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Biomechanical Analysis of Stress and Stiffness of New Load-Bearing Implants
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Medical implants are essential products for saving lives and improving life quality. Nowadays, the demand for implants, especially biocompatible and personalized ones, is increasing rapidly to deal with factors like congenital malformations, aging, and increasing prevalence of cancer. To facilitate their clinical applications, better understanding of their biomechanical properties is important. This thesis focuses on tubular and mandibular implants, and aims at studying stiffness properties and assessing stress distributions.

Tubular implants with coupled helical-coil structure, which can be potentially used as tubular organ constructs, were manufactured by winding polycaprolactone filaments. Tensile and bending stiffnesses were evaluated through mechanical testing and finite element simulations. By increasing the number of helical coils, we could realize a new type of tubular implants which could be used in applications like trachea and urethra stents. Stiffness properties of such implants were investigated analytically, due to the geometrical periodicity. Through computational homogenization, the discrete mesh structures were converted to equivalent continua, whose structural properties were studied using composite beam theories. The numerical and analytical models developed can serve as tools for the mechanical design of implants.

A patient-specific mandibular implant, additively manufactured of titanium alloys, failed shortly after surgery. The failure was studied using a numerical approach. Finite element models were generated from the 3D bone reconstructed from computed tomography data and implants processed by computational homogenization. The failure location and that of the numerically predicted largest von Mises stress agree well, which confirms the feasibility of using finite element simulations to quantitatively analyze implant failures and assist in implants design.

For implant failures caused by local bone loss, analytical studies were also carried out to assess the stress distribution around screw-loaded holes in bones. The mandibular bone was treated as a laminate of which elastic properties were obtained by classical laminate theory. The stress profiles were predicted using a complex stress function method. The loading direction was found to have a minor influence on the stress distributions, while the friction coefficient has non-negligible influence. The stress state can serve as starting point to predict bone remodeling and be compared with criteria for bone strength.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 61 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1290
National Category
Applied Mechanics
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
URN: urn:nbn:se:uu:diva-262688ISBN: 978-91-554-9342-4 (print)OAI: oai:DiVA.org:uu-262688DiVA: diva2:854886
Public defence
2015-11-06, Sal 80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2015-10-15 Created: 2015-09-18 Last updated: 2015-10-27
List of papers
1. Parametric elastic analysis of coupled helical coils for tubular implant applications: Experimental characterization and numerical analysis
Open this publication in new window or tab >>Parametric elastic analysis of coupled helical coils for tubular implant applications: Experimental characterization and numerical analysis
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2014 (English)In: Journal of The Mechanical Behavior of Biomedical Materials, ISSN 1751-6161, E-ISSN 1878-0180, Vol. 29, 462-469 p.Article in journal (Refereed) Published
Abstract [en]

Coupled helical coils show promising mechanical behavior to be used as tubular organ constructs, e.g., in trachea or urethra. They are potentially easy to manufacture by filament winding of biocompatible and resorbable polymers, and could be tailored for suitable mechanical properties. In this study, coupled helical coils were manufactured by filament winding of melt-extruded polycaprolactone, which was reported to demonstrate desired in vivo degradation speed matching tissue regeneration rate. The tensile and bending stiffness was characterized for a set of couple helical coils with different geometric designs, with right-handed and left-handed polymer helices fused together in joints where the filaments cross. The Young's modulus of unidirectional polycaprolactone filaments was characterized, and used as input together with the structural parameters of the coupled coils in finite element simulations of tensile loading and three-point bending of the coils. A favorable comparison of the numerical and experimental results was found, which paves way for use of the proposed numerical approach in stiffness design under reversible elastic conditions of filament wound tubular constructs.

Keyword
Coupled helical coils, Finite element simulation, Structural stiffness, Tubular implants
National Category
Natural Sciences Engineering and Technology
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-218968 (URN)10.1016/j.jmbbm.2013.09.026 (DOI)000330085700042 ()
Available from: 2014-02-20 Created: 2014-02-20 Last updated: 2017-12-06Bibliographically approved
2. Elastic analyses of periodic rhombic mesh structures
Open this publication in new window or tab >>Elastic analyses of periodic rhombic mesh structures
(English)Manuscript (preprint) (Other academic)
National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-262684 (URN)
Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2015-10-27
3. Applicability of a two-stage analytical model of the stiffness of mesh tubes
Open this publication in new window or tab >>Applicability of a two-stage analytical model of the stiffness of mesh tubes
(English)Manuscript (preprint) (Other academic)
National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-262685 (URN)
Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2015-10-27
4. Failure location predictoin by finite element analysis for an additive manufactured mandible implant
Open this publication in new window or tab >>Failure location predictoin by finite element analysis for an additive manufactured mandible implant
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2015 (English)In: Medical Engineering and Physics, ISSN 1350-4533, E-ISSN 1873-4030, Vol. 37, no 9, 862-869 p.Article in journal (Refereed) Published
Abstract [en]

In order to reconstruct a patient with a bone defect in the mandible, a porous scaffold attached to a plate, both in a titanium alloy, was designed and manufactured using additive manufacturing. Regrettably, the implant fractured in vivo several months after surgery. The aim of this study was to investigate the failure of the implant and show a way of predicting the mechanical properties of the implant before surgery. All computed tomography data of the patient were preprocessed to remove metallic artefacts with metal deletion technique before mandible geometry reconstruction. The three-dimensional geometry of the patient's mandible was also reconstructed, and the implant was fixed to the bone model with screws in Mimics medical imaging software. A finite element model was established from the assembly of the mandible and the implant to study stresses developed during mastication. The stress distribution in the load-bearing plate was computed, and the location of main stress concentration in the plate was determined. Comparison between the fracture region and the location of the stress concentration shows that finite element analysis could serve as a tool for optimizing the design of mandible implants.

National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-262683 (URN)10.1016/j.medengphy.2015.06.001 (DOI)000361409100005 ()
Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2017-12-04Bibliographically approved
5. An analytical study of stress distributions around screws in mandibular bone
Open this publication in new window or tab >>An analytical study of stress distributions around screws in mandibular bone
(English)Manuscript (preprint) (Other academic)
National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-262686 (URN)
Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2015-10-27

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