Exploring The Biological/Biomechanics Interface - The Arup Campus, 7th May 2009

Simulation of the Facial Construct: Soft Tissue Modelling and Surgical Outcomes

Liliana Beldie, Arup, Solihull, UK Brian Walker, Arup, Solihull, UK Yongtao Lu, Cardiff University, Cardiff, UK

Contents

• Software Tools• Simpleware• LS-DYNA

• Muscle Model

• Computer Simulations• Facial expressions• Surgery

• Next steps

Why FE simulation of a human face?

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

• Bimaxillary Osteotomy:• Le Fort I Osteotomy of the Maxilla • Sagittal Split Osteotomy of the

Mandible

• The computer simulation of the surgery would provide:

• Tool for preoperative planning• Prediction of the resulting facial

appearance

• The FE facial model can also be used for facial expressions and speech simulation

Why FE simulation of a human face?

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

Model of a human face

Muscles – STL – from muscle database for forensic facial reconstruction

• Data received as STL (Stereo-lithography) files

Scanning

Finite Element Model (LS-DYNA)

+ScanFEMeshing & Material Properties

+ScanCADCAD import & positioning

ScanIPImageProcessing

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

Software - Simpleware

Building the FE model of the face – ScanIP• Example of using ScanIP:

• Create a mask that fills in the open gap between face and muscles, i.e. create the subcutaneous fat

Gap to fill in

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• Finite Element model – output form Simpleware Scan FE

• 2,000,000 tetrahedral elements 1mm size

Building the FE model of the face • Oasys PRIMER – used for modifying connections and general

model set-up

• Working with facial surgeons to create and confirm the muscle connections of the FE model

TemporalisOrbicularis Oculi

Masseter

Buccinator

Depressor Anguli Oris

Levator Labii Superioris Alaeque Nasi (LLSAN)

Levator Labii Superioris

Zygomaticus major and minor

Orbicularis Oris

Depressor Labii Inferioris

Mentalis

The FE model with the muscles listed – page 1:

Risorius

Geniohyoid

MylohyoidStylohyoid

Posterior DigastricAnterior

DigastricHyoid bone

The FE model with the muscles listed (21) – page 2:

Medial Pterygoid

Lateral Pterygoid

FE simulations using LS-DYNA

• Two types of analyses:

• Maxillofacial osteotomy – the maxilla and mandible are ‘cut’ and repositioned; the muscles are non active during this simulation

• Facial expressions – user defined material to capture the muscle contraction

• Muscle – non-linear, anisotropic and viscoelastic

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

CE

PE

SEE

FMFM

vMa(t)

L

PE = Parallell (passive) element

CE = Contractile (active) element

SE = Serial element

a(t) = Activation function

LM = Force-length function

vM = Force-velocity function

LM

Constitutive muscle model

PE

CESE

Isometric Contraction (constant muscle length):

- CE shortens - SE lengthens and - PE is constant

Concentric contraction (muscle shortening):

- CE, SE and PE shorten

CESE

PE

Constitutive muscle model

• User Defined muscle material model for LS-DYNA

• The FE constitutive model of the muscle is active, quasi-incompressible, fibre-enforced and hyperelastic

• The FE analysis run with LS-DYNA Explicit

• Work under development together with Yongtao Lu of Cardiff University

Constitutive muscle model

Passive elongation Activated elongation

• A couple of validation results shown below

• Comparison to test results from Davis et al (2002) and Myers et al (1998) of rabbit Tibialis Anterior muscle

Constitutive muscle modelE

ngin

eerin

g S

tress

(Pa)

Eng

inee

ring

Stre

ss (P

a)

Engineering Strain (%)Engineering Strain (%)

Facial expressions

Expression Muscle activatedSmile - Zygomaticus Minor

- Zygomaticus Major - Rizorius - Orbicularis Occuli

Disgust - LLSAN - Depressor Anguli Oris - Orbicularis Occuli

Jaw lowering - Anterior Digastric - Mylohyoid

• LS-DYNA simulation – facial expressions

• Smile – Pre-surgery

• LS-DYNA simulation – facial expressions

• Disgust – Pre-surgery

• LS-DYNA simulation – facial expressions

• Open – Pre-surgery

Mandibular Sagittal Split Osteotomy

Maxillofacial osteotomy

Maxillary Le Fort I Osteotomy

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

Maxillofacial osteotomy – Comparison FE simulation (grey) & Patient images 6 month post surgery (green)

Maxillofacial osteotomy – Comparison FE simulation (grey) & Patient images 6 month post surgery (green)

Model of a human face – Next steps

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

• Further development of muscle material model

• The aim for the future is to build patient specific FE models and be able to assist in real facial surgery