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2 2019

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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.

Mechanical response at peri-implant mandibular bone for variation of pore characteristics of implants: A Finite Element Study

Sarkar Sulagna, Sahu Tikeshwar Prasad, Datta Arijit, Chandra Nimesh, Chakraborty Arindam, Datta Pallab, Majumder Santanu, Chowdhury Amit Roy

Acta of Bioengineering and Biomechanics

2019

Vol. 21, no. 2

83--93

In this paper, the mechanical response of generic dental implants having calculated porosities with varying pore-sizes has been evaluated. The purpose of this study was to compare the developed stress-strain of designed porous implants (i.e., stress at the implant and strain at the peri-implant bone) with that of the non-porous implant. Methods: 3D model of a mandible was prepared from CT scan data and nine generic dental implant models have been designed having 10%, 20%, and 30% porosity with 500, 700, and 900 micron pore size along with a non-porous model for carrying out FE analyses. First, failure analyses of implants, under a biting force of 250 N have been performed. Next, the remaining implants have been further evaluated under average compressive chewing load of 100 N, for mechanical responses at bone-implant interface. Results: Von Mises strain at the peri-implant mandibular bone increases with the increase in percentage porosity of the implant material and maximum implant stress remained much below the yield stress level. Conclusion: Implant stiffness and compressive strength vary as a function of porosity and pore size. Strain obtained on the peri-implant bone is sufficient enough to facilitate better bone growth with the 700 micron pore size and 30% porosity, thus reducing the effect of stress shielding.