= CONGENITAL DYSPLASIA OF HIP

= deformity of acetabulum ← disrupted relationship between femoral head and acetabulum

◊Acetabular dysplasia (without femoral subluxation / dislocation) can be determined only by imaging!

Etiology:

1. Late intrauterine event (98%)
   1. mechanical:
      • oligohydramnios (restricted space in utero)
      • firstborn (tight maternal musculature)
        • in 60% of patients with DDH
      • breech position (hip hyperflexion results in shortening of iliopsoas muscle; L÷R = 4÷1)
        • in 30–50% of patients with DDH
        • only 2–4% of deliveries are breech
   2. physiologic (females are more sensitive to):
      • maternal estrogen (not inactivated by immature fetal liver) blocks cross-linkage of collagen fibrils
      • pregnancy hormone relaxin
2. Teratologic (2%) ← neuromuscular disorder (myelodysplasia, arthrogryposis) occurring during 12^th–18^th week GA
3. Postnatal onset (<1%)

Prevalence: 0.15% of neonates (Australia 1%, Netherlands 3.7%, Poland 3.9%, Israel 5.9%, Austria 6.6%, Norway 16.9%)

Age: most dislocations probably occur after birth; M÷F = 1÷4–1÷8; Caucasians >Blacks

Increased risk:

1. infants born in frank breech position (25%; risk of breech÷vertex = 6–8÷1)
2. congenital torticollis (10–20%)
3. skull-molding deformities; scoliosis; generalized joint laxity (Larsen syndrome, Ehlers-Danlos syndrome, Down syndrome [5%]); neuromuscular disorders (eg, myelodysplasia, spina bifida, sacral agenesis, arthrogryposis multiplex)
4. family history of DDH (6–20%): 6% risk for subsequent sibling of normal parents, 36% risk for subsequent sibling of one affected parent; 12% risk for patient's own children
5. foot deformities [metatarsus adductus, clubfoot (2%)]
6. neonatal hyperextension of hips: swaddling of infants in hip extension / strapping to cradle board

Anatomy: acetabulum has a small bony component + a large cartilaginous component at birth; acetabulum highly susceptible for modeling within first 6 weeks of age + less susceptible >16 weeks of age

Classification:

1. Normal hip
2. Lax = subluxable hip
   • Subluxability up to 6 mm is normal in newborns (still under influence of maternal hormones); decreasing to 3 mm by 2^nd day of life
3. Concentric dislocatable unstable hip
   = joint laxity allowing nondisplaced femoral head to become subluxed / dislocated under stress
   Prevalence: 0.25–0.85% of all newborn infants (⅔ are firstborns)
   • Barlow positive
   • slight increase in femoral anteversion
   • mild marginal abnormalities in acetabular cartilage
   • early labral eversion
   
   Prognosis: 60% will become stable after 1 week; 88% will become stable by age of 2 months
4. Decentered subluxed hip
   = femoral head shallow in location
   • loss of femoral head sphericity
   • increased femoral anteversion
   • early labral inversion
   • shallow acetabulum
5. Eccentric dislocated hip
   = femoral head frankly displaced out of acetabulum
   1. reducible = Ortolani positive
   2. irreducible = Ortolani negative
   • accentuated flattening of femoral head
   • shallow acetabulum
   • limbus formation (= inward growth + hypertrophy of labrum)

• “hip click” = usually result of joint capsule and tendon stretching + snapping (often confused with “hip clunk”)
• positive examination result (up to 3 months of age):
  • positive Ortolani reduction test = reduction of dislocated femoral head into the acetabulum by lifting the flexed thigh + pushing the greater trochanter anteriorly; may be associated with audible “clunk”
  • positive Barlow dislocation test = posterior displacement of nondislocated proximal femur by progressive adduction with downward pressure (piston maneuver) on flexed hips and knees associated with audible “clunk”
• warning signs on physical examination:
  • limited hip abduction on affected side
  
  
  
• shortening of thigh on affected side:
  • asymmetric thigh / buttock creases
• Allis sign = Galeazzi sign = affected knee is lower with knees bent in supine position
• Trendelenburg test = visible drooping + shortening on dislocated side with child standing on both feet, then one foot

Location: left÷right÷bilateral = 11÷1÷4

Radiologic lines:

1. Line of Hilgenreiner
   = line connecting superolateral margins of triradiate cartilage
2. Acetabular angle / index
   = slope of acetabular roof = angle that lies between Hilgenreiner's line and a line drawn from most superolateral ossified edge of acetabulum to superolateral margin of triradiate cartilage
3. Perkin line
   = vertical line to Hilgenreiner's line through the lateral rim of acetabulum
4. Shenton curved line
   = arc formed by inferior surface of superior pubic ramus (= top of obturator foramen) + medial surface of proximal femoral metaphysis to level of lesser trochanter
   • disruption of line (DDx: coxa valga)
5. Center-edge angle of Wiberg = angle subtended by one line drawn from the acetabular edge to center of femoral head + second line perpendicular to line connecting centers of femoral heads
   • <25° suggests femoral head instability

AP pelvic radiograph: >4– 6 months of age (von Rosen view = legs abducted 45° + thighs internally rotated)

◊Not reliable first 3 months of life!

• proximal + lateral migration of femoral neck:
  • eccentric position of proximal femoral epiphysis (position estimated by a circle drawn with a diameter equivalent to width of femoral neck)
  • interrupted discontinuous arc of Shenton's line
  • line drawn along axis of femoral shaft will not pass through upper edge of acetabulum but intersect the anterior-superior iliac spine (during Barlow maneuver)
  • apex of metaphysis lateral to edge of acetabulum
  • femoral shaft above horizontal line drawn through the Y-synchondrosis
  • unilateral shortening of vertical distance from femoral ossific nucleus / femoral metaphysis to Hilgenreiner's line
  • femoral ossific nucleus / medial beak of femoral metaphysis outside inner lower quadrant of coordinates established by Hilgenreiner's + Perkin's lines
• acetabular dysplasia = shallow incompletely developed acetabulum:
  • acetabular angle >30° strongly suggests dysplasia
  • development of false acetabulum
  • delayed ossification of femoral epiphysis (usually evident by 4 months (range, 2^nd–8^th months) of life

US (practical only):

Screening period: >2 weeks and up to 4–6 months of age

• Instability often resolves spontaneously by 2 weeks of age!
• Examination impractical beyond 4–6 months of age

1. static evaluation (popularized in Europe by Graf)
2. dynamic evaluation (popularized in USA by Harcke)

• Relationship of femoral head & acetabulum
  • femoral head position at rest in neutral position
  • hip instability under motion + stress maneuvers
  • dislocated (= eccentric) hip can be reduced (Ortolani positive):
    • hypoechoic femoral head not centered over triradiate cartilage between pubis + ischium (on transverse view)
    • increased amount of soft-tissue echoes (“pulvinar”) between femoral head and acetabulum
    • cartilaginous acetabular labrum interposed between head and acetabulum (inverted labrum)
      
    • posterior + superior dislocation of head against ilium
  • “equator” sign = <50% of femoral head lies medial to line drawn along iliac bone (on coronal view): >58% coverage is normal; 58–33% coverage is indeterminate; <33% coverage is abnormal
• Femoral head
  • disparity in size of directly visualized unossified femoral head
  • disparity in presence + size of ossific nucleus
• Acetabulum
  • delayed ossification of acetabular corner
  • wavy contour of bony acetabulum with only slight curvature
  • abnormally acute alpha angle (= angle between straight lateral edge of ilium + bony acetabular margin)
    • α>60° in an infant is normal
    • α55–60° can be normal <4 weeks of age
    • α<55° occurs in an immature acetabulum
    • 4°–6° interobserver variation!
  
  Prognosis: alpha-angle <50° at birth / 50°–59° after 3 months indicates significant risk for dislocation without treatment; follow-up at 4-week intervals are recommended

CT (during cast treatment / attempted closed reduction):

• sector angle = angle between line drawn from center of femoral head to acetabular rim + horizontal axis of pelvis (= reflects acetabular support)
• anterior acetabular sector angle <50°
• posterior acetabular sector angle <90°

Cx:

1. Degenerative joint disease
2. Avascular necrosis of femoral head

Obstacles to reduction:

1. Intraarticular obstacle to reduction
   1. pulvinar = fibrofatty tissue at apex of acetabulum
   2. hypertrophy of ligamentum teres
   3. labral hypertrophy / inversion
2. Extraarticular obstacle to reduction (iliopsoas tendon impingement on anterior joint capsule with infolding of joint capsule)

Prognosis: 78% of hips become spontaneously normal by 4^th week + 90% by 9^th week; >90% of abnormalities identified by ultrasound resolve spontaneously

Rx:

1. Flexion-abduction-external rotation brace (Pavlik harness) / splint / spica cast
2. Femoral varus osteotomy
3. Pelvic (Salter) / acetabular rotation
4. Increase in acetabular depth (Pemberton)
5. Medialization of femoral head (Chiari)