Introduction

The Caton-Deschamps (CD) ratio is the preferred method of assessing patella alta and patella baja. Measurement of the CD ratio is often determined on either X-ray or MRI; however, the agreement between these two measurement modalities remains unclear. Accurate assessment of the CD ratio and patella alta is an important component of surgical decision making in the setting of patellar instability. The purpose of this study is to compare the measurements of the CD ratio on pre-operative MRI and X-ray of patients undergoing operative management of patellar instability.

Methods

A retrospective case series was conducted. Overall, 72 patients (73 knees) with available preoperative imaging who underwent primary medial patellofemoral ligament (MPFL) reconstruction and/or tibial tubercle osteotomy between January 1st 2015 and November 30th 2019 were assessed. CD ratios were measured by three independent reviewers on both X-ray and MRI. Intra-class correlation coefficients (ICCs) and a Bland-Altman analysis were calculated to assess inter-rater reliability and measurement agreement between X-ray and MRI.

Results

The average CD ratio was 1.23 ± 0.18 (95% confidence interval: 1.20 – 1.25) on X-ray and 1.26 ± 0.18 (95% confidence interval: 1.24 – 1.28) on MRI. Strong reliability was observed between reviewers for both X-ray and MRI CD ratios (ICCs: 0.700 and 0.715, respectively). There was moderate agreement between X-ray and MRI for patella to tibia distance, weak agreement for patellar articular cartilage distance, and strong agreement for the CD ratio (ICCs: 0.687, 0.485, and 0.749, respectively). Bland-Altman analysis demonstrated a mean difference in CD ratio of -0.03 ± 0.15 (95% limits of agreement: -0.29 – 0.23) between X-ray and MRI, meaning that CD ratios were on average 0.03 lower on X-ray than on MRI.

Conclusion

The CD ratio has strong agreement between X-ray and MRI in patients who undergo patellar stabilization. While inconsistencies exist in the current literature, the inter-rater reliability was strong for both MRI and X-ray calculations of the CD ratio.