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Difficulty: HardPediatric Musculoskeletal Disorders

A 12-year-old boy is brought to the clinic for a 4-week history of right-sided knee pain and a progressive limp. The pain is characterized as a dull ache that is exacerbated by walking and participating in sports. He has no history of trauma, falls, or fever. His medical history is significant for growth hormone deficiency, for which he has been receiving somatropin injections for the past 6 months. His body mass index is at the 60th percentile for his age. His temperature is 37.1°C (98.8°F), blood pressure is 110/70 mm Hg, and pulse is 78/min. On physical examination, there is no swelling, erythema, or localized tenderness over the right knee, and ligamentous testing is stable. Range of motion of the right knee is full and painless. When the patient walks, he exhibits an antalgic gait with the right foot pointing outward. During examination of the hips, passive flexion of the right hip results in obligate external rotation of the thigh. Internal rotation of the right hip is limited compared to the left. Which of the following is the most appropriate next step in management?

  1. Obtain bilateral anteroposterior and frog-leg lateral pelvic radiographsAnswer
  2. B
    Obtain anteroposterior and lateral radiographs of the right knee
  3. C
    Obtain an ultrasound of the right hip
  4. D
    Measure serum erythrocyte sedimentation rate and C-reactive protein
  5. E
    Recommend knee immobilization and nonsteroidal anti-inflammatory drugs

Answer

Obtain bilateral anteroposterior and frog-leg lateral pelvic radiographs
The correct answer is to obtain bilateral anteroposterior and frog-leg lateral pelvic radiographs. This patient's clinical presentation is highly suspicious for Slipped Capital Femoral Epiphysis (SCFE). While the classic demographic for SCFE is an obese adolescent boy, it frequently occurs in children with underlying endocrinopathies, such as hypothyroidism or those receiving growth hormone therapy, which weakens the physeal plate. A classic diagnostic pitfall in SCFE is referred pain to the anterior thigh or knee via the sensory branches of the obturator nerve, causing the patient to complain primarily of knee pain. However, physical examination will reveal a normal knee but an abnormal hip, characteristically demonstrating obligate external rotation of the thigh during passive hip flexion and loss of internal rotation. When SCFE is suspected, the most appropriate next step is to obtain bilateral AP and frog-leg lateral radiographs of the pelvis. Bilateral imaging is crucial because contralateral asymptomatic slippage occurs in up to 40% of patients. Once the diagnosis is confirmed, patients must be strictly non-weight-bearing to prevent further slippage and urgently referred for surgical pinning in situ.

Step-by-Step Solution

1
Analyze the patient's clinical presentation and underlying risk factors.
The patient is a 12-year-old boy presenting with insidious-onset right knee pain, an antalgic gait, and obligate external rotation of the hip upon passive flexion. He is receiving growth hormone therapy and has a normal BMI.
Identifying these key demographic and clinical features is necessary to recognize atypical presentations of common pediatric orthopedic conditions.
2
Localize the source of the pain based on physical examination findings.
Despite the complaint of knee pain, the knee examination is entirely normal. Conversely, the hip examination reveals limited internal rotation and obligate external rotation during flexion, indicating the true pathology is in the hip.
Referred knee pain via the sensory branches of the obturator nerve is a classic diagnostic pitfall in pediatric hip disorders.
3
Synthesize the findings to formulate the most likely diagnosis.
The combination of hip pathology in an adolescent exacerbated by growth hormone therapy strongly points to Slipped Capital Femoral Epiphysis (SCFE).
Growth hormone therapy causes rapid bone growth, which alters the biomechanics of the physis and is a known risk factor for SCFE, even in non-obese children.
4
Determine the most appropriate initial diagnostic test for the suspected condition.
The diagnostic modality of choice for SCFE is bilateral anteroposterior and frog-leg lateral radiographs of the pelvis.
Bilateral imaging is required to identify the physeal displacement and to check for asymptomatic contralateral slippage, which is common in these patients.

Key Concept

Patients with Slipped Capital Femoral Epiphysis (SCFE) often present with referred thigh or knee pain and a normal knee examination. The physical exam finding of obligate external rotation with passive hip flexion is characteristic. Diagnosis requires bilateral anteroposterior and frog-leg lateral pelvic radiographs.
Estimated Time:1m 30s
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Topics

Pediatric Musculoskeletal Disorders

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