WHAT THE PATELLA IS AND WHAT IT DOES

 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 are 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 down over 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.

 Many dogs show no outward signs of this problem until they are about five our six years old when arthritis sets in. By then, who would think about stifle problems being the cause? Other show outward signs as early as eight weeks old by holding up one hind leg and skipping a step or two, holding one leg up as if injured, or by moving with the hind legs crouched under the body just slightly.

 Loose patellas can come about in several different ways. One day, suddenly, a puppy or adult is found to be carrying one of his hind legs. Later in the day, he is again on all fours. Upon examination, it is found that the puppy is affected with patella luxation and that the channel on the thigh bone of the dog in which the patella runs, is unusually shallow. The kneecap in inclined to slide out of position if the dog twists it's body around or suddenly turns on that particular leg.

 When there is little angulation between the long bones of the hind leg, when the dog is said to have little rear angulation, when the dog is straight in stifle, the band of muscle (patellar tendon) in which the patella is seated, is under reduced tension and can lead to the patella sliding out of its groove on the femur. We then have another form of the same trouble - luxation. This does not mean that all dogs who have little angulation automatically have loose patellas. Nor does it mean that a well angulated dog cannot have the problem. There are variables in this as well as other things. Dogs can have this defect in different degrees from a slight looseness to actual displacement of the patella. It can show up at any time; however, too often not until the dog is older and by then severe degenerative changes can already occured that cannot be corrected by surgery. Arthritic changes set in and the dog must be put down or live in pain for the rest of its life.

 Patellas can be checked for looseness as early as 4 to 5 weeks of age. Sometimes, even earlier. Loose patellas DO NOT tighten with age. Your vet can show you how to check, or have him check your puppies. Granted those patellas are tiny at 5 weeks and those fingers checking them are large, but it can and should be done.

Do not feel too secure in an early diagnosis of normal patellas. As the dog matures and gains weight, the stress put on the patellar tendon and shallow grooves may result in loosening patellas at a later age, especially on a stud dog. Patellas should always be rechecked before a dog is bred.

 In spotting an unangulated leg, it is helpful to note the narrow thigh and often a long hock. The dog that stands or moves somewhat bowlegged is suspect. Since unangulated rear legs can sometimes be hidden by coat or disguised by a clever handler, it is useful to watch toplines when the dog moves. Often dogs with knee problems do not seem to be able to extend the back legs out behind them or bend at the knee and hock easily when moving. These dogs seem to move as if a pencil was attached at the hip rather than a jointed, flexible leg. As a result, the dog is often higher in rear than at the shoulders when in motion.

MEDIAL PATELLAR LUXATION

 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.

LATERAL PATELLAR LUXATION

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

SURGERY

 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.

ETHICAL CONSIDERATIONS

 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.

RECOGNIZING THE PROBLEM

 And one of my personal favourites paragraph here: 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.

THREE STEPS TO SOUND STIFLES

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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Luxating Patella

Race Foster, DVM

Marty Smith, DVM

Drs. Foster & Smith, Inc.

 A dog from one of the smaller breeds runs across the yard chasing a tossed ball. In mid-stride, it yelps in pain and pulls its left hind leg off of the ground. After a second, he continues limping on in three-legged fashion. After ten minutes, the rear leg drops back down to the ground and he uses it normally. This episode occurs maybe once a week. It never really seems to bother him that much – a yip of pain, a short period of lameness, and in a few minutes he is back to his old self. Typically, he is a small or toy breed such as a Lhasa Apso, Pekingese, Pomeranian, Poodle, or Boston Bull.

 A luxating patella may affect some animals much more severely, They may hold the leg up for several days and show considerable discomfort. Dogs who have a luxating patella on both hind legs may change their entire posture, dropping their hindquarters and holding the rear legs farther out from the body as they walk. Those most severely affected may not even use their rear legs, walking by balancing themselves on their front legs like a circus act, holding their hindquarters completely off the ground.

 Normal knee anatomy

 The patella is the bone we know as the knee cap. A groove in the end of the femur allows the patella to glide up and down when the knee joint is bent back and forth. In so doing the patella guides the action of the quadriceps muscle in the lower leg. The patella also protects the knee joint.

 Looking at the lower front portion of the femur (the thigh bone) in a normal dog, you will notice two bony ridges that form a fairly deep groove in which the patella is supposed to slide up and down. These structures limit the patella’s movement to one restricted place and, in so doing, control the activity of the quadriceps muscle.

 The entire system is constantly lubricated by joint fluid. It works so that there is total freedom of motion between the structures.

What occurs when the patella is luxated?

 In some dogs, because of malformation or trauma, the ridges forming the patellar groove are not prominent, and a too-shallow groove is created. In a dog with shallow grooves, the patella will luxate (jump out of the groove) sideways, especially toward the inside. This causes the leg to 'lock up' with the foot held off the ground.

 When the patella luxates from the groove of the femur, it usually cannot return to its normal position until the quadriceps muscle relaxes and increases in length. This explains why the affected dog may be forced to hold its leg up for a few minutes or so after the initial incident. While the muscles are contracted and the patella is luxated from its correct position, the joint is held in the flexed or bent position. The yip is from the pain caused by the knee cap sliding across the bony ridges of the femur. Once out of position, the animal feels no discomfort and continues its activity.

 Which dogs are at risk of having a luxated patella?

 Smaller breeds of dogs, especially Miniature and Toy Poodles, have the highest incidence of patella luxation. Genetics can play a role.

 In certain breeds that have extremely short legs such as the Basset or Dachshund, patellar luxation is thought to be secondary to the abnormal shape of the femur and tibia. The curvatures of the bones in these breeds work in conjunction with the forces of the quadriceps muscles to displace the patella to the inside. Please do not misunderstand – not all members of these breeds are affected with patellar luxation, only a small portion.

 What are the symptoms?

 Most dogs are middle-aged, with a history of intermittent (on-again-off-again) lameness in the affected rear leg(s). An affected dog commonly stops and cries out

i n pain as he is running. The affected leg will be extended rearward, and for a while the dog is unable to flex it back into the normal position.

 What are the risks?

 Uncorrected, the patellar ridges will wear, the groove will become even shallower and the dog will become progressively more lame. Arthritis will prematurely affect the joint, causing a permanently swollen knee with poor mobility. Therefore, a good evaluation needs to be done by your veterinarian early in the condition to prevent long-term arthritic crippling.

 Treatment for Luxating Patellas

 As would be expected, medical therapy has little corrective ability in this disorder and surgery is therefore required and is the treatment of choice. A surgical treatment is not necessary in every individual with this condition.

 Surgery can alter both the affected structures and the movement of the patella. The groove at the base of the femur may be surgically deepened to better contain the knee cap. The knee cap itself may be "tied down" laterally (on the outside) to prevent it from deviating medially (toward the inside). The bony protuberance at the site of the attachment of the quadriceps tendon on the tibia may be cut off and then re-attached in a more lateral position. All of these procedures work well and the type performed depends on the individual case and the clinician. The animal should respond quickly after surgery and is usually completely recovered within thirty days, using its legs in normal fashion.

 Breeding Considerations

 Because of the strong genetic relationships, we really feel that animals with this disorder should not be used for breeding. They can still be excellent pets - and those that do require surgery will usually lead perfectly normal lives without any restrictions on activity.

© 2001 Drs. Foster & Smith, Inc.