A 11-year-old, Haflinger breed mare developed a slowly growing mass, protruding on the ventral aspect of the mandible. On palpation, the lesion was pedunculated, very firm, cold and not painful. Radiographic examinations showed a radiopaque, multi-lobular, "mushroom-like" lesion, which arose from the mandibular cortex without involving the bone marrow cavity. Thereafter, the mass was excised for cosmetic purposes. Microscopically, the lesion mainly consisted of bone trabeculae, covered by a thick layer of connective tissue resembling the periosteum. History and clinical findings allowed the temptative diagnosis of mandibular osteoma, which was further supported by diagnostic imaging and pathological features. In particular, the presence of pedunculated lesions arising from the bone surface appeared as highly suggestive of osteoma. Equine osteomas usually affect paranasal sinuses and the rostral mandible and share many similarities with other neoplastic and neoplastic-like conditions, such as ossifying fibroma and fibrous dysplasia. The prognosis is good whenever the neoplasm is fully removed, that depending on the localization and the size of the mass.
Keywords: Bone; Differential diagnosis; Horse; Mandibular osteoma; Neoplasm.
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