Design factors of lumbar pedicle screws under bending load: A finite element analysis

In this study, 84 finite element (FE) models of the pedicle screw were generated having 7 pitch lengths, 3 major diameters, 2 thread profiles and 2 geometries. The assembly of pedicle screw and CT scan based half section FE model of 4th lumbar vertebra was loaded with a 200 N force on the screw head which is equivalent to a bending moment of 11 Nm.With triangular thread profile and cylindrical geometry, for 300% increase in pitch length (1 mm to 4 mm), von Mises stress in screw and von Mises strain in bone increased by 65% and 117% respectively, for a 26% decrease in major diameter (7.6 mm to 5.6 mm) and correlations were proposed among screw stress (r2 = 0.992) or bone strain (r2 = 0.986), pitch length and major diameter. Similar correlations were also proposed for trapezoidal thread profile and tapered geometry (r2 = 0.994 for screw stress and r2 = 0.986 for bone strain).Hence, a combination of tapered pedicle screw with lower pitch length, higher diameter and trapezoidal thread profile may serve better under bending load for lumbar vertebral implant.
Source: Biocybernetics and Biomedical Engineering - Category: Biomedical Engineering Source Type: research