Phenotypic variations in knee osteoarthritis: insights from MRI and radiographic comparisons.

Objective: To investigate the correlation between MRI-based phenotypes (cartilage-meniscus, subchondral bone, and inflammatory) and radiography-based atrophic and hypertrophic phenotypes, aiming to demonstrate MRI's diagnostic capability in identifying complex osteoarthritis phenotypes that radiography cannot fully capture.

Materials and methods: This single-center retrospective study examined knee radiographs and MRIs of patients from November 2021 to April 2023 to identify osteoarthritis phenotypes. Radiographs were staged by the Kellgren-Lawrence system, and both modalities were classified into atrophic or hypertrophic phenotypes. MRIs were further classified into three phenotypes: cartilage-meniscus, subchondral bone, and inflammatory. Associations between phenotypes, Kellgren-Lawrence stage, age, and gender were analyzed with Pearson chi-square test and student T-test. Reliability measurements were evaluated using kappa statistic.

Results: A total of 214 knees from 187 individuals (73.3% women, 26.7% men; mean age 57.1 ± 9.1 years) were included. The hypertrophic MRI phenotype was significantly associated with cartilage-meniscus and subchondral bone phenotypes (p < 0.001). Cartilage-meniscus and subchondral bone phenotypes were less prevalent in Kellgren-Lawrence stage 2 than in stages 3 and 4 (p < 0.001 and p = 0.004, respectively). The subchondral bone phenotype was more common in men (p = 0.022), and the cartilage-meniscus phenotype in the elderly (p < 0.001). Radiography and MRI had substantial agreement (Kappa = 0.637, p < 0.001) in diagnosing hypertrophic and atrophic phenotypes.

Conclusion: The hypertrophic phenotype was associated with cartilage-meniscus and subchondral bone phenotypes, with lower prevalences in Kellgren-Lawrence stage 2 knees. MRI offers enhanced phenotypic characterization, which facilitates more precise and individualized management in osteoarthritis care. Despite limitations compared to MRI, radiography remains valuable for the evaluation of hypertrophic and atrophic phenotypes.