The patella is the kneecap, which is located on the front of the stifle joint. Luxated patellas, also called genu valgum or slipped patellas, result from improper formation of the groove at the lower end of the femur in which the patella slides when the stifle joint is flexed and straightened. One or both stifles can be affected. In larger breeds like the Aussie the luxation is lateral (to the outside.) Sometimes patellar luxation can be caused by injury, but a vet would be able to detect that. The condition can be associated with straight stifles, ruptured cruciate ligaments, and hip dysplasia. Patellar luxation can lead to degenerative joint disease in the stifle in older dogs which is painful and can cause lameness.
Dogs with patellar luxation have abnormal carriage (the stifle will be kept flexed and the foot may or may not touch the ground) or abnormal gait in the hind limbs (crouched, bowlegged stance with inward-turned feet). Primary patellar luxation (that not caused by other diseases like CCL or HD) is inherited. Affected dogs should not be bred and near relatives need to be bred away from the disease. Selecting for proper stifle angulation and screening related dogs for patellar luxation will also help in breeding away from luxated patellas.