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research-article

Residual stresses in titanium spinal rods: effects of two contouring methods and material plastic properties

[+] Author and Article Information
Francesca Berti

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
francesca.berti@polimi.it

Luigi La Barbera

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
luigi.labarbera@polimi.it

Agnese Piovesan

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
agnese.piovesan@mail.polimi.it

Dario Allegretti

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
dario.allegretti@polimi.it

Claudia Ottardi

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
claudia.ottardi@polimi.it

Tomaso Villa

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
tomaso.villa@polimi.it

Giancarlo Pennati

Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
giancarlo.pennati@polimi.it

1Corresponding author.

ASME doi:10.1115/1.4040451 History: Received January 10, 2018; Revised May 24, 2018

Abstract

Posterior spinal fixation based on long spinal rods is the clinical gold standard for the treatment of severe deformities. Rods need to be contoured prior to implantation to fit the natural curvature of the spine. The contouring processes is known to introduce residual stresses and strains which affect the static and fatigue mechanical response of the implant, as determined through time- and cost-consuming experimental tests. Finite Element (FE) models promise to provide an immediate understanding on residual stresses and strains within a contoured spinal rods and a further insight on their complex distribution. The present study aims at investigating two rod contouring strategies, French bender (FB) contouring (clinical gold standard) and uniform contouring, through validated FE models. A careful characterization of the elasto-plastic material response of commercial implants is led. Compared to uniform contouring, FB induces highly localized plasticizations in compression under the contouring pin with extensive lateral sections undergoing tensile residual stresses. The sensitivity analysis highlighted that the assumed post-yielding properties significantly affect the numerical predictions, therefore an accurate material characterization is recommended.

Copyright (c) 2018 by ASME
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