Bioengineering

Research Samples

Student: Narek Akopyan
Professor/Sponsor: Professor Liwei Lin
Mentor: Dr. Ryan Sochol
Research Project Title: Micropost Traction Force Quantification

 

Abstract:
Microfabricated posts were designed to advance cell handling techniques, which is useful for research in biology. By creating stiffness and interpost spacing gradients along the micropost array, bovine aortic endothelial cells (BAECs) were observed to unidirectionally migrate. The cells migrated in directions of increasing micropost stiffness and decreasing interpost spacing. The goal was to quantify the forces that the cell pushed or pulled on the microposts in order to move in one direction. These forces were calculated by taking microscopic images of the immovable bottom of the micropost array which was stuck to the substrate and the top of the micropost array which moved due to the forces applied by the cell. By applying the general Hooke's Law, forces were related by the displacement each micropost moved since each cantilever could be approximated as a spring. With aid of image processing software, micropost traction forces were quantified, and the edges of the cells were found to pull more strongly on the microposts compared to the center of the cell. The forces were found to pull inwards towards the center of the cell causing unidirectional cellular migration due to the variable stiffness and spacing gradients.

 

Student: Alex Belinski
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Farzana Ansari and Hannah Gramling
Research Project Title: Investigating the possibility of cell-induced corrosion on metallic bearing surfaces in total shoulder arthroplasty devices

 

Abstract:

 

Introduction:
Evidence of direct cellular attack on orthopedic implants has been observed in knee and hip implants [1], but has not been reported in shoulder implants. This work documents signs of direct cellular attack on cobalt chromium humeral heads retrieved from shoulder arthroplasty patients. These findings may provide evidence that cells initiate corrosive sites on metal bearing surfaces in the shoulder.

 

Methods:
Visual evidence of cellular attack was observed macroscopically on eighteen humeral head implants (Figure 1). Ten of the samples showing possible cellular attack (Depuy Global) were further examined using scanning electron microscopy (FEI Quanta, Hillsboro, OR). Following Gilbert et al., samples were imaged at 5 kV and 20 kV to highlight possible biological material. Sites were characterized using EDS line scans at 20 kV to determine whether changes in element concentrations spatially correlated with boundaries of possible cellular attack (“attack sites”).

 

Results:
Two types of attack sites were observed. The first showed increased concentrations of carbon overlapping areas previously identified as possible biological material (Figure 2), and the second showed increased concentrations of iron (Figure 3). Both instances were coupled with significant dips in concentrations of constituent materials cobalt, chromium, and molybdenum. Due to the low excitation energy of carbon, lower energy electron beams scattered by biological material resulted in brighter regions within the SEM image, while
using higher energy beams produced comparatively darker regions. Carbon-rich regions were found on a total of 7 samples and iron-rich regions were found on 3.

 

Discussion:
The presence of carbon may be indicative of biological material remnant from direct cellular attack. As shown previously [1], increased levels of iron may be evidence of a Fenton reaction. These observations have been observed in hip and knee retrievals; this is the first reported observation in shoulder retrievals. Future work will examine additional samples and reverse shoulder arthroplasty retrievals.
 
Reference:
[1] Gilbert, J., et al. (2014). Journal of Biomedical Materials Research Part A,
103(1), pp.211-223.

 

Student: Bita Behziz
Professor/Sponsor: Professor Masayoshi Tomizuka
Mentor: Daisuke Kaneishi, Robert Matthew
Research Project Title: Torsional Stiffness Characterisation for the APEX Gamma Exoskeleton
Research Areas:  Biomechanical Engineering, Controls

 

Abstract:

Weakness or paralysis of body muscles are one of the common side effects for stroke survivors. Assistive devices can be to support these individuals, aiding them in functional daily tasks. Developing devices to assist upper limb movement would depend on precise characterization and control of the assistive device and on the needs of the user. This paper investigates the mechanical impedance of a pneumatic cylinder under several air pressure conditions for use in assistive devices. The characterization is based on fixing the number of moles of gas on each side of the cylinder and measuring the associated torque. This static torque is evaluated at different angular positions, corresponding to the flexion/extension of the elbow. It was shown experimentally that the maximum stiffness that can be applied to the user is 2.18Nm/rad when the air pressure is initialized at 50 psi in both chambers and the minimum stiffness is below 0.01 Nm/rad when the system is initialized at atmospheric pressure. This study offers deeper insight into how linear pneumatic cylinders can be used to semi-passively provide assistance to individuals with limb weakness, and supports previous publications which tested different assistive pressures on human subjects without a model of the associated stiffness’s response.


Student: Connor Benton
Professor/Sponsor: Professor Tony Keaveny
Mentors: Megan Pendleton and Alex Baker
Research Project Title: Design and Implementation of an Apparatus for Flexural Testing of Trabeculae

 

Abstract:
The purpose of this research is to identify the most efficient and precise method of measuring the strain-stress properties of trabecular bone tissue at a very small scale. The heterogeneity inherent in bone tissue across different people suggests that there is value in large-scale testing in order to better understand how factors such as age, disease, and disease treatment impact the material properties of the bone itself. We first evaluated the differences between strain tests and their feasibility in regard to testing specimens of our ideal size, eventually coming to the conclusion that three-point flexural testing was the best approach. We designed a test bed for this research, using high-resolution micrometers and actuators in order to give us the control we desired, and fabricated the apparatus in the student machine shop. In order to verify the accuracy of our solution, we conducted material tests of aluminum samples with known strain-stress properties. Our test bed is a compact, accurate, easy-to-use platform that provides the means to test large quantities of specimens and establish a better understanding of their material properties. This research will be built upon by our team in future testing and evaluation of different impacts on the strength of trabecular bone tissue.

 

Student:  Christopher Berthelet
Professor/Sponsor:  Professor Lisa Pruitt
Mentors:  Farzana Ansari and Louis Malito
Research Project Title:  Design Considerations for Total Shoulder Replacements: An Analysis of Glenoid Contact Stresses

 

Abstract: 

 

INTRODUCTION

The total shoulder replacement (TSR) is the third leading reconstructive orthopedic procedure behind hip and knee replacements, and the fastest growing arthroplasty device in the market today [1]. The ball-and-socket design typically consists of a cobalt chrome (CoCr) humeral head that articulates against an ultrahigh molecular weight polyethylene (UHMWPE) glenoid. While hip and knee replacements can last 10-15 and 18-20 years respectively, TSR’s only last around 5-13 years. This may be related to the fact that TSRs encounter complex rotational and translational joint kinematics; for example, superior-inferior motion of the humeral head along the glenoid occurs through abduction-adduction. These complex loading scenarios can lead to failure via implant loosening, joint instability, infection, implant wear, and device fracture. Fracture along the glenoid rim is a particularly unique failure, where excessive translational motion results in eccentric loading that can cause subsurface cracking and eventual fracture along the circumference [2]. While the effect of conformity and glenoid thickness on contact stresses has been documented, the interplay of these design factors with differing material properties and eccentric loading scenarios has not been thoroughly examined. This research seeks to utilize computational analysis and Finite Element Analysis (FEA) methods to assess the contact stresses that develop in the glenoid as a function of device geometry, material properties, and translational motion.

 

METHODS

The glenoid contact stresses that develop due to translation of the humeral head were computationally investigated using MATLAB. A standard elasticity solution (SES) developed by Bartel et al. [3] was used to calculate contact stress. Following Sweiszkowski et al. [4], the translation of the humeral head from the central axis was modeled as a rotation of the head about the origin of the glenoid. Maximum glenoid contact stress data was generated by the MATLAB code at each degree of angular translation. In addition to testing the effects of humeral head translation on contact stresses in the glenoid component, the effects of material composition (elastic modulus), glenoid thickness, component conformity, and specific humeral-glenoid radii combinations were investigated.

 

RESULTS & DISCUSSION

For all variables evaluated, higher contact stresses were seen farther away from the central axis (i.e. greater translation). Contact stresses near the rim of the glenoid were on average 11.3% greater than those at the center of the component for all conformities, material compositions, and thicknesses evaluated. This has important clinical implications considering that neutral positioning results in stresses ranging from 12 to 40 MPa. Combined with translation, these elevated stresses can easily exceed the yield stress of UHMWPE and contribute to crack initiation that under cyclic loading can propagate to fracture.

Decreasing conformity (increasing radial mismatch) caused an overall increase in contact stress and enhanced the effect of humeral head translation. Contact stresses were greatest for highly crosslinked and sub-melt annealed formulations. Contact stress also increased as glenoid thickness decreased or as device size decreased. Overall, the findings demonstrate that TSRs experience a complex stress state that subjects the UHMWPE to both yield and fracture.

 

ONGOING WORK:

Current work is focusing on developing an FEA model of a humeral-head/glenoid system (ABAQUS) to confirm and enhance the above computational analysis.

 

ACKNOWLEDGEMENTS:

I would like to also acknowledge Robin Parrish for her assistance with code development and analysis for this project.

 

[1] Day, JS, et al., J Shoulder Elbow Surg, 19(8):1115–20, 2010.

[2] Ansari, F, et al. World Congress on Biomechanics, June 2014

[3] Bartel, DL, et al. J BoneJoint SurgAm., 68(7):1041–51, 1986.

[4] Swieszkowski, W et al., J Engr in Medicine, 217:49-57, 2003.


Student: Matt Cameron
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Cynthia Cruz
Research Project Title: Improved Method for Specified Motion Monitoring While Conducting Wear Testing on Biomedical Thermoplastic Polycarbonate-Urethane

 

Abstract:
In current medical devices, cobalt chromium (CoCr) and ultra-high molecular weight polyethylene (UHMWPE) materials are used to minimize fatigue and wear of joint replacements while in vivo. However due to the nature of the polymer, UHMWPE particulates can wear off and be released into the body and subsequently loosen the implant with the potential to fail. A new polycarbonate-urethane material, Bionate®, is considered to have little to no wear compared to UHMWPE [1]. Studies have only focused on Bionate® 90A and 55D. DSM, Inc. is looking at the application of Bionate® 80A and 75D in the shoulder, yet no studies focused on the wear characteristics of these specific Bionate® chemistries. Hence, the Medical Polymer Group has a Multi-Directional Tribo-System (ELVIS), which will conduct wear testing on the 80A and 75D Bionate® material.

 

ELVIS is a converted CNC mill controlled by a custom a National Instruments LabVIEW Virtual Instrument (VI). The VI controls the type of motion, speed, and amount of translation and rotation to conduct wear tests as well as mimic motions within a joint. The VI simultaneously monitors the position of the point of contact, the temperate and electric current of the CNC motors, and cycle information. In preparation for the Bionate® wear testing, improvements were made to how the VI monitors cycle count and cycle time. This was achieved by adding new features to the motion code in addition to timing functions that execute when motion parameters change. This allows for more accurate testing and estimation for how long the test will take. The revamped VI in combination with other adjustments will allow ELVIS to accommodate samples of the new material and then the wear characteristics of Bionate® 80A and 75D can be established. Future studies will use ELVIS to mimic the gait cycle in a shoulder joint, which has translation, abduction/adduction, and elevation all at varying rates within one motion cycle. This would be a better model at predicting the wear in the shoulder joint.

 

[1] Smith SL, Ash HE, Unsworth A. A tribological study of UHMWPE acetabular cups and polyurethane compliant layer acetabular cups. J Biomed Res J Appl Biomech (2003); 813B: 710- 716.

 

Student: Jiayang Cao, Joyce Huang, Tatiana Jansen, and Rohan Konnur
Professor/Sponsor: Professor Grace O'Connell
Research Project Title: Modifying the existing Patient Controlled Analgesia (PCA) pump to explore non-chemical pain relief

 

Abstract:

Following research conducted at UCSF, under Ben Alter and Walter German, our group had the task of designing and building a device to attach to the existing PCA pump, in order to assess the effect of audiovisual cues on pain relief. Currently, the commonly-used PCA pump includes a handset that only the patient has control over. When the button on the handset is pressed, a very small dose of pain relieving medicine is dispensed into an IV in the patient’s arm. There is a specified lockout time as well that varies patient to patient. Our device is comprised of a Raspberry Pi Model 3B as our computer, enclosed in an acrylic box that we designed and made to attach to the IV pole. We also had to add a second button to the existing handset to interact with our device. Our attachment to the pump reliably plays an audiovisual cue, which researchers have made, on a monitor screen any time the button is pressed outside the lockout period. In addition, our device stores the patient data associated with these button presses into a csv file that can later be analyzed.

 

Student: Wan Fung Chui
Professor/Sponsor: Professor Grace O'Connell
Mentor: Megan Pendleton
Research Project Title: Data Processing for MTS Fatigue Testing
Research Areas:  Biomechanical Engineering, Mechanics

 

Abstract:

As ever more ambitious projects for human spaceflight are planned by both governmental and commercial organizations, the effects of long-term exposure to ionizing radiation on the mechanical properties of bone has emerged as an active area of research. Our team in the O’Connell lab strives to test the hypothesis that ionizing radiation encountered in space leads has an embrittling effect on bone tissue.

 

The central source of data on the experimental side of our study this semester involved conducting fatigue tests on L5 bones extracted from irradiated and non-irradiated rats. These tests, carried out using an MTS testing system, resulted in many large, unwieldy raw data files. In an attempt to automate and streamline our data collection process across the multiple samples we tested, I was tasked with writing several computer programs in MATLAB which accepted raw data as input and produced processed numbers and graphs as output. Metrics of interest included number of cycles to failure, strain to failure, slope of the secondary region in the fatigue process, and stiffness within the secondary region, all of which shed light into how, and to what extent, ionizing radiation affects the mechanical properties of bone.

 

The process of writing efficient, generalizable and user-friendly code involved navigating a series of challenges including lengthy file-parsing times, accounting for formatting difficulties in the raw data input, and recognizing the secondary and tertiary regions in the strains-cycles graphs, among other issues. Ultimately, this culminated in three polished, well-commented MATLAB programs which can now be easily used by members of our team and, potentially, other members of our lab conducting fatigue studies, to produce clean graphs and extract desired metrics in a user-friendly and timely manner.


Student: Patrick Holmes
Professor/Sponsor: Professor Tony Keaveny
Research Project Title: Effects of space-relevant levels of ionizing radiation on rat trabecular bone

 

Abstract:
Ionizing radiation is often used to treat cancer by applying a large dose of radiation locally to targeted tissue. This causes a number of destructive effects on bone in the affected area, which have been fairly well studied. The effects of very small doses of radiation on bone is less well documented. On a deep space mission, astronauts will be constantly exposed to radiation that is blocked for the rest of us by Earth's magnetosphere. The cumulative whole body dose they are likely to receive is very small; below what is used locally on a cancer patient in a single sitting. However, in conjunction with musculoskeletal disuse, this small amount of radiation could have a significant effect on the astronaut's bone. After helping with a literature review last semester, this semester I aided in the setup of an experiment to quantify changes to the material properties of rat vertebrae exposed to low doses of radiation. Individual vertebra will be tested in compression and at the same time simulated in a finite element model. In order to receive accurate compression results, the vertebrae must be prepared such that they have parallel ends. Otherwise, bending can occur and skew our data. To do this, several jigs were employed to first cement the vertebra in PMMA (bone cement) and then to saw off the ends of the vertebra with a slow moving ISOMET saw, yielding parallel sides. Physical tests will determine the parameters of our finite element model, with which we hope to explore the changes to the post yield properties of rat trabecular bone. We expect low doses of radiation to embrittle the trabecular bone, and to cause it to fracture earlier.

Student: Joe Felipe
Professor/Sponsor: Professor Grace O'Connell
Research Project Title: Design of Large Scale Waterbath For Mechanical Testing of Soft Tissue

 

Abstract:

Mechanical testing of the intervertebral disc requires that the soft tissue maintain hydration for accurate and meaningful results. The aim of this project was to build a large scale waterbath compatible with the lab's MTS machine for usage in soft tissue biomechanics. Prior to using a bath for mechanical testing, experiments that were performed could only be executed for 2-3 hours before samples of bone-disc-bone segments no longer maintained proper hydration. A new testing configuration was to be implemented in which the load cell was to be moved from the bottom to the top of the MTS machine. Grips were prepared to hold samples of bone-disc- bone segments in place for testing. With this new configuration, the bath and grips could be used for all sorts of mechanical testing such as compression, tension, and torsion. A 3-D model of the bath was made in Solidworks prior the machining of the bath. The implementation of a waterbath has greatly improved the accuracy of our results. Studies in the O'Connell lab have been focused on understanding the mechanical function of the healthy, injured and degenerated disc with the goal of developing viable repairment strategies. It is essential to have accurate and repeatable data to meet this goal. For example, a recent study "Osmotic loading environment alters intervertebral disc mechanical function" focused on comparing the mechanical properties of the intervertebral disc when soaked in a 1X vs 20X (.1M or 2M) saline solution. The difference in salinity would mimic two different states of hydration experienced in diurnal loading. Prior to the bath, bone-disc-bone segments were soaked overnight and then tested for only 2.5 hours. A prediction model was used in MATLAB which determined the samples would take about 16 hours until they reached equilibrium (no longer displacing during creep). With the implementation of the waterbath, results will no longer need to be "predicted" based off the limited data that could be collected. The next study using the bath will be on understanding the effects of space flight on spine biomechanics.

 

Student: Benjamin Glaser
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Noah Bonnheim
Research Project Title: Machining Methods for Carbon Fiber PEEK Composite
Research Areas:  Biomechanical Engineering, Materials

 

Abstract:

Carbon fiber reinforced polyether ether ketone (PEEK) is a polymer composite used in orthopedic implants and screws because it offers benefits over metals in some cases. Because there are multiple manufacturers of carbon fiber reinforced PEEK, it is important to understand their relative material properties, and also to compare those samples to a non-carbon-fiber PEEK for a control. A way to identify important material properties is through monotonic tensile testing, which requires material samples of specified sizes, as per ASTM documentation. In preparing for the machining of the PEEK sample testing “dogbones”, three manufacturing methods were researched for their benefits and effectiveness. Water jet cutting, CO2 laser cutting, and CNC milling were looked at. Water jet cutting was discounted because of machine limitations for handling polymer waste. CO2 laser cutting was not chosen because of the impact of high temperatures on the material properties of PEEK, specifically samples containing carbon fiber. Traditional machining was found to be more expensive for both cost and time, but was chosen for machining dogbones as the remaining viable option. Samples containing carbon fiber need are, however, more difficult to machine because of an increased wear on machining tools, and also because of the potential health hazards from carbon fiber dust being released into the air during the subtractive manufacturing process. A machine shop was chosen based on the capability of handling carbon fiber PEEK, as well as cost per specimen. ASTM testing will be performed following the ASTM document D638, which specifies the geometric shape of the dogbone specimens as well as the strain rate and method for testing. Because the strain rate is expressed in terms of time to failure, some dogbones must be sacrificed at different strain rates to find the appropriate rate for the remaining samples.


Student: Landon Henson
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Cynthia Cruz
Research Project Title: Polishing UHMWPE for use in experiments

 

Abstract:
The purpose of our current research is focused on various aspects of ultra high molecular weight polyethylene (UHMWPE) and Bionate® as it relates to wear, life and early failure in orthopedic implants. It is well known in the orthopaedic community that UHMWPE in combination with cobalt chromium (CoCr) are good counter bearing materials for joint replacements. However, UHMWPE does have a finite life expectancy. Historically wear and damage of UHMWPE has affected the longevity of orthopaedic implants. Thus, it is our goal to gain a better understanding of how the wear characteristics of Bionate® compares with the wear characteristics of UHMWPE. It has been proposed that Bionate® 80A and 75D should be used as a counter bearing material for CoCr in the shoulder joint by DSM, inc.

 

In order to run experiments and thus get a better understanding of how the wear characteristics of Bionate® and UHMWPE effect the life of implants, we must reproduce as close as possible a medical grade finish that manufacturers achieve on their implants. One such property is the "smoothness" or roughness average (Ra) number of the sample to be used in experiments. Ra is the measure of the texture of a surface. To quantify the surface roughness we use a profilometer to measure the profile of the UHMWPE surface.

 

Previously in our lab, UHMWPE samples were polished to an Ra number of approximately 0.2~.4 µm. To achieve this finish with repeatability a new SOP for polishing was needed. The new process is a two-part wet sanding procedure. Once the appropriate geometric tolerances are obtained from machining the samples, they are polished. Optimal conditions for polishing show that abrading the sample with 800-grade sandpaper followed by 1200-grade result in consistent Ra of .2~.3 µm.


Student: Naomi Kibrya
Professor/Sponsor: Professor Grace O'Connell
Research Project Title: Effect of Injury and Axial Compression Preload on Intervertebral Disc Torsional Mechanics

Abstract

Student: Divya Kulkarni
Professor/Sponsor: Professor Tony Keaveny
Mentor: Shashank Nawathe
Research Project Title: Influence Of Typical Population-Variations In Tissue-Level Ductility On The Femoral Strength

 

Abstract:
The strength of the whole bone is widely known to have a direct correlation with aging, disease and treatment. However there is not much work on the effect of tissue level ductility on whole bone strength. It makes sense that a change in individual tissue ductility would affect the overall failure of the bone whether it be the femur or the vertebrae. There have been studies in the past for which the tissue level ductility is manipulated to be either fully ductile or fully brittle and the effect of these cases on the strength of the whole bone are studied. In the real world case such extreme behaviors would most likely not be seen. In our study, we focus on human proximal femurs to study the whole bone strength and varying values of ultimate strain for the bone tissue ductility. The distinction between cortical and trabecular bone is made to find a deeper correlation between tissue level ductility and femoral strength. Relating the tissue level ductility on a micro scale with whole bone strength will be vital in understanding the cause of hip fractures and its risk-assessment.

 

Four cadavers are chosen to test various values of ultimate strain for both trabecular and cortical tissues of these bones. The values used are based on the previous studies of general ultimate strain values in the human population. It was assumed that ultimate strain values in tension and compression were equal. We performed our non-linear finite element analyses using the iterative quasi-nonlinear technique that has also been previously used in our fully brittle analyses.

 

The femoral strength was determined from each set of ultimate strains on both the cortical and the trabecular bone. This strength was determined using the force strain curve for a structure-level and calculating the 0.2% offset. Tissue level failure included both yielding and fracture. It seems as though during a sideways fall, only about 10% to 12% of the femoral strength is actually affected by the changing tissue ductility. The trabecular bone seems to have a larger effect on the entire bone strength. It seems the cortical bone ductility only plays a large role when the trabecular bone ductility is already low.

 

Student: Siyang Liu
Professor/Sponsor: Professor Liwei Lin
Mentor: Eric Sweet
Research Project Title: 3D Printed Three-flow Microfluidic  Concentration Gradient Generator  for Clinical E.Coli Antibiotic Drug Screening
Research Areas:  Biomechanical Engineering, Design, Fluids, MEMS/Nano

 

Abstract:

In the spring 2017 semester, I worked as an undergraduate researcher in Micro-Mechanical Method for Biology (M3B) program in Lin Lab of University of California, Berkeley under the supervision of Ph.D Student Eric Sweet. The project I conducted research on is the 3D Printed Three-flow Micro-fluidic Concentration Gradient Generator for Clinical E.Coli Antibiotic Drug Screening.

 

Specifically, I am to develop a device, through means of 3D printing, to mix three species of bio-fluids and obtain flow outputs with various concentration compositions. The design research process is comprised of CAD designing, 3D printing and cleaning, and Testing with Fluigent micro-fluidic system. The primary goal is to obtain equal flow and linear concentration gradient from the outputs. Through the semester, I went through multiple mixer designs with different design parameters, and reached an agreement with Eric that the design with equilateral tetrahedrons units will produce the ideal outcome. For the final testing design, we have three layers of tetrahedron units, which mixes 3 inlets into 15 outlets.

 

We used the Projet 3500 HDMax printer to print the mixer and went through a series of cleaning process including hot mineral oil bath, hot water cleaning and room temperature water cleaning. Then we used the Fluigent Micro-fluidic system to input fluid with uniform pressure into the mixer and try to obtain even flow rate out of all outlets. Surprisingly, it turns out the task is more difficult than expected due to uncontrollable disturbances coming from gravity and flow resistance in micro-channels. By varying fluid supply volume and refining cleaning process we were able to obtain even flow out of the mixer ultimately.

 

Student: Ruben Maldonado
Professor/Sponsor: Professor Tony Keaveny
Mentor: Arnav Sanyal
Research Project Title: Multi-Axial Strength Testing of Human Femoral Trabecular Bone

 

Abstract:
Since multiaxial stresses can develop in trabecular bone during falls and at bone-implant interfaces, multiaxial strength behavior is of fundamental relevance to a number of orthopaedic problems. Building on the work of other student researchers in the lab who developed a 3D multiaxial failure criterion for human trabecular bone, the goal of this research is to extend the work to low-density trabecular bone and subsequently validate it using experiments. The experimental results will be used to validate the finite element models.

Student: Audrey Martin
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Farzana Ansari
Research Project Title: Evaluation of Damage on Retrieved Humeral Head Prostheses

 

Abstract:
Introduction: Over 53,000 patients in the United States each year receive a Total Shoulder Replacement (TSR), a synthetic metal-polymer bearing system that serves to reproduce the function of a diseased or injured glenohumeral joint [1]. On average, 10 percent of these patients will undergo a risky and costly revision due to premature wear, loosening, and fracture of the ultrahigh molecular weight polyethylene (UHMWPE) glenoid component [2]. Studies have shown that there is a strong correlation between the presence of UHMWPE-wear debris and bone loss (osteolysis) which can induce loosening of the glenoid component [3, 4]. The purpose of this ongoing study is to analyze the relationship between damage on Cobalt-Chrome (CoCr) humeral head prostheses and glenoid component wear. The goal for this term was to collect more scoring data for and prepare for a counter-bearing wear analysis.

 

Methods: The Medical Polymers Group (MPG) houses a collection of retrieved humeral head prostheses, many with matching glenoid components. Samples were prioritized for scoring based on the presence of (1) a matching glenoid component, (2) a damage evaluation for the glenoid component, and (3) an orientation marking on the CoCr component. Three undergraduates were trained in a previously developed, detail-oriented scoring methodology to evaluate damage on the retrieved humeral heads. The scoring methodology segregates damage modes into six categories: hairline scratching, curvilinear abrasion, pitting, dimpling, striated scratching, and linear abrasion. The data was analyzed by determining the percent of samples exhibiting each damage mode and the percentage of identification variation between scorers as compared to previously collected scores. Preparations were also made for a counter-bearing wear analysis by evaluating the capabilities of MPG's custom multidirectional tribological-system and designing fixtures for testing.

 

Results: In total, seven new scores were collected. Striated scratching continues to be the most commonly found damage mode with 100% of samples exhibiting this damage mode followed by curvilinear abrasion at 94.1%. Dimpling was found to be the least common at 61.8%. At least 88% of scorers per sample showed agreement on the presence of a particular damage mode for n > 2 scorers. The test parameters for the counter bearing analysis were determined. Given the current capabilities of the test frame, a 200/20N load profile was deemed appropriate for preliminary testing.

 

Discussion and Conclusions:MPG's scoring methodology continues to yield consistent results. With striated scratching and curvilinear abrasion being the most commonly found damage modes, these would be appropriate parameters to isolate for upcoming counter-bearing analyses. Future work will include performing these wear analysis using a sample with an isolated region of striated scratching, and abrasion as compared to an unused sample with no damage.

 

Student:  Ariana Moini
Professor/Sponsor:  Professor Tony Keaveny
Mentor:  Saghi Sadoughi

Research Project Title:  Structure-Function Relations for Calcaneal Trabecular Bone - Comparison with other Sites
 

Abstract:

 

There are various methods used to detect osteoporosis, a growing disease that leads to low bone density and an increased risk of bone fracture. A common modality used to detect osteoporosis is Dual Energy X-ray Absorptiometry (DXA), which measures the bone mineral density of the patient. DXA is most often performed on the lower spine and hips. However, it is expensive and exposes patients to small doses of ionizing radiation. An alternative to DXA is calcaneal ultrasound, which is non-ionizing and inexpensive. It is easier to use and is widely accessible to the public. However, it is not clear how well the calcaneal trabecular bone relates to the mechanical behavior of the trabecular bone in the hip and spine. In this experiment, we want to understand the structure-function relations of the calcaneal trabecular bone and compare them to previously measured trabecular bone properties from other anatomic sites. The trabecular bone in the calcaneus specimens is not oriented the same way as in the vertebral body. Therefore, uniform compression loading configuration will not be along the main axis of trabeculae and as a result will be off-axis. Therefore, to be able to have a reasonable comparison between the structure-function relations of trabecular bone from different anatomic sites, we need to account for this on-axis versus off-axis loading. For that, the orientation of each trabecula in calcaneal specimens must be found. Individual trabeculae segmentation (ITS) software was utilized to categorize each individual trabecula within each specimen as a plate or rod and return its corresponding coordinates as a vector of its orientation. The data was then imported into Matlab to calculate the angle of the rod and plates with the horizontal axis to then compare the structure-function relations of calcaneal trabecular bone with that of the vertebral trabecular bone. This research is currently in the process of comparing the data to the vertebral body.

 

Student: Robin Parrish
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Farzana Ansari
Research Project Title: Analysis of Stresses in Glenoids

 

Abstract:
Introduction: Ultra high molecular weight polyethylene (UHMWPE) is the most commonly used bearing surface in total join arthroplasties. However, failure of the UHMWPE component is a common cause of device failure. Therefore, novel materials are being developed in an attempt to increase the life of these devices. This study set out to determine stresses in the bearing surface used in total joint arthroplasty as a function of material, geometry, and loading condition.

Methods: This study was carried out computationally using the simplified elasticity solution and focused on the glenoid component of total shoulder arthoplasties. The following parameters were varied to determine their effects on stresses: elastic modulus of material used, backing material, and radial mismatch. Glenoid radius of curvature was also investigated for consideration of its effects on stresses in the glenoid.

 

Results and Discussion: It was shown that stresses in the glenoid increase as the modulus of elasticity of the glenoid increases. Glenoid stresses also increase with decreasing radial mismatch between the glenoid and humeral components. However, due to the increased contact area associated with lower moduli, effects of conformity are minimized in systems containing glenoids with lower moduli. Finally, it was shown that for any given geometric configuration, there is a polynomial relationship between modulus and maximum stress. This relationship was used to isolate the effects of backing thickness and humeral geometry and to demonstrate that increasing backing thickness increases the effective modulus and maximum stress in the glenoid. Finally, our findings suggest that biomaterials with lower moduli may be able to decrease stresses in the glenoid, subsequently reducing wear rates and leading to lower device failure rates.

 

Future Work: Finite element analysis (FEA) will be performed, and results from the simplified elasticity solution will be compared to the results of the simulation. Following the validation of this FEA model, a glenoid with variable radius of curvature will be investigated. Finally, conclusions will be drawn concerning the efficacy of novel biomaterials.

Student: Robin Parrish
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Farzana Ansari
Research Project Title: Finite Element Analysis of Crack Propagation in UHMWPE

 

Abstract:
Introduction: Ultra high molecular weight polyethylene (UHMWPE) is commonly used as a bearing surface in total join arthroplasties. However, failure of the UHMWPE component is a common cause of device failure. Several material modifications can be made to increase wear resistance, fracture resistance, and oxidative resistance. However, each compositional change has trade-offs. We are interested in characterizing the structure-property relationships that govern crack propagation because fracture is a common cause of catastrophic device failure.

 

Methods: This study was carried out computationally in conjunction with mechanical crack-propagation tests. A crack test specimen was modeled in Abaqus FEA software (Dassault Systèmes Simulia Corp), and various loading conditions were applied. The radius of the notch tip was varied, and a side-groove was added to the model. Complementary mechanical tests were carried out with the same set-up as the Abaqus model.

 

Results and Discussion: The conclusions that were drawn from the results of the simulations are as follows: (1) Stresses near the notch tip increase with decreasing notch radius. (2) Stresses near the notch tip increase with movement through the depth of the sample into the center. (3) Sharper notch radius results in lower stresses away from the notch tip. (4) Stresses at the surface and in the center do not change proportionally with movement away from the notch tip.

 

Future Work: Material data is being collected on the specific formulations of UHMWPE that are of interest to us. Connections are being drawn between the FEA model and the mechanical tests. We will calculate the size of the plastic zone in front of the notch tip to better design the mechanical tests to result in cracking rather than yielding.

 

Student: Sam Pliska
Professor/Sponsor: Professor Grace O'Connell
Mentor: Ben Werbner
Research Project Title: Effects of chABC Treatment on Annulus Fibrosus Biochemical Composition

 

Abstract:

The integrity of the intervertebral disc (IVD) is dependent on many structural factors. Failure occurring in and around the disc can manifest in many different forms, from fractures to tears and herniation [1]. The annulus fibrosus (AF) specifically is susceptible to multiple forms of tears that can increase in frequency and severity with age [1]. These tears can allow for the herniation of the nucleus resulting in pinching of spinal nerves, causing pain throughout the lower back and leg [2]. Due to its increased prevalence with age and it being genetically inherited [1], IVD degeneration is a large and growing problem. From what is known, a key contributor to deterioration is a loss of proteoglycans or glycosaminoglycan (GAG) chains [3].

 

GAGs are hydrophilic meaning they attract water which assists in absorbing and distributing compressive loads [3]. One method of characterizing IVD degeneration consists of analyzing the biochemical composition such as GAG and water content [4]. At this point in time, however, there is still a gap in understanding regarding the relationship between composition and mechanics. Analysis of the impact of GAG content on the mechanical properties of IVDs could help reveal some of the mechanisms associated with deterioration.

 

In the past, chondroitinase ABC (chABC) has been used in studies to enzymatically digest GAG in the AF to simulate the natural degeneration of the IVD [1, 3, 5]. To validate the efficacy of this degeneration process, comparisons will be made between the GAG content of chABC digested and non-digested specimens.

 

Through the utilization of chABC to enzymatically digest GAG, the exact impact to the biochemical composition of the AF was calculated. Along with the value of percent GAG content by dry weight, the weight percent of the samples that were water was also discovered. Due to GAG’s ability to attract water, it makes sense that the chABC digestion protocol would impact water content as well.     

 

Our study shows that with a very high statistical significance, the samples treated with chABC had a reduced GAG content. This initial result makes sense as it matches results of past studies. The statistical significance between the control water content and dGAG water content makes sense as well as it also matches past works [4]. This decrease in water content associated with the chABC digestion process is also intuitive. Water is retained in the matrix of the AF by the attraction of the GAG’s. When these GAG’s are digested out, the ability of the AF to keep the same hydration levels decreases.

Knowing the degree to which the GAG was digested by the chABC is important for future works in being able to know the exact changes in GAG content. By knowing the impact of GAG loss on the mechanical properties of the IVD, and knowing the specific GAG loss generated by the chABC digestion protocol, a model can be generated to describe how the IVD’s mechanical properties will degenerate based on the GAG content.

 

Seeing how the water content of the AF is positively correlated to the GAG concentration, it becomes apparent why the chABC process has such a large impact on the mechanical properties of the IVD. Water is a key factor in protecting the disc. The water being held assists in absorbing compressive loads and distributes the load more evenly around the circumference of the annulus [3].

 

REFERENCES

[1] Adams, M. and Roughley, P., Spine, 2006; [2] Ando, T., et al., Clin Orthop Relat Res, 1995; [3] Isaacs, J., et al., JMBBM, 2014; [4] O’Connell, G. et al., J Biomech Eng, 2009; [5] Lyons, G. et al, Biochem Changes, 1981;

 

Student:  Steven Roth

Professor/Sponsor:  Professor Shawn Shadden

Mentor:  Jessica Oakes

Research Project Title:  Particle Deposition in Human Lungs due to Varying Cross-Sectional Ellipticity of Left and Right Main Bronchi

 

Abstract: 

Particle deposition in the human lungs can occur with every breath. Airbourne particles can range from toxic constituents (e.g. tobacco smoke and air pollution) to aerosolized particles designed for drug treatment (e.g. insulin to treat diabetes). The effect of various realistic airway geometries on complex f ow structures, and thus particle deposition sites, has yet to be extensively investigated using computational fluid dynamics (CFD). In this work, we created an image-based geometric airway model of the human lung and performed CFD simulations by employing multi-domain methods (Oakes et al. (2014), Annals of Biomedical Engineering, 42: 899-914). Following the flow simulations, Lagrangian particle tracking was used to study the effect of cross-sectional shape on deposition sites in the conducting airways. From a single human lung model, the cross-sectional ellipticity (the ratio of major and minor diameters) of the left and right main bronchi was varied systematically from 2:1 to 1:1. The inf uence of the airway ellipticity on the surrounding flow field and particle deposition was determined.

 

Student:  Gerald Santos

Professor/Sponsor:  Professor Grace O’Connell
Mentor:  Megan Pendleton
Research Project Title:  Understanding Spine Biomechanics When Exposed to Spaceflight Radiation

 

Abstract:

Examining the changes in bone quality after exposure to spaceflight radiation is the interest of this research. Bone quality properties of Young’s modulus, fracture and yield stresses, and number of cycles to failure are studied through mechanical testing methods. Rat spines were obtained in the lab, with certain specimens having an exposed radiation rating, while others served as controls. Proper care and dissection measures were taken to remove all non-bone tissue from the rat spines, without imposing any cuts or fractures on the bone. This tissue removing process averaged 2.5 hours to complete. The following step involved separating the L3, L4, and L5 vertebrae sections by cutting through the vertebral discs. The vertebrae were then secured with PMMA in fixtures to allow parallel cuts on each end with an Isomet Diamond Saw. After being cut, the samples to be used in the data analysis were Micro CT scanned to allow finite element analysis. Mechanical testing was performed on multiple samples, with a combination of three test methods. One test method obtained the Young’s modulus value of the bone, the second executes a compression to failure test, and the third is a cycles to failure test. The modulus obtaining method was successful in repeating Young’s modulus values through cyclic compression testing at stresses lower than 20% of the fracture stress. The compression to failure test provided the fracture stress of roughly 100 N. The cycles to failure test was not run successfully due to the modifications of the modulus obtaining method, which is a prerequisite test. Further modifications to the modulus obtaining method and cycles to failure test will be done, while the data thus far will serve as base values for future tests.


Student: Joanna Scheffelin
Professor/Sponsor: Professor Tony Keaveny
Mentor: Arnav Sanyal
Research Project Title: Multiaxial Failure Criterion of Trabecular Bone

 

Abstract:
This semester I worked with Arnav Sanyal on the "Multi-axial Strength Criterion" project in which micro-CT scans of trabecular bone cube specimens were crushed in FE simulations by applying displacements in the x, y, and z directions. Data was collected for failure (Principal stress at failure) for all 3 directions. The ultimate goal is to fit this data to a closed ellipsoid in which the failure stresses in each direction are superimposed to create a super ellipsoid to show failure criteria of the bone specimen. I wrote various algorithms in MATLAB to fit this code to a closed surface. The best fit is a quartic ellipsoid translated and rotated by 3 Euler angles and with an additional variable term to alter the fit. The fit is done using the "fmincon" function in MATLAB with 10 variables.

Student: Colin Shanahan
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Farzana Ansari
Research Project Title: Compression Testing of Cross-linked Vitamin E Enriched Ultra High Molecular Weight Polyethylene

 

Abstract:
Vitamin E enriched Ultra-high molecular weight polyethylene (UHMWPE) is growing in popularity as a material for knee and other joint replacements due to its anti-oxidation properties. However, there have not previously been any studies done on its compressive properties which greatly determine its quality as a material in joints such as the knee. Using a methodology developed in previous tests which was based off of ASTM standard D695 for compressive testing of rigid plastics a series of tests were performed using an Instron machine. Conventional GUR 1020 UHMWPE was tested for baseline comparison purposes, both cross linked and non-cross linked. The same was performed for GUR 1020 UHMWPE enriched with Vitamin E, both cross linked and non-cross linked. Also of interest was the orientation of samples cut from stock material to confirm isotropy. Results so far have not shown any clear correlations and so further testing is required.

Student: Gregory Slatton
Professor/Sponsor: Professor Liwei Lin
Mentor: Dr. Ryan Sochol
Sub Area: Microfluidics
Research Project Title: Kidney-on-a-Chip: Biophysical Biomimicry via Micro/Nanoscale 3D Printing

 

Abstract:
With nephrotoxicity, or kidney failure, accounting for nearly 20% of pharmaceutical drug development failures during clinical trials, in vivo kidney systems could render costly, time-consuming (and sporadically inaccurate) animal testing obsolete. Current state-of- the-art platforms are typically fabricated with multi-layer soft lithography and contain two planar channels separated by a permeable membrane. In contrast to their in vivo counterparts, which include complex architectural geometries, state-of-the-art kidney-on- a-chip platforms have overly simplified geometries. Additionally, biophysical stimuli, including micro-environmental geometric cues, have been shown to greatly influence a wide array of cellular functions, thus necessitating a better model of biomimetic architecture to enhance the predictive capabilities of kidney-on-a-chip technologies. Current micro-and-nanoscale 3D printing-based methodologies are uniquely suited for mimicking the complex geometries of in vivo kidney structures, making an artificial kidney-on-a-chip substitute more attainable than ever before. Utilizing multi-jet 3D printing, we have set out to demonstrate this process by fabricating microscale fluidic channels that are lined with kidney cells and permeable membranes to mimic tubules in the kidney. Once simple geometries are successfully demonstrated with our process, the next step is to achieve the functions of a permeable membrane and cell lining in complex geometric architectures to create an artificial kidney-on-a-chip structure functional enough to replace its in vivo counterpart in clinical drug trials.

 

Student: Nisha Subramanian
Professor/Sponsor: Professors Tony Keaveny and Grace O'Connell
Mentor: Megan Pendleton, Shannon Emerzian
Research Project Title: Effects of Ionizing Radiation on Bone Biomechanics

 

Abstract:

The underlying motivation behind this research is to gain a better understanding of the effects of spaceflight radiation on bone biomechanics. The qualitative changes to tissue that result from exposure to ionizing radiation can be especially important for radiotherapy patients and astronauts embarking on deep space missions to Mars. This study seeks to examine the susceptibility to bone fracture after exposure to radiation through the use of cyclic fatigue testing. Irradiated mouse and rat models were used as our samples to experimentally measure changes in tissue quality. Previous methods used to determine a correlation between applied load and cycles to failure showed too much variation to allow us to draw meaningful conclusions.  Therefore, this semester a novel approach was devised to have computational data drive experimental procedure. Computational stiffness values were determined for each rat L5 vertebra sample using micro-CT based finite element analysis. These stiffness values were then used to obtain the force limits to be applied during fatigue testing in load control. This method will ideally yield more telling results than our previous procedures in regards to the correlation between fatigue data and tissue quality.


Student: Amelia Swan
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Farzana Ansari
Research Project Title: Comparison of Scratching and Abrasion Damage on Retrieved Cobalt Chrome Humeral Heads

 

Abstract:
The in vivo damage observed on the counterbearing cobalt chrome (CoCr) surface of total joint replacements (TJR) can increase the volume of wear debris released from the ultra-high molecular weight polyethylene (UHMPWE) glenoid surface. Consequently, osteolysis and implant loosening can occur [1]. The previous study investigated metallic damage on a microscale, scanning retrievals for striated and hairline scratches with a Phaseshift 3D Optical Profilometer. MapVue and Vision 32 software were used to retrieve 2D profiles of the surface. Matlab uses this data to gain values for average roughness (Ra), minimum valley depth (Rv), maximum peak height (Rp), skewness (Rsk), and kurtosis (Rku) [2]. This investigation applies the same methodology to scratches within abrasion patches found on the CoCr surface. The abrasion data will be compared to scratch data to determine if damage modes have different severities. Additionally, after testing different profiling methods in Vision 32, a more global abrasion analysis has also been developed. This analyzes a whole patch of abrasion as opposed to just one of its components. Future studies will include a comparison of the damage found on shoulder retrievals with that of hips and knees using the same procedures, as well as examining damage trends on CoCr surfaces of total arthroplasties versus hemiarthroplasties. Thanks to the principle investigator Lisa Pruitt, graduate mentor Farzana Ansari, and the Biomedical Nanotechnology Center for the use of their optical profilometer.

 

[1] Willert et al (1977) J Biomed Materials 11(2): 157-64.
[2] Ansari, F. et al., ORS 2013 Annual Meeting. San Francisco.

Student: Amelia Swan
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Farzana Ansari
Research Project Title: Development of Roughness Parameter Analysis for Retrieved Humeral Heads

 

Abstract:
Once retrieved, total shoulder replacements display damaged counterbearing cobalt chrome (Co-Cr) humeral heads. This damage varies in geometry and severity, from hairline and striated scratching to curvilinear and linear abrasion. It is theorized that this damage accelerates the wear of the bearing ultra-high molecular weight polyethylene (UHMWPE) surface in vivo. These wear particles can lead to implant loosening and an inflammatory response called osteolysis [1]. Previous studies in the lab have developed a macroscale damage scoring system, as well as a damage analysis method that determines roughness parameters over 2D profiles of the microscale surface. [2] The study shows that scratching has a higher peak height, kurtosis (peak sharpness), and average roughness compared to abrasion, although abrasion has higher skewness. It is likely that the third-body wear mechanisms differ between the damage modes. The large peaks from scratch profiles likely generate larger UHMWPE particles, but their negative skewness indicates that some peak material may be worn away over time. Abrasion with a linear geometry commonly occurs at the center of the humeral head, which experiences the largest contact stresses; this explains the reduced peak heights and kurtosis values found for this damage type. Meanwhile, curvilinear abrasion has blunter, shorter peaks and positive skewness but covers a large portion of the head's surface area. This could generate smaller wear particles but a larger overall volume of wear debris, which can worsen the immune response. [3] The study is currently ongoing. Future steps include increasing the sample size for statistical analysis; comparing damaged surfaces from different fixation methods, implant geometries, and causes of failure; coupling the damage between UHMPWE and metal surface; expanding the roughness analysis to hips and knees; and testing retrieved implants to see how scratch morphologies change after wear testing. Thank you to Professor Lisa Pruitt, graduate mentor Farzana Ansari, the employees of the Mechanical Engineering Student Machine Shop, and the Medical Polymers and Biomaterials Group for their advisement and facilities, as well as the Biomedical Nanotechnology Center for the use of their optical profilometer.

 

[1] Willert et al (1977) J Biomed Materials 11(2): 157-64.
[2] Ansari, F. et al., ORS 2013 Annual Meeting. San Antonio
[3] Swan, A. et al., ORS 2014 Annual Meeting. New Orleans

 

Student: Amelia Swan
Professor/Sponsor: Professor Lisa Pruitt
Mentor: Farzana Ansari
Research Project Title: Damage Analysis of Cobalt Chrome Humeral Head Retrievals using 3D Profilometry

 

Abstract:
After a total shoulder replacement is retrieved, damage is observed on both the bearing glenoid surface and the counterbearing humeral head. These are made of ultra-high molecular weight polyethylene (UHMWPE) and cobalt chrome (Co-Cr), respectively. The damage accelerates UHMPWE wear debris generation when the two surfaces articulate in vivo. This can lead to implant loosening and a painful immune response called osteolysis [1]. Previous studies have used a Phaseshift Optical Profilometer to scan the surface of the Co-Cr component, and MapVue and Vision 32 software to collect 2D surface profiles. Matlab imports this data and calculates average roughness (Ra), minimum valley depth (Rv), maximum peak height (Rp), skewness (Rsk), and kurtosis (Rku) [2]. This methodology has been applied to different severity levels of hairline scratching, striated scratching, and linear abrasion patches. It has been shown that certain damage modes and severities have some significant differences in roughness parameter values. Logically, it follows that the differing roughness values on the Co-Cr surface generate variably-sized UHMWPE wear particles [3]. However, a macroscale analysis should also be considered, as the damaged area's size and density will also affect the volume of wear debris. A multi-directional tribotester will be used for preliminary wear testing of retrieved Co-Cr humeral heads against UHMWPE disks. The tests will focus on comparing results from contact areas covered by varying damage modes and severities. This will illuminate the volume of UHMWPE debris that is worn away based on damage mode, and how metallic damage modes change after articulation against UHMWPE disks. Future studies will include additional roughness parameter statistics, the continuation of wear testing, and the expansion of this analysis to hip and knee retrievals.

 

[1] Willert et al (1977) J Biomed Materials 11(2): 157-64.
[2] Ansari, F. et al., ORS 2013 Annual Meeting. San Antonio
[3] Swan, A. et al., ORS 2014 Annual Meeting. New Orleans

 

Student:  Minhao Zhou
Professor/Sponsor: 
Professor Grace O'Connell
Mentor:  Bo Yang

Research Project Title:  Study On A Novel Hip Joint Replacement Surgical Technique

 

Abstract

 

Student:  Shan Zhu
Professor/Sponsor:  Professor Tony Keaveny
Mentor:  Saghi Sadoughi
Research Project Title:  Micromechanics of the Human Calcaneus Bone

 

Abstract: 

One of the highest priorities of osteoporosis research is to define measures of bone quality that are better predictors of clinical fracture risk than bone mineral density (BMD) measurements. Within osteoporosis research, osteoporotic fractures represent a biomechanical breakdown of the bone, therefore, a detailed understanding of the biomechanical mechanisms of such fractures is required in order to move beyond BMD in fracture risk assessment. To better understand additional fracture risk predictors, we sought to determine the dependence of bone strength on bone volume fraction by performing high-resolution micro–computed tomography (micro-CT), and micro–finite-element analysis on a heterogeneous cohort of 25 human calcaneus bones. Although, elastic modulus used in linear studies has been reported to be well correlated with strength over a range of bone densities, we also conducted non-linear analysis because yield stress may be a superior indicator of strength since failure behavior generally involves nonlinear phenomena. High resolution images were acquired for each sample. The resulting images were segmented using a global threshold. Using these high-resolution scans, a 3D voxel-type finite element model was generated for each sample. All elements were cube-shaped. Displacement-type boundary conditions were applied to simulate the loading configuration. Individual finite element models were solved using an implicit, parallel finite element framework. The calcaneus stiffness was calculated from the linear analysis resultant force and calcaneus strength was determined using the nonlinear analysis force strain curve with 0.2% offset. The results showed that both stiffness and yield stress scale reasonably with bone volume fraction. Additionally, nonlinear analysis visualizations showed local failure starting in the most porous regions. Finally, it was observed that tensile failure is the dominant failure mechanism in bone since bone tissue is stronger in compression.