NCLE Part 2: Optical Principles of Contact Lenses

iris

pupil

sclera

conjunctiva

limbus

outermost layer

anterior chamber

posterior chamber

crystalline lens

ciliary muscles

vitreous

choroid

retina

optic nerve

cornea

provides 2/3 of refractive power

with the rule astigmatism

horizonal meridian is the flattest, vertical meridian is the steepest

against the rule astigmatism

horizontal meridian is the steepest, vertical meridian is the flattest

oblique astigmatism

major axes are found closer to 45 and 135 degrees

irregular astigmatism

major axes are not 90 degrees apart

average horizontal diameter

11.7mm

average vertical diameter

10.6mm

epithelium

surface layer of cornea, regenerated every 3-7 days

Bowman’s Membrane

resistant to injury but does not regenerate

Stroma

90% of corneal thickness

Descement’s Membrane

lies between the stroma and endothelium

Endothelium

controls the 78% eater content of the stroma and keeps the cornea clear

pleomorphism

changed endothelial cell shapes

polymegathism

reduced number of endothelial cells

corneal edema

a buildup of fluid on the cornea

keratometer

measures central 2-4mm of cornea

corneal astigmatism

difference between the flattest and steepest meridians

normal keratometer range

36.00D-52.00D