Alisa Sivak, MA, DipEd

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

Part one of the BCLA synopsis reviewed presentations relating to clinical trials; tears, wettability, comfort and dryness; and deposits and solutions. Part two of the synopsis includes presentations relating to physiology, oxygen permeability and other material properties, and adverse events.

Adverse events and their resolution

Robin Chalmers and colleagues presented the results of a 1-year, multi-site study of subjects wearing lotrafilcon A lenses on a 30-night continuous wear (CW) schedule. Patient age, refractive error, smoking, and difficulty achieving the full CW schedule (> 21 nights in a row) were associated with a greater risk for the development of corneal infiltrative events. Joe Shovlin (Northeastern Eye Institute) reported a low incidence of visual acuity loss resulting from MK among subjects wearing lotrafilcon A on a CW (30-night) schedule.

Melanie George and colleagues (CIBA Vision) examined the ability of a silver-impregnated anti-bacterial case to reduce adhesion of Pseudomonas aeruginosa (PA) to silicone hydrogel and conventional lenses. Less PA adhered to both silicone hydrogel and conventional lenses when soaked in the test lens case in a PHMB-based solution compared to standard lens cases used with polyquad- and PHMB-based solutions.

Carol Lakkis and Shubha Lakkola (University of Melbourne) evaluated the incidence and level of microbial contamination of a silver-impregnated lens case compared to a standard case when used with lotrafilcon B lenses worn on a DW basis and stored nightly over 30 days. There was no significant difference between the cases in terms of overall rate of contamination: 90% of the test cases and 100% of the control cases became contaminated after one month. Significantly higher levels of contamination, greater gram-negative contamination and a greater numbers of bacterial species were however observed in the control cases compared to the silver-impregnated cases,. Forty-three percent of the control cases were moderately or heavily contaminated, compared to only 19% of the silver-impregnated cases. The majority of isolates recovered from the silver-impregnated cases were normal ocular flora.

Oxygen permeability and other material properties

Noting that the ideal UVR-blocking contact lens should block radiation up to the UV-visible boundary at 400nm, James Walsh and colleagues (DIT) described their use of a novel fibre optic spectrometer to measure the transmission curves of senofilcon A, etafilcon A, galyfilcon A, and lotrafilcon A and B lenses. The newer lens types cut off at approximately the 385nm mark, while the older lens types transmitted down to 370nm. Lenses without a UVR blocker did not block significantly until below 250nm and were transparent across the entire width of the lens in the UV region.

Nancy Keir and colleagues (CCLR) reported the results of a study comparing corneal swelling after 8 hours of overnight wear, in eyes wearing two types of silicone hydrogel lenses. Results showed no significant difference between comfilcon A and lotrafilcon A,.

A team led by Brien Holden measured oxygen uptake rates (OUR) on the surface of both contact lenses and the cornea, immediately after lens wear. They found measurable differences in oxygen uptake rates and oxygen availability between the currently available silicone hydrogel lenses.

Kathy Dumbleton and colleagues (CCLR) found that central and peripheral swelling was higher with alphafilcon A toric lenses compared to lotrafilcon B toric lenses, after the lenses were worn overnight on two separate nights. The contralateral control eye (no lens) swelled more when an alphafilcon A toric lens was worn simultaneously on the contralateral eye as compared to when a lotrafilcon B toric lens was worn.

Yu-Chin Lai (Bausch & Lomb) reported on a series of prototype silicone hydrogels with significantly higher oxygen permeabilities than many of the current silicone hydrogel lenses whose Dk is limited with higher water content. These increases are possible as a result of different silicone hydrogel formulations in combination with specific hydrophilic monomers. Rajni Sing (CIBA Vision) introduced prototype lenses of a silicone hydrogel material with a Dk of 82 in custom parameters for high plus, minus and astigmatic corrections.

Physiology

Working with a simple 3-dimensional model of the cornea, Noel Brennan (Brennan Consultants) and Philip Morgan (University of Manchester) calculated that CW of silicone hydrogel lenses provides close to normal corneal energy production, but that hydrogel lenses fall short under both open- and closed-eye wearing conditions.

Brennan and colleagues determined oxygen consumption across the corneal profile beneath soft contact lenses, and plotted the outcome as colour-coded maps. This tool indicated that silicone hydrogel lenses provide 100% of the cornea’s oxygen needs for DW but do not meet its needs for extended wear, whereas conventional hydrogels do not meet the cornea’s oxygen needs even for daily wear.

Stay tuned for our synopsis of the 2006 AAO meeting, in Denver, Colorado.