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(1) Consider non-articular mimics of joint pain

  • 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

(2) Characterize the joint pain

  • 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

(3) Analyze synovial fluid

Synovial fluid analysis is particularly critical in patients with acute monoarthritis.

  • 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.

TABLE 9.1Differential Diagnosis for Joint Pain Based on Clinical Characteristics


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