Toric Corneal RGP

Question 1

What is the cause of astigmatism?

Answer 1

When the cornea or lens becomes an irregular shape and as a result light passing through is distorted, causing images to focus at different lengths from the retina.

Question 2

What percentage of refractive errors does astigmatism count for?

Answer 2

It accounts for 13% of all refractive errors

Question 3

What factors does the prevalence of astigmatism vary by?

Answer 3

Age and Ethnicity

Question 4

What are the different types of astigmatism a px can have?

Answer 4

• Corneal
• Lenticular
• Combination – beware of cancelling out when you prescribe RGPS
• • Regular - astigmatism where the principle meridians are 90 degrees apart
• Irregular - astigmatism where the principle meridians are not 90 degrees apart

Question 5

What is residual astigmatism?

Answer 5

Residual Astigmatism = Ocular Astigmatism - Corneal Astigmatism

When a spherical RGP lens is placed on the cornea, it will correct corneal astigmatism but not lenticular astigmatism. This lenticular astigmatism gets to be known as residual astigmatism.

Question 6

What is induced astigmatism?

Answer 6

Induced Astigmatism is created when a toric back surface is placed on a toric cornea. It is characterized by the differing refractive indices of the contact lens and the tear film beneath

Question 7

What is corneal astigmatism?

Answer 7

Astigmatism arising from just the cornea. - It can be measured using a keratometer.

Question 8

What is ocular astigmatism?

Answer 8

Astigmatism taken directly from the spectacle astigmatism after adjusting for BVD

Question 9

What is spectacle astigmatism?

Answer 9

Can occur either from corneal and/or lenticular astigmatism. Measured in the spectacle plane.

Question 10

We can fit a spherical RGP contact lens on a toric cornea - what is the advantage of this?

Answer 10

It is simple and inexpensive

Question 11

What is the disadvantage of fitting a spherical RGP (spherical base curve) on a toric cornea?

Answer 11

– Fluctuations in vision as a result of:

• There may be excessive movement
• Lens may not centre well

– May cause corneal distortion:

– Lens flexure

Question 12

How can you improve the fitting of a spherical lens on a toric cornea?

Answer 12

• Choosing a lens with a small diameter will help to minimise the exaggeration between the two meridians on the corneal surface. (Which in turn will avoid excessive edge clearance in the steeper meridian).

[Note that smaller diameter lenses often feel more uncomfortable - furthermore smaller diameter lens means a smaller back optic zone which if too small for the pupil will result in flare/halo].

• Aspheric designs - these generally have a narrower edge lift, reduced edge clearance along the steeper meridian which should encourage the lens to centre more accurately and give a better visual result.
• Thin lenses - If your px has WTR corneal astigmatism and ATR lenticular astigmatism use a thin lens which will flex and correct the residual astigmatism
• Steeper BOZR - Some practitioners believe steepening the BOZR to halfway or a third of the corneal astigmatism will create a more stable fitting lens.

Question 13

What is the rule of thumb for fitting a toric cornea with a spherical RGP lens where corneal astigmatism exceeds 1.50D?

Answer 13

BOZR should be decreased by 0.05 mm for each 0.50 D that the corneal astigmatism exceeds 1.50 D.

[Bear in mind though that this isn’t recommended - at this point a toric lens should be chosen - using a sph RGP despite the rule will cause longterm staining]

Question 14

What are some problems with fitting a spherical RGP lens on a toric cornea?

Answer 14

• Lens tends to ride high if held up by eye lid or drops and rides low
• Lens may be easily lost from eye due to lid catching lens edge
• Excessive movement and ‘rocking’ on blinking
  *

Question 15

What may be some indications that an astigmatic px cannot be fitted with a spherical lens?

Answer 15

• Px notes Reduced comfort – Area of alignment is reduced; excessive edge clearance leads to unwanted lid interaction
• 3 and 9 o’clock staining – Due to a lack of lens movement caused by reduced edge clearance
• Poor centration – The lens will ‘rock’ or decentre
• Corneal moulding which leads to → spectacle blur
• Residual astigmatism

Question 16

When would you choose a spherical RGP lens?

When would you choose a back surface toric RGP lens?

When would you choose a front-surface toric lens?

When would you choose a bitoric RGP lens?

Answer 16

Question 17

True or false - spherical soft contact lenses can correct small amounts of astigmatism up to 1D.

Answer 17

False - Soft contact lenses need to be toric in order to counteract any astigmatism as they need to be stabilised to avoid rotation.

Question 18

True or false- One of the biggest struggles with soft contact lenses is that they need to be stabilized in order to avoid rotation

Answer 18

True - which if fails to be done properly is the biggest reason px don’t adapt to soft contact lenses well

Question 19

True or False - If we put a back surface toric RGP lens on an eye, the fluorescein pattern should be the same as one of a spherical RGP.

Answer 19

True

Question 20

When prescribing a back surface toric lens do we need to be concerned with stabilising the lens, and if so why?

Answer 20

No - this is because the lens radii are aligned with each of the principal meridians of the cornea- with a closely aligned fit, lens rotation is therefore minimal.

Question 21

What are your three main steps for ordering a back surface toric RGP lens?

Answer 21

• Measure the steepest and flattest corneal radii using keratometry
• Calculate ocular prescription ( from spectacle rx - some labs may do this for you already)
• Specification for your TD (Total Diameter) should be based on the same criteria used for a spherical lens design

Question 22

Why will any Back Surface toric RGP lens result in induced astigmatism?

Do we need to worry about this, and if so what do we need to do?

Why may some people argue that a back surface toric is technically a bi-toric lens?

Answer 22

Any back surface Toric lens will result in induced astigmatism due to the toroidal back surface and the difference in n value between the tears and the lens material.

This requires the need for a small neutralizing correction on the front surface of the lens - but you don’t need to be concerned about that as the manufacturer will calculate the amount required to correct induced astigmatism and pop it on the front surface.

Because the front surface contains the corrective cyl for the induced astigmatism that comes along with any back surface RGP lens

Question 23

True or False- An aligned fitting back surface toric should not require any stabilisation.

Answer 23

True

Question 24

What is the difference between a Bi-toric RGP lens and a Back Surface Toric RGP lens?

Answer 24

A (true) Bi-toric RGP lens corrects both Lenticular Astigmatism (on the front surface) and Corneal astigmatism ( on the back surface).

A Back Surface Toric RGP lens corrects only Corneal Astigmatism.

Question 25

Will a rotating back surface toric cause visual problems?

Answer 25

A rotating back surface toric is aligned to both principal meridians of the cornea and so will not cause any visual problems unless the lens has also been corrected for (small amounts) of residual astigmatism ( i.e. lenticular astigmatism on the front surface).

[Note that a back surface toric can be adapted to correct small amounts of residual astigmatism yet a TRUE Bi-toric is one that corrects significant amounts of lenticular astigmatism]

Question 26

True or false - Toric RGP lenses tend to be thicker than spherical lenses.

Depending on whether this is true or false what would you, therefore, recommend to your px when prescribing them a toric lens?

Answer 26

It is true - as a result you would recommend a lens with a higher DK value

Question 27

What is a front surface toric?

Answer 27

It is essentially a Spherical RGP lens ( for good-aligned fit) with a toroidal front surface in order to counteract any residual astigmatism ( i.e. lenticular astigmatism).

Question 28

Will stabilisation be required in order to fit a front-surface toric lens?

Answer 28

Yes - in order to maintain the correct axis

Question 29

What is a typical method for prescribing a Front Surface RGP lens?

Answer 29

You will order a front surface trial lens from the manufacturer that is aligned to the flattest k (which is essentially a spherical lens with a dot on it).

You assess the fit of the lens - (which should have an appearance similar to a spherical RGP lens).

And then you perform an over-refraction, both of sphere and cyl. You make an estimation of the lens rotation by observing the movement of the dot on blink.

Typically some movement/rotation of around 5-10 degrees shall be seen nasally.

Ideally, you want the dot to not move (i.e. the px blinks causing the lens to move up then down but you want the dot to return IMMEDIATELY back into the same position it was in before).

Question 30

How do you know if a front surface toric RGP lens is fitted correctly?

Answer 30

The dot on the lens does not change axis after a blink ( i.e. when the px blinks the lens moves up and then falls back down, however, the dot should go back to its original position)

Question 31

True or false - Flatter lenses don’t rotate as much

Answer 31

False - Flatter lenses rotate more , steeper lenses rotate lens

Question 32

What are our two methods for stabilising a toric lens?

Answer 32

Prism Ballast and Truncation - both of these methods apply to stabilizing front surface torics or Bi-torics.

[Technically back Surface torics shouldn’t need stabilisation as they are aligned perfectly to the principal meridians of the cornea]

Question 33

What are the two disadvantages of the Prism Ballast Method for stabilising a lens?

Answer 33

The thicker lower edge of the lens may cause it to be more uncomfortable and it may cause the lens to drop.

Question 34

What are the drawbacks of the truncation method for stabilising a lens?

Answer 34

Discomfort for the px.

Ineffective for lens with a small TD (Total Diameter) or lid below the limbus

Question 35

True or False- Front surface Torics are used to improve VA where corneal astigmatism is present

Answer 35

False - Front surface torics are used to improve VA where residual Astigmatism is present

Question 36

What is prism Ballast?

Answer 36

A method of stabilising a toric lens.

It uses prism of 1.5-3.00 △D and attaches this to the bottom of the lens ( it is attached base down - so thickness lies at the bottom of the lens).

Question 37

If only one eye requires a front-surface toric why would using prism ballast to stabilise this lens be a bad idea?

Answer 37

Prism ballast stabilisation of only one lens may disrupt binocular vision ( as literally only one eye is receiving prism).

Question 38

Why may front surface stabilisation methods induce flare?

Answer 38

As front surface stabilisation methods typically inolve the lens sitting a little lower (inferiorly decentered).

Question 39

Despite an RGP toric lens giving better vision than a soft toric contact lens - when might a soft toric lens be a better option?

Answer 39

For Pxs that have high degrees of lenticular astigmatism only

For Pxs that have high corneal Sensitivity

Question 40

When talking to the patient about fitting Toric RGPs what must you make sure they are aware of?

Answer 40

That toric RGP fittings require more appts.

Adaptation may be required even for existing (sph) RGP wearers due to the extra thickness (especially if prism ballast method is used to stabilise the lens).

Manage expectations - especially in terms of comfort!

Question 41

What is truncation?

Answer 41

A method used for stabilising a toric lens.

[It can be sued in conjunction with prism ballast]

It is essentially the process of removing a chord from the lens (this is typically done at the bottom but a chord can also be removed from the top - in this case, it is called double truncation).

Question 42

What does it mean for a lens to be stable?

Answer 42

Every lens will move upon blink in the vertical meridian i.e. when u blink the lens moves up then back down. This is normal.

When we talk about stabilising a lens we want to reduce rotation of the lens upon blinking.

For a lens to be stable you want least lens roattion on blink as possible ( this depends on the power though - whereas 5 degrees of rotation on blink would be acceptable for a lens of -1.00D it would not be acceptable for a lens of -8.00D as it would cause a significant decrease in VA)

Question 43

What is overrefraction?

Answer 43

It is when you check the power of a contact lens by conducting subjective (Best Vision Sph mainly) on a px with their contacts already on - to see if you can improve vision.

(In a toric lens you would also be checking Cyl axis.

Question 44

What is it about having a toroidal front surface that throws stabilisation off?

Answer 44

EVERY lens rotates to some degree however with toroidal lenses, the weight of the lens is not evenly balanced all over (it is thicker in the cy meridian) thus this causes more rotation of the lens.

Question 45

To what degree can a Back Surface toric correct residual astigmatism and still be referred to as a back surface toric?

Answer 45

We can get away with about 0.50D to 0.75D front surface correction and still call the lens a back surface toric.