A 14-month-old boy is brought to the orthopedic clinic by his parents with complaints of an asymmetric gait and frequent falls over the past 2 months. The child was born via normal vaginal delivery and is the second child in the family (first child had no orthopedic issues). On examination, the left hip shows limited internal rotation (15° instead of normal 40°) and positive Trendelenburg sign. Plain radiographs of the pelvis show a shallow acetabulum on the left side with delayed ossification of the femoral head epiphysis. The child has no history of trauma or systemic illness. What is the most likely diagnosis?

Explanation

## Clinical Presentation of Late-Diagnosed DDH **Key Point:** A 14-month-old with asymmetric gait, limited internal rotation, positive Trendelenburg sign, and radiographic evidence of acetabular dysplasia represents undiagnosed DDH with femoral head subluxation or dysplasia. ## Pathophysiology of DDH Progression ```mermaid flowchart TD A[Acetabular dysplasia]:::outcome --> B[Femoral head subluxation]:::outcome B --> C[Altered weight-bearing mechanics]:::outcome C --> D[Limited internal rotation & abduction]:::outcome D --> E[Positive Trendelenburg sign]:::outcome E --> F[Asymmetric gait & limp]:::outcome F --> G[Progressive cartilage damage]:::outcome G --> H[Early osteoarthritis risk]:::outcome ``` ## Clinical Findings in Undiagnosed DDH at 14 Months | Finding | Mechanism | |---------|----------| | Limited internal rotation | Femoral head positioned anteriorly in shallow acetabulum | | Positive Trendelenburg sign | Gluteus medius weakness due to altered hip biomechanics | | Asymmetric gait / limp | Avoidance of weight-bearing on affected side | | Delayed femoral head ossification | Chronic subluxation impairs epiphyseal blood supply | | Shallow acetabulum on X-ray | Dysplasia—acetabular index >30° is abnormal | **High-Yield:** The Trendelenburg sign (pelvic drop on the contralateral side during single-leg stance) indicates hip abductor weakness or instability, a hallmark of chronic DDH. ## Why This Is DDH, Not SCFE **Warning:** SCFE is a slippage of the femoral capital epiphysis relative to the femoral neck, occurring in adolescents (typically 10–16 years, peak 13–14 years). However: - SCFE presents with hip pain, groin pain, or knee pain (referred) - SCFE shows **loss of internal rotation with flexion** (pathognomonic) - SCFE is associated with obesity, endocrine disorders (hypothyroidism, growth hormone deficiency) - SCFE radiographs show epiphyseal slippage, NOT acetabular dysplasia This 14-month-old has **acetabular dysplasia** (shallow acetabulum) and **femoral head subluxation**, not epiphyseal slippage. ## Why This Is DDH, Not JIA **Clinical Pearl:** Juvenile idiopathic arthritis (JIA) presents with: - Joint swelling, warmth, and effusion (not seen here) - Systemic symptoms (fever, rash) in systemic-onset JIA - Elevated inflammatory markers (ESR, CRP) - Bilateral involvement is common - Radiographs show joint space narrowing and periarticular osteopenia, not acetabular dysplasia This child has no systemic illness, no joint swelling, and unilateral findings consistent with DDH. ## Radiographic Features of DDH 1. **Acetabular index** (angle between horizontal and acetabular roof) - Normal: <30° - Dysplasia: >30° - Severe dysplasia: >40° 2. **Femoral head position** - Subluxation: laterally displaced relative to acetabulum - Dislocation: complete loss of contact 3. **Delayed ossification** - Femoral head epiphysis may be hypoplastic or delayed in DDH - Results from chronic subluxation and altered blood supply ## Management at 14 Months **High-Yield:** At 14 months (beyond the optimal window for Pavlik harness, which is <6 months), management depends on reducibility: - **Reducible subluxation** → Abduction bracing or traction, then possible surgery - **Irreducible subluxation** → Surgical reduction (open or arthroscopic) + pelvic/femoral osteotomy - **Goal** → Achieve concentric reduction before secondary osteoarthritis develops **Tip:** Late-diagnosed DDH (>6–8 months) has lower success rates with conservative treatment and often requires surgical intervention to prevent long-term disability and early osteoarthritis.