A Novel Quantitative Index for Objective MRI Grading of Gastrocnemius Muscle Strain

This study presents a quantitative MRI-based method to improve the objectivity and diagnostic accuracy of grading calf muscle strains. Ninety-six patients with MRI-confirmed medial gastrocnemius injuries who underwent bilateral lower leg MRI between January 2018 and December 2023 were retrospectively analyzed. Injuries were graded using the Dai classification (Grades I–III). Axial T2-weighted SPAIR images were used to measure the medial gastrocnemius cross-sectional area (CSA) at the distal one-third between the knee joint and the musculotendinous junction. The Gastrocnemius Medial Head Ratio (GCM ratio) was calculated by dividing the CSA of the injured side by that of the uninjured side. Two experienced radiologists independently performed the measurements twice. Intra- and interobserver reliability were assessed using intraclass correlation coefficients (ICC) and Cohen's kappa. A one-way ANOVA with the Bonferroni correction was used for group comparisons. ROC analysis evaluated the diagnostic performance of the GCM ratio. The GCM ratio significantly differed among Grade I (1.03 ± 0.02), Grade II (1.18 ± 0.06), and Grade III (1.35 ± 0.05) injuries (p < 0.001). Ipsilateral CSA increased with injury severity, whereas contralateral CSA remained consistent. CSA measurements showed excellent intra- and interobserver reliability (ICC > 0.87), and lesion grading demonstrated substantial to almost perfect agreement (κ = 0.75–0.88). ROC analysis revealed strong diagnostic accuracy (AUC = 0.986 for Grade I vs. II; AUC = 1.000 for Grade II vs. III). The GCM ratio provides a reproducible and objective MRI marker to aid in grading gastrocnemius strains, offering excellent diagnostic performance and clinical applicability.