Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Four The Appendicular Skeleton 525
Fig. 4-66 A 7-year-old male domestic short-haired cat with right
forelimb lameness. A swelling was noted in the distal radial area.
The distal radius is widened. The cortex is smooth. The lucent
lesion is well demarcated from the normal bone. No fractures are
identified. Diagnosis: Bone cyst.
C AV I TAT I N G B O N E L E S I O N S
Primary Bone Cysts. Bone cysts may be monostotic or polyostotic and usually are located
in the metaphysis and diaphysis (Figs. 4-13 and 4-66). 245-251 The condition is seen most
often in young dogs, 5 to 24 months old, although bone cysts may be asymptomatic, and
therefore an animal of any age may be affected. Bone cysts also occur in cats.
Radiographically, the bone is expanded, with thin cortical margins. In multicameral cysts,
thin bony partitions within the medullary canal divide the lesion into compartments. If a
pathologic fracture occurs, periosteal proliferation and callus may be observed. Recurrence
or malignant transformation has not been observed after curettage.
Aneurysmal Bone Cyst. Aneurysmal bone cyst is a rare lesion. It usually is located eccentrically
within the metaphysis and produces bony lysis with minimal cortical or periosteal
reaction (Fig. 4-67). 130,252-255
Epidermoid Cyst. Epidermoid cysts of bone are rare neoplasms. They consist of an island
of squamous cells embedded in a bone. 178 Reported cases involved vertebral bodies and
terminal phalanges. 256-258 All were characterized by localized radiolucent areas with sclerotic
margins.
Subperiosteal Cortical Defect. Subperiosteal cortical defect, or fibrous cortical defect,
is a symptomless rarefaction of cortical bone. 259-260 These defects are predominately
metaphyseal lesions arising close to the growth plate (Fig. 4-68). In humans they usually
arise from the posterior wall of a tubular bone and affect the medial osseous surface,
producing focal, shallow, radiolucent areas in the cortex with normal or sclerotic
adjacent bone. 260 Their exact cause is unknown.
Osteoid Osteoma. Osteoid osteoma is a very uncommon lesion that has been reported to
consist of a focal radiodensity, surrounded by a small lucent area that is bordered by a fine
line of sclerosis (Fig. 4-69). 261