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PATIENT 
Mrs. K is a 75-year-old woman who complains of a painful left knee.
What is the differential diagnosis of joint pain? How would you frame the differential?
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CONSTRUCTING A DIFFERENTIAL DIAGNOSIS
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The causes of joint pain range from common to rare and from not particularly dangerous to joint and life-threatening. Even the most benign causes of joint pain can lead to serious disability. The evaluation of a patient with joint pain calls for a detailed history and physical exam (often focusing on extra-articular findings) and occasionally the sampling of joint fluid and possibly analyzing radiologic and serologic tests.
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The differential diagnosis of joint pain can be framed with the use of 3 pivotal questions. First, is a single joint or are multiple joints involved (is the joint pain articular or polyarticular). If the pain involves just 1 joint, the next question is, is the pain articular or extra-articular? Although this distinction may seem obvious, abnormalities of periarticular structures can mimic articular disease. Finally, are the involved joints inflamed or not? Further down the differential, the acuity of the pain may also be important.
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Figure 27-1 shows a useful algorithm organized according to these pivotal points. Because periarticular joint pain is almost always monoarticular, the first pivotal point differentiates monoarticular from polyarticular pain. Periarticular syndromes are discussed briefly at the end of the chapter.
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The differential diagnosis below is organized by these 3 pivotal points as well. When considering both the algorithm and the differential diagnosis, it is important to recognize that all of the monoarticular arthritides can present in a polyarticular distribution, and classically polyarticular diseases may occasionally only affect a single joint. Thus, this organization is useful to organize your thinking but should never be used to exclude diagnoses from consideration.
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Monoarticular arthritis
Inflammatory
Infectious
Crystalline
Noninflammatory
Osteoarthritis (OA)
Traumatic
Avascular necrosis
Polyarticular arthritis
Inflammatory
Rheumatologic
(1) Rheumatoid arthritis (RA)
(2) Systemic lupus erythematosus (SLE)
(3) Psoriatic arthritis
(4) Other rheumatic diseases
Infectious
(1) Bacterial
(2) Viral
(a) Rubella
(b) Hepatitis B
(c) HIV
(d) Parvovirus
(3) Postinfectious
(a) Enteric
(b) Urogenital
(c) Rheumatic fever
Noninflammatory: OA
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Mrs. K’s symptoms started after she stepped down from a bus with unusual force. The pain became intolerable within about 6 hours of onset and has been present for 3 days now. She otherwise feels well. She reports no fevers, chills, dietary changes, or sick contacts.
On physical exam she is in obvious pain, limping into the exam room on a cane. Her vital signs ...