Accuracy of the short radiograph in the measurement of the tibiofemoral angle

Abstract

To determine which axes on the ‘short’ radiograph (36 cm × 43 cm) correlate best with the corresponding axes on the ‘long’ radiograph (36 cm × 130 cm), 100 long anteroposterior radiographs were taken postoperatively following total knee arthroplasty. The central 43 cm of the long radiograph were used to simulate a short radiograph of the knee. For the femur, the short-radiograph axis drawn from the midpoint at the proximal end to a point 10 cm above the joint line had the highest coefficient of correlation ( r = 0.97) with the anatomical axis of the femur on the long radiograph. For the tibia, the short-radiograph axis drawn from the midpoint of the distal end of the tibia to a point 10 cm below the joint line had the best correlation ( r = 0.93) with the anatomical axis of the tibia on the long radiograph. The tibiofemoral angle created by these two axes was 5.3°, which, in comparison to eight other axis combinations tested on the short radiographs, correlated best ( r = 0.94) with the mean tibiofemoral angle observed on the long radiographs. We find this angle to be a satisfactory indication of the tibiofemoral angle on a long radiograph. This finding will allow a more accurate determination of the optimal tibiofemoral angle that is so important to the long-term success of total knee arthroplasty.