Experimental and clinical assessment of the accuracy of knee extension measurement techniques

The purpose of this study was to analyze the accuracy of commonly used techniques for the measurement of knee extension and to compare them with a new measurement device. The bars of an external fixator were used to determine reference knee extension angles of 15 human cadavers. These angles were compared with measurements of knee extension on radiographs limited to the knee joint. Extension was determined in various knee positions using a generic goniometer and a novel long arm goniometer. In a clinical study, two independent examiners categorized knee extension performance according to the IKDC. Sixteen knees with deficits in the range of motion were rated using a generic goniometer, a long arm goniometer and the novel extension measurement device. The radiological measurement of knee extension angles that were restricted to the shaft of femur and tibia had a systematic error of -5.2 +/- 1.9 degrees compared with the lines created by the centers of rotation. In the experimental setup, the mean absolute deviations were 3.92 +/- 1.41 degrees with a generic goniometer and 1.22 +/- 0.20 degrees with the extension measurement device. The variance of the measurements was significantly lower (2.64 +/- 0.28) than with the generic goniometer (23.72 +/- 4.39; P < 0.05). Correspondence in the IKDC rating was 63% using a standard goniometer, 50% with the long arm goniometer, and 96% using the novel device. Radiological measurements of knee extension limited to the area of the knee joint deviates systematically from measurements of the total axis of the bones. A precision goniometer that utilizes bony landmarks of the tibia and femur is superior in accuracy compared with standard and long arm goniometer techniques.