Optical Zone Size Changes and GP Lens Performance
BY CATHERINE TUONG, OD, & JOHN MARK JACKSON, OD, MS, FAAO
When fitting corneal GP lenses, it is easy to overlook the importance of the optical zone diameter (OZD). With most stock lens designs used today, the labs set the OZD, and adjustments are mostly made to the base curve radius (BCR), the overall diameter (OAD), and the flat/steep relationship of the peripheral curves. But changes in the OZD can also be used to affect the lens fit.
When to Change the OZD
One reason to change the OZD would be to combat lens flare. This happens when the edge of the OZ gets too close to the pupil, and stray light refracted by the peripheral curves enters the eye. First, check the OAD and the alignment relationship of the lens. If you cannot increase the OAD and the alignment is as desired, consider enlarging the OZD (Kojima, 2015).
Theodoroff and Lowther (1990) looked at the position and movement of a 9.5mm OAD with three different OZDs and found that the lens with the smallest OZD resulted in superior and horizontal decentration. This lens also had the most movement. Clearly, the OZD can affect the fit, position, and movement of a GP contact lens. If the OAD is ideal, but the lens shows excessive movement and/or excessive edge clearance, increasing the OZD would push the edge of the optical zone peripherally, increasing lens stability and decreasing edge clearance (Kojima and Caroline, 2009). Conversely, a too-large OZD can result in excessive bearing peripherally, causing inadequate movement and seal-off (Figures 1 and 2).
Figure 1. Hard bearing at the edge of the optical zone with 7.8mm OZD on a 9.8mm lens.
Figure 2. Softer landing at the edge of the optical zone with 7.4mm OZD on a 9.8mm lens.
Rules for OZD Changes
There are a few things to keep in mind when changing the OZD. First, altering the OZD will alter the sag depth of the lens and change the alignment relationship. A good rule of thumb is to flatten the BCR by 0.05mm/0.25D for every 0.4mm increase in OZD (or steepen if you go to a smaller OZD). Second, when you make this BCR change, you must also adjust the power of the lens because the tear film power changes, too (remember SAM-FAP—steeper add minus, flatter add plus). Third, changing the OZD will alter the peripheral curve width (smaller OZD will yield wider peripheral curves), so you may need to ask the lab to make an edge clearance compensation for this.
Corneal GPs offer lots of flexibility in lens fitting due to the many parameter choices. Sometimes all of the possibilities can be overwhelming, so don’t forget to reach out to your GP lab for their expert advice. A customized lens fit is worth the extra effort. CLS
For references, please visit www.clspectrum.com/references and click on document #237.
Dr. Tuong is the current cornea and contact lens resident at Southern College of Optometry. Dr. Jackson is a professor at Southern College of Optometry, where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at email@example.com.