9 RGP Material Selection & Properties

What oxygen transmissibility criterion is used for RGP lenses compared with soft contact lenses?

RGP lenses use the same Dk/t criteria as soft contact lenses.

• Dk = oxygen permeability of the material

• t = lens thickness

• Higher Dk/t = more oxygen reaches the cornea.

What Dk/t values are associated with preventing corneal swelling during daily wear vs overnight wear?

• Daily wear: aim for essentially no corneal swelling, requiring about Dk/t ≥ 25.

• Overnight wear: swelling should be limited to physiologic normal, around 4%, requiring about Dk/t 87 to 135.

• Overnight wear requires much higher oxygen transmission because the closed eyelid reduces oxygen availability.

Why can RGP lenses be better for corneal oxygen access than soft lenses?

RGP lenses are usually smaller in diameter, allowing better tear exchange and more oxygen access around the lens.

• Smaller size → less corneal coverage

• Tear exchange under/around lens improves oxygen delivery

What three material properties contribute to contact lens durability and dimensional stability?

• Hardness = resistance to scratching/indentation

• Modulus = stiffness or resistance to deformation

• Toughness = resistance to breaking or cracking

What does hardness describe in GP contact lenses, and what is its major clinical issue?

Hardness describes resistance to surface scratching or indentation.

• Most GP lenses are still relatively scratchable.

• Scratches usually occur on the front surface.

• Proper handling can help prevent scratching.

How is hardness measured, and does hardness predict dimensional stability?

Hardness can be measured by methods such as the Rockwell indentation method.
However, hardness is not predictive of dimensional stability.

• A lens can resist scratching but still change shape.

• Dimensional stability depends more on material stiffness, flexibility, and resistance to warpage.

What is modulus in GP/RGP lens materials, and what does it clinically represent?

Modulus is the slope of the stress-strain curve and represents material stiffness, measured in MPa.

• Higher modulus = stiffer material

• Lower modulus = more flexible material

Why is modulus important for RGP lens fitting and durability?

Modulus affects the lens’s ability to mask astigmatism and resist flexure.

• Higher stiffness → better shape retention

• Better shape retention → better masking of corneal astigmatism

• Too much flexibility can allow lens flexure and reduce optical performance.

How does polymer chemistry affect modulus, durability, and oxygen transmission?

Modulus is determined by polymer chemistry.

• More cross-linking → improved durability/stiffness

• But more cross-linking → usually less O₂ transmission

What is toughness in RGP lens materials?

Toughness is the material’s resistance to breakage.

• Units: MPa mm/mm

• Higher toughness = better ability to resist cracking or breaking

• Clinically important for reducing lens fracture.

What material changes can reduce RGP lens breakage?

Breakage can be reduced by:

• Increasing lens thickness

• Increasing cross-linking in the material

Tradeoff: increasing thickness or cross-linking may improve durability but can reduce oxygen delivery.

How does silicone affect RGP lens toughness and flexibility?

Silicone tends to make RGP lenses more flexible but weaker.

• Silicone improves oxygen permeability

• But increased flexibility means poorer ability to resist flexure

• This can reduce toughness and make the lens more vulnerable to breakage.

Why are edge chips clinically important in RGP lenses?

Edge chips create weak points that make RGP lenses highly vulnerable to cracking.

• Even if the material itself is durable, a chipped edge can initiate cracks

• Lens breakage risk increases regardless of material properties

What does surface wettability describe in contact lens materials, and how is it related to wetting angle?

Surface wettability describes how well tears spread over the lens surface.

• Smaller wetting angle = better wettability

• Better wettability → smoother tear film → better comfort, optics, and lens surface performance

• Same basic wettability principles apply to RGPs and soft lenses.

What are common laboratory methods used to assess contact lens wettability?

Common methods include:

• Contact angle in air

• Captive bubble method

• Wilhelmy plate method

Why are laboratory wettability tests poor predictors of on-eye wettability?

Laboratory tests do not fully reproduce the in vivo tear environment.
On-eye wettability is affected by:

• Tear film composition

• Binding of proteins and lipids

• Interaction with the ocular surface

• Lens surface treatments

• Deposits and lens care products

What factors most influence on-eye wettability of RGP lenses?

On-eye wettability is influenced by:

• Binding to eye proteins

• Binding to lipids

• Surface treatments, such as plasma treatment or PEG-based coatings

• Tear film and deposit interactions

What are the main forces acting on an RGP lens on-eye?

The major forces acting on an RGP lens are:

1. Surface tension

2. Lid interactions

3. Gravity

Why are forces acting on an RGP lens important?

These forces determine how the RGP lens behaves on-eye, including:

• Lens centration

• Lens movement with blinking

• Lens position after blink

• Lens adherence or suction

• Comfort and visual stability

How does surface tension affect an RGP lens on the eye?

Surface tension helps suction the lens to the eye through interactions of water molecules between the lens and cornea.

• Tear film creates adhesive/capillary forces

• This helps hold the lens against the ocular surface

• The effect is strongly influenced by the edge tear meniscus

What role does the tear meniscus at the edge of an RGP lens play?

The tear meniscus at the lens edge contributes to surface tension and lens adherence.

• The tear meniscus forms where the tear layer meets the lens edge

• It influences how strongly the lens is held to the eye

• Edge lift changes the size/shape of the tear meniscus

What happens if an RGP lens has too little vs too much edge lift?

• Too little edge lift: reduced tear exchange, tight lens fit, increased suction/adherence, possible corneal staining or discomfort

• Too much edge lift: excessive lens movement, poor centration, edge awareness, discomfort, possible bubble/debris entry

How do tight lids or high lid tension affect RGP lens positioning?

Tight lids or high lid tension can significantly influence RGP lens position and movement.

• The upper lid can hold a GP lens up

• The upper lid can move the lens vertically during blinking

• This is important in lid-attached fitting

What is a lid-attached RGP fitting relationship?

In a lid-attached fit, the upper lid interacts with and helps position the RGP lens.

• Upper lid can hold the lens superiorly

• Lens moves vertically with the blink

• Can improve lens positioning in some patients, especially when lid anatomy supports the lens

How does the lower lid affect an RGP lens?

The lower lid can push up on the bottom of a GP lens.

• This can help support lens position

• Lower lid interaction may influence centration and vertical lens stability

• Important when evaluating lens movement and fit on-eye

What is the center of mass of an RGP lens, and why is it clinically important?

The center of mass, also called the center of gravity, is the lens’s balancing point.

• It is not the same as gravity.

• It influences lens position, centration, and stability of fit on the eye.

• A more favorable center of mass can help the lens position better.

How does the location of the center of mass affect whether an RGP lens drops?

If the center of mass is closer to the back surface of the lens, the lens is more likely to drop inferiorly.

• Center of mass affects how gravity acts on the lens

• A lens design that shifts the center of mass more favorably can improve vertical positioning

• Key idea: center of mass affects stability and lens drop

What lens factors move the center of mass farther from the back surface of an RGP lens?

Factors that move the center of mass farther from the back surface include:

• Larger overall lens diameter

• More minus power

• Thinner lens design

How do lenticular designs improve RGP lens fit through center of mass?

Lenticular designs make lenses thinner, which helps shift the center of mass backward/farther from the back surface and can improve lens positioning.

• Thinner lens → altered center of mass

• Better center of mass location → improved fit stability

• This is one reason lenticulars can improve RGP lens positioning.

How does plus vs minus power affect center of mass and lens drop?

Plus lenses are more likely to drop compared with minus lenses because their center of mass is less favorable.
Example from slide:

• -3.00 D GP: center of mass about 0.6 mm

• +3.00 D GP: center of mass about 0.5 mm

Which factor is most important for center of mass: diameter, center thickness, or power?

Diameter is the most important factor.

• Diameter is about 4 to 7 times more important than center thickness or lens power

• Larger diameter can significantly influence center of mass and lens positioning

• High-yield: if asked what most affects center of mass, pick diameter.

What determines the mass of an RGP lens?

Lens mass = volume × specific gravity.

What is specific gravity, and why does it matter for RGP lenses?

Specific gravity is a dimensionless ratio comparing a material’s density to a reference substance. Clinically, specific gravity matters because changing specific gravity changes lens mass without necessarily changing lens shape or volume.

If an RGP lens drops low on the cornea, should specific gravity be increased or decreased?

If the lens drops low, use a material with lower specific gravity.

If an RGP lens rides high on the cornea, should specific gravity be increased or decreased?

If the lens rides high, use a material with higher specific gravity.

Why may prism ballast lenses benefit from higher specific gravity?

Prism ballast lenses may benefit from higher specific gravity because extra mass can improve gravitational stabilization.

• Higher specific gravity → more mass

• More mass in the prism ballast area → better orientation/stability

• Useful when gravity is being used to help position or stabilize the lens

How does light transmission through contact lenses compare with spectacles?

Contact lens light transmission is generally greater than spectacle lens transmission.

• Contact lenses sit directly on the eye

• Less reflection and optical loss compared with spectacles

• Clinically, contact lenses usually allow excellent light transmission unless filters or tints are present

What is important to know about UV filters in GP/RGP lens materials?

Some GP/RGP materials contain UV filters.

• UV filters reduce transmission of ultraviolet light

• This can be beneficial for ocular protection

• Not all GP materials have UV-filtering properties

Why may a filter be needed when viewing fluorescein patterns in UV-protected GP lenses?

UV-protected GP lenses can make fluorescein patterns harder to view because the UV filter may reduce the excitation of fluorescein.

• Fluorescein requires blue/UV light excitation

• UV-filtering materials can interfere with pattern visibility

• A viewing filter may be needed to better assess the fluorescein pattern

How important is index of refraction when selecting a GP lens material?

Index of refraction is not as important in GP material selection as it is for spectacles.

• GP materials have relatively similar refractive indices

• Lens material selection usually depends more on oxygen permeability, wettability, durability, and specific gravity

• Spectacle lens thickness and optics are more strongly affected by refractive index

Do the refractive index values for GP materials matter?

Common GP materials have similar refractive indices.

What monomers form the PMMA backbone used in classic contact lens materials?

The PMMA backbone is based on acrylate chemistry, especially:

• Methacrylic acid (MA)

• Methyl methacrylate (MMA)

Why is methyl methacrylate important in GP lens material chemistry?

Methyl methacrylate (MMA) is the main monomer that polymerizes to form PMMA.

• MMA → polymerization → PMMA

• PMMA provides a rigid, clear, machinable plastic backbone

• This backbone became the historical foundation for rigid contact lenses

What are the main thermoplastic materials used in rigid contact lenses?

Rigid contact lens thermoplastics include:

1. PMMA

2. RGP materials, including:

   • Silicone acrylates

   • Fluorosilicone acrylates

How do RGP materials differ chemically from PMMA?

RGP materials build on rigid acrylate chemistry but add components such as silicone and/or fluorine.

• Silicone acrylates: improve oxygen permeability

• Fluorosilicone acrylates: improve oxygen permeability and may improve deposit resistance/wettability properties

• These modifications make RGPs more physiologically compatible than PMMA

What were the major advantages of PMMA as a contact lens material?

PMMA was historically useful because it was:

• Readily machined

• Low cost/cheap

• Fairly wettable

• Easy to care for

• Optically clear and rigid

What is the major limitation of PMMA contact lenses?

PMMA has zero oxygen transmissibility.

• No oxygen passes through the material

• Corneal oxygen must come mainly from tear exchange around the lens

• This limits safe wear time and increases risk of corneal hypoxia

Why did RGP materials replace PMMA for rigid contact lenses?

RGP materials replaced PMMA because they allow oxygen transmission while maintaining useful rigidity.

Compare PMMA, silicone acrylates, and fluorosilicone acrylates as rigid lens materials.

• PMMA: old rigid material; cheap, machinable, fairly wettable, easy care, but zero O₂ transmissibility

• Silicone acrylates: RGP materials with improved oxygen permeability due to silicone

• Fluorosilicone acrylates: RGP materials with silicone + fluorine modifications, improving oxygen transmission and material performance

What chemical change defines silicone acrylate GP materials, and what does it do to oxygen permeability?

Silicone acrylates use a methyl methacrylate backbone with added silicone containing Si-O-Si bonds.

• Silicone increases oxygen permeability.

• Typical Dk = 12 to 60, which is low to medium.

• This made silicone acrylates more physiologically useful than PMMA.

What surface properties of silicone acrylate materials create clinical problems?

Silicone acrylates tend to have a negatively charged, hydrophobic/lipophilic surface.

Clinical effects:

• More prone to lipid deposits

• Reduced surface wettability

• Surface can be scratched more easily

• Deposits and scratches can reduce comfort, optics, and lens performance

What are the main advantages and disadvantages of silicone acrylate GP lenses?

Advantages:

• Higher Dk than previous materials

• Reduced rigidity, making the lens somewhat more flexible

Disadvantages:

• More lipid deposit prone

• Surface easily scratched

• Higher breakage rate

• Flexure problems

• Parameter instability

Why can reduced rigidity in silicone acrylates be both helpful and harmful?

Reduced rigidity can improve handling comfort and flexibility, but too much flexibility can cause problems.

Clinical downsides:

• Lens flexure on-eye

• Reduced ability to mask corneal astigmatism

• Warpage or parameter instability

• Increased breakage risk

What chemical modification creates fluorosilicone acrylate GP materials?

Fluorosilicone acrylates are made by adding a fluorine monomer to silicone acrylate material.

Effects of fluorine:

• Lowers surface charge

• Reduces lipid deposition

• Increases oxygen permeability

• Improves material performance compared with older silicone acrylates

How do fluorosilicone acrylates compare with silicone acrylates in Dk and deposit resistance?

Fluorosilicone acrylates generally have:

• Higher Dk: about 40 to 100+, medium to high

• Less lipid deposition

• Lower surface charge than silicone acrylates

Why were fluorosilicone acrylates an improvement over silicone acrylates?

Fluorosilicone acrylates improved the major weaknesses of silicone acrylates.

• Silicone acrylates: higher Dk than PMMA, but hydrophobic/lipophilic and deposit prone

• Fluorosilicone acrylates: higher Dk plus fewer lipid deposits due to fluorine

What are the disadvantages of fluorosilicone acrylates?

Surface is easily scratched and greater lens flexure compared with PMMA.

What are the main advantages of fluorosilicone acrylate GP materials?

Fluorosilicone acrylates improve on older GP materials by having:

• Higher Dk than previous materials

• Fewer lipid deposits

• Better wetting

What are the main disadvantages of fluorosilicone acrylate GP materials?

Main disadvantages include:

• Surface is easily scratched

• Greater lens flexure compared with PMMA

Why are fluorosilicone acrylates generally better clinically than silicone acrylates?

Fluorosilicone acrylates keep the oxygen benefit of silicone while improving surface behavior.

• Silicone → increases oxygen permeability

• Fluorine → lowers surface charge, reduces lipid deposits, improves wetting

• Result: higher Dk + better deposit resistance

What is Menicon Z, and why is it important among GP materials?

Menicon Z is part of the Menicon family of fluorosilicone acrylate materials and was the first “hyper-permeable” GP material.

• Fluorosilicone acrylate with silylstyrene

• Very high oxygen permeability

• Important for extended-wear GP lens designs

What are the oxygen values for Menicon Z, and what do they mean clinically?

Menicon Z has very high oxygen transmission values:

• Dk = 163

• Dk/t ≈ 125

What is the FDA-approved extended wear duration for Menicon Z?

Menicon Z is FDA approved for up to 30 nights of wear.

Compare silicone acrylates, fluorosilicone acrylates, and Menicon Z.

• Silicone acrylates: Dk 12 to 60; higher oxygen than PMMA but hydrophobic/lipophilic, lipid deposit prone, scratchable, and less stable

• Fluorosilicone acrylates: Dk 40 to 100+; fewer lipid deposits, better wetting, but still scratchable and more flexible than PMMA

• Menicon Z: hyper-permeable fluorosilicone acrylate; Dk 163, Dk/t ~125, FDA approved for up to 30 nights