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Contact Lens Case Reports
Three Rules of Scleral Fitting
By Patrick J. Caroline, FAAO, & Mark P. André, FAAO
When it comes to modern scleral lens fitting, three rules have emerged that seem to be universal for lens designs that exceed 15.0mm in diameter. The rules are as follows.
The Three Rules
Rule #1 The lens center should clear the corneal apex by 200 to 300 microns. When the appropriate clearance is present, the pupil margins should be almost obscured by the fluorescein beneath the lens (Figure 1A). If the pupil margins are clearly discernible, clearance may be inadequate (Figure 1B). Figure 2 shows the best way to view apical clearance.
Figure 1. (A) The desired amount of apical clearance as seen with fluorescein. (B) This lens is suspected of having inadequate sagittal depth.
Figure 2. Apical clearance is best estimated with slit lamp, optic section and comparing the lens thickness with that of the post-lens tear film.
Rule #2 The lens should clear the limbus 360 degrees around (or as much as possible) (Figure 3). This, too, can be viewed in optic section or with fluorescein and either white light or cobalt blue light. If the limbal clearance is inadequate, a darkened area or ring will be present at the peripheral cornea and limbus (Figure 4). It is important to remember that both the center and the periphery of the lens will “drop” (decrease in sagittal height) approximately 80 to 120 microns during the first day of lens wear, so it is necessary to “overshoot” the lens clearance both apically and peripherally during the diagnostic lens fitting.
Figure 3. Desired limbal clearance with both fluorescein evaluation and anterior segment OCT imaging.
Figure 4. Inadequate limbal clearance as noted by the dark band of fluorescein thinning.
Rule #3 The full lens bearing should rest on the bulbar conjunctiva/sclera. The lens periphery should incorporate a concave “ski” radius to position the lens edge away from the bulbar conjunctiva (Figure 5). If the lens edge is rolled-in toward the conjunctival surface, the lens will impinge on the conjunctival vessels (Figure 6). CLS
Figure 5. Optimum edge clearance should demonstrate 50 percent of the edge thickness onto the conjunctival surface and 50 percent above the surface.
Figure 6. Peripheral clearance is inadequate when the entire lens edge rests below the conjunctival surface.
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.
Contact Lens Spectrum, Issue: September 2011