Traumatic Ossifying Fasciitis Presenting as a Large Flank Mass in a Standardbred Gelding

Introduction: An 8-year old pacer was presented with a large, firm pendulous swelling off the right flank. The swelling had been present for at least 3 years.

Physical Examination The gelding was healthy. The swelling was ovoid, ~7 × 5 inches in size, very firm, and adherent to the underlying musculature. Ultrasonographically, multiple ‘shadows’, indicating mineralized tissue, were evident, deep to the capsule. The ribs beneath the mass had a roughened, irregular surface, suggesting previous trauma.

Case Management The skin around the mass was desensitized and the mass was excised via a lenticular incision. The mass was radiographed after the excision, demonstrating an inner ‘spheroid’ of irregularly ossified tissue.

Histologic Findings Cellular reactive fibrous tissue was present within the lesion’s fibrous capsule. Large fragments of maturing bone, intermediate between woven and lamellar bone were present. Small irregular spicules of woven bone were also evident, set in a loose amorphous matrix. Cartilaginous nodules were also present in the central region. The diagnosis was ‘traumatic ossifying fasciitis’.

Discussion Ossifying fasciitis/myopathy lesions develop at sites of previous trauma. We speculate that thoracic wall trauma was responsible for the initial fasciitis. In horses, ossifying myopathy is most commonly encountered within and between the semimembranosus/semitendinosus muscles, where the surrounding fascia and intermuscular planes restrict the lesions to a flattened, linear shape. In this case, flank musculature and fascia provided little or no constraint to the lesion expansion, generating this unusual presentation.

Acknowledgement: There was no proprietary interest or funding provided for this project.