Why is PMMA no longer used for contact lenses?

PMMA has zero oxygen permeability, meaning the cornea receives no oxygen through the lens material. Oxygen reaches the cornea only through tear exchange during blinking, which is inadequate. Chronic PMMA wear causes corneal edema, spectacle blur, neovascularization, and endothelial cell loss. Modern fluorosilicone acrylate materials provide full oxygen permeability while maintaining good optics and durability.

What are the three main components of fluorosilicone acrylate?

Silicone provides oxygen permeability (higher amounts = higher Dk). Fluorine improves wettability and deposit resistance while also contributing to oxygen permeability. Acrylate (the PMMA component) provides structural rigidity, durability, and optical clarity. The ratio of these three components is adjusted to create materials with different property balances.

Why might a moderate Dk material be better than a hyper Dk material?

Very high Dk materials contain more silicone, which can reduce wettability (making the lens surface hydrophobic), decrease dimensional stability (causing lens flexure), and lower deposit resistance. A moderate Dk material (30-60) with excellent wettability and stability often provides better all-day comfort and visual performance for standard daily wear patients than a hyper Dk material with compromised surface properties.

GP Contact Lens Materials: PMMA to Fluorosilicone Acrylate

Evolution of Rigid Lens Materials

Rigid contact lens materials have undergone a significant evolution from impermeable plastics to modern gas permeable polymers. Understanding this progression and the properties of current materials helps you select the best rigid lens for each clinical situation.

PMMA: The Original Rigid Lens

PMMA (polymethyl methacrylate) was the first rigid contact lens material, used from the 1940s through the 1970s. While it offered excellent optical clarity and durability, it had a critical limitation: zero oxygen permeability.

Because PMMA allows no oxygen transmission, the cornea received oxygen only through tear exchange beneath the lens during blinking. This was inadequate for many patients, leading to:

Corneal edema: Chronic stromal swelling from oxygen deprivation
Spectacle blur: Temporary blurred vision with glasses after lens removal, caused by corneal edema and warpage
Neovascularization: Blood vessel growth into the cornea in response to chronic hypoxia
Corneal exhaustion syndrome: Progressive endothelial cell loss from long-term hypoxic stress

PMMA is no longer used for new fits, but understanding its limitations provides context for why modern materials were developed.

Modern GP Materials: Fluorosilicone Acrylate (FSA)

Fluorosilicone acrylate is the dominant material for modern gas permeable contact lenses. It combines three components, each contributing specific properties:

Silicone: Provides oxygen permeability. More silicone = higher Dk
Fluorine: Improves deposit resistance and wettability. Also contributes to oxygen permeability
Acrylate (PMMA component): Provides structural rigidity, durability, and optical clarity

By adjusting the ratio of these three components, manufacturers create materials with different balances of properties to suit various clinical needs.

Key Material Properties

Oxygen Permeability (Dk)

The Dk value measures how readily oxygen passes through the material. GP materials are available across a wide range of Dk values:

Low Dk: 15-30 (adequate for limited daily wear)
Medium Dk: 30-60 (suitable for standard daily wear)
High Dk: 60-100 (suitable for extended daily wear)
Hyper Dk: 100+ (suitable for extended/overnight wear)

Higher Dk materials generally contain more silicone, which improves oxygen transmission but can affect other properties.

Wettability

Wettability describes how well the lens surface attracts and maintains the tear film. A wettable surface allows tears to spread evenly, creating a smooth optical surface and comfortable wear. A poorly wettable surface causes the tear film to bead up, leading to blurred vision, dryness, and discomfort.

Higher silicone content tends to reduce wettability because silicone is naturally hydrophobic. The fluorine component helps counteract this by improving surface wetting. This trade-off is a fundamental challenge in GP material design: more silicone means more oxygen but potentially less wettability.

The GP material trade-off: increasing silicone content raises Dk (more oxygen) but can decrease wettability and mechanical stability. Fluorine helps offset this by improving wettability and deposit resistance. The best material for a given patient balances these properties based on their wearing schedule and needs.

Deposit Resistance

Deposit resistance measures how well the material resists accumulation of proteins, lipids, and other tear components on its surface. Deposits on GP lenses cause:

Reduced visual clarity
Decreased comfort
Increased risk of inflammatory reactions (GPC)
Reduced wettability

Fluorine in the material improves deposit resistance. Regular cleaning with appropriate GP lens cleaners also manages deposit buildup.

Dimensional Stability

Dimensional stability refers to the material's ability to maintain its shape over time without warping. Higher silicone content can reduce stability, making the lens more prone to flexure (bending) on the eye or during handling. Materials with more acrylate (PMMA component) tend to be more dimensionally stable.

Hardness and Durability

GP materials are harder than soft lenses but less hard than PMMA. They are more resistant to scratching than soft lenses but can still be damaged by improper handling. Higher Dk materials tend to be slightly softer and more flexible, making them somewhat more susceptible to scratches.

When selecting a GP material, match the Dk to the patient's wearing schedule. A patient who wears lenses 8-10 hours daily usually does well with medium Dk materials (30-60), which offer good wettability and stability. Reserve hyper-Dk materials for patients requiring extended wear or those with signs of corneal hypoxia.

Comparing GP and Soft Lens Materials

Property	GP Lenses	Soft Lenses
Optical quality	Superior (rigid surface)	Good (conforms to cornea)
Initial comfort	Lower (adaptation needed)	Higher (immediate comfort)
Deposit resistance	Better (non-absorbent)	Variable (material dependent)
Tear lens correction	Yes (corrects corneal astigmatism)	No (conforms to cornea)
Handling	More durable	More fragile
Replacement frequency	Annual or longer	Daily to monthly

Assuming the highest Dk material is always the best choice. Very high Dk materials may have reduced wettability, flexibility that causes lens flexure, and lower deposit resistance. The optimal material balances all properties for the individual patient's needs. A moderate Dk material with excellent wettability often outperforms a hyper-Dk material with poor wettability.

Key Takeaways

PMMA had excellent optics but zero oxygen permeability, causing chronic corneal complications
Modern GP lenses use fluorosilicone acrylate (FSA), combining silicone (oxygen), fluorine (wettability, deposit resistance), and acrylate (rigidity)
Dk ranges from low (15-30) to hyper (100+); match to the patient's wearing schedule
Higher silicone content improves oxygen but can reduce wettability and stability
Fluorine counteracts silicone's hydrophobic tendency and improves deposit resistance
GP lenses provide superior optics and deposit resistance compared to soft lenses but require an adaptation period