Went sledding 3days ago & hit my leg on a walking bridge very hard. can't see any bruises but it indented about a 1/4 inch. is that an internal bruise?

Indentation: There may be some bruising, but it sounds like an injury to the skin and the underlying soft tissue some of which is fat. A hematoma or bruise is likely swollen out and purple at the start. All the best.

A Dental Problem??: I saw a guy a while back who hit his head like that and then had a permanent dent in his skull, about the size you describe. So, it may be just that in your leg, unless you have a lot of soft tissue overlying the bone in that area that may have a temporary dent in it. In either case, there should be no serious long term problem. If you have more questions it should probably be seen in person.