Osteochondritis Dissecans of the Knee An Interrater Reliability Study of Magnetic Resonance Imaging Characteristics Peter D. Fabricant, * MD, MPH, Matthew D. Milewski, MD, Regina O. Kostyun, MSEd, ATC, Eric J. Wall, MD, Andrew M. Zbojniewicz, MD, The Research in Osteochondritis of the Knee (ROCK) Study Group, and Gregory D. Myer, PhD Investigation performed at the Hospital for Special Surgery, New York, New York, USA, and Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA Background: Imaging characteristics of osteochondritis dissecans (OCD) lesions quantified by magnetic resonance imaging (MRI) are often used to inform treatment and prognosis. However, the interrater reliability of clinician-driven MRI-based assess- ment of OCD lesions is not well documented. Purpose: To determine the interrater reliability of several historical and novel MRI-derived characteristics of OCD of the knee in children. Study Design: Cohort study (diagnosis); Level of evidence, 3 Methods: A total of 42 OCD lesions were evaluated by 10 fellowship-trained orthopaedic surgeons using 31 different MRI char- acteristics, characterizing lesion size and location, condylar size, cartilage status, the interface between parent and progeny bone, and features of both the parent and the progeny bone. Interrater reliability was determined via intraclass correlation coef- ficients (ICCs) with 2-way random modeling, Fleiss kappa, or Krippendorff alpha as appropriate for each variable. Results: Raters were reliable when the lesion was measured in the coronal plane (ICC, 0.77). Almost perfect agreement was achieved for condylar size (ICC, 0.93), substantial agreement for physeal patency (ICC, 0.79), and moderate agreement for joint effusion (ICC, 0.56) and cartilage status (ICC, 0.50). Overall, raters showed significant variability regarding interface characteristics (ICC, 0.25), progeny (ICC range, 0.03 to 0.62), and parent bone measurements and qualities (ICC range, –0.02 to 0.65), with reli- ability being moderate at best for these measurements. Conclusion: This multicenter study determined the interrater reliability of MRI characteristics of OCD lesions in children. Although several measurements provided acceptable reliability, many MRI features of OCD that inform treatment decisions were unreliable. Further work will be needed to refine the unreliable characteristics and to assess the ability of those reliable characteristics to predict clinical lesion instability and prognosis. Keywords: OCD; skeletally immature; pediatric; adolescent; cartilage; magnetic resonance imaging Osteochondritis dissecans (OCD) is defined as a focal idio- pathic alteration of subchondral bone with a risk for insta- bility and disruption of adjacent articular cartilage that may result in premature osteoarthritis. 8,11 The American Academy of Orthopaedic Surgeons developed a clinical practice guideline for the diagnosis and treatment of OCD, but the recommendations were largely weak or inconclusive. 3 The Research in Osteochondritis of the Knee (ROCK) Study Group was founded comprising ortho- paedic surgeons, radiologists, physical therapists, and researchers to help strengthen the evidence behind OCD diagnosis, prognosis, and treatment strategies. Magnetic resonance imaging (MRI) has become the mainstay for the diagnosis and staging of knee OCD lesions. Previous studies have been performed to stage OCD lesions through use of MRI. 1,4-7,10,12,13,15,16,18 De Smet et al 5,6 described several MRI signs of instability, including high T2-weighted signal intensity rim equal to fluid 5 mm or more in length, presence of a cyst 5 mm or larger, high T2-weighted signal line equal to fluid tra- versing the articular cartilage and subchondral bone plate into the lesion, and osteochondral defect 5 mm or larger. Kijowski et al 14 found differences in the sensitivity and specificity of previously published MRI criteria for determination of instability in skeletally immature versus mature populations with OCD. However, to our The American Journal of Sports Medicine 1–9 DOI: 10.1177/0363546520930427 Ó 2020 The Author(s) 1