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Optical Science 6 > lens > Flashcards

lens Flashcards

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1
Q

About how much power does the cornea contribute?

A

40D

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2
Q

Fundamental building block of lens is

A

is the many lens fiber cells arranged in a hexagonal pattern.
-similar to epithelial layer of cornea

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3
Q

Transparency of the crystalline lens is governed by the following characteristics:

A
  1. Light-scattering nuclei are near equator (constant
    refractive index across lens fiber cell)
  2. Close packing reduces light scatter at cell boundaries.
  3. lens absorbs little light
    (no pigments; avascular)
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4
Q

Lens fibers are too coarse (10 to 16 microns)

to act as what?

A

diffraction grating

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5
Q

Lack of nutrients means these cells are

A

metabolically inactive. This is conducive for transmitting light, but cell damage cannot be reversed, e.g., cataract.

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6
Q

Crystalline lens has a refractive index that radially varies, being largest (1.402) at the lens ________ and smallest at the lens ________ (1.386).

A

center; edge

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7
Q

A lens with a varying refractive index is called

A

Gradient-Index Lens, or GRIN for short.

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8
Q

How does the lens refract light?

A

vary thickness and refractive index

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9
Q

Wheres the highest refractive index of the lens?

A

nucleus (center)

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10
Q

Human crystalline lens generates refraction by varying its

A

axial thickness and varying its refractive index

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11
Q

Refraction occurs at both ________ and _______ surfaces of lens as well as ________. The combined refraction is ______.

A

anterior; posterior; within; 21.35 D (relaxed)

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12
Q

Does an intraocular lens perform optically the same way as the natural crystalline lens?

A

he didnt answer

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13
Q

Under relaxed conditions, does the lens radius curvature increase or decrease?

A

increase

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14
Q

Under accommodation conditions, what is the power of the lens?

A

31.85 D

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15
Q

Under relaxed conditions, whats the power of the lens?

A

21.35D

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16
Q

Where does the increase in power come from when accommodating?

A

surface radius curvature change

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17
Q

Which surface of the lens contributes more for total power increase of lens during accommodation?

A

anterior surface

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18
Q

peripheral light rays focus where in a spherical glass lens?

A

in front of central (paraxial) light rays

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19
Q

where do center light rays focus in a spherical glass lens?

A

behind peripheral light rays (positive spherical aberration)

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20
Q

the presence of the gradient refractive index generates what?

A

additional optical power

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21
Q

where do the rays focus in our gradient crystalline lens?

A

rays come to a single focus

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22
Q

what are the 3 physical changes that occur in lens?

A

Helmholtz theory

  1. front surface curvature
  2. back surface curvature
  3. thickness changes
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23
Q

near point

A

closest point the patient will have a single image

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24
Q

far point for myopic

A

in front of eye

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25
Q

far point for emmetrope

A

infinity

26
Q

Accommodative amplitude monotonically decreases with what?

A

age

27
Q

Duane accommodative amplitude equation

A

AA=15 - (0.25age)

28
Q

Two theories for accommodation in the eye and the impact of aging. Which answer do you give your patient?

A
  1. Helmholtz theory (more classical)

2. Schachar’s Theory (we teach this one)

29
Q

After 60 yo how much AA do you have?

A

1 D

30
Q

Helmholtz Theory

A
  • 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.)
31
Q

Schachar’s theory

A

• 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.

32
Q

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.

A

Helmoholtz

33
Q

If surface curvature increases, what happens to power of lens?

A

increases

34
Q

In the accommodated eye, the ciliary muscle _______ and _______ the tension on the equator of the lens.

A

constricts; relaxes

35
Q

current solutions for presbyopia

A
  • Bi- and tri-focal lenses

* Progressive lenses

36
Q

Potential solutions (in research & clinical phases) for presbyopia:

A
  • Sclera expansion
  • Pinhole inlay
  • Elastic crystalline lens
  • Electronic spectacles
37
Q

How does sclera expansion work?

A

increases gap b/w lens and cil m.

–> restores increased zonule tension that is lost over age

38
Q

How do charged eyeglasses work?

A

change voltage of a layer of lens –> this changes crystal phase, which changes refractive index

39
Q

Impact of aging on optical performance

A
  • Accommodation
  • Retinal image quality decreases (aberrations increase )
  • Scatter increases
  • absorption increases
40
Q

What options are available to restore normal vision after cataract surgery?

A
  1. 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.
  2. Intraocular lenses
    -best and most common solution (magnification is minimal and peripheral vision normal).
41
Q

If we remove the lens, what happens to the eye refractively?

A

hyperopic

42
Q

With accommodation, if curvature decreases, what does it do to the power?

A

increase power

43
Q

If thickness of lens increases (during accommodation), what does it do to the overall power?

A

decrease power

  • but overall change is that the total power will increase, since the other factors increase the power
  • this is opposite for cornea?
44
Q

4 things that change during accommodation

*which one will decrease power?

A
  1. front surface curvature decreases (10 to 5)
  2. back surface curvature decreases (6 to 5)
  3. thickness increases*
  4. anterior depth decreases
45
Q

IOL vs crystal lens
weight
diameter
thickness

A

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

46
Q

iol transmission vs. crystal lens transmission

A

includes a UV absorber to block < 400 nm; while crystal lens just blocks < 320 nm

47
Q

iol reflection vs. crystal lens reflection

A

iol: 0.301%
crystal lens: 0.0337%

*iol has 100% more reflection than crystal lens

48
Q

Entrance pupil range of IOL equals what?

A
  1. 13 x real pupil

* 13% bigger than real pupil

49
Q

Aren’t these IOLs too small for dim viewing conditions, even for the aging population who are most likely to have cataract surgery?

A

5-7 mm is sufficient for a patient

50
Q

What physical parameters do we need to know to predict the correct IOL power? Which parameters are actually clinically measured?

A
  1. Back vertex power of cornea (keratometer measures it)
  2. refractive index of aqueous and vitreous
  3. depth of anterior and vitreal chambers (pachometry eg ultrasound is used to measure)

*back vertex power and depth of chambers are clinically measured

51
Q

Goal of IOL:

A

rays from distant object intersect at retina!

52
Q

why does the keratometer give back vertex power instead of front vertex power?

A

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

53
Q

how do we find power of cornea given a 7.8 mm radius in clinic?

A

r=337.5/K

  1. 5/(7.8) –>43.27 D
    * NO NEED TO CONVER TO M with this equation
54
Q

if given power of K of 43, how do we find radius?

A

337.5/43

55
Q

What is 1.3375?

A

empirical number; not a real refractive index of anything in the eye!!!

56
Q
  1. Based on the SRK regression formula (developed in 1980s).
  2. 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.
  3. 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)
A

Empirical approach to calculating implant optical power

57
Q

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.

A

regression

58
Q

IOL decentration is almost universal with the mean decentration of what?

A

0.7 mm

59
Q

IOL tilt is also universal with a mean of what?

A

7.8 degree

10 degree tilt induces about 0.5 diopters of astig

60
Q

IOL tilt and decentration are what?

A

secondary optical complications following correct optical power

61
Q

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?

A

24 mm; hyperopic

Optical Science 6 (13 decks)

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