Definition

Osteoarthritis is primarily a noninflammatory disorder of movable joints characterized by an imbalance between the synthesis and degradation of the articular cartilage, leading to the classic pathologic changes of wearing away and destruction of cartilage.

Osteoarthritis (OA) is one of the oldest and most common diseases in humans and the most common form of joint disease in the world.

The term "osteoarthritis" means "joint inflammation." Because OA does not present clinically as an inflammatory disorder, some have suggested that the term "osteoarthrosis," or "joint disease," is more appropriate. Although OA is considered a noninflammatory form of arthritis, there can be a small inflammatory component. However, this inflammation is much less intense than that seen in rheumatoid arthritis, which is considered an inflammatory form of arthritis.

In some respects, OA appears to be a normal response of the joint to cartilage wear. OA was once thought to occur when the joints were just "worn out." Consequently, the phrase "wear and tear" may be used to describe OA, and it is sometimes referred to as degenerative joint disease. Some researchers no longer use the term "wear and tear" arthritis as a synonym for OA as newer information regarding the pathogenesis of OA has become available.

Although the etiology of OA remains elusive, the imbalance between the synthesis and degradation of the articular cartilage results in increased deterioration or breakdown of the cartilage and the formation of new bone at the joint surfaces and margins. This breakdown of joint cartilage often results in joint pain and loss of mobility, which may lead to long-term disability. This is of major concern because the prevalence of OA is expected to increase significantly due to the aging of the U.S. population.

Classification

OA may be classified according to its cause or major predisposing factor as either primary or secondary. It may be further subclassified according to the anatomic site that is affected, the number of joints involved, or by the presence of specific features.

Primary vs. Secondary

Primary, or idiopathic, is the most common type of OA and has no identifiable underlying etiology or predisposing cause. Although secondary OA has an underlying cause, pathologically it is indistinguishable from primary OA. In some cases, the distinction between primary and secondary disease is unclear because the clinical presentation and symptoms of both classifications are often very similar. However, clinically, it is generally not important to make a distinction between the two forms.

Several disorders are well recognized as causes of secondary OA. They can be grouped into the four basic categories as shown below in the following table.

Potential Causes of Secondary OA

Metabolic Calcium crystal deposition, hemochromatosis, acromegaly

Anatomic Leg length inequality, congenital hip dislocation

Traumatic Major joint trauma, chronic joint injury (occupational), joint surgery

Inflammatory Ankylosing spondylitis, septic arthritis

Data from Dequeker and Dieppe [1998].

Subclassifications

Site

OA can also be classified by the anatomic area or joint that is affected, such as OA of the hip, hand, or knee. There is a particular predilection of OA for the distal interphalangeal joint of the hand, base of the thumb, knee, hip, and intervertebral facet joints. There are differences in the degree of association of OA at each site.

Number of involved joints

There is variation in the pattern of joint involvement in different individuals. Patients with multiple joint involvement can be described as having oligoarticular or polyarticular OA. Monoarticular refers to the involvement of only one joint. Involvement of more than one joint is common. Also, many population surveys have reported that patients with OA in one joint have a higher frequency of involvement in other joints that cannot be explained by age or chance alone. Furthermore, there are differences in the degree of association of OA at each site. For example, in Caucasians, there is a stronger association between hand and knee OA than between hip and knee OA.

Presence of specific features

OA has also been subclassified based on the presence of specific features such as:

Inflammation: Inflammatory OA is characterized by visible inflammation seen in multiple joints.
Erosions: Erosive OA may be secondary to an aggressive form of the disease and usually occurs in middle-aged women. This form of OA is usually associated with acute features of inflammation and subsides over months to years, leaving joint deformity and ankylosis. Erosions frequently occur in the interphalangeal joints of the hand.

Reference

Dequeker J, Dieppe PA, eds. Disorders of bone cartilage and connective tissue. In: Klippel JH, Dieppe PA, eds. Rheumatology. 2nd ed. London: Mosby, 1998.

20108063(1)-03/01-EBS-PHY