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DIAGNOSTIC APPROACH TO JOINT PAIN
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(1) Consider non-articular mimics of joint pain
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Articular structures: Synovium, synovial fluid, articular cartilage, intraarticular ligaments, joint capsule, and juxtaarticular bone
Non- or peri-articular structures: Extraarticular ligaments, tendons, bursae, muscle, fascia, bone, nerve, and overlying skin
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(2) Characterize the joint pain
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Temporality: Acute versus chronic
- Acute: <6 weeks
- Chronic: >6 weeks
Inflammation: Inflammatory or non-inflammatory; assess for inflammatory features:
- History: Morning stiffness lasting >30–60 min
- Exam: “Rubor, calor, tumor” – Peri-articular erythema, warmth and joint swelling
- Labs: Elevated ESR and/or CRP, synovial fluid WBC >2K cells/μL
Number of joints involved: Mono-, oligo-, and polyarthritis
- Monoarthritis: Single joint
- Oligoarthritis: 2–4 joints
- Polyarthritis: ≥5 joints
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(3) Analyze synovial fluid
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Synovial fluid analysis is particularly critical in patients with acute monoarthritis.
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Cell count:
- WBC <2K cells/μL: Noninflammatory
Bloody: Trauma, coagulopathy, tumor
Non-bloody: OA, avascular necrosis, Charcot arthropathy (peripheral neuropathy resulting in unwitting pathologic joint trauma typically in the feet and ankles)
- WBC 2–10K cells/μL: Inflammatory arthritis, gout, and pseudogout
- WBC >20K cells/μL: Very high concern for infectious arthritis; gout and pseudogout are also possible, but orthopedic surgical consultation for consideration of washout is essential
Crystals:
- Monosodium urate: Needle-shaped, negatively birefringent crystals (yellow when parallel to the microscope polarization axis). Suggestive of gout in the setting of acute monoarthritis.
- Calcium pyrophosphate dihydrate: Rhomboid crystals with weak positive birefringence (blue when parallel to the polarization axis). Suggestive of pseudogout in the setting of acute monoarthritis.
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RHEUMATOLOGIC DIFFERENTIAL DIAGNOSIS FOR COMMON ...