Pathophysiology

•synonyms: cartilaginous exostosis, osteochondromatosis, diaphyseal aclasia, and hereditary exostosis

•multiple cartilaginous exostoses is a developmental condition of growing dogs

•2 forms: single (= osteochondroma) or multicentric (= osteochondromatosis)

•breed predisposition: Alaskan Malamute (± hereditary) and Terriers

•pathogenesis is unknown but possibilities include:

•fragments of the epiphyseal plate are pinched off and grow independently

•aberrant activity (i.e., cartilaginous metaplasia) of the osteogenic layer of the periosteum

•defect in the perichondral ring, bone growth, and remodeling

•physeal dysplasia

•incidence is difficult to determine as many dogs are asymptomatic

•lesions arise from areas of endochondral ossification where new bone is formed on cartilage caps (i.e., metaphysis)

•lesions can occur in the appendicular and axial skeleton and trachea

•lesions start in the metaphyseal region but move away from the physis with longitudinal bone growth

•large lesions can cause remodeling of adjacent bone

•lesions stop growing at skeletal maturity and they usually remain as unchanged mature bony projections

•30% lesions are observed < 3 months, 39% between 3-13 months, and 30% > 13 months

•malignant transformation to OSA or CSA has been reported in 6 dogs with multiple cartilaginous exostoses (4 with metastases) and 2 dogs with solitary spinal osteochondromas

•majority of dogs with metastatic transformation have persistent cartilaginous caps on their apical margins

•chondrosarcomatous transformation is reported in 1%-5% of humans with solitary osteochondromas and 10%-25% with multiple cartilaginous exostoses

•malignant potential is related to the thickness of the apical cap on MRI

Clinical Signs

•common sites: vertebrae, ribs, and long bones

•no clinical significance unless lesion interferes with joint movement, compresses vital structures (i.e., spinal cord, nerves, blood vessels, or trachea), or undergoes malignant transformation

•multiple sites of spinal cord compression is common

•palpable masses are variably painful due to mechanical irritation from overlying soft tissues

Diagnosis

Survey Radiographs

•bony mass with a fine trabecular on the surface of the affected bone with no evidence of bony lysis or proliferation and the cortex is smooth ± thin and medullary cavity is contiguous with host bone

•bilateral and often located on the outer cortical surface adjacent to the metaphysis ± diaphysis but not the epiphysis

•eccentric outgrowths of radiodense cancellous bone with regular contours merging into the parent bone

•vertebral and rib exostoses are usually circular with a sessile base while appendicular exostoses are varied

•exostoses are covered by a cap of radiolucent cartilage which may later calcify and become radiodense

•malignant transformation is characterized by indistinct, poorly mineralized, irregular, and thickened cartilage cap (CSA > 2-3 cm) which may require CT or MRI for diagnosis

Histology

•histologic diagnosis requires biopsy of the cartilaginous cap and underlying bony stalk

•histologic features include cartilaginous cap giving rise to an array of maturing bone according to the sequence of endochondral ossification with the cortical bone surface and surface of the mass continuous

Treatment

•conservative surgical resection is recommended if clinical signs do not abate following skeletal maturity

Prognosis

•38% mortality rate < 12 months due to progression of clinical signs

•malignant transformation in up to 83% (5/6) dogs with a single osteochondroma surviving > 6 years