Silky Terrier limping

Question
Hi, I have noticed over the last week or so that my Silky will hold one of his hind legs up when he is outside walking around, and even when he runs.  He doesn't usually do it inside, and what is really strange is that he doesn't do it all the time.  Also, he doesn't seem to be in pain, and I have touched/moved the leg/hip around with no reaction from him.  His paw is not sore or red.  The only thing I can think of is that he does play with my Lab and sometimes they do get rough.  I am not sure whether or not he needs to be seen by a vet since he isn't in pain, and this isn't all the time.  Ever heard of something like this?

Answer
This sounds exactly like a dog with a luxating patella. What that means is that his knee cap is popping in and out and when it goes out it will lock up his leg in the air. Once the knee pops back in he can put it back down and walk on it. What usually is the cause is that the groove that the kneecap rides in on the tibia bone is too shallow so it pops out of the groove.

Surgery is usually recommended for this condition if it becomes a problem. If it isn't a problem often or the dog is older, then most vets opt to leave it alone. Surgery makes the groove deeper so the knee cap stays in the groove.

I would have it looked at to find the cause for sure then you can decide what to do about it.