Luxating Patella


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What Is A Luxating Patella Problem ?

When the structures that make up your pet's knees (stifles) are misaligned or misshapen, a problem called "trick knee" or patellar luxation often occurs. Your pet’s kneecaps are an important component of a normally-functioning knee joint. These kneecaps patella) are meant to ride in a groove on the face of the femur. The patella acts as a pulley, giving leverage to extend the knee as your pet walks.

Look at this drawing I made of your pet’s knee. The kneecap should rides smoothly in a groove (trochlear groove) over the femur, (the large bone of the thigh). On the end nearest your dog’s body, a strong ligament attaches the patella to the large thigh muscles. On the other end, a ligament attaches the patella to the pet’s shinbone or tibia . Alignment ligaments located on the inner (medial patellar ligaments) and outer side (lateral patellar ligaments) of the knee help keep the patella riding in its track.

When a pet has a luxating (out of place) patella, this small bone jumps out of its normal grove as the leg is in motion. In over 90% of these cases in dogs, the patella jumps out of its tract to the inside of the pet’s knee (medial patellar luxation or MPL).

This problem is one of the five most common genetic (birth defect) problems we see in dogs. (ref) Three leg abnormalities in your pet lead to this:

The first abnormality that is usually present is a trochlear groove that is too shallow. In order for the patella not to jump out of this tract, the groove must be deep enough to accommodate and cradle the patella as it moves up and down in the groove. Some dogs are born with an abnormally shallow trochlear groove.

The second problem is probably a result of the first. It is a weakened and stretched lateral patellar ligament. Again, although only one knee may appear to be affected, in most cases, both knees of your pet share some degree of this problem.

The third problem occurs when the lower attachment of the kneecap ligament is too far to the inner side of the shinbone or tibia. This is a frequent problem in dogs that are bred to have exceptionally short legs (like dachshunds). When dogs develop luxating patellas before the tibia has reached maturity, it is also possible for this lower point of attachment to shift inward – throwing off the entire joint alignment.

One or all of these three problems are responsible for your pet’s knee locking up as it walks. These problems were not present when your pet was born. They resulted from this group of abnormalities involving your pet's entire hind limb that slowly change the contour of the bones as the pup grew. You can read about that here.

On very rare occasions, patellar problems are the result of direct or strain-injuries to the knee cap or accidental trauma to the knee joint that tear the collateral ligaments that keep the patella moving in line. (ref)

Is The Problem More Common In Some Breeds ?

Yes, the problem is quite common in Pomeranians, dachshunds, toy and miniature poodles, Yorkshire terriers and Boston bulldogs. It is also seen occasionally in Boykin spaniels, cocker spaniels, chow chows, Belington terriers, Australian terriers, Japanese chin, shar-pei, mi-ki, Lhasa Apsos, Tibetans spaniels, Tibetan terriers, and Labrador retrievers (more or less in that order of frequency).

ou can read 2011statistics on how often the problem is likely to occur in 105 common dog breeds here.

These tiny dogs are thought to be 12 times more likely to have luxating patellas (MPL) than larger breeds; but they are often about three years old before their knees begin to lock up and owners become concerned. You can read more about their special problem here.

There are veterinarians that believe that knee problems, particularly when they occur in larger dogs, actually start as hip problems. You can read a UK study that discusses that here. In another UK report, Labrador retrievers made up the bulk of the large dog patellar luxation problems. You can read about how those cases were handled and what their outcomes were here.

What Are The Signs Of A Luxating Patella In My Pet ?

The signs you will see in your dog depend on how severe the problem is and how long the problem has been present.

Most owners just notice that their pet begins to occasionally skip when it runs. There is no sign that there is any pain - your dog just doesn’t let one rear leg touch the ground as it runs or walks. Then, after a few steps, the leg usually returns to normal and the pet seems unconcerned.

In other pets, owners notice a pop while the dog is shifting around on the owner’s lap or being handled. It is quite often groomers or veterinarians that first bring the problem to the owner’s attention.

Most pets I have seen with this problem are over six months old when their gait problems were first noticed; but it can occur as early as 8-10 weeks of age when the defects are severe. On its first post-purchase health examination, gentle finger pressure, directed inward (medially) against your puppy's patella, can usually tell an experienced veterinarian if that pup is at increased risk of developing the problem later in life.

Patellar luxation is often divided into four grades of seriousness or severity. There is quite a bit of subjectivity (decision leeway) in deciding when a pet moves from one stage to the next:

Grade 1

Grade one pets do not experience pain. Their kneecap pops out of place intermittently and can be easily massaged back into place when the leg is fully extended.

Grade 2

Grade II pets have less stable knees. The kneecap can be massaged back into its groove - but it pops back out again once the knee is manually flexed or after the pet has taken a few steps. With time, many of these pets will develop knee pain and arthritis associated with their problem.

Grade 3

Those in which the problem seems more pronounced or when persistent pain or arthritic changes are already present are placed in grade III.

Grade 4

These are pets whose kneecap will not stay in its groove even for short periods. These dogs have a hard time walking. Dogs that have suffered this degree of joint damage for more than a year or two usually have pain, developing arthritis and degenerative joint disease. They usually walk with a crouching stance and stand knock-kneed with their toes turned inward.

Grade 3 & 4 dogs never walk normally. Although their problem is generally present early in life, they may not be brought in to a veterinarian for the problem until they are middle-aged. In these severe or advanced patellar luxation cases, changes are occurring that you cannot see. The slick, bony surfaces of the patella and trochlear groove become inflamed and eroded in a process called chondromalacia . At time passes, this inflammation becomes more generalized to involve most of the supportive cartilage and fibrous tissues of the knee. Surgery in these severe more advanced cases is less likely to be successful or stop the progression of arthritis in the joint. You can read more about that here.

It is also not uncommon for one of the cruciate ligaments of the pet’s knee to subsequently snap. (ref)

Does The Patella Ever Luxate To The Outside ?


Although the vast majority of cases that veterinarians see involve luxation to the inside (medial side) of the knee, the kneecap of some dogs does occasionally luxates laterally (lateral patellar luxation). These lateral luxations tend to be more pronounced and debilitating that those that luxate medially. When lateral luxation occurs in small breeds, it tends to occur later in life.

In younger dogs, lateral luxation is more frequent in large and giant breeds. When it does occur in these dogs, they usually have multiple misaligned bones in their rear legs from the hip down. These are thought to relate too little angulation in the hip, hock, and stifle joints.

The most common breeds to suffer that problem are Irish Wolfhounds, Great Danes and St. Bernards. Large dogs with this problem tend to suffer from a number of concurrent problems - all related to poor skeletal alignment. Because of this, most of these pets stand knock-kneed. Both knees are usually affected.

Giant breeds of dogs have tremendous nutritional needs during their early rapid growth. In these dogs, good nutrition, dietary mineral balance (Calcium 1.1%, Phosphorus 0.9% ref ) and a controlled growth rate can minimize many forms of skeletal deformities. (ref)

Some veterinarians believe that a dog whose patella rides too high on its leg (patella alta) is at higher risk of medial patellar luxation (MPL) (ref) and that those that have patellas that ride too low (patella baja) is more likely to luxate laterally (to the outside). (ref)

How Will My Veterinarian Diagnose This Problem ?

In most cases, gentle thumb pressure by your veterinarian on the pet’s kneecap while the knee is extended is sufficient to diagnose patellar luxation. There is a distinctive pop or jerk as the patellar jumps out of its groove to the inner surface of the thigh where it can be readily felt in its abnormal position. Your veterinarian may want to take an x-ray of both knees to determine if arthritic changes are already present. Determining if your pet has a stage 2 or 3 problem is more difficult. This is a subjective decision so it could differ between veterinarians you consult. Since grading the problem determines the need for surgery – have more than one veterinarian examine your pet before deciding what needs to be done.

Do All Dogs With This Problem Need Surgery ?


I do not believe that pets that limp only occasionally (grade 1) need surgery. Grade 2 pets are a harder decision. They probably do not need surgery either. Just feed them a balanced diet, keep them lean, keep their toenails trimmed short. Give them a chondrotin/glucosamine supplements if you wish. If you do elect surgery for these pets, there are minimally invasive arthroscopic techniques that you might consider. (ref)

Dogs carry the majority of their weight on their front legs and seem not to be inconvenienced even when running on three legs. However, patellar problems do not go away on their own - so you will have to judge how much of an inconvenience grade 2 problems are to your pet.

Dogs showing pain, dogs showing the beginning changes of knee arthritis and those that fall in categories 3 and 4 do need surgery. Just be sure that radiographs (x-rays) and other tests are performed to be sure that the lameness they are experiencing is directly attributable (caused by) a patellar problem and not to some other concurrent joint abnormality your pet may also have.

What Type Of Surgery Does My Pet Need ?

In contrasted with cruciate ligament or hip surgery , patellar surgery is less invasive, less expensive and generally has much better outcomes. When patellar surgery is performed before arthritic changes have occurred in the knee, the surgery is usually very successful. Once arthritic changes have developed, surgery is much less likely to produce a pain-free leg.

What needs to be done depends on what structures in your pet’s knees are abnormal and how abnormal they are. No two cases are exactly alike. There are three surgical procedures that are used to treat patellar luxation. Most cases can be cured with the first. If your veterinarian decides that ligamental reinforcement (lateral imbrication) will not be sufficient, the vet will add the second procedure, deepening of the trochlear groove. If that will not be sufficient, the surgeon may move the point where the patella's ligament attaches to the tibia (tibial crest transposition).

In 2011, a veterinary surgeon with a particular interest in luxating patellas published an article on his experiences correcting them. His conclusion was that deepening the pet's trochlear groove did not improve the surgical outcome. These pets were not followed for very long after surgery and it is quite common for studies of this type to reach opposite conclusions from one another. But it is also common for widely accepted surgical techniques to be later proven not to do what we thought the did. You can read his study here.

Reinforcement of the lateral collateral ligaments (lateral imbrication)

In many mild cases, it is sufficient to simply reinforce the weak lateral ligaments that keep the kneecap in alignment. When the inner or medial ligament has contracted or pulls too hard to the inner side of the knee, that ligament can be “stretched” (medial desmotomy) at the same time to allow the patella to glide in its groove in a straighter course.

Deepening of the trochlear groove trochlear modification or recession, sulcoplasty)

Your pet’s patella rides in a channel on the face of the femur. Sometimes, this channel or groove is not deep enough in toy breeds to hold the patella in track (course). This track is coated with a slippery cartilage that allows for smooth motion. In this surgery, this cartilage (hyaline cartilage) is carefully lifted off to one side while the notch is deepened, or a wedge of bone and cartilage are removed and then replaced in a deepened channel. You can read about the procedure as it has traditionally been done here.

Relocation of the tibial attachment (=tibial crest transposition)

In some pets, the conformation (shape) of the knee structures is so out of line that deepening the trochlear groove will still not allow for smooth action of the knee. In these cases, the kneecap’s attachment to the tibia (shinbone) is too far to the inside of the leg. Sometimes, this has occurred bit by bit as a persistently luxated patella tugs on the attachment, slowly causing it to bend and grow inward. At other times, the dog was just born with poor knee alignment. In both cases, the solution is to remove the point of attachment (tibial tuberosity) along with some underlying bone and reattach it farther lateral so the patella moves more normally in a straight up-down line.

Some pets are born with tibias so distorted that the whole upper section of bone needs to be rotated on its shaft. These are procedures best left to a veterinary orthopedic specialist. It is not unusual for veterinarians attempting to repair these complex cases to discover other problems in the course of surgery. That is when their special expertise is so important. You can read about some of these compound and difficult cases here and here . The earlier in your pet’s life that they are performed, the more positive the outcome tends to be.

You can read a veterinary text on how all these surgical techniques are taught and performed here.

What Will Happen If I Do Not Have This Problem Fixed ?

If your pet only suffers from a grade 1 condition, it should do fine without surgery. However, the higher the grade, the more likely your pet is to eventually develop arthritis in the knee leading to pain. We cannot predict which pets will develop painful knees in grade 2. The problem with delaying surgery in these pets is that arthritis, once it occurs, is irreversible. I am inclined to suggest lateral collateral ligament reinforcement to my clients with young-to-middle age pets in this situation to avoid this. This is because collateral ligament surgery in your pet is a relatively non-intrusive, safe surgery. It is often successful and does not preclude more extensive surgery later if it does not solve the problem.

Should The Knees Be Operated On One-At-A-Time ?

I generally suggest that knees be operated on one at a time. This way the pet is never unable to get about. If both knees need surgery, the second one can be operated on eight weeks after the first. In pets that are still growing extensively, it is better to operate on both knees simultaneously because inactivity in one leg will cause bone and muscle in that leg to develop slower and differently than the other.

Is Postoperative Care Required ?

Yes, your pet will most likely leave the hospital bandaged and in need of exercise restraint until some healing has occurred. Take it out-of-doors only on a leash. Depending on its personality, it may need some tranquilization to keep it from initially overusing the repaired leg and it may need a special collar to prevent it from chewing at its incision.

Because it is so hard to maintain pets hygienically, many veterinarians give antibiotics to all their post-surgical cases. Because the surgery is relatively straight forward, few post surgical complications occur. If only the knee ligaments were reinforced, the dog is usually back to its old self in three or four weeks. If more extensive procedures were performed, it might take six to eight weeks of limited exercise to begin using the leg.

Most vets suggest that beginning three weeks or so after surgery, physical therapy, swimming, hydrotherapy and range of motion exercises should begin to help prevent muscle contraction and reluctance to use the leg.

Should I Breed My Pet If It Has This Problem ?

That would not be a good idea.

We know that dogs that have this problem tend to pass it on to their puppies. Most dogs that develop luxating patellas appear normal at birth and as puppies. But the genes they inherited from their dame and sire make it likely the problem will occur later. If you are a serious breeder or a conscientious occasional breeder, have your dogs certified free of the trait by the OFA or ,at the very least, have your veterinarian thoroughly examine your pet for a tendency to patellar luxation before considering breeding. A technique that measures “Q-angle” might be helpful in choosing dogs least likely to pass on patella luxation tendencies


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