Design and biomechanical testing of pedicle screw for osteoporotic incidents

In this study, geometrical features of pedicle screws have been modified and their performances are compared. Performance analysis has been made in terms of pull-out strength and torsional strength. The parameters investigated are core diameter, holes drilled normal to screw axis, angle between sequential holes and distance between holes. Three different core diameter have been studied, which are 4 mm (normal core diameter), 5 mm (medium core diameter) and 5.5 mm (high core diameter). Distance between sequential holes has been arranged such that there is either one hole per pitch or one hole per two pitches. Angle between sequential holes is either 90 degrees or 120 degrees. According to the test results, the screw, with medium core diameter (5 mm) containing one hole per two pitches with 90 degrees angle between sequential holes, has exhibited the optimum performance considering torsional strength and pull-out strength requirements. Its torsional strength is slightly higher than and, when Grade 40 polyurethane foam was used as bone simulating material, its pull-out strength is as good as, an undrilled normal core diameter screw, which is already being used in surgical operations. The fatigue performance of this best performed screw has also been found satisfactory according to the related standard. Its pull-out strength is also tested on a calf vertebra and a promising result has been obtained.