Signalment:  

Two year old, male castrate, Maine Coon cat.Femoral neck fracture. Femoral head osteotomy performed. Radiographically the femoral neck appeared less dense and slightly lytic.

Histopathologic Description:

The femoral head has a normal shape and the articular cartilage is microscopically normal. There is variable lack of differential staining in marrow, bone lining cells and osteocytes interpreted to be artifacts of decalcification. The marrow is mostly fatty with limited hematopoiesis and areas acute hemorrhage. At the deep specimen margin there is bone debris within the marrow spaces that is presumed to reflect method of surgical removal since no sawing was done to the specimen once received. There are shredded fragments of hyaline cartilage present on the deep margin in what would be the location of the growth plate. Compared with a normal active growth plate, the cartilage is hypocellular and chondrocytes are present in irregular groups rather than columns.

Morphologic Diagnosis:  

Femoral head: Dysplasia and fracture of physis

Condition:  

Feline coxofemoral physeal dysplasia

Contributor Comment:  

This particular case of atraumatic fracture of the femoral capital epiphysis (in a cat over one year of age) was selected to submit to the Wednesday Slide Conference because, in our experience, it is typical of the appearance of such specimens. While the most important lesions are in the growth plate, little growth plate is present on many of these specimens received for histopathologic evaluation. Only fragments of the physis remain on the femoral head and these fragments have variable hypocellularity and disorganization. The remainder of the femoral head has no significant lesions. Subtle marrow or bone lining-cell changes are not possible to detect in this specimen due to over-decalcification. Most important in considering the diagnosis of physeal dysplasia at this site in the cat is the PRESENCE of a growth plate relative to the age of the cat. The microscopic appearance of the plate might actually be non-specific and reflect that although the plate has not closed, it is not contributing to longitudinal growth. On average, the growth plate at the femoral neck in cats closes at 40 weeks¹⁰. Castration delays this by about 6 weeks but this delay is NOT associated with increased length of the bone (reported for the radius)¹¹. Therefore, although it is open longer, the growth plate is not significantly adding to longitudinal growth. The age of presentation of cats with physeal fractures appears to have changed from earlier to more recent literature. Reports of physeal fractures of the femoral neck in cats in 1993 and 1996 have no cat older than 12 months of age affected^3,9. Publications in 2001², 2002⁶, 2004⁵ and 2006⁸ report femoral capital physeal fractures mostly in cats older than 12 months (one paper restricted itself to cats over 12 months of age)⁶ and one cat in these reports was 4 years old and still had several open physes⁸. Most of these cats are castrated over-weight males and have other growth plates open well beyond the age expected for normal closure. One study reported the contralateral physis open in 13 of 18 cats with slipped physes of the femoral head⁶. Many of these cases of fracture of the physis of the femoral head in cats older than 12 months appear not to be associated with trauma. This is similar to the condition in the pig for the physis of the femoral head which is considered a form of osteochondrosis (epiphysiolysis)⁴. In the pig however, the lesions develop mostly between ages 6-18 months. At 18 months, skeletal maturity is reported to be reached⁴. 

The classification of this lesion as a dysplasia in cats, appears to be appropriate but the disorganization and hypocellarity seem more likely to be secondary to failure to properly close than a primary chondrodysplasia of the growth plate. The conclusion that the growth plates WERE normal during growth is supported by the fact that the cats appear to have reached normal skeletal growth with normal appearing skeletons within normal time. Since the signals for longitudinal growth have apparently appropriately ceased in these cats but the signals for closure have either not been recognized or sent, it is understandable that the chondrocytes remaining in the non-functional growth plate would not have their normal arrangement and density. Likely the persistence of the plate and not its abnormal arrangement and density of chondrocytes is predisposing it to slip with minimal trauma in these heavy fully grown cats.

JPC Diagnosis:  

Femoral head: Dysplasia and fracture of physis, Maine Coon (felis domesticus), feline. 

Conference Comment:  

Feline physeal dysplasia is characterized by the observation of irregular clusters of chondrocytes that are separated by abundant matrix on both the epiphyseal and the metaphyseal side of the physeal cartilage cleavage site.^1,2 This is in contrast to a traumatic fracture, in which the chondrocytes retain their linear arrangement on both sides of the fracture site.²

Although the underlying cause of feline physeal dysplasia is not known, it has been associated with various factors including genetics, nutrition, obesity, endocrine imbalances, and other factors.¹ Due to causing a delay in physeal closure, neutering has been previously considered associated with feline physeal dysplasia⁶, although in more recent literature this observation has been challenged.⁸ It is not known if the association with obesity is due to the increased stresses placed on the physis from the additional weight, causing failure under conditions of minimal trauma, or if there is an underlying endocrine abnormality that results in both obesity as well as a weakened physis.²

Epiphysiolysis in pigs is a manifestation of osteochondrosis, which has many clinical manifestations. The growth plate in affected pigs is usually characterized by a focal failure of endochondral ossification in which retained cartilage extends into the metaphysis.^2,12 The chondrocytes of the cartilage core usually maintain their normal alignment. This differs from feline physeal dysplasia where the entire physis is usually affected, and the physis consists of irregular clusters of chondrocytes that have lost their normal alignment.²

The Salter-Harris classification system has been used to classify fractures of the growth plate in animals. In this system, fractures are divided into five types based on their location. 

Salter-Harris classification system¹³

Type 1	Fracture through the physis without involvement of the epiphysis or metaphysis
Type 2	Fracture involving the metaphysis and extending into the physis
Type 3	Fracture involving the epiphysis and extending into the physis
Type 4	Fracture involving the epiphysis and metaphysis going through the physis
Type 5	Compressive fracture of the physis, crushing the growth plate

References:

1. Burke J: Physeal dysplasia with slipped capital femoral epiphysis in a cat. Can Vet J 44:238-239, 2003
2. Craig LE: Physeal displasia with slipped capital femoral epiphysis in 13 cats. Vet Pathol 38:92-97, 2001
3. Culvenor JA, Black AP, Lorkin KF, Bradley WA: Repair of femoral capital physeal injuries in cats 14 cases. Vet Comp Orthop Traumatol 9:182-185, 1996
4. Dewey CE: Disease of the nervous and locomotor systems. In: Diseases of Swine, eds. Straw BE, Zimmerman JJ, DAllaire S, Taylor DJ, 9th ed., pp. 92-95. Blackwell Publishing, Ames, IA, 2006
5. Fischer HR, Norton J, Kobluk CN, Reed AL, Rooks RL, Borostyankoi F: Surgical reduction and stabilization for repair of femoral capital physeal fractures in cats: 13 cases (1998-2002). J Am Vet Med Assoc 224:1478-1482, 2004
6. McNicholas Jr. WT, Wilkens BE, Blevins WE, Snyder PW, McCabe GP, Applewhite AA, Laverty PH, Breur GJ: Spontaeous femoral capital physeal fractures in adult cats: 26 cases (1996-2001). J Am Vet Med Assoc 221:1731-1736, 2002
7. Moores AP, Owen MR, Fews D, Coe RJ, Brown PJ, Butterworth SJ: Slipped capital femoral epiphysis in dogs. J Small Anim Pract 45:602-608, 2004
8. Newton AL, Craig LE: Multicentric physeal dysplasia in two cats. Vet Pathol 43:388-390, 2006
9. Per+�-�z-Aparicio FJ, Fjeld TO: Femoral neck fractures and capital epiphyseal separations in cats. JSmall Anim Pract 34:445-449, 1993
10. Smith RN: Fusion of ossification centers in the cat. J Small Anim Pract 10:523-530, 1969
11. Stubbs WP, Bloomberg MS, Scruggs SL, Shille VM, Lane TJ: Effects of prepubertal gonadectomy on physical and behavioral development in cats. J Am Vet Med Assoc 209:1864-1871, 1996
12. Thompson K: Bones and joints. In: Jubb, Kennedy, and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 1, pp. 136-145. Elsevier Limited, St. Louis, MO, 2007
13. Bailli+�-�res Comprehensive Veterinary Dictionary, Bailli+�-�re Tindall, Philadelphia, PA, 1988

A virtual slide is not available for this case.

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Femoral head, Maine Coon cat

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