Osteochondritis Dissecans of the Medial Femoral Condyle: MRI Findings of Instability

BACKGROUND. Osteochondritis dissecans (OCD) of the medial femoral condyle (MFC), a common cause of chronic knee pain, is often found to be stable on arthroscopy. OBJECTIVE. The purpose of this study was to investigate the performance of MRI in identifying MFC OCD lesion instability in children. METHODS. This retrospective study included 59 children (37 boys and 22 girls; mean age, 12.5 +/- 2.2 [SD] years) with 69 MFC OCD lesions who underwent knee MRI from January 2016 to October 2023. Two radiologists independently reviewed examinations to determine regional findings (effusion, skeletal maturity, secondary physis visibility, and progeny composition) as well as direct (osteochondral defect, intraarticular body, cartilage alteration, bone plate disruption, and receded ossification front) and indirect (presence of progeny-parent bone interface, altered surface curvature, parent-bone low-signal-intensity marginal rim, and extensive perilesional marrow edema) findings of instability. Disagreements were resolved through consensus. Two investigators assessed an additional indirect finding, cyst(s), in consensus. Information on demographic characteristics and lesion volume was collected. The reference standard for lesion stability was arthroscopy (n = 52) or follow-up evaluation (based on symptoms and imaging findings) among patients treated with conservative management (n = 17). Stable and unstable lesions were compared. RESULTS. Forty-nine lesions were stable, and 20 were unstable. Among features showing significant associations with lesion instability, the sensitivity and specificity for instability were, respectively, 70.0% and 100.0% for skeletal maturity, 75.0% and 75.5% for osteochondral progeny composition, 100.0% and 85.7% for cartilage alteration, 85.0% and 81.6% for presence of progeny-parent bone interface, 30.0% and 100.0% for extensive interface, 90.0% and 34.7% for cyst(s), 45.0% and 91.8% for altered surface curvature, 80.0% and 89.8% for parent-bone low-signal-intensity rim, and 50.0% and 81.6% for extensive perilesional marrow edema. In multivariable analysis, factors independently associated with instability included larger lesion volume and indirect findings (presence of progeny-parent bone interface, cyst[s], altered surface curvature, parent-bone low-signal-intensity rim, and extensive perilesional marrow edema). The frequency of instability was 0.0%, 7.4%, 16.7%, 75.0%, 83.3%, and 100.0% for lesions with zero, one, two, three, four, and five indirect findings, respectively. CONCLUSION. Lesion instability was associated with an increasing number of indirect MRI signs. CLINICAL IMPACT. Assessment of lesion stability by MRI can help guide decision-making regarding arthroscopy versus conservative management for MFC OCD lesions in children.