In-VivoIn-Vivo and and In-Situ In-Situ Compressive Properties of Compressive Properties of Porcine Abdominal Soft Porcine Abdominal Soft TissuesTissuesJeffrey D. Brown

MMVR2003biorobotics laboratory

university of washington23-january-2003

23-Jan-03 2

IntroductionIntroduction• Surgery training

¤ Expensive!» Financial ($1M) costs; ethical, societal concerns

¤ Different modalities

• VR surgical simulation technology¤ Reduced use of cadavers/animals/patients¤ No risk of error to patient¤ Allows training for difficult and/or rare scenarios

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Problem StatementProblem Statement• Accurate tissue models are essential

¤ For realistic haptic feedback¤ Need to represent in-vivo behavior¤ Current simulators lack in-vivo models

• Hypothesis: There is a significant difference in mechanical properties in-vivo vs. postmortem¤ No quantitative information on time course of

change

23-Jan-03 4

Background — Soft TissuesBackground — Soft Tissues• Highly nonlinear behavior

¤ Nonlinear stress-strain» not F = k*x!

¤ Anisotropic¤ Heterogeneous¤ Viscoelastic

» Rate-dependent stress, creep, relaxation

¤ Hysteresis¤ Strain history dependence

» Preconditioning

¤ Behavior depends on many factors» Age, gender, pH, tissue health…

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Other DevicesOther Devices• Ottensmeyer (2001, 2002)

¤ TeMPeST 1-D» in-vivo compression (indentation)

Ott

en

smeyer

(20

01

)

Carter et al. (2001)

Rose

n e

t al. (

19

99

)

• Carter et al. (2001)¤ in-vivo indentation¤ human data!

• Rosen et al. (1999)

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Preliminary Results — Preliminary Results — GraspingGrasping

• Measured grasping force applied during 3 surgical tasks for 5 expert surgeons¤ 97.1% of all grasps

(both hands) were held less than 10 sec

¤ Maximum force observed was 40 N (rarely)

¤ Majority of frequency content was below 3 Hz

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the MEGthe MEG

• Cyclic compressions up to 3 Hz• > 40 N grasping force• Weighs about 0.7 kg (1.5 lbs)

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MethodologyMethodology• Animal testing

¤ pig, female, avg. weight 39.5 kg¤ standard laparoscopic setup (11 mmHg)¤ in-vivo: 9 pigs; in-situ: 4 pigs; ex-vivo: 1 cow liver

» 2 pigs done both in-vivo and in-situ

¤ 7 organs tested» gallbladder» liver» small bowel » large bowel » spleen» stomach» urinary bladder

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Methodology — Cyclic Methodology — Cyclic LoadingLoading

• Haversine and constant velocity¤ 0.25 - 3 Hz

• No preconditioning!¤ We want 1st-squeeze behavior and squeeze

history¤ New site for each test

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Data CollectionData Collection• Data synchronized with video from camera

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Results — Cyclic TestingResults — Cyclic Testing• 1 Liver• 1 Hz haversine• in-vivo vs. in-situ• 10 squeezes• Same organ,

different locations

• Remarks:¤ More inter-squeeze

variability in-vivo¤ Beating heart, ventilator

motion, re-perfusion

Str

ess [

MP

a]

Str

ess [

MP

a]

StrainStrain 0.50.500

0.020.02

00

in-vivoin-vivo

in-situin-situ

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Results — Cyclic TestingResults — Cyclic Testing• 3 small bowels• in-vivo

• Remarks:¤ Low initial slope followed

by sudden increase¤ Squeezing contents (air,

fluid, solid) then walls contacting

Str

ess [

MP

a]

Str

ess [

MP

a]

StrainStrain 0.50.500

0.040.04

00

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Results — Cyclic TestingResults — Cyclic Testing• 8 livers• 8 spleens• 3 small bowels• in-vivo• 1 Hz• 1st squeezes only

• Remarks:¤ Large variability

Str

ess [

MP

a]

Str

ess [

MP

a]

StrainStrain 0.50.500

0.040.04

00

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Methodology — Step LoadingMethodology — Step Loading• Measure stress relaxation

¤ Single steps; held for 60 sec¤ Periodic steps

» 10 sec hold» 2.5, 5, 10, 20, 30 sec off

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Results — Stress RelaxationResults — Stress Relaxation• 1 Liver• in-vivo vs. in-situ• Single step, 60 sec

hold time• Strains 21% - 41%

• Remarks: ¤ Higher percent decay in-

situ¤ Steady-state not

reached in 60 sec, especially in-situ

Norm

alized

Str

ess

Norm

alized

Str

ess

Time [sec]Time [sec] 606000

100%100%

50%50%

in-vivoin-vivo

in-situin-situ

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Results — Stress RelaxationResults — Stress Relaxation• 1 Liver• in-vivo vs. in-situ• Periodic steps

¤ 10 sec on / 2.5 sec off

• Strains 17% - 40%

• Remarks: ¤ Higher percent decay in-

situ¤ Behavior is similar to

single step (little recovery between squeezes)

Norm

alized

Str

ess

Norm

alized

Str

ess

Time [sec]Time [sec] 353500

100%100%

50%50%

in-vivoin-vivo

in-situin-situ

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Results — Stress RelaxationResults — Stress Relaxation• 1 Liver• in-vivo vs. in-situ• Periodic steps

¤ 10 sec on / 30 sec off

• Strains 21% - 51%

• Note: ¤ More recovery between

squeezes than 10/2.5¤ More recovery in-vivo

» Returns to 100%» Some >100%

– Swelling?

¤ Some recovery in-situ

Norm

alized

Str

ess

Norm

alized

Str

ess

Time [sec]Time [sec] 909000

100%100%

50%50%

in-vivoin-vivo

in-situin-situ

23-Jan-03 18

LimitationsLimitations• Difficult to control in-vivo conditions

¤ Heartbeat, respiration, intra-abdominal pressure, anesthesia

¤ Hollow organs» Contents may be different in each location

¤ We want these effects included!

• Boundary conditions¤ Organs may slide and rigidly translate instead of deform¤ Hand tremor may be present in data

• Compliant mechanism• Friction• Contact detection• Thin samples

¤ Large error in strain

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ConclusionsConclusions• in-vivo compression of soft tissues

¤ Abdominal organs relevant to laparoscopy¤ Compressive loading in ranges of force and

deformation observed in real surgeries

• Understanding of how tissue mechanical properties change postmortem

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Future WorkFuture Work• Further verification of MEG accuracy

¤ Comparison of MEG to MTS

• Characterization of organ phantoms’ properties

• Sterilizable MEG for in-vivo human testing

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AcknowledgmentsAcknowledgments• Funding:

¤ Whitaker Foundation» Graduate Student Fellowship program

¤ Washington Research Foundation¤ UW Department of Surgery

» Center for Videoendoscopic Surgery

• Co-authors:¤ Jacob Rosen, Yoon Sang Kim, Lily Chang, Mika

Sinanan, Blake Hannaford

23-Jan-03 22

Thank You!Biorobotics Lab

http://brl.ee.washington.edu