motion blur only affecting a specific object

Hi any help would be most apreciated

Basically I have a mouth and have teeth spinning inside. I have applied a motion blur to it and it looks really cool except I think it would look better if only the inside was blurred.
This would show the teeth’s speed is faster then the speed of the mouth opening and closing I have looked through the threads on motion blur, but I can not figure out a way to achive this result, do you have any ideas? thanks in advance if you do.

Simple. Use a combination of render layers, and a composite node (vector blur) to achieve motion blur on just that object.