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Difficulty: HardOrthopedic Surgery

A 13-year-old boy is brought to the pediatric orthopedic clinic by his parents due to a two-month history of a persistent left-sided limp and intermittent, dull pain in his left knee. The pain is worse after walking long distances and improves with rest. He denies any history of trauma, recent illnesses, fevers, or night sweats. He is at the 95th percentile for weight and 60th percentile for height. Vital signs are within normal limits.

On physical examination, the patient has an antalgic gait. Inspection of the left knee reveals no swelling, erythema, or deformity. Palpation of the left knee and thigh elicits no tenderness, and knee range of motion is full and painless. However, when the examiner passively flexes the patient's left hip to 90 degrees, the thigh obligatorily rotates outward. Internal rotation of the left hip is limited compared to the right and elicits mild discomfort. Distal pulses are 2+ bilaterally, and sensation is intact throughout both lower extremities.

Which of the following is the most appropriate next step in management?

  1. Bilateral anteroposterior and frog-leg lateral radiographs of the pelvisAnswer
  2. B
    Magnetic resonance imaging of the left knee without contrast
  3. C
    Reassurance, rest, and administration of as-needed oral ibuprofen
  4. D
    Ultrasonography of the left hip followed by diagnostic arthrocentesis
  5. E
    Immediate initiation of a structured physical therapy and weight reduction program

Answer

Bilateral anteroposterior and frog-leg lateral radiographs of the pelvis
The correct next step in management is obtaining bilateral anteroposterior and frog-leg lateral radiographs of the pelvis. This patient's demographic profile (obese, adolescent male) and physical exam findings (obligatory external rotation with hip flexion, loss of internal rotation) are pathognomonic for Slipped Capital Femoral Epiphysis (SCFE). Because the obturator nerve provides sensory innervation to both the hip joint capsule and the medial aspect of the knee, patients with SCFE frequently present with isolated referred knee or thigh pain. A normal knee exam in the setting of persistent knee pain should always prompt a careful evaluation of the hip. Plain radiographs are the initial diagnostic modality of choice. Bilateral views are necessary because SCFE occurs bilaterally in 20% to 40% of cases. The frog-leg lateral view is particularly sensitive for detecting early, subtle posterior displacement of the femoral head relative to the femoral neck.

Step-by-Step Solution

1
Analyze the clinical presentation.
The patient is an obese adolescent male presenting with chronic, afebrile knee pain and a limp.
Identifying demographic risk factors (obesity, early adolescence) is crucial for narrowing the pediatric orthopedic differential diagnosis.
2
Correlate symptoms with physical examination findings.
Despite the complaint of knee pain, the knee exam is completely normal. The hip exam reveals restricted internal rotation and obligatory external rotation upon flexion.
This discrepancy indicates that the knee pain is referred from the hip, a classic presentation caused by the shared sensory distribution of the obturator nerve.
3
Formulate a diagnosis and select the initial diagnostic modality.
The clinical picture is highly suspicious for Slipped Capital Femoral Epiphysis (SCFE). The gold standard initial step is plain radiography of the pelvis.
Bilateral anteroposterior and frog-leg lateral views are required to visualize the posterior and inferior displacement of the femoral head. Both hips must be imaged because SCFE presents bilaterally in a significant percentage of patients.

Key Concept

Patients with Slipped Capital Femoral Epiphysis (SCFE) often present with referred knee pain and a normal knee exam; diagnosis requires high clinical suspicion, recognition of restricted internal hip rotation, and bilateral pelvic radiographs.
Estimated Time:1m 15s
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Orthopedic Surgery

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