A plate bearing against a bone has a number of clearance holes with sleeves. Openings provided in the sleeves include each a section diverging away from the bone and bounded by a conical surface. The sleeves are subdivided into tongues by means of slots cut into the free face of the collar. A screw is screwed into the bone at every clearance hole, the head of the screw being bounded by a conical or convex surface. By tightening the screw the tongues are forced apart by the head of the screw, thus clamping the sleeve in relation to the plate. This provides a guarantee, that the plate will stay rigidly connected with the screws, even if a reabsorption of bone takes place. Some of the sleeves comprise clamping parts excentric in relation to the screw axes, enabling the surgeon to press the bone pieces against each other.