Osteoarthritis: Pathophysiology and Classification of a Common Disabling Condition

Osteoarthritis (OA) is a common condition affecting nearly 600 million people across the globe, with a significant burden of disability, especially in the later stages of the disease. For patients, OA can result in pain, stiffness, and reduced function, with increased associated morbidity and mortality due to bidirectional relationships with metabolic syndrome and cardiovascular disease. OA also has a significant economic burden, with both direct medical (estimated at 1–2.5% of gross domestic product) and indirect medical costs due to work-absenteeism and early retirement. Previously thought to be solely a degenerative disease, it is now understood to be multi-causal, with modifiable and non-modifiable risk factors, including sex, age, body mass, inflammation, joint structure, strength, disease, and injury. There are many different ways to describe and classify this disease, including aetiological, clinical, or radiological, with implications for clinical care and research studies. These different descriptions influence the reported rates of OA, and it is possible that different classifications reflect discrete pathological mechanisms requiring targeted interventions and bespoke clinical management. This article aims to provide an overview of OA pathophysiology, classification, and epidemiology before briefly introducing future research priorities for understanding and classifying this common disabling condition.