Contact Lens Case Reports
Consider Sclerals When Corneal Depth is Compromised
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
In modern laser-assisted in situ keratomileusis (LASIK) surgery, the depth of the central ablation zone is determined based on the amount of myopic correction required. This is most commonly calculated using the Munnerlyn formula, which states that the depth of the ablation (in microns) per diopter of refractive change is equal to the square of the optical ablation zone measured in millimeters, divided by three.
For example, with an ablation zone of 6.0mm, the depth of ablation per diopter of correction would be 12.0 microns; however, with the addition of a 1.0mm-wide transition zone, the depth is slightly greater, approximately 13.3 microns per diopter. Using this formula, a −6.50D myope (−6.00D at the plane of the cornea) would require an ablation depth of approximately 80.0 microns.
Figure 1. Axial display map of our patient following −6.00D LASIK surgery. At a chord of 8.0mm, the patient’s radius of curvature is 7.80mm (43.25D).
Figure 2. Our patient’s elevation display map. When the 7.80mm (43.25D) radius is used as the reference sphere, the estimated amount of tissue ablations is −82 microns.
Figure 3. Our patient’s scleral lens and the optical coherence tomography image showing 340 microns of apical clearance.
A Case for Scleral Lenses
Figure 1 shows the axial map of a patient who underwent a −6.00D LASIK ablation in 2006. Using the axial power map, his peripheral radius of curvature at a chord of 8.0mm is 7.80mm (43.25D). Figure 2 shows the patient’s elevation display. Using the 7.80mm (43.25D) radius as our reference sphere, it is possible to estimate the depth of the ablation, which in this case was −82 microns.
We have mentioned in previous columns that when fitting modern scleral lenses, we like to target between 300 to 400 microns of apical clearance. Figure 3 shows that the scleral lens design easily accommodates the additional -82 microns of depth for our post-LASIK patient. This is why we rarely need to utilize reverse geometry scleral lens designs when fitting patients in whom the corneal depth has been compromised, as we often do when fitting such patients with corneal GP lenses.
It is important to remember that everything we do with corneal contact lens designs is predicated on the many height differentials present in both regular and irregular corneas that determine exactly where the lens will position and how it will move on the cornea. One of the major advantages of scleral lenses is that their limbus-to-limbus clearance allows the lens to vault over corneal elevations, and their fluid dynamics allow the post-lens fluid to “fill in” corneal depressions. CLS
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.