Outline of Veterinary Skeletal Pathology

contents Ch 3, p 2 Chapter 3, Page 3 Ch 4, p 1 

Outline of Veterinary Skeletal Pathology

Chapter 3 - Bone, Specific Diseases

F. Cat
1. Fibrous osteodystrophy.
a. Incidence. Young animals fed all meat diets, particularly young cats fed a diet consisting almost exclusively of beef hearts (low in calcium and high in phosphorus), develop a severe osteopenia.
b. Clinical features. The disease is characterized by pathologic fractures in kittens. Nutritional imbalance of calcium and phosphorus results in hypocalcemia and secondary hyperparathyroidism. Unless compensated, there is hypocalcemia, hyperphosphatemia, and elevated serum alkaline phosphatase concentrations. The signs of the disease are nervousness and irritability, reluctance to move, abnormal stance and gait, and spontaneous fractures.
c. Pathology.
(1) macroscopic appearance. In addition to pathologic fracture of long bones or vertebrae (fig. Ic-59), the bones have thin cortices, are friable and may be bent.
(2) microscopic appearance. Although the disease has features of fibrous osteodystrophy, there are other lesions because the disease occurs in an animal during an active growth phase (see osteoporosis). The physeal plate, rather than appearing rachitic, has only minimally increased depth, and the zone of columnar cartilage cells appears almost normal. The spaces between the metaphyseal trabeculae adjacent the physis contain fibrous tissue and may be almost devoid of trabeculae, or there may be a sparse number of thin cartilaginous trabeculae covered with inadequate amounts of bone matrix. The bone cortex is composed of plexiform bone, where maturation by the addition of bone on perivascular surfaces is delayed so that the cortex of the diaphysis, rather than being composed of compact bone, has the appearance of joining trabeculae lined by osteoblasts. There may be pronounced subperiosteal osteoclastic resorption, and wide zones of osteoclasts may be seen lining trabeculae adjacent to the physeal plate or articular cartilage.

2. Feline osteochondromas differ from the inherited disease (see osteochondroma) and may be associated with intralesional retrovirus. Lesions in the cat show progressive growth even in mature cats. Tumors may arise in flat bone such as the scapula and the sternum as well as long bones (figs. Ic-60, Ic-61). The histology of feline osteochondromas usually shows a cartilage cap and a base composed of trabecular bone (fig. Ic-62). The lesion differs from that of the inherited type because the hyperplastic periosteum may directly form bone, and the cartilage lesions are more disorganized.

3. Hypervitaminosis A.
a. Incidence. The condition occurs when cats are fed beef liver almost exclusively.
b. Clinical features. Animals are usually over two years of age. Cervical ankylosis causes a stiff neck and postural changes. There is cutaneous hyperesthesia, lameness and difficulty with defecation.
c. Pathology.
(1) macroscopic appearance. Extensive confluent exostoses develop on dorsal and lateral aspects of the spinous processes of the cervical and cranial thoracic vertebrae (fig. Ic-63). Although the pelvis is affected, the lumbar vertebrae and pelvic limbs are not usually involved. Periarticular osteophytes also occur in proximal joints of the limbs (fig. Ic-64).
(2) microscopic appearance. Exostoses and bone of fixed joints are composed of trabeculae that progress from woven to lamellar bone. A reversible osteopenia also is seen in long bones (fig. Ic-65).

4. Exostosis of Scottish-fold cats.
a. Incidence. The folded ear is an autosomal-dominant inherited trait, and the fold-ear allele is signified by Fd. FdFd homozygotes and Fdfd heterozygotes exhibit the trait of folded ears, while fdfd homozygotes do not have folded ears. Skeletal abnormalities develop in FdFd homozygotes.
b. Clinical features. Skeletal problems include short, thickened flexible tail and gross deformities of the extremities. Radiographs of the feet show exostosis that in the hind limb appear on the calcaneus, plantar surfaces of the tarsal bone, and around the joint margins of the digits.
c. Pathology.
(1) macroscopic appearance. Abnormalities have been found only in the skeletal system. Thickening of the tail base is due to abnormally thickened caudal vertebrae and generally precedes the development of any other skeletal abnormalities. Cubital, tarsal and carpal bones are enlarged by exostosis, and bones may be united to each other without a joint space.
(2) microscopic appearance. Pathologic descriptions are limited, but abnormal endochondral ossification is reported with thickening of the physis and disordered arrangements of chondrocytes. Biopsies taken from mature diseased cats show severe arthritis characterized by chondromalacia, chondrocyte necrosis, and cartilage erosion with replacement of the cartilage surface by fibrovascular tissue. There is no inflammation or hyperplasia of the synovial membrane. Loose fibrous tissue, dense bands of collagen, and adipose tissue lined by a synovial membrane occupy the space between the bone ends where there was a previous joint. Most bones at the margins of affected joints show remarkable exostosis with proliferation of fibrocartilage and osseous replacement.

5. Neoplasms. Osteosarcoma and chondrosarcoma are observed with about equal frequency in each of the fore and hind limbs and the head.

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