Joints and Bursae

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

Joints of the extremities are synovial articulations designed to allow the greatest amount of motion, but this makes them highly susceptible to degenerative, infectious, and inflammatory processes.

• Osteoarthritis (Degenerative): Characterized by chronic, progressive destruction of the articular cartilage. The loss of a smooth articular surface leads to joint space narrowing, subchondral bone exposure, and reactive inflammation without systemic symptoms.
• Septic Arthritis (Infectious): Caused by direct bacterial invasion (most commonly Staphylococcus aureus) into the joint capsule. The bacteria trigger a massive influx of neutrophils; the resulting purulent exudate contains destructive enzymes that rapidly and permanently destroy articular cartilage if not emergently decompressed.
• Crystal Arthropathies (Inflammatory): Driven by the precipitation of microscopic crystals in the synovial fluid—monosodium urate in Gout, and calcium pyrophosphate in Pseudogout. These crystals are phagocytosed by macrophages, triggering an intense, acute inflammatory synovitis.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Septic Arthritis Resuscitation: Any acute monoarticular arthritis must be treated as a septic joint until proven otherwise. If synovial fluid is purulent or clinical suspicion is high, immediately initiate empiric IV antibiotics (Vancomycin and Ceftriaxone) pending cultures, and consult orthopedics for emergent surgical joint washout.
• Crystal Arthropathy Management: For acute flares of gout or pseudogout, acute inflammation control is the priority. Administer systemic NSAIDs (Indomethacin, Naproxen, Ibuprofen) as first-line therapy. Corticosteroids (systemic or intra-articular) are an excellent alternative for patients with kidney disease, but should be avoided in patients with uncontrolled diabetes. Colchicine may be used but is limited by gastrointestinal side effects (vomiting and diarrhea).
• Aseptic Bursitis Management: For well-localized inflammation over a bursa without signs of infection, manage with a compressive dressing, rest, ice, NSAIDs, and consider a localized corticosteroid injection (commonly for prepatellar or anserine bursae).

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

Critical "Can't-Miss" Differentials:

1. Septic Arthritis (Surgical emergency)
2. Crystal-Induced Synovitis (Gout / Pseudogout)
3. Reactive Arthritis (Oligoarthritis following enteric or genitourinary infection)
4. Osteoarthritis / Rheumatoid Arthritis Exacerbation
5. Aseptic Bursitis / Tendinitis (Mimics true arthritis but with well-localized pain)

Prioritized Diagnostic Workup:

• Diagnostic Arthrocentesis: This is the absolute gold standard and must be performed if the diagnosis is unclear or infection is suspected.
• Synovial Fluid Analysis Cutoffs:
• Normal Joint: Colorless/straw, high viscosity, WBC < 150.
• Non-inflammatory Arthritis: Straw/yellow, high viscosity, WBC < 3,000.
• Inflammatory Arthritis: Yellow, low viscosity, WBC > 3,000.
• Septic Arthritis: Purulent, mixed viscosity, WBC > 50,000 with > 75% PMNs.
• Hemorrhagic Arthritis: Red, low viscosity, WBC = RBC count.
• Crystal Microscopy: Essential to differentiate negatively birefringent needle-shaped crystals (Gout) from positively birefringent rhomboid-shaped crystals (Pseudogout).

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• Point-of-Care Ultrasound (POCUS): Utilize bedside ultrasound on acutely painful joints to accurately identify the presence of a joint effusion, which helps dictate the need for diagnostic aspiration.
• Standard X-Ray (Gout): Look for classic "rat-bite" erosions around the joint.
• Standard X-Ray (Pseudogout): Look for thin, white lines of chondrocalcinosis (calcification of the articular cartilage).
• Standard X-Ray (Osteoarthritis): Look for narrowed joint spaces, bone spurs (osteophytes), and erosion of the meniscus and cartilage.
• Physical Exam Visuals: Evaluate for specific joint involvement patterns. Osteoarthritis classically causes Heberden nodes at the DIP joints and Bouchard nodes at the PIP joints. Rheumatoid arthritis classically involves the PIP and MCP joints while sparing the DIP joints, causing Swan neck and Boutonniere deformities.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

Pathogen Risk Stratification in Septic Arthritis:
When selecting empiric antibiotics, tailor coverage based on classic historical risk factors:

• Most Common Overall: Staphylococcus aureus.
• Healthy, Sexually Active Young Adult (with tenosynovitis & pustular rash): Neisseria gonorrhoeae.
• Sickle Cell Disease: Salmonella species.
• Foot Puncture Wound: Pseudomonas aeruginosa.
• Recent Tick Exposure: Borrelia burgdorferi (Lyme disease).

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• The "Crystal Exclusion" Trap: Finding uric acid or CPPD crystals in a joint aspirate and assuming the joint is not infected. Crystal arthritis and septic arthritis frequently coexist. The presence of crystals must never be used to exclude a concurrent joint infection if the WBC count is > 50,000.
• The "Negative Gram Stain" Trap: Relying on a negative synovial fluid Gram stain to rule out septic arthritis. The Gram stain is positive in only 50% of patients with a confirmed bacterial joint infection.
• The "Allopurinol" Trap: Initiating Allopurinol during an acute gout flare. This inhibits xanthine oxidase and can cause a transient increase in uric acid, worsening the acute attack. It is strictly for chronic management.
• CRITICAL ACTION: True arthritis generates poorly localized pain in and around the joint with passive range of motion. If pain is highly localized and easily reproducible with focal palpation, reconsider mimics like bursitis and tendinitis.

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzword: "Negatively birefringent and needle-shaped" = Gout (most common at the 1st MTP joint / Podagra).
• Buzzword: "Positively birefringent and rhomboid-shaped" = Pseudogout (most common in the knee).
• Buzzword: "Can't see (conjunctivitis/uveitis), Can't pee (urethritis), Can't climb a tree (arthritis)" = Reactive Arthritis. This usually occurs 1-4 weeks following a Chlamydia trachomatis or enteric infection, strongly associated with HLA-B27.
• Distractor Trap: An exam question presents an acutely swollen knee. Synovial fluid shows a WBC count of 75,000 and the presence of uric acid crystals. A distractor will suggest discharging the patient with NSAIDs. The correct answer requires admitting for IV antibiotics and orthopedic washout, as a WBC > 50,000 mandates treatment for septic arthritis regardless of the crystals.

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

"This patient presents with acute monoarticular arthritis of the knee. Given that true arthritis causes poorly localized pain with passive range of motion, my primary 'can't miss' concern is septic arthritis, though crystal arthropathies like gout and pseudogout are on the differential. I will use bedside ultrasound to confirm the effusion and immediately perform a diagnostic arthrocentesis. I will send the fluid for cell count, Gram stain, cultures, and crystal analysis. If the synovial WBC count returns greater than 50,000, I will not be falsely reassured by a negative Gram stain or the concurrent presence of crystals. I will immediately initiate IV Vancomycin and Ceftriaxone, make the patient NPO, and consult Orthopedic Surgery for emergent joint washout to prevent irreversible cartilage destruction."