The Scleral Lens Vault
Utilizing Reverse Geometry to Improve Prolate and Oblate Fits
BY GREGORY W. DENAEYER, OD, FAAO
Practitioners have a variety of designs and fitting techniques from which to choose when fitting scleral lenses. Scleral lens geometry is a critical design feature to consider in the fitting process.
Standard geometry lenses have peripheral curves that are relatively flatter compared to the base curve. These designs work best for prolate corneas, which are steeper in the center and flatter in the periphery. Examples would include regular corneas and many keratoconic corneas.
For reverse geometry designs, the first peripheral curve will be steeper than the base curve. This type of design should be used when fitting an oblate cornea, in which the cornea is relatively flatter in the center compared to the periphery. Examples include patients who have undergone myopic corneal refractive surgery, including radial keratotomy or laser vision correction. Additionally, up to 30% of corneas will have an oblate geometry post-penetrating keratoplasty (Ibrahim et al, 1996). Using reverse geometry lens designs is necessary in these cases to achieve a uniform vault of the entire corneal surface.
Reverse Geometry Benefits
Utilizing reverse geometry designs can be advantageous when fitting prolate corneas even though their geometries don’t match. Practitioners can use the reverse curve of the lens to adjust central lens vault without changing the base curve. Significantly steepening the base curve results in increased power. Reverse geometry can also help to optimize central vault when the apex of corneal steepness isn’t centered, which is common with keratoconus patients.
For oblate corneas, using a prolate design will require excessive central vault to obtain adequate vault of the midperipheral cornea. This will result in an increase in power and decreased transmissibility. Using a reverse geometry design will allow the lens to vault the midperipheral cornea without excessive central corneal vault.
Case Examples Figure 1 shows a scleral lens with 13.5D of reverse geometry that was virtually fit to a keratoconic eye to obtain a minimum paracentral vault of 226µm over the apex while keeping the central corneal clearance at 304µm.
Figure 1. Reverse geometry lens for keratoconus.
Figure 2 shows a virtually fit scleral lens for an oblate corneal transplant that uses 8.5D of reverse geometry to maintain a 305µm central clearance while vaulting the midperipheral cornea with a minimum of 202µm.
Figure 2. Reverse geometry lens for an oblate cornea post-penetrating keratoplasty.
Standard geometry scleral lenses are best used to fit regular corneas. Reverse geometry can sometimes improve the fitting relationship of irregular eyes that have prolate geometries. Reverse geometry is necessary to properly fit both regular and irregular oblate corneas. CLS
For references, please visit www.clspectrum.com/references and click on document #246.
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio. He is a shareholder of Precision Ocular Metrology LLC, has proprietary interest in Visionary Optics’ Europa and Elara Scleral Lenses, and is a consultant to Visionary Optics and Alcon. You can contact him at email@example.com.