Frequency and Co-Reported Pathologic MRI Findings of Cystic Lesions of the Adult Knee: A Retrospective Study in 4,764 Knees

Objectives: Cystic lesions of the knee comprise anatomically distinct subtypes that may be reported alongside different degenerative, traumatic, meniscal, and synovial abnormalities. We sought to determine the frequency of radiologist-reported cystic lesion subtypes on adult knee magnetic resonance imaging (MRI) examinations performed for suspected internal derangement and, among cyst-positive examinations, to explore subtype-specific patterns of co-reported intra-articular and periarticular MRI findings. Material and Methods: We conducted a retrospective review of adult knee MRI examinations performed at a single academic institution between January 1, 2022, and January 1, 2023. MRI reports were reviewed for documented cystic lesions and co-reported MRI abnormalities using search terms. Among cyst-positive examinations, examinations with each cyst subtype were compared with cyst-positive examinations without that subtype. Continuous variables were compared using Student’s t-tests, and categorical variables were compared using Pearson’s chi-square or Fisher’s exact test, where appropriate. Unadjusted odds ratios (ORs) with 95% confidence intervals (CIs) were calculated. Analyses were exploratory and hypothesis-generating. Results: Among 4,764 adult knee MRI examinations, 1,208 (25.4%) contained at least one documented cystic lesion. Among cyst-positive examinations, Baker’s cysts were reported in 923 (76.4%), ganglion cysts in 257 (21.3%), parameniscal cysts in 172 (14.2%), and meniscal cysts in 23 (1.9%); these subtypes were not mutually exclusive. Baker’s cysts were more frequently co-reported with joint effusion (OR 1.48, 95% CI 1.13-1.94). Ganglion cysts were more frequently co-reported with posterior cruciate ligament pathology (OR 4.36, 95% CI 2.31-8.25). Parameniscal cysts were more frequently co-reported with lateral (OR 1.97, 95% CI 1.42-2.73) and medial meniscal tears (OR 1.96, 95% CI 1.31-2.93) and less frequently with effusion (OR 0.51, 95% CI 0.37-0.71). Meniscal cysts were uncommon overall (n=23). Conclusion: In this single-institution review, cystic lesions were documented in approximately one-quarter of adult knee MRI examinations performed for suspected internal derangement, with differing patterns of co-reported MRI abnormalities across cyst subtypes. These findings are descriptive and exploratory and should be interpreted in light of the retrospective design and reliance on clinical reports.