Digital lens fitting based on corneal topography - the future of (R)GP lens fitting
The case that I will describe here was managed with a virtual fitting software based on topography data, which is particularly helpful in post-LASIK lens fitting. In post-LASIK cases I usually analyze the map and use the virtual fitting software, which previews (R)GP lenses “on-eye” virtually. In this case a reverse geometry back surface lens was used on the eye because corneal eccentricity is “negative” in these cases due to the oblate form of the surface after the surgical treatment.
Thanks to the availability of diagnostic technologies and applied simulation in the most recent topographers, a series of steps can be followed to determine the base curve of the contact lens and the size of the optimal optical zone. Activating the function “caliper” in the topographer, I first measured the treated “blue” surface area of the cornea, adding 0.5 mm in diameter to that area to establish the projected optical zone diameter. To determine the base curve of the zone, I averaged the spherical curve on three points on the oblate surface. My goal was to reach an apical clearance of about 20-25 microns beneath the central zone, which this system can control for quite easily.
The slope of the curve peripheral to the treatment area, the first reverse curve, was determined by measuring and then averaging three points within the red area on the topography map.
I used minimal flattening in the peripheral landing zone area to create a gradual peripheral curve that, for this specific case, created a second reverse curve.
Each peripheral curve width was fixed at 0.4 millimetres for easier calculation, then an arbritrary edge curve was added. The final contact lens was a five-curve design, with a double reverse geometry that provided 20/20 or 1.0 visual acuity and very good lens comfort. The material used was Boston XO.
The topographer’s virtual simulation of the fit complied very well with the on-eye observation, proving that topographers and modern virtual fitting software today represent an important instrument and a valuable aid in planning complex cases such as, but certainly not limited to, post-LASIK lens fits.
This is the corneal map after a LASIK treatment with a suboptimal outcome.
First I measured the optical zone size with the “caliper” function of the topographer. I added 0.5 mm to the measured area (figure 1, above)
With a three point spherical evaluation function on the topographer, I determined the base curve radius of the contact lens (figures 2 and 3, above).
After this I used the virtual fitting software of the topographer to simulate the base curve value and test the amount of clearance (above).
To determine the reverse curve value, I used the three point spherical evaluation function on the red area of the topography map to create the reverse curve with a width of 0.4 mm.
I made the width of each peripheral curve 0.4 mm to obtain the final lens (figures 6 and 7, above).
Sphere –6.50 Cyl –0.50 @90° 10/10
Sphere +3.00 Cyl –1.25 @65° 5/10
BOZR 9.30 mm
F’v –1.50 D
BOZD 6.50 mm
R1 7.85 mm ; Ø1 7.30 mm
R2 8.10 mm ; Ø2 8.10 mm
R3 7.80 mm ; Ø3 8.90 mm
R4 9.50 mm ; ØT 9.70 mm
Material: Boston XO violet
Visual acuity 20/20
Final lens – including edge lift – simulated (figure 8, above).
Final lens – on-eye (figure 9, above)
Marco Tovaglia graduated from the Galileo Galilei Institute in Milan to become an Optometrist. His professional experience started in 1987 as Professional Service Manager for several contact lens companies in Italy. He has also been fitting contact lenses since 1989 as a contact lens consultant and in his private practice. He writes for many Italian optometry magazines and is working on a company protocol about keratoconus fitting approaches. Marco is a member of IACLE.