Medial Patellar Luxation
By: Emily Hoppmann, DVM
Many dogs, especially small breeds, have a congenital issue with their knee cap or patella and often owners have no idea. Have you ever seen your dog running full-force across the yard and then all of a sudden he or she begins to skip or unexpectedly pick up one or both rear legs? At first it seems shocking, as if they have broken something or strained a muscle; but, within 15 – 30 minutes you realize your dog is back to running on four legs like nothing happened at all! Chances are what you are seeing when your dog does this acrobatic move is one of the clinical signs of this issue – what we call a medial patellar luxation. A mild medially luxation patella by itself is not a painful condition, but it is one that tends to get worse with age and can predispose dogs to arthritis or deformities in the limb later in life which can be painful. The gradual progression of the disease and development of secondary issues which may be painful to your pet is why it is important to monitor the affected limb and all possible musculoskeletal issues so that we can properly manage your pet.
The patella, or knee cap, on dogs is very similar to that of humans. On the dog, it is a small bone that is buried in the tendon portion of the quadriceps muscle—the large muscles that run down the front of the thigh. The patella bone sits in a groove located on the front aspect of the femur bone. When your dog experiences the luxation, or dislocation, the patella actually slips out of its normal place and is unable to easily return to its original groove on the femur bone. The patella can slip either to the inside or outside of the knee, but it most commonly slips to the inside – thus, it is a medial patellar luxation. This ability to slip out of the groove can be due to a number of causes, including: laxity in the tendons and ligaments, the groove in the femur bone being too shallow, and abnormalities in other joints or muscles of the leg.
Medial patellar luxation is one of the most common orthopedic conditions in the dog, occurring 75 to 80 percent of the time. It is routinely diagnosed during annual and bi-annual physical exams. Typically, this condition primarily affects small breed dogs including Boston Terriers, Chihuahuas, Pomeranians, Miniature Poodles, and Yorkshire Terriers; although it is not uncommon for large breed dogs such as Akitas, Chinese Shar Peis, Flat-coated Retrievers, and Great Pyranees to develop the condition. Almost 7% of puppies are diagnosed with some form of luxation of the patella, which is why we examine the joints at every visit.
Unfortunately, since it is seen so commonly in the breeds listed above it is thought that patellar luxation is generally caused by a congenital abnormality— a genetic defect that an animal is born with and is passed down through many generations of breeding. This is why it is always recommended that dogs diagnosed with a patellar luxation should never be considered for breeding due to the risk that they pass down the genetic defect to their puppies.
Patellar luxation is graded on a scale of I – IV, with grade IV representing the most severe of cases. Grade I and II patellar luxations are the most commonly seen stages of this disease and when owners are unaware of the disease because they are not seeing any clinical signs or are only seeing signs occasionally at home. At these grades, the dislocation only occurs occasionally so the patella is in the groove where it is supposed to be more than it is out of the groove, and it is generally not a painful event. It is very common to not see any clinical signs with a grade I luxating patella, and sometimes the only time the patella will move out of place is when a veterinarian manipulates the knee during a physical exam. Clinical signs will become much more apparent as the disease progresses and the grading is higher. These signs include a more persistent skipping gate, intermittent leg holding, and mild lameness.
It is well-known that luxating patellas tend to worsen with age, so knowing and understanding the clinical signs to look for will be helpful to monitor progression. When a luxating patella reaches higher grades this is when we may begin to see severe lameness, some pain, and even deformity of the limbs. Dogs that have the most significant form of this disease, a grade IV luxating patella, often classically have a “bow-legged” appearance because the patella essentially never goes back into the femur groove. Recommendations for medical treatment and/or surgical intervention of medial luxating patella should be discussed and considered if it is affecting your pet’s quality of life. Pets that do not display any clinical signs should simply be monitored for progression of the condition. If it does start to affect the quality of life, secondary arthritis occurs, or your dog is in pain as a result of the malformation of the leg, medical management is often the first step. There are surgical options available if necessary, but this condition is often well managed with anti-inflammatory/pain medications.