Alisa Sivak, MA, DipEd

Alisa assists the Centre for Contact Lens Research by writing and editing publications, reports, grant applications, and educational communications.

Part 1 (August 2005)

This is the second half of a synopsis of silicone hydrogel-related posters and presentations at the BCLA conference in Brighton, UK. This half of the synopsis will look at presentations relating to optics; friction, fit and material properties; adsorption of elements from the tear film; comfort and wettability, as well as lens-prescribing patterns and incidence of corneal infiltrates.

Optics

Spherical aberration can reduce the retinal image quality of the eye when it is present in sufficient magnitude. William Reindel and colleagues (Bausch & Lomb) compared change in spherical aberration with three silicone hydrogel lenses (-1.00 and -5.00D). Balafilcon A lenses consistently reduced the amount of positive spherical aberration with both -1.00 and -5.00D lenses. Of the remaining silicone hydrogel lenses, only the -5.00D galyfilcon A lens significantly reduced the amount of spherical aberration.

Friction, fit and material properties

Silicone hydrogel contact lenses are stiffer than conventional hydrogel lenses, a characteristic that can lead to mechanical complications and adaptation concerns for some patients. Lyndon Jones (CCLR) and colleagues compared friction forces of lotrafilcon A lenses with a pHEMA lens, using both protein-doped and non-doped lenses. Using a silicone eye-form approximating the geometry of the cornea and sclera, the lenses were tested with a custom-made apparatus designed to measure friction forces. Results indicated that friction forces increased with increasing normal force and decreased with protein doping. Friction forces were always higher with lotrafilcon than with pHEMA lenses, but the difference was not statistically significant.

Gareth Ross and colleagues (Aston University) investigated and compared the dynamic mechanical properties, dynamic wettability and frictional properties of commercial contact lenses. They found that silicone hydrogel materials are becoming more and more like the cornea in terms of physical properties, though not necessarily when it comes to dehydration resistance. The higher water content lenses tend to be less stiff, and all the lenses investigated had relatively low coefficients of hydrated friction, though different material surfaces produced different values. Galyfilcon A and senofilcon A, both of which contain PVP, had low friction values similar to those found with the human cornea. When fully hydrated, these lenses also had smaller advancing contact angles, though the PVP did not appear to improve dehydration resistance. Lotrafilcon B has a higher water content than lotrafilcon A, with a similar plasma-coated surface. Though this lens was initially less wettable than the PVP-modified lenses became less wettable on dehydration.

Graeme Young (Visioncare Research Ltd.) and colleagues investigated the correlation between fit and base curve in three soft lenses, including two silicone hydrogels (lotrafilcon A and galyfilcon A), fitting the steeper base curve of each lens type in one eye and the flatter curve in the other eye. With all lens types, the steeper base curve provided the best fit on the greatest proportion of eyes. Looking at corneal topography, specifically curvature, shape and height, the most consistent correlation between lens fit and corneal topography was that between centration and corneal height. Interestingly, there was no significant difference in mean K-reading between those eyes best fit with the steeper base curve and those best fit with the flatter base curve, indicating that best-fit base curve can not be predicted by keratometry.

David Austin and colleagues (Anglia Polytechnic University) used two-dimensional T1-T2 correlation spectroscopy to investigate water mobility and aqueous morphology of HEMA and silicone hydrogel lenses. Results suggest that the water in silicone hydrogels is located in a series of micro-scale domains interconnected by nano-scale channels, which form bottlenecks to water and solute permeation. Lotrafilcon A has a smaller number of aqueous domains than balafilcon A, but they tend to be larger in size. In contrast, the results found with HEMA lenses suggest a nano-scale homogeneous structure with no large aqueous domains.

Adsorption of elements from the tear film

Lakshman Subbaraman (CCLR) and colleagues used radiochemical analysis to determine lysozyme deposition as a function of time in Group IV, Group II and silicone hydrogel materials. Their results indicated that the kinetics of contact lens deposition depends on the chemical structure of lens materials: lysozyme deposition occurs rapidly with group IV materials (ionic, high water content e.g. etafilcon A) before reaching a maximum, while silicone hydrogel and Group II materials (non-ionic, high water content e.g. omafilcon A) tend to accumulate lysozyme progressively without reaching a plateau.

Immunological methods for measuring protein deposits on contact lenses, which require the removal of protein from the lens surface, indicate that silicone hydrogel materials adsorb only small amounts of protein. Lyndon Jones (CCLR) and colleagues investigated the ability of in situ imaging methods to characterize the protein adsorbed by silicone hydrogel and conventional hydrogel lenses exposed to a dilute mixture of lysozyme and albumin. Scanning Electron Microscopy (SEM) revealed that galyfilcon A and balafilcon A lenses adsorb lysozyme and albumin in relatively large aggregates, whereas lotrafilcon A lenses adsorb protein in a more evenly-spread, monolayer formation. SEM also revealed that the hydrogel lenses adsorb more protein than any of the silicone hydrogel materials. Atomic Force Microscopy (AFM) revealed that balafilcon A, lotrafilcon A and the hydrogel lens adsorb lysozyme in a similar manner, with very little deposited in discrete areas. Studies using AFM also found that lysozyme deposits on galyfilcon A lenses are grouped in both aggregate and string-like formations across the lens surface. These results indicate that deposition patterns differ between materials depending on their bulk and surface composition.

Lyndon Jones (CCLR) and Heather Sheardown presented the results of a study examining the in vitro adsorption of albumin and lysozyme from model tear solutions by a variety of silicone hydrogel and HEMA-based contact lenses. Silicone hydrogel materials adsorbed less lysozyme than HEMA materials but both materials adsorbed similar amounts of albumen. The silicone hydrogel materials adsorbed significantly greater amounts of albumin than lysozyme despite the fact that the simulated tear solution contained considerably more lysozyme.

Comfort and wettability

Ronan Rogers and Lyndon Jones (CCLR) used a contact angle measuring device to determine the in vitro wettability of daily disposable and silicone hydrogel lenses direct from their packing solution. They also measured the ex vivo wettability of etafilcon A lenses soaked in a variety of care regimens. Of the silicone hydrogel lenses used, balafilcon A had the highest contact angles; galyfilcon A was the most wettable initially but rapidly developed contact angles similar to balafilcon A; and lotrafilcon materials retained the most wettable surface overall. Though the silicone hydrogel materials were less wettable than pHEMA-based hydrogels, they could be modified by soaking in certain care regimens: contact angles were significantly lower after soaking in SoloCare® Plus (CIBA) and Opti-free® Express® (Alcon), where other solutions had minimal impact on wettability.

Conclusion

Notwithstanding ongoing research relating to silicone hydrogel lenses, these lenses continue to occupy a significant corner of the worldwide market. In fact, the results of Philip Morgan’s (Eurolens Research/University of Manchester) prospective survey of lens-prescribing habits in 14 countries indicate that silicone hydrogels remain popular. Extended wear lenses make up nine percent of prescribed lenses overall, and appear to be particularly popular in Australia (silicone hydrogels in particular) – 13% of all fits in that country. Where silicone hydrogel lenses have been launched for daily wear, they have been quickly taken up by the market: four percent of prescribed lenses overall and 13% of all lenses prescribed in the U.K.. The results of another study conducted by Morgan (12-month, hospital-based) indicated that patients wearing conventional hydrogel lenses on an extended wear basis were more likely to develop a corneal infiltrate – and a more severe version of the event – than patients wearing silicone hydrogels on an extended wear basis. These results add to the list of evidence indicating that providing adequate oxygen to the corneal surface has a positive effect on ocular health.

Part 1 (August 2005)