Back to Astigmatism Basics
contact lens case reports
Back to Astigmatism Basics
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Although we typically report some rather complicated contact lenses cases, this month we want to discuss patients who have routine, high, with-the-rule astigmatism. Our patient was a 24-year-old female with a seven year history of custom soft toric lenses. Despite trying numerous lens brands, the patient's chief complaint has always been inconsistent vision.
After a short discussion about different lens options, we mutually agreed that the patient might be an excellent candidate for toric GP lenses. Her spectacle Rx was OD –4.50 –3.50 x015 20/15, OS –4.25 –3.75 x170 20/15. Keratometric readings were OD 43.00 @ 015/47.00 @ 105 and OS 42.75 @ 175/47.00 @ 085.
Figure 1. Topographical map of the patent's right eye.
Figure 2. Bitoric lens design for the patient's right eye.
In fitting any GP lens, three fitting criteria should be present:
1. The base curve of the lens should be steep enough to clear the central cornea.
2. The base curve of the lens should be steep enough to allow the lens to land on the midperipheral cornea at 3 o'clock and 9 o'clock to prevent lateral decentration.
3. The lens should be flat enough along the vertical meridian to permit unobstructed vertical movement with the blink.
Single-radius lens designs on highly astigmatic, with-the rule corneas result in excessive clearance along the steep meridian at 12 o'clock and 6 o'clock, causing excessive rocking along the vertical meridian. This can also result in lens decentration, unstable optics and blink-induced flexure. A vertical meridian with a steeper radius will improve alignment along the vertical meridian (Figure 1).
Spherical Versus Bitoric
For the purposes of this case report, we diagnostically fit singleradius spherical lenses and empirically designed bitoric contact lenses using the power cross technique in Figure 2. The top two power crosses design the posterior surface of the lens while the lower three crosses calculate the powers along the two meridians.
Figure 3. Fluorescein pattern of the single-radius (upper) and bitoric (lower) lenses for the right eye.
The top photo in Figure 3 shows the spherical (single-radius) lens on the right eye. Note the dumbbell fluorescein pattern along the horizontal meridian and the excessive lens clearance at 12 o'clock and 6 o'clock. The lower photo shows the same eye with the bitoric design. CLS
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences and SynergEyes, Inc. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision and SynergEyes, Inc.
Contact Lens Spectrum, Issue: January 2008