Corneal Shape Flashcards
usual visible iris diameter range is
10.5 to 13.5 mm
HVID is about
11.7 mm
VVID is about
10.6 mm
HVID is about ___ less for females
0.1 mm
VID can be measured via:
- limbus-to-limbus topography (Medmont)
- OCT (optical coherence tomography)
when can VID (corneal diameter) have clinical indications?
- soft contact lens base curve (sagittal height)
- corneal GP overall diamter
- infants
- microcornea/megalocornea
- scleral GP contact lenses
sagittal height formula is
r- (square root r^2-(d/2)^2)
are corneal curvature and sagittal depth related?
they are not the same thing!
list 4 contact lens parameters that can vary with sagittal height (thereby changing fit)
- base curve (BC)
- peripheral curve (PC)
- overall diameter (OAD)
- optic zone diameter (OZD)
do changes to the CL front surface (F1) alter sagittal height?
no, they do not alter inside sagittal height or lens-to-cornea fitting relationship
what does “mechanical fit” mean?
matching the sagittal depth of the CL to the sagittal depth of the corneal surface
how is corneal thickness measured? (what instruments can be used)
- pachymetry (ulrasound or optical)
- orbscan topography
- inferometry
- OCT (optical coherence tomography)
where is corneal thickness the thinnest and how many microns is it?
thinnest: centrally, 536 microns
where is corneal thickness the thickest and how many microns is it?
thickest: peripherally, 650 to 1,200 microns
in the morning right as you wake up, how does corneal thickness change?
in a.m., 4% decrease upon waking
when calculating if a patient has a thick enough cornea for lasik, how much is the flap thickness in microns?
about 100 microns
when calculating the thickness of a cornea after LASIK, how much is vaporized per diopter of correction?
16 microns for every D of correction
95% of the population has a corneal thickness between ___ and ____
473 - 595 microns
average corneal thickness is approximately
534 microns
after lasik, the post-surgical corneal bend thickness has to be between ___ and ___ microns
250 to 300 microns
what could you consider if a patient is borderline for thickness of LASIK?
PRK (photorefractive keratectomy), a surface ablation procedure
how do thin corneas affect IOP readings?
thin corneas cause under-reading of true IOP
keratoconus patient IOP readings
many measurements are single digits, ocular rigidity??
Goldmann is calibrated for ___ microns corneal thickness
550 microns
the ocular hypertension treatment study (OHTS) showed what concerning data about thin corneas and IOP?
thin corneas cause under-reading of true IOP- could lead to delay in diagnosis and establishing the wrong target reading for glaucoma tx
average corneal curvature data is about
43.00 D (7.85 mm)
corneal curvature changes from infancy until 5 y/o
flattens from infancy ‘til about age 5
corneal curvature is pretty stable from about ___ to ___ age
5 to 95 unless corneal injury, corneal surgery, CL wear
why does the corneal not act like a steel ball?
- patient fixation, especially if decreased vision
- tear film
- change over time
- measurement variability
if cornea is steeper than 52D and you use a +1.25 D trial lens, how much do you have to +/- to get the reading?
add 8 to 9 D
if cornea is steeper than 52D and you use a +2.25 D trial lens, how much do you have to +/- to get the reading?
~ 16 D
if cornea is flatter than 38D and you use a -1.00 D trial lens, how much do you have to +/- to get the reading?
subtract 6 D from drum reading
how repeatable is your keratometer data for central cornea?
K = 0.50 D (or better)
Sim K= 0.50 D (or better
how repeatable is your keratometer data for peripheral cornea?
less accurate than central
tomography may not be how the cornea is in real life, but what data does it produce?
anterior surface flow, posterior surface flow, thickness map, change map, etc.
clinically, how could you test for possible keratoconus on the keratometer ?
measure central cornea and then inferior. if inferior steeper, then maybe keratoconus
the old school K Shape theory describes the spherical corneal cap as:
central 4mm of maximum (steep) and essentially constant (sphere) curvature
what is the K Eccentricity (e) value?
rate of flattening
K eccentricity (e) has an inverse relationship to:
shape factor
average K eccentricity value (e) and range
~ 0.5 eccentricity (0.48 +/- 0.11)
___ (lower/higher) e = more spherical
lower
a new study shows liable area and first part of sclera, which have always been assumed to be curved, may actually be
in some cases the sclera often continues in a straight line (tangential) from the peripheral cornea onward
with new data from OCT on the peripheral cornea, limbus, and sclera: the best way to now describe the geometry of these regions is
a tangent angle rather than a radius curve
typically in the average eye, the ___ portion is flatter compared with the rest
nasal
many eyes are non rotationally symmetrical in nature beyond the corneal borders. this may call for:
non rotationally symmetrical lenses such as toric and quadrant specific lenses
prolate=
normal, steeper centrally and flattens in periphery
oblate =
flatter in center, steeper in edge, post-refractive (LASIK, RK), or post orthoK
lens design needed for prolate corneas
spherical lens design
lens design needed for oblate cornea
reverse geometry lens design
