Optics of contact lenses - look at book notes

Question 1

What is the standard BVD distance when dispensing glasses?

Answer 1

12mm

Question 2

What does ocular Rx refer to?

Answer 2

Rx at ocular surface - so this would be used for contact lenses

Question 3

Do you need to convert from a Spec Rx to an Ocular Rx if the power is less than 4.00 in any meridian?

Answer 3

No because the difference is minimal ( but you do need to convert if power is 4.00D or above)

Question 4

What are the Two types of Astigmatism?

Answer 4

Lenticular Astigmatism and Corneal Astigmatism

Question 5

What is refractive astigmatism?

Answer 5

Refractive Astigmatism = Lenticular Astigmatism (LA) + Corneal Astigmatism (CA)

Question 6

America VS England in how they measure corneal astigmatism:

Answer 6

In America they measure it in Dioptres whereas in England we measure it via radius.

e.g. in America they would say: 45.00D at 180 and 46.00D at 90

Whereas in England we would say 7.50 mm at 180 7.30mm at 90

(The difference in both cases is -1.00 which is your Corneal Astigmatism)

Question 7

What is the rule of thumb for working out corneal astigmatisms from the radii reading we get from doing keratometry?

Answer 7

For every 0.05mm difference there is a change of 0.25D

Question 8

We get the Refractive astigmatism from Spectacle Rx in glasses wearers and from Ocular Rx from contact lens wearers.

Answer 8

So for contact lens equations we would use astigmatism from ocular Rx as our refractive astigmatism.

Ocular Astigmatism= Corneal Astigmatism (CA)+LenticularAstigmatism(LA)

Question 9

How can we work out Corneal Astigmatism (CA)?

Answer 9

From the difference in Keratometry readings at each meridian

Question 10

How can we work out Lenticular Astigmatism?

Answer 10

From the equation:

Ocular Astigmatism= Corneal Astigmatism (CA)+LenticularAstigmatism(LA)

Question 11

If all your refractive astigmatism is coming from the cornea then which lens is best?

Answer 11

RGP lens is very good at correcting corneal astigmatism

Question 12

How does adding a Spherical RGP contact lens mean corneal astigmatism is corrected?

Answer 12

PLEASE LOOK AT NOTES FOR ALL THE PROOF OF MATHS.

Adding a spherical contact lens means corneal astigmatism is corrected because interface at which we measure astigmatism has changed. When we place a Spherical RGP on the cornea, tears replace the air (and cornea replaces tears) in interface equations ( basically the equivalent of keratometry readings).

Basically when you fit an eye with a Spherical RGP the tears make up the gap between the cornea and contact lens thus neutralising corneal astigmatism.

Thus RGP corrects 90% of corneal astigmatism.

Question 13

What astigmatism does a Spherical RGP not neutralise?

Answer 13

Lenticular astigmatism

Question 14

What is the condition for using a spherical RGP to correct corneal astigmatism?

Answer 14

A spherical RGP can correct up to 2.50D of astigmatism.

(There is a limit to how many tears the eye has to fill that space between the cornea and lens)

Question 15

If a patient wants an RGP and the corneal astigmatism is above 2.50D what type of lens do you prescribe?

Answer 15

A TORIC RGP

Question 16

How do you correct lenticular astigmatism with an RGP?

Answer 16

You prescribe a FRONT SURFACE TORIC RGP lens to neutralise lenticular astigmatism.

Question 17

Can you have a toric RGP with a front surface toric too?

Answer 17

Yes this would be for a Px that has corneal astigmatism above 2.50D and lenticular astigmatism too.

Question 18

If a Px has astigmatism and wants a Soft Contact Lens (SCL) what do you prescribe?

Answer 18

Toric Contact lens is needed to correct ANY astigmatism in a SCL.

Question 19

What is a potential problem with a toric SCL?

Answer 19

Needs to be stabilised to avoid rotation

Question 20

In general are toric lenses as stable as spherical lenses?

Answer 20

No

Question 21

What does ATR or Against The Rule mean?

Answer 21

It lies along 90 degree axis

Question 22

What does WTR or With The Rule mean?

Answer 22

It lies along 180 degrees

Question 23

What are perceptual effects to consider when switching form Glasses to Contact lenses?

Answer 23

When Pxs convert from glasses to CLs there is some possible distortion especially if Px has particularly large oblique cells. If patients have large oblique cells the retinal image is actually a little tilted however, the Px’s brain is use to interpreting this as normal.

CLs correct the retinal image causing it to be straight thus this can feel like a distorted view to someone who’s brain has interpreted a tilted retinal image to be ‘normal’.

Question 24

How is visual acuity affected by wear of contact lenses as a pose to glasses?

Answer 24

Spectacle magnification is related to Back Vertex Power (BVP) and Back Vertex Distance (BVD).

Thus Myopes get more magnification with CLs as a pose to glasses so a potentially improved VA.

Hyperopes get less magnification with CLs as a pose to glasses (because BVD is n longer increasing power of the lens) so a potentially reduced VA

Question 25

How does prismatic effect differ with use of CLs as a pose to glasses?

Answer 25

With glasses myopes receive a slight base in at near (when reading) which is lost with CLs- this is something that might throw the Px off.

Opposite with hyperopes - it is easier for them to read with CLs as a pose to glasses.

Question 26

How does accommodation differ with use of CLs as a pose to glasses?

Answer 26

For Myopes - spectacles delay/postpone presbyopia. In fact there are earlier signs of presbyopia with CLs.

For Hyperopes- CLs delay presbyopia

Question 27

How does refractive spectacle magnification differ with use of CLs as a pose to glasses?

Answer 27

There are two types of anisometropia; refractive and axial.

Refractive anisometropia - works well with CLs

Axial anisometropia- doesn’t always work well with CLs.

Question 28

What are the two types of anisometropia?

Answer 28

Refractive anisometropia - a difference in power of the eye of equal to or greater than +/- 2.00D. This difference in power is caused by refractive problems of the eye e.g. lens and cornea.

Axial anisometropia - this is a difference in power of the eyes of equal to or greater than +/-2.00D. This is caused by one eye being larger thus it has a larger axial length ( distance from lens to retina).

Question 29

Despite concerns that axial anisometropia may not work as well with contact lenses why may you still prescribe?

Answer 29

Px may still want CLs because they are visually more comfortable as you avoid the heavy frame and bad cosmesis.

Question 30

How does FOV (Field of View) differ with use of CLs as a pose to classes?

Answer 30

Larger FOV with CLs as they are not limited to the frame size. Thus less blind spots.

This is appreciated by Px with a high positive Rx (as positive sph decreases field of view)

You achieve a more natural view with CLs

Question 31

How do priscribed prisms differ with CLs as a pose to glasses?

Answer 31

Spectacles allow for prism to be:

• divided between the eyes (Vertically AND horizonatlly)
• fresnel prisms for high values

With CLs, however:

• Normally only base down prism can be fitted and at a max of 3 prism dioptres.
• Vertical prisms cannot be divided
• Horizontal and base up prisms are only available to be prescribed in scleral lenses.