Contact Lens Case Reports
Should Most Orthokeratology Lenses Be Toric?
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
The topographical changes that take place in overnight orthokeratology (OK) result from trapped fluid forces (tears) that create a positive (push) force in the center of the cornea and a negative (pull) force in the midperiphery. Therefore, a key ingredient to this process is that every OK lens must “land” peripherally 360º around the cornea to effectively maintain the necessary hydraulic pressure forces beneath the lens (Figure 1).
Figure 1. A reverse geometry OK lens that is appropriately landing 360º around the cornea, creating a semi-closed, fluid-filled environment.
When fitting lenses that have a symmetrical periphery, maintaining this semi-closed fluid-filled system is only possible if the peripheral cornea exhibits a nearly spherical shape. If the cornea is toric or if the astigmatism extends limbus-to-limbus, the fluid forces can escape along the steep meridian, and the desired changes in tissue shape cannot be achieved (Figure 2).
Figure 2. The difference in peripheral landing with central astigmatism versus limbus-to-limbus astigmatism.
Peripheral Corneal Shape
In a poster presented at the 2016 Global Specialty Lens Symposium (Kojima et al, 2016), we measured the sagittal depths of 114 eyes in the flat and steep meridians along the two principle meridians at chords of 8mm, 9mm, and 10mm (Figure 3).
Figure 3. The height differential (in microns) between the primary flat and the steep corneal meridians of 114 eyes.
The results showed that, as expected, the sagittal differential increased with the degree of corneal astigmatism and with greater chord diameters. The data indicate that a 25μm differential in corneal height (between the flat and steep meridians) is equal to approximately 1.00D of toricity.
This analysis found that the vast majority of eyes have some degree of toricity in the periphery regardless of the central corneal astigmatism. This suggests that many OK lenses should incorporate some peripheral toricity to better align the elevation differences between the two meridians of the peripheral cornea. Furthermore, because it is necessary for OK lenses to “land” midperipherally (at a chord of 7.0mm to 8.0mm), any cornea with greater than 25μm of height differential (Figure 4) might be best managed with a toric OK design. CLS
Figure 4. A patient who has 1.37D of corneal toricity with a 38µm differential in height between the flat (blue) and steep (red) meridians in one eye and 1.25D of corneal toricity with 60µm of height differential in the other eye.
For references, please visit www.clspectrum.com/references and click on document #245.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Contamac. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.