A clue to cause and effect would be the word "since" as it starts a cause for something,a reason for something to happen
My mother has a great admiration and belief in my father. (try this)
This is best described as an essential past participle phrase.
It is essential, because the participle explains what happened to the bone to make it heal slowly. It is a past participle, because the bone is already broken, and has already healed (or started to heal).