Gas Permeable Lenses for Astigmatism - Advanced Contact Lenses Fall 2026

How to manage this patient?

MR: -2.00-1.50x180

Ks: 42/42.50 @ 090

-1.00 uncorrected cyl present. First would fit them into a soft lens. if not a fan of the soft toric then would fit into a front surface GP. This patient would benefit from a front surface toric GP lens design

How to manage this patient?

MR: -4.00-3.50x010

Ks: 43/46.50 @ 100

This patient will need a bitoric lens design and will not be a good candidate for a spherical GP

How to manage this patient?

MR: -1.00-3.00x020

Ks: 42.50/44.50@110

This patient has corneal cyl 2/3 of spec cyl. We are able to fit a back surface bitoric GP lens for this patient.

How to measure corneal cyl?

-keratometry

-OPD

-tomography

-topography

Where is lenticular cyl from?

from the crystalline lens

What is the standard power of the crystalline lens?

pl - 0.50 x 090

What is refractive cyl?

Corneal cyl + lenticular cyl

What is residual astigmatism?

Astigmatism that remains uncorrected

Tear/lacrimal lens from spherical GP will correct _____ astigmatism

corneal

True or False:

Residual astig can be a result of lenticular cyl

true

Can residual astig be induced by a certain lens design?

Yes -- possibly d/t lens flexure (lens flexure - when one BC meridian is on K but the other is flatter than K = lens will align with on K (flat) meridian and the lens will bend on steep meridian)

True or False:

A soft toric lens can always be used to correct astigmatism

true

What is the problem with using a soft toric lens to correct astigmatism?

The soft lens optics may not provide as high of quality vision as a GP option for some astigmatic patients. With higher spec cyl the less of a chance the patient will be happy with soft torics because they’re more sensitive to cyl induced by rotation.

For every 10º rotation of a soft toric Cls, you will induce _____ of the cyl power on the eye

1/3

Which induce LESS cyl power on the eye with rotation?

GPs or soft toric lenses

GPs

How do GP lenses correct astigmatism?

-with the lacrimal lens

-BC of a spherical lens is selected to fit the cornea

-power of the lens is determined by the curve on the front of the lens

Why would you select a toric GP lens?

When spherical GPs do NOT correct all of the refractive error

Does corneal cyl always = refractive cyl?

No

Does the tear lens correct all of the astigmatism?

No

What are the different types of GP Toric lenses?

-Front Surface GP toric

-Back Surface GP toric

-Bitoric GP (SPE and CPE)

What is the benefit of front surface toric lens?

corrects refractive astigmatism

When to choose a front surface toric lens?

>/= 1D residual astig

</= 2.00 corneal toricity

Front surface toric lenses are typically ____ lenses

soft toric

Do soft toric lenses sometimes result in a poor fit?

Yes -- cornea too steep or too flat for soft torics

What does a front surface toric lens require?

lens ballast

Why does a front surface toric lens require a lens ballast?

to maintain cyl axis

How is a lens ballast formed in a front surface toric lens?

BD prism (1.25 PD on avg)

Ballasted front surface toric lenses usually rotate 10-15º ____

nasal

You can adjust for rotation in front surface toric lenses that rotate with ______

LARS

Front surface toric lenses should be orded with a _____ at the base of the prism

dot

True or False:

front surface toric lenses can include truncation

true

How to order a front surface toric lens?

Requires BC, Diameter, Prism, & Power in Spherocyl notation form with axis indicated

When ordering front surface toric lens, you should always indicate the (front/back) vertex power

back

When measuring the power of a front surface toric lens, where should the dot be on the lens?

At 6 oclock

Why would you fit a GP toric lens instead of a Spherical GP?

-High amounts of corneal toricity

-Spec blur

Spherical GPs (do/ do not) fit well or center on a cornea with >2.50D corneal cyl

do not

Spherical lenses fit too flat or steep will cause what?

corneal molding

A GP Toric will improve fit when corneal cyl is above what amount?

>2.00D

Will Toric GP lenses correct ALL of the cyl power?

No

How to correct additional cyl that is NOT corrected by a GP Toric lens?

correct additional cyl on the front of the GP lens (bitoric design)

What is a bitoric GP?

Toric curves on the front and back surface of the lens

Back toric surface of a bitoric lens will fit what?

the cornea

Front toric surface of a bitoric lens will create what?

the toric lens power

Corneal cyl >2.50D results in a _____ fit of the spherical GP

poor

Bitoric GPs can be fit as low as _____D

1.50

Bitoric GPs can be fit as low as 1.50D if the toricity is of what type?

limbus to limbus

Limbal to Limbal vs Central Cyl (Pic)

Limbal to Limbal vs Central Cyl (Pic)

The majority of GP toric lenses are of the _____ type

Bitoric

What is a bitoric GP?

two curvatures on the back surface and two curvatures on the front surface

What do you have to specify for a bitoric GP lens?

Specify power and BC for each meridian

When fitting bitoric GP, you are essentially fitting a _____ lens on each meridian independently

spherical

True or False:

When fitting a bitoric GP, you decide what the lacrimal lens power will be

true

When fitting a bitoric GP, you should use what?

an optical cross

Is the Power and BC for a bitoric GP written in spherocyl notation?

No -- write 2 BCs w/ 2 powers

What are the 2 fitting philosophies for a GP with a toric posterior surface?

-Saddle Fit

-Low Toric simulation

What is a saddle RGP fit?

full alignment

When to use a saddle RGP fit?

Can be used with any bitoric GP, but often used over LTS with 2-2.50D K toricity, ATR, or Oblique astigmatism.

What is the benefit of a saddle fit?

Creates a spherical lacrimal lens

How to design a saddle fit?

Fit with the same amount of BC change in each meridian

How will a saddle fit lens fit on the eye?

-Tighter fit that is more centered

-Smaller diameter

-Inter palpebral fit

What type of fitting philosophy is used when toricity >2.50D?

Low toric simulation

The BC in a low toric simulation will (over/under) correct corneal toricity

under -- will align with the flat meridian and undercorrect the steep meridian

What will happen over the flat meridian in a low toric simulation fit?

The lens will rock over the flat meridian by a small amount

Diameters of RGPs can be (smaller/larger) with low toric simulations

larger

With a low toric simulation fit, the lacrimal lens will be ____

toric

Low toric simulation fit will work better with (WTR/ATR) astigmatism

WTR

How to calculate bitoric lenses diagnostically?

-spherical trial lens

-SPE trial lens

How to calculate bitoric lenses emperically?

-optical crosses

-GPLI website

-Mandell-Moore Bitoric Worksheet

Is diagnostic fitting for bitoric lenses practical?

No

Mandell-Moore Bitoric Guide (Pic)

Mandell-Moore Bitoric Guide (Pic)

What are the fitting philosophies for bitoric lenses?

-Saddle Fit

-Low Toric Simulation

What are the methods of fitting bitoric lenses?

-Diagnostic

-Empirical

Describe Spherical Power Effect (SPE) of bitoric lenses in terms of BC and Power

Change in BC = Change in Power

Describe Cylinder Power Effect (CPE) of bitoric lenses in terms of BC and Power

Change in BC does not equal Change in Power

How to figure out SPE or CPE fit?

After you design the bitoric lens, calculate the difference in the BC and the difference in the Power of the lens

What does an SPE fit essentially create?

An optically spherical lens

What is the benefit of fitting an SPE fit?

Rotation does not affect vision

Any cyl induced by rotation is corrected by the tear lens

What is the negative aspect of a CPE fit?

Rotation will induce cylinder power

How to make a CPE a SPE fit?

Alter power to make difference in Power the same as difference in BC

Why would you want to change a CPE fit to an SPE fit by altering power?

changes result in more visually acceptable residual astigmatism

We want to make sure the fit of a RGP is stable to minimize _____

rotation

If the patient is not 20/20 with spherical OR, what can we do?

SCOR

What to assess on a GP Toric when on the eye?

-Movement

-Centration

-Lid interaction

-Rotation

-Fluorescein pattern

Lens with a spherical front surface & toric back surface?

Back surface toric lens

Lens with a toric front surface & toric back surface?

Bitoric lens

Lens with a toric front surface & spherical back surface?

Front surface toric lens

A back surface toric will improve fit when corneal cyl is ____D

>2.00

Back surface torics can induce _____

cyl -- induced cyl creates the wrong power of the lens

Are back surface torics usually used?

No

What is the special circumstance in which back surface toric lenses are used?

Corneal cyl = 2/3 (66%) spec cyl

Describe a back surface toric RGP

Back surface is toric, front surface is spherical

Because there is only 1 front power, the RGP will induce cyl created by the ________ from lens material to the tears

changes in index of refraction

Induced cyl of a back surface toric lens is created by the _____

lens

Since a back surface toric ONLY has toric power on the back on the lens, how would you order this lens?

Only order the one spherical power w/ toric BCs

Why is cylinder power INDUCED when a back surface toric is on the eye? The cyl power will be different than what is measured on a lensometer...

D/t the toricity of the back surface resulting in differences in the power in each meridian and the change in index of refraction

Induced cyl is the cyl on ____

the eye

BC toricity is the cyl on the ______

radiuscope

Air cylinder is the cyl on the ____

lensometer

True or False:

A back surface toric lens is usually NOT good at correcting astigmatism

true

WHEN is a back surface toric lens good at correcting astigmatism?

When the cyl corrected predicted residual astigmatism, which is when spec astigmatism is approx 1.5x the corneal toricity (3:2:1 rule)