Utilizing reverse geometry lens designs is an emerging trend in the specialty contact lens field. Standard geometry lenses are set up to fit prolate surfaces and so their peripheral curves gradually flatten away from the base curve. However, not all eyes are prolate in shape. Corneas that have had myopic refractive surgery become oblate where the mid-peripheral cornea is relatively steeper than the center. Penetrating keratoplasty (PKP) may also result in an oblate surface. Ibrahim et al.1 reported in a study that 30% of eyes after PKP resulted in oblate-type cornea post-operatively. Rigid lenses are not able to drape over these bottle cap profiles like soft contact lenses. Using a prolate design for an oblate cornea forces the fitter to fit the lens excessively steep in order to align or vault the relatively steeper mid-periphery. This can result in a tighter fitting lens, bubbles, softer vision, and increased higher order aberrations. A reverse curve that is steeper than the base curve enables a reverse geometry GP lens to align with an oblate profile. As well, decentered cones or patients who have had intrastromal corneal ring segments can often be more easily vaulted in the mid-periphery by a reverse geometry lens design. Finally, reverse geometry lenses can allow the fitter to simply increase the sagittal depth of a lens design without changing the base curve.
A 26 year-old keratoconus patient reported for contact fitting. He previously had gone through a corneal collegen crosslinking procedure and superior/inferior intrastromal corneal ring segment plancement in both eyes. The superior ring segments had been removed prior to the evaluation. Post-operatively the patient had tried both corneal (R)GPs and hybrid lenses without success. After discussion, I fit the patient with full sclerals using diagnostic lenses. Interestingly, the spherical back surface lenses with a standard geometry design cleared the central section of his cornea without problem, but exhibited touch in the mid-periphery (figure 1, below).
Switching to a reverse geometry design allowed the lenses (figure 2, below) to vault over the mid-peripheral area while keeping the central section of the lens from being excessively steep.
The utilization of reverse geometry lens designs will continue to grow as more manufacturers adapt this design strategy.
1. Ibrahim, O. Bogan, S. Waring, GO. Patterns of corneal topography after penetrating keratoplasty Eur J Ophthalmol. 1996 Jan-Mar;6(1):1-5.