How Big Can High Index Get?
What's feeding the mid- and high-index market, and why the bubble isn't likely to burst
By Joseph L. Bruneni

Most experienced doctors and dispensers developed their professional skills in a world dominated by CR 39 lenses. The U.S. conversion to plastic lenses began in earnest 30 years ago when the FDA ruled that glass lenses had to be 2.2mm thick. This resulted in increased weight for all post-1972 glass lenses, which eventually made plastic lenses the U.S. standard.

For most of the past 20 years, CR 39 has been the material of choice, and today, glass represents less than 5 percent of all lenses dispensed in this country. Recently, however, CR 39 has been losing market share to lens materials with a higher refractive index.

The following review examines mid- and high-index lens materials and the reasons for their increasing popularity.

WHY ALTERNATIVE MATERIALS?

Five factors are influencing which lens materials are recommended to the patient. Traditionally, high index has created thinner edges for minus corrections. Cosmetic need was what inspired the growth of mid- and high-index lens materials, but that is not always the primary reason for choosing lens materials other than CR 39.

The cosmetic factor. This is the basic reason high-index materials were created. When labs surface a minus lens in a substrate having a higher index than CR 39 (index 1.50), the resulting lens has thinner edges. This fact has led to an escalation of new materials possessing higher and higher refractive indices. Currently, the highest plastic index is 1.71. A 1.74 index was available briefly, but was later taken off the market. Dispensers can order plastic lenses in nine indices with a higher index than CR 39.

The economic factor. As the index increases, so does cost. Higher fees usually mean better profits, producing another reason for using high-index materials. Increased cost sometimes governs which index is recommended to the patient.

However, the cost of photo-chromics, progressives, aspheric lenses, and AR coatings has increased eyewear fees considerably. Each option adds cost, and recommending higher index materials can sometimes push the eyewear beyond a patient's means. This helps explain why more lower-priced, mid-index (1.53 to 1.57) lenses are being dispensed than higher index ones.

The technology factor. Patients can choose from a growing variety of high-tech lens options involving special substrates. Ordering advanced lenses such as photo-chromics, which all vary chemically from the basic CR 39 formula, automatically eliminates CR 39 as an option. Many of the newest photochromic lenses feature a mid-index.

The liability factor. One of the earliest issues impacting CR 39 turned out to be the liability factor. Courts awarded judgments to injured eyeglass wearers who were not informed that one lens material (poly) was more resistant to impact (and possible eye injury) than other lens materials.

Poly had other advantages besides strength. It qualified as high index (1.59) and offered 100 percent UV protection. Poly's most important advantage was that it proved to be the least expensive high-index material. These advantages helped polycarbonate capture more than 30 percent of the U.S. market.

Recently, another lens material, Trivex�, became available, offering eye protection similar to polycarbonate. Produced by Hoya and Younger Optics from a resin by PPG, other lens manufacturers are also planning to produce Trivex lenses--a further sign of the continuing trend away from CR 39.

Trivex has a relatively high Abbe value and qualifies as the lightest of all lens materials. Its index is 1.53, barely qualifying as mid index. With strength comparable to polycarbonate, however, the new Trivex lenses can be surfaced to 1.0mm centers.

*The dedicated use factor. This issue may, in time, influence choice of lens material more than any other factor. Alternative methods of producing ophthalmic lenses are now available, and each requires dedicated substrates created to meet lens fabrication requirements. Some forecasters predict that ophthalmic lenses in the future will be produced in totally different ways than today's conventional processes.

ALTERNATIVE METHODS

Here are a few examples of how lens design and lens fabrication processes can govern the practitioner's choice of lens substrate.

Rodenstock/2C lenses are cast-to-prescription, using a special 1.56 liquid monomer to create lenses for each patient's precise requirements. The monomer is proprietary and designed to meet the special demands of this type of lens casting.

Optical Dynamics has become dominant in the field of in-house casting. Producing lenses with its process involves 1.56 plastic monomers developed to meet the requirements of its casting process. The company uses several variations of monomers, including a 1.56 photochromic resin.

In a technology exchange between Essilor and Nikon, a complete lens line features what Nikon calls "index optimization," meaning that the index increases as the power increases, completely eliminating any need to select an index. The lenses are also identifiable to the consumer for marketing purposes.

Essilor also independently produces millions of lenses in mid- and high-index substrates. Recently they announced a new lens casting process for labs--not involving CR 39--that will gradually be introduced into Essilor-owned laboratories.

In the polycarbonate category, Optima has introduced a line called Resolution�. The lenses are produced in a way Optima representatives say produces lenses that are "birefringence" free. What distinguishes them is the fabrication process used for producing the lenses.

At SOLA, a division called Contour� Optics has produced a totally new form of polycarbonate ophthalmic lens called Enigma. It has a deep, 16 diopter base curve and provides improved peripheral vision, along with a unique appearance.

The newest entry in alternative lens fabrication is Johnson & Johnson, whose long-awaited release of Definity 2� features the most complex substrate, actually a composite laminate of a special plastic combined with polycarbonate. The Definity 2 lenses are now available in several markets and will eventually be available nationwide.

With so many choices, it's now possible to recommend lenses for each patient that are specifically designed to meet their individual needs--needs ranging from cosmetics to multiple pairs designed specifically for their multiple lifestyle needs. Dispensing eyewear may have been easier in the days when the only question was "Glass or plastic?," but the lens options today produce eyewear superior to the best available only a few years ago.

CR 39� and Trivex� are registered trademarks of PPG Industries, Definity 2� is a registered trademark of Johnson & Johnson, and Contour� is a registered trademark of SOLA International.