Written by: Dr. Chip Coombs
One of the more common lameness issues that a dog owner might be facing is a knee condition called luxating patella. In half the cases, both knees are affected. The patella is the knee cap that is imbedded in the large tendon, and it attaches the main upper leg muscles (quadriceps) to the shin bone (tibia), just below the knee. Every time the knee flexes and extends the knee cap slides smoothly up and down, neatly contained within grooves of the lower thigh bone (femur). The knee joint in people works the same way.
Unfortunately, when any one of a number of anatomical abnormalities come into play, this once very efficient joint is no longer very functional. Puppies that end up developing a luxating patella are usually born with the problem (i.e. it is congenital). Although it is possible that a patella can begin to luxate in response to severe trauma, this is very uncommon and it would require very significant trauma; trauma that would cause damage to all the ligaments that hold the patella in place in a normal joint. Some studies have indicated as many as 7% of puppies are born with patella issues. It usually is found in toy and small breeds, but even some larger breeds can be predisposed.
When the patella slides along the groove in the lower femur, it does so because all the forces pulling on the patella are nicely balanced and the bones are in ideal alignment. As soon as the upper (femur) and lower (tibia) bones are not lined up (rotated/ twisted), then there are abnormal forces that tends to pull the kneecap out of its groove and luxate to the inside (medially), or outside (laterally). If the grooves in the femur are shallow, then there is very little holding the knee cap in place. The result is that it takes very little pressure for the patella is slip in and out of its groove. Veterinarians grade the ease with which the patella slips out of position (usually medially) and whether it stays there.
Grade 1 - when one can manually luxate the patella when the knee is in full extension, but it easily returns to its normal position when released.
Grade 2 - when the patella luxates frequently on its own with no manual influence. The patella usually will return to its normal position on its own.
Grade 3 - when the patella is, more or less, permanently luxated out of its normal position and the grooves in the femur are quite shallow.
Grade 4 - when the patella is permanently luxated; there are no grooves in the lower femur and usually the tibia is significantly twisted.
The grading system is usually applied to adult dogs whose bone growth has matured, because as a puppy develops, a lower grade can worsen to a higher grade.
An inward (medial) patellar luxation in a toy or small breed dog is the classic occurrence that veterinarians will most commonly diagnose. However, there are variations to this depending upon the breed and age of the affected dog and the manner and degree of the torsion/ abnormal twisting of the two leg bones. Older, small breed dogs, due to what is believed to be a degenerative process with the knee ligaments, can develop an outward or lateral luxation. Giant breeds, like the St. Bernard, Great Dane or Irish Wolfhound can develop lateral patellar luxation due to the manner in which the two leg bones develop, literally pulling the patella out of position to the outside.
As one can readily imagine, the major clinical sign with luxating patella is varying degrees of lameness. With Grade 1 there may not be any clinically obvious lameness and these dogs are usually completely functional with few, if any, problems long term. With Grade 2 there is usually an intermittent lameness where a dog will suddenly show lameness in their back leg for a few steps (the patella is now out of position), and then starts to walk normally (when the leg extends fully and the patella clicks back into position). With Grade 3 and 4 a dog usually has an obvious, permanent limp or abnormal gait in the affected leg and the leg is bowed. This would be very uncomfortable for the dog.
Dogs who have a Grade 1 can usually avoid surgical intervention to correct the luxation, although they may benefit from being given joint supplements. Those with Grade 2 may not be so fortunate. Over time, the patellar movement can wear down the joint cartilage, as it moves in and out of its groove. This can lead to painful arthritis over time. The need for surgery varies with how often the patella luxates out of the groove. However, dogs who have Grade 3 or 4 will need surgery as both conditions are painful for the dog. Due to its increased severity, the surgery to repair a Grade 4 luxation is the most extensive and complicated. Another factor to consider, in addition to the pain, is that dogs who have a permanently luxated patella may have an increased likelihood (15% chance) of tearing their anterior cruciate ligament in the same knee due to abnormal stresses on the cruciate ligament itself.
From an owner’s perspective, there is no means by which this knee condition can be prevented. Unfortunately, it is congenital and is not caused by diet or lifestyle choices and only very rarely by severe trauma. Because it is widely believed that patellar luxation has a genetic component to its cause and development, any dog that has this condition should never be bred.
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