Osteoarthritis: A disease of the joint as an organ

Osteoarthritis (OA) is the most common form of arthritis and a major cause of pain and disability in older adults (1). Often OA is referred to as degenerative joint disease “(DJD)”. This is a misnomer because OA is not simply a process of wear and tear but rather abnormal remodeling of joint tissues driven by a host of inflammatory mediators within the affected joint. The most common risk factors for OA include age, gender, prior joint injury, obesity, genetic predisposition, and mechanical factors, including malalignment and abnormal joint shape (2, 3). Despite the multifactorial nature of OA, the pathological changes seen in osteoarthritic joints have common features that affect the entire joint structure resulting in pain, deformity and loss of function. The pathologic changes seen in OA joints (Figures 1 and ​and2)2) include degradation of the articular cartilage, thickening of the subchondral bone, osteophyte formation, variable degrees of synovial inflammation, degeneration of ligaments and, in the knee, the menisci, and hypertrophy of the joint capsule. There can also be changes in periarticular muscles, nerves, bursa, and local fat pads that may contribute to OA or the symptoms of OA. The findings of pathological changes in all of the joint tissues are the impetus for considering OA as a disease of the joint as an organ resulting in “joint failure”. In this review, we will summarize the key features of OA in the various joint tissues affected and provide an overview of the basic mechanisms currently thought to contribute to the pathological changes seen in these tissues. Open in a separate window Figure 1 Sagittal inversion recovery (A–C) and coronal fast spin echo (D–F) images illustrating the magnetic resonance imaging findings of osteoarthritis. (A) reactive synovitis (thick white arrow), (B) subchondral cyst formation (white arrow), (C) bone marrow edema (thin white arrows), (D) partial thickness cartilage wear (thick black arrow), (E–F) full thickness cartilage wear (thin black arrows), subchondral sclerosis (arrowhead) and marginal osteophyte formation (double arrow). Image courtesy of Drs. Hollis Potter and Catherine Hayter, Hospital for Special Surgery, New York, NY