a n s y s biomedical industry

A D V A N T A G E

Spotlight on Engineering Simulation in the

Biomedical Industrys10 Standing Up Right

s12 Designing with Heart

s14 Going with the Flow

s15 Battle of the Bulge

s2 Making Life Longer and Better

s4 Turning Up the Volume

s6 Hip to Simulation

s7 Walking Pain Free

s8 Engineering Solutions for Infection Control

SWEET SOUNDS FROM SIMULATIONCOCHLEAR IMPLANTS

www.ansys.comANSYS Advantage • Volume I, Issue 2, 2007s2

BIOMEDICAL: OVERVIEW

Recent analyses show that leading biomedical com-panies around the world are continuously growing theirinvestment into research and development (R&D), with anincrease of 12.5 percent in 2006 that reached total R&Dexpenses exceeding $9 billion [1]. This is no surprise, giventhe need for advanced medical treatments and care due to alarge and growing population of aging individuals, the needto find minimally invasive treatments for conditions such asdiabetes and heart disease, and the increasing demand forartificial organs. As medical product innovation continues tobecome more complex, there is a strong emerging need forSimulation Driven Product Development, which has beenseen and is broadly accepted in the semiconductor, aerospace and automotive industries.

Simulation is becoming an integral part of the productdesign cycle in biomedical applications ranging from prosthetics and artificial organs to endovascular techniquesto surgical devices, medical equipment and diagnostic

Simulation DrivenProduct Development:Making Life Longer and Better

By Thierry Marchal and Kumar Dhanasekharan, ANSYS, Inc.

products. There are a number of reasons for such simulationto continue its entrenchment in biomedical product develop-ment. First, the advancement in technologies such ashigh-performance computing (HPC) is able to meet thedemands of biomedical product development, allowinghealthcare institutions, life science researchers and theindustry to conduct large-scale simulation studies. Theincreasing ability to import computed tomography (CT)scans and magnetic resonance imaging (MRI) into simulationsoftware — a process now becoming routine — makes itfeasible to address in vivo device design needs (such as withrespiratory drug delivery and endovascular devices), essen-tially enabling virtual prototyping. In addition, the integrationof simulation techniques across multiphysics, from structuralanalysis to flow modeling to thermal analysis, is enhancingthe virtual prototyping needs of the biomedical industry. Forexample, in studying aneurysms, ANSYS simulation toolshave been used to import CT scans into the simulation

The biomedical industry is emerging as astrategic user of engineering simulation.

Simulation Driven Product Development is being applied regularly in the biomedical industry. This aneurysm study was performed within an integrated environment to analyze coupled fluid flow and structural simulation. The steps are: 1) CT scan; 2) segmentation from scans to extract branches; 3) cuts are written in form of splines; 4) creation of solid geometry composed of arterial wall/thrombus and automatic creation of fluid volume from the solid geometry; 5) independent mesh for each simulationtechnique (flow modeling and structural modeling); and 6) coupled fluid and structural model with model setup, analysis and post-processing in a single environment.

1 2 3

Arterial wall Thrombus

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BIOMEDICAL: OVERVIEW

environment, allowing researchers to study a structural analysis of the weakened arteries along with the flow patternsin a single virtual environment, truly creating a virtual proto-type model with multiphysics, all in an integrated manner.

Another growing area is drug delivery, particularly withmedicines that are released into the bloodstream or respi-ratory system. There is a need to better understand theprocess and how adjustments can be made to acceleratedrug delivery to the point of highest efficacy, which then willallow healthcare companies to design better devices thatadminister appropriate dosages.

Similarly, orthopedic departments are paying moreattention to the virtual prototyping approach brought bycomputed-aided engineering (CAE). Bones are criticalpieces of the body, having complex, specific geometries;they are made of different materials exhibiting strongly nonlinear behavior. Until now, scientists have lacked proper,robust models that can be used to bring together, into a single simulation, characteristics as complex as poro-elasticity, nonlinear viscoelasticity and linear elasticity, whichare needed for an accurate description of an intervertebraldisc (ID), for example. The improved robustness of existingmodels together with the availability of reliable material properties now provides evidence that these numericalresults can bring new, invaluable information to doctors. As aresult, healthcare institutions now are studying how a hipprosthesis will perform related to a comfortable walk over along period of time as well as investigating — prior to plan-ning spinal surgery or even designing an ID implant —whether the remodeling procedure leading to the unificationof the pedicle screw and the vertebra is likely to progress smoothly. [See Standing Up Right on page s10.]

To illustrate recent concrete progress in addressing real-life problems and pain relief via CAE, this biomedicalspotlight describes applications in which simulation tech-nology has made a major difference. Both fluid flows andsolid mechanics, or the combination of the two, appear in surprising applications. Some are critical to patient life orfunction, such as lung air flow and spine implant; others simply make life more comfortable through better earimplants and insole design.

For the future, imagine the impact of simulation to drivethe development of patient-specific medicine and medicalcare. For example, tomorrow’s surgeons may be able to takeCT scans of patient physiology and use simulation to conduct virtual surgery as well as study the procedure’seffectiveness as part of the overall process. This is enabledthrough automation of simulation along with rapid designcomparisons through automated parametric studies — andit is rapidly becoming reality. The era of simulation in the biomedical world is rising. ■

References

[1] The R&D Scoreboard 2006, Volume 2, Department of Trade andIndustry (DTI), U.K.

Proper design of a medical insole required to develop an accurate modeling of the foot at different stance phases during required ambulation: 1) the initial contact state; 2) the mid-stance state; and 3) the toe-off state. The resulting data was used to calculate thepressure and stress induced on the plantar surface as well as inside deep tissues.

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BIOMEDICAL: COCHLEAR IMPLANTS

Cochlear implants (CIs) are elec-tronic hearing devices designed torestore partial hearing to those who aredeaf or severely hearing-impaired. Thedevices consist of three external andtwo internal components. The externaldevice comprises a microphone thatpicks up sounds from the environment,a speech processor and a transmitter.The internal components include twosurgically implanted devices: a receiverthat works with the transmitter to convert speech processor signals intoelectronic impulses and an electrodearray that uses those signals to stimu-late the auditory nerves within the ear.One of the traditional limitations of the electrode array is the inability toachieve optimal depth of insertion intothe cochlea, the auditory portion of theinner ear. A German team including

CADFEM GmbH, the Hannover University of Applied Sciences andArts, and the Leibniz University of Hannover has found that an improve-ment might be possible using shapememory alloys (SMA).

Shape memory materials displaydistinct thermo-mechanical behavior.In the case of shape memory effect(SME), a body that has undergoneplastic deformation will return to theoriginal shape or form that it had priorto deformation by heating it above acritical temperature. After being heatedand returning to its original form, ashape memory material will not changeback to its deformed shape if cooled.This phenomenon can be observed inmany shape memory alloys, specific-ally nickel-titanium (Nitinol), which hasa wide range of applications in theautomotive and aerospace industries.In addition, due to its high biocompat-ibility, high resistance to corrosion and,above all, the thermal-induced SME,Nitinol is very useful in the field of medical engineering.

In the case of the CI, the researchteam thought that by taking advantageof the thermally induced shape memory behavior of Nitinol, greaterimplantation depth for the electrode

array could be achieved. The conceptwas to design an SMA componentwhose shape matched that of thecochlea. Prior to the insertion process,the component would be deformedpseudo-plastically, and then, relying on heating from the body itself, itwould return to its original form duringimplantation. To pursue this idea,implant simulations that accounted forthe pseudo-plastic deformation andshape memory behavior were carriedout using ANSYS Multiphysics tools.

For these simulations, the team cre-ated a material model for SMA andimplemented it in ANSYS Multiphysicsvia user-interface USERMAT for three-dimensional finite elements. The phenomenological material model was developed using stress–strain– temperature data for SMA and wasbased on a linear kinematic hardeningmodel. The stress–strain behavior ofshape memory materials, which is highly nonlinear in nature and varieswith temperature, was incorporated into the simulation with the addition of a temperature-dependent scalar parameter: the middle stress σm.

The shape memory stress–straincurve differs from the standard linearkinematic model in that the shape of

Turning Up the VolumeThe use of shape memory alloys offers the promise ofbetter functioning in cochlear implants.

By Dieter Kardas, Institut für Baumechanik und Numerische Mechanik (IBNM), Leibniz Universität Hannover, GermanyWilhelm Rust, Fachhochschule Hannover, Germany Ansgar Polley, CADFEM GmbH, Burgdorf, GermanyTilman Fabian, Hannover Medical School, Germany

Cochlear implant diagram: implant components (left) and insertion in the cochlea (right)

Image From Hals-Nasen-Ohren-Heilkunde, Boenninghaus, Hans-Georg, Lenarz, Thomas, 2005, Kapitel 5 “Klinik des Innenohres,” p 116. Published by Springer Berlin Heidelberg, ISBN 3-540-21969. With kind permission of Springer Science and Business Media.

Demonstration of one-way shape memory effect, from leftto right: initial shape of a component, deformed shape,shape on warming, shape on cooling after warming

Deform Heat up Cool down


the stress–strain hysteresis — whichone gets by periodically changing forcedirection — is ripped in a manner that varies with temperature. Shapememory alloys exhibit pseudo-plasticity at a low temperature rangeand pseudo-elasticity at a high temper-ature range. These temperature rangesdepend on the percentage compositionof nickel and titanium; generally both are equiatomic, which means thatthe rip of the curves increases withincreasing temperature.

The degree to which the curve isripped is determined by the mentionedmiddle stress, σm. If σm is set to zero,then the hysteresis experiences no rip,and pseudo-plasticity can be repre-sented. If σm is set to a higher valuethan the so-called amplitude stress σy*

(half value of the distance from upperflow curve to lower flow curve), pseu-do-elasticity can be represented. Theactual value of the middle stress wasdetermined using experimental datataken at various temperatures. In orderto obtain a smooth, nondiscontinuousrepresentation of the flow curve, atanh-function was included in theequations that describe the offset/ripbehavior as a function of σm.

By incorporating this offset- function σoff (tensor-function of ordertwo) into the material model, the shapememory behavior was effectively captured with only two sets of materialconstants: one set for pseudo-plasticityand another for pseudo-elasticity.ANSYS Multiphysics software itselfinterpolates between these parametersets to provide the material constantsfor the actual temperature. With this technique, it was possible to reproduceany intermediary state between pseudo-plastic and pseudo-elasticstress–strain behavior.

By including this shape memorybehavior, the CI development team wasable to simulate implantation of a shapememory cochlear implant (SM-CI) intothe cochlea. The results of a 65-secondsimulation of the implantation processsupported the idea that the temperatureof the human body could have enoughof a thermal effect on the array that,when implanted, it could return to theoriginal shape: that of the cochlea.These findings support the possibility of a solution that can provide deeper implantation and, thus, betterfunctionality for the CI. ■

Time-spaced results of the implant simulation for a shape memory cochlear implant. The red colorindicates that body temperature has been reached by the implant.Cochlear geometry data courtesy Hannover Medical School, Dr. Omid Majdani.

Pseudo-plasticity σm = 0 (left) and pseudo-elasticity σm > σy* (right). The middle stress (σm) rips theshape memory alloy stress–strain hysteresis as temperature increases.

BIOMEDICAL: COCHLEAR IMPLANTS


BIOMEDICAL: ARTIFICIAL JOINTS

Hip to SimulationEvaluation of designs for a hip replacement prosthesis overcomes physical and scientific limitations.

By Joel Thakker, Integrated Design and Analysis Consultants, U.K.

Hip replacement surgery involvesreplacing the damaged or diseasedball-and-socket joint configuration with artificial parts. During surgery, acup or hip socket — a dome-shapedshell/liner — is implanted into theacetabulum portion of the pelvic girdleafter the bone has been hollowed outusing a grater. The thigh, or femoral,portion of the hip replacement pros-thesis is composed of aball, which acts like abearing where it fits intothe cup and is attachedto a stem that furtherattaches to the femur.The Duraloc® unce-mented acetabular hipsocket, a replacementcup developed byDePuy Orthopaedics,Inc., in the U.K., uses an interference fitto hold the socket in place in the hipbone. To assist DePuy in the design ofthe Duraloc product, Integrated Designand Analysis Consultants (IDAC) used ANSYS Mechanical software todevelop parametric models that areused to establish both the necessaryimplantation and disassembly forcesfor variations of the replacement joint.

IDAC performed a two-dimensionalanalysis on the cup assembly in order

X-ray of a hip showing a prosthesis, including the socket,ball and stem. Image courtesy DePuy Orthopaedics, Inc.

Contour plot of stresses induced by the inter-ference fit between the prosthesis and the bone;the areas colored in grey illustrate the region of the bone that could be expected to yield during the assembly process.

Three-dimension finite element model meshof bone and prosthesis

Illustration of stress distribution in the hipjoint assembly after the prosthesis has beenpressed into place

to model the force required to removethe socket axially. A three-dimensionalmodel was used to analyze rotationalremoval of the joint, since a two-dimensional case would not representthe behavior fully. The ANSYSMechanical simulation used nonlinearcontact elements in the prosthetic hipsocket and accounted for frictionbetween the cup and bone. In all

analyses, the implantcup was modeled in titanium while the bonewas treated as an aniso-tropic material.

For both analyses,IDAC created parametricmodels in order to evalu-ate different bone andimplant cup geometries,material properties and

boundary conditions. The assemblyconditions involved inserting the cupinto the bone to overcome inter-ference, allowing the frictional effects to hold the cup in place, and subse-quently removing, either axially orrotationally, the cup from the bone toestablish disassembly loads.

This form of modeling allowsDePuy to evaluate different configura-tions of implant design numericallyrather than by physical testing, which

is time-consuming and expensive incomparison. Physical testing is limitedas real bone materials are not highlyavailable. Some synthetic and naturallyoccurring materials can be used, buttheir material properties do not pre-cisely match that of human bonematerials. Numerical modeling allowsDePuy to view detailed stress anddeflection distribution plots and loadversus time history plots that cannotbe created easily from physical tests.Comparisons between the resultsobtained through simulation and thoseobtained from previous testing reveal a close correlation.

As a result of this study, DePuy hasused this type of design evaluation inother orthopedic implant products,including artificial knee joints. ■

The Duraloc® uncemented acetabularhip socket is made from titanium andhas a porous coated shell.

ANSYS Advantage • Volume I, Issue 2, 2007

BIOMEDICAL: BIOMECHANICS

www.ansys.com s7

Walking Pain FreeNew insoles designed with the ANSYS mechanicalsuite relieve pain from foot disease.By Bum Seok Namgung, Dohyung Lim, Chang Soo Chon and Han Sung KimYonsei University, Seoul, Korea

During ambulation (top to bottom), the highestpressure progressively shifts from the plantarregion under the heel bone forward to themetatarsal head bone.

Von Mises stress distributions on the plantarsurface of the foot using the flat (top) andtotal contact insoles (bottom)

The human foot does more thansimply enable mobility. Feet are animportant part of the body because theybear weight, absorb shock and stabilizebody structure, but they usually get littleof our attention. When foot diseaseappears and pressure and stressexceed a given limit, pain occurs —making a person suddenly aware of justhow critical a function the feet provide.For people with diabetes, subject topoor circulation and neuropathy, evenordinary foot problems can get worseand lead to serious complications.

One research project designed to benefit such patients involves developing insoles that will prevent pres-sure sores on the deep tissues inside theplantar surface of the foot. A team at theInstitute of Medical Engineering at Yonsei University in Korea is finding newways to gather information on themechanical response of the foot to vari-ous insole designs. They are utilizingfinite element analysis (FEA) softwarefrom ANSYS, Inc. to design new patient-specific insoles that reduce bothpressure during ambulation and stresswithin the feet, ultimately relieving pain. The team selected the ANSYSmechanical suite because of its reliabilityand flexibility for handling complex andirregular geometries. Furthermore, itsnonlinear, hyper-elastic models andadvanced contact conditions provide arealistic alternative to experimentalapproaches for gait analysis.

Using the ANSYS technology, theresearchers first created a three-dimensional model using computerizedtomography (CT) images obtained fromthe right foot of a subject with hallus valgus, commonly called a bunion. Commercial software, CANTIBio™(CANTIBio, Inc., Korea) and meshingsoftware were used to fine tune the contours of the foot.

Two insoles, one flat (left) and one shaped to contact the entire sole of the foot (right), were compared in this analysisto understand the impact of the geometry on foot pain.

Three geometries representing threeprimary states (initial contact, mid-stanceand toe-off) during ambulation then werecreated. The simulation models incorpo-rated two insole designs: one flat and one contoured to contact the entire bottom of the foot. Each design was analyzed at various values of elastic modulus (0.3 MPa, 1.0 MPa and 1 GPa) inorder to represent a variation in insolefirmness and identify which more effec-tively redistributed von Mises stresses onthe plantar, or bottom, surface of the footduring standing.

During ambulation, ANSYS softwareshowed that high pressures first appearon the plantar surface region overlyingthe heel bone for the initial contact state,progresses through the middle of the footfor the mid-stance state, and finally, forthe final toe-off state, is concentrated inthe vicinity of the metatarsal head bone atthe front of the foot. These results are inagreement with those obtained from afoot scan system used in experimentalgait analysis.

The results found that stresses on theplantar surface are significantly lower withthe total contact insole compared withthose of the flat insole; stresses also aredependent on the insole elastic modulus.This confirms that customized design ofan insole for patients with foot diseasemay be necessary, and the solutionshould include biomechanical and clinicalpoints of view. ■

Hospital Nacional Dos de Mayo in Lima, Peru, was thesite of a TB ward ventilation system redesign.

Extract (low, wall)

Bed 2Bed 1

Supply(ceiling)

Extracts (high, wall)

Bed 2Bed 1

Supply(ceiling)

Supply(ceiling)

BIOMEDICAL: INFECTION CONTROL


Hospital-acquired infection poses amajor problem in healthcare facilitiesaround the world. Although many infections are transmitted through hand-to-hand contact, airborne transmissionalso may play an important role; this isthe primary mechanism for a number ofinfections, including tuberculosis (TB)and influenza. Airborne routes also havebeen implicated in the transmission ofhospital-acquired infections such asmethicillin-resistant Staphylococcusaureus, Acinetobacter spp and noro-virus. Successful control of infectioninvolves breaking the chain of trans-mission. To do so, it is necessary to understand both the mode of trans-mission as well as the nature of thepathogen and its behavior in the environment.

The role played by airborne transport of pathogens has been the driving force behind the researchcarried out by the Pathogen ControlEngineering Group at the University ofLeeds in the U.K. for the past 10 years.The multi-disciplinary team of engi-neers, mathematical modelers and

Engineering Solutions for Infection ControlSimulation assists in designing a hospital ward to reduce the airbornetransmission of diseases such as tuberculosis and influenza.

By Cath Noakes and Andrew Sleigh University of Leeds, U.K.

Original room layout and ventilation system (top) and proposed new layout (bottom) showing the location of thepartition between the two beds, the additional ventilationsupply diffuser and the modified extract locations

microbiologists is based in the Schoolof Civil Engineering, with strong links to clinicians at the Leeds Teaching Hospitals and to academics and scientists around the world. Originallyset up to investigate ultraviolet (UV) airdisinfection devices to combat TB, thegroup now focuses on understandingairborne transmission routes with astrong emphasis on the hospital environment. This knowledge is usedto aid the development of new infection control technologies and tooptimize engineering strategies toreduce the risk of disease.

The suitability of a ward ventilationsystem design was the subject of arecent study carried out using ANSYSCFX computational fluid dynamics(CFD) software [3]. The two-bed ward inHospital Nacional Dos de Mayo, located in Lima, Peru, is one of a number of similar rooms housingpatients with TB. Unusual to a hospitalin this part of the world, the wards aremechanically ventilated. Any airbornetransmission of TB within the hospitalwill be strongly influenced by theimposed ventilation flow. As part of awider project researching TB trans-mission, led by Dr. Rod Escombe ofImperial College in London, U.K., theCFD study was carried out to examinewhether changes to the ward layout andventilation system could reduce the riskof cross-transmission between patients,staff and visitors in the hospital.

A simplified geometry representedthe key features in the ward, including

the basic furniture, the ventilation supply and extract vents. Isothermalairflow was modeled on an unstruc-tured tetrahedral grid using a standardk–ε turbulence model. Supply airvelocities were defined to ensure aroom ventilation rate of 6 AC/h for allsimulations, and a pressure of –10 Pawas imposed on the extracts to

BIOMEDICAL: INFECTION CONTROL


Contaminant concentration contours, at an elevation of 1.4 mabove the floor originating from patient 1. The figure on the tophas no partition, while the figure on the bottom uses a partitionand ventilation systems local to each patient.

Streamlines originating from patients 1 (red) and 2 (blue) show how a partitioned room with modified ventilation system (bottom) moreefficiently extracts contaminated air than the original room (top) does.

simulate the negative pressure that ismaintained in the real facility. As the study focused on the risks ofcross-infection, it was important toinclude a model to represent therelease of infectious material from TBpatients. To relate the CFD study topublished outbreak data, a scalarinfectious particle production variablewas defined in terms of units of infec-tious dose, known as “quanta.”

To represent a patient’s productionof TB bacteria, a small inlet conditionwas located close to the head of each bed. Scalars, representing the infectious particles produced by eachpatient, were introduced into the roomat a constant rate of 14 quanta/hour in order to represent the typical pro-duction rate of a pulmonary TB patient.

The CFD study made it quick andeasy to compare the impact of a number of proposed modifications tothe ward. The original room layout withits single ceiling-mounted supply

diffuser and wall-mounted extractresulted in significant mixing of TBcontamination throughout the room,demonstrating the high risk of cross-infection between patients. The simpleaddition of a partition between the twobeds yielded an immediate benefit,providing a physical barrier that limitedthe transfer of infection between thetwo areas. As a low-cost intervention,this could prove beneficial in resource-poor countries, although it may not be suitable for naturally ventilated environments. Combining the partitionwith a new ventilation system layout,comprising ceiling supply diffusersabove the foot of each bed with wall-mounted extracts at the head of eachbed, yielded the best results. Despitethe ventilation rate remaining constant,the transfer of infectious materialbetween the two beds was reduced byover 75 percent, representing a significantly reduced risk of cross-infection between patients. These

findings were of immediate benefit tothe architects redesigning the ward,who based the new ventilation systemand ward layout directly on the studyresults. ■

www.efm.leeds.ac.uk/aerobiology

References

[1] Noakes, C.J.; Sleigh, P.A.; Fletcher, L.A.;Beggs, C.B., Use of CFD Modeling in Optimising the Design of Upper-Room UVGIDisinfection Systems for Ventilated Rooms.Indoor and Built Environment, 2006 15(1),pp. 347-356.

[2] Noakes, C.J.; Fletcher, L.A.; Beggs, C.B.;Sleigh, P.A.; Kerr, K.G., Development of aNumerical Model to Simulate the BiologicalInactivation of Airborne Microorganisms inthe Presence of UV Light. Journal of AerosolScience, 2004, Vol. 35(4), pp. 489-507.

[3] Noakes, C.J.; Sleigh, P.A.; Escombe, A.R.;Beggs, C.B., Use of CFD Analysis inModifying a TB Ward in Lima, Peru. Indoorand Built Environment, 2004, 15(1),pp. 41-47.


BIOMEDICAL: SPINAL DISORDERS

The human spine is a wonder ofengineering work, one that is heavilyused in daily activities. An importantpart of it, the intervertebral disc (IVD), isone of the most sophisticated suspen-sion and shock absorption systemsever found. When disorders arise, backpain quickly can become a nightmare.The National Technical University ofAthens (NTUA) in Greece conducted a study using ANSYS Multiphysics software that revealed some secrets ofhow this precious structure works, aswell as ways to fix it efficiently when itmalfunctions.

Simulating the Intervertebral DiscThe IVD is located between the ver-

tebrae in the spine. In performing dailyactivities, it acts as a cushion andtherefore is exposed to a combinationof compression, bending and torsionstresses. Each disc consists of thenucleus pulposus, a gel-like inner por-tion of the disc; the annulus fibrosus,the outer portion made of about 20lamellae of coarse collagen fibers; andthe two cartilaginous endplates, com-posed of hyaline cartilage, located oneither side of the nucleus and annulus.

The IVD simulation model comprisedfour distinct volumes corresponding tothe disc’s regions: The nucleus wasmodeled as a nonlinear viscoelasticmaterial in a kidney-like cross section;the two cartilaginous vertebral endplateswere considered linear elastic bodies;and the annulus surrounding the nucleuswas simulated as dual laminated shellelements whose outer surfaces were viscoelastic in nature. The study analyzed various scenarios in order todetermine the contribution of each section of the IVD to the viscous char-acter of the entire structure.

The numerical model revealed thatthe maximum stresses appeared in thefibers of the intermediate volumes of theannulus, in the vicinities of the endplates.The nucleus was almost stress-free, asexpected due to its gel-like nature. The NTUA study also investigated thebehavior of the IVD during daily activities;the results found that the reduction ofdisc height related to a person’s 24-hourdaily cycle was in very good agreementwith the respective experimental data byTyrell et al (L3–L4 discs) [1].

Standing Up RightANSYS Multiphysics sheds light on the wonders of thehuman spine and how to fix it.

By Stavros Kourkoulis, Satraki Margarita and Chatzistergos Panagiotis, National Technical University of Athens, Greece

a b c

The numerical model of the intervertebral disc: a) nucleus pulposus, b) annulus fibrosus and c) cartilaginous vertebral endplates

The spine’s intervertebral disc is exposed to a combination of compression, bending and torsionstresses.


BIOMEDICAL: SPINAL DISORDERS

The von Mises stress distribution through the center of the disc horizontally (left) and at the point of minimumvertical cross-sectional area (right)

The distribution of the Mises equivalent stress in a typicalvertebra for a pull-out displacement of 0.02 mm

The two phases of model construction: (left) the screw and surrounding bone implantedinto the verterbra and (right) the regions of the verterbra (yellow: canceious bone; red:subcortical bone; blue: cortical shell)

Studying the Surgical RemedySpinal stabilization using pedicle

screws and rods (or plates) is one of themost common invasive treatments forspinal disorders and injuries. In this procedure, the surgical team implantsscrews posteriorly into a number of vertebrae and bolts them to a rod orplate. This assembly actively fixes thevertebra in place, with respect to eachother, and thus stabilizes that section ofthe spine. After such a procedure, someserious problems can still exist. Pain inthe IVD adjacent to the fixed vertebraecan occur due to failure of the spinalinstrumentation, from either a fracture instructural elements or a loosening ofthe screws. Experimental and clinicalstudies alone cannot provide a com-plete view of the mechanical behaviorof such complex structures. Numericalsimulations introduce a unique tool forthe thorough and parametric study ofsuch systems.

From the moment a pedicle screwis implanted into the vertebra, the bonebegins to regrow around the screw.This regrowth leads to the eventualcomplete unification of the bone andthe implant, which occurs about twoyears postoperatively. A fundamentalrequirement for the success of thisprocedure is the stability of the screw’sfit into the bone. NTUA used mech-anical simulation to investigate theinfluence of the vertebra structure andscrew specifications — such as depthof implantation, pitch and inclination of the thread — on the value of theforce required to loosen the screw fromthe spine.

The parametric study assumedthat the vertebra consisted of cortical,subcortical and cancelous bone assuggested by measurements of bonemineral density of typical human lumbar vertebrae. The simulations estimated the force required to pro-duce a pull-out displacement of 0.02mm, the stress distribution onto thebone, and the contact pressure on thebone–screw interface. The results indi-cated that the pull-out resistance couldbe amplified significantly by ensuringthat the screw was anchored into theregions of stronger materials locatednear the cortical shell. Furthermore, theparameter found to have the strongestinfluence on the pull-out force was thescrew pitch. For pitch values varyingfrom 2 to 5 mm, the pull-out forceincreased linearly by approximately 30 percent. The variation of the screwdepth and the thread inclination hadlimited impact on the pull-out force.

A comparison of the numerical resultswith the experimental results foundthem to be in very good agreement,within the tolerance of experimentalerror.

The main advantage of the numerical models lies in the accuratesimulation of both the structure and theshape of the various portions of thebiological disc or vertebra as well as of the constitutive behavior of the different materials. In order to further improve the accuracy of thesenumerical analyses, researchers mustdevelop studies using models ofincreasing sophistication adapted tospecific groups of people with mor-phology and properties varying withage, sex, type of activities, degenera-tions and other factors. ■

References

[1] Tyrell, A; Reilly, T; Troup, J., CircadianVariation in Stature and the Effects of SpialLoading, Spine, 1985, 10(2), pp. 161-164.


BIOMEDICAL: ARTIFICIAL ORGANS

An important challenge facing thedesign of turbodynamic ventricularassist devices (VADs) intended forlong-term cardiac support is the opti-mization of the flow geometry tomaximize hydraulic efficiency whileminimizing the peak shear stress in theblood flow. High efficiency reduces therequired battery size while low shearreduces the number of red blood cellsthat are ruptured by the pump. A pedi-atric heart-assist pump is particularlychallenging. Due to its small size(about 28 mm diameter by 51 mmlength), the design laws for adult-sizedpumps do not apply, and they cannotbe scaled. Therefore, the design ofpediatric blood pumps must rely onmodern design approaches to opti-mize the flow path. Computational fluiddynamics (CFD) has been widely usedin the field of artificial heart pumps forthe analysis of internal flow because itoffers an inexpensive and rapid meansof acquiring detailed flow field informa-tion that is expensive and painstakingthrough in vitro testing. LaunchPointTechnologies, Inc., in the UnitedStates, which developed the first mag-netically levitated (maglev) heart pump(the Streamliner ventricular assistdevice that reached animal trials in1998), finds that CFD is a powerful toolin the performance assessment andoptimization of artificial heart pumps.

LaunchPoint has developed a CFD-based design optimization approach

Designingwith HeartCFD-based design optimization for a miniature ventricular assistimplant can shave years off themedical device development cycle.

By Jingchun Wu, LaunchPoint Technologies, Inc., California, U.S.A.and Harvey Borovetz, McGowan Institute for Regenerative MedicinePennsylvania, U.S.A.

that integrates internally developed 3-D inverse blade design methods,parameterized geometry models, automatic mesh generators and math-ematical models of blood damage withthe commercial ANSYS CFX solver.The system provides rapid optimiza-tion for various types of centrifugal,mixed-flow and axial-flow bloodpumps. The ANSYS CFX solver waschosen because of its robustness forcomputations with multiple frames ofreference (MFR) (the coupling betweenrotating and stationary components).

A new LaunchPoint VAD, Pedia-Flow™ is intended to deliver a flow rateof 0.3 to 1.5 l/min against 100 mmHgpressure rise to neonates and infantsweighing 3 to 15 kg. The PediaFlowwas designed with a magnetically sus-pended, mixed-flow style impeller witha single annular flow gap between therotor and housing to avoid unfavorableretrograde flow and separation. Theshear stress transport (SST) model, alow Reynolds number turbulencemodel, was selected for the turbulentflow simulation, which was justified by the representative Reynolds number of ~30,000 based on theimpeller outlet diameter and the pumptip speed. Although blood exhibits non-Newtonian behavior at very lowshear rates, many studies have shownthat blood can be modeled as a Newtonian flow at a shear rate largerthan the threshold of a 100 s -1. The

shear rate in the computational modelof the PediaFlow is much larger thanthis threshold, so Newtonian blood witha constant viscosity of 0.0035 Pa-s anda density of 1040 kg/s3 was assumedfor the simulations.

The CFD-predicted velocity vectorsat both the mid-span blade-to-bladeregion of the impeller and the vane-to-vane region of the stay-vanes show avery smooth distribution without anyvortices at the nominal flow conditionfor the optimized PediaFlow model. Asliterature is replete with anecdotal evi-dence that recirculating flows lead toattachment of platelets to biomaterialsurfaces — which in the clinical VADsetting can promote blood clot forma-tion — reverse flows and vortices areundesirable. The CFD results foundthat a smooth and gradual transition inthe secondary flow velocity was present at the curvature of one inflowand outflow cannula geometry. Thisgraduation helps to prevent separationand reversal flow for the primary flowvelocity. In addition, the predictedpathlines of representative particlesthrough the entire flow region did notexhibit any vortices.

The exposure of blood elements toshear stress above a certain thresholdas a function of exposure time cancause hemolysis, which actively breaksopen the red blood cells; activateplatelets, which can cause clottingproblems; and denature proteins, which

The PediaFlow ventricularassist device provideslong-term cardiac supportfor infants.


BIOMEDICAL: ARTIFICIAL ORGANS

alters the proteins so they can no longer carryout their cellular functions. Thus, it is desirableto minimize the shear stress that blood passing through the pump may experience.Using the results of the CFD simulation, a plotof shear stress versus exposure time for particles passing through the pump demon-strates relative uniformity within the annularflow gap region, but it is less uniform withinboth the impeller and stay-vane regions. The overall mean blood damage through the entire domain of the model is dividedaccording to the three main regions of theflow path: impeller, annular gap and the stay-vane. The analysis reveals that the hemolysislevel in the annular gap region is highest,accounting for more than 50 percent of thetotal, while the level of hemolysis in theimpeller region and stay-vane region is almostthe same, each causing approximately 20 to25 percent of the total blood damage.

CFD-based design optimization with theintegration of the ANSYS CFX solver can significantly reduce the design optimizationcycle from years, compared to the traditionaltrial-and-error methods, to just severalmonths. It provides detailed and useful flowfield information from which blood damagemay be computed, and it also predicts thehydrodynamic characteristics such as therelationship of developed pressure and efficiency to flow rate. ■

This research was supported in part by NIH ContractNo. HHSN268200448192C (N01-HV-48192).

PediaFlow is a trademark of WorldHeart, Inc.

Shear stress history from impeller inlet to stay-vane outlet Proportion of total blood damage at different pump components undernominal flow condition

Predicted smooth velocity vectors at mid-span blade-to-blade region of the impeller (left) andmid-span vane-to-vane region of stay-vanes (right)

Secondary flow streamlines at sections of inflow cannula (left) and sections of outflowcannula (right)

Pathlines of particles at inflow cannula and impeller side (left) and stay-vanes side andoutflow cannula (right)

0.0025

0.0020

0.0015

0.0010

0.0005

0.0000Impeller Annular Gap Stay Vane Total

Giersiepen

Heuser

0.002337 0.002341

21.20% 19.5%

52.5%57.7%

26.27%22.8%

dHb

300

250

200

150

100

50

00.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18

Shea

r Stre

ss (P

a)

Diseases such as asthma, chronicobstructive pulmonary disease (COPD)and cystic fibrosis can have a signifi-cant adverse impact on the structureand integrity of the lungs’ airways.While functional magnetic resonanceimaging (MRI) allows for measure-ment of air flow, computational fluid dynamics (CFD) provides highlydetailed information of local flow characteristics and resistances. Thefirst requirement of a patient-specific analysis is knowledge of the bounding walls of thepatient’s flow domain — their lunggeometry. This type of informationusually comes from computedtomography (CT), a scan that indi-cates detailed information aboutlung geometry because of thenatural contrast between air andthe lung walls. The main draw-back of CT is that the resultingscan is a static image. Couplingcomputational analyses of air flowwith the lung scan has the poten-tial to provide significant addedvalue to the clinical evaluation of lung function.

FluidDA, a spin-off of theAntwerp and Ghent universities inBelgium, has successfully devel-oped a workflow for predicting airflow in healthy and diseased lungs

simulate and examine the air flow. Flowpatterns, relative pressure drops anddrug delivery profiles are readilyextracted from the simulation results.The resistance distribution — definedas the total pressure drop over variouslung segments — also is available.

The pharmaceutical and medicaldevice sectors also can benefit frompatient-specific flow analysis as a wayto evaluate performance and efficacy ina virtual patient population. In clinical

studies, it is possible to analyzethe effect of bronchodilating medication, which widens lung airpassages and relaxes bronchialsmooth muscle to ease breathing,on airway volume and flow resist-ance. A researcher then can beginto establish correlations betweendrug deposition patterns and clini-cal outcomes, thereby providingan indication as to why the drugdoes or does not work. Functionalimaging also can be used toassess the placement of intra-bronchial devices such as stentsand valves.

Coupled with CFD, suchimaging can dramatically increaseinsight into medical assessmentand improve the accuracy of medical interventions. ■

www.ansys.comANSYS Advantage • Volume I, Issue 2, 2007s14 www.ansys.coms14

BIOMEDICAL: IMAGING

Going with the FlowFunctional biomedical imaging through CFD provides a new way of looking at pathological lungs.

By Jan De Backer and Wim VosFluidDA nv, Antwerp, Belgium

Reconstructed airway of a patient with cystic fibrosis:The red arrows indicate regions in which inflammationhas restricted the airways.

Contour plots show the effect that the use of a bronchodilator has on the local values for airway (left) volume andresistance (right); red indicates high values and blue indicates low values.

For patients with deformation of the spinal column (kyphoscoliosis),simulation can be used to determine the site of obstruction and/or respiratory function.

Obstruction site (and subsequent location) of an intrabronchial stent,which re-inflated the blocked lower right lung lobe. Pressure contoursare plotted in the airway.

An increase in the volume of the lower lobe is clear in time followinginsertion of a stent.

Stent location

Lower lobe

using CFD. The fluid and structuraldynamics company combines clinicalexperience and capabilities withnumerical simulations to offer a varietyof services to the healthcare industry.

The workflow process begins withthe conversion of CT scan data into a3-D computer model of the airway,performed with the Materialise productMimics. FluidDA then uses TGrid software to create surface and volumemeshes and FLUENT technology to

ANSYS Advantage • Volume I, Issue 2, 2007www.ansys.com s15www.ansys.com s15

BIOMEDICAL: SURGICAL TOOLS

The SpineJet repairs a herniated intervertebral disc by removing a portion of thenucleus. The tool uses the Venturi effect created by high-velocity saline jets tocut and then aspirate targeted tissue. Image courtesy T.G. Communications

In the United States, back pain is one of the most com-mon reasons for healthcare visits and missed work. Fourout of five adults have at least one bout of back pain atsome point in their lives.

A common source of pain is from a bulging interverte-bral disc impinging on spinal nerves, which can cause backpain or sciatica (pain down the leg) — a condition known as herniated disc. The intervertebral disc is sandwichedbetween the vertebrae of the back and acts as a shockabsorber during spinal movement. The disc is made of twoparts: a tough outer wall called the annulus and a gelatinousinner core called the nucleus. Trauma or aging of the disccan cause the annulus to bulge.

Most occurrences of lower back pain resolve with restand medication. For many people, though, the pain can bedebilitating and last for several months to years. Suchpatients typically require surgery.

Minimally invasive surgical techniques offer many bene-fits, since traditional back surgery can cause further painand complications. HydroCision, which develops and man-ufactures fluidjet-based surgical tools in the United States,used computational fluid dynamics (CFD) to improve anovel minimally invasive surgical treatment calledHydroDiscectomy™.

The goal of HydroDiscectomy is to decompress theherniated disc. When performing the procedure, a physi-cian uses a tool called the SpineJet® to remove a portion ofnucleus, which debulks the disc and retracts the bulge.The device uses a high-pressure jet of sterile water directed into an evacuation tube. The jet is attuned to cutthe softer nucleus but protect harder surrounding tissuessuch as the vertebrae and the annulus. The water jet natu-rally provides cutting and a low-pressure Venturi to drawthe nucleus to the jet, cut it and aspirate it through anevacuation tube.

As physicians adopt new technologies, their productdemands increase. HydroCision saw CFD as a technologythat could reduce development time and improve productperformance. Manufacturing limitations with the existingSpineJet nozzle affected the flow divergence, directionalityand alignment with the evacuation tube. By redesigning theSpineJet nozzle for better flow characteristics and greaterease of manufacture, the surgical device could be made more consistent and cost-effective. HydroCision’s productdevelopment team used FLUENT software in analyzing theperformance of the existing nozzle geometry. CFD simula-tions allowed new geometries to be designed and analyzedfor performance in a matter of hours to days. Optimizationof the device was faster and less expensive than the tradi-tional method of making and testing prototypes.

The CFD model included flow simulations through thesupply tube, nozzle orifice and evacuation region. CFDresults helped the HydroCision team visualize critical flowcharacteristics such as the velocity profile, pressure distri-bution and flow divergence (cone angle).

The team modeled six alternate SpineJet designs thatincorporated significant changes to the nozzle and/or thesupply tube. Engineers selected velocity magnitude andgeneral jet shape as the primary means for comparing thedifferent designs, since these two parameters are con-sidered the most accurate predictors of overall SpineJetperformance.

By Joe Richard, HydroCision, Massachusetts, U.S.A.Brenda Melius, consulting firm, New Hampshire, U.S.A.

Battle of the BulgeRapid prototyping results in a new surgical tool to treat back pain.

Supply and evacuation tube of the original SpineJet Image courtesy T.G. Communications.


BIOMEDICAL: SURGICAL TOOLS

CFD results for the existing SpineJet showed the influ-ence of a sharp-edge orifice and its location on the flowcharacteristic. As expected, the orifice creates a flow sepa-ration at the corner, and a vena contracta is formed. Inaddition, the proximity of the orifice to the 90-degree-bend inthe supply tube and the additional supply tube length pastthe orifice create a non-uniform flow condition at the orifice entrance. As a result, the region of highest flow velocity is concentrated in the lower portion of the orifice;therefore, the flow is neither symmetrical nor well developed.

CFD results for the alternate SpineJet designs showedsubstantial improvement compared to the existing design.Three of the alternate configurations had 20 percent highermass flow rates than the existing design as well as a 40percent reduction in cone angle (flow divergence). Thesedesigns had general jet shapes that were symmetrical andwell developed. They also retained higher flow velocitiesover longer distances from the orifice exit.

Historically, HydroCision manufactured prototypes ofnew geometries for testing to examine the feasibility of producing a new and improved design. Although fairlyeffective, this method was costly (more than $15,000 foreach design tested) and time-consuming (taking approxi-mately six months). Furthermore, testing did not alwayslead to a full understanding of the fluid flow characteristicsthat occur.

Computer modeling utilizing FLUENT software pro-vides a different approach to the problem. The onlyexpenses are computing and software costs; creating aCFD model and running it takes just a few days. This allowsHydroCision to model and refine many designs in a fractionof the time it would take to manufacture and test a singleprototype. In addition, computer simulation can yield betterinsights into the interactions between the geometry and thefluid flow. Finally, the graphics generated by FLUENT soft-ware help stakeholders better understand the operation ofthe surgical tool. ■

Cross-sectional view of all fluid volumes for original SpineJet design(top) with close-up section indicated by the red box at orifice (bottom).

Cross-sectional view of SpineJet alternative design colored by velocity magnitude

s16

For ANSYS, Inc. sales information, call 1.866.267.9724, or visit www.ansys.com.To subscribe to ANSYS Advantage, go to www.ansys.com/subscribe.

ANSYS Advantage is published for ANSYS, Inc. customers, partners and othersinterested in the field of design and analysis applications. Neither ANSYS, Inc.nor the editorial director nor Miller Creative Group guarantees or warrants accuracy or completeness of the material contained in this publication. ANSYS,ANSYS Workbench, CFX, AUTODYN, FLUENT, DesignModeler, ANSYSMechanical, DesignSpace, ANSYS Structural, TGrid, GAMBIT, and any and allANSYS, Inc. brand, product, service and feature names, logos and slogans areregistered trademarks or trademarks of ANSYS, Inc. or its subisdiaries located inthe United States or other countries. ICEM CFD is a trademark licensed byANSYS, Inc. All other brand, product, service and feature names or trademarksare the property of their respective owners.

© 2007 ANSYS, Inc. All rights reserved.

About the Industry Spotlight

Cover image: Simulation demonstrates shape memory for a cochlear implant.Photo courtesy Cochlear GmbH. Simulation courtesy Fachhocshule Hannover –University of Applied Sciences and Arts, CADFEM GmbH and Dr. Omid Majdani– Hannover Medical School.

Cross-sectional view of all fluid volumes

Supply tube volume

Supply tube 90°bend volume

Supply tubevolume

Orifice volume Evacuation tube volume

a n s y s biomedical industry

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