The depth of the hole is determined, the threads are cut and a 4.5 mm cortex screw 4 mm shorter than the depth of the hole is inserted and tightened. Tightening should be stopped when the side of the screw head makes contact with the bone. This leaves the screw head distant from the onset of the screw hole.
Note: the countersink is not used to facilitate recognition of the screw head for screw removal and to avoid making a bigger defect on the bone surface, which may develop a periosteal reaction that may be seen at a pre-purchase examination.
Tightening the screw until the screw head meets the onset of the screw hole leads to bending of the screw, which provides a risk for screw breakage during its removal.
The use of 3.5 mm screws bears the risk of screw breakage, especially if the screw is bent.