Malrotated lateral radiographs do not allow for proper assessment of patellar height using the Caton-Deschamps Index.

Abstract

To compare patellar height measurement (PHM) using the Caton-Deschamps Index (CDI) on malrotated versus properly positioned lateral knee radiographs within the same patients. It was hypothesised that malrotation would significantly alter CDI values. A secondary aim was to establish an anterior-posterior distance (APD) cut-off for malrotation corresponding to a minimally clinically important difference (MCID) of 0.1 in ΔCDI. This retrospective analysis included patients with lateral knee radiographs between January 2020 and March 2023 at a single academic institution. Patients were included if at least two radiographs were available: one with malrotation (APD ≥ 1 mm) and one properly aligned (APD < 1 mm). Radiographs with tilt, prior osseous surgery, or fractures were excluded. Patellar height was measured using the CDI. Inter and intrarater reliability were assessed via intraclass correlation coefficients (ICC). Paired <i>t</i>-tests compared CDI values. Cases with a ΔCDI exceeding the MCID of ≥0.1 were identified. The APD cut-off corresponding to a ΔCDI of 0.1 was determined by regression of ΔCDI on APD. A total of 126 lateral knee radiographs from 63 patients (57% female) were analysed. Inter and intrarater reliability was excellent (ICC > 0.8 for all comparisons). The mean CDI was 0.96 ± 0.06 on properly aligned and 1.03 ± 0.08 on malrotated radiographs, with a significant mean difference of 0.07 ± 0.05 (<i>p</i> < 0.001). In 34.9% of patients, CDI differed by ≥0.1 between imaging conditions. Linear regression of ΔCDI on APD showed a slope of 0.016 per mm (<i>R</i>² = 0.57). The APD cut-off corresponding to a ΔCDI of 0.1 was 6.3 mm (95% CI, 5.7-6.9 mm). Malrotation significantly alters PHM using the CDI on lateral knee radiographs. A clinically relevant difference of ≥0.1 in ΔCDI occurred in over one-third of patients. Malrotated radiographs should be used with caution, and repeat imaging should be considered when CDI values approach clinical decision thresholds. Level III, diagnostic study.