contact lens case reports
Correcting Toricity After LASIK
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Patient P.M. is a 34-year-old female with a 17-year history of toric soft contact lens wear. She underwent bilateral LASIK in January 2000 for correction of OD 6.25 1.25 x 170 and and OS 6.50 1.25 x 10. Pre-op K readings were OD 44.75 @ 180/46.00 @ 90 and OS 44.50 @ 14/45.50 @ 104 (Figure 1). Two months post-op her uncorrected visual acuities were OD 20/20, OS 20/60, manifest refraction OS was 0.50 1.50 x 180, 20/20 (Figure 2). The patient was interested in correcting her post-surgical astigmatism.
Figure 1. Pre-op corneal map OS
Figure 2. Post-op corneal map OS
The Post LASIK Corneal Shape
Following refractive corneal surgery, the endpoint visual acuity is ultimately affected by a number of complex optical characteristics: defocus (post-op refractive error) and optical aberrations.
With myopic photo ablations, the topography of the mid-peripheral cornea remains unchanged. The major concern in fitting any contact lens is the relative difference between the flatter central cornea and the steeper (normal) mid-peripheral cornea.
In our patient, approximately 72 microns of tissue were removed to correct the myopia. This resulted in a difference between the central and the peripheral cornea such that a rigid lens of 44.50 diopters (designed to align the mid-peripheral cornea), would result in 70 microns of apical clearance (Figure 3).
Figure 3. Simulated fluorescein map illustrating excessive central corneal clearance created by a RGP lens aligned to the mid-peripheral cornea
In the management of post LASIK astigmatism, a thick, ballasted toric soft lens may also vault over the central cornea. This often results in lens flexure, unstable optics and fluctuating vision.
Choosing a Lens
The Acuvue Toric's advantage for the post-LASIK cornea is its back surface, thin zone design which results in a center thickness of 0.07mm (50 percent thinner than a traditional toric soft lens). This allows the lens to better collapse over the central cornea, lessening lens flexure induced during blinking. We fitted the patient with an Acuvue Toric, base curve 8.7mm, power 0.75 1.25 x 180 and diameter 14.4mm (Figure 4). VA was 20/20.
Figure 4. Acuvue Toric OS
Following LASIK, we attempt to postpone lens fitting until eight weeks after surgery when refraction and topography have stabilized, and flap integrity is sufficient to withstand lens insertion and removal.
Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.
Mark André is director of contact lens services at the Oregon Health Sciences University.