Porth's Essentials of Pathophysiology, 4e

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Musculoskeletal Function

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Osteoarthritis Osteoarthritis (OA), also called degenerative joint dis- ease, is the most prevalent type of joint disease and is one of the 10 most disabling conditions in developing nations. 3,4 The disorder, which is characterized by degen- erative changes of the articular cartilage, can affect any one of 200 or so synovial joints in the body, and can occur as a primary idiopathic disorder or as a second- ary disorder. In most instances, OA appears insidiously, without an apparent initiating cause, as an aging phe- nomenon (idiopathic or primary OA). 3,4 In these cases the disorder is usually oligoarticular, but may be gen- eralized. Secondary OA has a known underlying cause such as congenital or acquired defects of joint structures, trauma, metabolic disorders, or inflammatory diseases. Joint changes associated with OA include a gradual loss of articular cartilage, combined with thickening of the subchondral bone, bony outgrowths (osteophytes) at joint margins, and mild synovial inflammation. These changes are accompanied by joint pain, stiffness, and limitation of motion, and in some cases by joint instabil- ity and deformity. Etiology and Pathogenesis Osteoarthritis is a multifactorial disease that has genetic and environmental risk factors. Studies of families and twins suggest that the risk of OA is related to the net impact of multiple genes, each with a small effect. 3,4 Age, gender, and race interact to influence the time of onset and the pattern of joint involvement in OA. Age is one of the strongest risk factors for OA of all joints. The increase in incidence and prevalence with age prob- ably is the consequence of cumulative exposure to the various risk factors and biologic changes that occur in a life time. Women are more likely to have OA than men, and they tend to have more severe OA as well. The prevalence of OA and pattern of joint involvement vary among racial and ethnic groups. Hand OA is more likely to affect white women, whereas knee OA is more com- mon in black women. The incidence of hip OA is lower among the Chinese than Europeans, perhaps represent- ing the influence of other factors such as occupation, obesity, or heredity. Bone mass may also influence the risk of developing OA. In theory, thinner subchondral bone mass may provide a greater shock-absorbing func- tion than denser bone, allowing less direct trauma to the cartilage. Obesity is a particular risk factor for OA of the knee in women and a contributory biomechani- cal factor in the pathogenesis of the disease. Weight loss reduces the risk of developing symptomatic arthritis of the knee. The pathogenesis of OA resides in the homeostatic mechanisms that maintain the articular cartilage. 3,4,59 Articular cartilage plays two essential mechanical roles in joint physiology. First, it serves as a remarkably smooth weight-bearing surface. In combination with synovial fluid, the articular cartilage provides extremely low fric- tion during movement of the joint. Second, the carti- lage transmits the load down to the bone, dissipating

is mild, and the person seeks medical care only with the onset of definite arthritis. Short antibiotic courses at this time are not effective. Psoriatic Arthritis Psoriatic arthritis is a seronegative inflammatory arthrop- athy that occurs with variable frequency in people with psoriasis. 56–58 It is a disease with various and similar fea- tures of the spondyloarthropathies in some persons, in whom an asymmetric sacroiliitis and spinal involvement predominate. In others, the disease is polyarticular and resembles RA, and in some, features of both disorders coexist. Although the arthritis can antedate a detectable skin rash, the definitive diagnosis of psoriatic arthritis cannot be made without evidence of skin or nail changes typical of psoriasis (see Chapter 46). The etiology of psoriasis and psoriatic arthritis is unknown. Genetic, environmental, and immunologic factors appear to affect susceptibility and play a role in expression of the psoriatic skin disease and the arthri- tis. Environmental factors that may play a role in the pathogenesis of the disorder include infectious agents and physical trauma. T-cell–mediated immune responses seem to play an important role in the skin and joint man- ifestations of the disease, as indicated by the observation that there is improvement in disease status after treatment with immunosuppressant agents such as cyclosporine. Psoriatic arthritis falls into five subgroups: (1) an oligoarticular form affecting four or fewer joints; (2) a spondylitis form in which sacroiliitis and spinal involvement predominate; (3) a polyarticular, or sym- metric, form that resembles RA; (4) a form in which the distal interphalangeal joints are primarily affected; and (5) arthritis mutilans, a very destructive form of arthritis. 56,57 The joint involvement of peripheral psori- atic arthritis is inflammatory in nature, presenting with swelling and stiffness of the affected joints. Early in the disease, the arthritis tends to be oligoarticular, but may become polyarticular over time. The affected joints of persons with psoriatic arthritis are often less tender than those of persons with RA. Thus, persons with psoriatic arthritis may present with deformity and joint damage, not having perceived any pain during the inflamma- tory phase of the disease. Sacroiliac involvement, when it occurs, tends to be asymmetric, involving only one sacroiliac joint and sparing the other or with differ- ent degrees of radiologic involvement. Likewise, spinal involvement tends to be asymmetric, with skip lesions. Dactylitis , or sausage digit , is a typical feature of distal interphalangeal joint disease and reflects inflammation of the entire digit. 56,57 Basic management is similar to the treatment of RA. Suppression of the skin disease may be important in helping control the arthritis. 56–58 Often, affected joints are surprisingly functional and only minimally symp- tomatic. The biologic response modifiers, specifically the TNF inhibitors (e.g., etanercept, infliximab, and adalimumab), have been found to be beneficial in con- trolling the arthritis as well as the psoriasis in patients with psoriatic arthritis. 56