8 Astigmatism Optics RGPs & Soft Lenses

How does vertex distance affect the amount of cylinder/astigmatism when converting from spectacle plane to corneal plane?

Vertexing changes the effective power of each principal meridian.

• Minus lenses/myopes: minus power becomes less minus at the corneal plane, so they have less astigmatism at the corneal plane.

• Plus lenses/hyperopes: plus power becomes more plus at the corneal plane, so they have more astigmatism at the corneal plane.

When vertexing an astigmatic spectacle Rx, what is the correct general approach?

Convert the Rx into its two principal meridians, vertex each meridian separately, then compare the two corneal-plane powers.

What are the major ocular sources of astigmatism?

Astigmatism can come from:

1. Cornea

2. Cornea & Crystalline lens

3. Crystalline lens only

If the spherical refractive error is caused by axial length, does that explain the cylinder?

No. Axial length can explain the spherical myopic component, but cylinder requires unequal power in different meridians. Therefore, the cylinder must come from differences in curvature or power from the cornea, crystalline lens, or both.

Why might spectacle cylinder not match corneal cylinder in amount or axis?

Spectacle cylinder represents the total refractive astigmatism of the eye, not just the cornea.

Why can corneal cylinder and spectacle cylinder be at completely different axes?

The cornea and crystalline lens can each contribute cylinder at different axes. These astigmatic components combine vectorially, so the final spectacle cylinder may shift toward a different axis or amount than the cornea alone.

Why is it clinically important to compare corneal astigmatism with spectacle astigmatism before fitting contact lenses?

Comparing corneal and spectacle cylinder helps determine whether astigmatism is mainly corneal or internal/lenticular.

• If spectacle cyl ≈ corneal cyl: astigmatism is mostly corneal.

• If spectacle cyl differs in amount or axis from corneal cyl: internal/lenticular astigmatism is likely contributing.
  This matters because contact lenses, especially rigid lenses, can neutralize corneal astigmatism, but not internal/lenticular astigmatism.

What contact lens factors can induce or alter astigmatism on the eye?

Contact lenses can induce astigmatism through changes in lens shape, position, or alignment. Major causes include:

1. RGP or scleral lens warpage

2. Lens flexure, meaning bending of an RGP/scleral lens on the eye

3. Lens decentration

4. Lens tilt, causing oblique astigmatic aberration

5. Toric contact lens or lacrimal lens surface effects

How can an RGP or scleral lens create unexpected astigmatism even if the lens is designed correctly?

An RGP or scleral lens can create unexpected astigmatism if the lens warps, flexes, decenters, or tilts on the eye.

• Warpage/flexure: changes the effective optical surface of the lens

• Decentration/tilt: causes light to pass through the lens obliquely

What is oblique astigmatic aberration in contact lens wear?

Oblique astigmatic aberration occurs when the contact lens tilts on the cornea, causing incoming rays to pass through the lens obliquely. It is usually a minor source of astigmatic aberration compared to other causes.

How can toric contact lenses or lacrimal lenses induce residual astigmatism?

Toric contact lenses or toric lacrimal lenses can induce residual astigmatism if the cylinder is:

• Obliquely crossed, meaning the cylinder axis is wrong relative to the needed correction

• Under-corrected

• Over-corrected

Why do crossed cylinders matter in contact lens over-refraction?

A contact lens cylinder and the patient’s residual/spectacle cylinder may not line up at the same axis. When cylinders are at different axes, they combine obliquely, producing a new net cylinder amount and axis.

What method is used clinically to determine residual cylinder when a contact lens does not match the spectacle cylinder?

Clinically, the slide says we do not usually calculate using M, J0, and J45. Instead, we use over-refraction to determine the residual refractive error over the contact lens.

Why is over-refraction especially useful when contact lens cylinder and spectacle cylinder are obliquely crossed?

Over-refraction saves time because it directly measures the net residual refractive error after all optical effects are combined, including:

• Contact lens cylinder

• Lacrimal lens effects

• Lens rotation or axis mismatch

• Residual internal/lenticular astigmatism

How is an astigmatic eye’s refractive error neutralized by a contact or lacrimal lens?

An astigmatic eye has excess plus cylinder power at a specific axis. To neutralize that astigmatism, the contact/lacrimal lens must provide an equal minus cylinder at the same axis.
Example:

• Eye has +1.00 × 180 excess power

• Needs −1.00 × 180 contact/lacrimal lens cylinder

When two cylinders are at the same axis, how do you determine the expected over-refraction?

If the eye cylinder and contact/lacrimal lens cylinder are at the same axis, simply add the cylinder powers.

What happens if the eye’s plus cylinder and contact/lacrimal lens minus cylinder are equal in magnitude and at the same axis?

The cylinders cancel each other out, so residual astigmatism is 0.
Example:

• Eye: +1.00 × 180

• Contact/lacrimal lens: −1.00 × 180

• Over-refraction: plano

What happens if the contact/lacrimal lens cylinder is greater in magnitude than the eye’s cylinder at the same axis?

The contact/lacrimal lens overcorrects the astigmatism.
The residual cylinder appears at an axis 90° away from the original axis.
Example concept: too much minus cylinder at 180 leaves residual astigmatism at 090.

What happens if the contact/lacrimal lens cylinder is less in magnitude than the eye’s cylinder at the same axis?

The contact/lacrimal lens undercorrects the astigmatism.
Residual astigmatism remains at the same axis as the original eye cylinder.

Why can’t you use simple cylinder addition when the eye cylinder and contact/lacrimal lens cylinder are at different axes?

Simple addition only works when cylinders are at the same axis.
When cylinders are at different axes, they combine obliquely, so the resultant cylinder has a new:

• Magnitude

• Axis

What is the rule for estimating the axis of the residual cylinder when two cylinders are crossed at different axes?

The residual cylinder axis is approximately:

45° from the midpoint between the plus cylinder axis and the correcting cylinder axis, toward the plus cylinder axis.

Example from the slide:

• Eye plus cylinder axis: 090

• Contact/lacrimal lens cylinder axis: 070

• Midpoint between 090 and 070 = 080

• Move 45° away from the midpoint toward the plus cylinder side

• Expected residual axis ≈ 125

How does the angle between crossed cylinders affect the amount of residual cylinder on over-refraction?

The larger the axis mismatch, the larger the residual cylinder. Approximate rules:

• 30° crossed: residual cyl ≈ 100% of correcting cylinder power

• 15° crossed: residual cyl ≈ 50% of correcting cylinder power

• 5° crossed: residual cyl ≈ 17% of correcting cylinder power

• Values in between can be estimated, such as 20° ≈ 70%

What should you do clinically if a toric contact lens is rotated by about 30°?

A 30° rotation creates a large crossed-cylinder effect, so you should usually try a different fit rather than trying to compensate with small power changes.

Why is a spherical RGP not ideal if the lacrimal lens axis is 30° away from the eye’s cylinder axis?

If the lacrimal lens cylinder axis is about 30° off from the eye’s cylinder axis, the residual cylinder on over-refraction can be large, approximately 100% of the correcting cylinder power.
This means a spherical RGP may not neutralize the astigmatism well. Clinically, consider moving to a soft toric lens or another better-fitting design.

For most lenses in clinic, what should you expect when the cylinder axis mismatch is small, such as 5°?

A small axis mismatch usually produces a very small residual cylinder power on over-refraction.
However, the residual cylinder axis may appear far away, often around 45° from either original axis.
High-yield takeaway: Small axis error = small OR cyl, but weird-looking OR axis. Expect this.