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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Septic arthritis is an orthopedic/infectious disease emergency
- Any acute hot swollen joint requires arthrocentesis unless contraindicated
- Synovial fluid: WBC, Gram stain, culture, and crystals
- Most common adult pathogen is Staphylococcus aureus
- Treatment requires drainage plus empiric IV antibiotics after cultures
Overview
Septic arthritis is infection within a joint space causing neutrophilic inflammation and rapid cartilage destruction. The knee is the most common native joint. Crystal disease can coexist, so crystals do not eliminate the need for culture.
Epidemiology
Risk factors include older age, diabetes, RA, prosthetic joints, immunosuppression, HIV, injection drug use, skin infection, bacteremia, joint injection, and pre-existing joint disease.
Clinical Features
Symptoms
Acute monoarticular pain with swelling, warmth, and severe limitation of motion
Fever, chills, malaise, or systemic toxicity
Hip infection may cause groin pain and inability to bear weight
Gonococcal disease: migratory polyarthralgia, tenosynovitis, dermatitis
Back pain with fever/neurologic symptoms suggests spinal infection
Signs
Hot swollen joint held in position of comfort with pain on passive ROM
Effusion and marked tenderness
Overlying cellulitis or skin breakdown may indicate source
Tenosynovitis and pustular lesions in disseminated gonococcal infection
Prosthetic joint drainage or sinus tract
Investigations
First-line
Urgent arthrocentesisCell count/differential, Gram stain, culture, and crystals
Blood cultures x2Often positive in nongonococcal septic arthritis
CBC, ESR, CRP, BMPSupport severity and monitoring but cannot exclude disease
Second-line
Plain radiographBaseline joint status; early films may show only effusion
UltrasoundDetects effusion and guides aspiration
NAAT for gonorrhea/chlamydiaTest exposed sites when DGI suspected
Specialist
MRIIf adjacent osteomyelitis, abscess, or spinal involvement suspected
Orthopedic drainageHip, shoulder, prosthetic joint, loculated infection, or failed aspiration
1
Immediate management
- Aspirate urgently before antibiotics if feasible
- Start empiric antibiotics after synovial and blood cultures unless septic
- Consult orthopedics early for hip, shoulder, prosthetic joint, or severe infection
2
Empiric antibiotics
- Vancomycin for MRSA coverage when risk or severe disease
- Add ceftriaxone/cefepime if Gram-negative risk
- Suspected gonococcal arthritis: ceftriaxone plus chlamydia coverage if not excluded
3
Source control
- Serial aspiration may suffice for accessible uncomplicated joints
- Arthroscopic or open drainage for hip, shoulder, prosthetic, loculated, or nonresponding infection
Complications
- Cartilage destruction: Permanent joint damage can occur rapidly
- Osteomyelitis: Contiguous bone infection
- Sepsis: Bacteremia, shock, metastatic infection
- Loss of function: Chronic pain, stiffness, disability
USMLE Step 2 CK Exam Tips
- 1Acute hot swollen joint = arthrocentesis first
- 2Crystals do not rule out septic arthritis
- 3Most common adult pathogen = Staphylococcus aureus
- 4Young sexually active patient + migratory polyarthralgia + tenosynovitis + dermatitis = DGI
- 5Hip septic arthritis needs urgent orthopedic drainage
- 6RA is a major risk factor
- 7Antibiotics alone are not enough — drainage is essential
practicetest your knowledge on septic arthritisApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — musculoskeletal and beyond.
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