Neck vs Head Interspace Ratios in Hallux Valgus: A Reliability Analysis of First Metatarsal Radiographic Measurements

Background The relationship between the intermetatarsal space and first metatarsal width has been proposed as a radiographic parameter for surgical planning in hallux valgus. However, the reproducibility of the previously described Head Interspace Ratio (HIR) has not been formally evaluated, limiting confidence in its clinical application.Purpose To compare the intraobserver and interobserver reliability of the Head Interspace Ratio (HIR) and the newly proposed Neck Interspace Ratio (NIR).Study design Retrospective cross-sectional reliability study.Methods Fifty standardized weight-bearing anteroposterior radiographs were independently evaluated by three observers with different experience levels during two sessions. Reliability was assessed using intraclass correlation coefficients (ICC). Agreement and measurement precision were evaluated using Bland-Altman analysis, standard error of measurement, and minimal detectable change at the 95% confidence level.Results NIR demonstrated good to excellent intraobserver reliability (ICC, 0.876-0.963) and good interobserver reliability (ICC = 0.886; 95% CI, 0.826-0.929). HIR showed greater variability, with good reproducibility for the two more experienced observers but poor reliability for the least experienced observer (ICC = 0.236; 95% CI, -0.043 to 0.481). Interobserver reliability was lower for HIR (ICC = 0.464; 95% CI, 0.276-0.633). Measurement error was lower for NIR than HIR (MDC95, 0.167 vs 0.277).Conclusions Under the conditions evaluated in this study, NIR demonstrated greater reproducibility and lower measurement error than HIR. These findings suggest that NIR may represent a more robust radiographic parameter and support further clinical investigation before its use in surgical decision-making for hallux valgus.