Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Four The Appendicular Skeleton 503
Motion or infection, or both, will result in absorption of bone around orthopedic
devices (Fig. 4-34). 120,121 Motion usually results in a smoothly marginated, uniform loss of
bony density around the device. When infection is present a more uneven pattern of bone
density loss occurs. Soft-tissue swelling also is frequently associated with infection. The
presence of metallic fixation devices complicates treatment. 122,123 Because radiographic
changes lag behind clinical signs, it is best to treat for infection based on clinical signs
rather than waiting for the radiographic diagnosis to become evident.
Loss of a bone fragment’s blood supply due to the original trauma or subsequent infection
may result in sequestrum formation. 124 A sequestrum cannot be identified radiographically
at the time of the injury; however, as the fracture heals, the sequestered
fragment will remain dense and its edges will remain distinct. An area of relative radiolucency,
or involucrum, will form around the sequestrum while other fracture fragments show
evidence of bony proliferation or absorption (Fig. 4-35). A large sequestrum will interfere
with fracture healing, while smaller sequestra may be revascularized eventually. A persistent
sequestrum will interfere with or prevent complete fracture healing (Fig. 4-36). 125 Therefore
sequestrum recognition is important, because its removal usually is necessary.
Delayed union, which is the failure of a fracture to heal in the normally expected time, is
difficult to define precisely because of the many factors that normally affect the rate of healing.
However, this diagnosis may be apparent in some situations. A nonunion occurs when
the fracture remains and there is cessation of bone healing. The diagnosis of nonunion
should not be tendered as long as there is continued evidence of healing progress (i.e.,
additional callus present at the fracture site when sequential radiographs are compared).
A nonunion is identified by a persistent fracture line and smooth, rounded, and dense
fracture fragment margins with obliteration of the marrow cavity by callus (Figs. 4-37 and
Fig. 4-34 A 13-month-old male Golden Retriever had transverse fractures
of the distal one third of the radius and ulna that were repaired by
means of a Kirschner-Ehmer device 1 month previously. Callus is present
bridging the fracture. There are areas of radiolucency around the
Kirschner-Ehmer pins (arrows). There is bony proliferation on the caudal
aspect of the ulna at the site of proximal pin penetration and on the
cranial surface of the radius in association with the Kirschner-Ehmer
pins. Diagnosis: Infection. The irregular nature of the radiolucencies
indicates that infection rather than motion is responsible for the loss of
bone.