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Many mechanisms provoke acute joint symptoms: degradation and degeneration of articular cartilage (osteoarthritis), deposition of immune complexes or immune system–related phenomena (rheumatoid arthritis, rheumatic fever, and, possibly, a component of gonococcal arthritis), crystal-induced inflammation (gout and pseudogout), seronegative spondyloarthropathies (ankylosing spondylitis—primarily genetic, and reactive arthritis—postinfectious with HLA-B27 susceptibility), and bacterial invasion (gonococcal and nongonococcal septic arthritis, including Lyme arthritis), or viral invasion (viral arthritis). These processes impact joint capsules and surfaces, resulting in a cascade of reactive and inflammatory events. The term septic arthritis refers to the invasion of a joint by an infectious agent with proliferation and associated inflammation; bacterial arthritis is a subset of septic arthritis. Under ideal conditions, the infectious agent is recoverable from the joint fluid in septic arthritis, but in practice, this may not occur. This chapter reviews the common causes and treatments of acute nontraumatic joint pain. Joint injuries are discussed in Section 22, Injuries to Bones and Joints, and disorders due to repetitive use syndromes are discussed in Section 23, Musculoskeletal Disorders, by anatomic site.

Clinical Features and Risk Factors

The first step in diagnosis is distinguishing monoarticular from polyarticular arthritis (Table 281-1) and determining if joint pain is migratory or not (Table 281-2).1 The most concerning diagnosis of acute joint pain is septic arthritis due to bacterial invasion, so decision making focuses on its exclusion or inclusion. The two most important diagnostic considerations for acute nontraumatic monoarthritis are nongonococcal or gonococcal septic arthritis contrasted with crystal-induced arthropathy (gout and pseudogout). Patient age, gender, and comorbid illnesses are aids to diagnosis. Risk factors for nongonococcal and gonococcal septic arthritis are listed in Table 281-3.2–4Figure 281-1 provides a diagnostic algorithm for the nontraumatic, acute monoarticular arthritis.

Table 281-1 Classification of Arthritis by Number of Affected Joints
Table 281-2 Common Joint Disorders with a Migratory Distribution Pattern
Table 281-3 Risk Factors for Nongonococcal and Gonococcal Septic Arthritis

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