What is Patellar Luxation?

Pictured here is THE ORIGINAL Cairn Terrier of years ago. This is how the dog was presented in the show ring - zero grooming; no plucking, no stripping, no nothing. Just a true working terrier.

The patella, or kneecap, is part of the stifle joint (knee). In patellar luxation, the kneecap luxates, or pops out of place, either in a medial or lateral position.
Bilateral involvement is most common, but unilateral is not uncommon. Animals can be affected by the time they are eight weeks of age. The most notable finding is a knock-knee (genu valgum) stance. The patella is usually reducible, and laxity of the medial collateral ligament may be evident. The medial retinacular tissues of the stifle joint are often thickened, and the foot can be seen to twist laterally as weight is placed on the limb.
Patellar luxations fall into several categories:

Medial luxation (toy, miniature, and large breeds)
Lateral luxation (toy and miniature breeds)
Lateral luxation (large and giant breeds)
Luxation resulting from trauma (various breeds, of no importance to the certification process)
Numbers 1-3 are either known to be heritable or strongly suspected. 