Andrew Fiske‐Jackson

Summary A 4‐week‐old Warmblood foal presented with a rapidly growing firm mass on the dorsomedial aspect of the distal left third metacarpal bone. The initial examination revealed no pain on palpation or associated lameness. Radiography demonstrated a solitary expansile large osseous cyst‐like lesion with a coarse intra‐luminal trabecular pattern associated with the cortical bone. Computed tomographic imaging was performed prior to surgery and revealed a monostotic, eccentric, diaphyseal and likely periosteal nonaggressive osseous lesion. A transcutaneous surgical approach was used to excise the mass followed by curettage of the parent bone. The foal recovered uneventfully and no re‐occurrence of the mass was noted after 12 months. Histopathology of the mass revealed a reactive process consisting of trabecular bone, haemosiderophages, granulation tissue and mesenchymal tissue proliferation consistent with an aneurysmal bone cyst. Aneurysmal bone cysts should be included in the differential diagnosis of rapidly growing masses on long bones in foals and are amenable to surgical resection.