contact lens materials
Fitting and Evaluating a Custom Silicone Hydrogel Lens
The first article in this series described development of the O2Optix Custom lens (CIBA Vision), including its material and manufacture. Here we will look at how to fit the lens.
The Fitting Process
The fitting process for this custom lens is different from most modern lenses. Due to safety concerns surrounding the re-use of diagnostic lenses on different patients, trial fitting sets won't be available. Instead, you fit the initial lens empirically. This may seem daunting if you've spent decades using diagnostic lenses before making a final order, but a fitting guide (Table 1) is available to help the decision process.
Choose an initial lens diameter at least 2mm larger than the horizontal visible iris diameter. Order the back vertex power as the vertexed spectacle lens power and select the base curve from the fitting guide. The rigidity of the lens allows reasonable masking of corneal cylinders, and we've found surprisingly good visual acuity with corneal cylinders up to 1.25D.
Once you decide on the base curve, diameter and power, order diagnostic fitting lenses to use for an extended fitting trial. After the patient returns for the diagnostic lens fitting evaluation, you can feel confident that the fitting characteristics observed within 20 minutes of application will remain constant throughout the lens' lifespan.
The lens should fit just as any other soft lens, providing suitable centration, movement and optimal visual acuity. If the diagnostic lenses fit appropriately, then you can dispense that pair of lenses. It may be prudent for this extended trial to last two to three weeks, because as in many instances where patients are refit into silicone hydrogel lenses, there may be a short period of adaptation before they report equivalent or superior comfort to their previous, low-Dk lenses. This may result from the increased lens rigidity, change in surface properties from their habitual lens or the rapid oxygenation of the cornea, resulting in hypoxic rehabilitation.
After the trial period, carefully evaluate the visual acuity and over-refraction to determine any prescription changes and to perform a mandatory slit lamp examination. Look particularly for the presence of any microcysts, regression of neovascularization and epithelial staining due to solution incompatibility.
After this visit, decide on any minor alterations to the lens parameters. If a large alteration is required, then choose another pair of diagnostic lenses and repeat the fitting process. If there's no change or only a small change (as occurs in most instances), then you can order a four-pack of lenses for each eye (with modifications incorporated), which will provide the patient with lenses for one year.
A Look Ahead
The final article in this series on the O2Optix Custom lens will describe some case examples.
Dr. Jones is the associate director of the Centre for Contact Lens Research and a professor at the School of Optometry at the University of Waterloo, Ontario, Canada. Dr. Dumbleton is a senior clinical scientist at the Centre for Contact Lens Research. Jill Woods is a clinical scientist at the Centre for Contact Lens Research.