Improving Ocular Health and Comfort With Silicone Hydrogel Contact Lenses
Improving Ocular Health and Comfort With Silicone Hydrogel Contact Lenses
Experts discuss the attributes of various lens materials and the tools used to measure lens wettability in the laboratory and in practice.
Understanding the Link Between Wettability And Lens Comfort
Learn why a wettable surface is a comfortable surface and how the latest generation of silicone hydrogels measures up.
By Lyndon Jones, PhD, FCOptom, DipCLP, DipOrth, FAAO (Dip CL), FIACLE
In a perfect world, all contact lens patients would remain happy with the level of comfort their lenses provide and never feel the need to go back to wearing spectacles full time. Currently, however, about 30% of contact lens wearers stop wearing contact lenses, particularly at the end of the day, because of discomfort.1-4 Can silicone hydrogel lenses help stem the tide of dropouts? We are seeing positive signs that this may be the case.
First, silicone hydrogels provide patients with substantially more oxygen than conventional hydrogel lenses.5 As patients wear silicone hydrogel lenses, we see reduced limbal redness and neovascularization; corneal swelling is equivalent to that of no-lens wear, and patients have a minimal microcystic response.6-12
Nonetheless, even though many silicone hydrogel wearers recognize the benefits of these new materials, many would like the lenses to be even better. A major reason why patients express dissatisfaction with silicone hydrogel lenses is because their eyes still feel more comfortable without them. Only 20% of patients report no symptoms with their lens wear, regardless of lens type.8
So, how can we address this issue? A variety of factors come into play, including dehydration,13 deposition,14 solution interactions15 and wettability,16 to name a few. In this article, I will discuss the role of wettability in contact lens comfort.
ROLE OF WETTABILITY
Contact lens wettability is critical to contact lens comfort for several reasons. We need a good wetting surface to prevent changes to the under surface of the eyelid as it blinks over the contact lens. Soft lens wearers blink approximately 11,000 times in a 24-hour period,17 so it is very important to maintain a highly wettable lens surface for as much of the day as possible. Wettability also affects lens deposition. A poorly wettable surface will cause dry patches on the lens that will increase the deposition of lipid and denatured protein from the tears.
The earliest silicone hydrogel lenses used surface treatments to counteract the hydrophobic quality of silicone and to improve wettability.18 Basically, the goal was to hide the hydrophobic silicone signal on the lens surface from the tear film, thereby improving wettability and controlling deposition. This is a technically difficult and expensive two-phase process, with the silicone hydrogel lens being molded and then subsequently plasma-treated. More recently, contact lens companies have tried to develop silicone-based materials that don't require a secondary process to modify the surface of the lens,19 but are inherently wettable immediately following manufacture.
Clinically, most practitioners determine contact lens wettability by observing the lens surface with a slit lamp biomicroscope. In some patients, particularly those who have meibomian gland dysfunction, the relatively hydrophobic surface of these silicone hydrogel materials may produce a poorly wettable surface. In these patients, using either a dedicated surfactant cleaner or ensuring they adhere to their care regimen with a rub-and-rinse step may be required to keep the lens surface clean and wettable.20
Another method of assessing wettability involves determining the ease with which a fluid spreads over the contact lens surface and measuring the angle made by the fluid on the lens surface. This so-called "contact angle" (CA) is used in many other areas of biomaterial research21 and with contact lens materials.16,22,23 The more a fluid spreads over the lens surface, the lower the CA and the better the wettability24 A number of methods exist to measure CA, including Wilhelmy plate,16 captive bubble22 and sessile drop methods,23 each of which provides information about a material's surface.
Given the recent release of nonsurface-treated silicone hydrogel materials, it is interesting to compare these newer materials, in terms of their wettability, with tried-and-tested, surface-modified silicone hydrogels, because historically nonsurface-treated silicone-based lenses have been very hydrophobic.25
Figure 1: Graph of sessile drop advancing contact angle for a variety of silicone hydrogel lens materials measured directly from the packaging solution and after a soak in saline to remove the influence of any surface active agents within the packaging products.
LABORATORY CONTACT ANGLE ASSESSMENT
Figure 1 shows advancing sessile drop water CA measures, in which a digital Optical Contact Angle Analyzer was used upon removing various silicone hydrogel contact lens materials from their packaging (blue bars) and after rinsing them for several minutes in unpreserved saline (green bars). This was done to remove the influence of the packaging solutions in which the lenses were stored following manufacture.
The graph clearly shows there are differences in wettability as determined by advancing CA between the lens materials. The material with the lowest CA was Acuvue Advance, followed by Biofinity and the two CIBA surface-coated silicone hydrogels (O2Optix and Night & Day). Acuvue Oasys and PureVision had very high advancing CA. Following a saline soak, the angles for most of the materials changed slightly, with most increasing in CA by only a few degrees. However, the CA for Acuvue Advance increased substantially, implying that surface active agents within the packaging solution may have been rinsed away by the saline rinsing procedure.
INFLUENCE OF SOLUTIONS
When it comes to wettability, practitioners also must consider the lens care products used with lens materials. Most modern multipurpose care products contain surfactants, which both clean the lenses and improve surface wettability.16,23 In Figure 2, the blue bars show the CA for three silicone hydrogel materials after soaking in saline to remove packaging solution effects. What happens if these materials are then soaked overnight in modern care systems, such as OptiFree Replenish (Alcon), a multipurpose care product that uses a combination of various wetting agents and surfactants? The green bars represent the advancing sessile drop water CA measured after the various lens materials have been soaked overnight in OptiFree Replenish and then rinsed in saline to remove any loosely bound solution. It is clear that Replenish has little impact on the CA measures on some of the silicone hydrogel lens materials. However, Replenish has a significant impact on wettability on the comfilcon A material (Biofinity) as assessed by advancing CA. The clinical relevance of these differences in CA remains to be investigated in longer-term clinical studies. But the contact lens industry may see the development of targeted solutions that work optimally with certain lens materials — particularly silicone hydrogel lens materials.
Figure 2: Graph of sessile drop advancing contact angle for three silicone hydrogel lens materials measured after soaking in saline to remove the influence of any surface active agents within the packaging products and after an overnight soak in OptiFree Replenish.
Ultimately, clinical performance of the contact lens materialis of much greater value than any laboratory-based measure. Thirty-one existing soft-lens wearing subjects were enrolled in a prospective, double-masked, contralateral-eye study in which they wore the nonsurface-modified silicone hydrogel material comfilcon A (Biofinity) and the surface modified silicone hydrogel material balafilcon A (PureVision) continuously for 30 nights.26
Of these subjects, researchers selected 12 wearers randomly at the 12-month visit. After the participants wore the lens materials for 30 nights continuously, investigators measured ex vivo wettability with the sessile drop technique and assessed clinical objective and subjective measures.
Researchers graded subjective comfort on a 0 to 10 scale, with 0 being extremely uncomfortable and 10 being extremely comfortable. Vision and corneal physiology were similar between the two materials (p>0.05), as were the subjective comfort scores (8.9±0.9 for comfilcon vs. 8.3±1.2 for balafilcon; p>0.05). Ex vivo wettability measures on lens removal were statistically different, with the comfilcon lenses exhibiting lower CA (60±10° vs 94±13°; p<0.001). A statistically significant correlation was found between comfort and contact angle (r=-0.43, p=0.04), with higher comfort scores associated with lower contact angles. This study demonstrates that a nonsurface-treated silicone hydrogel lens can provide a surface that exhibits excellent wettability and subjective comfort. It also shows that measures of ex vivo wettability appear to be related to contact lens comfort, at least in this study, although other trials have shown that this is not always the case.27
|Dr. Jones is a professor at the School of Optometry and the departments of physics, biology, chemical engineering and chemistry at the University of Waterloo, Ontario, Canada. He is also associate director at the School of Optometry's Centre for Contact Lens Research.|
As the contact lens industry continues to introduce ever "smarter" polymers and lens care products, it is likely that these will have a positive impact on the number of contact lens dropouts. In addition, wettability likely will be just one factor to consider as clinicians try to enhance the quality of their patients' contact lens experience. CLS
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Contact Lens Spectrum, Issue: June 2007