Understanding the Nuances of Contact Lens Materials

BY BRAD GIEDD, OD
July 1999

Learn just how important your choice of contact lens materials is to a successful fit.

Whether for a soft, rigid gas permeable or hybrid contact lens, the issue of material is often overlooked. In the case of RGPs, some practitioners even leave it up to their labs to make the important determination of lens material for them. The fact of the matter is that the success of many contact lens patients can hinge on the proper determination of the most appropriate contact lens material for that patient's eye.

I will review a few of the basic characteristics of soft and RGP lens materials that can impact clinical decision-making for both symptomatic and asymptomatic contact lens patients. A thorough understanding of the nuances of various materials can help round out a practitioner's trouble-shooting arsenal and perhaps even be the difference between maintaining and losing borderline patients.

Where We Are With Soft Lens Materials

Since their introduction in the 1970s, hydrogel contact lenses have become the lens of choice for most contact lens practitioners, comprising as much as 85 percent of the contact lens marketshare. Concurrently, the number of different hydrogel materials available has grown to accommodate nearly any physical or physiological fitting need, and now includes designs and materials especially made for dry eye, giant papillary conjunctivitis (GPC) and even presbyopia. At the same time, a number of different replacement schedules in addition to conventional wear has evolved, adding frequent replacement, planned replacement, disposable use and extended wear to today's soft contact lens vocabulary. The choices are so numerous that many practitioners become familiar with only a few contact lenses and use them exclusively. But disregarding the less familiar lenses isn't always in every patient's best interest.

Material Classification

In order to better understand some of the characteristics of today's soft contact lens materials, the clinician must be familiar with the classification of various lenses and the significance of these classifications. The FDA classification system for hydrogel materials is based on the water content and the ionic nature of the lens matrix of the contact lens material. This system is outlined in Table 1.

Water content -- In general, hydrogel lens materials derive their oxygen permeability from their water content. Higher water content materials have higher oxygen permeabilities. Strategies used to increase water content include: adding small quantities of charged groups such as methacrylic acid or neutral groups such as polyvinyl alcohol (PVA) or N-vinyl pyrrolidone (NVP) to the base material, polyHEMA or methyl methacrylate (MMA). These strategies raise water contents to 60 percent or greater. Typical hydrogel materials are available in water contents ranging from 37.5 percent to 79 percent. However, as water content increases, physics dictates that lens thickness must also increase in order to preserve lens strength and to prevent the contact lens from tearing easily.

Deposits -- Surface deposition on hydrogel materials is an area of great interest and extensive research. Specifically, the tendency of different material groups to collect protein and lipid deposits has been examined in detail, which produced some interesting results.

FDA group I materials, including polymacon, tetrafilcon A and glycerol methacrylate, comprise the group of polymers found to collect lesser amounts of protein. Due to their low water content, these polymers are typically involved in the manufacturing of the thinnest types of lenses. It's interesting to note that polymacon (polyHEMA 38% water), the original soft lens material, is still used in the manufacture of nearly 50 percent of all soft contact lens types!

FDA group IV materials are relatively porous, negatively charged networks that are more susceptible to tear film constituents entering the lens matrix. They are the only group of contact lens materials into which the low molecular weight, positively- charged protein, lysozyme, is able to diffuse. Group IV materials predominantly deposit protein due mostly to the ionic carboxylate groups of the matrix. This material charge also influences the location of the protein, with deposition occurring deep within the material matrix of group IV materials and more on the surface of group II materials. With protein deposition, the charge and the water content of the lens material are the main influencing factors, with minimal inter-subject differences. Furthermore, protein deposits can be detected almost immediately upon insertion of these contact lenses, though deposition has been shown to reach a plateau between 1 and 7 days post-insertion.

FDA group II materials predominantly deposit lipids due mainly to the presence of hydrophobic N-vinyl pyrrolidone. Contrary to the protein deposition seen with group IV materials, group II materials show considerable inter-subject differences in lipid deposition. The deposition of lipids is cumulative in these materials and does not reach a plateau, unlike the protein deposition of group IV materials.

Another issue affecting the use of hydrogel contact lenses is that of lens dehydration and its affect on lens comfort. Among currently available hydrogel lens materials, those with higher water contents are thought to dehydrate more upon exposure to low humidity environments. This is especially true for thin, high water content lenses, as compared to their thicker counterparts. People who are constantly in low humidity, air conditioned environments and those who live in colder climates, where hot air heating systems often lower humidity, tend to experience the most discomfort. The anterior surface drying associated with this dehydration also contributes to binding lens deposits, while overall soft contact lens dehydration may bind the lens to the eye. Researchers believe that in addition to the overall water content of a lens, other factors, including the structure of the water within the lens matrix, determine the rates of dehydration among various contact lenses. Hydrogel contact lenses containing more bound water, as opposed to free water, are believed to dehydrate more slowly because the more structured bound water is less likely to evaporate.

A Change is All it Takes

The implications of many of these material characteristics can be significant for many borderline contact lens patients. For patients with comfort issues and overt lipid or protein deposition problems, changing materials can be a quick solution. For patients with complaints of dryness related to environmental factors, changing materials can again be a logical first step. When hypoxia is a critical issue for a patient, knowing Dk properties of materials can provide an immediate alternative. For patients who have difficulty handling their lenses or who frequently damage them, prescribing a thicker or lower water content material can help.

RGP Materials

Much has changed in the area of rigid gas permeable materials since the days of PMMA, CAB and the first silicone/acrylates. Today's variety of materials offers a wide range of options in parameters such as Dk, specific gravity, wettability and material stability. A broad understanding of these material characteristics is invaluable in fitting and refitting RGP contact lens patients.

First generation RGP contact lenses were made of silicone and methacrylic acid polymers, known as silicone acrylates. At the time, this material offered some distinct advantages to its predecessors, PMMA and CAB, including a higher oxygen permeability. Silicone acrylate was not only a better thermal conductor than PMMA, it experienced less warpage than CAB. The sacrifices at the time were decreased optical quality, decreased wettability and greater protein deposition.

In order to improve wettability and oxygen transmissibility, the use of fluoro siloxane methacrylates (fluorosilicone acrylates) became popular. These second generation polymers combined the silicone acrylate material with fluorinated monomers, giving the material better wetting characteristics. This combination also helped reduce protein deposition and improve oxygen transmission, allowing for reduction in the siloxane component. Conversely, the material was more susceptible to lipid deposits and was found to be more sensitive to harsher cleaning agents.

In general, the silicone/fluorine part of the polymer gives the material its high oxygen transmissibility, while the methacrylate enhances optical quality and stability. As previously mentioned, higher amounts of silicone tended to have detrimental effects on lens performance, including poor surface wettability, greater protein deposition, increased flexure and instability, and decreased lens durability. The incorporation of fluorine helps to overcome many of these shortcomings.

Dk Versus Dk/t

When we think of the various RGP contact lens material properties, probably none is more considered than that of Dk. Hundreds of studies have focused on this determination, its significance and the ideal value needed to maintain overall ocular health. Oxygen permeability, or Dk, is defined as the product of the diffusion coefficient of oxygen in a material (D) and the solubility coefficient of oxygen in a material (k). It is a property inherent to any polymer matrix and is independent of lens thickness. Oxygen transmissibility, or Dk/L, is defined as the oxygen permeability (Dk) per lens thickness (L), and varies for each particular contact lens.

A working knowledge of Dk issues and the ability to make an intelligent Dk lens material selection can make a significant difference for some patients. Patients being refit from PMMA lenses, for example, are typically happier in lower Dk materials which will more closely resemble their previous lenses in optical quality and durability. In moderate to high myopes, the dimensional stability provided by the low Dk fluorosilicone acrylates is advantageous, as relatively thin center thicknesses might otherwise result in flexure. Conversely, the increased permeability provided by the high Dk fluorosilicone acrylates can benefit hyperopes, where increased center thicknesses minimize the need for the more dimensionally stable materials. But how much permeability is enough? And who's to say that more is better?

According to Robert Grohe, O.D., a multi-dimensional private practitioner and contact lens researcher based in Chicago, Ill., many practitioners have forgotten about some of the benefits of high and hyper Dk materials. Besides offering UV monomer protection, which is thought to be helpful in macular degeneration and cataract prevention, these materials also provide the optimum oxygen levels to ocular tissues. Dr. Grohe believes that these materials have been unfairly labeled as unstable, and that the real problem stems from patient and practitioner mishandling. In fact, when it comes to extended wear, Dr. Grohe says that hyper Dk RGPs are the way to go and that with the proper design can be worn safely for weeks at a time.

Other important material properties that can impact the fit of a lens include: lens thickness, specific gravity and on-eye wettability. Several new materials have evolved and are designated exclusively for thin contact lens designs. However, Dks for these materials tend to remain in the low to moderate range due to stability issues. Dr. Grohe reminds us that one of the easiest ways to move an inferior positioning lens, be it a high plus, bifocal or ortho-k lens, is to decrease lens mass with a material of less specific gravity. Table 2 attempts to list some of the more common RGP lens materials and several of the parameters thought to be most clinically significant.

Silicone Hydrogels

The newest type of contact lens material, the silicone hydrogel, is designed specifically for the extended wear patient, and incorporates the best characteristics of both RGP and hydrogel materials. By incorporating silicone into the hydrogel matrix, extremely high oxygen transmission, or hypertransmissibility, is combined with the comfort and dehydration resistance of conventional hydrogels.

Bausch & Lomb's PureVision lens was the first lens of this type to acquire FDA approval (7-day extended wear). Its material, balafilcon A, incorporates B&L's advanced AerGel technology and has been shown to outperform conventional extended wear contact lenses on key parameters, including overnight corneal swelling, hypoxia-related effects and the level of microbial adherence to exfoliated corneal cells. The Dk/t of Purevision is 110 (@
-3.00D) and its water content is 36 percent.

Marketed as Focus Night & Day abroad, CIBA Vision's new material, lotrafilcon A, a biphasic block co-polymer comprising a highly permeable fluorosiloxane-based polymeric phase coupled with a water phase, is pending FDA approval in the United States. The Dk/t of lotrafilcon A is 175 (@ -3.00D) and its water content is 24 percent. Clinical trials have shown very low amounts of corneal swelling when compared to currently available 7-day extended wear contact lenses.

These new hybrid contact lenses offer oxygen permeability in the ranges previously seen only in hyper Dk RGP materials, making them ideal for patients with hypoxia-related conditions and adding another tool to the contact lens practitioner's assortment. As an extended wear lens, they can also be thrown into the mix as yet another alternative for patients who are considering refractive surgery.

Keeping abreast of the latest contact lens designs and materials provides practitioners with the best opportunity to ensure a successful contact lens fit and to maintain a happy contact lens patient. A comprehensive knowledge of contact lens materials is imperative in solving comfort and lens fit issues and in providing patients with the best alternatives to meet their needs.

Dr. Giedd is in the second year of his cornea and contact lens graduate fellowship at the Ohio State University College of Optometry in Columbus, Ohio.