Using toricity with scleral lenses
Change the curve
Scleral lenses are indispensable in a specialty contact lens practice. The indications for scleral lens use are well established in the literature and range from visual rehabilitation of irregular corneas to severe ocular surface disease management.1-4 Many more uses may still be revealed.
The hallmark of scleral lens fitting is vaulting the cornea and limbus and landing on the conjunctiva overlying the sclera. They are called scleral lenses for a reason: they fit the sclera. Much attention is placed on proper corneal clearance; however, equally important is the landing of the periphery of the scleral lens on the ocular surface. We are fitting the scleral shape after all.
Front toric vs. back toric: What’s the difference?
Toricity may mean different things depending on the practitioner. Remember, toricity is two different curves separated by 90 degrees and oriented at a given axis. In optics those two different curves have dioptric power that induces astigmatism.
When employing front surface toricity, the effect is optical. Front surface toricity is utilized when astigmatism is revealed with a sphero-cylindrical over-refraction. Orientation is important, and the lens requires stabilization either by way of prism ballasting, dual thin zones, or double slab-off prism in order to provide stable vision.
Back surface toricity is used for lens alignment. Scleral lens back surface toricity is typically found in the peripheral curve/landing zone/haptic system. Figure 1 shows the profiles of a scleral lens with back surface toric curves. It has been shown that scleral asymmetry is increased further from the limbus.5 If the landing zone of a spherical or rotationally symmetric lens is producing asymmetric compression, impingement, or edge lift, then the fit may be improved by employing back surface toricity.
To summarize, front surface toricity is an optical feature, and back surface toricity is a fitting feature. Both front and back toricity can be manufactured in the same scleral lens. The result can be excellent lens stability (from the back surface toricity scleral lens alignment) with outstanding optical performance (from the front surface optical toricity).