Lecture 4

Question 1

how does the lens grow throughout life?

Answer 1

new cells form at the equator, then elongate as “fibers” that wrap around peripheral surface

Question 2

what is a fundamental building block the crystalline lens?

Answer 2

many lens fiber cells arranged in a hexagonal pattern

Question 3

what 3 things create a transparent lens?

Answer 3

light scattering nuclei are near equator, close packing reduces light scatter at cell boundaries, and lens absorbs little light (no pigments)

Question 4

what is a disadvantage of the lens being avascular?

Answer 4

cells are metabolically inactive - cell damage cannot be reversed (cataracts)

Question 5

what is the refractive index of the lens - how does it change?

Answer 5

it radially varies (largest 1.402 at the center and smallest 1.386 at the edge) = GRIN or gradient index lens

Question 6

how does the crystalline lens refract light?

Answer 6

varies its thickness and refractive index (thickest/higher refractive index in center)

Question 7

what parts of the lens are involved in refraction/accommodation?

Answer 7

anterior, posterior surfaces and center of lens

Question 8

which surface, anterior or posterior change more during accommodation?

Answer 8

the anterior surface changes more (goes from 10mm to 5mm)

Question 9

during accommodation, which changes produce an increase in power?

Answer 9

curvature decreases and anterior chamber depth decreases

Question 10

during accommodation, which change produces a decrease in overall power?

Answer 10

thickness increases = decreases power

Question 11

why do spherical surfaces produce spherical aberrations?

Answer 11

the peripheral (marginal) rays focus in front of the paraxial (central) rays

Question 12

how does the human eye reduce spherical aberration?

Answer 12

by having a gradient refractive index (increases from periphery to center) = makes the paraxial rays focus closer (where the peripheral rays are)

Question 13

what is the range of accommodation?

Answer 13

the range from the far point to the near point of an eye (relaxed to accommodated)

Question 14

what is the Duane’s formula for amplitude of accommodation?

Answer 14

AA = 15 - 1/4 age

Question 15

what is the Helmholtz’s theory for accommodation?

Answer 15

contraction in the ciliary muscle decreases tension in all zonules to increase optical power

Question 16

what is Helmholtz’s reason for losing accommodation as we age?

Answer 16

increase in mass and stiffness of the lens with age reduces lens deformation

Question 17

what is Schachar’s theory of accommodation?

Answer 17

contraction of the ciliary muscles increases tension in equatorial zonules and decreases tension in anterior and posterior zonules to increase optical power

Question 18

what is Schachar’s reason for losing accommodation as we age?

Answer 18

decrease in space between ciliary muscle and lens with age causes the zonules to become slack

Question 19

what is the most common option to restore normal vision following cataract surgery?

Answer 19

Intraocular lenses (magnification is minimal and peripheral vision is normal)

Question 20

how are IOL’s different from the crystalline lens (weight, diameter, thickness)?

Answer 20

IOL is heavier, smaller in diameter and thinner than the crystalline lens

Question 21

are IOL’s going to be smaller than the pupil size in dim environments and cause optical problems?

Answer 21

no- the pupil diameter measured is the entrance pupil (image) and is 13% larger than the real pupil

Question 22

to use an analytic approach to calculating an IOL power, what parameters need to be known?

Answer 22

back-vertex power of cornea, refractive index of aqueous and vitreous, and depth of AC and vitreal chamber

Question 23

what is the equation for finding the back vertex power from K readings?

Answer 23

K = (1.3375 -1)/r or r anterior (mm) = 337.5/K

Question 24

how was the SRK regression formula created?

Answer 24

represents a curve of best fit IOL powers (claims 90% of patients should be within 2D with axial length 22-24.5mm)

Question 25

what is the SRK formula?

Answer 25

F IOL = A - 2.5 x axial length (mm) - 0.9 Keratometry reading (K - average)

Question 26

what 2 things are secondary optical complications following IOL surgery?

Answer 26

IOL decentration and IOL tilt

Question 27

what is the average amount of IOL decentration that occurs?

Answer 27

0.7mm

Question 28

what is the average amount of IOL tilt that occurs?

Answer 28

7.8 degrees (10 degrees induces about 0.5D of astigmatism)