Effects of Malpositioning of the Knee on Radiographic Measurements: The Influence of Adduction, Abduction, and Malrotation on Measured Tibial Slope

Abstract

Background: Increased posterior tibial slope (PTS) is a risk factor for knee pathology. Accurate measurement of PTS is predicated on a quality lateral knee radiograph; however, little is known about how the quality of the radiograph affects the measured PTS. Purposes: To (1) describe a method for measuring malalignment on lateral knee radiographs, (2) assess the effects of malpositioning of the knee on radiographic measures of malalignment, and (3) determine any correlations between malalignment and the measured PTS. Study Design: Descriptive laboratory study. Methods: Using a setup similar to that of a standard radiology suite, 25 sets of radiographs were taken using 5 sawbone models. Each set included a true lateral view and separate malpositioned radiographs at 5°, 10°, and 15° of adduction, abduction, internal rotation, and external rotation. Malalignment for each radiograph was quantified as the anterior-posterior distance (APD) and proximal-distal distance (PDD) between femoral condyles. The medial PTS was measured in duplicate, and the interrater reliability was calculated. Results: The interrater reliability was excellent, with intraclass correlation coefficients of 0.92, 0.91, and 0.96 for the APD, PDD, and PTS, respectively. Malrotation significantly affected the APD (P <5 mm and an |APD| of <15 mm when measuring the PTS.