Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

524 Small Animal Radiolo g y and Ultrasono graphy
Fig. 4-65 A 2-year-old male English Pointer with an acute lameness of
the left forelimb that developed during a field trial. The lateral radiograph
of the distal left radius and ulna reveals a small, broad-based
bony projection extending from the distal ulnar metaphysis (arrow).
The projection tapers to a small tip and is oriented at a 90-degree angle
to the long axis of the ulna. Diagnosis: Osteochondroma. This apparently
was not significant, because the dog recovered completely with 48
hours of rest.
dorsal spinous processes of the vertebrae, or the costochondral junctions. 135,233-242
Growth of the lesions usually stops at the time of physeal plate closure. Radiographically,
osteochondromas are typified in dogs by one of two radiographic appearances. One form
has a wide base attached to the metaphysis of a long bone, which projects to a narrower
tip. The osteochondroma usually is at right angles to the axis of the host bone (Fig.
4-65). The other form has an eccentric expansile lesion of the bone, usually metaphyseal,
with smooth cortex, a trabecular pattern, and no periosteal reaction. These usually are
asymptomatic unless they cause a mechanical interference with locomotion. Another
unusual form may involve the pelvis or vertebrae. These can impinge upon nervous
structures and result in neurologic signs. In dogs, osteochondroma is thought to be a
hereditary disease.
A viral etiology for osteochondroma has been suggested in some cats. 235, 243 The radiographic
appearance of the viral-associated disease in the cat is that of polyostotic, poorly
defined, variably sized, and irregularly mineralized lesions with large, radiolucent areas
interspersed within mature trabecular bone. These occur in both diaphyseal and
metaphyseal regions. In the cat, the lesions usually appear after skeletal maturity, grow progressively,
and often increase in number.
Some osteochondromas may undergo transformation to chondrosarcoma at a later
time. Malignant transformation is reportedly more frequent in multiple osteochondroma
than solitary osteochondroma. Rapid growth of the lesion after physeal closure and the
presence of an irregularly mineralized mass suggest malignant transformation.
Enchondroma. Enchondroma is an extremely rare, benign neoplasm that may produce
radiolucent defects with cortical thinning in the metaphysis or diaphysis of a long bone. 243
These lesions may be the result of a failure of normal endochondral ossification with retention
of physeal growth cartilage. 244 In humans the lesion may grow slowly, thinning and
expanding the cortex, and resulting in a pathologic fracture.