The Osteoarthritis Initiative (OAI) was a longitudinal cohort study which investigated the natural history of knee osteoarthritis across the spectrum of the disease through the collection of imaging, biochemical, genetic, and risk markers of knee OA. Measures included knee symptoms and function (evaluated using the West Ontario and McMaster Universities Arthritis Index (WOMAC) and the Knee injury and Osteoarthritis Outcome Score (KOOS), hip and other joint symptoms, general quality of life, physical performance, knee examination, risk factors, health behaviors, psychosocial measures, and medication and supplements taken.

OAI enrolled a total of 4,796 participants aged 45 to 79 years old at time of recruitment. Biospecimens collected include blood, urine, DNA, and lymphocytes. Images collected throughout the study include baseline and annual bilateral knee x-ray and bilateral knee MRI and other joint imaging (e.g., hip, pelvis, and hand x-rays; MRI of the thigh; and full limb MRI for knee alignment). The schedule of completed assessments is available on the OAI website.