Calculate Cross-Cylinder Effects with Lensometry

BY ROBERT CAMPBELL, M.D. & PATRICK CAROLINE, C.O.T., F.A.A.O.
JAN. 1997

Patient L.W. is a 47-year-old woman who underwent bilateral hexagonal keratotomies in 1994 to correct hyperopia. Her preoperative cycloplegic refraction was OD +3.50 -0.75 x 100, 20/20-; OS +3.25 -0.75 x 75, 20/25.

Twelve months postoperatively, her manifest refraction was OD
-0.75 -1.25 x 105, 20/25; OS -1.00
-3.00 x 104, 20/30. Central keratometric readings had dramatically steepened from OD 42.25 @ 180/41.50 @ 90 to 46.25 @ 180/45.12 @ 90 and OS 42.50 @ 180/41.50 @ 90 to 47.87 @ 10/45.00 @ 100.

FIG. 1: HEXAGONAL KERATOTOMY OS WITH EPITHELIAL INCLUSION CYSTS.

FIG. 2: SPHEROCYLINDER POWER EFFECT CALCULATED BY LENSOMETRY

FIG. 3: TORIC SOFT CONTACT LENS OD FOLLOWING HEXAGONAL KERATOTOMY.

L.W.'s postoperative complaints included headaches, photophobia, night glare and difficulties with depth perception. Slit lamp examination revealed bilateral 4.5mm hexagonal scars with epithelial inclusion cysts and intersecting or touching incisions (Fig. 1). The patient had reported intolerance to RGPs after numerous fitting attempts, and minimal success with the Softperm lens. We eventually fitted her with diagnostic custom toric soft contact lenses, base curve 8.6mm, diameter 15.0mm. The powers were OD
-0.75 -1.00 x 90, OS -1.00 -1.50 x 85. Following equilibration, we performed a spherocylinder overrefraction over the diagnostic lenses.

THE LENSOMETRY TECHNIQUE

When a toric soft contact lens positions to either side of 6 o'clock, the lens axis is not in the same position as the refractive axis and, therefore, a cross-cylinder effect results. Even when the axes are aligned (marker at 6 o'clock), a cross-cylinder effect can be induced secondary to lens draping, lens flexure, induced minus lens power and cylinder masking.

For this case, we used the following simple, cross-cylinder calculation technique to determine the final lens power:

• Place the sphere, cylinder and axis powers of the diagnostic soft contact lens into a trial frame.
• Place the resultant spherocylinder overrefraction on top.
• Use the lensometer to read the new lens power (Fig. 2).

In our case, the spherocylinder overrefraction was: OD +0.50 -1.00 x 135, 20/25; OS +0.75 -2.00 x 127, 20/30.

This simple technique calculates the cross-cylinder effects of the spherocylinder overrefraction and eliminates the many power and axis errors associated with the LARS technique (which we sometimes refer to as the 'Less Apt to Result in Success' technique).

The final lens powers for L.W. were: OD -0.50 -1.50 x 110, 20/25; OS -0.75 -2.75 x 106, 20/30. Her wearing time was 14 hours a day and her asthenopic symptoms were significantly reduced (Fig. 3).

For practitioners who don't have access to a programmed cross-cylinder calculator, or for those of us who can't remember the cross-cylinder formula, the lensometry technique offers a simple and accurate way of calculating the cross-cylinder effects of refractions over toric soft contact lenses. CLS

Dr. Campbell is medical director of the Park Nicollet Contact Lens Clinic & Research Center, Minnetonka, Minn. Patrick Caroline is an assistant professor of optometry at Pacific University, Forest Grove, Ore., and director of contact lens research at Oregon Health Sciences University in Portland.