I developed a special interest in fitting hybrid contact lenses during my cornea and contact lens residency. I’ve since had the opportunity to educate students on hybrid lenses manufactured and distributed across North America. Here are three key points related to where the soft skirt meets the GP center.
1) The Junction Bond Is Essentially Unbreakable
Unlike hybrid lenses of the distant past, today’s hybrid lenses can withstand a pressure greater than the maximum force that would be exerted by four fingers digitally manipulating a lens (Doan, 2018). This study also showed that even when breakage did occur, the hybrid lens tore in the soft silicone hydrogel skirt and not at the junction.
2) The Junction Does Not Land on the Cornea
There are points of visible touch before the junction at the inner landing zone (ILZ) as well as after the junction at the outer landing zone (OLZ), but the point at which the two materials join together vaults over the cornea (Figure 1). This is evident in the fluorescein pattern as a ring of edge lift around the perimeter of the typically 8.5mm GP center (Figure 2). This ring of clearance is very characteristic of an ideal hybrid lens fit. The darker circular band corresponds to the ILZ in the GP center; this area, along with the OLZ in the soft skirt, helps to bear the weight of the lens on the eye.
3) There Is Clearance Before and After the Junction
A poster presented during the 2015 Global Specialty Lens Symposium (Ramdass, 2015) evaluated various hybrid lenses on the eye after 30 minutes of wear time using optical coherence tomography (OCT) to obtain high-resolution images (Figure 3). Using the caliper software of the device, the observed clearance over the cornea was measured for pockets of vault after the ILZ and OLZ. The steeper the skirt curve of the hybrid lens, the greater the measured pocket of clearance in the soft skirt.
This is an important aspect of hybrid lens fitting, as hybrid lenses have historically been known to decrease in movement on the eye with continued wear, especially the steeper the skirt. These pockets of space fill with tears and help to promote tear exchange and ensure oxygen delivery to the cornea. The study authors acknowledged that further testing is needed to measure clearances at the end of the day (post-eight hours of lens wear). CLS
For references, please visit www.clspectrum.com/references and click on document #287.