Measuring flexion in knee arthroplasty patients.

Flexion following total knee arthroplasty can be visually estimated, measured with a goniometer placed against the patient's leg, or measured from a lateral radiograph of the flexed knee. Three examiners, in a blinded fashion, estimated the degree of maximal knee flexion and measured the flexion with a goniometer for 27 knees in 16 patients. A lateral knee radiograph then was taken and the flexion angle was measured from the radiograph by 2 different methods. Although interobserver and intraobserver correlation coefficients were high (0.79 and 0.92), 45% of the visual estimates and 22% of the goniometer measurements differed by 5 degrees or greater from the radiographic measurements. These differences increased as the flexion angle increased. Body mass index did not affect the accuracy of the estimates or goniometer measurements.