Correcting Irregular Astigmatism After RK/PK

BY ROBERT CAMBELL, M.D., & PATRICK CAROLINE, C.O.T., F.A.A.O.
Aug. 1997

This 56-year-old man had bilateral radial keratotomies resulting in irregular corneal astigmatism that was partially correctable with RGPs. He wore the lenses intermittently for 10 years and discontinued lens wear in 1993.

More recently, the patient's left eye developed a posterior subcapsular cataract. He underwent cataract extraction, IOL implantation and corneal transplant surgery OS. Postoperatively, his uncorrected visual acuities were OD 20/80 and OS 20/300. Manifest refraction OD did not improve acuity due to central irregular astigmatism (Fig. 1). The left eye was correctable to 20/30 with a prescription of -2.00 -4.25 x 172.

FITTING THE POST-RK EYE

There is a common misconception that the midperipheral cornea steepens after RK. However, topography indicates that symmetrically placed radial incisions flatten virtually the entire cornea. This diffuse flattening suggests that RK is a tissue-addition procedure and that the resulting optical effects relate to wound gape and the subsequent increase in anterior corneal surface area. The degree of wound gape and the amount of corneal flattening is dictated by the number, depth and length of the incisions, intraocular pressure, biochemical and stress properties within the corneal tissue, and individual healing response.

The post-RK midperipheral cornea generally does not steepen, but typically flattens one-half to one diopter, so it's important that the contact lens align with the midperipheral corneal topography about four millimeters from center.

In this case, we chose a diagnostic lens with a base curve radius of 40.00D, equal to the horizontal temporal radius four millimeters from center. Using fluorescein, we verified slight midperipheral bearing at 3 and 9 o'clock and unobstructed vertical movement (Fig. 2).

The final lens OD was: base curve 40.50D, power -2.50D, diameter 10.4mm, optic zone 9.0mm. Distance visual acuity was corrected to 20/30.

FIG. 1: IRREGULAR CORNEAL ASTIGMATISM POST RK OD.	FIG. 2: FLUORESCEIN PATTERN OF THE DIAGNOSTIC LENS OD.
FIG. 3: 9 MONTHS POST TRIPLE PROCEDURE OS.	FIG. 4: DIAGNOSTIC LENS OS.

FITTING THE POST PK EYE

Videokeratography of the left eye nine months post PK revealed central keratometric readings of 44.12 @ 144/47.75 @ 54 (Fig. 3). The corneal map showed a flat central zone with midperipheral steepening.

Videokeratography four millimeters from center revealed midperipheral radii of: temporal 48.00D, superior 46.50D and inferior 52.50D. Using a diagnostic lens with a base curve radius equal to the temporal midperipheral radius, we noted approximately 180 degrees of superior fluorescein thinning at 9, 12 and 3 o'clock (Fig. 4), obstructing vertical lens movement. We ordered a base curve 0.50D flatter than the diagnostic lens. Final lens specifications were: base curve 47.50D; power -6.00D; diameter 10.4mm; and optic zone 9.0mm. Distance visual acuity was corrected to 20/25. CLS