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

Part two of this synopsis focuses on sessions addressing lysozyme and lipid deposition, adverse events, and the physiological effects of silicone hydrogel lens wear.

Lysozyme and lipid deposition

Holly Lorentz (CCLR) analyzed the effect of in vitro lipid deposition on conventional hydrogel and silicone hydrogel lens wettability, determining contact angle measurements with a sessile water drop technique. The contact angles of balafilcon, galyfilcon and senofilcon were unaffected after soaking in a lipid doping solution, whereas lotrafilcon A and B (surface treated materials) had markedly reduced contact angles after lipid exposure, as did the conventional hydrogels. These researchers note that exposure to lipid may actually improve the wettability of certain lens materials, particularly those that are surface treated.

Fiona Carney (CIBA) reported that lipid adsorption varies depending on lens material. Lotrafilcon A adsorbs less lipid, indicating that hydrophobicity drives adsorption and may be hindered by a uniform surface environment, such as that found on lotrafilcon A and B. After soaking a variety of silicone hydrogel lenses in saline-based cholesterol or phosphotidylethanolamine (PE) solution, results showed that though all lenses adsorbed more cholesterol than PE, lotrafilcon A and B adsorbed the lowest amounts of PE; in contrast, galyfilcon A and senofilcon A adsorbed significantly higher amounts.

S. Zhang (Georgia State University) noted that unworn and biofilm-coated silicone hydrogel lenses of all types absorb significantly less lysozyme compared to etafilcon A lenses after soaking in either lysozyme or a solution of lysozyme mixed with albumin, mucin or lactoferrin. Residual enzymatic activity of lysozyme found on the surface of the lenses did not change significantly in the presence of other tear proteins.

Two groups from the CCLR noted that lotrafilcon A and B materials attract lower levels of lysozyme compared to conventional and other silicone hydrogel lenses. Frank Zhang used lysozyme incubation and confocal microscopy to determine that lysozyme penetrated the entire depth of hydrogel, vifilcon and balafilcon materials incubated in a lysozyme-saline solution, while no lysozyme penetration was evident with lotrafilcon A or B after the same incubation period. Galyfilcon lenses were unique, with only a partial penetration profile after 4 weeks. Maciej Suwala used an in vitro model to determine the activity of hen egg lysozyme deposits on conventional and silicone hydrogel materials. Results also showed the least amount of lysozyme and lowest activity on lotrafilcon A and B. The greatest amount and highest activity was found on hydrogel lenses.

Adverse Events

Cherry Radford (Moorfields Eye Hospital) reported that even with high-Dk lenses, overnight wear continues to be a major risk factor for contact lens-related MK. This study
assessed risk factors for MK among silicone hydrogel wearers over a 2-year period.

T.K. Beattie and Alan Tomlinson (Glasgow Caledonian University) investigated the attachment of Acanthamoeba to lotrafilcon B lenses, to determine if it is affected by the presence of a bacterial biofilm coating. After incubating unworn and biofilm-coated lenses in a suspension of plate-cultured A. castellanii trophozoites, they found no significant difference in attachment between lotrafilcon B, lotrafilcon A and a conventional hydrogel lens, but both silicone hydrogels had significantly more amoebal attachment than the hydrogel lens. The biofilm coating significantly increased attachment to the hydrogel lens, but no such increase was found with the silicone hydrogel lenses.

To date, culture techniques have been inconsistent and difficult to reproduce, making it difficult to evaluate changes in the microbial content of the conjunctiva with contact lens wear. M.F. Chan (University of Rochester Eye Institute) evaluated a method of detecting and quantitating endotoxin levels in tears, comparing levels between wearers and non-wearers of silicone hydrogels. Results indicated that silicone hydrogel contact lens wear does not change the microbiota of the normal external eye.

Loretta Szczotka-Flynn (Case Western Reserve University) reported the results of a 19-site study using survival analysis to model the probability of developing corneal infiltrates through 3 years of CW with lotrafilcon A. Out of 317 study participants, there were 16 occurrences of corneal infiltrates within the first year, 7 in the second year and 4 in the third year. Results also indicated that the previous presence of corneal staining and limbal redness may predict the subsequent development of an infiltrative event with CW. The probability of remaining free of corneal infiltrates for up to 1 year of CW with lotrafilcon A lenses was 94.3% at the end of one year, 91.5% at the end of 2 years, and 89.7% at the end of three years.

Carol Lakkis and Kate Weidemann (University of Melbourne) investigated the clinical and subjective performance of comfilcon A lenses during CW over 9 months. The performance of these lenses was not significantly different from that of lotrafilcon A and balafilcon A. The incidence of adverse responses with comfilcon A was significantly lower than that found with balafilcon A.

Effects of lenses on physiology

The corneal epithelial sub-basal nerve fibre layer provides innervation to the surface of the epithelium, but previous research has indicated that corneal sensitivity declines due to contact lens-induced hypoxia. Using confocal microscopy, Krystynne Harvey (CCLR) reported that the appearance of this layer is the same in both silicone hydrogel and non-contact lens wearers.

Work by Noel Brennan (Brennan Consultants) confirmed that there is minimal hypoxia with CW of silicone hydrogels and that conventional hydrogels do not provide adequate corneal oxygenation under DW conditions, calculating corneal ATP generation from oxygen diffusion equations with and without contact lens wear using a 3-dimensional corneal model constructed from standard curvature and thickness values.

Blanca Golebiowski (Vision CRC) found a significant decrease in central corneal sensitivity 1 week after habitual hydrogel lens wearers ceased hydrogel wear. Sensitivity remained low even after 6 and 12 months of silicone hydrogel wear, though confocal microscopy revealed no significant change in any indicators of corneal nerve fibre morphology and no association between nerve fibre morphology and corneal sensation threshold.

Stephanie Duench (CCLR) investigated the relationship between corneal staining and bulbar and limbal redness in subjects wearing silicone hydrogels on a DW basis while using various care regimens. A Spectrascan650 photometer and conventional redness measurements revealed no significant correlation between subjective bulbar, limbal or objective redness and staining in any of the studies conducted, indicating that asymptomatic subjects exhibiting low levels of ocular redness should be examined for signs of corneal staining; staining may not be correlated with other signs and symptoms.

Y. Zhang (Wayne, Detroit) used a rat model to examine the biocompatibility of various lens care products with silicone hydrogel lenses. Lewis rats were fitted with lotrafilcon B lenses that had been soaked in a variety of lens care solutions. Results showed that in conjunction with silicone hydrogel lenses, solutions can elevate host cytokine levels in the cornea to different degrees. A PHMB solution (Aquify MPS) and a peroxide solution (ClearCare) were the least likely to stimulate inflammatory mediators when used with lotrafilcon B lenses.

T.N. Yeh (University of California) studied the effects of 30-day CW of silicone hydrogel lenses on epithelial barrier function by measuring the permeability of the corneal epithelium to sodium fluorescein. Results show that there is a substantial decrease in epithelial barrier function during CW. The investigators also found a disparity in corneal epithelial barrier function between patched and unpatched eyes, suggesting that blinking upon awakening mechanically agitates the accumulated debris in the post-lens tear film against the corneal epithelium during lens movement. 

F.C. Abib (Curitiba, Brazil) reported that switching to lotrafilcon A lenses may decrease severe corneal polymegathism in long-term hydrogel wearers.

Silicone hydrogels as piggyback lenses

Finally, B.B. Fresco and D.S. Rootman (Toronto) described the successful use of silicone hydrogel lenses as base lenses in a piggy back system for use by patients with advanced keratoconus or non-keratoconic distorted corneas unable to tolerate GP lenses.

Part 1