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Difficulty: MediumOrthopedic Surgery

A 14-year-old boy with a BMI of 31 kg/m231 \text{ kg/m}^2 is brought to the clinic by his mother due to a 3-week history of right knee pain and a progressive limp. He denies any recent trauma, fever, or weight loss. Vital signs are within normal limits. On physical examination, there is no effusion, erythema, or tenderness to palpation over the right knee. Ligamentous testing of the right knee is intact. When the patient's right hip is passively flexed to 90∘90^\circ, the right thigh obligatorily rotates externally. Internal rotation of the right hip is significantly limited and elicits pain. Which of the following is the most appropriate next step in management?

  1. Bilateral hip radiographyAnswer
  2. B
    Magnetic resonance imaging of the right knee
  3. C
    Arthrocentesis of the right knee
  4. D
    Complete blood count and erythrocyte sedimentation rate
  5. E
    Reassurance and referral for outpatient physical therapy

Answer

Bilateral hip radiography
The correct answer is bilateral hip radiography. This patient's presentation of an obese adolescent male with knee pain, a limp, and significantly limited internal rotation of the hip is highly suspicious for slipped capital femoral epiphysis (SCFE). The hip and knee share sensory innervation via the obturator nerve, which frequently leads to isolated referred knee pain in patients with primary hip pathology. The classic physical examination finding of SCFE is obligatory external rotation of the thigh when the hip is passively flexed. The essential initial step in diagnosing suspected SCFE is obtaining anteroposterior and frog-leg lateral radiographs of both hips, as the condition presents bilaterally in up to 40% of patients. Early diagnosis and strict non-weight bearing are critical to prevent further slippage and complications such as avascular necrosis.

Step-by-Step Solution

1
Identify the key clinical features in the presentation.
An obese adolescent male presents with knee pain, a limp, and obligatory external rotation of the hip during passive flexion.
Recognizing the demographic risk factors (obesity, adolescence) and classic physical examination findings is crucial for generating a differential diagnosis.
2
Differentiate the source of the pain based on the physical examination.
The knee examination is completely benign, but the hip examination is grossly abnormal, indicating that the knee pain is referred from the hip.
The hip and knee share sensory innervation via the obturator nerve; primary hip pathology frequently presents as isolated distal thigh or knee pain.
3
Establish the most likely clinical diagnosis.
The presentation is highly characteristic of slipped capital femoral epiphysis (SCFE).
Obligatory external rotation with hip flexion and limited internal rotation in an obese adolescent are hallmark signs of SCFE.
4
Select the appropriate next step in management.
Obtain anteroposterior and frog-leg lateral radiographs of both hips.
Imaging is required to confirm the posterior and inferior displacement of the femoral epiphysis. Both hips must be imaged because SCFE is often bilateral.

Key Concept

Slipped capital femoral epiphysis (SCFE) often presents with referred knee pain and is characterized by obligatory external rotation of the hip during passive flexion; the initial diagnostic step is bilateral hip radiography.
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Topics

Orthopedic Surgery

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