The intense pain associated with a felon occurs because of

A closed space infection : A felon is an infection of the distal pulp space of a digit, usually secondary to a puncture wound. This is a closed space at the level of the distal interphalangeal joint. Since the infection cannot spread proximal to this point, pain is throbbing and intense. Late osteomyelitis may develop in the distal phalanx if the felon is not opened appropriately.