Best Next Step in Evaluating a 13-Year-Old Obese Boy with Hip Pain
Bilateral hip radiographs (Option A) are the best next step in evaluating this 13-year-old obese boy with right hip pain, limited internal rotation, and normal WBC count. 1, 2
Clinical Reasoning
The patient's presentation has several key features that guide our diagnostic approach:
- 13-year-old male with obesity
- 1-month history of right hip pain and limp
- Limited internal rotation due to pain
- No joint swelling or erythema
- Normal white blood cell count
- Afebrile
Why Radiographs First?
Radiographs should be the initial imaging study for several reasons:
- They provide essential baseline information about bone structure and alignment
- They can detect common causes of hip pain in obese adolescents, particularly slipped capital femoral epiphysis (SCFE)
- They help rule out other conditions like fractures, Legg-Calvé-Perthes disease, and advanced osteoarthritis
The American College of Radiology (ACR) Appropriateness Criteria for chronic hip pain consistently recommend radiographs as the initial imaging study before proceeding to more advanced imaging 1.
Why Bilateral Radiographs?
Bilateral hip radiographs are specifically indicated because:
- They allow for comparison between the symptomatic and asymptomatic sides
- SCFE can be bilateral in up to 20-40% of cases, even when symptoms are unilateral
- They establish a baseline for the contralateral hip which may develop symptoms later
Differential Diagnosis
The most concerning diagnosis in this case is slipped capital femoral epiphysis (SCFE), given:
- Adolescent age (13 years)
- Obesity (major risk factor)
- Insidious onset of pain
- Limited internal rotation
- Limp
Research shows a high prevalence of SCFE-like deformities in obese children and adolescents, with one study finding that 2.7% of a cohort of obese children had radiographic evidence of prior silent SCFE 3.
Other conditions in the differential diagnosis include:
- Femoroacetabular impingement
- Labral tear
- Transient synovitis
- Juvenile idiopathic arthritis
- Legg-Calvé-Perthes disease
- Stress fracture
Next Steps After Radiographs
If radiographs are negative or equivocal but clinical suspicion remains high:
MRI without contrast would be the next appropriate imaging study (not option D initially) 1, 2
- MRI has superior sensitivity (99%) for detecting occult fractures, bone marrow edema, and soft tissue injuries
- Particularly useful for detecting early SCFE that may not be visible on radiographs
CT without contrast could be considered if MRI is contraindicated 2
- Sensitivity 79-94% for occult fractures
Why Not the Other Options Initially?
Ultrasound (Option B): Limited utility for evaluating intra-articular hip pathology in this case. The ACR rates ultrasound as "1" (usually not appropriate) for evaluating chronic hip pain with suspected intra-articular pathology 1.
CT (Option C): While useful for evaluating bony structures, it should not be the first-line imaging study before plain radiographs.
MRI (Option D): While highly sensitive, MRI is more expensive and less readily available than radiographs. It should be reserved for cases where radiographs are negative but clinical suspicion remains high.
Pitfalls to Avoid
Delaying diagnosis: SCFE is an orthopedic emergency that requires prompt diagnosis and treatment to prevent complications like avascular necrosis and early osteoarthritis.
Missing bilateral disease: Always image both hips, even when symptoms are unilateral.
Attributing symptoms to muscle strain: Hip pain in obese adolescents should raise suspicion for SCFE until proven otherwise.
Overlooking referred pain: Hip pathology can present as knee pain, so a thorough hip examination is essential in adolescents with knee pain.