Associated with: tibia vara

Age: 9–28 months

Histo: dense hypocellular fibrous tissue resembling tendon with lacuna formation

• slight shortening of affected leg

Location: insertion of pes anserinus (= tendinous insertion of gracilis, sartorius, semitendinosus muscles) distal to proximal tibial physis; unilateral involvement

• unilateral tibia vara
• well-defined elliptic obliquely oriented lucent defect in medial tibial metadiaphyseal cortex
• sclerosis along lateral border of lesion
• absence of bone margin superomedially
  Prognosis: resolution in 1–4 years

DDx:

1. Unilateral Blount disease (typically bilateral in infants, varus angulation of upper tibia, decreased height of medial tibial metaphysis, irregular physis)
2. Chondromyxoid fibroma, eosinophilic granuloma, osteoid osteoma, osteoma, fibroma, chondroma (not associated with tibia vara, soft-tissue mass)