Evaluation of locking bone screw heads

Abstract eng:
When extracting a trauma/orthopaedic titanium alloy implant, there are some difficulties in trying to release some locking bone screws from locking holes of the angularly stable plates (stripping the internal hexagon in the screw heads or twisting off the screw). Experimentally, the magnitude of the torque that causes stripping of the internal hexagon (the Inbus type head) of 3.5 mm locking cortical bone screw made of titanium alloy Ti6Al4V according to ISO 5832-3 was determined. It has been experimentally found that slipping the end of a screwdriver with a hexagonal end inside the head of the locking cortical bone screw when stripping the internal hexagon causes a deformation of the screw head circumference and thus increases the thread friction. The influence of the tightening torque on the ability to unscrew the locking cortical bone screw from the locking hole of the angularly stable plate was evaluated experimentally. The numerical assessment of the five alternatives of the head shapes of the locking cortical bone screws in terms of stresses and resulting deformations has shown that the screw with the Torx type head, which reaches the lowest values of reduced stress and plastic deformation, is the best option.

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