Alisa Sivak, MA, DipEd

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

At a conference boasting more than 9 thousand in attendance and over 6,000 presentations, 17 posters offered information relating to silicone hydrogels. Topics included wettability, lysozyme deposition, risk and avoidance of infection, and mechanical and material properties of these lenses.

Wettability and lysozyme deposition

Wettability of contact lens materials can have a significant impact on in-eye comfort. Two groups of presenters used contact angle analysis to determine that different polymer combinations interact differently with the surfactants in multipurpose solutions, producing a range of results with regard to wettability . David Meadows et al. (Alcon) found that unworn lotrafilcon A and B materials cycled through ATS-air exposures to simulate blinking conditions retain lower contact angles compared to galyfilcon A materials. Ronan Rogers and Lyndon Jones (Centre for Contact Lens Research, University of Waterloo) found that soaking unworn silicone hydrogel lenses - direct from their packing solution improved their wettability.

Lakshman Subbaraman (CCLR, University of Waterloo) et al. 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.

Zhang et al. (Georgia State University) reported the results of a study aimed at determining the in situ activity of lysozyme on unworn etafilcon A, balafilcon A and RGP lenses using a direct (no extraction) micrococcal assay and a micro-BCA assay. Results demonstrated that lysozyme deposited on etafilcon A lenses was mostly absorbed by the lens, with reduced activity on the lens surface. In contrast, lysozyme deposited on balafilcon A lenses was mostly adsorbed by (or adhered to) the lens surface . These results also suggest that lysozyme adsorbed by the lens surface is less active than the equivalent amount examined while in solution.

Risk and avoidance of bacterial infection

Nathan Efron and colleagues (University of Manchester) conducted a 12-month, prospective, hospital-based study in which the clinical severity of documented cases of contact lens-associated keratitis were scored and categorized as ‘severe’ (traditionally termed “microbial keratitis’) or ‘non-severe,’ with hospital catchment population, wearing modalities and lens types estimated from relevant demographic and market data. Results ‘indicated that there is a higher incidence of severe keratitis in patients who sleep in their contact lenses. In addition to lens type and modality, significant risk factors included gender, smoking, season and relevant eye and health problems. Infiltrative events occurring in the corneal periphery were less severe, and none of the patients suffered significant visual loss.

Katie Edwards (Vision CRC) and colleagues conducted a 12 month, prospective, population-based case control study and found that increased risk of infection is associated with showering in lenses, smoking, occasional overnight use of daily wear lenses and inappropriate handling of lenses while swimming.

C.J. Clark (Mayday Hospital) and colleagues conducted a 2-year, prospective study of 32 patients with contact lens related ulcers in order to evaluate risk factors contributing to occurrence and severity of microbial keratitis (MK). Results indicated that all extended wear and monthly replacement lenses pose a higher risk of MK compared to daily disposable lenses.

Pit Vermeltfoort (University of Groningen) and colleagues characterised lens surfaces and measured bacterial adhesion after 1 and 4 weeks of continuous wear with 2 types of silicone hydrogels. Bacteria (Staphylococcus aureus and Pseudomonas aeruginosa) adhered in lower numbers and less tenaciously to worn vs. unworn lenses - except S. aureus, which adhered in higher numbers to worn balafilcon A lenses.

Reporting the results of two studies, Alan Tomlinson, Tara Beattie and colleagues (Glasgow Caledonian University) reported a significant increase in microbial attachment with both worn and biofilm-coated galyfilcon A materials compared to unworn lenses, but noted that these lenses still display less Acanthamoebal attachment than lotrafilcon A and balafilcon A materials.

Mary Mowrey-McKee and Colleagues (CIBA Vision) used flow cytometry to evaluate the viability of human epithelial cells after exposing them to an assortment of silicone hydrogel lenses soaked in a variety of lens care solutions as well as the care solutions themselves. Cells exposed to a PHMB-based and a peroxide-based lens care system were more viable than those exposed to a polyquad-based care system.

Material properties and physiological effects

Jose Gonzalez-Meijome ( University of Minho) et al. reported the results of a study comparing 3 microscopic techniques that provide information about the surface polymer structure of silicone hydrogel materials. Atomic force microscopy allowed the researchers to study materials in their hydrated state and at a higher resolution. The high magnification of scanning electron microscopy and cryo-scanning electron microscopy revealed surface features such as differences in polymer assembly patterns, but had limited resolution and induced serious damage to materials.

Cheryl Skotnitsky (Vision CRC / University of NSW) and colleagues reported the results of retrospective clinical trials in Australia and India revealing that the incidence of local CLPC is higher in silicone hydrogel lens-wearers than with low-Dk lens wear, but that there is a significantly greater incidence of general CLPC in low-Dk hydrogel lens wear. A 22-study meta-analysis performed by Loretta Szczotka-Flynn and M. Diaz-Insua ( Case Western Reserve University) supported these conclusions.

Barbara Fink and colleagues (The Ohio State University) found that, for piggyback lens systems, differences in total thickness and material permeability are fundamental to the impact of the blink on oxygenation of the covered corneal surface. Measuring corneal oxygen uptake rates of various GP-silicone hydrogel combinations under blink and non-blink conditions, the researchers noted that there was a greater difference in response between non-blink and blink conditions with piggyback combinations that included higher Dk/t lenses.

Research has demonstrated that wearing soft lenses inside-out can induce corneal changes similar to those achieved with rigid orthokeratology lenses. Jennifer Choo (Institute for Eye Research/University of NSW) and colleagues confirmed that wearing inverted lotrafilcon A lenses resulted in corneal topographic changes consistent with traditional orthokeratology, whereas as an un-inverted lens produced very few topographic changes. Lenses with a higher minus power induced a higher number of corneal topographic changes in addition to inducing a mid-peripheral ring of epithelial erosion.

Conclusion

These presentations are an excellent demonstration of current trends in research relating to silicone hydrogel lenses.