Alisa Sivak, MA, DipEd

Alisa is Communications Coordinator at the Centre for Contact Lens Research, University of Waterloo, Canada

Physiological effect of lens wear

Researchers at the Centre for Contact Lens Research (University of Waterloo, Canada) demonstrated the physiological benefit of refitting conventional hydrogel wearers with silicone hydrogel lenses by using Adobe Photoshop to manipulate and analyze the images.  Jill Woods and colleagues refit highly myopic hydrogel wearers with a lathe-cut sifilcon A lens, which was worn on a daily wear basis for 9 months.  Photographs were taken of the superior, inferior, temporal and nasal limbal regions, at 12x magnification and then imported into Adobe Photoshop, where a red-free filter was applied to enhance contrast.  They then measured the length of the longest visible blood column and counted the number of blood columns measuring over 0.5 mm in each quadrant. 

Contrary to common perception, Gerard Cairns and colleagues (Bausch & Lomb) reported that SEALs do not occur at a high rate in an Asian population wearing balafilcon A lenses with centre thickness 90 and 70µ.  Subjects included 206 adapted contact lens wearers recruited from sites across Asia.  Varying lens thickness by nearly 30% also did not increase the incidence of SEALs.

Chantal Coles and colleagues (Brennan Consultants, Australia) tested the proposal that that the law of diminishing returns with increasing Dk/t also applies to the corneal endothelial bleb response in subjects of East Asian origin.  Galyfilcon A, lotrafilcon A and senofilcon A produced a low bleb response in comparison to a conventional hydrogel lens.

Gary Gunderson and colleagues (Illinois College of Optometry, USA) investigated the effect of lens material and design on image quality and higher order aberrations in 12 subjects with <0.75D of corneal astigmatism and different pupil diameters.  Results found decreased spherical aberration with senofilcon A, galyfilcon A, lotrafilcon B and balafilcon A, particularly balafilcon A.  Only balafilcon induced coma compared to no lens.


Oxygen

Colour-coded maps were used by Noel Brennan (Brennan Consultants) to illustrate corneal oxygenation (and local areas of deficiency) during toric contact lens wear.  Daily wear of silicone hydrogels allowed normal oxygen consumption across the entire lens area, but under closed eye conditions the regions affected by reduced oxygen differed between products, as might be expected from the different toric designs.  These lenses were compared to one hydrogel lens, which demonstrated inferior performance for both open and closed eye.

Barbara Fink and colleagues (Ohio State University, USA) wondered whether lens power influences oxygen uptake rate at the epithelial surface.   Using a polarographic electrode, they observed hypoxic stress with blinking on an individual basis, indicating that factors such as tear viscosity, lid tension and movement have an influence in addition to lens modulus.

Amir Moezzi and colleagues (Centre for Contact Lens Research) investigated the effect of lens power on central and peripheral corneal swelling after eight hours of sleep in lenses with a variety of silicone hydrogel oxygen transmissibility values.  Central corneal swelling was significantly greater for all three lens powers investigated, but lenses in +6.00 power induced significantly greater central swelling.  Both +6.0 and -10.0 D lens powers induced significantly greater peripheral corneal swelling than that induced by -3.00 lenses.  Overnight wear of each lens power induced more corneal swelling than that found in control eyes wearing no lens.  This same group also found that lotrafilcon induced the smallest and galyfilcon the highest degree of central corneal swelling.


Comfort

Karen Perekalsky and Ralph E. Gundel (ODs, New York City) conducted a trial to determine the optimal replacement cycle for comfortable daily wear of senofilcon a lenses in patients with mild to moderate dry eye symptoms.  Results indicated that a 10-15-day replacement schedule is adequate with these lenses.

Sameena Haque-Khan and colleagues (CIBA Vision) compared eight types of silicone hydrogel lenses with respect to comfort at insertion.  “Enhanced” Lotrafilcon A and lotrafilcon B lenses were rated highly compared to balafilcon A, senofilcon A and galyfilcon A, as well as the original (not “enhanced”) lotrafilcon A.

Victoria Evans and colleagues (Institute for Eye Research, Australia) conducted a retrospective analysis of 40 subjects wearing 20 combinations of five silicone hydrogels lenses and a variety of care solutions over three months.  Twice as many subjects discontinued with no adverse events (compared to the number who discontinued due to adverse events).  Common factors included poor comfort, dryness, self-reported redness, self-reported poor vision, and reduced wear time.  Discontinuations due to adverse events tended to be linked to solution factors, whereas discontinuation unrelated to adverse events were more likely to be related to lens factors.


Deposits

Lakshman Subbaraman reported that the kinetics of lactoferrin deposition on contact lens materials varies depending on the chemical structure of the lens material.  After artificially doping silicone hydrogel and conventional hydrogel (group II and IV) lenses with labelled lactoferrin solution and performing radiochemical analysis, these researchers found a gradual increase in lactoferrin deposition across all time points.  After 28 days, etafilcon A and balafilcon A had adsorbed the highest amounts of lactoferrin, while lotrafilcon and B adsorbed the smallest amounts of lactoferrin.