Classification systems for assessing acute muscle injuries: a retrospective comparison of inter-reader agreements

The purpose of this study is to compare three commonly used classification systems for MRI grading of acute muscle injury concerning their inter-reader reliability.

Ethical committee approval was obtained. Inclusion criteria comprised patients with acute muscle injury, age ≥ 18 years, and signed informed consent. MR examinations were evaluated by four independent musculoskeletal radiologists. Muscle injuries were graded according to the British Athletics Muscle Injury Classification (BAMIC), the Munich Consensus Injury Classification (MCIC), and the Chan et al. Injury Classification (CIC). Inter-reader reliability was quantified with Fleiss' Kappa (κ) and associated 95% confidence interval (CI).

One hundred eleven acute muscle injuries in 110 patients (84% males) were assessed. Injured muscle groups included 85 thigh injuries (44 hamstrings, 41 non-hamstrings), 19 lower leg injuries, and 7 injuries in other locations. κ values (CI) were 0.506 (0.499, 0.514) for BAMIC, 0.566 (0.549, 0.584) for MCIC, and 0.306 (0.302, 0.311) for CIC. The highest reproducibility was seen for non-hamstring injuries in the thigh using MCIC 0.749 (0.720, 0.777), the lowest for lower leg injuries using CIC 0.199 (0.185, 0.213). Injury severity showed greater reproducibility (κ = 0.594-0.696) than the location of the injury within the muscle (κ = 0.349-0.576).

The MCIC and BAMIC demonstrate moderate inter-reader reliability, whereas the CIC demonstrates fair inter-reader reliability. The challenge with the classifications is the reproducibility of localizing the injury anatomically within the muscle, rather than classifying injury severity. Non-hamstring thigh injuries were most reproducible with MCIC, while lower leg injuries were least reproducible with CIC.