What is hip dysplasia?

Developmental Dysplasia of the Hip (DDH)

Developmental dysplasia of the hip (DDH) is a spectrum of abnormalities ranging from hip instability and mild subluxation to fixed dislocation, characterized by abnormal acetabular shape and malposition of the femoral head that can lead to early degenerative joint disease if left untreated. 1

Definition and Spectrum

• DDH, formerly known as congenital dislocation of the hip, encompasses various abnormalities including abnormal acetabular shape (dysplasia) and malposition of the femoral head 1
• The spectrum ranges from hip laxity and mild subluxation to complete dislocation 1
• DDH affects approximately 1.5 per 1,000 Caucasian Americans and occurs less frequently in African Americans 1

Etiology

• The pathophysiology is multifactorial and not completely understood 1
• Two leading causes are hormone-induced laxity and limited in utero hip mobility 1
• Maternal hormones (relaxin) can cause abnormally increased laxity of the hip capsule and surrounding ligaments 1, 2
• Mechanical factors such as breech positioning, oligohydramnios, and firstborn status can restrict hip mobility in utero 1
• Breech positioning produces extreme hip flexion with knee extension, leading to shortening and contracture of the iliopsoas muscle, promoting femoral head dislocation 1
• Genetic predisposition is suggested by increased prevalence among monozygotic twins compared to dizygotic twins 1

Risk Factors

• Female gender (relative risk 2.5), breech positioning (relative risk 3.8), and positive family history (relative risk 1.4) are the most important risk factors 1
• DDH is 4 to 8 times more common in females 1
• Left hip is affected three times more frequently than the right hip (relative risk 1.5), likely due to the common left occiput anterior in utero position 1
• Infant swaddling is another recognized risk factor 1
• Preterm infants are not at increased risk for DDH 1

Natural History and Consequences

• The natural history depends on the type and degree of abnormality 1
• Most borderline "abnormal" hips during the neonatal period represent physiologic immaturity 1
• 60-80% of abnormalities identified by physical examination and >90% identified by ultrasound spontaneously normalize at follow-up 1
• Late presentation is a major negative prognostic factor, increasing likelihood of complex treatment, surgical intervention, and long-term complications 1
• Untreated subluxation and dislocation inevitably lead to early degenerative joint disease 1
• DDH is estimated to cause up to one-third of all total hip arthroplasties performed in patients under 60 years of age 1, 3

Diagnosis

• Clinical evaluation should be performed at every well-baby visit according to the recommended schedule (2-4 days for newborns discharged in <48 hours, 1 month, 2 months, 4 months, 6 months, 9 months, and 12 months) 1
• Key physical examination findings include:
  • Positive Ortolani test (relocating an already dislocated hip) or Barlow test (dislocating a reduced but unstable hip) 1, 4
  • Limited hip abduction (especially after 3 months of age) 1, 4
  • Asymmetric skin folds and leg length discrepancy 1
• Imaging plays an integral role in screening, diagnosis, and monitoring 1
• Ultrasound is the preferred imaging modality for infants under 4-6 months of age 4, 5
• Radiographs become more reliable after 4-6 months when ossification centers develop 1, 4

Screening Recommendations

• The American Academy of Pediatrics recommends selective screening rather than universal ultrasound screening 1
• Selective ultrasound screening should be performed in infants with:
  • Risk factors (family history of DDH, breech presentation) 1
  • Inconclusive findings on physical examination 1
• Ultrasound for DDH should be performed after 2 weeks of age, as laxity is common after birth and often resolves spontaneously 1

Treatment Approach

• Treatment depends on the age of the patient and the reducibility of the hip joint 6, 3
• Early diagnosis usually leads to low-risk treatment with a harness, while late diagnosis may require surgical intervention 6
• For infants up to 6 months of age, the Pavlik harness is typically the first-line treatment 3
• If harness treatment fails, closed reduction and spica casting is usually performed 6
• After 18 months of age, treatment usually consists of open reduction and hip reconstruction surgery 6, 3
• The most serious complication of treatment is avascular necrosis, which is a predictor of poor prognosis 1, 3

Long-term Implications

• Persistence of hip dysplasia into adolescence and adulthood may result in abnormal gait, decreased strength, and increased rate of degenerative hip disease 6, 7
• Hip dysplasia is seen in 20-40% of patients with osteoarthritis of the hip 8
• Early and correct identification is crucial to offer timely joint-preserving treatment 7
• Modern total hip replacement remains an excellent option for more arthritic joints, though difficulties can arise from anatomical abnormalities and previous operations 8