9. How to Select an RGP Material - Contact Lenses I Summer 2026

Tyler's Quarterly Gas Perm Button Manufacturers (Pic)

Tyler's Quarterly Gas Perm Button Manufacturers (Pic)

Is considering what your patient needs when deciding an RGP Material important?

Yes

What are some needs of patients that must be considered when deciding an RGP Material?

-Overnight wear?

-Only wear during the day?

-Dry eye?

-Do they have a lot of astig?

Is the Dk/t different in RGPs vs soft lenses?

No -- the Dk/t is the same as soft CLs

What is the Dk needed for no daily wear corneal swelling?

25

What is the Dk needed to limit overnight swelling to normal (4%)?

85-135

Why are RGPs better for the cornea than soft CLs?

-More tear exchange under the RGP than soft lenses (25% more)

-The lens is smaller & does not cover the entire cornea

What are the 3 components of durability in an RGP lens?

-hardness

-modulus

-toughness

True or False:

Most GPs are quite scratchable

true

Where do scratches usually occur?

usually on the front surface of the RGP

How can scratches be prevented on RGPs?

can be prevented by proper handling

How is Hardness of an RGP measured?

By the Rockwell indentation method usually (drop a ball on the lens)

Is hardness predictive of dimensional stability?

No -- lenses do not stay the same over time. The plastic will change shape

What is the modulus of a RGP?

stiffness of a lens

Why is modulus important in RGPs?

In order to mask astigmatism

How is modulus determined?

by polymer chemistry

More cross linking of polymers in a RGP = (improved/worsened) durability

improved

More cross linking of polymers in a RGP = (more/less) O2 transmission

less

Modulus -- Higher is Stiffer (Pic)

Modulus -- Higher is Stiffer (Pic)

What is the break point of an RGP?

The point in which a lens will break

If patients are breaking their lenses frequently, what change may need to be made to the lens material?

Lower Dk -- material is not stiff enough and modulus needs to be increased

What is toughness of an RGP?

resistance of a material to breakage

How to reduce breakage of an RGP?

-Increase thickness

-More cross linking

Toughness & Dk (Pic)

Toughness & Dk (Pic)

PMMA (does/does not) bend

does not

**Lowest Dk

As the Dk increases in an RGP lens, the modulus of the lens will be (greater/less)

less

As the Dk increases in an RGP lens, the lenses will be (more/less) likely to break

more

____ makes RGPs flexible and weak

Silicone

Why does silicone make RGPs flexible and weak?

poor ability to resist flexure

Chips in the edge of RGPs can make them very vulnerable to ______ regardless of material properties

cracking

Does surface wetability also apply to RGPs?

Yes -- same as soft CLs

A (smaller/larger) wetting angle will equal a better wettability

smaller

True or False:

Lab tests are great indicators of on eye wettability

false -- poor indicators

What is on-eye wettability impacted by?

-Binding to the eye, proteins, and lipids

-Surface treatments (plasma, PEG)

What are the forces that are acting on an RGP Lens?

-surface tension

-lid interactions

-gravitational (center of mass, mass, gravity)

What is the function of surface tension?

suctions a lens to the eye

What creates surface tension on an RGP?

Interaction of water molecules between lens and cornea

There is a _____ at the edge of the lens that is d/t the edge lift

tear meniscus

What is an effect of too much edge lift?

Surface tension will draw water under lids & can dry out the cornea

What are the impacts to a upper lid attached fit of an RGP?

-Can hold a GP lens up

-Moves lens in a vertical direction during a blink

What are the impacts of a lower lid interaction with an RGP fit?

can push up on bottom of a GP lens

What is center of mass also referred to as?

center of gravity

Center of Mass is the ____ point of the lens

balancing

What does center of mass influence?

the lens position on the eye and the stability of fit

A center of mass that is ______ the lens helps the lens position better

further behind

What happens if a center of mass is closer to the back surface of a lens?

the lens will drop

What are the factors that lead to a center of mass that is further from the back surface of the RGP lens?

-minus power

-LARGER LENSES *****

-thinner lenses

How to make lenses thinner?

lenticular

How does a lenticular improve fit of an RGP?

-improve center of mass

-improve edge thickness

Thinner lenses with a lenticular will move the center of mass where?

backwards

(Plus/minus) lenses are more likely to drop on the eye

plus

Diameter is ____x more important than center thickness or power when determining the center of mass on an RGP lens

4-7x

Center of Mass (Pic)

Center of Mass (Pic)

What is the equation for mass of an RGP?

volume * specific gravity

What is the specific gravity of a RGP?

a dimensionless ratio of a substance's density compared to a reference substance

If we want to alter the mass of an object, we must change the ______

specific gravity

If the contact lens drops low on the cornea, we will want to choose a lens with a (higher/lower) specific gravity meaning (more/less) mass

lower; less

If the contact lens rides high on the cornea, we will want to choose a lens with a (higher/lower) specific gravity meaning (more/less) mass

higher; more

_____ lenses may benefit from a higher specific gravity measurement

Prism ballast

There is more transmission of light though a (CLs/spec)

Cls

True or False:

Some RGPs have a UV filter

true

The index of refraction is (more/less) important in Cls than specs

less

True or False:

Index of refraction is very similar in all RGP materials

true

All RGP materials are _____

plastics

What was the 1st RGP material?

PMMA

What are the thermoplastics available for RGP materials?

-PMMA

-Silicone Acrylates

-Fluorosilicone Acrylates

What are the pros of PMMA?

-readily machined

-low cost/cheap

-fairly wettable

-easy to care for

What is the con of PMMA?

zero O2 transmissibility

What is the backbone of silicone acrylates?

methyl methacrylate

What is the Dk of Silicone Acrylates?

12-60 (low to medium)

What are the surface characteristics of silicone acrylate lenses?

-surface is negatively charged d/t monomer

-Hydrophobic

-Lipohillic

What are the advantages to the silicone acrylates?

-Higher Dk than PMMA

-Reduced rigidity

What are the disadvantages to the silicone acrylates?

-more lipid deposit prone

-surface easily scratched

-higher breakage rate than PMMA

-flexure problems/bending on the eye

-parameter instability

What type of monomer is added to silicone acrylate to create fluorosilicone acrylates?

fluorine monomer

Fluorosilicone acrylates have a _____ surface charge

lower

What is the Dk of fluorosilicone acrylates?

40-100+ (medium to high)

What are the advantages of fluorosilicone acrylates?

-Higher Dk than previous materials

-Fewer lipid deposits d/t lower surface charge

-Better wetting

What are the disadvantages of fluorosilicone acrylates?

-Surface is very easily scratched

-Greater lens flexure compared to PMMA

What was the 1st hyper-permeable lens?

Menicon Z (superfluorosilicone acrylate)

Superfluorosilicone acrylate lenses have a very (high/low) Dk

high

What is the monomer that is added to the Superfluorosilicone acrylate lenses?

silylstyrene

Superfluorosilicone acrylate lenses are FDA approved for up to ________

30 nights of wear