If the groove is too shallow, the knee cap will slip out when the knee bends. When the knee cap slips out of the groove this is called "patellar luxation". Patellar
luxation is usually an inherited defect which occurs during the developmental stages of the fetus and is rarely acquired as a result of trauma.
There are 2 different types of patellar luxation: medial and lateral patellar luxation, as can be seen in the diagram below.
When the patella slips out and goes to the inside of the leg, this is called "medial patellar luxation". This type of luxation is more common in the smaller breeds
of dogs such as, toys and miniatures. In some dogs it may be seen in puppies as they begin to walk. In other breeds it may not be seen until later in life. Pugs
will tend to have this particular type of luxation. A dog with luxating patellas may have a gait which is sometimes normal and sometimes abnormal, since the
patella may slip in and out of place. When it is out of place, the affected leg is usually carried with the joint bent and the foot turned inward. 25% of cases are
seen in both legs.
If the patella slips out and goes to the outside of the leg, this is called "lateral patellar luxation" and is more common in large or giant breeds of dogs such as
the Saint Bernard, Rottweilers, and German Shepards and is usually seen at between 5 and 6 months of age. It can be observed as a "knock-kneed" type of
stance. The foot often twists outward as weight is placed on the limb. Both knees are almost always involved.
Diagnosis of luxation is made by attempting to push the patella out of the trochlear groove. The degree of luxation is graded between 1 and 4, depending on
how easy it is to dislocate the patella and whether or not the patella returns spontaneously to the trochlear groove. The table below shows the grades of
severity and the definition for each grade.
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Grade I Patella can be manually luxated, but patella returns to position when pressure is released.
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Grade II Patella can be manually luxated or can spontaneously move to the side when the joint is flexed; patella remains luxated until it is manually reduced or
the joint is extended and the leg rotated in the opposite direction of the luxation.
* Grades I and II - Dogs with these grades, intermittently carry their affected limb with the joint flexed.
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Grade III Patella remains luxated most of the time, but can be manually reduced with the joint extended; flexion and extension of the joint results in reluxation
of the patella.
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Grade IV Patella is permanently luxated and cannot be manually repositioned. The trochlear groove is either too shallow or is missing all together.
* Grades III and IV crouching, bowlegged, or knock-kneed stances are seen for either medial or lateral luxations. Most of the weight is transferred to the front
limbs.
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Screening and Treatment:
Preliminary veterinary screening for medial patellar luxation should be done on toy and small breeds as early as 6 - 8 weeks of age, before the pups are sent
to their new homes.
Treatment for patellar luxation involves surgical procedures geared towards redefining the trochlear groove, realigning the patella, and reinforcing the soft
tissues, cartliages, and some procedures also include the use of pins and wires to assist in the stabilization of the joint structures. The specific operation done
depends a great deal on the age of the dog and the type of luxation.
Post-op Care: This usually includes limiting exercise (especially preventing jumping) for 4 weeks and then gradually increasing active use of the affected limb.
Possible Complications: Recurrence after surgical stabilization is reported to be as high as 48%, usually of a lower grade than the original luxation. 90 of
patients are free from lameness and clinical dysfunction.
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This was an excerpt from the book, "Dog Owner's Home Veterinary Handbook" 3rd Edition by James M. Giffin MD & Liisa D. Carlson, DVM with additional
notes taken from "The 5 Minute Veterinary Consult" 2nd Edition by Larry P. Tilley and Francis W. K. Smith, Jr.
Patellar Luxation
(1) Dislocation of the kneecap, mode of inheritance is unclear, thought to be polyenic (controlled by several genes), breeds affected; Cocker Spaniel,
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.
