Another thing that you may consider is that you're joint flexability/muscle limberness is not up to snuff. I only suggest that because you say it's not a prob with dumbells though I'm sure you're not handling 180 lb dumbs. Next time you bench try warming up a bit more and stretch a little. Then stretch well after you've finished benching. I have seen guys who's muscles are stronger than their joints can handle.

Do however ask your MD. You can never be too safe bro...it's the only body you have.