Article Date: 8/1/2009

CRT For Limbal-to-Limbal Astigmats
contact lens management

CRT For Limbal-to-Limbal Astigmats

Meet the visual and comfort needs of all these patients.


When corneal astigmatism extends out to the limbus, it can disrupt the uniform tear fluid forces along a standard corneal reshaping lens' mid-periphery and periphery, resulting in poor centration, lens flexure and/or under treatment. As a result, practitioners have often had to have many of these motivated patients discontinue wear.

The good news: Paragon Vision Sciences now offers the CRT Dual Axis lens, which is specifically designed to enable all limbal-to-limbal astigmatism patients to achieve successful wear. A limbal-to-limbal astigmatic pattern is seen when one or both sides of the astigmatic ridge either reach or approximate the limbal region.

Expanded fitting parameters

The CRT Dual Axis lens incorporates a dual mid-peripheral Return Zone Depth (RZD) system that includes a deeper RZD to align with the steeper corneal meridian, and a shallow RZD to line up with the flat corneal meridian. These two different RZD values are oriented 90° apart to help create a sealed peripheral system to maintain the necessary tear fluid forces beneath the new lens.

Further, you can increase the Dual Axis' lens Landing Zone Angle (LZA) in one axis to provide a uniform circumferential edge clearance to achieve the desired sealed system. This is a real benefit for those limbal-to-limbal patients who were unable to achieve success in a standard orthokeratology lens due to the lens' peripheral edge clearance manifesting significant differences along the cornea's steep and flat meridians.

Optimal centration

Because of differences in sagittal height between the vertical and horizontal meridians of a limbal-to-limbal astigmatic cornea, the standard orthokeratology lens doesn't always result in optimal centration for these patients. This becomes evident when you observe the lens touching the mid-periphery of the cornea's flat meridian and then rocking, flexing or tilting over the cornea's steep meridian. The result: asymmetrical topographic changes with induced aberration.

CRT Dual Axis

MATERIAL: Paragon HDS 100 (paflucon D)

DK/T: 100

WEARING SCHEDULE: Overnight six to eight hours


BASE CURVE: 7.9mm to 8.9mm standard; custom parameters also available

DIAMETER: 10.5mm

RZD/LZA: Varies with desired treatment

PRACTITIONER COST: $102.50 per lens

With the CRT Dual Axis lens, however, you can increase the RZD by 25 microns or more in the cornea's steep meridian. This evenly aligns the lens periphery with it, resulting in symmetric topographic changes with minimal induced aberration.

In fact, several patients I've fit in the CRT Dual Axis lens have commented on the enhanced visual acuity they experience upon removal of the lens and throughout the day.

(Note: The CRT Dual Axis lens is intended for limbal-to-limbal astigmatism, not for apical astigmatism, irregular astigmatism or lenticular astigmatism. I've found that apical astigmatism patients often achieve success with the standard CRT design. Irregular or lenticular astigmatism patients, however, aren't ideal corneal reshaping candidates [neither standard CRT nor Dual Axis]).

Improved treatment

The standard CRT, or other orthokeratology lenses, aligns with the peripheral steep meridian by compressing, or flexing, along the flat meridian. So, limbal-to-limbal astigmatism patients often experience an uneven, oval treatment zone with these lenses. Further, tear fluid leakage beneath this unsealed peripheral system results in an incomplete treatment effect.

With the Dual Axis CRT, however, you can order an increased RZD to align with the steep meridian or a decreased RZD to align with the flat meridian to ensure a sealed system. This creates an even treatment zone beneath the lens, enhancing the treatment effect.

Fitting made easy

I've found you can easily fit the CRT Dual Axis design by using topographic elevation maps. Just send the limbal-to-limbal astigmatism patient's map to Paragon Vision Sciences for consultation, and they'll help you design the RZD and LZA parameters.

Notice the Dual Axis design's deep Return Zone Depth (RZD) along the steep cornea meridian and the shallow RZD along the flat cornea meridian.

Or, carefully observe the NaFl patterns of a poorly fit standard CRT lens, and consult with the company to determine the proper parameter changes.

Finally, consider using Paragon Vision Sciences 16-lens diagnostic set or their 40- to 80-lens Dual Axis Fitting and Dispensing System to ensure an optimal fit. The 16-lens set contains a variety of RZD combinations with a limited number of base curves. It's intended for fitting purposes only and aides you in determining the optimal RZD combination to order. The 40- and 80-lens Dual Axis dispensing sets offer expanded RZD and LZA combinations and an expanded amount of base curves.

With the CRT Dual Axis lens, you can now confidently offer corneal reshaping to all your limbal-to-limbal astigmatism patients — a wonderful opportunity to enhance their quality of life and your practice's finances. OM


Optometric Management, Issue: August 2009