Hindlimb lameness associated with a focal osseous metaplasia in an 18‐year‐old Welsh Section D mare

This report describes a case of chronic lameness secondary to an extraskeletal osseous mass located in the plantaromedial aspect of the right hind pastern in a mature Welsh Section D mare. The lesion was confirmed to represent metabolically active osseous tissue in close apposition with the adjacent plantar digital neurovascular bundle and digital flexor tendons. Surgical resection of the mass resulted in a complete resolution of lameness and return to previous level of activity. Histopathological examination classified the mass as a focal osseous metaplasia, which was most likely to be the result of previous trauma causing local haemorrhage, which resulted in subsequent dystrophic mineralisation and eventually osseous metaplasia. Similar lesions have been described in man, but have not been previously reported in the horse.