What Causes A Pus Filled Hole On The Finger After Amputation?

Ok, long question. in july 2009 i had a wodworking accident with a jointer which "scalloped" the tops of my two middle fingers on left hand. lost partial bone and most of nail on each tip but pad on each finger remained intact. Hand surgeon "modified" the amputation by removing nail and matrix and cleaning up severed bone then folded the finger pad up and stitched about where cutical used to be. healed fine, have full use and 90% sensation, but there is a tiny hole on one of the fingers that never closed and puss/fluid can be squeezed out. Never painful, never inflamed or swolen, and never seems infected. Any idea what might have happened?