Musculoskeletal System (MSK) Module — 3rd Year MBBS

AIM STEP 6 • CONCEPT INTEGRATION

Infectious Arthritis, Rheumatic Fever and Bone–Joint Infections

3rd Year MBBS • KMU Curriculum

A concise synthesis connecting infection, immune injury, morphology, clinical clues, investigations, treatment and prevention for rapid revision.

1. THE TOPIC IN ONE CONNECTED FLOW

Bone and joint inflammation may result from direct bacterial or mycobacterial infection, or from an immune reaction after group A streptococcal pharyngitis. Direct infection damages tissue through suppuration or granulomatous inflammation, while rheumatic fever produces sterile inflammation through molecular mimicry. Recognizing this difference guides the correct specimen, investigation, treatment and prevention.

Cause or Trigger
Pyogenic bacteria, mycobacteria, bacteremia, trauma or group A streptococcal pharyngitis

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Core Mechanism
Neutrophilic suppuration, macrophage-driven granulomas or molecular mimicry

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Tissue Change
Pus, raised pressure, ischemia, cartilage erosion, bone necrosis, caseation or Aschoff bodies

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Clinical Presentation
Hot swollen joint, chronic cold swelling, bone pain, sinus, migratory arthritis or carditis

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Diagnostic Clue
Synovial aspirate and culture, tissue biopsy, imaging or evidence of preceding streptococcal infection

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Treatment or Prevention
Prompt antimicrobial therapy, drainage, treatment of streptococcal infection and prevention of recurrence

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Outcome
Recovery or progression to ankylosis, chronic osteomyelitis, pathological fracture, deformity or valvular disease

Central distinction: Suppurative arthritis causes rapid neutrophil-mediated destruction, mycobacterial arthritis causes slow granulomatous destruction, and rheumatic fever causes sterile immune-mediated inflammation after pharyngitis.

2. KEY CLINICAL CONNECTIONS

Acute Suppurative Arthritis

Hot, tender monoarthritis

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Neutrophilic suppuration and raised intra-articular pressure

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Rapid cartilage inflammation and ischemic injury

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Turbid aspirate and culture guide urgent drainage and antimicrobial therapy

Mycobacterial Bone–Joint Disease

Chronic cold swelling or monoarthritis

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Delayed-type cellular immune response

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Caseating granulomas and slow tissue destruction

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Tissue biopsy and mycobacterial evaluation support diagnosis

Acute Rheumatic Fever

Recent streptococcal pharyngitis

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Molecular mimicry and cross-reactive immune injury

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Migratory arthritis with possible carditis

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Evidence of preceding infection and cardiac assessment guide management and recurrence prevention

3. AIM HIGH-YIELD INTEGRATION REVIEW

⭐ Purulent synovial fluid + severe restriction → suppurative arthritis: Neutrophilic inflammation rapidly damages cartilage, so aspiration, culture and urgent treatment are essential.

⭐ Raised intra-articular pressure → reduced perfusion: Septic arthritis injures cartilage through both inflammatory destruction and ischemia.

⭐ Cold swelling + caseating granulomas → mycobacterial arthritis: This combination indicates chronic, slowly progressive granulomatous destruction rather than acute suppuration.

Bone necrosis → sequestrum → persistent infection: A sequestrum is avascular dead bone with poor antimicrobial penetration, while involucrum is reactive new bone formed around it.

⭐ Streptococcal M protein → molecular mimicry: Cross-reactive immunity links preceding pharyngitis with sterile rheumatic inflammation of joints and the heart.

Migratory arthritis + carditis after pharyngitis → acute rheumatic fever: The joint inflammation is immune-mediated rather than caused by direct bacterial invasion.

Aschoff bodies → acute carditis: In contrast, leaflet thickening and commissural fusion indicate chronic rheumatic valvular damage.

Early treatment + recurrence prevention → reduced disability: Prompt organism-directed therapy and drainage limit bone–joint destruction, while treating streptococcal infection and preventing recurrence reduce chronic rheumatic valve disease.

AIM Exam Trap: Migrating arthritis after streptococcal pharyngitis is caused by a sterile immune reaction. It does not mean that bacteria are physically spreading from one joint to another.