confused about vets advice re: luxating patella

Question
I just took my 2 1/2 year old chihuahua mix to the vet because I started noticing her skipping once in a while on one of her hind legs during walks.  She is very active, and has not showed any signs of slowing down because of this limp.  It started about 3 months ago when it got cold out, and seems to come and go.  The vet said that she has a level 2 medial luxating patella, but he said he wasn't sure whether she might be limping because of something else, like legg-perthe disease (which i thought was usually caught early in younger dogs?).  He prescribed Rimadyl for ten days and specifically told me not to limit any activities.  If she doesn't improve then he will take an x-ray.  Why would he have prescribed pain meds, and why wouldn't he tell me to rest her leg by restricting jumping etc.?  What do you think he is hoping to learn/rule out from her reaction to the meds?  I don't really understand what he prescribed them for.  Would Cosequin be useful?  And what are the common differences in symptoms between luxating patella and the femur necrosis?  I need to know what I should specifically be asking about next time, because I was sort of confused at my visit today and didn't know how to ask the right questions.  Thanks for your help!

Answer
Luxating patellas are very common in this breed of dog and in most small dogs. Legg-perthe disease, on the other hand, is pretty uncommon.

Xrays will rule out the Legg-perthe disease, but a simple in house manipulation of the legs will show if the dogs patella is popping in and out. This is caused by the groove that the patella rides in being too shallow. I would venture to say that once the dog is old enough and the muscles are well developed is when the luxation starts to show up.

Skipping is the most common presentation of this. The reason they skip is because when the patella moves out of the groove, it essentially 'locks' the leg in that position until the dog moves around enough to let the patella slide back into position.

Here is a link to an article about Leggs-Perthe:
http://www.vetsurgerycentral.com/legg_calve.htm

I would get a second opinion if he insists that this dog has that. An xray of the hip would show it but if she isn't showing these symptoms and at her age, I would think the rule out would be a luxating patella.

Now most of the time this does require surgery IF the dog is having pain when the leg locks or it is a real problem, e.g, the leg won't stay straight and the patella is constantly luxating.

Some owners never have it done and the dog just skips around forever. It's all a decision you will need to make.
I have no idea why he put her on pain meds if she isn't in pain.
Considering the toxicity that Rimadyl carries I would ask that first of all.

If this is a younger vet then maybe he hasn't seen any luxating patellas. I don't know, but you are entitled to more than one opinion on this.

I hope you find the final answer. Please let me know how it goes.