contact lens design and materials
A New Take on a Hybrid Lens Design
BY RONALD K. WATANABE, OD, FAAO
The SynergEyes line of hybrid contact lenses are options for many contact lens wearers. The latest addition to the SynergEyes family is the ClearKone lens for keratoconus and other corneal irregularities. You might wonder why this lens is necessary when SynergEyes already has keratoconus (KC) and post-surgical (PS) lenses. Experience with the SynergEyes KC has taught us that it works best for smaller, central cones, but not as well for larger, decentered cones.
ClearKone was developed to better fit the more common decentered oval cones and more advanced cones that couldn't be vaulted by the KC design.
SynergEyes took a completely different approach from the KC design in developing ClearKone and went through 30 design iterations before arriving at the final design (Figure 1). It has a reverse geometry GP portion with a spherical base curve radius over a 6.00mm optic zone. The optic zone transitions to a reverse curve zone which extends out to 7.40mm. The central zone of the lens is specified by sagittal depth that ranges from 100 microns to 600 microns in 50-micron steps, as opposed to a more traditional base curve radius.
Figure 1. The ClearKone lens design for keratoconus.
Fitting this lens on the eye takes into account the sagittal height of the cornea and not the curvature. In fact, the GP portion is not designed to align with the cornea, but simply to vault it. As a result, the lens can fit very steep corneas with relatively flat base curves, which means lower minus powers, thinner lens profiles, and fewer lens-induced aberrations. The three skirt curvatures (steep, medium, and flat) are the same regardless of the central curve of the lens, unlike the KC design.
One final unique feature is that the diagnostic lens set is made so that all of the lenses have the same effective power on the eye. This means that as the sag (and base curve) changes, the lens power also changes. A patient should therefore have the same over-refraction regardless of the diagnostic lens that is on the eye.
By comparison, the KC design has a prolate aspheric back surface and a soft skirt available in three curvatures tied to the base curve radius. The PS design is a reverse geometry lens, but the back surface is different from that of the ClearKone design. The PS lens has a central spherical base curve that transitions to an oblate aspheric reverse curve available in three different lifts (steep, medium, and flat), and the soft skirt is available in only two curvatures.
A New Option
ClearKone is a new option for keratoconus patients who may benefit from the hybrid platform but cannot successfully wear the other SynergEyes designs. CLS
Dr. Watanabe is an associate professor of op-tometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry's Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass.