17-year-old chestnut Hanoverian mare (Equus caballus).This mare presented approximately seven years prior to euthanasia with lameness which progressed to swelling over the splint bones of the left forelimb. A bone scan showed intramedullary sclerosis of the left distal humerus occupying approximately one third of the bone and a suspected lesion of the glenoid cavity of the left scapula. The mare was treated with non-steroidal anti-inflammatory drugs (NSAIDs) and stall rest. No follow-up examination was recorded with reference to the lesions in the humerus and scapula.

Nearly two years prior to euthanasia, the mare presented with an approximately 10 cm diameter, rounded, boney tumor on the caudal aspect of the left mandible. The mandible was thickened in the horizontal plane, with a craterous depression immediately caudal to the mass. Radiographs revealed an altered trabecular pattern of bone extending dorsally into the mandibular cheek tooth roots.

Previous clinical history included a left-sided ethmoid hematoma diagnosed approximately eight years prior to euthanasia, which was treated with serial surgeries and formalin injection. Following complete recovery, the mare later developed a chronic, low-volume, bilateral, mucoid nasal discharge due to a mucocele, which resolved in response to 10% formalin injection. No further mucoid discharge or endoscopic changes to the ethmoid turbinates were observed following treatment. The reason for eventual euthanasia was not specified by the case contributor.

Gross Description:  

Necropsy examination was performed by the attending clinician, who reported boney masses in the left mandible and left scapula; the masses effaced pre-existing bone and contained coalescing pockets of soft, white material. The clinicans working diagnosis included osteosarcoma and fibrosarcoma.

Histopathologic Description:

Bone, mandible (slide 6) and scapula (slide 7): Extending to the margins of the submitted tissue, expanding the medulla, separating and surrounding regularly-spaced, irregularly-shaped trabeculae of woven bone, and invading and effacing overlying cortical bone is an unencapsulated, infiltrative, well-demarcated moderately cellular neoplasm composed of spindle to stellate mesenchymal cells arranged in haphazard streams on a moderately edematous fibromyxomatous matrix. Trabeculae are lined by a single layer of plump, hypertrophied, active osteoblasts, and frequently, one or more osteoclasts within Howships lacunae (osteoclastic osteolysis). Proliferating mesenchymal cells have indistinct cell borders, scant eosinophilic cytoplasm, and one oval to elongate nucleus with finely-stippled chromatin and 1-2 generally indistinct nucleoli. Mitoses are rare (less than 1 per ten 40X HPF). Scattered throughout the connective tissue are moderate numbers of osteoclasts, few aggregates of hematopoietic precursors (marrow elements), few perivascular foci of lymphocytes and plasma cells, and rare hemosiderin-laden macrophages. Multifocally, the spindle cells invade and replace the overlying woven or compact cortical bone, which also undergoes osteoclastic osteolysis. In some slides (particularly slide 7), there are aggregates of lymphocytes and plasma cells in the overlying fascia, and attached skeletal muscle myocytes exhibit sarcoplasmic pallor, swelling, vacuolation, with formation of contraction bands (degeneration) or are shrunken, angulated, fragmented, and have hypereosinophilic sarcoplasm with loss of cross striations and pyknosis (necrosis).

Morphologic Diagnosis:  

Bone, mandible and scapula: Ossifying fibroma.


Ossifying fibroma

Contributor Comment:  

The lesions in the submitted section of mandible (slide 6) and scapula (slide 7) are essentially identical. Ossifying fibroma is a benign, but invasive, lytic, and expansile fibro-osseous proliferation that is most commonly reported in the mandible in horses less than one year of age, where it is classified as equine juvenile mandibular ossifying fibroma. The lesion is less frequently seen in cats, dogs, and sheep, has been reported in greater kudus, a llama, a goat, and a rabbit, and has not been reported to undergo malignant transformation or metastasize.(reviewed in 4,6,8) This case is unusual in that the affected mare was much older than is typical, and scapular involvement is uncharacteristic of this tumor, which classically presents as a well-demarcated, expansile, non-painful mass protruding from the rostral mandible.(4,6,8) However, ossifying fibroma has been reported in sites other than the mandible, including the maxilla, and the os penis of a dog.(5) Like the most commonly affected bones of the skull, the proximal scapula develops by intramembranous ossification, so its involvement in this case is not completely unexpected. Moreover, the lesion has been reported in tubular bones in humans and cats.(2)

JPC Diagnosis:  

Bone, mandible and scapula: Ossifying fibroma.

Conference Comment:  

During the conference, participants discussed the differential diagnosis for this case in detail. Without the benefit of knowing the clinical history and radiographic findings, most conference participants diagnosed fibrous osteodystrophy (FOD), a fairly common metabolic bone disease caused by persistent primary or secondary hyperparathyoridism. Primary hyperparathyroidism results from a functional parathyroid gland adenoma, and is far less common than secondary hyperparathyroidism in all domestic species. Secondary hyperparathyroidism can result from renal disease or a nutritional deficiency or imbalance (i.e. simple dietary calcium deficiency, dietary phosphorus excess, or vitamin D deficiency). Nutritional secondary hyperparathyroidism is most common in horses, a species exquisitely sensitive to the effects of high dietary phosphorus. Horses fed a diet containing a calcium to phosphorus ratio of 1:3 or wider are highly susceptible to FOD; this most commonly occurs in horses fed a diet consisting primarily of grain, giving rise to the colloquial term bran disease for the condition. The disease is also reported in horses grazing pastures high in oxalate, even when calcium and phosphorus are normal. Disease prevalence is highest in young growing horses, and the characteristic clinical presentation is bilateral swelling of the mandibles and maxillae, prompting another colloquial term, big head. Serum PTH concentrations and urinary fractional clearance of phosphorus are increased in affected horses, and are more useful for antemortem diagnosis than are serum calcium and phosphate concentrations.(7)

Microscopically, FOD is characterized by the triad of increased osteoclastic bone resorption, fibroplasia, and increased osteoblastic activity with the formation of trabeculae of woven bone; all three features are also seen in normal bone remodeling and certain stages of fracture repair, and are present in this case as well.(7) Therefore, the diagnosis requires correlation with the clinical and radiographic findings, and examination of sections of bone distant from sites of bone remodeling or fracture repair. The numerous osteoclasts present in this case compelled some participants to discount ossifying fibroma entirely in favor of a diagnosis of FOD; however, the conference moderator cautioned participants to remember that any mesenchymal cell can secrete RANK-L, resulting in osteoclast differentiation and activation. In this case, fibrous osteodystrophy can be excluded only in light of the clinical history and radiographic findings.

The foci of inflammation within marrow spaces seen in this case are unusual, and are not typical of FOD or ossifying fibroma, urging some participants to consider osteomyelitis in the differential diagnosis. While inflammation in the marrow spaces and reactive new bone growth may result from chronic, productive osteomyelitis, the conference moderator and participants noted several features that argue against osteomyelitis in this case: 1) inflammation in osteomyelitis would be expected to occur in larger pockets, whereas in this case the foci of inflammatory cells are small and generally confined to perivascular areas; 2) the reactive trabecular bone in osteomyelitis would generally exhibit a distinct orientation around pockets of inflammation, whereas in the examined slides the new bone is arranged haphazardly; and 3) productive osteomyelitis generally yields reactive periosteal new bone growth that is oriented perpendicular to the cortex initially (though it may orient parallel to the cortex after remodeling), and would not typically produce the degree of cortical osteoclasis seen in this case.

Microscopically, ossifying fibroma is often described as intermediate between fibrous dysplasia and osteoma, two other benign bone-forming tumors considered by conference attendees and discussed during the conference. Fibrous dysplasia has been ascribed to a mesenchymal developmental abnormality, hamartomatous anomaly, or neoplastic process. While relatively common in humans, it has been only rarely reported in the horse, including once in the accessory carpal bone causing sudden-onset lameness,(2) and once in the nasal cavity causing epistaxis.(3) Fibrous dysplasia closely resembles ossifying fibroma in all respects except one: while spicules of bone in ossifying fibroma are lined by a single layer of plump osteoblasts which apparently arise from the proliferating mesenchymal cells that separate them, spicules of bone in fibrous dysplasia form directly from mesenchymal cells and are not lined by osteoblasts.(2-4,6-8) At the other end of the spectrum, osteomas are composed primarily of dense trabeculae of woven bone that, as in ossifying fibroma, are lined by a layer of plump osteoblasts; however, osteomas generally project from the surface of the bone, and their bony trabeculae are more separated by smaller spaces with more loosely-arranged mesenchymal tissue than is present in ossifying fibroma.(4,6,8)

Because reports of fibrous dysplasia, ossifying fibroma, and osteoma in horses remain sparse, differences in prognosis among these benign fibro-osseous proliferations are not yet apparent, and the more practically important duty of the veterinary histopathologist is distinction between these proliferative lesions and malignant neoplasia. Affirming this, the submitting clinicians working diagnosis, based on the gross and radiographic findings, included osteosarcoma and fibrosarcoma. Osteosarcoma, while less common than ossifying fibroma in horses, is occasionally reported in the bones of the face and jaw and shares with ossifying fibroma the tendency to replace alveolar and cortical bone, causing tooth loosening and increased susceptibility to pathological fracture. Osteosarcoma is excluded in this case by the presence of regularly-spaced, irregularly-shaped spicules of welldifferentiated woven bone, a feature characteristic of ossifying fibroma but not osteosarcoma. Moreover, the spicules of bone are lined by plump osteoblasts that are clearly distinct from the proliferating mesenchymal cells that separate the bone trabeculae. In contrast to the usually overtly malignant neoplastic cells of osteosarcoma, the proliferating mesenchymal cells in this case are microscopically bland, with a low mitotic rate, minimal anisocytosis and anisokaryosis, and absence of osteoid.(4,5,7,8) Similarly, fibrosarcoma is excluded based on the bland microscopic features of the proliferating mesenchymal cells and the gradual transition between the proliferative cell population and adjacent tissue.

Finally, though not associated with the degree of cortical lysis present in this case, idiopathic enostosis-like lesions have been reported in the long bones of horses(1) and should be considered when a proliferation of woven bone is encountered in a diagnostic setting. An example from the right distal humerus of a Swedish Warmblood riding horse was reviewed in WSC 2008-2009, Conference 21, case IV.


1. Bassage LH, Ross MW: Enostosis-like lesions in the long bones of 10 horses: scintigraphic and radiographic features. Equine Vet J 30:35-42, 1998
2. Jones NY, Patterson-Kane JC: Fibrous dysplasia in the accessory carpal bones of a horse. Equine Vet J 36:93-95, 2004
3. Livesey MA, Keane DP, Sarmiento J: Epistaxis in a standardbred weanling caused by fibrous dysplasia. Equine Vet J 16:144-146, 1984
4. Miller MA, Towle HAM, Heng HG, Greenberg CB, Pool RR: Mandibular ossifying fibroma in a dog. Vet Pathol 45:203-206, 2008
5. Mirkovic TK, Shmon CL, Allen AL: Urinary obstruction secondary to an ossifying fibroma of the os penis in a dog. J Am Anim Hosp Assoc 40:152-156, 2004
6. Speltz MC, Pool RR, Hayden DW: Pathology in practice: ossifying fibroma of the right mandible. J Am Vet Med Assoc 235:1283-1285, 2009
7. Thompson K: Bones and joints. In: Jubb, Kennedy, and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 1, pp. 82-88, 110-124. Elsevier Saunders, Philadelphia, PA, 2007
8. Thompson KG, Pool RR: Tumors of bones. In: Tumors in Domestic Animals, ed. Meuten DJ, 4th ed., pp. 248-255, Iowa State Univ. Press, Ames, IA, 2002

Click the slide to view.

1-1. Ossifying fibroma, mandible and scapula

1-2. Ossifying fibroma, mandible and scapula

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