Has your dog ever been running and suddenly picked up one of his back legs and started running on three legs? Usually, this will be for a short distance. Then, he will give his leg a shake and go back to running on all four legs.
Your dog is experiencing something called Patellar Luxation. Sounds complicated, but it is really very simple. The patella (knee cap) is a small bone that sits in a groove at the front of the knee. For some dogs, the groove the patella sits in is too shallow. So, when the patella is experiencing a lot of movement (i.e. the dog is running), it will slip out of the groove and cause the knee to lock up. The dog will hold the leg up and sometimes give it a little shake causing the patella to slip back into its groove.
Patellar Luxation is a congenital problem caused by the failure of the bone to develop correctly. It is a common problem for small breed dogs such as Pomeranians, Poodles, and Chihuahuas. The luxation itself isn't painful. It is more of a nuisance. However, over time, the action of the patella sliding up and over the groove will cause “wear and tear” on the cartilage of the knee joint. Eventually, this will lead to arthritis.