Accuracy of leg alignment measurements from antero-posterior radiographs

After a knee replacement procedure, postoperative radiological assessment is carried out to evaluate outcome and predict procedure success. For this assessment, long-standing load-bearing antero-posterior radiographs are used to carry out manual identification of anatomic landmarks. These landmarks are subsequently used to estimate leg alignment. The positions of the landmarks in the radiographs are affected by the patient pose and the X-ray projection center. Although there is some past work exploring the impact of patient pose on the landmarks in the radiographs, there is no previous work on the impact of the X-ray projection center on the estimated leg alignment. In this work, we carried out a study of the impact of patient foot rotation, and X-ray projection center on landmark measurement errors, and estimation of leg alignment. In this evaluation, landmarks were first identified in three-dimensional computed tomography scans. Digitally reconstructed radiographs were then obtained from these scans under varying rotation and projection centers. Subsequently, landmarks were manually identified in these radiographs and leg alignment was estimated from these landmarks. We found that foot rotation leads to increased errors in certain landmarks. We also found that variations in the X-ray projection center do not lead to significant (p<0.01) errors in landmark measurements. Also, errors as large as 13.1 mm for the femoral knee center and 13.6 mm for the lateral malleolus led to a maximum error of 1.46° for the femoral mechanical axis and 0.66° for the tibial mechanical axis.