A luxated patella is a condition in which the kneecap slides out of place. Instead of gliding within the natural groove in

the upper bone of the knee joint (the femur) it becomes displaced either to the inside or the outside of the joint. 

The displacement, or luxation, can be temporary or permanent, and it can be partial or complete.


In the case of most Toy Dogs, the luxated patella is much more likely to become displaced on the inner side of the

knee joint rather than the outer side. This inside displacement is called medial patella luxation, or MPL.


Although MPL can result from trauma, itís much more likely to occur as a result of a genetic predisposition - and, 

in such cases, will often show up within the first year of a Toy Dogís life. The condition is prevalent in many Toy Dogs,

including Affenpinschers, Brussels Griffons, Cavalier King Charles Spaniels, Chihuahuas, English Toy Spaniels,

Havanese, Italian Greyhounds, Japanese Chins, Maltese, Miniature Pinschers, Papillons, Pekinese, Pomeranians, 

Toy Poodles, Silky Terriers, and Yorkshire Terriers.


Whatís not clear is why Toy Dogs are prone to this condition. Many experts believe that just being small makes the

Toy Dog vulnerable to the patellas luxation. "Luxation may be a normal consequence of the miniaturization of the

legs", says Denis Marcellin-Little, D.E.D.V., associate professor or orthopedic surgery at North Carolina State

Universityís College of Veterinary Medicine.   


And the problem isnít really in the knees, according to Kurt Schulz, D.V.M., DACVS, associate professor of surgical 

and radiological sciences at the University of California-Davis School of Veterinary Medicine. "The origin of patellar

luxation is not in the ( knee ) joint primarily but in the long bones on either side of the joint" he explains.


"Breeding small dogs results in a higher incidence of torsion and bending of the tibia than in large dogs.  Why this

occurs, we are not certain."


Although experts believe that the patellar luxation is genetic in origin, "the genetic mechanisms are not very simple,"

Marcellin-Little says. "The genes have a much more subtle influence (on luxation) than those that are involved with

other (health) conditions".


Thatís because there are many different reasons why Toy Dogsí kneecaps pop out of place. One dogís knee may 

have a groove that is too shallow for the kneecap to stay in place. Another may have ligaments that are too tight  

to allow for sufficient flexibility. Other factors that affect patellar luxation include the time it takes for a dogís legs 

to grow to maturity, and the time it takes for cartilage to mineralize. All of these structures and processes are

affected, at least to some extent, by genetics.


Treating Patellar Lucation

The treatment of a Toy Dogís patellar luxation depends on two factors: the severity of the condition and the 

precise cause of the condition.


Veterinary orthopedists grade an animalís luxation according to severity. A grade of 1 is the least severe. 

In such cases, the knee generally is in the correct position but can be pushed out of place, however, 

the knee will immediately return to the correct position. A grade of 4 indicates that the knee is always out 

of position and cannot be popped back manually.


Dog with Grade 1 luxations may respond to medical management; restricted exercise, non-steroidal,

anti-inflammatory medications, and conscientious monitoring. In more severe cases, however, surgery 

may be needed to return the kneecap to its proper position and correct the anatomical abnormality that prompted 

the luxation in the first place. However, because patellar luxations can result from a variety of causes, 

"there are many opinions about the best way to treat patellar luxations, and these vary with the surgeon 

and the severity of the condition," Schulz says.


Grading a Luxated Patella

Grade 1

The patella is usually in a normal position but can be pushed out of place. 

Once out of place (luxated), it promptly pops back to the normal position


Grade 2

The patella can be in either a normal or luxated position. If luxated, it can be placed into 

a normal position and will stay there. If in a normal position, it can be manually luxated 

and will remain in that position


Grade 3

The patella is usually in a luxated position. Although it can be placed manually into a 

normal position, it will promptly pop back into the luxated position


Grade 4

The patella is always in a luxated position and canít be manually pushed back into a normal position.



A new high-tech method could eventually result in more effective treatment of patellar luxations regardless 

of the cause. The technique, called rapid prototyping, uses computer images to develop models of body parts.

Veterinary surgeons could then practice on the models before operating on the animals. For a Toy Dog with 

a luxated patella, rapid prototyping could result in faster, more customized and less traumatic surgical 

treatment than conventional surgery.


Marcellin-Little is hoping to use the technique to investigate the mechanism of cranial cruciate ligament ruptures 

In dogs. These ruptures, which correspond to anterior cruciate ligament ruptures in people, affect one out of 

five dogs with luxated patellas, he says.


According to Marcellin-Little, rapid prototyping could not only help individual Toy Dogs that suffer from patellar

luxation, but also could help narrow the scope of research into the genetic roots of the condition. By developing

models that enable scientists to study exactly how patellas luxation affects the knee and leg apparatus, 

geneticists can target their inquiries to those genes that are known to affect various aspects of that apparatus



Unfortunately, surgical treatment for patellas luxation can be very expensive. According to Cynda Seibert, 

a member of the Chihuahua Club of Americanís health committee, standard surgical treatment for one 

luxated patella costs at least $1200, and currently, rapid prototyping is even more costly: when 

Denis Marcellin-Little used the technique to treat a German Shepherd Dogís hind leg cruciate ligament problems, 

the cost came to about $6000.


For those reasons, improved data and prevention of the condition are two important strategies in the fight against

patellar luxation. Some experts advise against breeding dogs with this condition because of its strong genetic

component. Others, however, are concerned that the condition is so common that removal of all affected dogs 

from the gene pool is virtually impossible.


Experts agree that much remains to be learned about patellar luxation, including a better understanding of genetics,

the physiological causes of the condition, the true prevalence of the condition, and effective ways to treat 

affected Toy Dogs.


Signs of 

Patellar Luxation


*   Sudden pain while running

*   Lifting of affected leg off the ground

*   Range of motion pain

*   Bowlegged stance

*   Reluctance to walk or jump

*   Intermittent lameness


Another kind of Luxation

Although medial patellar luxation (MPL) is one of the most common knee ailments in dogs - especially Toy Dogs - 

itís not the only trick that knees can play. Another form the condition can take is lateral patellar luxation (LPL). 

This occurs when the kneecap pops out of place and moves to the outside of the knee joint.


LPL Occurs less frequently than MPL, but its occurrence may be more widespread among various dog breeds. 

While MPL generally affects only Toy and small-breed dogs, LPL also affects very large breeds such as 

Great Danes and Irish Wolfhounds.


Large breeds affected by this condition tend to be affected in both knees, and show signs of the condition when

young. By contract, a Toy Dog or other small dog is not likely to show signs of LPL until it is well beyond puppyhood -

generally about 4 or 5 years of age.


Indications and treatment of LPL are similar to those for MPL.


Reprinted with permission from the Purina Pro Club Toy Group Update, Nestle Purina PetCare, 2003