The shaft end of a locking plate works in different bone from the head end. Down the diaphysis the cortex is thick and dense, and the screw that anchors there should be threaded for it: a fine, shallow thread that packs maximum engagement into hard bone. That is the cortical thread form — and on a locking screw it arrives with a head that fixes its angle in the plate.
The combination matters in long bridging constructs, where the shaft screws carry the working length of the plate. Anchored bicortically and locked to the plate, they hold the construct to length and alignment without being crushed against the bone. Screw length for unicortical or bicortical purchase and the number of shaft screws remain choices of the operating surgeon.
