A locking screw does two jobs at once. Its shaft grips the bone like any screw, while its threaded head engages the plate hole and locks there. The result is a construct in which screw and plate move as one: angle held, load shared across all screws at once, and no dependence on friction between plate and bone.
That is what makes locking fixation the default in osteopenic metaphyseal bone, and it is why plate systems define their screws as carefully as their plates. The universal locking range follows the plate families it serves — small diameters for hand and foot plates, large diameters for the periarticular plates of the wrist, elbow, knee and ankle. Diameter, length and configuration remain selections of the operating surgeon.
