Answer
a) Herbert Screw
Definition and principle
The Herbert screw is a headless, variable-pitch, cannulated compression screw designed primarily for fixation of intra-articular fractures, especially fractures of the scaphoid. It provides interfragmentary compression while allowing the implant to be completely buried beneath the articular cartilage or subchondral bone. Its essential biomechanical principle is that the distal thread has a greater effective pitch than the proximal thread; as the screw is advanced, the distal fragment is drawn towards the proximal fragment, producing compression across the fracture.
Design and biomechanics
- It is usually cannulated, permitting guidewire-directed insertion and accurate placement.
- It is headless, avoiding prominence within an articular surface and reducing irritation to tendons or opposing cartilage.
- The screw has differential thread pitch. The thread pitch difference, rather than simple tightening of a screw head, produces interfragmentary compression.
- The central unthreaded portion or variable thread arrangement permits the screw to act as a compression device across a fracture.
- Some modern Herbert-type screws have threads over both ends, with different pitch, and are available in multiple diameters and lengths.
Compression is greatest when the fracture surfaces are accurately reduced and the screw is inserted perpendicular, or as nearly perpendicular as possible, to the fracture…