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

Biswas Jayanta Kumar, Sahu Tikeshwar Prasad, Rana Masud, Roy Sandipan, Karmakar Santanu Kumar, Majumder Santanu, Roychowdhury Amit

Biocybernetics and Biomedical Engineering

2019

Vol. 39, no. 1

52--62

Loosening and breakage of lumbar pedicle screw are the most common complications affecting the spinal stability. The design factors of the pedicle screw that may affect the fixation strength under bending load are pitch length, major diameter, thread profiles and geometry. 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 (1–4 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.