Lecture 4 Flashcards
how does the lens grow throughout life?
new cells form at the equator, then elongate as “fibers” that wrap around peripheral surface
what is a fundamental building block the crystalline lens?
many lens fiber cells arranged in a hexagonal pattern
what 3 things create a transparent lens?
light scattering nuclei are near equator, close packing reduces light scatter at cell boundaries, and lens absorbs little light (no pigments)
what is a disadvantage of the lens being avascular?
cells are metabolically inactive - cell damage cannot be reversed (cataracts)
what is the refractive index of the lens - how does it change?
it radially varies (largest 1.402 at the center and smallest 1.386 at the edge) = GRIN or gradient index lens
how does the crystalline lens refract light?
varies its thickness and refractive index (thickest/higher refractive index in center)
what parts of the lens are involved in refraction/accommodation?
anterior, posterior surfaces and center of lens
which surface, anterior or posterior change more during accommodation?
the anterior surface changes more (goes from 10mm to 5mm)
during accommodation, which changes produce an increase in power?
curvature decreases and anterior chamber depth decreases
during accommodation, which change produces a decrease in overall power?
thickness increases = decreases power
why do spherical surfaces produce spherical aberrations?
the peripheral (marginal) rays focus in front of the paraxial (central) rays
how does the human eye reduce spherical aberration?
by having a gradient refractive index (increases from periphery to center) = makes the paraxial rays focus closer (where the peripheral rays are)
what is the range of accommodation?
the range from the far point to the near point of an eye (relaxed to accommodated)
what is the Duane’s formula for amplitude of accommodation?
AA = 15 - 1/4 age
what is the Helmholtz’s theory for accommodation?
contraction in the ciliary muscle decreases tension in all zonules to increase optical power
what is Helmholtz’s reason for losing accommodation as we age?
increase in mass and stiffness of the lens with age reduces lens deformation
what is Schachar’s theory of accommodation?
contraction of the ciliary muscles increases tension in equatorial zonules and decreases tension in anterior and posterior zonules to increase optical power
what is Schachar’s reason for losing accommodation as we age?
decrease in space between ciliary muscle and lens with age causes the zonules to become slack
what is the most common option to restore normal vision following cataract surgery?
Intraocular lenses (magnification is minimal and peripheral vision is normal)
how are IOL’s different from the crystalline lens (weight, diameter, thickness)?
IOL is heavier, smaller in diameter and thinner than the crystalline lens
are IOL’s going to be smaller than the pupil size in dim environments and cause optical problems?
no- the pupil diameter measured is the entrance pupil (image) and is 13% larger than the real pupil
to use an analytic approach to calculating an IOL power, what parameters need to be known?
back-vertex power of cornea, refractive index of aqueous and vitreous, and depth of AC and vitreal chamber
what is the equation for finding the back vertex power from K readings?
K = (1.3375 -1)/r or r anterior (mm) = 337.5/K
how was the SRK regression formula created?
represents a curve of best fit IOL powers (claims 90% of patients should be within 2D with axial length 22-24.5mm)
