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(1) Dislocation of the kneecap, mode of inheritance is unclear, thought to be polyenic (controlled by several genes), breeds affected; Pomeranian, Yorkshire Tenier, CKCS, Chihuahua, Poodle, Boston Tenier, Chow, Flug, and small breeds. This condition is fairiy common in some breeds. (2) The patella is the kneecap and in dogs it can be luxated or dislocated causing lameness. It can be intermittent or recurrent and is characterised by a dog that is normal one ininate and carries its hind limb the next. No pain is felt and the dog will usually tolerate palpation of the limb but lameness is obvious if the dog is asked to use the limb. In most cases the dislocation is medial (that is turning inwards towards the centre) and in only a few instances is lateral (turning outwards). This is shown by a survey of 169 dogs reported by Endres (1977) in which 143 had medial luxation, 21 lateral and 5 had both. The patella can be manipulated into place but will usually be dislocated again and in this case will led to persistent lameness and abnormal movement. It seems that the primary &etiology is due to a shallowness or total absence of the trochlea (femoral groove). This causes the patella to slip sideways to he temporarily or permanently dislocated. Priester (1972) has studied the sex, size and breed incidence of this problem in a population of 69,245 patients at an animal clinic of which 542 had patellae lexation. There appeared to be 1.5 times more problfms in females than mmales and small breeds (under 9kg adult weight) had some 12 times greater chance of the problem than the lane dogs with very little differential risk among medium, large or giant breeds. The relative risk of small breeds which confirms earlier reports from Britain (Hodgman 1963, Knight 1963). The high relative risk among Poodles of the Miniature or Toy varieties is supported by Endres (1977) whose sample of 169 cases in Germany included over 50% from these two varieties. Knight suggested that the abnormality is a consequence of dwarfing. Koditowakku (1962) considers the defect is due to a simple antosomal recessive whereas the view of Loeffler and Meyer (1961) and Loeffler(1964) is in favour of a polygenic mode of inheritance. Several reports (AUT 1965, CAVA 1967) consider this to be inherited condition as did Hodgman (1963) but no reliable genetic evidence exists. It seems most probable that a simple genetic explanation does not exist and that the mode of inheritance, if any, is likely to polygenic. Although small breeds are mostly at risk Hodgman(1963) did list 43 breeds in which the condition was known while Priester (1972) lists 34 and suggests that other non-recognised breeds also had the problem. It does not follow that the genetics will be identical in each case. (3) In small animals, patellae Taxation usually is seen in toy and miniature breeds of dogs, in which it should be considered heritable. Usually it is termed congenital since the predisposing conditions are present at birth. It is characterized by coxa varn and a decrease in femoral Beck anteversion. Signs may be intermittent. In older animals, the condition may appear acutely as a result of minor trauma, worsening of degenerative joint disease pain, or breakdown in Soft tissue. Luxation in lame and giant breeds, also called geimu vaigum, is seen in the same breeds that are affected by hip dysplasia. In these cases, it usually is bilateral and is apparent by 5-6 months of age. Clinical Findings - Small Animals: In small breeds lexation is medial in 75 - 80% of the cams, and nearly all traumatic cases are medial; lateral luxations are seen, but usually later in life (5-gyr).The condition in ImMe breeds, described above, usually results in lateral Taxation. In all cases, the gait is affected,- medial luxations usually cause a bow-legged appearance, and lateral laxatioims cause a knock-kneed stance. The stifle is flexed to varying degrees, and the degree of weight hearing also varies. Especially in the congenitally affected small breeds, lexation way be intermittent; even when lnzated, the effect on gait may ]mot he apparent at every stride. In 15-20% of middle-aged and older dogs with chronic luxation, the cranial ligament is ruptured. Diagnosis: Clinical signs are strongly suggestive, and palpation or radiographic visualization of the displaced patella is diagnostic. This condition way resemble femoral nerve paralysis, rupture of the cranial cruciate ligament, or gonitis of any fons. Radiography and arthroscopy are helpful in evaluating osteochondrotic lesions and deformities of the trochicar groove or ridges, especially when surgical treatment is contemplated. Treatment: Animals with profound congenital deformities of the trochicar ridges have a poor prognosis, especially if the patella does not stay within the trocklear groove for some time after manual replacement. Surgical techniques to deepen the trochlear groove and reconstruct the periadicular ligaments or the joint capable may be successful in selected cases. Sources: Genetics of the Dog & Practical Genetics for Dog Breeders - Malcolm B. Willis, The Merck Veterinm7 Manual 7th Edition. |