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

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 < .001), with a mean change of 5.6 mm per 5°. Malpositioning in abduction/adduction significantly affected the PDD (P < .001), with a mean change of 5.1 mm per 5°. There was no significant impact of rotation or APD on the PTS. Abduction/adduction did affect the PTS (P < .001) above a threshold of 5° of malpositioning. The PTS decreased as the PDD increased, moving from adduction to abduction (R(2) = 0.5687). CONCLUSION: The measured PTS was more sensitive to malpositioning by abduction/adduction than by malrotation. Malrotation affected the APD, while abduction/adduction affected the PDD. Thus, the accuracy of the measured PTS was compromised more by poorly aligned distal femoral condyles than it was by poorly aligned posterior femoral condyles. CLINICAL RELEVANCE: To minimize the effects of malpositioning, we recommend utilizing radiographs with a |PDD| of <5 mm and an |APD| of <15 mm when measuring the PTS.