SEPTIC ARTHRITIS PATHOPHYSIOLOGY
Joint Infection | Cartilage Destruction | Surgical Emergency
KOCHER CRITERIA (PEDIATRIC HIP)
Critical Must-Knows
- Septic arthritis is a surgical emergency - irreversible cartilage damage begins within 6-12 hours
- Synovial fluid WCC over 50,000 with over 90% PMNs strongly suggests septic arthritis
- Native joint sepsis = single washout + antibiotics. Prosthetic joint = DAIR or two-stage revision
- Kocher criteria (fever, non-weight bearing, ESR over 40, WCC over 12) predict pediatric septic hip
- Australian eTG recommends flucloxacillin plus ceftriaxone for empiric therapy
Examiner's Pearls
- "Gonococcal arthritis is the most common in sexually active young adults
- "Lyme arthritis is monoarticular large joint - serology positive
- "Prosthetic joint infection under 3 months = acute, debride and retain (DAIR)
- "Kingella kingae is common in children under 4 years - difficult to culture
Critical Septic Arthritis Exam Points
Pathophysiology
Proteolytic enzymes destroy cartilage rapidly. Bacterial toxins and host PMN release collagenase, elastase, and metalloproteinases. Irreversible damage begins in 6-12 hours. This is why septic arthritis is a true orthopaedic emergency.
Diagnosis
Synovial fluid analysis is diagnostic. WCC over 50,000 with over 90% polymorphs suggests sepsis. Gram stain positive in 50%. Culture gold standard but negative in 20-30%. Always send for crystal analysis to exclude gout.
Kocher Criteria
Four criteria for pediatric septic hip: Fever over 38.5°C, Non-weight bearing, ESR over 40, WCC over 12. All 4 present = 99.6% probability. Use to decide urgency of surgical drainage.
Treatment
Surgical drainage plus antibiotics. Native joint = arthroscopic or open washout, repeat if not improving. Prosthetic joint = DAIR if acute (under 3 months), two-stage if chronic. Empiric antibiotics must cover S. aureus.
At a Glance
Septic arthritis is an orthopaedic emergency—irreversible cartilage destruction begins within 6-12 hours due to proteolytic enzymes from bacteria and host PMNs. S. aureus is the most common organism in adults; Kingella kingae in children under 4 years; Neisseria gonorrhoeae in sexually active young adults. Diagnosis requires synovial fluid WCC over 50,000 with over 90% PMNs; Gram stain is positive in only 50% of cases. The Kocher criteria (fever, non-weight bearing, ESR over 40, WCC over 12) predict pediatric septic hip with 99.6% probability when all four are present. Treatment is urgent surgical washout plus empiric IV antibiotics (Australian eTG: flucloxacillin + ceftriaxone).
ASKINGOrganisms by Age Group
Memory Hook:When ASKING about septic arthritis organisms, think of patient age and risk factors!
WGCSynovial Fluid Findings in Septic Arthritis
Memory Hook:When you Get Synovial Fluid, check WGC - White cells, Gram stain, Culture!
Overview and Introduction
Septic arthritis is a bacterial infection of a joint that represents a true orthopaedic emergency. The condition results in rapid destruction of articular cartilage through the combined action of bacterial toxins and host inflammatory mediators. Understanding the pathophysiology is essential for recognizing the urgency of treatment.
Key Time Concept:
- Irreversible cartilage damage begins within 6-12 hours of bacterial invasion
- Surgical drainage required within 24 hours to prevent permanent joint damage
- Delayed treatment leads to chronic pain, stiffness, and potential need for arthrodesis or arthroplasty
Mechanisms of Joint Infection
Biofilm Formation and Bacterial Virulence
Bacteria can form biofilms on cartilage and synovial surfaces:
- Glycocalyx matrix protects bacteria from antibiotics and host immune cells
- Quorum sensing allows bacterial communication and coordinated toxin production
- Persistent infection develops if biofilm establishes on damaged cartilage
Host Inflammatory Response
The host response paradoxically contributes to cartilage destruction:
- PMN infiltration - neutrophils release proteolytic enzymes
- Collagenase and metalloproteinases - degrade cartilage matrix
- Cytokine cascade - IL-1, TNF-alpha drive continued inflammation
- Elevated intra-articular pressure - compromises cartilage nutrition
Classification
Classification of Septic Arthritis
By Mechanism of Infection
- Hematogenous: Bacteremia seeding synovium (most common in native joints)
- Direct inoculation: Trauma, surgery, injection
- Contiguous spread: From adjacent osteomyelitis or soft tissue infection
By Joint Type
- Native joint: Higher cure rates with appropriate treatment
- Prosthetic joint: More complex, often requires implant removal
Clinical Classification
Gächter Classification (Arthroscopic)
| Stage | Findings | Treatment Implications |
|---|---|---|
| Stage I | Turbid fluid, hyperemic synovium | Single arthroscopic lavage often sufficient |
| Stage II | Purulent fluid, fibrin deposits | Arthroscopic lavage, may need repeat |
| Stage III | Thick fibrin, compartmentalization | May require open synovectomy |
| Stage IV | Cartilage destruction, osteolysis | Open debridement, poor prognosis |
Clinical Relevance and Diagnosis
Understanding the pathophysiology directly informs clinical management:
Why Surgical Drainage is Mandatory:
- Removes bacterial load and proteolytic enzymes from joint
- Reduces intra-articular pressure
- Prevents biofilm establishment on cartilage
- Allows direct visualization and debridement
Why Antibiotics Alone Fail:
- Antibiotics cannot penetrate established biofilm
- Do not remove proteolytic enzymes already released
- Cannot reduce mechanical effects of elevated pressure
- Delay allows irreversible cartilage damage
Pathophysiology of Septic Arthritis
Routes of Infection
Haematogenous Spread
Most common route - 80% of cases, Transient bacteremia seeds synovium, Synovium is highly vascular with no basement membrane, Bacteria easily penetrate into joint space, Risk factors: IV drug use, immunosuppression, diabetes
Direct Inoculation
Iatrogenic - joint injection, arthroscopy, arthroplasty, Trauma - penetrating injury, open fracture, Extension - adjacent osteomyelitis breaking into joint, Typically lower bacterial load initially, May present with delayed symptoms
Contiguous Spread
From adjacent osteomyelitis - especially pediatric, Hip, shoulder, elbow have intra-articular metaphysis, Metaphyseal infection ruptures into joint, Neonatal osteomyelitis commonly causes septic arthritis, Adult vertebral osteomyelitis can seed disc
Mechanism of Cartilage Destruction
Progressive Joint Destruction
Bacteria bind to synovium and proliferate. Synovial membrane has abundant blood supply but lacks basement membrane - bacteria easily penetrate. Rapid bacterial multiplication in nutrient-rich synovial fluid. Inflammatory response triggered.
Massive PMN infiltration and enzyme release. Neutrophils release proteolytic enzymes: collagenase, elastase, metalloproteinases. Bacterial toxins directly damage chondrocytes. Synovial inflammation increases intra-articular pressure. Irreversible cartilage damage begins.
Proteoglycan depletion and collagen degradation. Loss of cartilage matrix leads to chondrocyte death. Subchondral bone exposed in severe cases. Joint capsule distends with purulent effusion. Vascular compromise from elevated pressure.
Pannus formation and fibrous ankylosis. Granulation tissue (pannus) grows over remaining cartilage. Fibrous adhesions form. May progress to bony ankylosis. Growth plate damage in children. Chronic pain and stiffness.
Time is Cartilage
Irreversible articular cartilage damage begins within 6-12 hours of bacterial invasion. This is why septic arthritis is classified as an orthopaedic emergency requiring urgent surgical drainage. Delays beyond 24 hours significantly worsen outcomes - permanent joint damage, chronic pain, and need for arthrodesis or arthroplasty.
Microbiology
Organism by Patient Population
| Age/Risk Group | Most Common Organism | Second Most Common | Key Clinical Feature |
|---|---|---|---|
| Neonate (under 3 months) | Group B Streptococcus | E. coli, S. aureus | Can spread from osteomyelitis via transphyseal vessels |
| Child (3 months to 4 years) | Kingella kingae | S. aureus | Difficult to culture - fastidious organism |
| Child (over 4 years) | Staphylococcus aureus | Streptococcus pyogenes | 70% of pediatric septic arthritis |
| Sexually active adult | Neisseria gonorrhoeae | S. aureus | Polyarticular, migratory arthritis, skin lesions |
| Adult (general) | Staphylococcus aureus | Streptococcus species | 60-70% of adult septic arthritis |
| IV drug user | S. aureus, Pseudomonas | Candida species | Unusual joints: sternoclavicular, sacroiliac |
| Immunocompromised | Gram-negative rods | Fungi, atypical mycobacteria | Indolent course, multiple joints |
| Prosthetic joint (under 3 months) | S. aureus | Coagulase-negative Staph | Acute presentation - DAIR candidate |
| Prosthetic joint (over 3 months) | Coagulase-negative Staph | Propionibacterium | Low virulence biofilm - two-stage revision |
Kocher Criteria for Pediatric Septic Hip
The Four Kocher Criteria
1. Fever
Temperature over 38.5°C (101.3°F)
- May be absent in immunocompromised
- Low-grade fever less predictive
- Documented fever most reliable
2. Non-Weight Bearing
Refusal to bear weight on affected limb
- Most specific clinical sign
- Also present in transient synovitis
- Inability to weight bear worse than limp
3. ESR over 40
Erythrocyte sedimentation rate over 40 mm/hr
- Less specific than CRP
- Slower to rise and normalize
- CRP over 20 mg/L is alternative cutoff
4. WCC over 12
White cell count over 12,000 per microliter
- Systemic inflammatory response
- May be normal in early infection
- Less reliable than synovial fluid WCC
Kocher Criteria Probability Table
| Number of Criteria | Probability of Septic Arthritis | Recommendation |
|---|---|---|
| 0-1 criteria | Less than 3% | Observe, serial examination |
| 2 criteria | 40% | Urgent aspiration and synovial fluid analysis |
| 3 criteria | 93% | Strong indication for surgical drainage |
| 4 criteria | 99.6% | Emergency surgical washout required |
Kocher Limitations
The Kocher criteria were derived for pediatric hip septic arthritis to distinguish from transient synovitis. They should NOT be used to diagnose septic arthritis in adults, other joints, or to exclude infection in high-risk patients. A child with 0-1 criteria still has 3% risk - clinical judgment remains essential.
Diagnosis
Synovial Fluid Analysis
Synovial Fluid Characteristics
| Parameter | Normal | Inflammatory (Gout) | Septic Arthritis | Clinical Note |
|---|---|---|---|---|
| WCC (per microL) | Under 200 | 2,000-50,000 | Usually over 50,000 | Overlap exists - over 90% PMN more specific |
| PMN percentage | Under 25% | 50-75% | Over 90% | Over 90% polymorphs highly specific for sepsis |
| Gram stain | Negative | Negative | Positive in 50% | Immediate result - guides empiric therapy |
| Culture | Sterile | Sterile | Positive in 70-80% | Gold standard - obtain before antibiotics |
| Glucose | Similar to blood | Normal | Less than half blood glucose | Bacteria consume glucose |
| Crystals | None | MSU or CPPD positive | None (may coexist) | Always check - gout can be septic too |
Septic Arthritis Can Coexist with Crystal Arthropathy
Finding crystals on synovial fluid analysis does NOT exclude septic arthritis. Gout and pseudogout can precipitate septic arthritis or coexist. If WCC is over 50,000 or clinical suspicion high, treat as septic arthritis regardless of crystal presence. Send culture and start antibiotics.
Imaging
Plain Radiographs
First-line investigation:
- Joint space widening from effusion
- Soft tissue swelling
- Loss of fat planes
- Exclude fracture or foreign body
- Chronic: joint space narrowing, erosions
- Cannot rule out sepsis on XR alone
Ultrasound
Useful for detecting effusion:
- Hip effusion in children
- Guides aspiration
- Differentiates effusion from synovitis
- Cannot distinguish septic from aseptic
- Operator-dependent
Investigations
Laboratory Investigations
Joint Aspirate (Most Important)
- WCC: Greater than 50,000/μL highly suggestive (greater than 25,000/μL suspicious)
- Gram stain: Positive in 50-70% of cases
- Culture: Gold standard, positive in 85-95%
- Crystal analysis: Rule out gout/pseudogout (can coexist)
Blood Tests
- WCC: Often elevated (may be normal in immunocompromised)
- CRP: Elevated (useful for monitoring response)
- ESR: Elevated but slower to change than CRP
- Blood cultures: Positive in 30-50%, always obtain if febrile
Kocher Criteria (Paediatric Hip)
- Fever greater than 38.5°C
- Non-weight bearing
- WCC greater than 12,000/μL
- ESR greater than 40mm/hr
- 4/4 criteria = 99% probability of septic arthritis
Management
Native Joint Septic Arthritis Management
Treatment Algorithm
Urgent joint aspiration before antibiotics. Send for WCC, Gram stain, culture (aerobic, anaerobic, fungal). Document gross appearance (purulent, cloudy, bloody). Check for crystals. Blood cultures if febrile.
Start antibiotics after cultures obtained. Australian eTG: flucloxacillin 2g IV 6-hourly PLUS ceftriaxone 2g IV daily. Covers S. aureus, Streptococcus, and Gram-negatives. Adjust when cultures available.
Arthroscopic or open washout within 24 hours. Copious irrigation (minimum 9 liters). Debride fibrin, synovial debris. Take multiple tissue samples. Hip requires open arthrotomy. Small joints may respond to repeat aspiration.
Repeat washout if not improving. Persistent fever, rising CRP, persistent joint swelling indicates inadequate drainage. Up to 3 washouts may be needed. Consider open if arthroscopy failing.
Total 2-6 weeks antibiotics. IV for 2 weeks, then oral if improving. Monitor CRP weekly - should fall by 50% at 2 weeks. Small joints 2 weeks, large joints 4 weeks, hip 6 weeks.
Hip Septic Arthritis Requires Open Arthrotomy
The hip joint is too deep for effective arthroscopic drainage in septic arthritis. Open anterior (Smith-Petersen) or anterolateral (Watson-Jones) approach allows complete debridement of fibrinous material and copious irrigation. Leaving a drain is controversial - most surgeons do not use drain to avoid retrograde infection.
Surgical Technique
Surgical Drainage Techniques
Arthroscopic Lavage (Preferred for Most Joints)
- Standard arthroscopic portals for the affected joint
- Thorough inspection and documentation of cartilage status
- Copious irrigation: Minimum 9-12 liters saline
- Debridement of fibrinous debris and synovitis
- Take multiple tissue samples for culture
Open Arthrotomy (Required for Hip, Some Failures)
- Standard surgical approach (anterior for hip)
- Complete capsulotomy for drainage
- Copious irrigation and debridement
- Leave drain controversial - most avoid (retrograde infection risk)
- Closure in layers
Repeat Aspiration (Small Joints)
- Serial daily aspirations may be adequate for fingers/toes
- Must demonstrate falling WCC and sterile cultures
- Convert to surgical drainage if not improving
Complications
Complications of Septic Arthritis
Cartilage Destruction
- Occurs within 48-72 hours of untreated infection
- Enzymatic degradation from bacterial toxins and host proteases
- Irreversible once established - leads to arthritis
- Prevention: Early diagnosis and aggressive treatment
Systemic Complications
- Sepsis and septic shock (5-10%)
- Multi-organ dysfunction
- Death (mortality 5-15% in adults, higher in elderly)
- Metastatic infection (other joints, endocarditis)
Joint-Specific Complications
- Hip in children: Avascular necrosis of femoral head
- Knee: Stiffness, secondary osteoarthritis
- Shoulder: Rotator cuff damage, stiffness
Postoperative Care
Postoperative Management
Antibiotic Therapy
- Continue IV antibiotics minimum 2 weeks
- Transition to oral when clinically improving and CRP falling
- Total duration: 2-6 weeks depending on joint and response
- Culture-directed therapy once sensitivities available
Monitoring
- Daily clinical assessment (pain, swelling, ROM)
- Temperature monitoring
- CRP every 2-3 days initially, then weekly
- Expect CRP to fall by 50% at 2 weeks
Rehabilitation
- Gentle passive ROM as soon as tolerated
- Protected weight-bearing until infection controlled
- Progressive strengthening after infection resolution
Outcomes
Outcomes of Septic Arthritis
Overall Prognosis
- Mortality: 5-15% in adults (higher in elderly, comorbidities)
- Joint destruction: 40-50% develop secondary osteoarthritis
- Full recovery: 50-60% with early aggressive treatment
Factors Affecting Outcome
Prognostic Factors
| Factor | Better Outcome | Worse Outcome |
|---|---|---|
| Time to treatment | Under 24 hours | Over 72 hours |
| Patient age | Young, healthy | Elderly, comorbid |
| Organism | Streptococcus | S. aureus, Gram-negative |
| Joint involvement | Small joint | Hip, shoulder |
Evidence Base
Kocher Criteria Validation Study
- Prospective study of 282 children with hip effusion
- Four independent predictors: fever over 38.5C, non-weight bearing, ESR over 40, WCC over 12
- Probability of septic arthritis: 4 criteria = 99.6%, 3 criteria = 93%, 2 criteria = 40%, 0-1 criteria = under 3%
- Combined criteria more accurate than any single test
Synovial Fluid WCC Thresholds
- Systematic review and meta-analysis of synovial fluid WCC for septic arthritis diagnosis
- WCC over 50,000 has sensitivity 62%, specificity 92%
- Over 90% polymorphs more specific than absolute WCC
- No single cutoff perfectly distinguishes septic from inflammatory arthritis
Timing of Cartilage Destruction in Septic Arthritis
- Animal model demonstrating time-dependent cartilage destruction
- Irreversible damage begins within 6-12 hours of bacterial inoculation
- PMN enzymes (collagenase, elastase) primary mediators of destruction
- Early drainage significantly improves cartilage preservation
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Pediatric Septic Hip Diagnosis
"A 3-year-old boy presents with 2 days of right hip pain and refusal to walk. Temperature 39.2°C. Held in flexion, abduction, external rotation. Blood tests: WCC 14,000, CRP 65, ESR 48. Ultrasound shows hip joint effusion. How would you assess and manage this child?"
Scenario 2: Prosthetic Joint Infection DAIR Decision
"A 68-year-old woman presents 6 weeks after primary total knee replacement with 1 week of increasing pain, swelling, and wound drainage. Temperature 38.1°C. Knee warm, effusion present, small amount of serous drainage from wound. CRP 145. The implant appears well-fixed on plain radiographs. Is she a candidate for DAIR or does she need two-stage revision?"
MCQ Practice Points
Synovial WCC Threshold Question
Q: What synovial fluid WCC threshold suggests septic arthritis? A: Over 50,000 cells per microliter with over 90% polymorphs - this combination has 92% specificity. However, overlap exists with crystal arthropathy - gout can have WCC over 50,000.
Most Common Organism Adult Question
Q: What is the most common organism causing septic arthritis in adults? A: Staphylococcus aureus - causes 60-70% of adult septic arthritis. Exception: sexually active young adults where Neisseria gonorrhoeae is most common.
Kocher 4 Criteria Probability Question
Q: A child has all 4 Kocher criteria present. What is the probability of septic arthritis? A: 99.6% probability - this mandates emergency surgical washout. The 4 criteria are: fever over 38.5C, non-weight bearing, ESR over 40, WCC over 12.
DAIR Window Question
Q: What is the time window for considering DAIR in prosthetic joint infection? A: Under 3 months from surgery OR acute hematogenous infection with symptoms under 3 weeks - beyond this window, biofilm is established and two-stage revision is required.
Australian Context
eTG Antibiotic Guidelines
Therapeutic Guidelines: Antibiotic (eTG version 17, 2024):
- Empiric therapy: Flucloxacillin 2g IV 6-hourly PLUS ceftriaxone 2g IV daily
- Covers S. aureus, Streptococcus, Gram-negatives
- If suspected gonococcal: ceftriaxone alone
- Adjust to organism-specific therapy when culture available
- Duration: 2-6 weeks depending on joint and response
ACSQHC Sepsis Recognition
Australian Commission on Safety and Quality in Health Care:
- Septic arthritis is time-critical infection
- Immediate aspiration and culture mandatory
- Antibiotics within 1 hour of recognition
- Surgical drainage within 24 hours
- Document time from presentation to aspiration
SEPTIC ARTHRITIS PATHOPHYSIOLOGY
High-Yield Exam Summary
Key Pathophysiology
- •Irreversible cartilage damage begins 6-12 hours
- •PMN enzymes (collagenase, elastase) destroy matrix
- •Elevated intra-articular pressure compromises blood supply
- •Pannus formation leads to fibrous ankylosis
Kocher Criteria (Pediatric Hip)
- •Fever over 38.5C, Non-weight bearing, ESR over 40, WCC over 12
- •4/4 criteria = 99.6% probability septic arthritis
- •3/4 criteria = 93% probability
- •2/4 criteria = 40% probability - urgent aspiration
Synovial Fluid Analysis
- •WCC over 50,000 with over 90% PMN suggests sepsis
- •Gram stain positive in 50% - immediate result
- •Culture gold standard - obtain before antibiotics
- •Crystals can coexist with sepsis - do not exclude
Microbiology
- •Adult: S. aureus (60-70%), N. gonorrhoeae in sexually active
- •Child: S. aureus, Kingella kingae (under 4 years)
- •Neonate: Group B Strep, E. coli, S. aureus
- •IV drug user: S. aureus, Pseudomonas, Candida
Treatment
- •Native joint: washout within 24h + 2-6 weeks antibiotics
- •Hip requires open arthrotomy (too deep for arthroscopy)
- •PJI acute: DAIR if under 3 months, well-fixed, under 3 weeks symptoms
- •PJI chronic: Two-stage revision (90% success)
- •eTG empiric: Flucloxacillin + ceftriaxone