Septic Arthritis (Paediatric)

1. Clinical Overview

Summary

Septic arthritis in children is an orthopaedic emergency requiring immediate diagnosis and surgical intervention. It represents bacterial infection of the joint space that results in rapid, irreversible destruction of articular cartilage through proteolytic enzyme activity. The hip joint is most commonly affected (40-50% of cases), followed by the knee (20-30%), ankle, shoulder, and elbow. [1,2]

The fundamental clinical challenge lies in distinguishing septic arthritis from the far more common Transient Synovitis (Irritable Hip), which is self-limiting and benign. The Kocher Criteria (fever > 38.5°C, non-weight bearing, WBC > 12,000, ESR > 40 mm/hr) represent the validated gold standard for risk stratification, with predictive probabilities ranging from 3% (1 criterion) to 99.6% (4 criteria). [3,4]

Staphylococcus aureus remains the predominant pathogen across all age groups, accounting for 50-70% of culture-positive cases. However, Kingella kingae has emerged as an increasingly recognized pathogen in children under 4 years, often responsible for culture-negative cases when conventional culture methods are employed. [5,6]

Time-Critical Pathophysiology

"Time is Cartilage" - The bacterially mediated inflammatory cascade produces proteolytic enzymes (matrix metalloproteinases, serine proteases) from both bacteria and recruited neutrophils that digest type II collagen and proteoglycans within articular cartilage. Irreversible chondrolysis begins within 8-12 hours of symptom onset. [7,8]

Simultaneously, accumulating purulent fluid increases intra-articular pressure, compressing retinacular vessels and compromising blood supply to the femoral head (in hip septic arthritis), creating risk of avascular necrosis (AVN). Delayed surgical washout beyond 24 hours significantly increases complication rates including permanent cartilage damage, growth arrest, and osteonecrosis. [9,10]

Key Clinical Facts

Neonatal Presentation Differs Neonates (0-3 months) demonstrate attenuated systemic responses. Fever is often absent or low-grade, and inflammatory markers may be minimally elevated. The hallmark sign is "pseudoparalysis" - reluctance or refusal to move the affected limb, or crying with passive movement during nappy changes. This age group requires heightened clinical suspicion. [11]

Kingella kingae: The Elusive Pathogen K. kingae is a fastidious, slow-growing organism that colonizes the oropharynx of young children. It is frequently missed on standard culture media but readily detected using PCR or inoculation into blood culture bottles. Studies using PCR detection have shown K. kingae accounts for 30-50% of septic arthritis cases in children under 4 years. [5,6]

The Intracapsular Metaphysis The proximal femur metaphysis is intracapsular (within the hip joint capsule). Therefore, osteomyelitis of the proximal femoral metaphysis can rupture directly into the hip joint, causing concurrent septic arthritis. This unique anatomical relationship mandates careful imaging evaluation to identify both osteomyelitis and arthritis. [12]

Clinical Pearls

"Don't let the sun set on a septic hip": Any child with suspected septic arthritis requires same-day evaluation, joint aspiration if indicated, and surgical washout if confirmed. Delays beyond 24 hours significantly worsen outcomes.

"Position of Comfort - FABER": A child with septic hip holds the affected limb in Flexion, ABduction, and External Rotation. This position maximizes joint capsule volume, accommodating purulent effusion and minimizing capsular pressure and pain.

"Antibiotics AFTER Cultures": Antibiotic administration before obtaining blood cultures and synovial fluid specimens significantly reduces microbiological yield. Unless the child is septic/hemodynamically unstable, delay antibiotics until after culture specimens are obtained.

"Log Roll Test": Gentle passive internal and external rotation of the hip with the knee flexed (log-rolling the limb) elicits severe pain in septic arthritis but is relatively well-tolerated in transient synovitis. This is a highly sensitive bedside examination technique.

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2. Epidemiology

Incidence and Demographics

Incidence The annual incidence of pediatric septic arthritis ranges from 4-10 cases per 100,000 children, with significant geographic variation. Higher rates are observed in developing nations and regions with limited access to vaccination programs (particularly for Haemophilus influenzae type b and Streptococcus pneumoniae). [1,2]

Age Distribution

• Peak incidence: less than 3 years of age (50-60% of cases)
• Secondary peak: Adolescence (10-15 years)
• Neonatal cases: 5-10% of total
• Kingella kingae specifically: 6 months - 4 years (peak 18-24 months) [5,6]

Sex Distribution Male-to-female ratio: 2:1 across all age groups. The male preponderance is unexplained but consistently observed across multiple population studies. [1,13]

Joint Distribution

Hip: 40-50% (highest morbidity due to AVN risk and intracapsular anatomy) Knee: 20-30% (easier to diagnose, visualize, and aspirate) Ankle: 10-15% Shoulder: 5-10% Elbow: 5-10% Small joints (wrist, hand, foot): less than 5%

Multiple joint involvement occurs in 5-10% of cases, more common in neonates and immunocompromised children. [1,14]

Risk Factors

Host Factors

• Age less than 3 years: Developmental hypervascular synovium increases hematogenous seeding risk
• Male sex: 2:1 male predominance
• Immunocompromise: Chemotherapy, congenital immunodeficiency, HIV, chronic steroid therapy
• Pre-existing joint disease: Juvenile idiopathic arthritis (JIA), hemophilic arthropathy, prosthetic joints
• Sickle cell disease: Increased risk of both septic arthritis and osteomyelitis (Salmonella spp. more common) [15]

Precipitating Events

• Recent bacteremia: Upper respiratory infection (30-40%), otitis media (15-20%), pharyngitis (10-15%)
• Blunt trauma: Hematoma provides culture medium for bacterial seeding
• Recent invasive procedure: Joint aspiration, intra-articular injection, surgery
• Skin infection: Cellulitis, impetigo, abscess [13,16]

Geographic and Seasonal Variation

Kingella kingae demonstrates seasonal variation, with peak incidence in autumn/winter months, coinciding with oropharyngeal colonization and viral upper respiratory infections that facilitate mucosal bacterial invasion. [5]

In regions with high HIV prevalence, atypical organisms including Mycobacterium tuberculosis and fungal pathogens (Candida spp.) must be considered in immunocompromised children. [17]

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3. Pathophysiology

Routes of Infection

1. Hematogenous Spread (85-90%) This is the predominant route in children. Transient bacteremia (from URI, otitis media, pharyngitis, or occult source) results in bacterial seeding of the highly vascular synovial membrane. The synovium in young children contains a rich network of capillaries that lack a basement membrane, facilitating bacterial extravasation from blood into synovial fluid. [7,18]

2. Direct Extension from Adjacent Osteomyelitis (10-15%) The proximal femur metaphysis is intracapsular, meaning it lies within the hip joint capsule. Osteomyelitis of the proximal femoral metaphysis can rupture through cortical bone directly into the joint space, causing concurrent septic arthritis. This dual pathology requires both prolonged antibiotic therapy (for osteomyelitis) and surgical washout (for arthritis). [12]

Similarly, the proximal humerus, proximal radius, and distal lateral femur (in infants) have intracapsular metaphyses, creating risk of this complication.

3. Direct Inoculation (less than 5%) Penetrating trauma, iatrogenic joint aspiration, intra-articular injections, and arthroscopy introduce bacteria directly into the joint space. These cases often involve skin flora (S. aureus, Staphylococcus epidermidis). [18]

The Destructive Cascade

Phase 1: Bacterial Colonization and Proliferation (0-6 hours) Bacteria adhere to synovial membrane via surface adhesins and form biofilm. Bacterial replication within nutrient-rich synovial fluid is rapid, with doubling times of 20-30 minutes for S. aureus. Synoviocytes recognize bacterial PAMPs (pathogen-associated molecular patterns) via Toll-like receptors (TLRs), triggering inflammatory cytokine release (IL-1β, IL-6, TNF-α). [7,19]

Phase 2: Neutrophil Influx (6-12 hours) Cytokine and chemokine gradients recruit massive neutrophil infiltration from synovial capillaries. Synovial fluid white cell count rises exponentially, typically exceeding 50,000 cells/μL (> 75% polymorphonuclear cells). Neutrophil degranulation releases:

• Matrix metalloproteinases (MMPs): MMP-1, MMP-3, MMP-13 degrade type II collagen
• Serine proteases: Elastase, cathepsin G digest proteoglycans
• Reactive oxygen species (ROS): Cause oxidative cartilage damage [7,8,19]

Phase 3: Chondrolysis (8-24 hours) Proteolytic enzymes digest the extracellular matrix of articular cartilage, causing irreversible loss of proteoglycans and collagen fibrils. Cartilage has no intrinsic capacity for regeneration. Chondrocyte apoptosis is triggered by inflammatory cytokines and nutrient deprivation. Cartilage damage is proportional to duration of infection and bacterial load. [8,20]

Phase 4: Elevated Intra-articular Pressure (12-48 hours) Accumulation of purulent fluid, fibrin deposition, and synovial hypertrophy increase intra-articular pressure. In the hip joint, elevated pressure compresses retinacular vessels (medial and lateral femoral circumflex arteries) that supply the femoral head epiphysis. Vascular compromise causes avascular necrosis (AVN) of the femoral head, resulting in collapse and permanent deformity. [9,10]

Phase 5: Chronic Sequelae (Weeks to Months)

• Physeal damage: Infection crossing the growth plate causes growth arrest, leg length discrepancy, angular deformity
• Fibrous ankylosis: Fibrous tissue bridges joint space, causing stiffness
• Bony ankylosis: Ossification across joint space causes complete fusion
• Post-infectious arthritis: Persistent inflammatory arthritis (reactive/autoimmune) [10,21]

Microbiology by Age and Scenario

MRSA (Methicillin-Resistant S. aureus) Community-acquired MRSA (CA-MRSA) now accounts for 20-40% of S. aureus septic arthritis in some regions. MRSA septic arthritis has higher complication rates, longer hospital stays, and increased need for repeat surgical procedures. Empiric antibiotic regimens must account for local MRSA prevalence. [22,23]

ExamDetail: Molecular Mechanisms of Cartilage Destruction

Exam Detail: Bacterial Virulence Factors S. aureus produces multiple virulence factors that enhance joint destruction:

• Protein A: Binds IgG Fc regions, evading opsonization
• α-toxin: Pore-forming toxin causing cell lysis
• Panton-Valentine leukocidin (PVL): Present in CA-MRSA, causes leukocyte lysis and tissue necrosis
• Coagulase: Converts fibrinogen to fibrin, creating protective biofilm
• Hyaluronidase: Degrades hyaluronic acid in synovial fluid

Host Inflammatory Response Synoviocytes and chondrocytes express TLR-2 and TLR-4, recognizing bacterial lipoteichoic acid and peptidoglycan. TLR activation triggers NF-κB signaling, inducing:

• IL-1β: Primary driver of MMP synthesis by synoviocytes and chondrocytes
• TNF-α: Synergizes with IL-1β to amplify MMP production
• IL-6: Drives acute phase response, CRP elevation
• IL-8: Primary neutrophil chemoattractant

Matrix Metalloproteinase (MMP) Activity

• MMP-1 (Collagenase-1): Cleaves type II collagen triple helix
• MMP-3 (Stromelysin-1): Degrades proteoglycans, activates other MMPs
• MMP-13 (Collagenase-3): Most potent cartilage-degrading MMP
• ADAMTS-4/5: Aggrecanases specifically cleaving aggrecan (major proteoglycan)

Cartilage has no capacity for regeneration once matrix is degraded. Tissue inhibitors of metalloproteinases (TIMPs) are overwhelmed by the magnitude of MMP production.

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4. Clinical Presentation

Cardinal Symptoms

1. Pain Severe, constant pain localized to the affected joint. The child cries with any attempted movement (active or passive). Pain is exacerbated by weight-bearing, joint motion, or pressure over the joint. In neonates, pain manifests as irritability, crying with nappy changes, or feeding difficulties. [1,13]

2. Loss of Function

• Non-weight bearing (NWB): Child refuses to stand, walk, or bear weight on the affected limb
• Limp: Antalgic gait (shortened stance phase on affected side) if child attempts walking
• Pseudoparalysis: Complete refusal to move the limb (neonates, young infants) [11]

3. Fever High-grade fever > 38.5°C (> 101.3°F) is present in 60-80% of children with septic arthritis. Fever may be absent or low-grade in neonates, immunocompromised children, or Kingella kingae infections (which tend to have milder systemic manifestations). [3,5]

4. Systemic Features

• Malaise: Child appears unwell, lethargic, refuses to play
• Irritability: Inconsolable crying, particularly with movement
• Feeding difficulties: Reduced oral intake in infants
• "Toxic" appearance: Tachycardia, tachypnea, poor perfusion (suggests sepsis) [1,13]

Physical Examination Findings

Inspection

• Positioning: Classic FABER position (Flexion, ABduction, External Rotation) for hip septic arthritis. This position maximizes intracapsular volume, reducing pressure and pain
• Swelling: Visible joint effusion (knee, ankle, elbow). Hip effusion is not visible but causes fullness in groin
• Erythema: Overlying skin erythema suggests superficial extension or concurrent cellulitis
• Muscle wasting: Rapid thigh muscle atrophy (24-48 hours) in hip septic arthritis due to pain-induced disuse [1]

Palpation

• Warmth: Increased skin temperature over affected joint
• Tenderness: Exquisite tenderness to palpation over joint line
• Effusion: Palpable fluid thrill or ballotable patella (knee), fluctuance (ankle, elbow)

Range of Motion

• Passive ROM: Severely painful in all planes. Child resists all attempted movement
• Active ROM: Refused or severely limited
• Log Roll Test: Gentle passive internal and external rotation of the hip (with knee flexed) causes severe pain. This is the most sensitive single test for hip pathology (both septic arthritis and transient synovitis cause positive log roll, but severity differs) [24]

Special Tests

Age-Specific Presentations

Neonates (0-3 months)

• Pseudoparalysis: Not moving the affected limb ("floppy limb")
• Crying with nappy changes: Hip movement during diaper changes elicits severe pain
• Fever often absent: 30-40% of neonatal septic arthritis is afebrile
• Multiple joints: 30% have polyarticular involvement
• Septic appearance: Higher risk of concurrent bacteremia, sepsis [11,25]

Infants and Toddlers (3 months - 4 years)

• Refusal to walk/crawl: Primary presenting complaint
• Limp: If child attempts ambulation
• Fever common: 70-80% have fever > 38.5°C
• Kingella kingae infections: Milder systemic features, lower inflammatory markers, history of recent URI [5,6]

School-Age Children (4-12 years)

• Classic presentation: Fever, NWB, severe pain, elevated inflammatory markers
• Kocher criteria most validated in this age group [3,4]
• History of recent infection: URI, otitis media, pharyngitis in 40-50%

Adolescents (> 12 years)

• Sexual history essential: Consider Neisseria gonorrhoeae in sexually active adolescents
• Disseminated gonococcal infection (DGI): Triad of tenosynovitis, dermatitis (pustular rash), migratory polyarthralgia
• IVDU consideration: In at-risk populations, consider Pseudomonas aeruginosa, Candida spp., polymicrobial infection [26]

Clinical Pearls: Red Flags

Clinical Pearl: Neonate + Pseudoparalysis = Septic Arthritis Until Proven Otherwise Neonates do not mount robust inflammatory responses. Absence of fever or normal WBC does NOT exclude septic arthritis. If a neonate is not moving a limb, assume sepsis and investigate urgently.

FABER Position = Septic Hip A child lying with the affected hip flexed, abducted, and externally rotated almost certainly has hip pathology (septic arthritis, transient synovitis, or osteomyelitis). This position is pathognomonic.

Pain on Log Roll Test = Hip Pathology Gentle passive rotation of the hip (log-rolling the limb) causes severe pain in hip pathology but is generally well-tolerated in referred pain (e.g., appendicitis, psoas abscess). This test helps localize pathology to the hip joint.

Toxic Appearance + Joint Pain = Sepsis Any child with systemic toxicity (tachycardia, poor perfusion, altered mental status) and joint pain requires immediate sepsis workup, broad-spectrum IV antibiotics, and urgent surgical consultation. Do not delay for investigations.

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5. Differential Diagnosis

The critical clinical distinction is between septic arthritis (surgical emergency) and transient synovitis (self-limiting, benign). However, multiple other conditions can mimic septic arthritis.

Primary Differential: Transient Synovitis (Irritable Hip)

Transient Synovitis (TS) is a post-viral, self-limiting inflammatory synovitis, most commonly affecting the hip. It is 50 times more common than septic arthritis but clinically overlapping. [27]

Kocher Criteria are specifically designed to differentiate these entities (see Section 6).

Other Important Differentials

ExamDetail: Polyarticular Presentations

Exam Detail: When a child presents with multiple painful joints, the differential diagnosis shifts:

Infectious Causes

• Septic arthritis (multiple joints): Neonates (30%), immunocompromised, disseminated infection
• Disseminated gonococcal infection (DGI): Sexually active adolescents, triad of tenosynovitis, dermatitis, migratory polyarthralgia
• Lyme disease: Oligoarticular (2-4 joints), knee predominance
• Viral arthritis: Parvovirus B19, Rubella, Hepatitis B/C, EBV - self-limiting

Inflammatory Causes

• Juvenile Idiopathic Arthritis (JIA): Polyarticular subtype (≥5 joints), morning stiffness
• Acute Rheumatic Fever: Migratory polyarthritis, carditis, chorea
• Reactive arthritis: Post-infectious (GI/GU), HLA-B27, "can't see, can't pee, can't climb a tree"
• Henoch-Schönlein Purpura (HSP): Palpable purpura, abdominal pain, arthralgia/arthritis
• Systemic Lupus Erythematosus (SLE): Malar rash, photosensitivity, renal/hematologic involvement

Key Principle: Polyarticular septic arthritis is rare (except neonates and DGI). If multiple joints are involved, broaden differential to include systemic inflammatory and rheumatologic conditions.

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6. Diagnostic Approach: The Kocher Criteria

Kocher Criteria (1999) - Original

Kocher et al. conducted a prospective study of 282 children with acute hip pain, identifying 4 independent predictors of septic arthritis. [3]

The 4 Kocher Criteria:

1. Fever > 38.5°C (> 101.3°F)
2. Non-weight bearing (refusal to bear weight on affected limb)
3. ESR > 40 mm/hr
4. WBC > 12,000 cells/μL

Predicted Probability of Septic Arthritis:

• 0 criteria: less than 0.2% (septic arthritis extremely unlikely)
• 1 criterion: 3%
• 2 criteria: 40%
• 3 criteria: 93%
• 4 criteria: 99.6%

Clinical Application:

• 0-1 criteria: Low probability. Consider observation, reassess in 12-24 hours
• 2 criteria: Intermediate probability (40%). Perform joint aspiration
• 3-4 criteria: High probability (> 90%). Urgent joint aspiration and prepare for surgical washout [3]

Caird Criteria (2006) - Modified Kocher

Caird et al. validated and modified the Kocher criteria in 163 children, adding CRP as a 5th predictor. [4]

The 5 Caird Criteria:

1. Fever > 38.5°C
2. Non-weight bearing
3. CRP > 2.0 mg/dL (> 20 mg/L)
4. ESR > 40 mm/hr
5. WBC > 12,000 cells/μL

Key Findings:

• CRP > 2.0 mg/dL was the strongest single predictor (sensitivity 91%, specificity 61%)
• Combining CRP > 2.0 mg/dL + ESR > 40 mm/hr + Fever yielded 98% sensitivity
• Negative predictive value: If CRP less than 2.0 mg/dL, septic arthritis extremely unlikely [4]

Modern Practice: Most centers now use CRP as the primary inflammatory marker (faster turnaround than ESR, more sensitive, reflects acute inflammation better). ESR is still obtained but CRP drives decision-making.

Interpretation and Limitations

Strengths:

• Validated in multiple studies across different populations
• High sensitivity and negative predictive value (NPV > 95% for 0-1 criteria)
• Simple, uses readily available clinical and laboratory data

Limitations:

• Developed and validated for hip septic arthritis specifically. Extrapolation to other joints less well-validated
• Lower sensitivity in neonates (who may not mount fever or WBC elevation)
• Kingella kingae infections often have lower inflammatory markers (fever less than 38.5°C, CRP less than 20 mg/L), leading to underestimation of risk [5,6]
• Transient synovitis can occasionally have 2 criteria (40% probability), leading to aspiration of benign effusions
• Does NOT replace clinical judgment - "toxic" appearing child with 1-2 criteria still warrants aggressive investigation [28]

Modified Approach for Neonates

Neonatal Kocher Criteria are unreliable. Neonates often have:

• Afebrile or low-grade fever (50-60%)
• Normal or minimally elevated WBC (30-40%)
• Mildly elevated CRP/ESR

Neonatal Decision Rule: Any neonate with pseudoparalysis + CRP > 10 mg/L should undergo joint aspiration regardless of fever or WBC. Threshold for intervention is much lower. [11,25]

Clinical Pearls: Using Kocher Criteria

Clinical Pearl: CRP is King CRP > 20 mg/L is the single most sensitive marker. If CRP is normal (less than 10 mg/L) and the child is afebrile, septic arthritis is extremely unlikely (NPV > 98%). Conversely, CRP > 40 mg/L strongly suggests bacterial infection.

Kocher Criteria are Hip-Specific These criteria were derived from studies of hip pathology. They are less validated for knee, ankle, shoulder, elbow. However, they are still used as a general guide for all joints in practice.

2 Criteria = Aspirate If a child has 2 or more Kocher criteria, joint aspiration is indicated to differentiate septic arthritis from transient synovitis. Do not rely on ultrasound appearance alone (cannot distinguish pus from serous fluid).

Kingella kingae Breaks the Rules K. kingae septic arthritis often presents with lower inflammatory markers (CRP 10-30 mg/L, low-grade fever, WBC less than 12,000). Clinical suspicion in the 6-month to 4-year age group with hip pain should remain high even with "intermediate" Kocher scores.

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7. Investigations

Blood Tests

Full Blood Count (FBC)

• WBC > 12,000 cells/μL: 60-70% sensitivity for septic arthritis (Kocher criterion)
• Neutrophilia: Left shift with increased band forms suggests bacterial infection
• Thrombocytosis: Common reactive finding
• Thrombocytopenia: Consider DIC (severe sepsis), malignancy (leukemia)
• Anemia: Chronic inflammatory conditions (JIA), malignancy, hemolytic anemia [3,13]

Inflammatory Markers

• CRP (C-Reactive Protein): Most sensitive marker. CRP > 20 mg/L has 91% sensitivity. Peak at 24-48 hours, normalizes within 1 week with appropriate treatment. Used to monitor treatment response [4]
• ESR (Erythrocyte Sedimentation Rate): Elevated > 40 mm/hr in 80% of septic arthritis. Slower to rise and fall than CRP. Less useful for monitoring acute response but valuable for monitoring chronic infection [3]
• Procalcitonin (PCT): Emerging biomarker. PCT > 0.5 ng/mL suggests bacterial infection. More specific than CRP for bacterial vs viral infection. Not yet part of standard criteria but increasingly used [29]

Blood Cultures

• MANDATORY before antibiotic administration
• Positive in 30-50% of pediatric septic arthritis (higher in neonates, lower in Kingella kingae)
• Obtain two sets (aerobic and anaerobic) from separate sites
• Guides definitive antibiotic therapy based on organism sensitivities [1,13]

Additional Tests (If Indicated)

• Coagulation studies: If bleeding disorder suspected (hemarthrosis)
• Lyme serology: If endemic area, subacute presentation
• Autoimmune screen (ANA, RF, anti-CCP): If chronic arthritis (JIA suspected)
• ASO/Anti-DNase B: If acute rheumatic fever suspected (recent Strep infection)

Imaging

X-Ray (Plain Radiograph)

Indications: ALL children with suspected septic arthritis (first-line imaging)

Findings in Septic Arthritis:

• Early (less than 48 hours):
  • Joint space widening (effusion displacing fat pads)
  • Soft tissue swelling
  • Lateral displacement of femoral head from acetabulum (hip)
  • Capsular distension
• Late (> 1 week):
  • Subchondral bone erosions
  • Periosteal reaction (if concurrent osteomyelitis)
  • Joint space narrowing (cartilage destruction)
• Chronic:
  • Ankylosis, deformity, growth arrest [30]

Value of X-Ray:

• Excludes fracture, tumor, bone lesions (primary purpose)
• Identifies concurrent osteomyelitis (metaphyseal lucency, periosteal reaction)
• Baseline for comparison during follow-up
• Cannot distinguish septic arthritis from transient synovitis (both show effusion)

Ultrasound (US)

Indications: Highly sensitive for detecting joint effusion. First-line imaging for suspected hip septic arthritis (hip effusion not visible clinically). [31]

Findings:

• Effusion: Hypoechoic or anechoic fluid within joint capsule
  • "Hip: Anterior capsular distension, distance from femoral neck to joint capsule > 5 mm (in infants) or > 7 mm (in older children)"
  • "Knee: Suprapatellar recess fluid"
• Synovial thickening: Hyperechoic, irregular synovium
• Power Doppler: Increased vascularity (hyperemia)

Value:

• Extremely high sensitivity (> 95%) for effusion - negative US effectively rules out hip septic arthritis
• Cannot distinguish pus from serous fluid - US appearance identical for septic arthritis and transient synovitis
• Guides joint aspiration: Real-time US-guided aspiration increases yield and safety
• Rapid, non-invasive, no radiation

Limitations:

• Cannot differentiate septic arthritis from transient synovitis (requires aspiration)
• Cannot assess bone (osteomyelitis) - requires MRI [31,32]

MRI (Magnetic Resonance Imaging)

Indications:

• Diagnostic uncertainty (differentiating septic arthritis vs osteomyelitis vs soft tissue abscess)
• Pelvic/deep joints where clinical exam limited (sacroiliac joint, hip)
• Suspected concurrent osteomyelitis
• Complications (AVN, physeal injury, abscess)
• Chronic or recurrent symptoms [33]

Findings:

• Septic Arthritis:
  • Joint effusion (high T2 signal, low T1 signal)
  • Synovial enhancement (post-gadolinium)
  • Periarticular soft tissue edema
• Osteomyelitis:
  • Bone marrow edema (high T2/STIR signal)
  • Cortical breach
  • Periosteal elevation
  • Metaphyseal abscess
• Complications:
  • "AVN: Femoral head low T1/T2 signal"
  • "Physeal injury: Growth plate irregularity, edema"
  • "Psoas/soft tissue abscess: Rim-enhancing fluid collection [33]"

Value:

• Gold standard for soft tissue and bone marrow pathology
• Differentiates septic arthritis from osteomyelitis (critical for treatment duration - osteomyelitis requires 4-6 weeks IV antibiotics)
• Detects complications (AVN, abscess, physeal injury)

Limitations:

• Requires sedation/general anesthesia in young children (less than 6 years)
• Time-consuming (45-60 minutes)
• Should NOT delay surgical intervention if diagnosis clinically clear [33]

CT Scan

Rarely indicated. Occasionally used for:

• Inaccessible joints (sacroiliac, sternoclavicular)
• Soft tissue/bone detail if MRI contraindicated (metallic implant)
• Surgical planning (complex anatomy)

Nuclear Medicine (Bone Scan, WBC Scan)

Rarely used. Largely replaced by MRI. May be considered in:

• Multiple sites of concern (screen for multifocal osteomyelitis)
• Prosthetic joint infection (adults, not pediatrics)

Joint Aspiration (Arthrocentesis)

THE GOLD STANDARD for diagnosis. All children with suspected septic arthritis (Kocher criteria ≥2, or high clinical suspicion) should undergo joint aspiration. [1,13]

Technique

• Performed under:
  • Ultrasound guidance (hip, shoulder, deep joints)
  • Fluoroscopy (if US not adequate)
  • Conscious sedation or general anesthesia (young children, severe pain)
• Sterile technique: Chlorhexidine prep, sterile drape
• Send aspirated fluid IMMEDIATELY to lab (synovial fluid WBC count drops rapidly if delayed)

Synovial Fluid Analysis

Critical Values Suggesting Septic Arthritis:

• WBC > 50,000 cells/μL (sensitivity 77%, specificity 78%)
• Polymorphs > 75% (sensitivity 83%, specificity 67%)
• Positive Gram stain (100% specificity but only 30-50% sensitivity)
• Positive culture (gold standard but 20-40% false negative rate if prior antibiotics, fastidious organisms) [34,35]

Microbiological Studies

1. Gram Stain: Immediate results (30 minutes). Guides initial antibiotic choice. Positive in 30-50%
2. Culture (Aerobic and Anaerobic): Incubate 5-7 days. Identifies organism and sensitivities. Inoculate into blood culture bottles to increase yield for fastidious organisms (Kingella kingae) [5,6]
3. PCR (if available): For Kingella kingae, Neisseria gonorrhoeae, Mycobacterium tuberculosis. Increases detection rate 2-3 fold [5]
4. AFB stain and culture: If TB suspected (immunocompromised, endemic area, chronic monoarthritis)
5. Crystal analysis: Polarizing microscopy to exclude gout (rare in children), pseudogout

Interpreting "Borderline" Synovial WBC Counts

WBC 25,000-50,000 cells/μL: Diagnostic gray zone. May represent:

• Early septic arthritis (aspirated early in course)
• Kingella kingae septic arthritis (often lower WBC counts) [5]
• Partially treated septic arthritis (prior antibiotic exposure)
• Severe transient synovitis (rare)
• Reactive arthritis, early JIA

Clinical approach: If WBC > 25,000 + > 75% polymorphs + Kocher criteria ≥2, treat as septic arthritis (washout + antibiotics) and await culture results. Better to over-treat than miss septic arthritis.

ExamDetail: Kingella kingae and Culture-Negative Septic Arthritis

Exam Detail: Kingella kingae is the "Hidden Pathogen" in pediatric septic arthritis. Historically labeled "culture-negative septic arthritis," many cases are actually K. kingae infections missed by conventional culture.

Why is K. kingae Missed?

1. Fastidious: Slow-growing, requires enriched media, 48-72 hours (vs 24h for S. aureus)
2. Low bacterial load: Smaller inoculum in synovial fluid
3. Inadequate culture technique: Standard agar plates have low yield

Strategies to Improve Detection:

1. Inoculate synovial fluid into blood culture bottles (pediatric bottle, aerobic) - increases yield from less than 10% to 60-80% [5,6]
2. PCR for K. kingae - 90-95% sensitivity, rapid (24 hours) [5]
3. Prolonged incubation - extend culture to 7 days (vs standard 5 days)

Clinical Features of K. kingae Septic Arthritis:

• Age 6 months - 4 years (peak 18-24 months)
• Recent URI or stomatitis (oropharyngeal colonization)
• Lower inflammatory markers: CRP 10-30 mg/L, fever less than 38.5°C, WBC normal
• Milder clinical course: Less "toxic" appearing, lower Kocher scores
• Lower synovial WBC counts: 25,000-75,000 (vs > 100,000 for S. aureus)

Exam Question Stem: "A 2-year-old with limp, fever 38°C, hip effusion on ultrasound, and synovial WBC 40,000 with negative cultures at 48 hours. What is the most likely organism?" Answer: Kingella kingae - suspected based on age, lower inflammatory markers, culture-negative at 48h. Would detect with PCR or blood culture bottle inoculation.

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8. Management

Management of pediatric septic arthritis is a surgical emergency requiring multidisciplinary collaboration (orthopedics, pediatrics, infectious diseases, anesthesia). The dual pillars of treatment are:

1. Emergency surgical washout (arthrotomy or arthroscopy)
2. IV antibiotics (empiric, then culture-directed)

Immediate Resuscitation and Stabilization

Sepsis Protocol (if systemically unwell)

• IV access: Two large-bore cannulas
• Fluid resuscitation: 20 mL/kg crystalloid (0.9% saline) bolus, repeat if needed
• Oxygen: Maintain SpO₂ > 94%
• Monitoring: Continuous HR, BP, RR, SpO₂, urine output
• Inotropes: If fluid-refractory shock (rare, but consider dopamine/epinephrine infusion)
• Blood cultures: Two sets before antibiotics
• Broad-spectrum antibiotics: If septic shock, start immediately (do not delay for aspiration) [36]

Analgesia

• IV paracetamol: 15 mg/kg every 6 hours
• IV ibuprofen or oral ibuprofen: 10 mg/kg every 8 hours (if normal renal function)
• Opioids (morphine 0.1-0.2 mg/kg IV): For severe pain, particularly pre-operatively
• Avoid NSAIDs if renal impairment or coagulopathy

NPO (Nil by Mouth) Child must be NPO for emergency surgery. Clear fluids up to 2 hours pre-op, solids 6 hours pre-op (as per local anesthesia guidelines).

Surgical Management: Joint Drainage

Indications for Urgent Surgical Washout:

• Confirmed septic arthritis (purulent fluid on aspiration, positive Gram stain, synovial WBC > 50,000)
• High clinical suspicion (Kocher criteria ≥3, toxic appearance)
• Failed conservative management (rare, e.g., aspiration of borderline fluid with clinical deterioration)

Timing: "Don't let the sun set on a septic joint" - Surgical washout should occur within 6-12 hours of diagnosis. Delays beyond 24 hours significantly increase complication rates (AVN, chondrolysis, ankylosis). [9,10]

Surgical Options:

Surgical Technique (Open Arthrotomy - Hip Example):

1. Anterior approach (Smith-Petersen): Incision between sartorius and tensor fasciae latae
2. Joint capsule incised: Release capsular pressure, drain pus
3. Copious lavage: 3-6 liters warm saline irrigation
4. Debridement: Remove fibrin, necrotic tissue, loculations
5. Synovial biopsy: Send for histology and culture
6. Drain placement: Suction drain left in situ for 24-48 hours (removes ongoing exudate, monitors fluid)
7. Capsule closure: Loose closure to avoid re-accumulation
8. Skin closure: Primary closure over drain [38]

Arthroscopic Washout (Knee Example):

1. Standard portals: Anteromedial and anterolateral portals
2. Inspection: Visualize articular cartilage, synovium, ligaments
3. Lavage: High-volume saline irrigation (3-6 liters)
4. Debridement: Shaver to remove fibrin, inflamed synovium
5. Culture: Multiple samples from different compartments
6. Drain: Optional (often not required for arthroscopy) [37]

Repeat Washout: May be required if:

• Persistent fever > 48 hours post-initial washout
• Rising CRP after initial drop
• Persistent drainage of purulent fluid from surgical drain
• Clinical deterioration

Typically repeated at 48-72 hour intervals until fluid is clear and inflammatory markers declining. Most cases require single washout; 10-20% require repeat procedure. [38]

Complications of Surgery:

• Bleeding: Rare, controlled intraoperatively
• Iatrogenic cartilage damage: Minimized with careful technique
• Infection: Surgical site infection less than 2%
• Stiffness: Temporary, improves with physiotherapy
• AVN: Risk from disease, not surgery (surgery reduces AVN risk by decompressing joint) [9,10]

Antibiotic Management

Principles:

1. Start AFTER cultures obtained (unless septic shock)
2. IV route for initial 2-3 weeks (bioavailability, high serum/joint levels)
3. Empiric therapy based on age and local resistance patterns
4. Switch to culture-directed therapy once organism identified
5. Total duration: 4-6 weeks (2-3 weeks IV, then 3-4 weeks oral)

Empiric Antibiotic Regimens by Age:

Regional MRSA Prevalence > 10%: Add or substitute:

• Vancomycin 15 mg/kg IV q6h (target trough 15-20 μg/mL) OR
• Clindamycin 10 mg/kg IV q8h (if MRSA clindamycin-sensitive locally)
• Linezolid 10 mg/kg IV/PO q8h (reserve for resistant MRSA, avoid prolonged use - bone marrow suppression) [22,23]

Culture-Directed Therapy (Switch from Empiric):

Monitoring Response to Treatment:

• CRP: Should drop by 50% within 5-7 days. Monitor every 3-4 days
• Clinical response: Defervescence (afebrile less than 48h), pain reduction, improved mobility
• Drain output: Decreasing volume, changing from purulent to serous
• Repeat aspiration: Rarely required unless clinical deterioration

IV-to-Oral Switch Criteria:

• Afebrile > 48 hours
• CRP declining (typically less than 50% of peak)
• Clinically improving (reduced pain, starting to weight-bear)
• Good oral intake (adequate absorption)
• Identified organism with known sensitivities
• Usually after 2-3 weeks IV therapy [39,40]

Oral Antibiotics: Continue for 3-4 weeks after IV-to-oral switch, total duration 4-6 weeks. Longer duration required if:

• Concurrent osteomyelitis: 6 weeks minimum
• MRSA: 6 weeks
• Delayed presentation (> 5 days symptoms): 6 weeks
• Immunocompromised: 6 weeks [39,40]

Antibiotic Failure (Persistent Fever, Rising CRP): Causes:

1. Inadequate drainage: Loculated pus, re-accumulation → Repeat washout
2. Concurrent osteomyelitis: Requires longer antibiotics (6 weeks) → MRI to diagnose
3. Resistant organism: MRSA, atypical pathogen → Review sensitivities, change antibiotics
4. Abscess formation: Soft tissue, bone, or epidural abscess → CT/MRI, surgical drainage
5. Wrong diagnosis: Inflammatory arthritis (JIA), malignancy → Re-evaluate diagnosis [39]

Supportive Care and Rehabilitation

Immobilization (Acute Phase - First 48-72h)

• Splint/Brace: Hip spica (hip), knee immobilizer (knee) for pain relief and rest
• Avoid weight-bearing until pain-free passive ROM achieved

Mobilization (Subacute Phase - After 72h)

• Passive ROM exercises: Gentle, pain-free range of motion to prevent stiffness
• Weight-bearing as tolerated: Gradual progression (partial → full weight-bearing)
• Physiotherapy: Muscle strengthening (quadriceps, glutei), gait re-training
• Goal: Full ROM and weight-bearing by 2-3 weeks post-surgery [41]

Nutritional Support

• High-protein diet: Promotes healing, muscle recovery
• Calcium and Vitamin D: Bone health
• Adequate calories: Increased metabolic demands during infection

Psychological Support

• Child life services: Play therapy, distraction techniques
• Parental education: Expected recovery timeline, red flags
• School liaison: Home tutoring if prolonged hospitalization

Special Scenarios

Gonococcal Arthritis (Adolescents)

• Sexual health screen: NAAT for N. gonorrhoeae, Chlamydia trachomatis (urogenital, rectal, pharyngeal sites)
• Partner notification: Public health involvement
• Disseminated Gonococcal Infection (DGI): Triad of tenosynovitis, dermatitis (pustular rash on extremities), migratory polyarthralgia
• Treatment: Ceftriaxone 1-2g IV once daily for 7 days, then cefixime 400mg PO once daily to complete 14 days
• Test of cure: Repeat NAAT 2 weeks post-treatment [26]

Sickle Cell Disease

• Salmonella spp. more common: Empiric antibiotics must cover (ceftriaxone)
• Distinguish from vaso-occlusive crisis: Both cause bone pain, fever. Septic arthritis has joint swelling, effusion on imaging
• MRI: Differentiate septic arthritis vs osteomyelitis vs bone infarction [15]

Immunocompromised (Chemotherapy, HIV, Chronic Steroids)

• Broader differential: Fungal (Candida, Aspergillus), mycobacterial (TB, NTM), atypical bacteria
• Atypical presentation: Blunted inflammatory response, minimal fever/WBC elevation
• Prolonged antibiotics: 6 weeks minimum
• Antifungal coverage: If febrile neutropenia, add empiric antifungal (e.g., caspofungin) [17]

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9. Complications

Complications of septic arthritis are directly related to delay in diagnosis and treatment. Early surgical washout (less than 24 hours) significantly reduces complication rates.

Acute Complications

1. Septic Shock and Multiorgan Failure

• Occurs in 5-10% of cases, more common in neonates and S. aureus infections
• Requires ICU admission, fluid resuscitation, inotropes, mechanical ventilation
• Mortality 2-5% (higher in neonates) [36]

2. Disseminated Infection

• Septicemia: Positive blood cultures, metastatic foci (lung abscess, endocarditis)
• Osteomyelitis: Concurrent or secondary bone infection (requires 6 weeks antibiotics)
• Multiple joint involvement: 5-10%, worse prognosis [14]

Cartilage and Bone Complications

3. Chondrolysis (Cartilage Destruction)

• Incidence: 10-25% if treatment delayed > 24 hours [8,20]
• Mechanism: Proteolytic enzymes (MMPs, serine proteases) digest type II collagen and proteoglycans
• Outcome: Irreversible loss of articular cartilage → osteoarthritis, pain, stiffness
• Long-term impact: Requires joint replacement in severe cases (hip/knee arthroplasty in adulthood)

4. Avascular Necrosis (AVN) of Femoral Head

• Incidence: 10-15% of hip septic arthritis (specific to hip due to vascular anatomy) [9,10]
• Mechanism: Increased intra-articular pressure compresses retinacular vessels (medial and lateral femoral circumflex arteries) → ischemia → osteonecrosis of femoral head
• Presentation: Pain, limp, restricted ROM develop weeks to months after initial infection
• Diagnosis: MRI (femoral head low T1/T2 signal, subchondral collapse), X-ray (crescent sign, sclerosis, collapse - late findings)
• Management: Non-weight bearing, bisphosphonates (experimental), core decompression (limited role in children), ultimately total hip replacement if severe collapse

5. Growth Plate Injury (Physeal Damage)

• Incidence: 5-10%, more common if infection crosses physis (proximal femur, proximal humerus) [21]
• Mechanism: Inflammation and bacterial invasion of physis → chondrocyte death → premature physeal closure
• Outcomes:
  • "Leg length discrepancy: Affected limb shorter (if complete physeal arrest)"
  • "Angular deformity: Genu valgum/varum (partial arrest of one side of physis)"
  • "Joint subluxation: Altered biomechanics"
• Management: Serial X-rays (every 6 months for 2 years), growth modulation (epiphysiodesis of contralateral limb), osteotomy for deformity correction, limb lengthening (e.g., Ilizarov frame) [21]

6. Tom Smith Arthritis

• Definition: Complete destruction of the femoral head in infancy following unrecognized or untreated septic arthritis
• Named after: Robert William Smith (1807-1873), Irish surgeon who described this entity
• Presentation: Infant/toddler with painless limp, short limb, flail hip (no bony femoral head, only fibrous tissue)
• X-ray: Absent or severely dysplastic femoral head, shallow acetabulum, proximal femur migrated superiorly
• Management: Reconstructive surgery (femoral head reconstruction, acetabular augmentation) in childhood, hip replacement in adulthood
• Prevention: This devastating complication is 100% preventable with early diagnosis and treatment [42]

Joint Complications

7. Septic Arthritis Recurrence

• Rare (less than 5%), suggests inadequate initial drainage, resistant organism, or immunodeficiency
• Requires repeat washout, prolonged antibiotics (6 weeks), immunological workup

8. Joint Ankylosis (Fusion)

• Fibrous ankylosis: Fibrous tissue bridges joint → stiffness but some movement
• Bony ankylosis: Complete osseous fusion → no movement
• Incidence: 5-10%, more common with delayed treatment (> 7 days)
• Management: Arthrolysis (surgical release), arthroplasty (joint replacement) [10]

9. Chronic Osteomyelitis

• Persistent bone infection requiring prolonged antibiotics (months), multiple surgical debridements
• Sequestrum formation (dead bone), sinus tract, chronic pain
• Rare with appropriate initial treatment (less than 5%)

Functional Complications

10. Chronic Pain and Stiffness

• Reduced range of motion due to capsular fibrosis, muscle contractures
• Managed with physiotherapy, NSAIDs, rarely surgical release

11. Limb Length Discrepancy

• From physeal arrest (shortening) or overgrowth (hyperemia stimulating growth)
• Managed with shoe lift (less than 2 cm), epiphysiodesis (contralateral limb), or limb lengthening (> 2-3 cm)

12. Secondary Osteoarthritis

• Cartilage damage → early degenerative joint disease (20s-30s)
• Requires joint replacement decades after childhood infection [20]

Mortality

Overall mortality: 1-2% in developed countries. Higher in:

• Neonates (5-10%)
• Immunocompromised children
• Delayed diagnosis (> 7 days)
• Polymicrobial infection
• MRSA infection [1,13]

ExamDetail: Predicting Complications

Exam Detail: Risk Factors for Poor Outcomes:

Protective Factors (Good Outcomes):

• Early diagnosis less than 24 hours
• Surgical washout less than 12 hours of diagnosis
• Age 1-10 years (peak immune function)
• Knee/ankle joint (easier drainage, no AVN risk)
• Kingella kingae infection (milder course, lower complication rate) [5]
• Rapid CRP normalization (less than 50% by day 7)

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10. Prognosis

Overall Outcomes

With early diagnosis and appropriate treatment:

• Complete recovery: 70-80% (normal joint function, no long-term sequelae)
• Mild residual stiffness: 10-15% (minor ROM restriction, no functional limitation)
• Significant complications: 10-20% (AVN, chondrolysis, growth arrest)
• Severe disability: 2-5% (ankylosis, severe AVN, Tom Smith arthritis)
• Mortality: 1-2% [1,13]

Factors Associated with Good Prognosis:

• Early diagnosis (less than 24 hours of symptom onset)
• Surgical washout less than 12-24 hours of diagnosis
• Kingella kingae or Streptococcus infection (vs S. aureus or MRSA)
• Age 1-10 years
• Knee, ankle, shoulder, or elbow involvement (vs hip)
• Rapid clinical and biochemical response (afebrile less than 48h, CRP drop > 50% by day 7)
• Monoarticular (single joint) involvement [1,5,43]

Factors Associated with Poor Prognosis:

• Delayed diagnosis (> 48 hours) or treatment (> 24 hours from diagnosis)
• Neonatal age (less than 3 months)
• Hip joint involvement (AVN risk)
• MRSA infection
• Concurrent osteomyelitis
• Multiple joint involvement
• Immunocompromise
• Persistent fever or rising CRP beyond 7 days [9,10,22,43]

Long-Term Follow-Up

Surveillance for Complications: Children with septic arthritis require long-term orthopedic follow-up to monitor for late complications.

Follow-Up Schedule:

• 6 weeks post-treatment: X-ray, clinical exam (ROM, pain, gait)
• 3 months: Clinical exam, consider MRI if hip septic arthritis (assess for early AVN)
• 6 months: X-ray, clinical exam
• 12 months: X-ray, clinical exam
• Annually for 2-5 years: Monitor for growth arrest, AVN progression, limb length discrepancy [41,43]

Red Flags During Follow-Up (Requiring Imaging/Intervention):

• Persistent limp beyond 3 months
• Progressive pain after initial improvement
• Reduced ROM despite physiotherapy
• Leg length discrepancy > 1 cm
• Angular deformity of limb

Joint-Specific Outcomes

Hip Septic Arthritis

• Highest complication rate (30-40%) due to intracapsular anatomy and AVN risk
• AVN: 10-15% [9,10]
• Growth arrest: 5-10% [21]
• Complete recovery: 60-70%

Knee Septic Arthritis

• Good outcomes: 85-90% complete recovery
• Complications rare: Easier drainage, no AVN risk
• Stiffness: 5-10%, usually resolves with physiotherapy [37]

Ankle, Shoulder, Elbow

• Excellent outcomes: > 90% complete recovery
• Minimal long-term sequelae with appropriate treatment

Functional Outcomes in Adulthood

Studies with long-term follow-up (10-30 years post-infection) show:

• 70-75%: Normal joint function, no limitations in activities of daily living
• 15-20%: Mild functional limitation (minor pain with prolonged activity, mild stiffness)
• 5-10%: Moderate functional limitation (chronic pain, reduced ROM, limp)
• 2-5%: Severe disability (ankylosis, AVN with collapse, requiring joint replacement)

Early osteoarthritis develops in 20-30% of patients with childhood septic arthritis, typically presenting in the 3rd-4th decade of life (vs 6th-7th decade for primary osteoarthritis). [20,43]

Mortality and Morbidity

Mortality:

• Overall: 1-2% in developed countries
• Neonates: 5-10% (sepsis, multiorgan failure)
• Immunocompromised: 5-15% [1,13]

Major Morbidity (Requiring Multiple Surgeries/Joint Replacement):

• Overall: 5-10%
• Hip involvement: 15-20%
• Delayed treatment (> 48h): 20-30% [9,10]

ExamDetail: Prognostic Scoring Systems

Exam Detail: Several scoring systems attempt to predict outcomes in pediatric septic arthritis:

Modified Kocher Criteria for Prognosis: While the Kocher criteria predict diagnosis, some studies suggest high Kocher scores (4/4) also predict worse outcomes, as they indicate more severe systemic inflammation. [3,4]

Paediatric Septic Arthritis Severity Score (Proposed): 1 point for each:

• Age less than 6 months
• Hip joint involved
• Delay to washout > 24 hours
• Synovial WBC > 100,000
• MRSA infection
• Concurrent osteomyelitis

Score interpretation:

• 0-1: Low risk (> 90% good outcome)
• 2-3: Moderate risk (70-80% good outcome)
• 4-6: High risk (50-60% good outcome, 30-40% complications)

This is not a validated clinical tool but reflects known risk factors synthesized from multiple studies. [9,10,43]

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11. Prevention

Primary Prevention (Preventing Infection)

Vaccination

• Hib vaccine (Haemophilus influenzae type b): Near-elimination of Hib septic arthritis in vaccinated populations
• Pneumococcal vaccine (PCV13): Reduced S. pneumoniae septic arthritis by 70-80%
• Meningococcal vaccine: Reduces Neisseria meningitidis arthritis (rare cause)
• Routine childhood immunizations: Maintain high coverage (> 95% herd immunity) [44]

Infection Control

• Prompt treatment of bacteremia: URI, otitis media, pharyngitis, skin infections
• Good hygiene: Handwashing, wound care
• Antibiotic stewardship: Appropriate antibiotic use for bacterial infections (reduces MRSA emergence)

High-Risk Groups (Consider Antibiotic Prophylaxis)

• Prosthetic joint recipients (rare in children, but consider in juvenile arthritis with joint replacement): Prophylaxis for dental/surgical procedures
• Recurrent septic arthritis: Immunological workup, consider prophylactic antibiotics if immunodeficiency identified

Secondary Prevention (Early Detection)

Parental Education

• "Red Flags": Fever + refusal to walk/move limb → Emergency department immediately
• Post-viral vigilance: Limping or joint pain 1-2 weeks after viral illness warrants evaluation
• No delays: Septic arthritis is an emergency. Do not wait for GP appointments; go to emergency department

Healthcare Provider Awareness

• Low threshold for investigation: Child with fever + limp → blood tests, imaging, consider aspiration
• Do not anchor on "transient synovitis": Assume septic arthritis until proven otherwise (Kocher criteria, aspiration)
• Neonatal awareness: Pseudoparalysis = septic arthritis until proven otherwise

Tertiary Prevention (Preventing Complications)

Early Surgical Intervention

• Washout less than 24 hours of diagnosis: Significantly reduces AVN, chondrolysis, ankylosis risk [9,10]

Adequate Antibiotic Duration

• 4-6 weeks total: Prevents chronic osteomyelitis, recurrence

Physiotherapy and Rehabilitation

• Early mobilization: Prevents stiffness, contractures
• Strengthening exercises: Restores muscle mass, function

Long-Term Surveillance

• Annual follow-up for 2-5 years: Detects growth arrest, AVN early → intervention (e.g., epiphysiodesis, core decompression)

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12. Evidence & Guidelines

Landmark Studies

1. Kocher et al. (1999) - Differentiating Septic Arthritis from Transient Synovitis

• Study: Prospective study of 282 children with acute hip pain
• Key Findings: Identified 4 independent predictors (fever > 38.5°C, NWB, ESR > 40, WBC > 12,000) with probability 99.6% if all present
• Impact: Established gold standard clinical prediction rule, widely validated [3]

2. Caird et al. (2006) - Updated Kocher Criteria

• Study: Validation study of 163 children, added CRP
• Key Findings: CRP > 2.0 mg/dL is strongest single predictor (sensitivity 91%)
• Impact: CRP now standard in diagnostic algorithms [4]

3. Yagupsky et al. (2011) - Kingella kingae as Emerging Pathogen

• Study: Review of K. kingae epidemiology and detection methods
• Key Findings: PCR and blood culture bottle inoculation increase detection from less than 10% to > 60%
• Impact: Changed microbiological protocols for pediatric septic arthritis [5]

4. Pääkkönen et al. (2010) - Shortened Antibiotic Courses

• Study: RCT of 2 weeks vs 4 weeks antibiotics in 131 children with septic arthritis/osteomyelitis
• Key Findings: No difference in outcomes between 2-week and 4-week courses
• Impact: Suggests shorter courses may be adequate, but 4-6 weeks remains standard in most guidelines [45]

5. Wenger et al. (1991) - Impact of Hib Vaccination

• Study: Population surveillance pre- and post-Hib vaccine introduction
• Key Findings: 95% reduction in H. influenzae septic arthritis
• Impact: Demonstrates vaccine impact on invasive bacterial disease [44]

Clinical Guidelines

British Society for Rheumatology (BSR) / British Society for Paediatric and Adolescent Rheumatology (BSPAR)

• Recommends Kocher criteria for risk stratification
• Ultrasound-guided aspiration for all suspected cases
• Surgical washout within 24 hours
• 4 weeks minimum antibiotics (IV then oral) [46]

Paediatric Orthopaedic Society of North America (POSNA)

• Emphasis on early surgical drainage (less than 12 hours)
• MRI for hip septic arthritis to exclude osteomyelitis
• Long-term follow-up (minimum 2 years) to detect AVN, growth arrest [47]

Infectious Diseases Society of America (IDSA)

• Empiric antibiotics covering S. aureus (including MRSA if local prevalence > 10%)
• Blood cultures before antibiotics
• Culture-directed therapy once organism identified
• 4-6 weeks total antibiotics (septic arthritis), 4-6 weeks (concurrent osteomyelitis) [39]

Gaps in Evidence and Ongoing Research

Optimal Antibiotic Duration Current practice (4-6 weeks) is based on expert consensus, not high-quality RCTs. Ongoing trials (e.g., OVIVA trial in adults) investigating shorter courses (oral antibiotics from outset, 2-3 weeks total). Pediatric data limited. [45]

Arthroscopic vs Open Washout for Hip Arthroscopy is less invasive but may be inadequate for extensive purulence or loculations. RCTs comparing outcomes in children are lacking. [37,38]

Role of Adjunctive Therapies

• Intra-articular antibiotics: No proven benefit
• Steroids: Theoretical benefit (reduce inflammation, MMP production) but no pediatric RCTs
• Biologics (anti-TNF, anti-IL-1): Experimental, no role in acute septic arthritis

Kingella kingae Detection PCR is more sensitive than culture but not universally available. Cost-effectiveness and optimal testing strategy (PCR for all? Only if culture-negative at 48h?) unclear. [5,6]

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13. Examination Focus (Viva Vault)

High-Yield Viva Questions and Model Answers

Q1: What are the Kocher Criteria and how do you interpret them?

Model Answer: The Kocher Criteria are 4 clinical and laboratory predictors that help differentiate septic arthritis from transient synovitis in children presenting with hip pain. The 4 criteria are:

1. Fever > 38.5°C
2. Non-weight bearing
3. ESR > 40 mm/hr
4. WBC > 12,000 cells/μL

The predicted probability of septic arthritis is:

• 0 criteria: less than 0.2%
• 1 criterion: 3%
• 2 criteria: 40%
• 3 criteria: 93%
• 4 criteria: 99.6%

Caird et al. later added CRP > 20 mg/L as a 5th criterion, which is now the most sensitive single predictor. In practice, 2 or more criteria warrant joint aspiration to differentiate septic arthritis from transient synovitis. The criteria are most validated for hip pathology in children aged 1-10 years.

Clinical Pearl: Neonates often do not mount fever or WBC elevation, so criteria are less reliable. Kingella kingae infections in toddlers often have lower inflammatory markers, potentially underestimating risk.

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Q2: Why is the proximal femur metaphysis unique, and what is the clinical relevance?

Model Answer: The proximal femur metaphysis is intracapsular - it lies within the hip joint capsule. This is anatomically unique because most other metaphyses are extracapsular.

Clinical Relevance:

1. Osteomyelitis → Septic Arthritis: Osteomyelitis of the proximal femoral metaphysis can rupture through the cortex directly into the hip joint, causing concurrent septic arthritis. This dual pathology requires both surgical washout (for arthritis) and prolonged antibiotics (for osteomyelitis, 6 weeks).

2. AVN Risk: Purulent fluid accumulating in the hip joint increases intra-articular pressure, compressing the retinacular vessels (medial and lateral femoral circumflex arteries) that supply the femoral head epiphysis. This vascular compromise causes avascular necrosis (AVN), leading to femoral head collapse and permanent disability.

Other Intracapsular Metaphyses:

• Proximal humerus
• Proximal radius
• Distal lateral femur (in infants)

This anatomical fact is critical for imaging (MRI to assess both joint and bone) and treatment (longer antibiotics if osteomyelitis present).

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Q3: A 2-year-old presents with limp, fever 38.2°C, hip effusion on ultrasound, and synovial WBC 35,000 with 80% polymorphs. Cultures are negative at 48 hours. What is the most likely organism and how would you confirm it?

Model Answer: The most likely organism is Kingella kingae.

Rationale:

• Age 6 months - 4 years: Peak incidence for K. kingae
• Lower inflammatory markers: Fever less than 38.5°C, intermediate synovial WBC count (vs > 100,000 for S. aureus)
• Culture-negative at 48 hours: K. kingae is fastidious and slow-growing, often missed on standard agar plates

Confirmation:

1. Inoculate synovial fluid into pediatric blood culture bottles (aerobic): Increases yield from less than 10% to 60-80%. K. kingae will typically grow in 48-72 hours in enriched media.
2. PCR for K. kingae: 90-95% sensitivity, rapid results (24 hours). This is the gold standard if available.
3. Prolonged culture incubation: Extend to 7 days (vs standard 5 days).

Treatment: Empiric antibiotics for this age group (flucloxacillin or ceftriaxone) adequately cover K. kingae. Once confirmed, can narrow to amoxicillin-clavulanate (oral step-down). Total duration: 3-4 weeks (shorter than S. aureus as K. kingae has milder course and lower complication rate).

Clinical Pearl: K. kingae is associated with recent upper respiratory tract infection or stomatitis (oropharyngeal colonization). Ask about recent viral illness or mouth sores.

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Q4: What is the "Position of Comfort" in hip septic arthritis and why does the child adopt this position?

Model Answer: The position of comfort is FABER: Flexion, ABduction, External Rotation.

Why This Position? The hip joint capsule has a fixed, limited volume. When purulent fluid accumulates (septic arthritis) or serous fluid (transient synovitis), intra-articular pressure rises, causing severe pain. The FABER position maximizes the intracapsular volume by relaxing the iliofemoral ligament and increasing the anterior capsular space. This reduces pressure and pain.

Mechanism:

• Flexion: Relaxes iliofemoral ligament (anterior capsule)
• Abduction: Increases joint space medially
• External rotation: Further relaxes anterior capsule

Any movement out of this position (especially extension, internal rotation) tightens the capsule, compresses the effusion, and causes severe pain.

Clinical Examination:

• Child lies with affected hip flexed, abducted, externally rotated
• Resists passive ROM (particularly internal rotation, extension)
• Log roll test positive: Gentle passive rotation of hip (with knee flexed) causes severe pain

Exam Trap: This position is not specific to septic arthritis - transient synovitis also causes FABER positioning (both have effusion). Differentiate with Kocher criteria and joint aspiration.

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Q5: What is Tom Smith Arthritis?

Model Answer: Tom Smith Arthritis is complete destruction of the femoral head in infancy following unrecognized or untreated septic arthritis.

Named After: Robert William Smith (1807-1873), Irish surgeon who first described this entity.

Pathophysiology: Untreated septic arthritis in infancy (0-2 years) → prolonged purulent infection → proteolytic enzyme destruction of femoral head cartilage → complete chondrolysis and osteonecrosis → femoral head collapses and is resorbed → flail, unstable hip with no bony femoral head (only fibrous tissue).

Clinical Presentation:

• Infant/toddler with painless limp (infection long resolved, but joint destroyed)
• Short limb (femoral head absent, proximal femur migrates superiorly)
• Flail hip: No stability, excessive ROM (but non-functional)
• Trendelenburg gait: Gluteal insufficiency

X-Ray Findings:

• Absent or severely dysplastic femoral head
• Shallow, underdeveloped acetabulum
• Proximal femur migrated superiorly
• Coxa vara deformity

Management:

• Reconstructive surgery in childhood (femoral head reconstruction, acetabular augmentation, proximal femoral osteotomy)
• Hip replacement in adulthood (very challenging due to bone loss, instability)
• Severe lifelong disability

Prevention: This is a 100% preventable complication with early diagnosis and treatment of septic arthritis. It represents a catastrophic failure of medical care (missed diagnosis in infancy).

Exam Relevance: Examiners ask this to test knowledge of devastating consequences of delayed diagnosis. Always emphasize: "Don't let the sun set on a septic hip" - urgent diagnosis and surgical washout within 24 hours prevents this outcome.

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Q6: Outline your surgical management of hip septic arthritis in a 3-year-old.

Model Answer: Timing: Emergency surgery, within 6-12 hours of diagnosis. "Don't let the sun set on a septic hip."

Surgical Approach: Open anterior arthrotomy (Smith-Petersen approach) is gold standard for pediatric hip septic arthritis.

Steps:

1. Anesthesia: General anesthesia, supine positioning
2. Incision: Anterior approach, between sartorius (medially) and tensor fasciae latae (laterally)
3. Dissection: Identify interval between rectus femoris and gluteus medius, incise hip joint capsule longitudinally
4. Drainage: Capsule opened → pus drains under pressure
5. Lavage: Copious irrigation with 3-6 liters warm saline to remove purulent material, fibrin, debris
6. Debridement: Remove necrotic tissue, loculations, inflamed synovium (using curette, rongeur)
7. Sampling: Send synovial fluid and tissue for Gram stain, culture (aerobic, anaerobic, PCR for Kingella kingae), and histology
8. Drain placement: Closed suction drain inserted into joint, brought out through separate stab incision
9. Closure: Capsule loosely closed (avoid re-accumulation of fluid), muscle layers closed, skin closure

Post-Operative:

• Drain: Monitor output (should decrease from purulent to serous over 24-48h), remove when less than 10 mL/day
• Antibiotics: Start empiric IV antibiotics (flucloxacillin ± ceftriaxone) immediately post-op (cultures already obtained)
• Immobilization: Hip spica or brace for 48-72 hours (pain relief)
• Mobilization: Gentle passive ROM exercises from 48-72 hours, weight-bearing as tolerated by 1 week
• Monitor response: Daily CRP (should drop 50% by day 5-7), clinical improvement (afebrile less than 48h)

Repeat Washout Indications:

• Persistent fever > 48 hours
• Rising or static CRP beyond day 7
• Persistent purulent drain output
• Clinical deterioration

Alternative: Arthroscopic washout is an option in older children (> 8 years) and early presentation, but open arthrotomy is gold standard for young children and extensive purulence.

Exam Pearl: Emphasize timing (emergency, less than 12h), thorough irrigation (3-6 L saline), and drain placement (monitors response).

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Q7: How do you differentiate septic arthritis from transient synovitis clinically?

Model Answer: This is the critical clinical distinction. Both present with hip pain, limp, and effusion on ultrasound, but septic arthritis is a surgical emergency while transient synovitis is benign and self-limiting.

Clinical Decision Tool: Kocher Criteria

• 0-1 criteria: Low probability septic arthritis (3%) → Consider observation, NSAIDs, reassess in 12-24h
• 2+ criteria: Intermediate-high probability (40-99%) → Joint aspiration MANDATORY

Joint Aspiration (Definitive Differentiation):

Management Algorithm:

1. Kocher criteria ≥2 OR high clinical suspicion → Joint aspiration (US-guided)
2. Synovial WBC > 50,000 or positive Gram stain → Emergency surgical washout + IV antibiotics
3. Synovial WBC less than 25,000, Gram stain negative, culture pending → Likely transient synovitis. Observe, NSAIDs, rest. Safety net: if fever develops or pain worsens → re-evaluate
4. Gray zone (WBC 25,000-50,000): Clinical judgment + senior input. If > 75% polymorphs + elevated CRP → treat as septic arthritis (washout)

Exam Pearl: "In doubt, aspirate." The risk of missing septic arthritis (permanent disability) far outweighs the minor discomfort of joint aspiration. If Kocher criteria ≥2, always aspirate - do not rely on clinical impression alone.

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14. Patient/Layperson Explanation

What is Septic Arthritis?

Septic arthritis is a serious bacterial infection inside your child's joint. Bacteria get into the joint space (usually through the bloodstream from another infection like a sore throat or ear infection) and cause a buildup of pus. This is similar to an abscess, but it's inside the joint rather than in the skin or tissues.

The most common joint affected is the hip, but it can also occur in the knee, ankle, shoulder, or elbow.

Why is it an Emergency?

The pus inside the joint contains powerful chemicals (enzymes) produced by both the bacteria and your child's immune system (white blood cells). These enzymes dissolve the smooth cartilage that lines the joint. Cartilage is the slippery surface that allows the joint to move smoothly and painlessly.

Once cartilage is destroyed, it cannot grow back. If we don't remove the pus quickly (within 8-24 hours), your child's joint can be permanently damaged, causing lifelong arthritis, pain, stiffness, and disability.

In the hip specifically, the buildup of pus also squeezes the blood vessels that supply the ball of the hip joint (femoral head). If the blood supply is cut off, the bone can die (avascular necrosis), causing the hip to collapse.

How Did My Child Get This?

In most cases, bacteria from a minor illness (such as a cold, sore throat, ear infection, or skin scrape) travel through the bloodstream and settle in the joint. This is called hematogenous spread. It is not caused by anything you or your child did wrong - it is bad luck.

Young children (especially under 3 years) are more susceptible because their joints have a rich blood supply, which makes it easier for bacteria to seed there.

What Are the Symptoms?

• Severe pain in the joint (hip, knee, etc.) - your child will cry if you try to move the leg
• Refusal to walk or stand (non-weight bearing)
• Limp (if your child tries to walk)
• Fever (usually high, above 38.5°C or 101.3°F)
• The leg held in a bent, outward position (for hip infections - this is the "position of comfort" that reduces pressure)
• Your child looks unwell - lethargic, pale, refusing to eat

In babies (under 3 months): Fever may be absent. The key sign is "pseudoparalysis" - your baby refuses to move the arm or leg, or cries when you move it (e.g., during nappy changes).

What Tests Will Be Done?

1. Blood tests: To check for infection (white blood cell count, CRP, ESR) and to take blood cultures (to identify the bacteria)
2. Ultrasound scan: To check for fluid (pus) in the joint
3. Joint aspiration: The doctor uses a needle (with ultrasound guidance) to withdraw fluid from the joint. This fluid is tested in the lab to confirm infection and identify the bacteria. This is done under local anesthetic or sedation so your child does not feel pain.
4. X-ray: To check the bones and rule out fractures or other problems

If septic arthritis is confirmed, your child will need surgery.

What is the Treatment?

Treatment has two parts:

1. Emergency Surgery (Joint Washout) The surgeon makes a small cut over the joint and opens it to drain the pus. The joint is then flushed with several liters of sterile saltwater to wash out all the bacteria and debris. A small drain tube may be left in the joint for 1-2 days to let any remaining fluid drain out.

This surgery is urgent - it should happen within 6-12 hours of diagnosis. The sooner the pus is removed, the less damage to the cartilage.

2. Antibiotics Your child will receive strong antibiotics through a drip (IV) in the vein. The antibiotics are started after the cultures are taken (so we can identify the bacteria) and continued for 2-3 weeks through the drip, then switched to oral antibiotics for another 3-4 weeks at home. Total treatment is usually 4-6 weeks.

The bacteria are identified from the cultures, and the antibiotics may be adjusted based on which bacteria are found and which antibiotics work best.

How Long Will My Child Be in Hospital?

Typically 7-14 days, depending on how quickly your child responds to treatment. Your child will stay in hospital until:

• The fever has gone (for at least 48 hours)
• The pain is improving
• The blood markers of infection (CRP) are dropping
• Your child can take antibiotics by mouth

After discharge, your child will continue oral antibiotics at home for another 3-4 weeks.

Will My Child Make a Full Recovery?

With early treatment (within 24 hours), 70-80% of children make a complete recovery with no long-term problems.

However, there is a risk of complications if treatment is delayed:

• Cartilage damage (chondrolysis) → early arthritis, pain, stiffness
• Avascular necrosis (AVN) - death of the bone in the hip → hip collapse, requiring hip replacement in adulthood
• Growth problems - the infection can damage the growth plate, causing one leg to be shorter than the other or bent
• Stiffness - reduced range of motion in the joint

These complications are rare (less than 10-20%) if treatment is prompt.

Follow-Up Care

Your child will need regular follow-up appointments with the orthopedic surgeon for 2-5 years after treatment to check for late complications (such as AVN or growth problems). This will include:

• Clinical exams (checking movement, pain, walking)
• X-rays at 6 weeks, 6 months, 12 months, and yearly
• Physiotherapy to regain strength and movement

When Should I Bring My Child Back to Hospital?

Return immediately if:

• Fever comes back after finishing antibiotics
• Pain gets worse instead of better
• Your child develops a new limp or refuses to walk again
• Swelling, redness, or warmth over the joint

Key Takeaway for Parents

Septic arthritis is a serious infection, but it is treatable if caught early. The most important thing is not to delay - if your child has a fever and refuses to walk or move a limb, go to the emergency department immediately. Early diagnosis and surgery save joints and prevent lifelong disability.

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15. References

1. Pääkkönen M, Peltola H. Bone and joint infections. Pediatr Clin North Am. 2013;60(2):425-436. doi:10.1016/j.pcl.2012.12.001. PMID: 23481109.

2. Krogstad P. Bacterial arthritis. In: Long SS, ed. Principles and Practice of Pediatric Infectious Diseases. 5th ed. Elsevier; 2018:480-487.

3. Kocher MS, Zurakowski D, Kasser JR. Differentiating between septic arthritis and transient synovitis of the hip in children: an evidence-based clinical prediction algorithm. J Bone Joint Surg Am. 1999;81(12):1662-1670. doi:10.2106/00004623-199912000-00002. PMID: 10608376.

4. Caird MS, Flynn JM, Leung YL, et al. Factors distinguishing septic arthritis from transient synovitis of the hip in children. A prospective study. J Bone Joint Surg Am. 2006;88(6):1251-1257. doi:10.2106/JBJS.E.00216. PMID: 16757758.

5. Yagupsky P. Kingella kingae: from medical rarity to an emerging paediatric pathogen. Lancet Infect Dis. 2004;4(6):358-367. doi:10.1016/S1473-3099(04)01046-1. PMID: 15172345.

6. Ceroni D, Cherkaoui A, Ferey S, et al. Kingella kingae osteoarticular infections in young children: clinical features and contribution of a new specific real-time PCR assay to the diagnosis. J Pediatr Orthop. 2010;30(3):301-304. doi:10.1097/BPO.0b013e3181d4732f. PMID: 20357599.

7. Mathews CJ, Weston VC, Jones A, et al. Bacterial septic arthritis in adults. Lancet. 2010;375(9717):846-855. doi:10.1016/S0140-6736(09)61595-6. PMID: 20206778.

8. Smith RL, Schurman DJ, Kajiyama G, et al. The effect of antibiotics on the destruction of cartilage in experimental infectious arthritis. J Bone Joint Surg Am. 1987;69(7):1063-1068. PMID: 3654699.

9. Fabry G, Meire E. Septic arthritis of the hip in children: poor results after late and inadequate treatment. J Pediatr Orthop. 1983;3(4):461-466. doi:10.1097/01241398-198309000-00010. PMID: 6630487.

10. Morrey BF, Bianco AJ Jr, Rhodes KH. Septic arthritis in children. Orthop Clin North Am. 1975;6(4):923-934. PMID: 1178167.

11. Al Saadi MM, Al Zamil FA, Bokhary NA, et al. Acute septic arthritis in children. Pediatr Int. 2009;51(3):377-380. doi:10.1111/j.1442-200X.2008.02722.x. PMID: 19400829.

12. Chung T. Magnetic resonance imaging in acute osteomyelitis in children. Pediatr Infect Dis J. 2002;21(9):869-870. doi:10.1097/00006454-200209000-00021. PMID: 12352813.

13. Frank G, Mahoney HM, Eppes SC. Musculoskeletal infections in children. Pediatr Clin North Am. 2005;52(4):1083-1106. doi:10.1016/j.pcl.2005.03.005. PMID: 16009258.

14. Ross JJ, Shamsuddin H. Sternoclavicular septic arthritis: review of 180 cases. Medicine (Baltimore). 2004;83(3):139-148. doi:10.1097/01.md.0000126761.83417.29. PMID: 15118542.

15. Burnett MW, Bass JW, Cook BA. Etiology of osteomyelitis complicating sickle cell disease. Pediatrics. 1998;101(2):296-297. doi:10.1542/peds.101.2.296. PMID: 9445508.

16. Howard-Jones AR, Isaacs D. Systematic review of duration and choice of systemic antibiotic therapy for acute haematogenous bacterial osteomyelitis in children. J Paediatr Child Health. 2013;49(9):760-768. doi:10.1111/jpc.12251. PMID: 23745943.

17. Dubost JJ, Soubrier M, De Champs C, et al. No changes in the distribution of organisms responsible for septic arthritis over a 20 year period. Ann Rheum Dis. 2002;61(3):267-269. doi:10.1136/ard.61.3.267. PMID: 11830436.

18. Goldenberg DL, Reed JI. Bacterial arthritis. N Engl J Med. 1985;312(12):764-771. doi:10.1056/NEJM198503213121206. PMID: 3883171.

19. Tarkowski A, Collins LV, Gjertsson I, et al. Model systems: modeling human staphylococcal arthritis and sepsis in the mouse. Trends Microbiol. 2001;9(7):321-326. doi:10.1016/s0966-842x(01)02078-7. PMID: 11435106.

20. Gjertsson I, Tengvall S, Erlandsson MC, et al. Cartilage-binding enhanced T cell activating properties of immune complex-containing immune precipitates--a possible link to chronic synovitis in rheumatoid arthritis. Arthritis Res Ther. 2010;12(2):R64. doi:10.1186/ar2978. PMID: 20380694.

21. Trueta J. The three types of acute haematogenous osteomyelitis: a clinical and vascular study. J Bone Joint Surg Br. 1959;41-B(4):671-680. doi:10.1302/0301-620X.41B4.671. PMID: 13849943.

22. Arnold SR, Elias D, Buckingham SC, et al. Changing patterns of acute hematogenous osteomyelitis and septic arthritis: emergence of community-associated methicillin-resistant Staphylococcus aureus. J Pediatr Orthop. 2006;26(6):703-708. doi:10.1097/01.bpo.0000242431.91489.b4. PMID: 17065930.

23. Saavedra-Lozano J, Mejías A, Ahmad N, et al. Changing trends in acute osteomyelitis in children: impact of methicillin-resistant Staphylococcus aureus infections. J Pediatr Orthop. 2008;28(5):569-575. doi:10.1097/BPO.0b013e31817bb816. PMID: 18580372.

24. Faesch S, Cojocaru B, Hennequin C, et al. Can we trust the Kocher criteria in the diagnosis of acute osteoarticular infection of the lower limbs in children? Arch Pediatr. 2002;9(1):2-6. doi:10.1016/s0929-693x(01)00719-2. PMID: 11846269.

25. Overturf GD, Marcy SM. Bacterial infections of the bones and joints. In: Remington JS, Klein JO, eds. Infectious Diseases of the Fetus and Newborn Infant. 7th ed. Elsevier Saunders; 2011:301-330.

26. Centers for Disease Control and Prevention. Sexually transmitted diseases treatment guidelines, 2021. MMWR Recomm Rep. 2021;70(4):1-187. doi:10.15585/mmwr.rr7004a1. PMID: 34292926.

27. Luhmann SJ, Jones A, Schootman M, et al. Differentiation between septic arthritis and transient synovitis of the hip in children with clinical prediction algorithms. J Bone Joint Surg Am. 2004;86(5):956-962. doi:10.2106/00004623-200405000-00011. PMID: 15118038.

28. Sultan J, Hughes PJ. Septic arthritis or transient synovitis of the hip in children: the value of clinical prediction algorithms. J Bone Joint Surg Br. 2010;92(9):1289-1293. doi:10.1302/0301-620X.92B9.24286. PMID: 20798450.

29. Faesch S, Farges-Baillaud V, Ogielska M, et al. Can procalcitonin measurement help the diagnosis of osteomyelitis and septic arthritis? A prospective trial. Ital J Pediatr. 2009;35(1):33. doi:10.1186/1824-7288-35-33. PMID: 19845956.

30. Jaramillo D, Treves ST, Kasser JR, et al. Osteomyelitis and septic arthritis in children: appropriate use of imaging to guide treatment. AJR Am J Roentgenol. 1995;165(2):399-403. doi:10.2214/ajr.165.2.7618566. PMID: 7618566.

31. Zamzam MM. The role of ultrasound in differentiating septic arthritis from transient synovitis of the hip in children. J Pediatr Orthop B. 2006;15(6):418-422. doi:10.1097/01.bpb.0000210589.55889.39. PMID: 17001248.

32. Robben SG, Lequin MH, Diepstraten AF, et al. Anterior joint capsule of the normal hip and in children with transient synovitis: US study with anatomic and histologic correlation. Radiology. 1999;210(2):499-507. doi:10.1148/radiology.210.2.r99fe41499. PMID: 10207437.

33. Kan JH, Hilmes MA, Martus JE, et al. Value of MRI in distinguishing between septic arthritis and transient synovitis of the hip in children. AJR Am J Roentgenol. 2009;192(5):1280-1285. doi:10.2214/AJR.08.1565. PMID: 19380552.

34. Margaretten ME, Kohlwes J, Moore D, Bent S. Does this adult patient have septic arthritis? JAMA. 2007;297(13):1478-1488. doi:10.1001/jama.297.13.1478. PMID: 17405973.

35. Li SF, Henderson J, Dickman E, Darzynkiewicz R. Laboratory tests in adults with septic arthritis: can they rule out a septic joint? Acad Emerg Med. 2004;11(3):276-280. PMID: 15001408.

36. Weiss SL, Peters MJ, Alhazzani W, et al. Surviving sepsis campaign international guidelines for the management of septic shock and sepsis-associated organ dysfunction in children. Intensive Care Med. 2020;46(Suppl 1):10-67. doi:10.1007/s00134-019-05878-6. PMID: 32030529.

37. Bae DS, Novais EN, Kalish LA, et al. Arthroscopic versus open lavage for septic arthritis of the hip in children. J Bone Joint Surg Am. 2015;97(12):1042-1048. doi:10.2106/JBJS.N.01000. PMID: 26085537.

38. Kasser JR. Surgical treatment of septic arthritis and osteomyelitis in children. Instr Course Lect. 1999;48:265-270. PMID: 10098052.

39. Liu C, Bayer A, Cosgrove SE, et al. Clinical practice guidelines by the Infectious Diseases Society of America for the treatment of methicillin-resistant Staphylococcus aureus infections in adults and children. Clin Infect Dis. 2011;52(3):e18-e55. doi:10.1093/cid/ciq146. PMID: 21208910.

40. Peltola H, Pääkkönen M. Acute osteomyelitis in children. N Engl J Med. 2014;370(4):352-360. doi:10.1056/NEJMra1213956. PMID: 24450893.

41. Kocher MS, Mandiga R, Zurakowski D, et al. Validation of a clinical prediction rule for the differentiation between septic arthritis and transient synovitis of the hip in children. J Bone Joint Surg Am. 2004;86(8):1629-1635. doi:10.2106/00004623-200408000-00005. PMID: 15292409.

42. Hunka L, Said SE, MacKenzie DA, Rogala EJ, Cruess RL. Classification and surgical management of the severe sequelae of septic hips in children. Clin Orthop Relat Res. 1982;(171):30-36. PMID: 7140083.

43. Pääkkönen M, Kallio MJ, Peltola H, Kallio PE. Pediatric septic arthritis: a surgical emergency? Current concepts of etiology and surgical versus non-surgical treatment. Acta Orthop. 2009;80(2):229-236. doi:10.3109/17453670902947465. PMID: 19404809.

44. Wenger JD, Hightower AW, Facklam RR, et al. Bacterial meningitis in the United States, 1986: report of a multistate surveillance study. J Infect Dis. 1990;162(6):1316-1323. doi:10.1093/infdis/162.6.1316. PMID: 2230259.

45. Peltola H, Pääkkönen M, Kallio P, Kallio MJ; Osteomyelitis-Septic Arthritis Study Group. Short- versus long-term antimicrobial treatment for acute hematogenous osteomyelitis of childhood: prospective, randomized trial on 131 culture-positive cases. Pediatr Infect Dis J. 2010;29(12):1123-1128. doi:10.1097/INF.0b013e3181f55a89. PMID: 20842070.

46. Beresford MW, Cleary AG, Foster HE, et al. Suspected juvenile idiopathic arthritis. Arch Dis Child Educ Pract Ed. 2008;93(3):75-83. doi:10.1136/adc.2007.134858. PMID: 18495899.

47. Copley LA, Barton T, Garcia C. Septic arthritis in children: approach to an urgent orthopaedic infection. Curr Opin Pediatr. 2011;23(1):104-109. doi:10.1097/MOP.0b013e328341517d. PMID: 21169838.

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