Screw is a cylinder with spiral threads running on its outer surface. It converts torsional forces into compression. The primary functional objective in the design of a screw is to dissipate and distribute the mechanical load. Thread design should maximise initial contact, enhance surface area, dissipate and distribute stresses at the screw-bone interface and increase the pull out strength. Screws can be used for attachment of implants to bone, bone to bone fixation or for soft tissue fixation or anchorage. In conventional plates they act by increasing the friction between the plate and the bone. Newer locking plates do not depend on the friction between the plate-bone interface; it acts as an internal fixator by using locking of screw head into the reciprocal threads of the plate to form a fixed angle construct.
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