Radiologists should use the hip-knee-ankle angle rather than the mechanical axis deviation to describe knee alignment

Preoperative and postoperative coronal knee alignment is an important predictor of total knee arthroplasty (TKA) failure. Radiologists often report the mechanical axis deviation (MAD) rather than hip-knee-ankle angle (HKAA) to describe coronal knee alignment. The aim of this study is to evaluate (i) how well the MAD predicts the HKAA; (ii) if patient height and sex affect the performance of the MAD; and (iii) if the MAD could be measured faster than the HKAA. Two hundred patients undergoing hip-to-ankle radiographs for TKA planning were retrospectively reviewed. The MAD and HKAA were measured using previously published methods by the Visage picture archiving and communication systems (PACS) tools. Receiver operator characteristic (ROC) curves were used to evaluate the performance of the MAD to predict HKAA by gender and height. The performance of a linear model was used to predict HKAA from MAD in a prospectively collected cohort of 40 patients. Paired t tests were used for the comparison of time measurement in MAD and HKAA in this cohort. MAD strongly correlated with HKAA (r = 0.99, p < 0.001); however, the performance of MAD differed by height (p = 0.005) and sex (p < 0.001). There was no significant difference in the time taken to measure HKAA versus MAD (p > 0.05). HKAA should be used instead of the MAD because it is more clinically relevant and takes the same amount of time to be measured.