Unit 1 Flashcards
With the rule astigmatism
-Greatest refractive power is w/in 30 degrees of the verticle meridian
-Minus cyl axis around the horizontal meridian
-Most common type
-Steeper in the verticle
-Most common in children
Against the rule
-Greatest refractive power is within 30 degrees of the horizontal meridian
-Minus cyl axis around vertical meridian
-Most common in adults
-Steeper in horizontal
Oblique astigmatism
-Greatest refractive power is w/in 30 degrees of the oblique meridians
-Between 30 and 60 or between 120 and 150
Irregular astigmatism
-The 2 principal meridians are NOT perpendicular to each-other
-Associated with trauma, disease or degeneration (kerataconus, pellucid marginal degeneration, scaring)
What is keratometer
An instrument used to measure the front surface of the cornea
-Curvature power, degree and type of corneal astigmatism
-DOES NOT measure any lenticular astigmatsim
-Average is +43D
What does keratometer measure
-Measures central 3mm of cornea
-36-52 diopters
-Addition of a -1.00D auxiliary lens extends range to 30D to get a flatter reading
-Addition of a +1.25 auxiliary lens extends range to 61 D to get a steeper reading
How do you calculate corneal astigmatism
- Find the difference in the Ks by subtracting (ignore signs)
2 Place a minus sign in front of your answer
3, The meridian of your SMALLER number becomes the axis
EX.. -1.00D x 180
How to transpose RX
- Add the first 2 numbers together algebraically
- Change the sign of the cylinder
- Change the cylinder axis by 90 degrees so the new number falls between 1 and 180
What lenses have Tear lens / lacrimal lens??
GP lenses have a layer of tears that form between the posterior surface of the CL and the anterior surface of the cornea
Does a soft CL have tear lens?
NO, it is plano power
Rule of thumb for base curves
0.1mm BC radius is equivalent to 0.50D
ex. if there is a 0.2mm change that means we change the power by 1.00
TRUE OR FALSE
if a patient has astigmatism <2.50D then a spherical GP lens might work
TRUE // the lower amount of astigmatism can be neutralized by the tear film
can we ‘ignore’ the cly value during sam/fap?
Yes
What is fluroscein used for ?
-Help visualize corneal damage - dry eyes, corneal abrasions, infections etc..
-Helps to visualize different characteristics of a GP lens fit
Fluroscien pattern for WTR cornea
Bow tie shape
-Steeper in the verticle / flatter in horizontal
Fluroscein pattern in ATR cornea
Figure 8 shape
-Steeper in horizontal flatter in verticle
What is residual astigmatism?
The amount of astigmatic refractive error that remains uncorrected with a CL applied to the cornea
“left over” astigmatism
Residual astigmatism formula
Residual astigmatism = spectacle astigmatism - corneal astigmatism
Types of residual astigmatism?
-Physiologic
-Induced
Physiologic RA
From curvature refractive index differences between the corneal and crystalline lens
Induced RA
Caused from lens application
EX.. warpage, flexure, decentration
Can RA very depending on what type of CL being used?
Yes it depends on whether it is spherical GP or spherical soft CL
To calculate RA what 2 pieces of info do you need?
- The spectacle astigmatism (cyl in the gls)
- The corneal astigmatism (calculated from the K values)
When solving for RA and it asks for minus cyl what do we do ?
We need to transpose it so we switch the power of the cyl and change the axis so it is 90 degrees different from current axis
T OR F
Do we need to vertex even when the spec rx is given?
YES
We need to find the spectacle rx that has been vertexed to the corneal plane
ALWAYS VERTEX
Why is over refraction needed ?
To determine if a change in the final lens power is necessary
Goal is to maximize the pxs VA
Why may we have a significant power in our over refraction?
-If tear lens is not fitting on alignment, unwanted power is created
-Fitting a spherical GP lens on a toric cornea
What data do we need to solve OR problems”?
-The spec correction-vertex if needed
-Pxs K readings
-The power of the diagnostic GP lens (often -3.00
Things to calculate/solve for during OR
-Residual astigmatism
-The spherical portion of the OR
-Impact of the tear lens from the CL base curve
Spherical OR formula
Sphere power @ the corneal plane - diagnostic sphere power of GP lens
Steps to solve OR problems
- Find the RA (cyl power and axis of the RA = cyl power and axis of the OR)
- Find the spherical power of the OR
(sphere power @corneal plane - diagnostic sphere power of GP) - Apply the SAM FAP rule to your spherical OR power from step 2 if the CL is NOT an on-k fit
- The final answer to the OR problem is the sphere power from step 3 combined with the cyl power and axis from step 1
