lens Flashcards
About how much power does the cornea contribute?
40D
Fundamental building block of lens is
is the many lens fiber cells arranged in a hexagonal pattern.
-similar to epithelial layer of cornea
Transparency of the crystalline lens is governed by the following characteristics:
- Light-scattering nuclei are near equator (constant
refractive index across lens fiber cell) - Close packing reduces light scatter at cell boundaries.
- lens absorbs little light
(no pigments; avascular)
Lens fibers are too coarse (10 to 16 microns)
to act as what?
diffraction grating
Lack of nutrients means these cells are
metabolically inactive. This is conducive for transmitting light, but cell damage cannot be reversed, e.g., cataract.
Crystalline lens has a refractive index that radially varies, being largest (1.402) at the lens ________ and smallest at the lens ________ (1.386).
center; edge
A lens with a varying refractive index is called
Gradient-Index Lens, or GRIN for short.
How does the lens refract light?
vary thickness and refractive index
Wheres the highest refractive index of the lens?
nucleus (center)
Human crystalline lens generates refraction by varying its
axial thickness and varying its refractive index
Refraction occurs at both ________ and _______ surfaces of lens as well as ________. The combined refraction is ______.
anterior; posterior; within; 21.35 D (relaxed)
Does an intraocular lens perform optically the same way as the natural crystalline lens?
he didnt answer
Under relaxed conditions, does the lens radius curvature increase or decrease?
increase
Under accommodation conditions, what is the power of the lens?
31.85 D
Under relaxed conditions, whats the power of the lens?
21.35D
Where does the increase in power come from when accommodating?
surface radius curvature change
Which surface of the lens contributes more for total power increase of lens during accommodation?
anterior surface
peripheral light rays focus where in a spherical glass lens?
in front of central (paraxial) light rays
where do center light rays focus in a spherical glass lens?
behind peripheral light rays (positive spherical aberration)
the presence of the gradient refractive index generates what?
additional optical power
where do the rays focus in our gradient crystalline lens?
rays come to a single focus
what are the 3 physical changes that occur in lens?
Helmholtz theory
- front surface curvature
- back surface curvature
- thickness changes
near point
closest point the patient will have a single image
far point for myopic
in front of eye
far point for emmetrope
infinity
Accommodative amplitude monotonically decreases with what?
age
Duane accommodative amplitude equation
AA=15 - (0.25age)
Two theories for accommodation in the eye and the impact of aging. Which answer do you give your patient?
- Helmholtz theory (more classical)
2. Schachar’s Theory (we teach this one)
After 60 yo how much AA do you have?
1 D
Helmholtz Theory
- Contraction of the ciliary muscle ↓ tension in all zonules → increase optical power
- Increase in mass and stiffness of the lens with age reduces lens deformation. (Lens continues to enlarge throughout life as new lens fibers are added.)
Schachar’s theory
• Contraction of the ciliary muscle increases tension in equatorial zonules and ↓ tension in the anterior and posterior zonules → increase optical power
• Decrease in space between ciliary muscle and lens with age causes the zonules to become slack.
What is the theory for:
The relaxed eye is under tension
at the equator from the ciliary body. This keeps the surfaces flat
enough so that for a typical eye distant objects focus on
the retina.
Helmoholtz
If surface curvature increases, what happens to power of lens?
increases
In the accommodated eye, the ciliary muscle _______ and _______ the tension on the equator of the lens.
constricts; relaxes
current solutions for presbyopia
- Bi- and tri-focal lenses
* Progressive lenses
Potential solutions (in research & clinical phases) for presbyopia:
- Sclera expansion
- Pinhole inlay
- Elastic crystalline lens
- Electronic spectacles
How does sclera expansion work?
increases gap b/w lens and cil m.
–> restores increased zonule tension that is lost over age
How do charged eyeglasses work?
change voltage of a layer of lens –> this changes crystal phase, which changes refractive index
Impact of aging on optical performance
- Accommodation
- Retinal image quality decreases (aberrations increase )
- Scatter increases
- absorption increases
What options are available to restore normal vision after cataract surgery?
- Spectacles
-rarely used today because the high power (~20D) creates visually annoying magnification (25%) and distortion for the patient.
2. Contact lenses
-a better alternative (only 7% magnification), but elderly patients often have trouble handling contact lenses. - Intraocular lenses
-best and most common solution (magnification is minimal and peripheral vision normal).
If we remove the lens, what happens to the eye refractively?
hyperopic
With accommodation, if curvature decreases, what does it do to the power?
increase power
If thickness of lens increases (during accommodation), what does it do to the overall power?
decrease power
- but overall change is that the total power will increase, since the other factors increase the power
- this is opposite for cornea?
4 things that change during accommodation
*which one will decrease power?
- front surface curvature decreases (10 to 5)
- back surface curvature decreases (6 to 5)
- thickness increases*
- anterior depth decreases
IOL vs crystal lens
weight
diameter
thickness
iol: 4-5 grams (heavier)
crystal: 0.2 grams
iol diameter: 5-7 mm (smaller)
crystal: 9 mm
iol thickness: 0.6 mm (much thinner)
crystal thickness: 3.6-4 mm
iol transmission vs. crystal lens transmission
includes a UV absorber to block < 400 nm; while crystal lens just blocks < 320 nm
iol reflection vs. crystal lens reflection
iol: 0.301%
crystal lens: 0.0337%
*iol has 100% more reflection than crystal lens
Entrance pupil range of IOL equals what?
- 13 x real pupil
* 13% bigger than real pupil
Aren’t these IOLs too small for dim viewing conditions, even for the aging population who are most likely to have cataract surgery?
5-7 mm is sufficient for a patient
What physical parameters do we need to know to predict the correct IOL power? Which parameters are actually clinically measured?
- Back vertex power of cornea (keratometer measures it)
- refractive index of aqueous and vitreous
- depth of anterior and vitreal chambers (pachometry eg ultrasound is used to measure)
*back vertex power and depth of chambers are clinically measured
Goal of IOL:
rays from distant object intersect at retina!
why does the keratometer give back vertex power instead of front vertex power?
we need to know power of cornea, so that when light passes through cornea we know how much is refracted?
*more useful than front vertex power
how do we find power of cornea given a 7.8 mm radius in clinic?
r=337.5/K
- 5/(7.8) –>43.27 D
* NO NEED TO CONVER TO M with this equation
if given power of K of 43, how do we find radius?
337.5/43
What is 1.3375?
empirical number; not a real refractive index of anything in the eye!!!
- Based on the SRK regression formula (developed in 1980s).
- SRK formula represents a curve of best fit through IOL results on over 1,000 patients (i.e. keratometry, axial length, lens type), and power of IOL needed for emmetropia.
- Claim is that 90% of patients should be within 2 diopters. (most errors occur in longer and shorter eyes)
(Within certain range of axial length: 22 ~ 24.5 mm)
Empirical approach to calculating implant optical power
Process for determining the statistical relationship between a random variable (IOL power) and one or more independent variables that are used to predict the value of the random variable.
regression
IOL decentration is almost universal with the mean decentration of what?
0.7 mm
IOL tilt is also universal with a mean of what?
7.8 degree
10 degree tilt induces about 0.5 diopters of astig
IOL tilt and decentration are what?
secondary optical complications following correct optical power
if we dont know a pt’s axial length of eye, what do we assume the length is? What happens to patients refractive error after surgery?
24 mm; hyperopic
