This condition is found in the large majority of smaller breeds and some larger breeds. The canine patella is equivalent to the human knee cap. It is the bony structure that sits in a groove on the femur in front of the stifle and has tendons from the femur and tibia. These tendons are what straightens or extends the leg. As the leg is extended and flexed the patella moves up and down in the trochlear groove. Ideally the groove should be deep enough so that patella fits comfortably in it. If all the tendons, muscles and bones are properly aligned then everything works smoothly.

Patella luxation (also called slipped stifles) results from abnormalities in the bones of the rear legs, such as a shallow trochlear groove. What is luxation in relation to the patella? Luxation is the movement of the patellar bone out of its normal position within the femoral groove. Patellar movement can be lateral (movement to the outside of the knee) or it can be medial (movement to the inside of the knee). In small breed dogs the large majority (98%) of luxation are of the media type.

This condition may be congenital/inherited or acquired (caused by trauma or injury), though in over 80% of dogs with luxating patellas it is considered hereditary. This is for all breeds and mixes not specific to the Cavalier King Charles Spaniels. According to OFA statistics the percentage of Cavaliers affected is 2.3%. This condition is easily checked by manipulation by a veterinarian.

Patella luxation is graded into 4 degrees of severity. No luxation would be Grade 0.:

Grade 1 the stifle joint is almost normal and luxation is found on examination but the joint returns to normal position upon being released. Usually there is no gait abnormality.
Grade 2 the patella lies loosely in its normal position but will luxate when the joint is flexed. Dogs with a Grade 2 may have a “bunny hop” gait where the patella moves out of the trochlear groove and the dog hops along on the good leg trying to kick the bad leg straight to move the patella back in place. Many dogs can live reasonably well with this though some crepitation (noise or sensation when joints are manipulated) may be apparent over time.
Grade 3 the patella is dislocated much of the time but can be manipulated back into the joint when the leg is extended. The dog may hold the limb in a semi flexed position with little or no weight bearing when standing.
Grade 4 the patella is dislocated all of the time.

In both Grade 3 and 4 the dog shows varying degrees of lameness, often with a bowlegged appearance with the toes pointed in due to the deformity of the rear legs. Surgery is usually indicated for both Grade 3 and 4.

Mode of inheritance is unknown though most likely polygenic. Environmental issues (such as jumping up and down off furniture) may be involved in the development of the condition.