Strong Agreement Between Magnetic Resonance Imaging and Radiographs for Caton–Deschamps Index in Patients With Patellofemoral Instability

PURPOSE: To compare the measurements of the Caton–Deschamps index on preoperative magnetic resonance imaging and radiographs of patients undergoing operative management of patellar instability. METHODS: Patients who underwent primary medial patellofemoral ligament reconstruction and/or tibial tubercle osteotomy between January 2015 and November 2019 were assessed. Caton–Deschamps indices were measured by 3 independent reviewers on both radiographs and magnetic resonance imaging. Intra- and interclass correlation coefficients and a Bland–Altman analysis were calculated to assess inter-rater reliability and measurement agreement between radiographic and magnetic resonance imaging. RESULTS: Seventy-two patients (73 knees) were identified. The average Caton–Deschamps index was 1.23 ± 0.18 on radiograph and 1.26 ± 0.18 on magnetic resonance imaging. Strong inter-rater reliability was observed between reviewers for both radiographic and magnetic resonance imaging Caton–Deschamps indices (intraclass correlation coefficients 0.700 and 0.715, respectively). Pooled observer measurements revealed a moderate agreement between radiographic and magnetic resonance imaging for patella to tibia distance, weak agreement for patellar articular cartilage distance, and strong agreement for the Caton–Deschamps index (intraclass correlation coefficients 0.687, 0.485, and 0.749, respectively). Bland–Altman analysis demonstrated a mean difference in Caton–Deschamps index of –0.03 ± 0.15 (95% limits of agreement: –0.29 to 0.23) between radiographic and magnetic resonance imaging, meaning that Caton–Deschamps indices were on average 0.03 lower on radiographic than on magnetic resonance imaging. CONCLUSIONS: The Caton–Deschamps index has strong agreement between radiographic and magnetic resonance imaging in patients undergoing patellar stabilization surgery. Either modality can be reliably used to preoperatively assess patellar height. LEVEL OF EVIDENCE: Level IV, diagnostic case series.