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Biomedical Engineering

Faculty of Engineering, LTH

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Biomechanics

Interested?

If you are interested in the projects below, please contact Hanna Isaksson.

Crack propagation in single bone trabeculae

Objective: Develop a computational finite element (FE) model to study damage and crack propagation in single bone trabeculae. The goal is to evaluate how the internal microstructure affects the crack path and the fracture resistance of the bone locally.

Background: Damage in trabecular bone starts as microcracks inside individual trabeculae. These are believed to be important for the overall mechanical integrity and for the remodeling of the trabecular bone tissue. Experimental studies have shown that microcracks tend to follow the internal tissue structure inside the trabeculae, however, how that affects the mechanical response or the resistance to fracture is not known.

Approach: The candidate will develop FE models of single trabeculae based on microscopy images. Starting from a homogeneous geometry, models with increasing complexity will be developed to analyze e.g. the effect of the lamellar microstructure, heterogeneous material distributions and anisotropy on crack propagation.  

Application: A modelling framework based on the extended finite element method (XFEM) in Abaqus will be used as a starting point. This framework has been previously developed in the research group and used to simulate crack propagation at the microscale in cortical bone models. Within the master’s project, alternative approaches to simulate crack propagation can also be evaluated, such as using zero-thickness cohesive elements. The results in terms of crack patterns and mechanical response will be evaluated. 

Figure. Schematic representation of project approach. A single trabecula is segmented and converted to a FE-model. The XFEM method is used to simulate crack propagation under tensile loading.  

Student background: Knowledge of finite element modeling (FHL066 or analogous course is highly valued), solid mechanics and fracture mechanics.

For more info, please contact: Biomechanics group, Professor Hanna Isaksson, hanna.isaksson@bme.lth.se

Analysis of bone-implant integration using digital volume correlation on micro-CT images

Objective: Analyse the on bone-implant integration strength in terms of crack propagation using digital volume correlation (DVC).

Approach: The candidate will determine local strain accumulation and track crack propagation in the interface between a PEEK implant and newly formed bone using DVC applied to micro-CT images acquired during in-situ pull-out of the implant.

Application: Proper integration of an implant is essential for its longevity and the mechanical integrity of the bone-implant interface. Using a hollow implant, filled either with pure bone cement or with a mixture of cement and bioactive molecules stimulating bone ingrowth, left to integrate for 6 weeks in rat tibiae, the effects of drug treatment on bone formation was studied by doing stepped in-situ pull-outs of the implant whilst acquiring micro-CT images after each load step. To access the internal strains leading to implant loosening, 3 representative specimens from each treatment group should be analysed using DVC. The overall aim is to evaluate the effects from treatment on the structural integrity of the newly formed bone around the implant.

Figure. a) Rat tibia with implanted hollow PEEK screw. To cover the entire region of interest 3 vertical scans were done. b) Image slice of tomography acquired at the base of the screw. c) Implant loosening and DVC analysis for load step 1 and 2 [1]. d) Crack propagation at the bone-implant interface [2].

Student background: Basic knowledge in image analysis and solid mechanics.

Relevant literature:

[1]         Le Cann, S. et al., “Characterization of the bone-metal implant interface by Digital Volume Correlation of in-situ loading using neutron tomography,” J. Mech. Behav. Biomed. Mater., vol. 75, no. March, pp. 271–278, 2017.

[2]         Le Cann, S. et al., “Investigating the Mechanical Characteristics of Bone-Metal Implant Interface Using in situ Synchrotron Tomographic Imaging,” Front. Bioeng. Biotechnol., vol. 6, no. January, Jan. 2019.

 

For more info, please contact: Biomechanics group, Professor Hanna Isaksson, hanna.isaksson@bme.lth.se

Characterization the microenvironment of mineralized regions in the Achilles tendon

Objective: Characterize structural and mechanical microenvironment around areas of mineral deposition found in the Achilles tendon

Approach:

-          Literature exploration: Geometrical characterization of the mineral deposition. How/when and why?

-          Experimental mechanical analysis: Quantify mineral deposition from microCT images and perform mechanical characterization of the microenvironment surrounding the mineral deposition by mapping strains during tensile testing of tendons

-          Finite element analysis: Testing the effect of introducing mineralized regions inside the soft tendon on spatial distributions of strains in an established Achilles tendon computational model

Background: The Achilles tendon transfers load between different calf muscles and the heel bone. Recently, a wide range of animal studies and human case studies have found significant large mineral formations (bone-ish) in the Achilles tendon. We would like to investigate what affect these regions have on the local mechanical response and the tendon architecture. Do these mineralized regions affect tendon rupture patterns?   


  

Figure. Bone formations in A) injured mice* and B,C) injured and noninjured rats**. The project will try to characterize the D) microenvironment of mineral inside the tendon and E) investigate how these regions affect the local mechanics, e.g. local strains around the bone region using an established tendon finite element model.

Student background: Basic knowledge in solid mechanics and experimental mechanics or finite element modeling (depending on focus above). Interest in biomechanics and image analysis is highly beneficial.

Important literature:

*Howell K et al. (2017) Novel model of tendon regeneration reveals distinct cell mechanisms underlying regenerative and fibrotic tendon healing. Scientific reports 7:45238

**Huegel et al. (2019) Quantitative Comparison of Three Rat Models of Achilles Tendon Injury: A Multidisciplinary Approach. Annual meeting Orthopaedic Research Society 2019, #0554

For more info, please contact: Biomechanics group, Professor Hanna Isaksson, hanna.isaksson@bme.lth.se

High definition 3D tracking of tendon collagen fibers

Objective: Develop a computer algorithm to 3D track fibers inside the complex Achilles tendon tissue. Determine how fiber sizes change in different regions of the tendon and under different loading conditions. Determine the (dis)continuity of fibers (which is currently under debate).

Background: Tendons are soft tissues composed of hierarchically arranged collagen fibers (30%) and water (70%). Achilles tendons are mechanoresponsive, i.e. they actively adapt to their in vivo mechanical loading environment over time. Using high-resolution tomography and automized 3D computer analysis this experiment aims to investigate how the microstructure is affected by altered in vivo mechanical loading/unloading. To date the full relationships between structure and function throughout the tendon hierarchy is yet to be elucidated. Understanding how loading influence the tendon tissue is essential to preserve a correct mechanical function of tendons which, in turns, affect the overall skeletal health. The data that will be analyzed in this project are high resolution volumes acquired by phase-contrast enhanced synchrotron X-ray tomography. To our knowledge this cutting-edge technique was never use before to study Achilles tendon structure before.

Approach: The candidate will develop and validate an algorithm to segment single fibers, calculate fiber sizes and fiber geometries, and follow fiber trajectories three-dimensionally.

 

Figure: A) schematic representation of an Achilles tendon. B) Phase-contrast enhanced synchrotron X-ray volume showing the tendon internal structure. C) volume rendering showing some collagen fibers.


Student background:
knowledge in 3D image analysis and programing.

For more info, please contact: Biomechanics group, Professor Hanna Isaksson, hanna.isaksson@bme.lth.se

Development of a dynamic FE model of a fall to the side

Objective: Develop a computational 3D finite element (FE) model of the hip impact during a fall to the side. The goal is to predict whether the fall would result in a hip fracture or not.

Background: Hip fractures are a very common issue among elderly people and an ever-increasing socio-economic issue in an ageing population. Computational 3D FE models can help to predict bone strength and consequently the risk of fracture. However, most current FE models simulate quasi-static loads on the bones, whilst over 90% of hip fractures result from a sideways fall, which is a clear dynamic event.

Approach: The candidate will develop and validate a computational 3D FE model using the determined global and local strains, trabecular bone microstructures, and mineral density distributions from µCT images that have been acquired while bone deforms in situ under compressive loading.

Application: An existing subject-specific FE modeling pipeline of human femurs that was developed in our group will be used as the starting point. This pipeline includes strain-rate dependent material properties with specific strain limit values for yield and failure. First, this material model will be adjusted to work in dynamic models that are solved using explicit solvers. The second part of the project consists in modeling the soft tissue around the hip using CT scans and including that in the FE model to evaluate the damping effect of soft tissue on the fracture outcome.  

Figure. The validated quasi-static FE models in developed at the biomechanics group at Lund University. This project will adjust the constitutive relations to work in case of a dynamic fall, where an impact speed to the ground is imposed, and add soft tissue padding obtained from CT scans to the model.

Student background: Knowledge of finite element modeling (FHL066 or analogous course is highly valued), image analysis and solid mechanics.

For more info, please contact: Biomechanics group, Professor Hanna Isaksson, hanna.isaksson@bme.lth.se

A computational model of the human Achilles tendon

Objective: Characterize the 3D Achilles tendon behavior by establishing a realistic human Achilles tendon model by describing realistic loading, geometry, material properties.

Approach:

-          Literature exploration: Geometrical characterization of the tendon fascicles and establish appropriate boundary conditions for the separate subunits of the Achilles tendon.

-          Experimental mechanical analysis: Tensile mechanical testing of the different tendon fascicles

-          Finite element analysis: Combining acquired information to build a fascicle-level Achilles tendon model  of the human tendon in a finite element framework

Background: The Achilles tendon transfers load between different calf muscles and the heel bone. This anatomy induces large inhomogeneities in external loading and internal properties. However, deep understanding of these inhomogeneities is lacking, and the tendons is mostly considered one cylindrical homogenous structure.

Collaboration opportunities: Potential collaboration with the biobank for obtaining biological samples for experimental characterization of geometrical properties and boundary conditions, or with Linköping University to utilize clinical CT images of the human lower limb.


 

Figure. A) Leg anatomy B) identifying the 3 subunits of the Achilles tendon and C) characterizing these dimensions and their D) orientations*. E) Comparison of the old simplified tendon geometry and a prototype for a more realistic geometrical model.

Student background: Basic knowledge in solid mechanics and experimental mechanics or finite element modeling, depending on focus above. Interest in biomechanics and image analysis is highly beneficial.

Important literature:

*Edama et al. (2015) The twisted structure of the human Achilles tendon. Scand J Med Sci Sports 25: e497-e503

For more info, please contact: Biomechanics group, Professor Hanna Isaksson, hanna.isaksson@bme.lth.se

Computational model on bone fracture mechanisms

Objective: Develop and validate a computational 3D finite element (FE) model on bone fracture mechanisms for improved predictions of fractures.

Approach: The candidate will develop and validate a computational 3D FE model using the determined global and local strains, trabecular bone microstructures, and mineral density distributions from µCT images that have been acquired while bone deforms under compressive loading.

Application: Trabecular bone tissue from cadavers with normal and low density bone is used. Using combined micro tomographic imaging, in situ mechanical loading, and digital volume correlation (DVC), the global mechanical characteristics and local tissue strains of human trabecular bone have been assessed. After development and validation of the FE model, the roles of local tissue mineral density and trabecular bone structure to the local and global fracture resistance in trabecular bone are evaluated. The overall aim is to predict the mechanical integrity of trabecular bone tissue and bone material for a better understanding of the damage and fracture mechanisms.

Collaboration opportunities: The work is part of a collaborative project between Lund University (Lund, Sweden, Department of Biomedical Engineering), and University of Eastern Finland (Kuopio, Finland, Department of Applied Physics). The thesis supervisors will be Mikael Turunen, Ph.D., (Kuopio) and Associate professor Hanna Isaksson, Ph.D., (Lund). The work will be performed mainly at University of Lund.

Student background: Basic knowledge in finite element modeling, image analysis and solid mechanics.

Simulate crack propagation in cortical bone with cohesive finite elements

Objective: Explore the potential of using cohesive finite elements to simulate crack propagation in cortical bone on the microscale and compare with existing models based on the eXtended Finite Element Method (XFEM).  

Approach: Create an automatic method for inserting cohesive elements into a mesh with “ordinary” finite elements. Evaluate possible damage models as well as mesh sensitivity, computational cost and convergence rate and compare with XFEM-models.

Application: The method can be tested on simplified microstructural models of cortical bone where Haversian canals, osteons and cement lines are the main components affecting crack propagation. 

Clinical relevance: We need better tools to model crack propagation in bone to understand the connection between bone quality and resistance to fracture.

Student background: Basic knowledge in solid mechanics with in-depth knowledge and interest in finite element modelling. This project will give hands-on experience with user-defined subroutines in Abaqus, which is one of the largest and most common commercial finite element softwares.

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