Feline physeal dysplasia

©L. E. Craig Physeal Dysplasia with Slipped Capital Femoral Epiphysis in 13 Cats Vet Pathol 38:92-97 (2001). http://www.vetpathology.org/cgi/content/full/38/1/92

Separation of the femoral capital epiphysis is usually associated with severe trauma in dogs,11,19,28 calves,20 and foals.6,15,26 The condition in pigs is termed epiphysiolysis and is considered a manifestation of osteochondrosis, with only minimal trauma required.48 Atraumatic slipped capital femoral epiphysis has also been described in the coypu (Myocastor coypus).25 In humans, the condition is well described as an atraumatic separation that occurs most often in obese adolescent males.7 This report describes a unique histopathologic lesion associated with physeal separation in 13 cats that have similar sex distribution and weight abnormalities as the human cases.

In small animals, the physes of the femoral head and greater trochanter contribute approximately 30–40% of the longitudinal growth of the femur.36 The physis is made up of five zones: 1) reserve chondrocytes, 2) proliferating chondrocytes, 3) mature chondrocytes, 4) hypertrophic chondrocytes, and 5) calcified cartilage.8 Chondrocytes in the reserve zone are scattered singly or in couples (mitotic pairs) within relatively abundant cartilage matrix. Cell division and matrix production occur at a slow rate within this population. In the proliferating, mature, and hypertrophic chondrocyte zones, the cells are arranged in rows that are parallel to the long axis of the bone. The cells are round in the reserve zone, flattened with their long axis transverse to the long axis of the bone in the proliferating zone, and polygonal with abundant cytoplasmic glycogen in the hypertrophic zone. The cells within the hypertrophic zone undergo an eightfold increase in cell volume without any increase in bone width. Therefore, the amount of matrix between the cells is markedly decreased.8 It is this zone of hypertrophied cartilage cells separated by thin strands of matrix that is the weakest and experimentally the most likely to fracture following trauma.28

Traumatic fractures of the growth plate have been classified by Salter and Harris into five types, with decreasingly favorable prognoses for normal growth plate function with each succeeding type.43 The true slipped epiphysis is classified as type I, in which only the physis is involved. In small animals, this is the most common type of physeal fracture and the proximal femur is the most common site.36 The histopathology of this type of fracture has been described in dogs euthanased following motor vehicle trauma. The findings in these dogs differed from experimental animals in that the fracture lines often crossed through multiple zones of the physis resulting in separation of the proliferative cartilage from the epiphysis. However, the chondrocytes retained their linear arrangement on both sides of the fracture site for 1–4 days following the injury.28

Slipped capital femoral epiphysis (SCFE) is a well-described entity in humans, which primarily affects overweight adolescent boys.7,30,33 The male-to-female ratio ranges from 2:1 to 4:1.30,37 The average age at diagnosis is slightly younger for girls (12 years) than boys (13 years).30,33 The onset is atraumatic and often insidious.7 Twenty-one to 80% of reported cases are bilateral.33 Undiagnosed cases are thought to be a frequent cause of degenerative joint disease of the hip in later life.7,44 Blacks are more often affected than whites, and the familial incidence ranges from 3 to 35%.5,37 Males and females are equally affected within family groups, and an autosomal dominant inheritance with variable penetrance is suggested.5,37 This report describes the histopathology of SCFE in cats with similar weight, sex, age, and clinical characteristics as SCFE in humans.

Fig. 1. Femoral capital epiphysis; cat 2. The entire slipped capital epiphysis with viable bone and marrow. Notice the abnormally thick physeal cartilage along the cleavage site. H&E. Bar = 1,111 µm.
Fig. 2. Femoral metaphysis; cat 2. The femoral neck metaphysis with abnormally thick physeal cartilage along the cleavage site. H&E. Bar = 1,111 µm.
Fig. 3. Epiphyseal side of physeal separation; cat 2. Higher magnification of rectangular area in Fig. 1 illustrating irregular chondrocyte clusters in an abundant extracellular matrix attached to the epiphysis. H&E. Bar = 84 µm.
Fig. 4. Metaphyseal side of physeal separation; cat 2. Higher magnification of rectangular area in Fig. 2 illustrating irregular chondrocyte clusters attached to the metaphysis. H&E. Bar = 84 µm.
Fig. 5. Normal physis; 8-month-old cat. Notice the regular linear arrangement of chondrocytes and the thickness of the physis compared with Figs. 3 and 4. H&E. Bar = 84 µm.

The higher than expected number of Siamese cats affected, the male predominance, and the suspected occurrence of SCFE in sibling cats (cat 8 had a male littermate that was also lame) are all supportive of a genetic aetiology. The pathogenesis of SCFE in humans is poorly understood,44 and pathological materials are rarely available.1,2 The similarity of the lesion in humans, pigs, Shetland sheepdogs, and cats suggests that these species have an analogous physeal dysplasia that results in persistence of an open, disorganized growth plate that cannot resist the shear forces associated with normal activity. This lesion is responsible for a small minority of SCFE in dogs, and the condition in pigs does not have the same male predominance as in cats and humans. Therefore, the cat may serve as an animal model in which to study the role of genetics, nutrition, obesity, endocrine imbalances, and other factors in the development of this lesion.

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