Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Four The Appendicular Skeleton 527
Fig. 4-69 A 6-year-old neutered female mixed breed dog with a
6-month history of left forelimb lameness that worsened with exercise.
The lateral view of the left humerus reveals a small area of very radiodense
bone (N, nidus). There is a small zone of relative lucency and a
fine line of sclerosis surrounding it (arrows). The nidus was surgically
removed and the dog made a full recovery. Diagnosis: Osteoid
osteoma.
OSTEOMYELITIS
Osteomyelitis, or infection of bone, can be caused by bacterial, fungal, or protozoal organisms.
Viral organisms also have been reported rarely. 262 Pathogens may gain access to bone by direct
inoculation, extension from an adjacent soft-tissue infection, or hematogenous spread. The
most common cause of osteomyelitis is complication of fracture healing or repair.
The radiographic changes caused by infection share many characteristics with
those caused by neoplasia. 178 Typically, soft-tissue swelling, bone lysis, and periosteal
reaction are seen. In some specific infections, one or more of these signs tend to predominate.
B AC T E R I A L O S T E O M Y E L I T I S
Bacterial osteomyelitis may occur anywhere within a bone. 263,264 One (monostotic) or
multiple (polyostotic) bones may be affected. 265-269 Monostotic lesions usually are due to
direct inoculation or extension of infections, whereas polyostotic disease usually is due to
hematogenous spread. 269 Both aerobic and anaerobic bacteria may be involved. 270-275
Anaerobes may be involved in up to 70% of osteomyelitis cases. 262
Radiographic findings center on bone destruction associated with extensive periosteal
and endosteal proliferation. Soft-tissue swelling usually is diffuse. 276,277 These findings may
mimic those seen with neoplasia. In rare cases both neoplasia and infection may occur
together. 278
Bacterial osteomyelitis associated with a fracture can lead to other complications. It
may cause the formation of a sequestrum, which is an avascular piece of bone that appears
abnormally radiodense and has distinct margins. If the sequestrum is within the bone, a
radiolucent area (involucrum) may be seen surrounding it. If the process breaches the cortex,
a defect (cloaca) may be seen, which may extend to the skin with a draining fistulous
tract (Figs. 4-70 to 4-72).