What is Luxating Patella (or Patellar Luxation)?
The patella, or "kneecap," is normally located in a groove on the end of the femur, or thighbone.
The term luxating means "out of place" or "dislocated". Therefore, a luxating patella is a kneecap that moves out of its normal location. It generally resumes its normal anatomical orientation after only a brief period of luxation in most dogs.
Most pet parents with small breeds are very familiar with the cute skip they may often see in their dogs’ step when their dogs frolic about. It may look a bit like Charlie Chaplin’s gait, silly and sweet, but this little hop can be more painful than humorous for some dogs. A skip in a dog’s step can indicate that the dog is suffering from a luxating patella that over time may progress into a condition requiring surgical intervention.
Luxating patella, also referred to by veterinarians as patellar luxation, is much more complicated than “the knee bone’s connected to the thigh bone.” The patella is the term describing a dog’s kneecap and when the patella luxates (shifts) in the complex structure of the knee joint, a misalignment of the knee has occurred, says Ron Hines, DVM, PhD.
“These kneecaps (or patella) are meant to ride in a groove [trochlear grove] on the face of the femur [the large bone of the thigh],” writes Dr. Hines. “The patella acts as a pulley, giving leverage to extend the knee as the pet walks.” With a malfunctioning “pulley system,” a dog’s left to deal with the agitating symptoms of luxating patella, which is also commonly referred to as a “trick knee.” To compensate for the patella’s dislocation, a dog may run on three legs for a period of time until the patella readjusts itself. For some dogs, this happens in mere seconds, for others in minutes and still others may never receive relief from the condition.
Hines continues by explaining that a sturdy ligament holds the patella to the thigh’s large muscles. Opposite this ligament, on the other end, another ligament exists that joins the patella to the pet’s shinbone (tibia). “Alignment on the inner (medial patellar ligament) and outer side (lateral patellar ligament) of the knee help keep the patella riding in its track,” says Hines.
The patella can dislocate in two ways, either laterally or medially. Medial luxating patella means that the dog’s kneecap has luxated toward the dog’s opposite knee, whereas lateral luxating patella means that the knee has dislocated away from the dog’s body, writes Wendy Brooks, DVM, DipABVP,.
“In over 90 percent of these cases the patella jumps out of its tract to the inside of the pet’s knee,” says Hines. In most cases a dog will suffer from medial luxating patella, which is less apparent than lateral, while those with lateral luxating patella suffer from a more debilitating condition. In fact, Hines writes that lateral luxation occurs more often in larger breeds as a result of poor skeletal alignment. Some large breeds commonly afflicted with this condition include Irish Wolfhounds, Great Danes and Saint Bernards.
About 50 percent of dogs with luxating patella will have it in both knees and the other 50 percent will carry the problem in one knee alone. If left untreated, luxated patella can lead to more stressful and painful conditions. “The entire weight-bearing stress of the rear leg is altered, which in time leads to changes in the hips, long bones, and ultimately leads to arthritis,” writes Brooks. “How severe the changes are depends on how severe the luxation is and how long that degree of luxation has been going on.”
What are the Symptoms of Luxating Patella?
Patellar luxation occurs within four levels of severity, with each level displaying similar as well as unique symptoms. One of the most obvious signs of luxating patella is one already mentioned, a skip in a dog’s step while running. Some pet parents, explains Hines, might note a pop in the dog’s knee joint area while holding their dogs.
The following list outlines the four levels of pattelar luxation in a dog’s knee(s) as described by Hines:
* Grade 1
Grade one pets do not experience pain. Their kneecaps pop out of place intermittently and can be easily massaged back into place when the legs are extended.
* Grade 2
Grade two pets have less stable knees. The kneecaps can be massaged back into their grooves, but they pop back out again once the knees are manually flexed or they have taken a few steps. With time, some of these pets will develop 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 arbitrarily placed in grade three.
* Grade 4
These are pets whose kneecaps will not stay in their grooves 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, and have developed arthritis and degenerative joint disease. They usually walk with crouching stances and stand knock-kneed with their toes turned inward.
Hines makes it clear that in the more severe levels, grades 3 and 4, a dog most likely developed the condition earlier in life but never saw a doctor until it was middle-aged. Most of the animals seen by veterinarians for this condition are over six months old, but severe defects can be pronounced as soon as eight to ten weeks of age.
“In severe or advanced patellar luxation, changes are occurring that you cannot see,” writes Hines. “The slick, bony surfaces of the patella and trochlear groove become inflamed in a process called chondromalacia. As time passes, this inflammation becomes more generalized to involve most of the supportive cartilage and fibrous tissues of the knee.”
What Causes Luxating Patella?
Genetics plays a large role in the causes behind patellar luxation. As previously discussed, several smaller breeds are more prone to this condition than others. Those breeds include: Boston Terrier, Chihuahua, Pomeranian, Miniature Poodle, Yorkshire Terrier, Dachshund and Basset Hound to name a few. Hines adds that the condition is occasionally seen in other breeds as well, such as the Boykin Spaniel, Cocker Spaniel, Chow Chow, Bedlington Terrier, Australian Terrier, Japanese Chin, Shar-Pei, Mi-Ki, Lhasa Apso, Tibetan Spaniel, Tibetan Terrier and Labrador Retriever.
“In certain breeds that have extremely short legs such as the Basset Hound or Dachshund, patellar luxation is thought to be secondary to the abnormal shape of the femur and tibia,” writes Race Foster, DVM, for PetEducation.com. “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.”
Dominique Griffon, DVM, MS, PhD Diplomate, of the American College of Veterinary Surgeons, says that this condition is one of the “most common congenital anomalies in dogs,” noting that 7 percent of puppies are diagnosed with patellar luxation. She goes on to explain that though luxating patella can be the result of traumatic injury to the knee, most luxating patella comes without a clear cause. Dogs diagnosed early on with patellar luxation that are predisposed to it and have not had any trauma to their knees most likely have the condition as a result of abnormalities in their entire hind limb. For this reason, as Dr. Griffon explains, “Congenital patellar luxation is therefore no longer considered an isolated disease of the knee, but rather a component/consequence of a complex skeletal anomaly affecting the overall alignment of the limb.”
Hines breaks down the specific abnormalities to the knee that leads to patellar luxation as follows: First, and most common, is a weak or stretched lateral patellar ligament. In almost all cases, this is a problem the pet is born with. Although only one knee may appear to be affected, in most cases, both knees share some degree of the problem.
Second 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.
Third is a problem that 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 the point of attachment to shift inward – throwing off the alignment.
How is Luxating Patella Diagnosed?
If you notice that your dog is exhibiting signs of luxating patella, such as skipping, yelping in pain while running, or popping sounds within the knee area, take her in to see a veterinarian. Your veterinarian will exam your dog by gently applying pressure to your dog’s kneecaps with her thumbs while the knee is extended, writes Hines. “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,” explains Hines.
The next steps involve determining the level of severity of the patellar luxation and determining whether or not any arthritic changes have occurred. Hines tells pet parents to seek the professional evaluation of at least two veterinarians to help determine the grade of the condition and whether or not your dog will need surgery.
In order to diagnose the condition fully, your dog’s veterinarian may run more tests and order X-rays. In order to determine the most appropriate treatment, writes Griffon, your vet may do any of the following tests:
Palpation of the knee under sedation to assess damage to ligaments
Radiographs of the pelvis, knee and occasionally the tibias to evaluate the shape of the bones in the rear leg and rule out hip dysplasia
Three-dimensional computed tomography to provide an image of the skeleton of the rear legs to help the surgeon plan surgery where the shape of the femur or tibia needs correcting
Blood work and urine analysis as a precaution before anesthesia
How is Luxating Patella Treated?
The only viable treatment for patellar luxation is surgery. Though it’s the only way to correct the condition as well as is possible, not all levels of the condition need surgery. Dogs with grade 1 or grade 2 patellar luxation (not exhibiting signs of pain) do not need surgery. Dogs in these two levels are those that will always have a hop in their step, but aren’t suffering the excruciating pain that would require surgery.
Hines recommends that these dogs be kept on a balanced diet and kept lean to alleviate any extra weight on the hips and knees that could aggravate the condition. He also suggests trimming the dog’s toenails to prevent any snagging that could cause trauma to the knee. Lastly, he suggests feeding the dog a glucosamine and chondroitin supplement to help provide some extra relief to the dog’s knees and to help slow the acceleration of arthritis.
“Dogs carry the majority of their weight on their front legs and seem not to be inconvenienced when running on three legs,” writes Hines. “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.”
Though grade 2 problems can be taken care of without surgery, Hines explains that sometimes a corrective surgery at this stage can be done successfully and can leave keep the condition from progressing further. If the surgery isn’t done well in advance of arthritic changes while the knee is still pain-free, the surgery is not as successful and a dog will still have pain in the leg. Hines explains that it becomes the pet parent’s call at this point based on the advice from the dog’s veterinarian.
Dogs that show signs of pain, are in the beginning stages of knee arthritis and those that fall in grades 3 and 4 require surgery. But before doing so, Hines recommends that pet parents rule out any other joint problems that may be causing the dog’s pain and symptoms.
As Dr. Foster explains, dogs at this stage will absolutely require surgical attention. Left untreated, patellar luxation can worsen and make it more and more difficult for a dog to walk. “Arthritis will prematurely affect the joint, causing a permanently swollen knee with poor mobility,” writes Foster.
If your dog requires surgery for this condition, she will undergo one or several of three surgeries: lateral imbrications, trochlear modification, or tibial crest transposition. As Griffon explains, the surgeries are as follows and involve the outlined procedures:
Lateral Imbrication (also called Lateral Reinforcement)
When the patella slips out of its groove, the joint capsule surrounding it is stretched to allow this motion. Imbrication simply involves taking a tuck in the joint capsule. The tightened joint capsule does not allow for the slipping of the kneecap and the kneecap is confined to its proper groove.
The patella rides in a groove at the bottom of the femur (thigh bone). In toy breed dogs this groove is shallow, which allows the patella to slip. If the groove is deepened, the patella stays where it belongs. The normal groove in the femur is lined by slippery lubricated cartilage, called hyaline cartilage. This cartilage is peeled or cut away, the bone underneath is sliced out to form a deeper groove, and the cartilage is replaced. Techniques that do not preserve the original cartilage are no longer recommended.
Tibial Crest Transposition
If the knock-kneed conformation has already started to set in, the tibias (or leg bones) will have rotated; specifically, the crest on the tibia where the thigh muscle (the quadriceps femoris) attaches may have migrated inward. If this is the case, the crest will have to be removed and pinned back where it belongs to straighten out the leg. Severe rotation of the tibias may involve actually cutting through the entire bone and de-rotating it back into place. Following the surgery, your dog will go through a period of recovery. Your dog will be sent home bandaged up and you will most likely be told to restrict her exercise. If your dog needs to go outdoors, use a leash and be careful not to allow too much activity. If your dog is difficult to keep calm, you may want to speak to your veterinarian about tranquilization to keep her from causing harm to the leg. As always, an Elizabethean collar will most likely go home with your dog to keep her from chewing at the stitches.
Some veterinarians, Hines writes, will give dogs antibiotics to deter infections. Complications after the surgery are minimal and a dog is usually up and back to her old tricks within three to four weeks if only the knee ligaments were reinforced. The recovery time is longer for the more complicated procedures, about six to eight weeks.
“Beginning three weeks or so after surgery, physical therapy, swimming, hydrotherapy and range of motion exercises will help prevent muscle contraction and reluctance to use the leg,” writes Hines.