A comparative assessment of alignment angle of the knee by radiographic and physical examination methods

Objective. To compare the knee-alignment angle from a full-limb radiograph (mechanical axis) with the anatomic-axis angle as measured by physical examination using a goniometer and by 2 other radiographic methods. Methods. The knee-alignment angle was measured in 114 knees of 57 subjects who had radiographic osteoarthritis (OA), with a Kellgren/Lawrence grade of >= 1 in at least one knee. The mechanical axis was defined as the angle formed by the intersection of 2 lines, one from the center of the head of the femur to the center of the tibial spines, and a second from the center of the talus to the center of the tibial spines. The anatomic axis was defined as the angle formed by 2 lines, each originating from a point bisecting the femur and tibia and converging at the center of the tibial spine tips. The anatomic-axis angle was measured by 3 methods: 1) physical examination using a goniometer, 2) a posteroanterior (PA) fixed-flexion knee radiograph (anatomic(PA) axis), and 3) an anteroposterior (AP) full-limb radiograph (anatomic(AP), axis). Results. Significant correlations were found between the mechanical-axis angle and the anatomic-axis angle measured by each of the 3 methods: by goniometer (r = 0.70, P < 0.0001), by anatomic(PA) axis (r = 0.75, P < 0.0001), and by anatomic(AP), axis (r = 0.65, P < 0.0001). The anatomic axis was offset a mean 4.21 degrees vagus from the mechanical axis (3.5 degrees in women, 6.4 degrees in men), which was consistent across all methods. Conclusion. Knee alignment assessed clinically by goniometer or measured on a knee radiograph is correlated with the angle measured on the more cumbersome and costly full-limb radiograph. These alternative measures have the potential to provide useful information regarding the risk of progression of knee OA when a full-limb radiograph is not available.