Patellar Luxation 
(Slipping Kneecap)

Patellar luxation is the dislocation (slipping) of the patella (kneecap).  In dogs the patella is a small bone that shields the front of the stifle joint.  This bone is held in place by ligaments.  As the knee joint is moved, the patella slides in a grove in the femur.  The kneecap may dislocate toward the inside (medial) or outside (lateral) of the leg.  This condition may be the result of injury or congenital deformities (present at birth).  Patellar luxation can affect either or both legs.
The most common occurrence of luxating patella is the medial presentation in small or miniature dog breeds.  Shallow femoral groove, weak ligaments and malalignment of the tendons and muscles that straighten the joint are all conditions that will predispose a dog toward luxating patellas.
Indications of patellar luxation are; difficulty in straightening the knee, pain in the stifle, limping, or the tip of the hock points outward while the toes point inward.
The diagnosis of this condition can usually be confirmed (by a veterinarian) by manipulating the stifle joint and pushing the patella in and out of position.
        Patellar luxations fall into several categories:
        1) Medial luxation; toy, miniature, and large breeds.
        2) Lateral luxation; toy and miniature breeds.
        3) Lateral luxation; large and giant breeds.
        4) Trauma induced luxation; various breeds 

Categories 1, 2, and 3 are either hereditaryor strongly suspected of being inheritable.
Beyond the categorization of patellar luxation there are 4 grades of deviation for this condition:

Grade 1:
Intermittent patellar luxation - occasional carrying of the affected limb.  The patella can easily be manually luxated at full extension of the stifle, but returns to proper position when pressure is released.

Grade 2:
Frequent patellar luxation - in some cases luxation is more or less permanent.  The affected limb is sometimes carried, although the dog may walk with the stifle slightly flexed.

Grade 3:
Permanent patellar luxation - even though the patella is luxated; many animals will walk with the limb in a semi-flexed position.

Grade 4:
Permanent patellar luxation - the affected limb is either carried or the animal walks in a crouched position, with the limb partially flexed.

Medial Luxation in Toy, Miniature, and Large Breeds
Termed "congenital" because the luxations occur early in life and are not trauma associated.
Clinical indications:

1) Neonates and older puppies - display clinical signs of abnormal leg carriage and function from the time they start walking.  These cases are generally grades 3 or 4.

2) Young to mature animals - usually exhibit intermittently abnormal or abnormal movement all their lives.  Generally evaluated when the symptomatic gait worsens.  Most often grade 2 or 3.

3) Older animals - may exhibit sudden lameness.  Usually due to further breakdown of soft tissues or the degenerative nature of joint disease.  These cases are usually grade 1 or 2.

Lateral Luxation in Toy and Miniature Breeds
Lateral luxation in small breeds is most often seen in dogs between 5 to 8 years of age.  The skeletal abnormalities in these cases is most often obscure.  Most of these cases are grade 1 or 2 and the dog tends to exhibit a more functional disability.

Lateral Luxation in Large and Giant Breeds
Animals usually appear to be affected by 5 to 6 months of age, with bilateral involvement being the most common presentation.  The most noticed symptom is a knock-knee stance.  The patella is usually reducible.

Related articles
Southern California Veterinary Surgical Group - Their article and a diagram.
Animal Clinics article on Luxating Patella
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