1

Face to Face

Computer Simulation and

Facial Movement

March 18th, 2009

Brian Walker ArupLiliana Beldie ArupYongtao Lu Cardiff Univ.

Contents – Computer Simulation and Facial Movement

• Why FE Modelling

• Software Tools• Simpleware

• LS-DYNA

• Muscle Model

• Computer Simulations• Facial expressions before surgery

• Surgery

• Facial expressions after surgery

• Summary

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Model of a human face – Why FE modelling?

• Maxillofacial surgery – a specialist surgical procedure involving the correction or rebuilding of the face following trauma or disease

• There are tools in place for planning the surgery – for example moulds of the upper and lower jaws are used to plan the relocation of the bones before a BimaxillaryOsteotomy – in this particular case a Le Fort I Osteotomy of the Maxilla and Sagittal Split Osteotomy of the Mandible

• Computer simulation of the surgery would provide an excellent tool for preoperative planning, allowing investigation of various possible strategies

• The FE simulation would also allow for prediction of the resulting facial appearance

Model of a human face – Why FE modelling?

• Maxillofacial surgery – a specialist surgical procedure involving the correction or rebuilding of the face following trauma or disease

• There are tools in place for planning the

surgery – for example moulds of the upper and lower jaws are used to plan the

relocation of the bones before a BimaxillaryOsteotomy – in this particular case a Le

Fort I Osteotomy of the Maxilla and Sagittal Split Osteotomy of the Mandible

• Computer simulation of the surgery would provide an excellent tool for preoperative planning, allowing investigation of various

possible strategies

• The FE simulation would also allow for prediction of the resulting facial appearance

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Scanning

Finite Element Model (LS-DYNA)

+ScanFE

Meshing & Material Properties

+ScanCAD

CAD import &

positioning

ScanIPImageProcessing

CT, Micro-CT, MRI, Tiff, Jpg ..etc.

Software - Simpleware

LS-DYNA

SCAN

MESH

SET-UP

ANALYSE

POST-PROCESS

ScanIP

• Import in scan data – Filter and

segment to determine different objects.

+ScanFE

• Mesh the data from Scan IP

+ScanCAD

• Integration of CAD models with

image

Software – LS-DYNA®

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Face/ Maxilla/ Mandible –CT/STL – from a specific patient

Model of a human face

Muscles – STL – from muscle database

• Data received as STL (Stereo-lithography) files

Building the FE model of the face – ScanCAD

• Reading the separate STL’s in ScanCAD and creating the masks

• Good features in ScanCAD:

• Translate/rotate functions –

sometimes the STL parts

do not come in the correct

position

• Several (three) options when converting CAD to masks depending on the type of the mask (see next slide)

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Building the FE model of the face – ScanIP

• Create the subcutaneous fat from an existing subcutaneous mask which does not fit the face

Building the FE model of the face – ScanFE

• Good features in ScanFE:

• All the masks that contact each other will be meshed in, i.e. the connection will consist of coincidental nodes – no need to use contacts if not required by the model

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Building the FE model of the face – Initial model

• After the model is exported from ScanFE, we need to make sure

that the attachments of the muscles reflect the anatomical

connections applying to a real person – need to undo/redo some of

the connections – used Oasys PRIMER

• We have been working with facial surgeons to create and confirm

the muscle connections of the FE model

FE simulations using LS-DYNA

• We want to run two types of analyses:

• 1. Facial expressions – model individual muscles by means of a user defined material to capture the muscle contraction

• 2. Maxillofacial osteotomy, in which the maxilla an mandible are ‘cut’

and repositioned; the muscles are non active during this simulation, modelled with standard LS-DYNA materials

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Facial muscles - Modelling

• Muscle – non-linear, anisotropic and viscoelastic

• Hill’s three-element model proposed in 1938 still used today:

CE

PE

SEE

FMFM

vMa(t)

LM

PE = Parallell (passive) element = tendon elasticity

CE = Contractile (active) element =

freely extendible at rest but capable of shortening when activated

SEE = series elastic element =

muscle elasticity in isometric conditions – due to cross-bridges

a(t) = activation level = specifies the level of muscle stimulation as a

function of time

LM = length of muscle

vM = shortening velocity of muscle

LM

Facial muscles - Modelling

• Hill’s three-element muscle model

CESE

PE

Isometric Contraction

(constant muscle length):

- CE shortens- SE lengthens and

- PE is constant

Muscle lengthening:

- CE, SE and PE lengthen

CESE

PE

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Facial muscles - Modelling• Diagrams from Zajac, 1989

LM

CESE

PE

C, D: When fully activated, muscle

tissue is subjected to a constant pull

(tension), it first shortens and then

stops (isotonic contraction); the length at which the shortening

terminates corresponds to the length

at which such a force can be

sustained in steady-state

An applied constant force less (or greater) than F0

M causes muscles to

shorten (or lengthen)

At vm (maximum shortening velocity)

the muscle cannot sustain any force, even when fully activated

Isometric Force-length (static - LM =const & vM =0 ):

A – 100% activated force

B – 50% activated force

Force-velocity at L0M

(dynamic):

C – 100% activated force D – 50% activated force

A, B: By definition, peak

active force F0M is developed

when fibres are at their optimal length, LM = L0

M

• The FE analysis will be run using LS-DYNA Explicit

• A user defined muscle material model (*MAT_USER_DEFINED_MATERIAL_MODELS) for LS-DYNA is defined

based on Hill’s three element model

• The FE constitutive model of the muscle is active, quasi-incompressible,

fibre-enforced and hyperelastic

• Work under development by Yongtao Lu of Cardiff University.

Facial muscles – FE Modelling

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Muscle Tests

Examples of Muscle Tests – Symbols Used

No Muscle Activity

Active Muscle

Passive Muscle – After Active (De-activated)

Muscle Tests

Passive Tests – No Activation

Muscle extended by applied displacement

and held.

Muscle extended by applied displacement

and returned to initial length

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Muscle Tests

Isometric Contraction

Muscle activated (Length held)

Muscle activated (Length held), followed by

deactivation (Length held).

Muscle activated (Length held), followed by deactivation (Length held), Finally one end

condition restraint release.

Muscle Tests

Activated - Elongation

Muscle activated (Length held). Muscle

extended whilst activated.

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Computer Simulations

Passive elongation Active elongation

• Some of the validation results are shown below for the Passive

elongation and Active elongation

• There are in same cases instabilities in the user defined material

and so the material model is still under development

Temporalis

Orbicularis

Oculi

Masseter

Buccinator

Depressor Anguli Oris

Levator LabiiSuperioris AlaequeNasi (LLSAN)

Levator LabiiSuperioris

Zygomaticus

major and minor

OrbicularisOris

Depressor LabiiInferioris

Mentalis

The FE model with the muscles listed – page 1:

Risorius

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Geniohyoid

Mylohyoid

Stylohyoid

Posterior DigastricAnterior

Digastric

Hyoid bone

The FE model with the muscles listed – page 2:

Medial Pterygoid

Lateral Pterygoid

Bones –Rigid material

Subcutaneous –Mooney-Rivlinmaterial

The FE model with materials used:

Face skin –

Mooney-Rivlin

Muscles – User defined material

Nose cartilage & eyeball – Linear elastic material

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Muscles active in various expressions:

- Anterior digastric

- Mylohyoid

- LLSAN

- Depressor anguli oris

- Orbicularis occuli

- Zygomaticus Minor

- Zygomaticus Major

- Rizorius

- Orbicularis occuli

Muscle activated

Jaw lowering

Disgust

Smile

Expression

• LS-DYNA simulation - user defined material model for contracting muscles

• Smile – Pre-surgery

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• LS-DYNA simulation - user defined material model for contracting muscles

• Disgust – Pre-surgery

Mandibular SagittalSplit Osteotomy

Bimaxillary osteotomy

Maxillary Le Fort I Osteotomy

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Maxillofacial osteotomy – FE simulation

• Maxilla 5.0mm Forward & 4.0mm Up

• Mandible 8mm Rearward & 4.0mm Up

Maxillofacial osteotomy – FE simulation

• Maxilla 5.0mm Forward & 4.0mm Up

• Mandible 8mm Rearward & 4.0mm Up

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Maxillofacial osteotomy – FE simulation

• Maxilla 5.0mm Forward & 4.0mm Up

• Mandible 8mm Rearward & 4.0mm Up

Maxillofacial osteotomy – FE simulation

• Maxilla 5.0mm Forward & 4.0mm Up

• Mandible 8mm Rearward & 4.0mm Up

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• LS-DYNA simulation - user defined material model for contracting muscles

• Smile – Post-surgery

• LS-DYNA simulation - user defined material model for contracting muscles

• Disgust – Post-surgery

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Maxillofacial osteotomy – Comparison FE simulation (grey) & Patient

images 6 month post surgery (green); Geomagic Qualify 7 software is

used for the comparison

Maxillofacial osteotomy – Comparison FE simulation (grey) & Patient

images 6 month post surgery (green); Geomagic Qualify 7 software is

used for the comparison

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Model of a human face – Next steps

• Understand the function of various muscles in facial expression and speech

• Further development of muscle material model

• Improve facial model• Muscle connections

• Material data

• The aim for the future is to build patient specific FE

models and be able to assist in real facial surgery