The influence of lateral knee X-ray positioning on the accuracy of full extension level measurements: an in vitro study

Abstract

The inaccuracy of full extension measurements can lead to errors, especially in reconstructive surgery of the knee and in the rehabilitation period following total knee replacement. The goal of this study was to first determine whether plain lateral X-rays of the knee can be used to measure the axis of the bones of the lower limb. The second goal was to determine whether the angle from which the X-ray is taken influences the true angle of the knee. We analyzed 620 digital photographs of cadaver knees to measure the difference between the true axis of the femur and tibia compared with lines drawn on the anterior and posterior cortex of the femur and tibia. We also analyzed 150 photographs of cadaver lower limbs to determine how the true flexion angle compares to the angles measured if the picture was taken from different rotated positions (maximum of 30°). Our results concluded that lines drawn onto the distal anterior cortex of the femur and proximal posterior cortex of the tibia can determine the true flexion angle with 2° of accuracy. We also concluded that lateral X-rays of the knee taken within ±30° of the true lateral position only causes a maximum of a 2% discrepancy in the true flexion angle.