Silicone Hydrogels at ARVO 2004


Alisa Sivak, MA, DipEd

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

Simone Bayer and colleagues (CCLR) presented the results of a study investigating the effect of Clerz ® Plus rewetting drops on comfort and protein deposition with silicone hydrogel lens wear. Thirty-two subjects wore silicone hydrogel lenses on a continuous wear basis for one month, instilling either saline or Clerz ® Plus four times per day. Lenses were then placed in an extraction buffer, after which total protein, lysozyme and denatured lysozyme were quantified. The investigators found that though the drops did not significantly lower lens comfort and dryness symptoms, they did reduce protein and lysozyme deposition and had an impact on mucous discharge on waking.

Silicone hydrogel lenses tend to accumulate significantly fewer protein deposits than conventional hydrogel lenses. Mary-Anne Glasier and colleagues (CCLR) presented the results of a study aimed at identifying a procedure sensitive enough to accurately and reliably quantify such small amounts of protein. The investigators evaluated the sensitivity and accuracy of four protein assays in addition to assessing their cross-reactivity to unworn silicone hydrogel materials. Results indicated that the amido black (AB) on nitrocellulose (NC) protein assay provides a reliable and sensitive means of quantifying total protein as low as 50 ng.

Another team from the CCLR has found that lysozyme deposits undergo a decrease in total mass relative to storage time after being extracted from silicone hydrogel contact lens materials, a factor that interferes with accuracy when quantifying total lysozyme deposition. Lakshman Subbaraman and colleagues presented the results of a study in which the investigators developed a procedure allowing them to preserve extracted lysozyme deposits without loss of any mass. A Modified Reconstitution Buffer (MRB) containing salt and BioStab TM, stored at -70° C, resulted in minimal loss of protein.

Holly Lorentz and colleagues (CCLR) presented the results of a study aimed at optimizing the extraction of lipid deposits found on balafilcon and lotrafilcon contact lens materials. After doping the lenses by immersing them in a lipid solution, the investigators performed a comparison of four extraction procedures. Results showed that 2:1 chloroform:methanol was significantly more efficient in the extraction of lipids from both materials, capable of removing more than 90% of lipid deposits. The study also found a greater quantity of lipid deposits on balafilcon lenses.


Adsorption and Release of Chemical Agents

Karen Sentell and Elizabeth Beaullieu, from CIBA Vision, presented the results of a study aimed at determining the preservative (PHMB) uptake and release profiles of balafilcon and lotrafilcon contact lens materials when paired with a PHMB-containing lens care product. The lenses were soaked in PHMB containing solutions, which were subsequently analyzed using High Performance Liquid Chromatography (HPLC). The lenses were then soaked in a phosphate buffered saline solution, which was analyzed to measure their release of preservatives. Results showed that though each lens care product tested contained the same amount of PHMB, its level of uptake onto the lenses was dependant on the formulation matrix for each solution: some solutions were more inclined to be adsorbed by silicone hydrogel materials. The study also found that the lotrafilcon lenses were less inclined to adsorb PHMB than balafilcon lenses.

Another poster presentation examined the cornea’s response to chemical agents released by both conventional hydrogel and silicone hydrogel lenses. Renée Garofalo and Nissanke Dassanayake from Alcon Research, Ltd. presented the results of a series of pilot studies examining the adsorption and release of chemical agents found in marketed multipurpose solutions and their effect on corneal staining and subjective symptoms. Subjects wore lenses pre-soaked in a variety of multipurpose solutions, rating comfort and ocular symptoms at the end of each wearing period. The ocular surface was examined with fluorescein at baseline and after lens removal. Results showed that clinically significant but asymptomatic corneal staining occurred with both hydrogel and silicone hydrogel lenses pre-soaked in solutions containing PHMB. The investigators concluded that clinically meaningful staining may appear when some types of lenses are paired with a PHMB based system on a daily basis, though patients may not report any subjective symptoms.


Dryness Symptoms and Subjective Comfort

Practitioners generally advise patients that they will become aware of reduced comfort or vision if they insert their lenses inside-out. Nina Tahhan and colleagues from Vision CRC at the University of Sydney (Australia) found, however, that this advice is not necessarily accurate with silicone hydrogel lenses. The investigators examined 12 lenses in a double-masked study in which subjects wore an inverted lens in one eye and the same lens type, correctly inserted, in the contralateral eye. Subjective ratings of comfort and vision were collected at baseline and after 20 minutes of wear. Though the conventional hydrogel lenses received significantly worse comfort and/or vision ratings when inverted, subject did not report any differences with the silicone hydrogel lenses.

Optimizing comfort and reducing dryness symptoms are an ongoing concern with soft contact lens wear. Jran Jerome Ozkan and colleagues at Vision CRC at the University of Sydney (Australia) displayed the results of a single-masked study investigating the influence of solutions of varying viscosity on comfort with the wear of silicone hydrogel and conventional hydrogel lenses. With a silicone hydrogel lens on one eye and a conventional hydrogel lens on the contralateral eye, three solutions were randomly instilled into subjects’ eyes following lens insertion, after two hours, and after four hours. After each instillation, subjects rated both comfort and dryness. All solutions were effective in improving initial comfort with both silicone hydrogel and conventional hydrogel lenses, but comfort at six hours was not significantly influenced by any of the solutions. Dryness with silicone hydrogel lenses was reduced more by the more viscous solutions than by saline.

David Meadows and colleagues from Alcon Laboratories presented the results of a study looking at the wettability of contact lens materials. Using a sessile water drop technique in conjunction with high speed video equipment, his team measured the contact angle of solution on both pHEMA-MAA and silicone hydrogel lenses. After exposing the lenses to saline solution and air for a number of cycles to simulate exaggerated blinking conditions, results showed that the two types of lenses have different dewetting patterns: the pHEMA lenses showed more dewetting properties with each cycle of saline solution-air exposure, while the wettability of the silicone hydrogel lenses was dependant on the type of surfactant used in the pre-soaking solution. These dewetting characteristics appear to be dependant on the surface chemistry of the lenses.


Corneal Infiltrates

CIBA Vision’s John McNally and Robin Chalmers, clinical trial consultant in Atlanta, Georgia, presented the results of a study relating the development of corneal infiltrates with continuous wear of lotrafilcon A lenses to factors reported by wearers at baseline. Subjects completed an extensive questionnaire, and responses from wearers who developed a corneal infiltrate since enrollment were compared with responses from wearers who did not develop infiltrates within the first 3 months of the study. The results show that the development of corneal infiltrates with continuous wear of silicone hydrogel lenses may be associated with age under 30 years, casual hand washing behaviour and swimming while wearing lenses, while new contact lens wearers tended not to develop infiltrates.


Measurement of Intraocular Pressure by Through a Silicone Hydrogel Lens

R. Allen, from Essex County Hospital in Colchester, UK and colleagues presented the results of a study investigating whether accurate intraocular pressure (IOP) measurements could be made through a therapeutic silicone hydrogel contact lens using applanation tonometry. Using a Goldmann applanation tonometer, the investigators measured IOP on a bare eye and with a -0.50D silicone hydrogel contact lens in place, a power commonly used as a bandage lens. There was no significant statistical difference between the two groups, indicating that accurate IOP measurements can be made through a bandage silicone hydrogel lens.