Lag Screws

Lag screws are used to compress fractures. A lag screw is not a particular type of screw, but rather refers to how a screw is inserted. To function as a lag screw, the proximal part of the screw slides freely in a hole drilled in the bone on the near side of the fracture while the tip is screwed into the bone on the distal side of the fracture. For cortical or cancellous screws, as the screw is tightened, the head forces the two sides of the fracture together. Fully threaded or partially threaded screws can be used as lag screws. Cortical screws are used as lag screws in fractures of long bones and  cancellous screws for metaphyseal fractures. These screws produce uniform static fracture compression.

The two illustrations on the right demonstrate how a partially threaded cortical screw functions as a lag screw in a long bone fracture. In the upper image, the jagged white area represents a fracture gap in a long bone. The cortex is dark gray and medullary cavity is light gray. The black area is a hole drilled in the bone across the fracture. In the lower image, a partially threaded cortical screw called a shaft screw was inserted into the threaded distal hole and screwed in. When the head impacted the near cortex, it pulled the fragments together. As the screw continued to be tightened, it closed the gap as indicated by the arrows, compressing the fracture. A lag screw inserted in this fashion is called an interfragmentary screw. Lag screws can also be inserted through the various types of fracture fixation plates.