What is the difference between HEMA hydrogel and silicone hydrogel lenses?

HEMA hydrogel lenses transport oxygen through their water content, so higher water content means more oxygen but also more dehydration. Their Dk values are limited. Silicone hydrogel lenses incorporate silicone channels that transmit oxygen directly, achieving 5-6 times higher Dk values regardless of water content. SiHy lenses are generally stiffer, have lower water content, and tend to attract lipid rather than protein deposits.

What do the four FDA groups classify?

The FDA classifies soft lens materials by water content (low 50%) and ionicity (ionic or non-ionic). Group I: low water, non-ionic. Group II: high water, non-ionic. Group III: low water, ionic. Group IV: high water, ionic. Groups affect deposit tendency (Group IV highest, Group I lowest), solution compatibility, and recommended replacement schedules.

Why do silicone hydrogel lenses sometimes cause problems with certain solutions?

Silicone hydrogel materials have different surface chemistry than conventional hydrogels. Some multipurpose solutions that work well with conventional lenses can cause solution-induced corneal staining (SICS) when used with certain SiHy lenses. This is typically a reaction between the solution preservative and the lens surface. Always verify that the care solution is approved for use with the specific SiHy material being prescribed.

Soft Contact Lens Materials: HEMA, Silicone Hydrogel, and FDA Groups

Evolution of Soft Lens Materials

Soft contact lens materials have evolved dramatically since their introduction. Understanding the differences between material types, their properties, and how they are classified helps you select the best lens for each patient's needs and anticipate potential complications.

Conventional Hydrogel (HEMA-Based)

HEMA (hydroxyethyl methacrylate) was the original soft contact lens material, introduced in the early 1970s. Conventional hydrogel lenses are water-absorbing polymers that derive their flexibility from their water content.

Oxygen Transport in Hydrogels

In conventional hydrogel materials, oxygen reaches the cornea by dissolving into and diffusing through the water content of the lens. This means:

Higher water content = higher oxygen permeability (Dk)
Water content typically ranges from 38% to 74%
However, there is a practical ceiling: very high water content makes the lens more fragile, causes faster dehydration on the eye, and the Dk still does not reach levels adequate for extended (overnight) wear

Even the highest water content conventional hydrogels achieve Dk/t values well below the Holden-Mertz criteria for safe extended wear (Dk/t of 87). This inherent limitation drove the development of silicone hydrogel materials.

Trade-offs of High Water Content

Higher water content increases initial comfort but also increases dehydration on the eye
As the lens dehydrates during wear, it can draw moisture from the tear film, worsening dryness symptoms
Higher water content lenses are more fragile and tear more easily during handling
Higher water content (especially ionic) lenses tend to accumulate more protein deposits

In conventional hydrogels, water carries oxygen. Higher water content means more oxygen, but also more dehydration, deposit susceptibility, and fragility. This trade-off is the fundamental limitation of HEMA-based materials.

Silicone Hydrogel (SiHy)

Silicone hydrogel lenses represent a major advancement in contact lens technology. They incorporate silicone into the polymer matrix, creating channels through which oxygen can pass directly without needing to dissolve in water first.

Oxygen Transport in SiHy

Silicone is inherently highly permeable to oxygen. In silicone hydrogel lenses, oxygen travels through both:

Silicone channels: The primary pathway, providing very high oxygen transmission regardless of water content
Water content: A secondary, minor pathway (same as in conventional hydrogels)

This dual pathway allows SiHy lenses to achieve Dk/t values of 100 or higher, well above the extended wear threshold. Some SiHy materials achieve Dk values 5 to 6 times greater than conventional hydrogels.

SiHy Properties

Higher modulus (stiffness): Silicone hydrogels are generally stiffer than conventional hydrogels, which can affect comfort, especially with early-generation materials. Newer formulations have reduced modulus to improve comfort
Lower water content: Because oxygen transport does not depend on water, many SiHy lenses have lower water content (24-48%) compared to conventional hydrogels. This reduces dehydration but may feel different initially
Surface treatments: Silicone is naturally hydrophobic (water-repelling). Manufacturers use surface treatments or internal wetting agents to make the lens surface wettable and comfortable. Examples include plasma surface treatment and embedded wetting agents
Lipid deposits: SiHy materials tend to attract more lipid (oil-based) deposits compared to conventional hydrogels, which attract more protein deposits

When switching a patient from conventional hydrogel to silicone hydrogel, be aware that the care solution compatibility may differ. Some solutions that work well with conventional hydrogels cause corneal staining when used with certain SiHy lenses. Always verify that the patient's care system is approved for use with their specific lens material.

FDA Classification Groups

The FDA classifies soft contact lens materials into four groups based on two properties: water content and ionicity (electrical charge of the polymer).

Group	Water Content	Ionicity	Deposit Tendency
Group I	Low (<50%)	Non-ionic	Lowest deposits
Group II	High (>50%)	Non-ionic	Low deposits
Group III	Low (<50%)	Ionic	Moderate deposits
Group IV	High (>50%)	Ionic	Highest deposits

Why FDA Groups Matter

Deposit prediction: Group IV lenses (high water, ionic) attract the most protein deposits because the ionic charge attracts the positively charged lysozyme in tears. Group I lenses (low water, non-ionic) attract the least
Solution compatibility: Different groups may interact differently with care solutions. Solution approvals often specify which FDA groups they are compatible with
Replacement schedule: Higher deposit-prone lenses (Groups III and IV) benefit from more frequent replacement

Assuming that higher water content always means better comfort and performance. While high water content provides good initial comfort and higher Dk in conventional hydrogels, it also leads to faster dehydration and more protein deposits (especially in ionic Group IV lenses). Silicone hydrogels with lower water content can provide superior oxygen transmission and less dehydration.

Key Takeaways

Conventional hydrogels (HEMA) transport oxygen through water content; higher water = more oxygen but more dehydration
Silicone hydrogels transport oxygen through silicone channels, achieving much higher Dk/t regardless of water content
SiHy materials are stiffer, tend to attract lipid deposits, and require surface treatments for wettability
FDA Group I (low water, non-ionic) has the lowest deposit tendency; Group IV (high water, ionic) has the highest
Ionic lenses attract more protein deposits due to charge interaction with tear lysozyme
Care solution compatibility should always be verified when changing lens materials