Mr. Jones seemed a little chagrinned in the exam room. His new Yorkshire Terrier puppy was happily wandering around on the floor sniffing the corners of the table then jumping up for attention as if it was just another day at the office. Not twenty minutes before Mr. Jones had called for an emergency appointment because something had happened to her right hind leg and she was unable to use it at all. When he called he was pretty sure she had broken something, and now here she was walking around without any problems at all. It is amazing how something so small can make you feel like such a big idiot.
While bending her hind legs at the knee I could feel the telltale popping of a kneecap, or patella, sliding out of place to the inside of the leg and then going back to its proper position when her leg was straightened again. The diagnosis was a luxating patella, a common problem in many small breed dogs who have an anatomical predisposition for wandering kneecaps.
Most of the big muscles on the front part of the thigh come together and form a tendon that runs over the top of the patella to attach on the shin bone, called the tibia, below. The actions of these muscles pulling on the tibia allows the dog to swing the lower leg forward when walking or running. When the patella slides out of its groove it pulls those upper muscles and the tendon attaching to the tibia below to the side as well, thus changing the normal forces on the leg and making it uncomfortable to put the foot on the ground and stand on it.
A dog that has just had a patella pop out of place will usually hold the leg up and refuse to use it. Usually the patella will slide back into place on its own and the dog will suddenly start using the leg normally again. This is about the only lameness in which a dog will go from perfectly normal, to completely non-weight-bearing on a hind leg, to perfectly normal again, sometimes within a matter of minutes.
Luxating patellas occur in a range of severity from mild, where the patella usually resides in its proper location and only occasionally slips to the side, to severe, where the patella prefers to live out of place and as a result of the twisting forces generated from the abnormal patellar location the leg bones themselves start becoming twisted and bowed. You may have seen the funny videos of Chihuahuas walking around on their front legs only while holding their rear ends up in the air. The dogs are actually not trying to be amusing, they usually have severe luxating patellas that cause them to be unable to use either hind leg, so they solve their problem the only way they can, by walking in a handstand position. A dog that hops occasionally on a hind leg may not need any intervention, but a dog with continual problems needs a surgical fix.
Surgical repair for luxating patellas usually involves deepening the groove that the patella lives in to help hold it in place. When there have been twisting changes to the leg some other procedures may be needed at the same time to encourage the patella to stay where it needs to be. The outcome is usually quite good for these cases, but the more severe and long term the changes to the bones in the leg are, the more difficult the problem is to fix satisfactorily.
So for Mr. Jones, what initially seemed like it would be a wasted visit turned out to be helpful. Now he knows what is happening with his puppy and what to watch for so that we can have a plan in place to correct this problem if needed before she joins the ranks of the hand-standing Chihuahuas.