Osteoarthritis (OA) is the most common form of joint disease. Osteoarthritis is the leading cause of disability in those over age 65. Osteoarthritis involves progressive loss of articular cartilage and reactive changes at joint margins and in subchondral bone.
Primary osteoarthritis is idiopathic (of unkown cause) and is divided into subsets depending on clinical features.
Secondary osteoarthritis can be caused by several mechanisms, including: Childhood anatomic abnormalities (e.g., congenital hip dysplasia, slipped femoral epiphyses); Inheritable metabolic disorders (e.g., alkaptonuria, Wilson's disease, hemochromatosis); Neuropathic arthropathy (Charcot's joints); Hemophilic arthropathy; Acromegalic arthropathy; Paget's disease; Hyperparathyroidism; Noninfectious inflammatory arthritis (e.g., rheumatoid arthritis, spondyloarthropathies); Gout, calcium pyrophosphate deposition disease (pseudogout); Septic or tuberculous arthritis; and Post-traumatic.
Biomechanical, biochemical, inflammatory, and immunological factors are all implicated in pathogenesis of osteoarthritis. Risk factors include: Age over 50; Obesity (weight bearing joints); Prolonged occupational or sports stress (Injury to a joint).
Standard lab tests are not helpful.
Joint aspiration may be helpful to distinguish between OA and chronic inflammatory arthritides. OA shows cell count usually < 500 cells/mm3, predominantly mononuclear. Inflammatory arthritis shows cell counts usually > 2000 cells/mm3, predominantly neutrophils.
Synovial fluid may have a slightly increased white blood cell count, predominantly mononuclear. Calcium pyrophosphate dihydrate and/or apatite crystals may occasionally be seen in effusions and require polarized light microscopy or special techniques to see. X-rays are usually normal early. Later they often show narrowed joint space, osteophyte formation, subchondral bony sclerosis, and cyst formation. Erosions may occur on surface of distal interphalangeal (DIP) and proximal interphalangeal (PIP) joints when OA is associated with inflammation (erosive osteoarthritis).
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