The diagnostic accuracy of a practical morphometric ratio on MRI for the discoid lateral meniscus

To determine the diagnostic accuracy of a practical morphometric MRI ratio (a/b ratio) for identifying discoid lateral meniscus (DLM).

This retrospective diagnostic accuracy study included 212 adults (mean age 44.8 ± 9.7 years; 56.1% female) who underwent 1.5-T knee MRI between 2020 and 2022. The DLM group (n = 106) was defined according to established MRI criteria, with an equal number of age- and sex-matched controls without discoid morphology. The index test was the a/b ratio, defined as the shortest lateral meniscal body width (a) divided by its medial offset from the lateral tibial spine (b). Additional evaluated morphometric parameters were tibial eminence width, tibial spine heights, and angles. The reference standard was consensus MRI diagnosis by two musculoskeletal radiologists. Diagnostic accuracy was assessed using ROC analysis, and measurement reproducibility was evaluated with intraclass correlation coefficients (ICC).

The a/b ratio was significantly higher in DLM than in controls (3.80 ± 5.4 vs. 0.47 ± 0.16; p < 0.001). A cut-off ≥ 0.764 yielded excellent diagnostic accuracy (AUC 0.996, 95% CI 0.991-1.000), with 100% sensitivity and 96.2% specificity. Complete DLM demonstrated markedly higher ratios than incomplete types (12.2 ± 7.4 vs. 1.9 ± 1.8; p < 0.001; AUC 0.990, 95% CI 0.972-1.000). Inter- and intra-observer agreement for the ratio was excellent (ICC 0.935 and 0.972, respectively). Tibial eminence width and lateral tibial spine angle were also greater in DLM (p < 0.001).

The a/b ratio is a practical and reproducible MRI-based metric with excellent diagnostic accuracy for DLM. Incorporation into routine MRI interpretation may enhance diagnostic confidence and aid preoperative planning.