I just saw in [this post](http://drhem.files.wordpress.com/2013/04/pic-2.jpg) the exposed bone and it started to wonder if it would hurt if someone tapped it or what it would feel like being there aren't any nerves on the bone.
If bone is exposed at any time there would be extreme pain at all times. In the picture the bone and all the tissue is completely necrosed and there is no tissue or nerves left. The bone and tissue are dead. The end of the bone itself probably at that point will have no feeling, but the area where there is still viable tissue would be causing the patient a great deal of pain.