Technology to Optimize GP Comfort and Success
The technology behind today's GP lenses makes them not only more comfortable, but much easier to fit.
By Cary M. Herzberg, OD, FOAA
In today's marketplace, technological breakthroughs seem to occur almost daily. The GP lens industry has been no exception. This period of advanced GP technology lists among its achievements second-generation hybrid lens designs, computerized lathing with micron accuracy, advanced scleral lens designs, advanced materials with hyper-oxygen permeability, high technology topography systems, and powerful computer software for contact lens design. These breakthrough products provide serious tools to answer almost any visual need that patients may have—all in an easy-to-wear, comfortable fashion (Bennett, 2011; Kollbaum, 2009; and others, full list available at www.clspectrum.com/references.asp).
Why GP Lenses?
Today GP lenses make up 10 percent of contact lens fittings in the United States (Nichols, 2011). Despite this, we have a cornucopia of new technological advances to discuss (van der Worp, 2010). Why this interest in the face of a shrinking marketplace for new products?
The fact is, when you are addressing a refractive error that has corneal aberrations in its makeup, or would like to fit the safest lenses to your patients, the gamut of best choices today still includes GP lenses (van der Worp, 2009). GP lenses, when properly fit, will deliver the best vision results of any medium—vision that is, for the most part, free of aberrations (Kollbaum, 2009). In fact, in almost any category in which vision with contrast sensitivity is the sole decision maker, GP technologies win hands down (Bennett, 2011; Coletta, 2005).
The development in GP technology has made it possible to now fit these more accurate lenses with less chair time, resulting in incredible vision with comfort approaching that of soft lenses. This is not limited to scleral designs, either; with powerful lens design software at your fingertips, corneal GP lenses can be fit so well that there is little or no adjustment period for even life-long soft lens wearers (Potter, 2008).
The best place to start when discussing GP technology is with the materials. Advances in polymer technology, combined with current understanding of eye physiology, have produced remarkable polymers over the last few decades that have changed the landscape in contact lens fitting. This material revolution has brought advances in oxygen permeability (Dk), wettability, and thinner designs with less flexure. With oxygen transmission (Dk/t) high enough to satisfy even the harshest critics, materials could be specified for even the most demanding wearing situations, whereas new generations of low-to-moderate Dk materials are available for daily wear myopes.
Plasma Treatment The wettability of contact lenses made with silicone-acrylate polymers has always been an issue due to the polymers' hydrophobic properties. Fluoro-silicone acrylates can be enhanced to bring more mucin interaction with the lens surface, which reduces the lens hydrophobic properties. To answer surface wetting issues, plasma treatment is utilized to increase lens wettability. Plasma treatment involves cold gas plasma in an energetic process utilizing electrical energy to remove debris and contaminants from a GP contact lens surface. This creates a more desirable surface, engineered to enhance wettability and comfort (Laurenzi, 2008).
High-Index Materials With the growth in the baby boomer generation and the increased near point demands made on computer users came the quest for the ultimate multifocal contact lens design. Current presbyopic lens designs often provide insufficient add power to meet the near demands of moderate-to-advanced presbyopic patients. Higher-oxygen materials often have a lower index of refraction, resulting in increased center thickness, heavier lenses, and lower effective adds for reading.
The development of higher-refractive-index materials such as Paragon HDS HI (Paragon Vision Sciences), which has a refractive index of 1.54, and Optimum HR (Contamac), which has a refractive index of 1.51 or 1.53, can help with this problem. With the subsequent reduction in edge and center thickness, the finished lens comfort and fit is often dramatically improved. These lens materials, in combination with anterior aspheric multifocal designs and advanced manufacturing techniques resulting in high optical quality and aberration control, have greatly improved GP multifocal contact lens designs (Bennett, 2011).
Improvements In Lathing Technologies: Surface Shape Accuracy
To develop GP technology to its full potential, improvements were needed in the lathing process. With the advent of the newer two-axis machine technology, scleral lenses, for example, can be manufactured more easily and accurately, thus reducing their cost. The older three-axis lathes (with a rotary axis under the diamond tool) could not machine lenses of large diameters and extreme sagittal depths without ruining their edges. For designs extending beyond the cornea, the newer technology allowed for quadrant-specific designs in nonrotationally symmetric lenses for better ocular health due to fewer pressure points on the sclera. For corneal designs, custom-designed lenses based on topographical data points became possible (Pantle, 2011).
The cube spline, which replaced aspheric curves used as blends to connect two spherical curves on the back surface of a lens, resulted in a seamless junction on the back surface with a smooth contour. The cube spline is actually a formula that determines the bridge or blends between the two spherical curves (Bennett, 2011).
Today's modern lathes provide submicron machine control, resulting in surface shape accuracy of 2 to 5 microns. These superb surface finishes allow for minimal surface polishing times, often less than 12 seconds. This reduces surface geometry changes during the polishing cycle.
The advancements in lathing technology have resulted in more comfortable lenses, in large part due to their being fabricated accurately within several microns of tolerance, and at a reduced cost when compared to their predecessors.
The standard procedure in contact lens practice used to measure the cornea is keratometry. Keratometers typically measure the average curve of the central three millimeters of the cornea in only two meridians. With contact lenses covering a much larger area of the cornea, more advanced diagnostic equipment was needed.
Corneal topographers can instantly provide up to a thousand data points and also a better understanding of corneal shape, which is important in corneal reshaping and also in fitting asymmetric corneas. Placido disk-based topographers (reflection) are the most common, but estimate radii of curvature to derive elevation. To solve this problem, modern topographers utilize height maps, which use algorithms combined with logical interpolation/extrapolation to derive reasonably accurate height data. This data is essential for both the manufacture and design of custom lenses. Projection topographers are somewhat more complicated, measuring sagittal height directly, but due to their high cost are typically found only in research settings (Legerton, 2011).
Because topographers typically measure only the surface of the cornea out to 8mm to 10mm, measuring the shape of the limbal area and the first part of the sclera was not possible. With the advent of anterior segment optical coherence tomography (OCT), not only can this surface be measured but also scleral lens fits can be analyzed for optimum alignment. Studies of the sclera with OCT have revealed that not only can the sclera be toric, but it can also have one segment steeper or flatter than the rest. This would require a quadrant-specific scleral contact lens for optimum fit and comfort, now possible with modern computerized lathing (DeNaeyer, 2011).
Scleral GP Lens Designs
The Contact Lens Manufacturers Association (CLMA) held a meeting in 1993 at which the organization asked its members to come up with a new GP design that would provide the vision of a rigid lens with the comfort of a soft contact lens. Bob Cotie of C&H Contact Lens, Inc. proposed a GP lens design that would exhibit minimal movement, have a larger diameter than the cornea while still being aligned with it, and exhibit a minimal edge standoff. He called his design Macrolens, and it began the modern era of scleral GP lens designs.
The initial success of the Macrolens has led to many different but related designs. One example of a versatile innovative design using these new technologies is the Digiform series from TruForm Optics. Digiform lenses are designed using OCT profiles of corneas specific to such conditions as keratoconic, post-surgical, and traumatized corneas; thus they accurately profile the corneal characteristics found in these conditions. This results in a marked improvement in successful first fit outcomes, even with empirical fitting. Practitioners who still prefer to trial fit these patients can borrow or purchase a fitting set from TruForm Optics for the specific condition being treated.
Even though the market today for scleral contact lenses has begun to mature, it remains a niche product used mainly for diseased, traumatized, post-refractive, keratoconic, and corneal transplant eyes. But with the growing demand, scleral lens education has become an important component of specialty contact lens symposiums, such as the Global Specialty Lens Symposium (GSLS). The Scleral Lens Education Society (www.sclerallens.org), which was launched in 2010, is dedicated to teaching practitioners how to fit and use scleral contact lenses in their specialty lens practices.
Advanced Scleral Contact Lens Design: Mainstreaming the Modality
Bob Cotie, who, as previously mentioned, designed the Macrolens contact lens design, has also designed a new generation of scleral contact lenses called Solace (C&H Contact Lens). To simplify the fitting paradigm, Solace is designed to fit the eye like a conventional GP contact lens with a similar fluorescein pattern (Figure 1). The Solace design has only two scleral skirt alternatives for each base curve, which is fit equivalent to the average keratometry value to make the fitting process as easy as that of traditional soft contact lenses. While 50µ of central clearance will compromise the fit of most scleral contact lenses, Solace can be successfully fit with as much as 200µ of clearance, giving it four times more latitude in fitting. Solace is intended to fit normal corneas and is designed to be fit empirically, according to C&H Contact Lens.
Figure 1. The Solace lens (C&H Contact Lens).
Another innovative design is the NormalEyes 15.5 lens (Paragon Vision Sciences) designed by Dr. Jerome Legerton and Bill Meyers. NormalEyes 15.5 is also targeted to fit normal corneas. The lens has three distinct posterior zones—the base curve radius (BCR), return zone depth (RZD), and the landing zone angle (LZA), which are the same terms used for Paragon CRT lenses—that can be changed completely independently from each other. The lens also features the dual-axis technology utilized in the Paragon CRT series, which successfully controls lens flexure. The lens was designed using corneal and scleral contour studies conducted at the University of Arizona and can be prescribed using topographic trend analysis or sagittal depth caliper values from OCT, resulting in higher first-time order success. The final lens parameters are achieved with one or two diagnostic lens observations from a trial set. NormalEyes 15.5 is intended for patients who have any refractive error, but is particularly useful for high refractive errors in which corneal lens centration is challenging, for dry eye and ocular surface disease, or when patients require better vision than they can achieve from current soft lens technology, according to Dr. Legerton.
Hybrid Lenses Come of Age
Two-phase technology or hybrid lenses offer soft lens comfort with the vision advantages of GPs. They consist of a GP center surrounded by a soft hydrophilic skirt. The original Precision-Cosmet Saturn II design, which later evolved into Softperm (Ciba Vision, lens no longer available) was made from a single button and featured two distinct phases with a transitional zone. The lens was also lathe cut on both surfaces, but suffered from a tearing problem at the soft/hard juncture. In addition, patients would experience edema and neovascularization due to the low Dk of the hydrophilic skirt.
In 2005, SynergEyes, Inc. received Food and Drug Administration (FDA) approval for its first hybrid lens. The SynergEyes A differed from Softperm in that it used the high-Dk Paragon HDS100 (Paragon Vision Science) material for its GP portion while its soft skirt portion also exhibited improved properties over its predecessor. To minimize the tearing problem, SynergEyes incorporated a hyperbond junction between the rigid and soft portions. The company has produced innovative designs for fitting keratoconus, post-refractive surgery, and other irregular corneas.
Recently SynergEyes introduced a new design called Duette, which is designed to fit regular astigmatic corneas (Figure 2). Duette features a new hyper-Dk (130) material (MaxVu) GP center that is flexure resistant and has a low wetting angle. The soft outer skirt has been improved with a higher-Dk (84) material. In addition, the design has been simplified in its fitting choices and will feature multifocal correction available this summer (Eiden and Davis, 2010; Davis, 2011).
Figure 2. The Duette hybrid lens design (SynergEyes).
Advanced Lens Design at Your Fingertips
By the late 1990s, technology in the form of powerful computer software was making itself felt in contact lens laboratories. Optometrist/programmer Dr. Jim Edwards had been working with a Keratron topographer (Eyequip) to create software that could custom design GP contact lenses. Dr. Edwards said that the software he developed, termed Wave, relied on the tear film layer for its calculations (Figure 3). Presented with a profile of the lens and eye, fitters could choose any number of profiles that they preferred in their final lens. For example, if you want to steepen the periphery, then 5µ can be added to the peripheral profile.
Figure 3. A multifocal Wave design.
Today many topographers provide lens design software with virtual images of potential fitting profiles to help optimize the lens-to-cornea fitting relationship. What made these custom designs possible was the development of software in the lens manufacturing process at about the same time. These powerful software programs became the core of the machine tool that is the heart of the machine functionality. This has enabled lens designers to create shapes that were unimaginable 10 years ago (Kojima and Caroline, 2009). This almost infinite ability to create different shape combinations gave lens designers the tools to create lenses of incredible comfort. The addition of Oscillating Tool Technology (OTT) allowed custom fit back-surface designs and provided an opportunity to reconfigure the front surface for multifocal additions (Kojima and Caroline, 2009). OTT's refinement over the years has added multiple new dimensions to the surface shapes of custom contact lenses (Figure 4). This technology has provided more accuracy to toric lenses and allowed for wave-front contact lenses as well as quadrant-specific designs (Pantle, 2011).
Figure 4. Lens manufacturing using Oscillating Tool Technology (OTT).
Using the Medmont Topography system (Precision Technology), Randy Kojima of Precision Technology and his team have set out to improve the efficiency and accuracy of initial lens selection while increasing contact lens comfort. This would be the ultimate fit of contact lens to corneal profile. Randy calls the use of asymmetrical back surfaces on asymmetric eyes “topography-derived surfaces” (TDS). This software achieves an accuracy level so high that it can produce the optimal tear profile in every meridian over 360 degrees. The goal is to determine the optimal apical clearance, a specific point and shape of bearing with a specific lift at a desired point, creating the best fitting contact lens (Kojima, 2011).
A Perfect Fit
The common theme throughout innovation. This technological revolution has expressed itself over the entire field. Best of all, with each new generation, the evolving technology has made the resulting contact lenses easier for practitioners to fit.
We have come a long way from ordering lenses using refractive data and just two points of information from a keratometer. Modern GP lenses are a result of the most advanced lathing technologies that involve powerful computer-driven software, which processes information provided by enhanced topography measurements of the corneal surface. This is accompanied by the use of innovative design software that keeps practitioners in the loop. Practitioners willing to consider this new technology will see, with their increase in GP skills, an increase in both satisfied patients and scope of practice. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #187.
The author would like to thank Chris Pantle (DAC); PM Hawkins, Dr. Jerry Legerton, and Ken Kopp (Paragon Vision Sciences); George Mera (TruForm Optics); Bob Cotie (C&H Contact Lens); Dr. Ed Bennett (GP Lens Institute); Jan Sevier (SynergEyes); Mark Cosgrove (C&E GP); and Randy Kojima (Precision Technology) for their help in putting together this article.
|Dr. Herzberg has been practicing orthokeratology for more than 25 years. He is the founder, president, a board member, and a fellow of the Orthokeratology Academy of America. He is an advisory board member of the Gas Permeable Lens Institute, a contact lens design consultant to C&H Contact Lens, and he shares a patent for the first scleral orthokeratology design.|