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Patellar Luxation

(Slipped stifles)


[What the patella is and what it does] [Medial Patellar Luxation] [Grade I] [Grade II] [Grade III] [Grade IV] [Lateral Patellar Luxation] [Ethical Considerations] [Recognizing the problem] [Three Steps to Sound Stifles]

The canine patella is equivalent to the human knee cap. It is a bony structure that sits in a groove in the femur (thigh bone) on the front of the stifle (knee joint).

Attached to the patella is the quadriceps tendon and the patellar tendon. The quadriceps tendon runs from the patella up to the muscle mass on the front of the thigh and to a lesser extent downover the patella to the patella tendon. The patella tendon runs from the patella down to a protuberance on the front of the tibia.

When the muscles contract, they pull the quadriceps tendon up which pulls the patella up, which, in turn pulls on the tibia via the patella tendon. The result of all this pulling is to straighten or extend the leg.

As the leg is extended and flexed, the patella rides up and down in the trochlear groove. Ideally, the groove should be deep enough for the patella to fit snugly into it. If the tendons, muscles and bones are all aligned properly, the result is a stable, sound stifle joint. Improper alignment means instability and this not only leads to degenerative joint disease, but also places increased stress on other ligaments and supporting structures.

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This condition, commonly called slipped stifles, results from several abnormalities of the bones of the hind legs. These include a shallow trochlear groove and twisting and bending of the femur and tibia.

Proper joint function relies on a perpendicular alignment of the quadriceps tendon, patella and patellar tendon, one right above the other when viewed from the front. If the upper end of the tibia is rotated inward, for example, it takes the attachment of the patellar tendon with it. The alignment is no longer perpendicular and patella is pulled to the inside (medially).

Medial patellar luxations (dislocations) can occur in varying degrees of severity and they have been classified into four grades:


Grade I:
The stifle joint is almost normal and the patella luxates only when the leg is extended and the patella is pushed. When the leg is extended, there is less tension on the tendons holding the patella in place, and it is therefore easier to push it out of place. Usually, there is no gait abnormality, but as the animal becomes older, it may develop arthritis.


Grade II:
The patella usually lies loosely in its normal position, but it will luxate medially with flexion of the joint. These animals have a "hopping" gait. The patella becomes trapped out of the trochlear groove on flexion and the dog hops along on the other leg while kicking the sore leg straight to allow the patella to move back into position. Animals with Grade II luxation are more likely to develop arthritis that those with Grade I because of the greater degree of abnormal patella movement.


Grade III:
The patella is dislocated most of the time, but can be put back manually if the leg is extended.


Grade IV:
The patella is dislocated all of the time and cannot be corrected without surgical intervention. Grades III and IV show varying degrees of lameness, often the dog assumes a crouched position due to the inability to fully extend the stifles; often they appear bowlegged with the toes pointed inward due to deformities in the rear limbs. TopBack to index


This problem is less common and is seen most often in large breeds. TopBack to index


There are various techniques for correcting patellar luxation, but all of them are designed to re-establish the proper perpendicular alignment of the quadriceps tendon, the patella, and the patellar tendon and to prevent sideways movement of the patella.

The veterinarian will not usually recommend surgery for Grade I cases, but surgery becomes more advisable with the increasing severity.

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Dogs can have Grade I or II slipped stifles and move quite normally. As these dogs become older, either with the breeder or in pet homes, they may develop lameness. Sold at a young age by an unknowing breeder, even the Grade II and IV dog can go undetected for awhile. If the new owner does not recognize the problem in time, severe degenerative changes can occur that cannot be corrected by surgery.

The conscientious breeder is aware - aware of the problem in the breed, aware of the stifle status of all breeding stock, aware of the consequences of not breeding the problem out, aware that soundness and health are just as important as breed type.

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I have heard many well-known breeders state that they do not have a problem with slipped stifles in their kennels. Further questioning revealed they did in fact have the problem but they couldn't SEE it. Because all their dogs were walking around normally, they could see no reason to be concerned. I suspect most breeders who feel this way do not keep their breeding stock into old age. To be honest is the key.

If we are to ever eliminate patellar luxation, everyone who breeds Chihuahuas must know about the problem and be able to recognize it in their dogs. As the patella moves in and out of the trochlear groove, there is a "clicking" feeling. All you have to do in some dogs is take hold of the back legs, one in either hand, and raise and lower the back feet off the ground to feel the clicking. Judges do this while checking for rear leg straightness, and they should discover patellar luxation more often this way than trying to feel a loose patella with the leg on the ground.

If your dog passes this test, try holding one hind leg in the hock area and stretching the leg out straight behind. While the leg is fully extended, twist the hock outwards, turning the toes inward and holding it in this position, push the leg so as to bend it. If you feel a "click", it is the patella popping back into the groove.

Detecting very slight luxations is more difficult and it would be a good idea to have your veterinarian show you how to push the patella with your thumb while holding the leg extended.

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1. Accept that patella luxation is a problem.

2. Learn to recognize it in your dogs.

3. Conduct a breeding program to eliminate it.

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Thanks to
Randy & Lynne George

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