Developmental Dysplasia of the Hip (DDH) - Pathogenesis

• Campbell's Operative Orthopaedics 15th Ed, p. 1402
• Miller's Review of Orthopaedics 9th Ed, p. 2797

Spectrum of Disease (Classification)

Theories of Pathogenesis

1. Mechanical / Intrauterine Positional Factors

• Breech presentation (especially extended breech) causes abnormal hip flexion forces, easily producing femoral head dislocation. Breech is found in 3-4% of all deliveries, but in up to 30-50% of DDH cases.
• The most common intrauterine position is left occiput anterior (LOA), which places the left hip in adduction against the rigid maternal lumbosacral spine - partially explaining why the left hip is more commonly affected (67% of cases).
• Oligohydramnios restricts fetal movement, increasing mechanical stress on the hip.
• Firstborn children have less intrauterine space (tighter primigravid uterus), restricting movement and predisposing to DDH.
• Campbell's, p. 1402; Bailey & Love, p. 636

2. Hormone-Induced Joint Laxity

• The maternal hormone relaxin, which relaxes the maternal pelvis during delivery, can cross the placenta and is transmitted via breast milk (in animal models).
• This produces ligamentous laxity in the neonate, allowing dislocation of the femoral head in the perinatal period.
• Girls are 4-5 times more commonly affected, presumably because they are more sensitive to the effects of relaxin and maternal estrogens.
• Generalized joint laxity (often dominantly inherited) is also associated with DDH.
• Campbell's, p. 1402; The Developing Human, p. 1007; Bailey & Love, p. 636

3. Primary Acetabular Dysplasia

• Abnormal development of the acetabulum occurs in approximately 15% of neonates with DDH.
• Breech posture during the terminal months of pregnancy may result in abnormal development of both the acetabulum and the head of the femur.
• Without a properly formed acetabulum, the femoral head lacks bony coverage and stability.
• The Developing Human, p. 1007

4. Genetic / Hereditary Factors

• A positive family history increases the risk of DDH significantly (approximately 10-20% incidence in first-degree relatives).
• The condition follows a multifactorial pattern of inheritance.
• DDH is more common in white and Navajo children and less common in Chinese and Black children, suggesting genetic predisposition.
• A 2024 systematic review (Jacobsen et al., BMC Musculoskeletal Disorders, PMID 39354451) confirms multiple genetic loci associated with DDH.

Risk Factors (in order of importance)

1. Breech positioning (strongest)
2. Positive family history / ligamentous laxity
3. Female sex (~85% of cases)
4. Firstborn child
5. Left hip predisposition
6. Oligohydramnios
7. Congenital torticollis (~8% coexistence), metatarsus adductus
8. Postnatal swaddling with hips in extension

Pathological Anatomy - Secondary Changes

• The acetabulum becomes increasingly shallow and dysplastic
• The acetabular labrum (limbus) becomes inverted and hypertrophied, blocking reduction
• The acetabular fossa fills with fibrofatty tissue (pulvinar), obstructing reduction

• The femoral head loses its normal round shape
• Increased femoral anteversion and valgus develop
• A pseudoacetabulum may form at the site of chronic dislocation

• The hip capsule becomes contracted (especially the inferomedial portion)
• The iliopsoas tendon constricts the contracted capsule (hourglass deformity)
• The ligamentum teres becomes hypertrophied and elongated
• The transverse acetabular ligament tightens across the acetabular floor

Iliopsoas tendon → Pulvinar → Hypertrophied ligamentum teres → Contracted inferomedial capsule → Transverse acetabular ligament → Inverted labrum

• Miller's Review of Orthopaedics, p. 2831

Cultural / Postnatal Factors

• Swaddling the legs together (hips in extension and adduction) exacerbates instability.
• Carrying babies astride the caregiver with hips in flexion and abduction improves hip stability.
• The relatively low incidence in Chinese children and high incidence in Navajo children likely reflects postnatal cultural practices combined with genetic susceptibility.

Recent Evidence Update

• Normal Hip - femoral head fully seated in a deep acetabulum
• Dysplasia - shallow acetabulum with inadequate femoral head coverage (still in joint)
• Subluxation - partial lateral/superior displacement, labrum begins to invert
• Dislocation - complete displacement with all secondary changes labeled:
  • Pulvinar filling the acetabulum
  • Inverted/hypertrophied labrum
  • Contracted hourglass capsule from iliopsoas
  • Pseudoacetabulum formation