|Although there are a large number of
different screws, there are two basic designs: cortical and cancellous
screws. As the names imply, cortical and cancellous screws are designed for
placement in cortical and cancellous bone respectively.
Cortical screws have closely-spaced, shallow threads and larger core-to-outer diameter ratios than cancellous screws. Cortical screws are stronger than cancellous screws of the same outer diameter. They are usually blunt ended. When used to fix plates to long bones, fully-threaded screws are used in order to fixate both cortices so that maximal stability is achieved. The blunt end should extend only a few millimeters into the soft tissues beyond the far cortex to minimize damage to the soft tissue.
Cancellous screws are designed for fixation of cancellous bone. They are most commonly used in the metaphyses of long bones where cancellous bone is abundant. They have more deeply cut and more widely spaced threads compared to cortical screw. Since cancellous bone is much less dense than cortical bone, the screw threads cut their path in the bone when the screw is inserted, i.e. cancellous screws are self-tapping. Partialy threaded cancellous screws are often used as lag screws for metaphyseal fractures.