Osteoarthritis is a mechanical condition — the cartilage wears down over time due to age, load, and joint use. Rheumatoid arthritis is an autoimmune condition — the body's immune system attacks the joint lining, causing inflammation that damages the cartilage and bone over time. Both can cause knee pain, swelling, and stiffness, but they have different causes, patterns, and treatment approaches.
Osteoarthritis typically develops gradually in older adults and often affects one knee more than the other. It is worsened by activity and relieved by rest, at least in the early stages. Rheumatoid arthritis tends to affect both joints symmetrically, is associated with morning stiffness lasting more than an hour, and may affect other joints and body systems simultaneously. Both conditions can progress to the point where knee replacement is considered — but the management of rheumatoid arthritis also involves disease-modifying medications that must be coordinated with a rheumatologist.
