Septic Arthritis

Overview

Septic arthritis, also known as infectious or bacterial arthritis, is a serious infection within a joint space that requires immediate medical attention. The condition occurs when bacteria, viruses, or fungi invade the synovial fluid and tissues of a joint, causing intense inflammation, pain, and potential destruction of cartilage and bone. Without prompt treatment, septic arthritis can permanently damage the joint within days to weeks and may spread to cause life-threatening systemic infection.

The incidence of septic arthritis ranges from 2 to 10 cases per 100,000 people annually, with rates significantly higher in individuals with predisposing conditions such as rheumatoid arthritis, diabetes, or immunosuppression. The knee is the most commonly affected joint (50% of cases), followed by the hip, shoulder, ankle, and wrist. While any joint can be involved, septic arthritis typically affects a single large joint, though polyarticular infection occurs in about 10-20% of cases, particularly in immunocompromised patients.

The urgency of septic arthritis cannot be overstated – it represents one of the few true emergencies in rheumatology. Bacterial enzymes and the inflammatory response can irreversibly damage articular cartilage within 48 hours of infection onset. Even with appropriate treatment, mortality rates range from 10-15%, rising to 30-50% in patients with underlying medical conditions. Long-term morbidity is also significant, with up to 40% of survivors experiencing chronic pain, reduced range of motion, or need for joint replacement. Early recognition, prompt joint aspiration, and immediate antibiotic therapy are crucial for preserving joint function and preventing systemic complications.

Symptoms

Septic arthritis typically presents with a triad of fever, joint pain, and impaired range of motion. Symptoms usually develop rapidly over hours to days.

Primary Joint Symptoms

Systemic Symptoms

Joint-Specific Presentations

• Knee:
  • Large effusion with inability to fully extend
  • Held in slight flexion for comfort
  • Unable to bear weight
• Hip:
  • Pain may radiate to groin, thigh, or knee
  • Leg held in flexion and external rotation
  • Particularly serious in children (can damage growth plate)
• Shoulder:
  • Arm held against body
  • Severe pain with any movement
  • May have associated bursitis
• Sacroiliac joint:
  • Lower back and buttock pain
  • Pain worse with weight bearing
  • Often misdiagnosed initially

Special Populations

• Infants:
  • Irritability and crying
  • Refusal to move affected limb (pseudoparalysis)
  • Poor feeding
  • May not have high fever
• Elderly:
  • May have minimal fever
  • Confusion or altered mental status
  • Less pronounced inflammatory signs
• Immunocompromised:
  • Atypical organisms more common
  • Multiple joint involvement more likely
  • Systemic symptoms may be blunted

Red Flag Symptoms

• Rapid progression of symptoms over hours
• Signs of sepsis (hypotension, altered consciousness)
• Multiple joints affected
• Skin lesions suggesting disseminated infection
• Neurological symptoms (suggesting epidural abscess)

Causes

Septic arthritis occurs when microorganisms invade the joint space through various routes. Bacterial infections are most common, but fungi and viruses can also cause joint infections.

Routes of Infection

• Hematogenous spread (most common):
  • Bacteria travel through bloodstream from distant site
  • From skin infections, UTIs, pneumonia
  • IV drug use
  • Endocarditis
• Direct inoculation:
  • Penetrating trauma
  • Joint injections or arthroscopy
  • Joint surgery
  • Animal or human bites near joint
• Contiguous spread:
  • From adjacent osteomyelitis
  • Soft tissue infections
  • Infected bursitis

Common Causative Organisms

• Staphylococcus aureus (most common - 40-50%):
  • Including MRSA (increasing prevalence)
  • All age groups
  • Associated with skin infections, IV drug use
  • Can cause rapid joint destruction
• Streptococci (20-30%):
  • Group A Streptococcus (S. pyogenes)
  • Group B Streptococcus (infants, diabetics)
  • Streptococcus pneumoniae
  • Viridans streptococci
• Gram-negative bacteria:
  • E. coli (elderly, immunocompromised)
  • Pseudomonas aeruginosa (IV drug users)
  • Klebsiella
  • Enterobacter
• Neisseria gonorrhoeae:
  • Sexually active young adults
  • Often with skin lesions
  • Migratory polyarthritis initially
  • Better prognosis than other causes

Less Common Organisms

• Mycobacteria:
  • M. tuberculosis
  • Atypical mycobacteria
  • Chronic presentation
• Fungi:
  • Candida species
  • Aspergillus
  • Cryptococcus
  • In immunocompromised hosts
• Anaerobes:
  • After penetrating trauma
  • Human or animal bites
  • Diabetic patients
• Viral (rare):
  • Parvovirus B19
  • Hepatitis B and C
  • HIV
  • Rubella

Age-Specific Organisms

• Neonates (0-3 months):
  • Group B Streptococcus
  • S. aureus
  • Gram-negative enterics
• Children (3 months-14 years):
  • S. aureus
  • S. pyogenes
  • S. pneumoniae
  • Kingella kingae (age 6 months-4 years)
• Adults:
  • S. aureus
  • Streptococci
  • N. gonorrhoeae (sexually active)
• Elderly:
  • S. aureus
  • Gram-negative bacteria
  • Streptococci

Risk Factors

Certain conditions and factors significantly increase the risk of developing septic arthritis:

Joint-Related Risk Factors

• Pre-existing joint disease:
  • Rheumatoid arthritis (major risk factor)
  • Osteoarthritis
  • Gout
  • Pseudogout
  • Previous joint surgery or trauma
• Prosthetic joints:
  • 1-2% lifetime risk of infection
  • Risk highest in first 2 years
  • Revision surgeries higher risk
• Recent joint procedures:
  • Intra-articular injections
  • Arthroscopy
  • Joint aspiration

Immunocompromising Conditions

• Diabetes mellitus:
  • Impaired immune function
  • Vascular compromise
  • Increased skin infections
• HIV/AIDS:
  • CD4 count <200 highest risk
  • Atypical organisms
  • Multiple joint involvement
• Malignancy:
  • Hematologic cancers
  • Chemotherapy
  • Neutropenia
• Immunosuppressive medications:
  • Corticosteroids
  • Biologics (TNF inhibitors)
  • Disease-modifying antirheumatic drugs
  • Transplant medications

Lifestyle and Social Factors

• Intravenous drug use:
  • Direct bacterial introduction
  • Often affects axial joints
  • Pseudomonas, S. aureus common
• Alcoholism:
  • Immunosuppression
  • Poor hygiene
  • Malnutrition
• Sexual activity:
  • Risk for gonococcal arthritis
  • Multiple partners
  • Unprotected sex

Medical Conditions

• Chronic diseases:
  • Chronic kidney disease
  • Liver cirrhosis
  • Sickle cell disease
  • Lupus
• Skin conditions:
  • Psoriasis
  • Eczema
  • Chronic ulcers
  • Recent cellulitis

Age-Related Risks

• Infants: Immature immune system
• Elderly (>65 years): Multiple comorbidities
• Very elderly (>80 years): Highest risk group

Diagnosis

Rapid and accurate diagnosis is crucial for septic arthritis. Joint aspiration (arthrocentesis) is the gold standard diagnostic procedure and should be performed emergently.

Clinical Assessment

• History:
  • Acute onset of symptoms
  • Recent infections or procedures
  • Risk factors assessment
  • Sexual history (if gonococcal suspected)
  • Travel history
• Physical examination:
  • Joint effusion, warmth, erythema
  • Range of motion (severely limited)
  • Position of comfort
  • Skin examination for entry sites
  • Full systemic examination

Joint Aspiration (Arthrocentesis)

• Urgent procedure:
  • Should be done before antibiotics if possible
  • Use ultrasound guidance if needed
  • Send fluid for multiple analyses
• Synovial fluid analysis:
  • Cell count and differential
  • Gram stain (positive in 50-75%)
  • Culture and sensitivity
  • Crystal analysis
  • Glucose and protein levels
• Findings in septic arthritis:
  • WBC count >50,000/mm³ (often >100,000)
  • >90% polymorphonuclear cells
  • Low glucose (<50% of serum)
  • Turbid or purulent appearance
  • Positive Gram stain or culture

Laboratory Tests

• Blood tests:
  • Complete blood count (leukocytosis)
  • ESR and CRP (markedly elevated)
  • Blood cultures (positive in 50%)
  • Procalcitonin
  • Liver and kidney function
• Special cultures:
  • Gonococcal (urethral, cervical, pharyngeal)
  • Mycobacterial (if suspected)
  • Fungal cultures
  • PCR for specific organisms

Imaging Studies

• Plain radiographs:
  • Usually normal early
  • Joint space widening
  • Soft tissue swelling
  • Late: erosions, joint destruction
• Ultrasound:
  • Detects effusion
  • Guides aspiration
  • Evaluates surrounding tissues
• MRI:
  • Most sensitive for early changes
  • Detects osteomyelitis
  • Evaluates extent of infection
  • Helpful for spine, hip, sacroiliac joints
• CT scan:
  • If MRI contraindicated
  • Guided drainage procedures
  • Evaluate for gas (rare)

Diagnostic Criteria

• Definitive diagnosis:
  • Positive synovial fluid culture
  • Positive Gram stain with compatible clinical picture
• Presumptive diagnosis:
  • Synovial WBC >50,000 with >90% PMNs
  • Clinical presentation consistent
  • Exclusion of crystal arthropathy

Differential Diagnosis

• Crystal arthropathies (gout, pseudogout)
• Reactive arthritis
• Rheumatoid arthritis flare
• Hemarthrosis
• Lyme arthritis
• Viral arthritis
• Avascular necrosis

Treatment Options

Treatment of septic arthritis requires immediate antibiotic therapy and drainage of infected joint fluid. Delay in treatment significantly increases morbidity and mortality.

Initial Management

• Immediate priorities:
  • Joint aspiration for diagnosis
  • Blood cultures before antibiotics
  • Start empiric antibiotics immediately after cultures
  • Pain control
  • Joint immobilization initially
• Hospital admission:
  • All cases require hospitalization
  • Often need orthopedic consultation
  • Infectious disease consultation helpful

Antibiotic Therapy

• Empiric therapy (adjust based on Gram stain/culture):
  • Gram-positive cocci or no organisms seen:
    • Vancomycin (for MRSA coverage)
    • OR Nafcillin/Oxacillin if low MRSA risk
    • Plus 3rd generation cephalosporin if immunocompromised
  • Gram-negative bacilli:
    • Ceftriaxone or cefotaxime
    • OR Fluoroquinolone
    • Consider antipseudomonal coverage
  • Gonococcal (suspected):
    • Ceftriaxone 1-2g IV daily
    • Plus azithromycin or doxycycline
• Duration of therapy:
  • IV antibiotics: 2-4 weeks typically
  • May switch to oral after clinical improvement
  • Total duration: 4-6 weeks
  • Longer for complicated cases

Joint Drainage

• Needle aspiration:
  • Daily or every other day initially
  • Continue until fluid clears
  • Monitor cell counts
  • Adequate for many cases
• Arthroscopic drainage:
  • Allows visualization and debridement
  • Irrigation of joint
  • Less invasive than open surgery
  • Good for knee, shoulder
• Open surgical drainage indicated for:
  • Hip joint infections
  • Failed needle drainage
  • Loculated fluid collections
  • Concurrent osteomyelitis
  • Prosthetic joint infections

Supportive Care

• Pain management:
  • NSAIDs if not contraindicated
  • Opioids for severe pain
  • Joint immobilization initially
• Physical therapy:
  • Start passive ROM early
  • Progress to active exercises
  • Prevent contractures
  • Maintain muscle strength
• Monitoring:
  • Daily joint examination
  • Serial inflammatory markers
  • Repeat aspiration if needed
  • Watch for complications

Special Situations

• Prosthetic joint infection:
  • Often requires removal of prosthesis
  • Two-stage revision typical
  • Antibiotic spacer placement
  • Prolonged antibiotics (3-6 months)
• Gonococcal arthritis:
  • Better prognosis
  • Shorter antibiotic course (7-14 days)
  • Test and treat partners
  • Screen for other STIs
• MRSA infection:
  • Vancomycin or daptomycin
  • Consider adding rifampin
  • May need longer treatment

Treatment Failure

• Consider if no improvement in 48-72 hours
• Reassess for undrained pus
• Review culture results and sensitivities
• Look for concurrent osteomyelitis
• Consider atypical organisms
• Evaluate for prosthetic loosening

Prevention

While not all cases of septic arthritis can be prevented, risk reduction strategies can significantly decrease incidence:

General Prevention

• Infection control:
  • Prompt treatment of skin infections
  • Good wound care
  • Dental hygiene
  • Treatment of UTIs
• Management of chronic conditions:
  • Optimal diabetes control
  • Rheumatoid arthritis management
  • Immunosuppression monitoring

Medical Procedure Precautions

• Joint injection guidelines:
  • Strict aseptic technique
  • Avoid through infected skin
  • Single-use vials
  • Proper site preparation
• Surgical prophylaxis:
  • Appropriate antibiotic timing
  • Proper surgical technique
  • Minimize operative time
  • Post-operative monitoring

High-Risk Groups

• Prosthetic joint patients:
  • Antibiotic prophylaxis for dental work (controversial)
  • Prompt treatment of any infection
  • Regular follow-up
• IV drug users:
  • Needle exchange programs
  • Education on safe practices
  • Addiction treatment referral
• Sexually active individuals:
  • Safe sex practices
  • Regular STI screening
  • Partner notification

Early Recognition

• Education about warning signs
• Prompt medical attention for joint symptoms
• Regular monitoring if high risk
• Clear instructions after joint procedures

When to See a Doctor

Septic arthritis is a medical emergency. Any suspicion of joint infection requires immediate evaluation.

Seek Emergency Care Immediately For:

• Classic triad:
  • Fever + joint pain + limited motion
  • Even one of these with risk factors
• Severe symptoms:
  • Inability to move or bear weight on joint
  • Severe joint pain with fever
  • Hot, swollen, red joint
  • Rapid onset (hours to days)
• Systemic signs:
  • High fever with chills
  • Confusion or altered mental status
  • Signs of sepsis
  • Multiple joints affected

High-Risk Situations Requiring Urgent Evaluation:

• Recent joint injection or surgery with new symptoms
• Prosthetic joint with new pain or fever
• Joint symptoms with recent infection elsewhere
• IV drug use with joint pain
• Immunocompromised with any joint symptoms
• Child refusing to use limb

Do Not Delay Because:

• Joint damage can occur within 48 hours
• Delayed treatment increases mortality
• Early treatment improves outcomes dramatically
• Can progress to life-threatening sepsis

What to Tell Healthcare Provider:

• Exact onset and progression of symptoms
• Any recent infections or procedures
• Current medications (especially immunosuppressants)
• Underlying medical conditions
• Recent travel or sexual activity
• IV drug use (be honest for proper treatment)

Related Conditions

References

1. Ross JJ. Septic Arthritis of Native Joints. Infectious Disease Clinics of North America. 2017;31(2):203-218.
2. Mathews CJ, et al. Bacterial septic arthritis in adults. Lancet. 2010;375(9717):846-855.
3. Coakley G, et al. BSR & BHPR, BOA, RCGP and BSAC guidelines for management of the hot swollen joint in adults. Rheumatology. 2006;45(8):1039-1041.
4. Sharff KA, et al. Clinical Management of Septic Arthritis. Current Rheumatology Reports. 2013;15(6):332.
5. Garcia-Arias M, et al. Septic arthritis. Best Practice & Research Clinical Rheumatology. 2011;25(3):407-421.
6. Margaretten ME, et al. Does this adult patient have septic arthritis? JAMA. 2007;297(13):1478-1488.