September Online Photo Diagnosis
By Patrick J. Caroline, FAAO, & Mark P. André, FAAO
What caused this rather peculiar (clown face) topographical map? Believe it or not—or better yet, no clowning around—this topographic map resulted from one night of orthokeratology on a highly astigmatic cornea.
Orthokeratology and Astigmatism
The topographical changes that take place in orthokeratology (OK) result from unequal fluid forces beneath the contact lens. The reverse geometry shape of these lenses results in positive fluid pressure forces (push force) in the center of the cornea, causing compression and flattening, and negative forces (pull force) in the midperiphery, causing midperipheral steepening (Figure 1).
Therefore, a key ingredient to this process is that every OK lens must "land" peripherally approximately 360 degrees around the cornea to prevent the rapid escape of the fluid from beneath the lens (Figure 2).
This semi-closed, fluid-filled fitting relationship is made possible when the cornea has minimal corneal toricity or when the peripheral cornea exhibits a near spherical shape. However, if the cornea is toric or if the astigmatism extends from limbus to limbus, the fluid forces escape along the steep meridian, and the desired topographic changes cannot be achieved (Figure 3).
To address the dilemma of astigmatic OK, manufacturers have developed astigmatic or dual axis reverse geometry lens designs. These lenses incorporate a shallower reverse curve (return zone) to "close down" the flat corneal meridian and a deeper reverse curve (return zone) to "close down" the steeper corneal meridian (Figure 4). These design features allow the lens to better maintain the necessary post-lens fluid forces for improved centration and enhanced OK treatment.
This month's Online Photo Diagnosis shows the case of a 25-year-old male who had 2.87D of corneal astigmatism in both eyes (Figure 5). The patient was fitted with a traditional Paragon CRT (Paragon Vision Sciences) lens on the right eye and a Paragon CRT Dual Axis design on the left eye.
Figure 6 shows the fluorescein pattern of the traditional CRT lens on the patient's right eye. Note the decentered lens position and excessive edge lift along the steeper vertical meridian. Worn overnight, the traditional CRT lens design resulted in a decentered, superiorly positioned treatment zone as well as an "unhappy" clown face (Figure 7).
Figure 8 shows the fluorescein pattern of the dual axis design on the patient's left eye. Note the improved horizontal and vertical lens alignment and the subsequent centration. Overnight, the dual axis lens design resulted in a centered treatment zone and a "much happier" clown face (Figure 9).
This case demonstrates how astigmatic (Dual Axis) OK lens designs can be used when lens centration is not possible with a traditional OK lens design or when corneal astigmatism prevents the lens from achieving meridian alignment both horizontally and vertically.