What is the most likely diagnosis for a 13-year-old with hip pain, a hip held in external rotation, and an antalgic gait?

Slipped Capital Femoral Epiphysis (SCFE)

The most likely diagnosis for a 13-year-old presenting with hip pain, external rotation positioning of the hip, and antalgic gait is slipped capital femoral epiphysis (SCFE). This is the most common hip disorder in adolescents and represents a true orthopedic emergency requiring urgent surgical referral 1, 2.

Classic Presentation

The clinical triad you describe is pathognomonic for SCFE:

• External rotation deformity is a hallmark finding—the affected limb assumes an externally rotated position at rest 1, 3
• Antalgic gait (limping that favors one leg) is present in the majority of cases 1, 4
• Hip pain in an adolescent, though pain may be referred to the thigh or knee rather than localized to the hip 1, 2, 5

Obligate external rotation with passive hip flexion is the pathognomonic physical examination finding—when you flex the hip, it automatically rotates externally 3. This occurs because the femoral head has slipped posteriorly and medially off the femoral neck through the growth plate.

Critical Diagnostic Considerations

Age and Risk Factors

• SCFE typically occurs between ages 8-15 years, with peak incidence during adolescent growth spurts 1, 5
• Obesity is the strongest risk factor, though SCFE occurs in non-obese children as well 1, 5
• Endocrine abnormalities (hypothyroidism, growth hormone supplementation, hypogonadism) are occasionally associated 1

Why This Diagnosis Cannot Be Missed

SCFE is one of the most commonly missed diagnoses in children, and delay in diagnosis leads to devastating consequences 1, 2:

• A stable SCFE can progress to an unstable slip if diagnosis is delayed 2
• Unstable SCFE carries a much higher risk of avascular necrosis (AVN) of the femoral head 2, 3
• Progressive deformity increases the risk of early-onset hip arthritis 2
• Chondrolysis and femoroacetabular impingement are additional complications of delayed treatment 1

Immediate Diagnostic Workup

Order bilateral hip radiographs immediately with the following views 1, 3:

• Anteroposterior (AP) pelvis view with the patella directed anteriorly
• Frog-leg lateral views if the patient can tolerate positioning (for stable SCFE) 1
• Cross-table lateral views if the patient cannot tolerate frog-leg positioning or if unstable SCFE is suspected 1

Radiographic Findings

Early signs include 4:

• Widening and blurred margins of the epiphyseal plate
• Klein's line abnormality—a line drawn along the superior femoral neck fails to intersect the femoral epiphysis (normally it should intersect a portion of the epiphysis) 4
• Loss of height of the femoral epiphysis due to posterior-medial displacement 4

Advanced cases show 4:

• Obvious posterior and inferior displacement of the femoral head relative to the femoral neck
• The slip is often more apparent on the lateral view than the AP view 4

Critical Pitfall to Avoid

Never dismiss hip pathology in an adolescent presenting with isolated knee pain 2, 5. Referred pain from the hip to the knee is extremely common in SCFE because both joints share innervation from the obturator nerve 2.

• Every child with knee, thigh, or groin pain must undergo a complete hip examination 2
• A "groin pull" is exceedingly rare in children and should be a diagnosis of exclusion 2
• Physical examination findings of out-toeing, decreased internal rotation, or obligate external rotation with flexion should prompt immediate hip radiographs 2

Differential Diagnosis Exclusions

While the guidelines emphasize excluding serious pathology in young adults with hip pain 6, SCFE itself IS the serious pathology that must be excluded first in a 13-year-old with this presentation:

• The consensus guidelines specifically list SCFE as a "serious hip pathological condition" that must be ruled out before considering other diagnoses like femoroacetabular impingement syndrome or labral tears 6
• Other conditions in the differential (septic arthritis, tumor, stress fracture, Perthes disease) would present differently and are less likely given the classic external rotation deformity 6

Immediate Management

This is a surgical emergency requiring urgent orthopedic referral 1, 3:

• Make the patient non-weight-bearing immediately to prevent progression from stable to unstable SCFE 1
• Refer urgently to pediatric orthopedics the same day 2
• In situ percutaneous screw fixation is the gold-standard treatment and should be performed urgently to prevent slip progression 1, 3
• The contralateral hip requires monitoring until skeletal maturity, as bilateral involvement occurs in a significant percentage of cases 3