She weighs 18 pounds, at our vet's suggestion she needs to lose weight. Her left knee slips in and out of place, and she was probably born that way. Now she has trouble with her right knee, too, sometimes limping a lot. Her X-rays showed a little arthritis in one knee.
Knees are the most vulnerable joints in the body.
Unlike a stable ball and socket like a hip or shoulder, the knee is simply the end of one bone sitting on top of the ends of two others. Thick straps of connective tissue called ligaments are all that hold the joint together. It gets worse. Your Chihuahua, like many small breed dogs, was born with bowed rear legs. Her kneecap slips out of place when her knee bends.