women. Although measures were precise to 1.0°, it appears a difference of 3° may be needed to detect a real difference in Q-angles when measured in this fashion.