Locking mechanism for a broken or fractured bone, comprising a medulla nail with a proximal portion which may incorporate perforations and a distal portion, and at least one locking aid, characterized in that the distal portion has a surface structure which is designed so that the bone can be stabilized by means of the medulla nail due to the at least one locking aid acting on the surface structure of the nail through the bone from outside, preferably preventing it from rotating and/or being subjected to axial stress.