10 Lacrimal Lens GP Optics

How can spherical equivalent be calculated using the two principal meridians?

Spherical equivalent can be calculated by averaging the powers in the two principal meridians.

Why do we use spherical equivalent in residual astigmatism?

Spherical equivalent is used to place the circle of least confusion on the retina.

• This gives the best possible vision when residual astigmatism remains.

• It does not eliminate astigmatism.

• It balances blur between the two focal lines.

What is the lacrimal lens, also called the tear lens, in RGP wear?

The lacrimal lens is the layer of tear fluid trapped between:

• The back surface of the hard/RGP contact lens, especially the base curve radius/BCR

• The front surface of the cornea, described by Ks

Why can the tear lens become toric even if the RGP lens is spherical?

The tear lens becomes toric if the cornea is toric.

• A spherical RGP lens does not perfectly match a toric corneal surface.

• Different corneal meridians create different tear layer thicknesses.

• Therefore, the tear layer has different powers in different meridians.

How does a spherical RGP align on a toric cornea, and where does tear pooling occur?

A spherical RGP tends to align with the flat corneal meridian rather than fully paralleling the corneal contour.

• This leaves excessive clearance in the steep meridian.

• Tears pool where the cornea is steepest.

• This creates a toric tear lens.

How does a spherical RGP align on a toric cornea, and where does tear pooling occur?

A spherical RGP tends to align with the flat corneal meridian rather than fully paralleling the corneal contour.

• This leaves excessive clearance in the steep meridian.

• Tears pool where the cornea is steepest.

• This creates a toric tear lens.

How can a spherical RGP correct corneal astigmatism?

A spherical RGP can correct corneal astigmatism by creating a toric tear lens that neutralizes the corneal toricity.

Mechanism:

• Spherical RGP aligns with the flat meridian.

• Extra tear pooling occurs in the steep meridian.

• The tear lens power helps neutralize corneal astigmatism.

What is residual astigmatism?

Residual astigmatism is the astigmatism that remains after placing a contact lens on the eye.

• It may remain because not all astigmatism is corrected by the contact lens.

• It can be intentionally left uncorrected or under-corrected.

• In RGPs, the lacrimal/tear lens may correct some or all of the corneal astigmatism.

Why might residual astigmatism be intentionally uncorrected or under-corrected?

Residual astigmatism may be left intentionally if full correction is not practical or tolerated.

Why is residual astigmatism with a spherical RGP not always the same as spectacle cylinder?

A spherical RGP can create a toric lacrimal lens, which may correct some or all of the corneal astigmatism.

• Spectacle cylinder includes total ocular astigmatism.

• RGP tear lens can neutralize corneal astigmatism.

• Any remaining astigmatism after the tear lens effect is often due to internal optics, especially the crystalline lens.

After a spherical RGP neutralizes corneal astigmatism, what usually causes the remaining astigmatism?

The remaining astigmatism is usually due to the crystalline lens.

• The toric lacrimal lens may correct corneal astigmatism.

• Astigmatism that remains after this correction is internal astigmatism.

• Internal astigmatism often comes from the crystalline lens.

What is measured residual astigmatism?

Measured residual astigmatism is the astigmatism measured by over-refraction after fitting a contact lens.

• It is found clinically after the lens is on the eye.

• It tells what cylinder remains after the contact lens and tear lens effects.

• This is the real-world measurement of remaining astigmatism.

What is predicted residual astigmatism?

Predicted residual astigmatism is the astigmatism mathematically predicted to remain after fitting a contact lens.

• It is calculated from pre-fitting optical measurements.

• It estimates what astigmatism should remain after the lens is placed.

• Ideally, it should match the measured residual astigmatism.

How should measured and predicted residual astigmatism compare?

They should be the same if all measurements are perfect.

• Measured residual astigmatism: found by over-refraction after fitting.

• Predicted residual astigmatism: mathematically predicted before/around fitting.

• In reality, the values are usually very close, but not always exactly identical.

What two formulas are needed to empirically calculate spherical RGP lens power?

The two key formulas are:

1. LL = BCR - K

   • LL = lacrimal lens power

   • BCR = base curve radius

   • K = central corneal curvature

2. CLP = SpecRx vertexed - LL - RA(or)OR

   • CLP = contact lens power

   • RA = residual astigmatism

   • OR = over-refraction, which represents residual astigmatism

What is the overall workflow for empirical spherical RGP lens power calculation?

• Vertex the spectacle Rx if needed.

• Calculate the lacrimal lens power using:
  LL = BCR - K

• Calculate the final contact lens power using:
  CLP = SpecRx vertexed - LL - RA(or)OR

What is the main question when deciding if a patient is a good candidate for spherical RGP lenses?

The key question is whether the patient will have low residual astigmatism (RA) with a spherical RGP lens.

• A good spherical RGP candidate typically has RA ≤ 0.75 DC.

• Low RA means the spherical RGP plus tear lens system can adequately correct the patient’s astigmatism.

• You do not need to use power crosses to answer this initial candidacy question.

What level of residual astigmatism is usually acceptable for spherical RGP lenses?

Typically, residual astigmatism should be ≤ 0.75 DC.

Clinical meaning:

• ≤ 0.75 DC RA: usually acceptable for spherical RGP

• > 0.75 DC RA: may need a toric design, front-surface toric, back-surface toric, bitoric, or other specialty lens option

How do you quickly estimate whether a patient will have low residual astigmatism with a spherical RGP?

Compare the patient’s spectacle cylinder to the patient’s corneal cylinder.

• If spectacle cylinder and corneal cylinder are close or match, the patient will likely have low residual astigmatism.

• If spectacle cylinder and corneal cylinder are very different, the patient may have higher internal astigmatism and higher residual astigmatism.

What does SAM FAP mean when changing spherical RGP lens fit?

SAM FAP is the rule for adjusting RGP power when the base curve radius (BCR) is changed:

• SAM = Steeper Add Minus

• FAP = Flatter Add Plus

If an RGP lens is made steeper, how does the tear lens change and how do you compensate?

Making the RGP steeper makes the tear lens more plus than before.

To compensate:

• Add minus to the RGP power by the same amount as the tear lens change.

If an RGP lens is made flatter, how does the tear lens change and how do you compensate?

Making the RGP flatter makes the tear lens more minus than before.

To compensate:

• Add plus to the RGP power by the same amount as the tear lens change.