in-vivo and in-situ compressive properties of porcine abdominal soft tissues jeffrey d. brown...
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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
23-Jan-03 3
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
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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