Orthokeratology contact lenses reshape the cornea overnight to provide clear vision during the day without the need for contact lenses or glasses during those daytime hours. These specialty contact lenses can be great for children, athletes, or those with jobs or lifestyles that make glasses or contact lens wear very difficult. As a practitioner, prescribing these contact lenses involves much more technical skill than the average contact lens, and adjusting the lens for better vision and fit requires a working knowledge of different parts of the design that are completely unique to orthokeratology lenses. These lenses employ a large reverse curve change, where the central part of the lens bears down on the cornea while the mid peripheral region of the lens vaults back up to align the contact lens properly on the eye. Today's post breaks down the basic design of orthokeratology contact lenses, and to help determine what parameters need to be changed to achieve that best fit for maximum lens performance and next day clarity!
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| A Perfect Fit with Paragon CRT lenses via |
Fitting orthokeratology contact lenses requires assessment of 3 zones of the contact lens:
1) Base Curve
The central area of the contact lens (6mm in size in the popular Paragon CRT lens design) is the base curve region. This area is the main pressure point for vision correction -- specifically calculated to negate the patient's myopic prescription. The only reason to change this curve is to get more vision correction -- don't change this to change the fit of the contact lens! With Paragon CRT lenses, the appropriate base curve will be selected by the patient's spherical equivalent prescription and their flat K using a nomogram, so very little calculation will be needed by you in initial lens selection. If you need more prescription correction later on, you can flatten the base curve to achieve more correction.
Increasing the Base Curve will flatten the contact lens (or create more treatment).
Increase the Base Curve (flatten) by 0.1 mm = 0.50D more myopic treatment
2) Return Zone or Reverse Curve
The Return Zone area is just outside of the central base curve region where the contact lens lifts back up off of the cornea. In the Paragon design it is ideally about 1 mm in size. The Return Zone is measured in microns of sagittal depth (much like when you fit scleral contact lenses!) so you are measuring the amount of lift off of vault away from the cornea. The Return Zone is essential to aligning the contact lens for centration, so this is an area that you may need to adjust to achieve best fit!
Higher Return Zone Depth = More Lift Off of Cornea
Typically change this by 25 micron units to achieve visible results
The key to Return Zone measurements is that you want alignment fit with the cornea underneath. If there are bubbles in this area, you have too much sag depth and you need to lower the Return Zone. If you don't have a well defined central pressure zone, you may need to increase the Return Zone to get more treatment and better alignment.
3) Landing Zone or Alignment Curve
The very edge of the contact lens is the landing zone, allowing the lens to fit comfortably on the cornea. The lower the landing zone angle, the more edge lift you will achieve as the edge of the lens lifts up and away from the cornea. Ideally you want 1-2 mm of tear film between the edge of the contact lens and the cornea. If you have too little tear film underneath, the edge would be too tight and will be increasingly uncomfortable with wear, not to mention that underneath your closed eyelid the contact lens may not be centered and you would awake with poor vision the next day. If the edge lift is excessive, again comfort will be a major issue as you blink, and centration will also be an issue.
Lower the Landing Zone Angle by 1 Degree to Increase Edge Lift
Increase the Landing Zone Angle by 1 Degree to Decrease Edge Lift
Here's another tricky part, if you change the Landing Zone angle, you will also change the Return Zone depth: Decreasing the Landing Zone by 1 degree will Decrease the Return Zone sag depth by around 15 microns. Typically if you want to make changes to contact lens alignment in small increments, you would start by changing the Landing Zone before you change the Return Zone. That's because changing this area will impact your Return Zone anyway.
Stay Tuned for the next post in this series: trouble shooting orthokeratology fits.
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