The biomechanics of screws.

Screws are simple machines, commonly used in orthopaedics. Understanding the mechanics by which a screw generates compression can facilitate screw selection and insertion techniques and help reduce failure rates. Screw strength depends on both the material and size of the screw. A screw's holding strength depends both on it's type and size and on the type of bone into which it is inserted. Other factors being equal, a larger screw will have greater holding strength than a smaller screw. Properly placed lag screws greatly increase fracture stability and lead to better fracture healing.