Patella. From the Latin patina, meaning pan or dish. So called because of its shape. Commonly referred to as the “kneecap.” Luxating. From the Latin luxus, meaning dislocated, or the Greek loxos, to slant. Put them together and what have you got?
Luxating patella is a common condition, particularly for little dogs and occasionally cats, in which the kneecap becomes displaced from its normal position. Let’s take a hypothetical small dog. Let’s call her Bee, the Brussels Griffon. (Yes, that’s a real breed, not a vegetable. Look it up.)
Before going further, we have to clarify one anatomical detail. Exactly where are Bee’s knees? Simple, you say, pointing to the middle of her front leg. If Bee were a horse, a camel, even an elephant, you would be linguistically correct. In these animals the front leg joint that bends to allow “kneeling,” is colloquially called the knee. Scientifically, however, that joint is analogous to the human wrist. No kneecaps there. In four-legged mammals the joint that matches our knees, i.e., where the thighbone’s connected to the shinbone, and where the patella is located, is in the hind leg and is officially called the “stifle.” So Bee’s knees are in her hind legs.
How exactly does the patella work? A smooth, oval-to-triangular-shaped bone, it rests in a channel at the lower end of the femur (thighbone) called the trochlear groove. The patella is held in place by various ligaments and tendons that allow it to slide up and down in a way that redistributes forces and gives the big thigh muscles better leverage for moving the lower leg. When it’s working right, it glides without impediment as Bee bends and straightens her legs. But in certain individuals the kneecap sometimes pops out of its designated groove. Voila! Trick knee.
Why does this happen? In dogs and cats, luxating patellas usually occur as the result of congenital anatomical abnormalities. If Bee’s trochlear groove is too shallow, the patella derails too easily. If tendons or ligaments attach too much to one side or the other, when muscles contract, they pull the kneecap out of alignment. If her leg bones are more curved than they should be, or things are not lined up precisely, all these situations can lead to luxating patellas. These anatomical details are usually genetically determined, hence the condition is considered to be inherited and severely affected individuals should not be used for breeding. In the majority of cases in dogs and cats, the kneecap luxates medially, i.e., toward the inside. Luxations to the outside are uncommon but when they do occur, it is usually in larger breeds. Typically both knees are affected, but it is not uncommon for one side to be more problematic than the other.
Which brings us to how veterinarians classify luxating patellas into four grades. In Grade 1, I can manually push the kneecap off the groove by manipulating Bee’s leg, but as soon as I let go, it spontaneously pops back in place. You probably don’t even know Bee has the condition. Animals with Grade 1 rarely show clinical signs and intervention is not often warranted.
In Grade 2, the kneecap does not automatically pop back in place when I stop messing with it. Bee is occasionally lame when her kneecap gets “stuck” in the wrong position. She may hop a few steps or hold the leg in a funny position, but eventually things go back into place. If this happens frequently or if Bee is chronically uncomfortable, you might consider surgery, but more about this later.
In Grade 3, the kneecap spends most of its time in the wrong location. I can manually push it back to the correct spot, but as soon as Bee moves, it spontaneously dislocates again. Bee exhibits frequent lameness or may simply hold up the affected leg and run on three. In her youth, the condition may not seem especially painful. She just can’t get the knee mechanism to work right.
But Mother Nature made kneecaps for a reason. Without a properly functioning patella, over time animals with Grade 3 luxations have a high risk of developing arthritis. Corrective surgery done early in life will not only improve mobility but may minimize discomfort in the senior years.
Finally, Grade 4. These poor animals have kneecaps that cannot be moved into the correct position at all. No amount of manipulation will get that patella where it should be. These animals cannot straighten their legs. They walk with their knees bent all the time and are poster pups for the surgeon’s motto, “A chance to cut is a chance to cure.”
There are several surgical options for luxating patellas depending on the grade and particulars of the case. Procedures range from a simple tightening of the surrounding connective tissue to elaborate surgeries that may include moving the tendon insertion, deepening the trochlear groove, even altering the shape of the bones. If your veterinarian suggests surgery, an orthopedic specialist can advise you as to the best choice. For non-surgical cases, we may suggest various nutritional supplements that are touted to promote joint health, such as fish oil, and products containing glucosamine, chondroitin, etc.. These won’t change Bee’s anatomy, but may help minimize the long-term impact of her trick knee. And if she develops arthritis later in life, we have medications for that, too.
In case your mind wanders off on tangents like mine does, and you got to wondering about the expression “the bee’s knees” (meaning something excellent), the origin of the idiom is the subject of some debate. Most sources attribute it to the fact that bees store treasured pollen in tiny, hairy basket-like appendages on their knees. Yes, bees have actual knees. Or at least they have segmented legs which include parts called the femur and the tibia, so the joint between those can reasonably be considered a “knee.” But they don’t have patellas. In case you were wondering.