Blount Disease

• Etiology: unknown but risk factors of early ambulation and obesity suggest biomechanical component, stress to medial compartment of knee, developmental disorder with disrupted endochondral ossification of the medial proximal tibial physis leading to abnormal development of the proximal medial tibial epiphysis / metaphysis
• Radiograph: tibia has increased metaphyseal-diaphyseal angle (Drennan) of > 11 degrees (< 11 degrees is physiologic bowing, borderline is 8-11 degrees), widened medial tibial physis, medial tibial metaphysis is depressed / beaked / irregular and fragmented, abnormal and delayed ossification of the medial tibial epiphysis, angular deformities of genu varum on standing AP radiograph (mostly from tibia), procurvatum of tibia on lateral radiograph, internal rotation of tibia, lateral subluxation of tibia, limb shortening leading to limb length discrepancy if asymmetric or unilateral
• MRI: medial proximal tibia
  — Physis has widening and downsloping, physeal bar
  — Metaphysis has irregularity, downsloping, increased SI on T2WI
  — Tibial epiphyseal cartilage – far medial is thick, central mid-coronal is thin leading to increased joint space
  — Medial meniscus is thickened with +/- abnormal signal
  — Angular deformities of medial and posterior downsloping
  — Soft tissues – perichondral membrane is thickened
• DDX: physiologic bowing which shows a curved tibia while Blount disease has sharp angle at medial tibial epiphysis
• Clinical: two forms – infantile or early onset in < 4 years (usually bilateral) and late onset in > 4 years (usually unilateral)

Radiology Cases of Blount Disease