Hardware Properties

Orthopedic hardware is designed for use at specific anatomic sites and for treatment of specific conditions. Properties of specific types of hardware are discussed under their individual entries in the hardware atlas. General properties are described here.

One of the most important hardware properties is its strength, i.e. how well hardware can withstand applied forces without breaking. Most hardware withstands compressive and tensile forces well but has much less strength with regard to shear or bending forces. The strength of hardware is a function of its composition and thickness. The thicker a screw or plate, the greater its resistance to bending or fracture. The strength of hardware and bone can be defined in several different ways. For fracture fixations, one of the most important measures of strength is the pullout strength of a screw. This is defined as the amount of force required to pullout a screw that has been placed in bone. The primary determinant of pullout strength is amount of bone-metal contact perpendicular to the long axis of the screw. This is determined by the surface area of the screw threads and the amount of bone contacting the threads. For the same surface area, cortical bone produces greater contact with the threads than cancellous bone.

The geometry of hardware (and bone) plays a major role in how forces are distributed through a structure. Places where forces tend to concentrate are called stress risers and are regions of reduced strength. Screw holes in plates and bones and sharp edges are stress risers, and fractures tend to occur in these locations.