Screws: Screws are best at resisting tension (applying compression).
The threads pull whatever they are screwed into against the head. Thus the screw itself is loaded in tension and applies compression (just like the tension wires discussed above)
In this example the midpoint of a spiral fracture is pulled back closed with a screw. The threads are in one side of the bone and the head pulls the other side of the bone to them. A screw which does this (pulls two fragments together) is called a “lag screw.” At the hardware store “lag screws” are partially threaded screws like this one. In Orthopaedics, the first partially threaded screws were all designed with a cancellous thread (most still are) and so are frequently called “cancellous screws” rather than “lag screws.
A fully threaded screw like this one can still be used as a lag screw by “overdrilling” the near cortex (the one under the head) to the outer diameter of the screw so that the threads don’t bite on that side.
A little thought will make you realize that plate screws are also lag screws. They are simply lagging the plate to the bone. Initially, when the screw is tight, it compresses the plate against the bone and friction prevents the plate from sliding around. Later, as the screw loosens from bone remodeling, the plate will slide and the screw can be loaded in a lot of shear at the plate bone interface – thus in nonunions you will often see the heads of screws sheared off as a failure mode for plating.
Screws as pins:
Screws can do the same job that pins do in resisting shear, acting as “threaded pins.” They will only be as strong as a pin with the same diameter as the root diameter of the threads, however (not the outer diameter). Also realize that a smooth pin cannot hold two bone fragments apart (“distracted”) while a threaded pin or fully threaded screw not