Has anyone had a horse that had a capped elbow?  If so how did you treat it?  

Shoeboil boot.

Usually DMSO and furacin will reduce the swelling, laser treatments will help. If it won't go down, the vet can drain it and put a little cortisone in to calm it down. Sometimes you can put a very thick bell boot on (like the old style Professionals Choice) after they wear the boot for awhile.

Yes I bought one with a bad shoe boil, we drained it a few times and injected it with cortisone a few times when it was drained but it always came back it, was horrible. The people that owned him never did anything for it so it got really bad, finally had to have it removed.

The shoe boil donut is your best friend for fresh capped elbows. If you don't have one, order ASAP and wrap the foot woth cotton on the heels at night when the horse is more likely to lay down.

Yes from what I have read they get it from their shoes, my horse is barefoot.

It happens when the horse lays down and then when they get up they will pinch or hit the elbow area with their back hoof no matter if wearing shoes or barefooted, my gelding was barefooted and he had one of the worst shoe boils I have ever seem. 

I can see if I can attach a picture.

Thats a nasty one.. 

On the gelding I had it got to be soft ball size, it was so ugly and he would pinch the heck out of it when he would start to get up after laying down are rolling. Did your vet open it up? 

Yes she did.  Cleaned it out and cut some flaps off that was just hanging there.  She seemed to think that it is a sarcoid that busted open.  Got him on antibotics and cleaning the wound and putting Crest toothpast on it.  Seems to be doing better and he is not really sore from it.