CUSTOM SILICONE HYDROGEL
A New Era of Specialty Soft Contact Lens Fitting
A new latheable silicone hydrogel material brings silicone hydrogel advantages to challenging lens fits.
By Timothy O. Koch, COT, FCLSA
Tim Koch is a professional services consultant for Contamac, US. He has more than 30 years of experience in the eyecare industry and is well known as an author, lecturer, and specialist in the contact lens field. He currently sits on the Board of the Contact Lens Society of America and on the Board of Directors of the Contact Lens Manufacturers Association of America.
Market reports indicate the steady increase of silicone hydrogel (SiHy) lenses as practitioners' modality of first choice. While this has been an easy transition for patients whose prescriptions fall into a relatively "normal" range of spherical and standard toric prescriptions, the availability of SiHy lenses for unique prescriptions and specialty designs has been very limited—which is frustrating as these are the most challenging clinical cases. SiHy lenses for patients who require unique designs and prescriptions, and who could most benefit from the added oxygen delivery of modern SiHy materials, have been more or less unavailable—until now.
Benefits of Silicone Hydrogels
Silicone hydrogel lenses were introduced in 1999, and their benefits in terms of oxygen performance have been well documented (Alvord et al, 1998; Tighe, 2007; Dumbleton and Jones, 2008). Several studies have reported reduced limbal hyperemia and corneal vascularization in daily wear with SiHy lenses compared to with traditional hydrogels (Papas et al, 1997; Fonn et al, 2002; Stern et al, 2004; Dumbleton et al, 2006; Malet et al, 2003). Other studies have also found improved comfort and reduced symptoms with SiHy lenses (Long and McNally, 2006; Fonn and Dumbleton, 2003; Riley et al, 2006).
Today, about one in every two lens fits in the United States is currently with a SiHy material (Morgan et al, 2009). The increased oxygen transmissibility of these materials supports corneal physiology better as compared to traditional hydrogel materials and is especially desirable in challenging cases:
● Corneas that have been exposed to lifelong low-Dk lens wear.
● Patients requiring corrections for irregular corneas, astigmatism, higher myopia, and hyperopia.
In these cases, the higher oxygen supply to the cornea available with SiHy materials can be appreciated even more. Unfortunately, these more challenging lens corrections are typically not available in SiHy materials. Patients who have optical needs beyond the limits of the currently available SiHy lenses have had to rely on traditional contact lens materials.
Growing Interest in Soft Specialty Designs
A whole range of new soft specialty lenses are entering the marketplace, and attention has been drawn to this topic in the ophthalmic media. At the Global Specialty Lens Symposium in January, for instance, soft specialty lens fitting was a major topic. More recently, a newsletter available at www.softspecialedition.com was launched that follows and explores the many options that soft specialty lenses offer.
Examples of currently popular soft specialty lens topics include new keratoconus soft lens designs and customized wavefront-designed soft lenses that optimize optical quality. The availability of this technology means two things: first, the back surface of soft lenses can now be optimized using the custom design technology available in SiHy material. This includes, for instance, sophisticated designs such as quadrant-specific soft lenses (Figure 1). Second, the front surface can be optimized to correct many higher-order aberrations (HOAs). This may mean, as an example, a rotationally symmetric lens that doesn't need stabilization to correct spherical aberration, or lenses with dynamic stabilization that can correct any other HOAs (of which vertical coma is important from a clinical standpoint because of its high frequency in keratoconus). These lenses need to become available in SiHy materials, as appropriate, so that the patients can benefit from the newest technology available.
Figure 1. Topography of a keratoconic eye (top), diagram of a quadrantspecific soft lens design for this eye (middle), quadrant-specific soft lens on patient (bottom).
There is also much to gain from specialty soft lens applications in multifocal lenses. Considering the individual characteristics of the eye (including the wavefront pattern again) can assist in designing the optimum multifocal lens for each individual patient. Higher Dk would definitely be desirable for the development of (typically thicker) translating soft lenses to keep up with the oxygen demands of the cornea.
Furthermore, high ametropia, high toricity, and combination lenses (such as toric bifocal lens designs, etc.) all could benefit tremendously from specialty lens innovation in a SiHy material.
But maybe the biggest potential of all, possibly involving millions of lens wearers, is the theoretical option to reduce myopia progression with soft lenses. At the recent Association for Research in Vision and Ophthalmology (ARVO) meeting, much attention was drawn to this topic, with one company even announcing "the soft myopia controlling lens of the future." While it may be just a bit early for that, there are very promising studies currently underway, mostly on the effects seen in orthokeratology lenses, that show a reduction in kids' myopia progression with the required special optics contact lenses. The main question at this point is: what is the optimum optical design to achieve maximum effect? Several soft contact lens designs have been proposed: concentric rings, center-near bifocal lenses, and other lens designs. But it seems only a question of time before we can actually apply some of this in clinical practice. Of course, it would seem obvious that these contact lenses should provide the highest oxygen delivery to the cornea possible.
Availability of Specialty Silicone Hydrogel Lens Materials
Apart from the cast-molded lenses available, how flexible are manufacturers in making any kind of specialty lens desired? The good news is that manufacturers are now able to lathe cut SiHy materials to better meet patients' needs. The two major brands that are available now for this purpose are the Ciba Vision O2Optix Custom, which has a 32 percent water content, Dk of 82, and modulus of 1.1 Mpa, and the Contamac Ltd. latheable Definitive material, which has a 74 percent water content, a Dk of 60 and a modulus of 0.39 MPa and can be manufactured in any lens design desired. O2Optix Custom is manufactured in the sifilcon A material, which has known and well documented characteristics and performance. But what do we know about the Definitive material? It is a newly developed material now available from the United Kingdom, specifically developed to serve the purpose described. Table 1 lists some of its characteristics. Recently, the Definitive material underwent both U.S. Food and Drug Administration (FDA) testing and a separate clinical comparative evaluation.
Studying a New Latheable Silicone Hydrogel
The FDA trial, "Three Month Evaluation of the Contamac Silicone Hydrogel Lens REC#08/H0206/45," was coordinated by Visioncare Research Ltd. It was performed at four different sites in the United Kingdom by appropriately qualified and registered optometrists with IRB approval of the protocol. The investigators' inclusion criteria included spherical distance correction in the range of −0.50D to −10.00D and cylindrical correction of 1.00D or less in both eyes, and only normal eyes with no evidence of abnormality or disease. They compared the Definitive material to a predicate device, the Biofinity (CooperVision) SiHy lens, which is not a latheable material. On average, subjects in the test lens group had −3.07D of sphere compared to −4.48D in the control lens group. Refractive astigmatism was 0.29D and 0.39D in the test and control groups, respectively. From the test results it was clear that the Contamac SiHy material performed well and proved to be safe and effective over a three-month period, performing equally as well as the control material in the test.
An additional independent study was initiated to test the new material, supervised and coordinated by Eef van der Worp, BOptom, PhD, FAAO, in the Netherlands. This study has been reported elsewhere and will be reviewed here (Koch et al, 2010 GSLS Poster SP41). In this multi-center study, custommade lenses (HydroWave, Ultravision) were manufactured in the Definitive material by Ultravision CLPL (Cambridge, UK) and compared to the described O2Optix Custom lenses. The aim in this three-month, patient-masked, prospective study was to evaluate comfort, vision, lens performance, and corneal/conjunctival physiology of the new material as compared to the control lens material worn on the fellow eye.
The investigators looked at subjects who had high myopia spherical corrections with a −6.00D minimum. The reason for choosing spherical corrections only was to purely compare lens material. If, for instance, toric lenses would have been used, this would have resulted in comparing lens designs rather than material performance, according to the investigators.
The investigators followed 34 subjects ranging in age from 17 to 65 years for three months, during which lens material was randomly assigned to one eye or the other. The mean myopic correction was −8.61 ± 2.00D (range from −5.50D to −15.25D), with a mean corneal astigmatism of 0.57 ± 0.55D.
From a clinical standpoint, there are two main points of interest: comfort and corneal physiology. So let's explore these, with comfort as the first focal point. It was found that contact lens comfort was slightly higher at dispensing in the test material group than in the control material group. The mean comfort score at dispensing was 6.4 ± 2.2 in the test group and 5.0 ± 2.3 in the control group (p=0.01), while the investigators found no differences in comfort at the later visits. Because there was absolutely no indication to the lens wearer as to which contact lens was worn on which eye, this information is of interest and is open to speculation as to why lens comfort was different at dispensing and why this difference was not present at the follow-up visits. Hypothetically, the clinically reported smooth surface of the Definitive material could be responsible for the initial higher rating in comfort. Overall preference and none of the other questions regarding the presence of discomfort (severity, time of day of discomfort, and frequency of discomfort) showed any difference between the two contact lens material groups. No difference in objective visual acuity at any of the visits was found.
Evaluation of corneal physiology in the high myopia test group in this study revealed no difference between the two SiHy lens materials. Staining type, depth, and extent were all equal at all visits, and ocular redness was also rated the same at all visits. The only slight difference was a very subtle but consistent difference in contact lens papillary conjunctivitis (CLPC) from baseline to 12 weeks, with the control material showing the highest rating.
Decreased surface quality characteristics and surface irregularities are believed to have an effect on the development of CLPC, and these would need to be monitored in future evaluations to see if the reported smooth surface of the Definitive material could be beneficial in this regard. Although no statistical significance was met at any point, overall the test material scored better with regard to surface wettability and debris buildup on the lens surface.
In conclusion, the results from these studies comparing SiHy materials intended for high myopic spherical correction indicate that the test material performed equal to or slightly better than the control materials. Comfort of lens wear was higher at dispensing in the control. Lens surface quality (both wettability and debris) and CLPC are points of interest for future studies evaluating the material.
The latheable nature of the tested SiHy material has great potential for custom-made specialty contact lens fits. For instance, currently nearly 400,000 soft lenses have been produced in the new high-Dk material and are worn by patients around the world. The Definitive material will also be available through authorized manufacturers in the United States.
Eyecare practitioners are increasingly choosing SiHy lenses as the modality of first choice. But SiHy lens options for unique prescriptions and specialty lens designs have had limited availability.
New, latheable SiHy materials have long been desired to provide patients with the latest technology available, especially for challenging clinical cases. The new SiHy lens material described in this article performed at least equally compared to existing available SiHy lenses, opening up the possibility of specialty lens modalities for a wider range of patients—including finally for those who could benefit the most. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #178.