Flashcards in Lecture 1 - Theory and Practice Deck (44)
what index of refraction does the keratometer use when making measurements?
1.3375 (not 1.376 to account for the small contribution from posterior cornea and corneal thickness)
what portion of the cornea does the keratometer measure?
the central 3-4mm (assumes axes are separated by 90 degrees)
what can you use if the dioptric measurment ralls outside of the keratometer range?
+1.25 or -1.00 sphere lens = extends range to 36-52D to 31-60D
what do you multiply the results by if you use a +1.25D lens? what about for a -1.00D lens?
+1.25 = 1.166
-1.00 = 0.8576
how do you calculate diopters from mm (radius)?
surface power equation (337.5/r mm)
how do you record the keratometry results?
flat radius (weak power) and meridian comes first and then the steeper power and meridian
what shape is the cornea? what are the HVID and VVID averages?
HVID = 11.7mm
VVID = 10.6mm
(both are 0.1mm less in females)
what is the average corneal thickness? central radius? sagittal depth?
thickness = 0.52mm (center) and 0.67mm (limbus)
radius = 7.80mm
sagittal depth = 2.6mm
what happens to the corneal curvature in the periphery?
the curvature flattens towards the periphery
what is the corneal front apical radius? back apical radius?
front = 7.8mm (43.27D)
back = 6.5mm (-6.15D)
what is the average corneal refractive index?
1.376 (not optically homogenous)
what is the cornea made of?
5% other parts
how much of the corneal thickness are the corneal epithelium and stroma?
epithelium = 50 microns (10% of cornea's wet weight)
stroma = 90%
what nerves are present in the cornea?
(about 30 nerves)
ophthalmic division of CN5
appear as axon bundles near limbus and lose myelin sheath before 1st division
what will a keratometry over a soft CL show?
the front surface's eye radius (not the front surface manufactured radius)
what does it mean if the mires appear irregular over a soft CL?
the front surface is irregular - keratoconus or the CL is inside out
what will the keratometer show over a GP lens?
gives the manufactured radius and will reveal unwanted lens flexure (induced astigmatism)
what is Dk?
oxygen permeability - one of most important properties of a CL material (inherent material property)
what is Dk/t?
oxygen transmissibility - Dk of the material divided by the lens thickness (10^-9)
what is the unit for Dk?
Barrer (10^-11 and CT is in cm)
what is an acceptable minimum Dk/t value for avoidance of corneal edema?
daily wear = 24
extended wear = 87
what is an acceptable minimum Dk/t value for avoidance of anoxia for open and closed conditions?
daily wear = 35
extended wear = 125
what 3 things occur with lack of oxygen to the cornea?
edema, swelling and change in K readings
what is wettability?
crucial for comfort and vision - can be detected in vitro and in vivo (determine wetting angle)
is a large angle good for wettability?
no - the lesser the angle = better wettability
what are the in vivo methods for determining wettability (visual analysis)?
tear coverage (look for dry spots), break-up time and drying time
what are plasma treatments, what does it do to wettability?
improves - uses a good cleaning of the lens (UV energy breaks organic bonds - lasts a few days to months)
what are some natural wetting agents in the human tearfilm?
mucin, lipids, lactoferrin and lysozyme
what is the stem filcon for?