Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Four The Appendicular Skeleton 505
Fig. 4-37 A 2-year-old spayed
domestic short-haired cat was hit by
a car 9 months previously, and a
fracture of the right humerus was
immobilized in a coaptation splint.
There is a fracture of the right
humerus at the junction of the middle
and distal thirds. The fracture
fragments are smooth: their margins
and medullary canals are sclerotic.
There is no callus crossing the fracture
site. Diagnosis: Nonunion
humeral fracture. There is remodeling
of the distal scapular glenoid and
proximal humeral head, which most
likely resulted from previous trauma
with malarticulation and degenerative
joint disease.
4-38). 116,125,126 With time, a nonunion may develop a pseudoarthrosis. This is characterized
by the development of a joint space and joint capsule, which is lined by synovium, between
the two bone fragments. Radiographically, a pseudoarthrosis may appear as two adjacent
fragments with smooth, sclerotic ends that have remodeled to form congruent surfaces,
which resemble a false ball and socket joint between the bone fragments.
Osteomedullography, injection of contrast media into the medullary canal of the distal fragment,
can be performed in fractures that are older than 4 weeks to confirm the suspicion of
a delayed union or nonunion. Failure of the contrast to cross the fracture line confirms the
diagnosis. 43 Fractures may heal with malaligned fragments. This is described as a malunion.
126
Stress protection may result in osteopenia beneath a bone plate or other rigid fixator
after a fracture has healed (Fig. 4-39). 127,128 This weakness can then result in a pathologic
fracture after the fixator is removed. Thin cortices and coarse trabeculation beneath or
around the bone plate are radiographic evidence of stress protection–induced osteopenia.
Disuse osteopenia, a more generalized form of stress protection–induced osteopenia,
can result from immobilization of a limb. The loss of bone density usually is more apparent
distal to the fracture (Fig. 4-40). Severe disuse osteopenia predisposes to a pathologic
fracture. This usually occurs if the animal exercises vigorously soon after the immobilization
device is removed. A gradual return to normal weight bearing should be prescribed
when disuse osteopenia is evident.
TUMORS
P R I M A RY B O N E T U M O R S
Most neoplasms of the appendicular skeleton are malignant and may be categorized as
primary bone tumors, tumors metastatic to bone, or primary soft-tissue tumors with
bone invasion. A definitive diagnosis requires both radiographic and histologic evaluation
of the neoplasm. The age, breed, and clinical history must be considered also. The
following lists several specific radiographic features to be considered when evaluating the
radiograph:
1. The soft tissues around the bone lesion.
2. The anatomical location of the lesion within the bone (e.g., metaphyseal, diaphyseal).