THE CALCANEAL X/Y RATIO: TRUE MORPHOLOGY OR A RADIOLOGICAL ILLUSION? A COMPARISON OF RADIOGRAPH AND CT

Introduction: Tourne et al. described the calcaneal X/Y ratio on weightbearing radiographs to assess ““overlong”“ calcanei in Haglund's syndrome, suggesting a ratio <2.5 predisposes to heel pain and supports the use of Zadek osteotomy. However, other series report good outcomes with Zadek osteotomy regardless of X/Y ratio. Aim: To evaluate the accuracy of the XY ratio measurement on the plain radiographs reflecting the morphology of the calcaneum. This was achieved by comparing the XY ratio on X-rays and CT scans. Method: 50 patients with Hindfoot CTs and Weightbearing X-rays, regardless of heel pain presence were selected. Two assessors (One resident & one fellow) were trained by a specialist Musculoskeletal Radiologist and Fellowship trained orthopaedic foot & ankle surgeon to measure the X/Y ratio on both plain radiographs and the sagittal cut of the CT scans. The reliability and correlation between the X/Y ratio measured on X-ray and CT as well as Inter-observer agreements between these measurements were calculated. Results: The mean X/Y ratio measured by both assessors was lower on X-ray (2.71±0.43, 2.69±0.41) than on CT (3.14±0.44, 3.10±0.44). X/Y ratio measured on X-ray and CT were poorly correlated for both reviewers (observer one ICC (95%CI) =.487 (-.099, .784) P< .001, observer two ICC (95%CI) =.476 (-.076, .734) P< .001) Good Inter-observer reliability of the X/Y ratio measured on X-ray (ICC (95%CI) =.808 (.662, .891) P< .001) and excellent when measured on CT (ICC (95%CI) =.956 (.992, .975) P<.001) was found. Conclusion: According to the findings of this study, the X/Y ratio measured on standing lateral Radiographs does not correlate to Calcaneal morphological length measured on CT. The published association between Haglund's Syndrome and an X/Y ratio < 2.5 is likely due to factors other than true calcaneal length such as calcaneal alignment.