Article

CONTACT LENS DESIGN & MATERIALS

HYBRIDS—THEN AND NOW

The quest for the ultimate contact lens that provides razor-sharp optics and perfect comfort has taken many paths through the past several decades. Rigid lenses have provided superior vision, while soft lenses have been better for overall comfort. Hybrid lenses have attempted to marry the best of both worlds, incorporating a GP center for vision and a soft periphery for stability and comfort.

In the late 1970s, a new technology that could cross-link a soft hydrophilic plastic to a rigid plastic was developed. With it, hybrid lenses were born. In the mid-1980s, the U.S Food and Drug Administration (FDA) cleared the first hybrid lens incorporated a rigid center made of pentasilcon P (Dk of 14) and a 25% water HEMA-based hydrogel skirt. It was relatively small (13.0-mm overall diameter, with a 6.5-mm single-cut GP center) and, unfortunately, tended to fit tightly, causing adherence and corneal edema. It also tore easily at the soft-GP junction.

In the late 80s, a larger (14.3mm overall, with an 8.0mm GP center), more viable design was introduced. Available in a number of base curve options, it eventually offered base curves as steep as 6.5mm with a few soft skirt curve options for mild-to-moderate keratoconus patients. However, it was still fairly fragile, tended to fit tightly, and sometimes caused problems due to hypoxia.

The Next Generation

In the mid-2000s, the FDA cleared a hybrid lens featuring entirely new materials and designs. This lens utilized a new technology that could create a much stronger bond between the rigid and soft phases of the lenses, significantly reducing the tearing problem of previous hybrids. It used a high-Dk GP center (paflufocon D, Dk of 100) and a low-Dk, 27% water poly-HEMA hydrogel skirt (Dk of 9). There were designs for both normal and irregular corneas, but it was primarily used for irregular corneas because it provided GP optics to patients who really needed them without the edge awareness of GP lenses.

About 5 years later, a reverse geometry design innovation was developed to better fit keratoconic corneas that were not successful with previous hybrid designs. With this design, decentered and steeper cones could be fit with relatively flat base curves. This created less peripheral tightness, and a relatively low optical power could be used, resulting in better optical and visual quality. It was also the first hybrid specified by sagittal depth rather than base curve radius, which made lens changes more intuitive.

Also at that time, new higher-Dk materials were introduced. The GP material (petrafocon A, Dk of 130) includes a UV blocker, and the soft skirt is made with a novel silicone hydrogel material (hem-larafilcon A, 32% water, Dk of 84) to improve oxygen transmission across the entire lens. A refined “normal” cornea design using these materials is currently available in both single-vision and multifocal versions. A refined reverse geometry design is also available, as is a design for post-refractive-surgery eyes.

Another hybrid design from a French manufacturer is gaining traction outside of the U.S. The lens has a 100-Dk GP center and a 50-Dk silicone hydrogel skirt.

Summary

Today’s hybrid offerings are an important part of contact lens practice. They can be excellent options for both irregular corneas and for patients who have myopia, hyperopia, astigmatism, and/or presbyopia. Consider hybrids for patients who are looking for the best of both worlds: sharp, GP-caliber optics in a comfortable design. CLS