contact lens case reports
Orthokeratology for Astigmats
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
The topographical changes that take place in corneal reshaping, or orthokeratology, result from unequal fluid forces beneath the lens. The posterior shape of these lenses results in fluid pressure forces that are positive (push force) in the center of the cornea and negative (pull force) in the midperiphery. Therefore, a key requirement is that an orthok lens must "land" peripherally approximately 360 degrees around the cornea to contain the fluid forces. If corneal astigmatism is limbus-to-limbus rather than central (Figure 1), the fluid forces escape along the steep meridian and the desired topographical changes cannot occur.
Figure 1. Central/incomplete corneal astigmatism versus limbus-to-limbus astigmatism.
Designing a Solution
To address this, Paragon Vision Sciences recently released its new Dual Axis lens. The design incorporates a dual return zone depth system with a shallower return zone depth to align the flat corneal meridian and a deeper return zone depth to align the steeper corneal meridian (Figure 2). This design allows the lens to better maintain the necessary fluid forces for improved centration and enhanced treatment.
Figure 2. The Dual Axis design with a deeper return zone depth along the steeper corneal meridian.
Our patient is a 25-year-old male with 2.87D of corneal astigmatism OD and OS. Figure 3 shows a traditional Paragon CRT lens OD. Note the decentered lens position and excessive edge lift along the steep vertical meridian. Overnight wear of this lens resulted in a decentered, superior-positioned treatment zone.
Figure 3. A traditional CRT design on a cornea with limbus-to-limbus astigmatism.
Figure 4 shows the Dual Axis design on the same cornea. Note the improved lens alignment and centration. Overnight, this lens maintained a centered position resulting in a centered treatment zone. In this case, a single landing zone angle (LZA) of 33 degrees aligned the peripheral cornea. However, if necessary the lens can be manufactured in a Dual Axis LZA design as well.
Figure 4. A Dual Axis design on the same eye as Figure 3.
Dual Axis lenses are fitted from topographical elevation maps in which the difference between the sagittal height of the horizontal and vertical meridians is calculated. This lens design may be ideal when you can't achieve lens centration with the traditional CRT design or when corneal astigmatism prevents the lens from achieving meridional alignment both horizontally and vertically. CLS
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.