Plain Radiographs Have Limited Utility in the Evaluation of Discoid Lateral Meniscus

BACKGROUND: Debate persists regarding which radiographic measurements are important when investigating a possible discoid lateral meniscus and the utility of x-rays in the diagnostic workup. HYPOTHESIS/PURPOSE: The purpose of this study was to identify differences in various measurements between healthy knees and those with a discoid meniscus while controlling for other patient characteristics. METHODS: Radiographs of patients with a confirmed discoid lateral meniscus were matched by age and sex to those with a healthy knee (verified by magnetic resonance imaging). The following parameters were associated with discoid meniscus in previous studies and were measured on x-ray for each subject: lateral joint space height (both in the central and medial portion of the compartment; LJSH-C and LJSH-M, respectively), medial joint space height, fibular head height (FHH), lateral tibial spine height, femoral inter-epicondylar distance, lateral tibial plateau obliquity, and chordal distance of the femoral condyle (medial and lateral). Ratios were calculated between LJSH-C/M and medial joint space height. Univariate analysis was followed by multivariate regression to adjust for confounders. RESULTS: The study included 57 patients with a discoid meniscus and 55 controls with a mean age of 11.0±3.5 years (50% female). The median LJSH-C for subjects with a discoid meniscus was 7.3 mm [interquartile range (IQR) 2.1] compared to 5.8 mm (IQR 2.2) in normal knees (p<0.001). A similar difference was found for LJSH-M [6.9 mm (IQR 2.9) vs. 5.4 mm (IQR 2.4), p=0.002]. Mean FHH was lower for discoid menisci than controls (12.5±4.0 mm vs. 14.9±5.9 mm, p=0.01). No other radiographic measurements were significantly different between cases and controls. When adjusting for covariates in regression analysis, the presence of a discoid meniscus was predictive of a higher LJSH-C and LJSH-M and lower FHH. Of note, age and sex were also significantly predictive in these models. Patients with a discoid meniscus had 3.5 times higher odds of LJSH-C ≥7 mm [95% confidence interval (CI) 1.5-8.3, p=0.005), 3.3 times higher odds of LJSH-M ≥6 mm (95% CI 1.4-7.9, p=0.006), and 3.1 times higher odds of FHH <16 mm (95% CI 1.2-7.8, p=0.02). CONCLUSION: After controlling for other factors, LJSH-C, LJSCH-M, and FHH on plain radiographs were associated with a discoid lateral meniscus. However, many previously reported measurements were not predictive. While these parameters may aid diagnostic decision making when there is suspicion for a discoid meniscus, age and sex must also be considered. Advanced imaging may be required to confirm the diagnosis.