Block 5 Flashcards
Which meridian is with-the-rule astigmatism steeper?
Vertically (90 degree)
Wider than it is tall
What should the keratometer read if calibrated correctly?
7.5/45
Which is steeper? 45.0 D or 42.0 D?
45 is steeper
Higher the diopter is steeper
Which is steeper? 7.5mm or 8.04mm
7.50mm is steeper
The lower the mm is steeper
Meters and diopters are inversely related
How many millimeters is 0.5 D?
0.1mm
How many millimeters is 5 D?
1mm
How many millimeters is 1 D?
0.2mm
If 42.00 D = 8.04mm, how many millimeters is 42.50 D?
- 94mm
- 5 - 42 = 0.5 D = 0.1mm
- 04 - 0.1mm = 7.94mm
If 45 D = 7.50mm, then how many mm are in 44.50 D?
- 60mm
7. 50mm + 0.10mm = 7.60
What is the index of refraction of the cornea?
1.3375
If calculating with radius of curvature, use 337.5
N-1(1000) = 337.5
At what power do you have to start worrying about vertex distance?
After +/- 4 Diopters
What do the hotter colors on the computer keratometry mean?
Those are the steeper areas
What are the Sim K values on the computer keratometry reading?
They are the equivalent to the manual keratometry reading
How do you expand the range of the ketatometer for a very high reading? (Above 52 diopters)
Place a +1.25 D trail lens over the objective opening to increase the range by 9 diopters
OR multiply the K reading by 1.185 for an exact reading
How do you expand the range of the keratometer for a very low reading? (Less than 36D)
Place a -1.00 D trail lens over the objective opening to shift it down by 6 D
OR
Multiply K reading by 0.840
What are the advantages of the keratometer?
Ease of use
Low cost
What are the disadvantages of the keratometer?
Only central 3 mm
Apex is not directly measured, ONLY the geometric center - which is not always the steepest K reading (contacts rest at the corneal apex)
Examiner error possible
The keratometer is change may not correspond with the refractive change
What is keratometry good for?
Initial selection of lens parameters
Initial base curve selection
Prediction of residual cylinder
What is the most important assessment in GP contact lens fitting?
Fluorescein pattern evaluation
What is the most important assessment for soft lens fit?
Contraction and lens lag
What does it mean if there is fluorescein pooling in a ring around the center and none at the edge?
Fluorescein needs 20 microns to show up, so it places like the middle where there is no fluorescein it means it is sitting tightly against the cornea there
No fluoro at the edges means CL is too tight
What are the regions of the cornea?
Optic cap - steepest area at the center - 3-4mm
Paracentral/mid peripheral region - flattens as you go out - 4-8mm
Peripheral region - 8-11mm
What should you do if you can’t get the keratometer in focus?
Move the head reset placement
What do you need to make sure you do in strabismic patients?
Definitely occlude one eye
Back surface is what shape?
Concave
MINUS power
-45
What shape is the front surface?
Convex
PLUS power
+45
The back surface is the ____ curve?
Fitting
The front surface is the _____ curve
Power
If you require a power of -5 D, what will the front and back surface powers be?
Back surface will be -45 and front surface will be +40 (flatter)
If you need a power of +5 D what should the front and back surfaces be?
Back is -45, front is +50
Steeper
To accurately measure the total corneal power, what do you need to measure? 3 things
Anterior corneal curvature
Posterior corneal curvature
Central corneal thickness
Which is the one surface that the keratometric equation takes into consideration?
Anterior corneal curvature
This is why it is only an approximate calculation of optical power of the cornea
The curvature (in diopters) is a reciprocal of what?
Radius of curvature (in meters)
A large radius of curvature means?
Small curvature
What is normal astigmatism?
2 principal meridians are perpendicular
Keratometry can not measure?
Asphericity
What is toricity?
A difference in the horizontal meridian vs the vertical meridian
Aka astigmatism
What is asphericity?
Steeper in the middle vs flatter in the outer or vice versa
Not a perfect sphere
Steeper in the vertical meridian is what kind of astigmatism?
With the rule
Steeper in the horizontal meridian is what kind of astigmatism?
Against the rule
What is the difference between axial vs tangential maps?
In axial maps all the circles are lined up on the same optical axis
(Most people prefer these maps)
In the tangential maps the radius is just measured from off the tangent to the surface (the radii may not be in a line)
What are elevation maps?
All the notches in or out have the same curvature
There is a best fit sphere placed on top, the cornea below it is a depression (show up as blue on map) and the cornea above it is an elevation (show up as red on the map)
What is the difference in the axial curvature maps vs the elevation maps?
The axial curvature maps look like the normal astigmatism maps (bow tie pattern)
The elevation maps show the elevation or depression at the edges, so there are red spots in the superior and inferior with yellowish color in the middle
What does bright green mean on elevation maps in a contact lens fitting?
Green means the more clearance between the lens and the cornea
Black means less clearance
What does the Scheimpflug do for centration that the OCT does not?
The Scheimpflug uses the corneal apex as a standard reference
The OCT relies on the examiner’s identification of the pupil center
Which machine takes several pictures across the cornea and then puts them together like pizza slices
Pentacam
But this one takes more time
This is the opposite of the pentacam.. it is quicker because it only takes one picture?
Corneal topography
What is Cassini’s elevation based topography?
It is asymmetrical with 7 trianglular plates
It is more sensitive to small differences
It can discern feature changes along both the radial and tangential directions
What is the usual radius of curvature for a contact lens?
7-9mm
What is the purpose of having bicurve or tricurve contact lenses?
Helps in fitting the lens
There are no optical properties
Most CLs are bi or tri curves
Where is the power located in CLs?
Optical zone
It must be larger than scotopic pupil size
What is the base curve of the contact lens?
The curvature of the central portion of the posterior surface of the lens
The cornea is _____ and the base curve is ______
Cornea is steeper
Base curve is flatter
What is the chord diameter?
The width of the contact lens
Usually 13-15mm
Some riding gas permeable (RGP) are smaller
Are the peripheral curves flatter or steeper than the base curve?
Usually flatter to blend and enhance comfort
Why is large thickness detrimental to contact lenses?
Thicker the lens means less oxygen permeability
What is the Sagittal depth of the contact lens?
The distance between the center of the lens to the plane connecting the edges
The sagittal depth decreases as the base curve increases(lens is flatter)
With the same diameter, an increased lens curvature means?
More sagitta
Black in a fluorescein picture means?
Contact, near zero clearance
What is the tear lens?
An optical lens formed by the entrapped tear film layer between the posterior surface of a contact lens and anterior surface of the cornea
As a rule of thumb for tear lens power, for every 0.05mm radius of curvature difference, what is the diopter power?
0.25 D
The steeper the base curve, the more plus lens power
The flatter the base curve, the less tear lens power
The steep part of the cornea that usually includes the geometric center
Apical zone
Usually 3-4mm
This is the steepest point of the cornea
Corneal apex
What is the usual degree between the pupillary axis and the light of sight?
3-5 degrees
What is the vertex distance?
The distance between the back surface of the lens and the front surface of the cornea
What is the formula for the downstream optical power?
Power/(1-transposition (in m) * power)
How do plus and minus lenses affect the retinal image?
Plus lens magnifies
Minus lens minifies
So with CLs, less magnification for a plus and less minification for a minus lens
When switching myopes from glasses to contacts what may happen?
Highly myopic patients may have better VAs in contacts b/c of the minification that happens with glasses
Hyperopes have the opposite effect - less VA with contacts
This is important for aniseikonia and aphakia***
In myopia, the need to converge is ___ with a CL
More
In hyperopia, the need to converge is ___ with CL
Less
In myopia there is ___ accommodative demand with CLs
More
A myope when viewing a near object accommodates more with CLs than white spectacles
In hyperopia ____ accommodative demand is needed with CLs
Less
A hyperope when viewing a near object will accommodate less with CLs
Who is credited to first thinking about contact lenses?
Leonardo DaVinci
What material supposed to have the best oxygen permeability
Silicone hydrogel
