What are the usual causes of having a foreign body in the knee joint?

BF knocked knee badly and has been diagnosed with the above - they are going to drain it out (presumably under local anaesthetic?) next week. He banged it badly. Is the foreign body likely to be a piece of dislodged bone or tissue? Or something else? He didn't cut it, so it doesn't seem that anything (like a wood splinter for example) went in at the time. Being a chap he was in great pain and didn't ask too many questions at the time - so grateful for any medical advice :)