Abduction and adduction malpositioned lateral radiographs cannot be used for proper assessment of the posterior tibial slope.

Abstract

To compare medial posterior tibial slope (mPTS) measurements on abduction/adduction malpositioned versus properly positioned lateral knee radiographs within the same patients. We hypothesized that malpositioning would significantly alter mPTS values and potentially change surgical classification at the commonly used 12° threshold for slope-correcting osteotomies. This retrospective analysis included patients with preoperative knee radiographs between October 2020 and March 2023 at a single academic institution. Patients were included if at least two lateral knee radiographs were available: one with abduction/adduction malalignment (proximal-distal distance [PDD] ≥5 mm) and one properly aligned (PDD < 5 mm). Radiographs with insufficient tibial length, rotational malalignment, or prior osseous surgery or fractures were excluded. mPTS was measured using the method of Dejour & Bonnin. Inter- and intra-rater reliability were assessed via intra-class correlation coefficients (ICC > 0.9 for all comparisons). A t-test was used to compare mPTS values. Cases with a measurement difference of ≥2° were identified. Bland-Altman analysis was performed, and classification shifts across the 12° mPTS threshold for slope-correcting osteotomies were reported. Eighty-eight lateral knee radiographs from 44 patients (57% female) were analyzed. The mean mPTS was 7 ± 3° in properly aligned radiographs and 9 ± 3° in malaligned radiographs (p < 0.001), with a mean difference of 2°. In 59.1% of patients, mPTS differed by ≥2° between imaging conditions. Bland-Altman analysis showed 95% limits of agreement from -3.62° to 7.80°. In 15.9% of patients, classification relative to the 12° mPTS threshold changed. Abduction/adduction malalignment significantly alters mPTS measurements on lateral knee radiographs. A clinically relevant difference of ≥2° was observed in over half of patients, and 15.9% changed classification relative to the 12° surgical threshold. Lateral radiographs with a PDD ≥ 5 mm should not be used for surgical planning, and repeat imaging should be considered when mPTS values approach surgical decision thresholds. Level III, diagnostic study.