Measuring and modeling elasticity distribution in the intraocular lens

1
Measuring and modeling elasticity distribution in
the intraocular lens
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Lens System
Zonules
Cornea
Intraocular Lens
Retina
Ciliary Muscle
3
Lens Anatomy
Lerman S., Radiant energy and the eye, (1980)
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Helmholtz Accommodation
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Colemans Theory of Accommodation
Schachar RA, Bax AJMechanism of human
accommodation as analyzed by nonlinear finite
element analysis ANNALS OF OPHTHALMOLOGY 33 (2)
103-112 SUM (2001)
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Presbyopia
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Presbyopia

• Onsets at about 40 years
• 100 prevalence
• Complicates Stabismus (cross eyed)
• Increases safety risks for pilots

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Conceptual Elastic Model
Zonules
Media
Capsule
Zonules
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Lasering
Laser
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Photodisruption

• Femtosecond pulsed laser
• Nonlinear absorption
• Breakdown only occurs above threshold
• Limited to focal spot
• No damage to surrounding tissue
• Small disruption sites 1 to 10 mm
• Precise location

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Acoustic Radiation Force
Gas Bubble
Acoustic Wavefront
Elastic Solid
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Advantages

• Reflection more efficient than absorption
• Bubbles
• Approximate perfect reflectors
• High spatial resolution
• High contrast for anechoic tissues like lens
• Potential in-vivo procedure
• Localized measurement

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Experimental Set-up
Focusing Lens
Shutter
ND Filter
Ultrafast Laser
Mirror
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Sampling
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Bubble Displacement (Porcine Lens)
40
30
Maximum Displacement (mm)
20
10
0
1
3
5
7
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Lateral Position (mm)
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Bubble Size Dependence
Push 1
Push 7
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Cumulative Normalized Bubble Displacement (N 12)
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Relative Stiffness Porcine Lens
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Youngs Modulus Porcine Lens
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Conclusions

• Acoustic radiation force displaces bubble
• Ultrasound tracks bubble
• Convert displacement into elasticity
• Lens elasticity
• Not homogeneous
• Function of radial distance

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Heys et. al., Experimental Setup
Heys KR, Cram SL, Truscott RJW Massive increase
in the stiffness of the human lens nucleus with
age the basis for presbyopia? Molecular Vision
(2004)
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Heys et. al., Results (65 year-old)
Heys KR, Cram SL, Truscott RJW Massive increase
in the stiffness of the human lens nucleus with
age the basis for presbyopia? Molecular Vision
(2004)
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Elasticity Distribution vs. Age
Heys KR, Cram SL, Truscott RJW Massive increase
in the stiffness of the human lens nucleus with
age the basis for presbyopia? Molecular Vision
(2004)
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Multilayer Model
Anterior
2
Light
1
I
H
G
F
D
E
C
Polar distance (mm)
A
B
0
Zonules
-1
Capsule
-2
Posterior
1
2
3
4
5
6
0
Radial distance (mm)
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Caution

• Not a direct model of presbyopia
• Ignore age-related geometry
• Separate biomechanical contributions
• Average elasticity
• Elasticity distribution

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Procedure
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Optical Power
the degree to which a lens converges or diverges
light, equal to the reciprocal of the focal
length
ra anterior radius of curvature rp posterior
radius of curvature t polar lens thickness n1
index of refraction for lens n2 index of
refraction for vitreous
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Elasticity Distribution (Varying Average
Elasticity)
Multiplier
A
B
C
D
E
F
G
H
I
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Average Elasticity (Varying Average Elasticity)
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Accommodation (Varying Average Elasticity)
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Elasticity Distribution (Varying Elasticity
Distribution)
I
H
G
F
E
D
C
B
A
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Average Elasticity (Varying Elasticity
Distribution)
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Accommodation (Varying Elasticity Distribution)
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Lens Biomechanics
Polar distance
Radial distance
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Elasticity Distribution (Example)
High Average Favorable Distribution
Low Average Unfavorable Distribution
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Accommodation (Example)
Low Average Unfavorable Distribution
High Average Favorable Distribution
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Conclusions

• Multi-layer model shows accommodation
• Two presbyopia mechanisms
• Increased average elasticity (known)
• Elasticity distribution change (new)
• Elasticity map needed for presbyopia surgery

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Colleagues

• Matthew ODonnell
• Todd Erpelding
• Jing Yong Ye
• Christine Tse
• Marwa Zhody

• Tibor Juhasz
• Gagik Jotyan
• Ron Kurtz

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• Biomedical Ultrasound Laboratory
• Biomedical Engineering Dept.
• bul.eecs.umich.edu
• Center for Ultrafast Optical Science
• www.eecs.umich.edu/CUOS/
• University of Michigan
• IntraLase Corporation, Irvine, CAwww.intralase.co
  m
• Supported by NIH grant
• R21 EY015876