In luxating patella the kneecap (the patella) slips out of the smooth groove in which it normally rotates on. If it slips medially, it slips towards the opposite leg. If it slips laterally, then it slips towards the outside of the leg. When it does this the dog cannot extend his leg properly, so he will hold his affected hind leg up. He will have difficulty bearing weight on the leg until the ligament snaps back into place.
This condition can be temporary where you will see your dog running and notice that he is running with one hind leg up. Then you may see him running as if nothing happened if the knee slips back to where it is supposed to be.
Half of the dogs who are affected by this have a problem with both knees. If your dog only has a minor case of this problem, the biggest problem will be the development of arthritis or a possible torn knee ligament. Severe cases usually require surgery, and the result will be better if the surgery is done before arthritis occurs. If your veterinarian is not well experienced with this type of surgery, ask for a referral to a surgical specialist.