Diagnosis: Legg-Calvé-Perthes Disease
The most likely diagnosis is Legg-Calvé-Perthes disease, based on the characteristic presentation of gradual onset limp in a child aged 4-10 years, Trendelenburg gait, limited hip rotation, and most critically, decreased perfusion at the femoral head on bone scan. 1
Clinical Reasoning
Key Diagnostic Features Present
Decreased perfusion on bone scan is pathognomonic for Legg-Calvé-Perthes disease, which represents juvenile osteonecrosis of the femoral head caused by disruption of blood flow to the anterior and superior femoral head. 1, 2
The gradual onset over weeks to months with progressive refusal to run followed by limping distinguishes this from acute processes like septic arthritis or transient synovitis. 3
Trendelenburg gait indicates hip abductor weakness secondary to femoral head involvement, a classic finding in Legg-Calvé-Perthes disease. 4
Limited hip rotation (particularly internal rotation and abduction) reflects early femoral head deformity and synovitis. 3
Normal plain radiographs are typical in early-stage Legg-Calvé-Perthes disease (Waldenström Stage 1), as radiographic changes lag behind the vascular insult by weeks to months. 1, 5
Why Other Diagnoses Are Excluded
Septic arthritis and osteomyelitis are ruled out by the absence of fever, normal appearance, and gradual (not acute) onset. These conditions require fever >101.3°F, systemic toxicity, and acute presentation. 6, 7
Transient synovitis presents acutely (hours to days), resolves spontaneously within 1-2 weeks, and would not show decreased perfusion on bone scan. 3
Slipped capital femoral epiphysis (SCFE) would be visible on plain radiographs (particularly frog-leg lateral views) and typically occurs in adolescents who are overweight, not the 4-10 year age group. 3
Juvenile idiopathic arthritis would present with morning stiffness, multiple joint involvement, and elevated inflammatory markers, not isolated hip symptoms with decreased perfusion. 3
Pathophysiology and Imaging Correlation
Legg-Calvé-Perthes disease results from interruption of blood supply to the femoral epiphysis, leading to avascular necrosis. The disease progresses through predictable stages: initial (decreased perfusion), fragmentation, reossification, and healed. 1, 2
Bone scan showing decreased perfusion reflects the initial ischemic stage before radiographic changes appear, making it highly sensitive for early diagnosis. 1
Perfusion MRI studies demonstrate that affected femoral heads initially show 5-70% perfusion (average 35%), with revascularization occurring in a horseshoe pattern from posterior, lateral, and medial aspects toward the anterocentral region over 10-11 months. 1, 5
Clinical Pitfalls to Avoid
Do not dismiss the diagnosis based on normal radiographs alone. Early Legg-Calvé-Perthes disease (Waldenström Stage 1) has normal radiographs in up to 100% of cases, with cystic and sclerotic changes appearing only after weeks to months. 5, 8
The absence of fever does not rule out serious pathology, but in this case, the gradual onset and bone scan findings point away from infection. 6, 7
Hip pathology frequently presents as knee pain in up to 30% of cases due to referred pain, but this patient's examination correctly localized to the hip. 7
Next Steps in Management
MRI of the hip should be obtained to quantify the extent of femoral head involvement, assess lateral pillar perfusion (which predicts prognosis), and guide treatment decisions. 5
Perfusion levels ≥90% in the lateral third of the epiphysis predict lateral pillar group A (best prognosis), while ≤55% perfusion predicts group C (worst prognosis). 5
Orthopedic referral is mandatory for consideration of joint-preserving procedures (core decompression, osteotomies, vascular grafts) if diagnosed before femoral head collapse. 2, 8
Early intervention prior to collapse is critical, as post-collapse disease typically requires total hip arthroplasty in young adulthood. 2, 8