CASE
A 6-month old boy presented for evaluation of right lower-extremity bowing. According to the mother, mild bowing present at birth had worsened as the boy grew. The baby was born at full term via vaginal delivery. The pregnancy was uneventful, although the mother was treated for preeclampsia. An initial Apgar score was reported as low, but it rebounded within 5 minutes and did not necessitate admission to a neonatal ICU. On subsequent examinations, the pediatrician found the baby's neurologic and physical development to be age-appropriate. The mother reported no fevers, pain, tenderness, swelling, or difficulties with feeding or sleeping. Family history was negative for inheritable diseases and limb deformities.
On examination, the patient had 35 degrees of anterolateral bowing and a right lower-leg segment that was clinically shorter than the left leg by 2.5 cm. Palpation at the deformity elicited no pain. The foot and ankle had some mildly increased varus positioning but demonstrated full active and passive range of motion. The patient was neurologically intact. Pulses were palpable, and capillary refill in the toes was well within 3 seconds. No skin lesions or wounds were observed. The spine, contralateral leg, and both upper extremities were normal in appearance and function. Plain radiographs revealed a lytic lesion where the middle and distal thirds of the tibia meet. Medial cortical disruption and 45 degrees of medial angulation were observed between the proximal third and the distal third of the tibia. The fibula exhibited some lateral bowing with mild cortical thickening (Figure 1). What is the likely diagnosis?
