Paediatric Hip Ultrasound

Paediatric hip ultrasound is the main imaging test for developmental dysplasia of the hip (DDH) in infants under 4 to 6 months, when the femoral head is still largely cartilaginous and not visible on a radiograph. A 7.5 to 12 MHz linear probe is used.

Static and dynamic assessment are combined. The static (morphological) method views the hip in the coronal plane (a fixed plane showing the iliac line, the lower limb of the ilium, and the labrum) in the lateral decubitus position, with the baby's hip flexed to 30 degrees.

It measures the alpha angle (the inclination of the acetabular roof, bony roof) and the beta angle (the cartilaginous roof), and classifies the hip into morphological types.

The dynamic method (stress method) examines the hip in the coronal flexion and transverse flexion views, with gentle stress manoeuvres (Barlow and Ortolani) to show instability, and measures femoral head coverage by the acetabulum (percentage of the head lying within the acetabulum; normal is 50% or more).

The examination takes 10 to 15 minutes, with no radiation or sedation.

DDH, a spectrum from instability, through dysplasia and subluxation, to complete dislocation, affects about 1 to 2 in 1000 live births (dislocation) and up to 10 to 20 in 1000 (sonographic immaturity or mild dysplasia, mostly resolving spontaneously). Risk factors are female sex, breech presentation, positive family history, oligohydramnios, firstborn status, swaddling, and foot deformities.

Untreated DDH leads to gait abnormality, limb length discrepancy, pain and early osteoarthritis. Early detection (before 3 months) allows treatment with a harness (abduction harness) with a success rate of over 90%, avoiding surgery. Ultrasound has a sensitivity of about 90% and a specificity of 97% for DDH in expert hands.

Indications. Clinical suspicion of DDH (positive Barlow or Ortolani test, limited hip abduction, leg length discrepancy, asymmetric skin folds). Risk factors with a clinically normal examination (breech presentation after 36 weeks regardless of mode of delivery, first degree family history, and in many countries female breech) at 4 to 6 weeks of age.

Universal screening in some countries (4 to 6 weeks of age, or 0 to 6 weeks in a selected approach). Follow up of known DDH on treatment (harness, abduction brace) and after reduction. Abnormal hip on antenatal scan or associated conditions (torticollis, metatarsus adductus, neuromuscular conditions, spinal dysraphism).

Not for infants over 6 months with ossified femoral heads (radiograph is used). Hip irritability with a limp or fever in older children: joint effusion.

Contraindications and precautions. None. The baby should be calm, warm, fed and comfortable (feeding during the scan helps). The examination is done with a gentle technique without forced stress; excessive stress can be painful and may injure the hip.

The infant's parents are informed about the procedure and the meaning of the results. A trained operator is needed, since errors in probe position (tilt, rotation) give false angles.

Preparation and technique. No preparation is needed. For the static method, the baby lies in the lateral position in a positioning device (cradle) with the hip flexed to 30 to 45 degrees and slightly internally rotated.

The probe is placed on the greater trochanter in the coronal plane, parallel to the body axis, to obtain the correct plane: the straight iliac line (os ilium), the acetabular fossa with the lower limb of the ilium (the lower edge of the ilium), and the labrum visible.

The baseline (along the iliac line), the bony roof line (from the inferior iliac edge to the bony rim), and the cartilage roof line (from the bony rim to the labrum) are drawn to measure the alpha and beta angles.

In the dynamic assessment (supine position), the probe is placed laterally; coronal flexion and transverse flexion views are obtained at rest and with gentle posterior stress, showing the position of the femoral head relative to the acetabulum, and the percentage of the head covered.

Both hips are examined, and the report states the type and the technique used.

Normal appearances. The femoral head is a round, hypoechoic structure (cartilage) with a small echogenic centre (ossification centre appears at 3 to 6 months), sitting well within the acetabulum. The acetabular roof is concave, with a well formed bony rim (sharp, angular) and a cartilaginous roof covering the head (the labrum).

Alpha angle is 60 degrees or more (normal), beta angle under 55 degrees; femoral head coverage is 50% or more and the head is stable under stress. In immature hips under 3 months, minor immaturity (alpha 50 to 59 degrees) may be physiological and resolves with follow up.

Interpretation.

• morphological type I (mature hip): alpha angle 60 degrees or more, well formed bony roof with angular rim; type Ia: beta under 55 degrees; type Ib: beta over 55 degrees. No treatment.
• Type IIa (physiologically immature hip, under 12 weeks): alpha 50 to 59 degrees, rounded bony rim with sufficient coverage; follow up scan in 4 to 6 weeks; usually resolves; type IIa (plus): near the border (alpha 55 to 59), and type IIa (minus): at risk of failing to mature (alpha 50 to 54).
• Type IIb (delayed ossification, over 12 weeks): alpha 50 to 59 degrees with deficient bony roof; treat with abduction harness.
• Type IIc (critical hip): alpha 43 to 49 degrees, beta under 77 degrees, rounded or flattened bony rim; a stable hip is at the risk of decentring; type IId: decentring (beta over 77 degrees), the head starts to be displaced; treatment with a harness.
• Type D (decentring): alpha 43 to 49 degrees and beta over 77 degrees; femoral head displaced laterally, labrum pushed up; this is a dislocatable hip needing treatment.
• Type III (eccentric, dislocated hip): alpha under 43 degrees, beta not measurable, bony roof flat, cartilaginous roof displaced upward (IIIa: echogenic cartilage roof with structural change, IIIb: hypoechoic or showing degeneration); the femoral head lies outside the acetabulum.
• Type IV (dislocated hip): alpha under 43 degrees, the labrum interposed under the head, with the cartilaginous roof pressed downward (inverted), the head is displaced and the acetabulum filled with fibro fatty tissue (pulvinar); irreducible by closed methods.
• Dynamic findings: instability (the head moves out of the acetabulum with posterior stress, then reduces, positive Barlow and Ortolani), subluxation (head displaced partially, with coverage under 50%), dislocation (the head lies posterior to the acetabulum, without contact), and a reducible versus irreducible dislocation (interposed labrum, pulvinar, ligamentum teres, or a tight iliopsoas).
• Joint effusion: anechoic fluid between the femoral neck and the joint capsule (capsule distance over 5 mm or an asymmetry of over 2 mm compared with the other hip) in transient synovitis, septic arthritis or Perthes disease in older children.
• Post-treatment assessment: concentric reduction in a harness, improvement in alpha angle and coverage, avascular necrosis (delayed ossification centre, later), and residual dysplasia.

Critical values and findings needing immediate action.

• Dislocated hip (morphological type III or IV, or an unstable hip on dynamic examination): urgent referral to a paediatric orthopaedic service within days for harness treatment (abduction harness) in the first 6 months, with monitoring of reduction; irreducible dislocations require closed or open reduction.
• Decentred or dislocatable hip (type D, IIc unstable): treatment in a harness and weekly or fortnightly scans until stable.
• Persistent dysplasia in a harness after 3 to 4 weeks (not centring): stop the harness and refer for alternative treatment (abduction brace, traction, closed reduction).
• Septic arthritis (effusion with fever, irritability, refusal to move the limb): urgent aspiration or washout and antibiotics.
• Complications of treatment: femoral nerve palsy, avascular necrosis, and inferior dislocation in the harness (leg held in excessive flexion and abduction); reassess.
• Late presentation (over 6 months) or missed dislocation: radiograph and specialist referral.

Limitations and artefacts.

• Measurement errors arise from incorrect probe positioning (tilt or rotation of the probe, wrongly chosen plane); the plane must show the straight iliac line, the lower limb of ilium and the labrum, and incorrect planes cause wrong alpha and beta angles of up to 10 degrees or more; image quality and documentation of landmarks are important.
• Operator dependence and the learning curve are large; training and accreditation, with audit, improve reliability.
• Over diagnosis and overtreatment of immature hips: scanning under 4 weeks of age leads to false positives, since many immature hips (type IIa) mature spontaneously; scanning at 4 to 6 weeks and observation for type IIa avoids unnecessary harness treatment.
• Dynamic examination is operator dependent, and excessive stress produces false instability.
• Screening results depend on timing: ultrasound after 4 to 6 months loses accuracy because of ossification of the femoral head, which shadows the acetabulum.
• Normal ultrasound at 6 weeks does not exclude later dysplasia (late onset acetabular dysplasia in a small percentage); clinical follow up and a pelvis radiograph at 4 to 6 months are used for those at risk.
• The method (static versus dynamic) affects classification, and the report must state which was used; different countries use different screening strategies.
• Ultrasound cannot assess the proximal femur ossification, and avascular necrosis is detected late.

Special groups.

• Breech infants: the highest risk group, a scan at 4 to 6 weeks regardless of clinical examination.
• Infants with a family history: risk raised 6 to 12 times; scan at 4 to 6 weeks.
• Infants with neuromuscular disorders (cerebral palsy, spina bifida, arthrogryposis): hip surveillance throughout childhood with radiographs.
• Preterm infants: correct for gestational age, and scan at an appropriate time.
• Infants with torticollis, metatarsus adductus or other postural deformities: lower threshold for scanning.
• Older infants and toddlers: radiograph with indices (acetabular index, centre edge angle) replaces ultrasound.
• Regions: DDH screening is limited; late presentation (walking age) with a limp is common; swaddling practices (tight leg extension) increase the risk, and counselling of safe hip healthy swaddling reduces it; where ultrasound is available, a targeted programme for breech and family history is worthwhile.

Follow up tests. Repeat hip ultrasound at 4 to 6 weeks for immature hips (type IIa), at 2 to 4 weeks during harness treatment, and at the end of treatment. Pelvis radiograph at 4 to 6 months (after ossification of femoral heads) to confirm normal development in those at risk or treated.

Magnetic resonance or arthrography for assessing reduction in complex cases. Blood tests and aspiration in suspected septic arthritis (C-reactive protein, erythrocyte sedimentation rate, white cell count, culture). Clinical screening of the contralateral hip and examination of the spine and feet. Orthopaedic surveillance until skeletal maturity for residual dysplasia.

And referral to paediatric orthopaedics.