The Patella is equivalent to the dog’s knee. When a dog has a luxating patella that means that the dog’s knee is dislocating. This is also known as having a trick knee or a floating patella. It is common in Boston Terriers and toy breeds, such as Pomeranians, Chihuahuas and other Terrier breeds, that are prone to weak ligaments. Most commonly this is a congenital problem, but can be caused by blunt trauma.
There are four grades of severity for the luxating patella.
In Grade one the patella will only slip out when manipulated by someone such as a vet.
In Grade two the patella will occasionally luxate when the dog is walking or running. In this instance the dog may not react much, or may act like it is in pain, but the patella will often slid back into place on its own.
In Grade three the knee will slip out of place frequently and can cause chronic lameness.
In Grade four the knee will luxate and can not be returned to its socket when manually manipulated.
Dogs with grade one luxations will often show little or no symptoms and the patella will only slip out if manipulated manually. Dogs with grade two luxations will often demonstrate spontaneous lameness that will quickly go away, for instance when a dog will occasionally carry its leg up for two or three steps then puts it down. Sometimes dogs with grade two luxations will yelp but often they do not display many symptoms. Dogs with grade three luxations may not want to jump or have more frequent episodes where they carry their legs up as described in grade two. Dogs with grade four luxations have the most pain and their legs can often not be fully straightened manually. They will show evidence of chronic pain and poor ability to jump. They may even have a stiff legged gait or won’t use the leg at all.
A Medial Luxating Patella is when the kneecap has slipped towards the opposite leg instead of up or down. Subluxation of the patella is a partial luxation, meaning that the knee will only partially slip out of the joint before it goes back in.
Over time dogs with low-grade luxation may worsen, for example a grade one could be come a grade two. This may lead to stiff joints as the dog ages and could also lead to arthritis because of the slippage of the joint over time. Surgery can correct this problem but as this is an orthopedic surgery there can be changes in bone or joints that are hidden. It is possible that the surgery to repair the luxation can cause other problems and lameness for the dog because of the hidden changes.
Another consideration for dogs with luxating patella is that they should never be bred as this is considered a congenital problem. Even if the dog has minor luxation it can be passed down to its descendants. If this problem was fixed with a successful surgery that does not repair the genes that cause the malformation and this should not make the dog a potential breeding animal.