Septic Arthritis of the Pediatric Hip

Blood Supply at Risk

The femoral head blood supply is unique and precarious.

1. Medial Circumflex Femoral Artery: Gives off retinacular vessels.
2. Retinacular Vessels: Travel along the femoral neck (intracapsular) to supply the head.
3. Intracapsular Pressure: Normal hip pressure is low. In septic arthritis, effusion pressure can exceed arterial perfusion pressure, leading to tamponade and AVN.

Implication: Urgent decompression (aspiration/arthrotomy) is essentially a "fasciotomy of the hip".

The Neonatal Hip (Tom Smith Arthritis)

In neonates (under 18 months), transphyseal vessels exist connecting the metaphysis to the epiphysis.

Consequence: Osteomyelitis of the metaphysis can spread directly to the epiphysis and destroy the physis (Growth Plate). Result: Complete destruction of the femoral head/neck (Tom Smith Arthritis) leading to unstable, flail hip.

This is a distinct pathologic entity from adult septic arthritis.

Kocher Criteria (1999)

A validated prediction rule for differentiating septic arthritis from transient synovitis.

The Criteria:

1. Fever (Temp over 38.5°C)
2. Non-weight bearing
3. ESR over 40 mm/hr
4. WCC over 12,000 cells/mm³

The probability increases exponentially with each added factor.

Caird Criteria (2006)

Added CRP over 20 mg/L as a 5th independent predictor.

Probability with 5 predictors: 98%.

Note: CRP is generally more useful than ESR as it rises and falls faster (better for monitoring response).

Reliability of MRI

Some centers use MRI to distinguish. Choi et al showed MRI features predictive of septic arthritis:

1. Signal intensity alteration in bone marrow.
2. Decreased perfusion of femoral epiphysis.
3. Thickening of synovium.

However, sedation requirements for MRI in children can delay treatment. Ultrasound + Aspiration remains the pragmatic gold standard.

Always prioritize clinical judgment over imaging findings.

Septic Arthritis vs Transient Synovitis

Transient synovitis is the most common cause of hip pain in this age group, but is a diagnosis of exclusion.

Rule of Thumb: If the child can walk into the clinic (even with a limp), it is unlikely to be septic arthritis.

Proximal Femoral Osteomyelitis

Can coexist with septic arthritis or mimic it.

• Pathology: Infection of the metaphysis (neck).
• Imaging: X-ray often normal early. MRI shows marrow edema.
• Treatment: Requires more prolonged antibiotics (6 weeks) and potentially drilling of the metaphysis if no abscess in joint.

Differentiation: High inflammatory markers but less joint restriction than septic arthritis (unless joint invaded).

Perthes Disease

Avascular necrosis of the femoral head.

• Onset: Insidious (weeks).
• Age: 4-8 years.
• X-ray: Sclerosis, flattening, fragmentation.
• Markers: Normal WCC/ESR.

Clinical: Reduced Abduction and Internal Rotation, but usually not "irritable" hip in the acute septic sense.

Psoas Abscess

Collection in the iliopsoas muscle.

• Signs: Psoas sign (pain on extension). Hip held in flexion.
• Differentiation: Hip rotation is often pain-free if the hip is kept flexed (unlike septic hip where rotation is painful).
• Imaging: Ultrasound or MRI confirms.

Treatment is usually drainage (percutaneous) and antibiotics.

Imaging Protocol

1. X-ray (AP Pelvis + Frog Lateral):

• Usually normal early.
• Look for: Widened joint space (Waldenstrom sign over 2mm asymmetry), osteomyelitis changes (rare early), subluxation.

2. Ultrasound:

• Gold standard screening. Detects fluid.
• Cannot reliably distinguish sterile vs infected fluid (though turbidity helps).
• Facilitates guided aspiration.

3. MRI:

• Diagnostic dilemma solver (e.g., Psoas abscess vs Septic Hip vs Osteomyelitis).
• Excellent for evaluating concomitant osteomyelitis.

MRI should be reserved for cases where the diagnosis is unclear or Psoas abscess is suspected.

Hip Aspiration (Arthrocentesis)

The Definitive Diagnostic Test.

• Indication: Kocher 2-3 or equivocal diagnosis.
• Technique: Ultrasound guided or Fluoroscopic (in theatre).
• Send for: MCS (Microscopy, Culture, Sensitivity), Cell Count, Gram Stain, Crystal (rare in kids).
• PCR: Request Kingella kingae PCR specifically in children under 4.

Core Principles

1. Decompression: Urgent removal of pus to reduce pressure and enzymatic damage.
2. Antibiotics: High dose IV therapy to sterilize blood and tissues.
3. Rest: Immobilization for symptom control and stability.

Empirical Antibiotics (organism- and age-directed, always after cultures):

• Default (over 3 months): Anti-staphylococcal agent — flucloxacillin or cefazolin; clindamycin if penicillin-allergic (also covers most Kingella).
• Neonate: Anti-staphylococcal penicillin PLUS a third-generation cephalosporin (cefotaxime) to cover Group B Streptococcus and Gram-negatives.
• Unimmunised / Hib risk: Add a third-generation cephalosporin (ceftriaxone or cefotaxime).
• MRSA risk (high-prevalence region or severe sepsis): Vancomycin or clindamycin per local antibiogram.

The goal is rapid sterilisation of the joint fluid; rationalise to a narrow-spectrum agent once cultures and PCR return.

Anterior Approach (Smith-Petersen)

Preferred for Septic Hip. Allows direct access to the joint and easy drainage.

1. Incision: Bikini line or longitudinal from ASIS.
2. Interval: Sartorius/Tensor Fascia Lata (Superficial), Rectus Femoris/Gluteus Medius (Deep).
3. Capsulotomy: Longitudinal or T-shaped incision in capsule.
4. Washout: Copious saline irrigation. Inspect head.
5. Closure: Leave drain? (Controversial, many close over drain). Loosely close capsule.

Pros: Excellent exposure, classic approach. Cons: Risk to Lateral Cutaneous Nerve of Thigh (LCNT).

Medial Approach (Ludloff)

Alternative. Often used for congenital dislocation but good for inferior drainage.

1. Interval: Adductor Longus / Pectineus.
2. Risk: Medial Circumflex Femoral Artery (MCFA) is at risk.

Use: Less commonly used for septic arthritis now compared to Anterior.

Arthroscopic Washout

Increasing Popularity.

• Minimally invasive.
• 2 or 3 portals (Anterior, Anterolateral).
• Requires pediatric distraction set.
• Warning: Can be difficult in small children (under 2 years).
• Evidence: Similar outcomes to open washout in older children.

Technique is demanding and visualization can be poor in the infant hip.

Management by Choi Type

Sequelae management depends on the deformity.

1. Type I (Normal): No treatment.
2. Type II (Coxa Magna): Observation. Usually remodeling is sufficient.
3. Type III (Coxa Vara): Valgus Osteotomy may be required if neck-shaft angle is under 110 degrees or progression occurs.
4. Type IV (Head destruction): Pelvic support osteotomy or Arthrodesis in severe cases. Total Hip Arthroplasty (THA) in adulthood.

Treatment must be individualized based on age and deformity severeity.

Salvage Procedures

For the destroyed hip (Tom Smith Arthritis):

• Trochanteric Arthroplasty (Colonna): Placing the trochanter into the acetabulum (historical).
• Pelvic Support Osteotomy: Stabilizes the hip without a joint.
• Arthrodesis: Reliable pain relief but functional limitation (back pain).
• Total Hip Arthroplasty: Excellent function but limited lifespan in young patients. Custom implants often required for small canals.

Arthroplasty is the last resort.