Soft contact lenses are known for conforming to the shape of the corneal surface. They almost always rest effortlessly past the limbus onto the encircling conjunctiva. But when fitting modern hybrid lenses, it is not uncommon for eyecare practitioners to observe edge fluting or puckering of the soft silicone hydrogel (SiHy) skirt.
Although a rare occurrence, if any type of edge lift is observed when fitting one of today’s standard soft SiHy lenses (Figure 1), we categorize this as a poor fit and likely switch to another comparable brand in our diagnostic lens room. However, when the SiHy skirt flutes in hybrid lens fitting (Figure 2), we are limited in our cleared brand options for alternative fitting. A parameter modification is required; let’s take a step back and consider why we would observe hybrid fluting in the first place before deciding what fit modification(s) are necessary.
What’s Causing the Fluting?
Could lens modulus and/or thickness be playing a role? The modulus of the SiHy soft skirt in U.S. Food and Drug Administration-cleared second-generation hybrid lenses is 0.50 to 0.80 MPa. To put that in perspective, reported modulus values of commonly prescribed commercial SiHy lenses today are in the 0.40 to 0.75 MPa range (Kim et al, 2017). As inherent lens stiffness largely influences how well a lens conforms to the shape of the eye, the minimal difference in modulus values does not explain why these two modalities may behave differently on an ocular surface.
Reported edge thickness for select SiHy lenses in the –3.00D range is 0.12mm to 0.19mm (Horst et al, 2012). There is minimal literature on edge thickness of hybrid SiHy skirts; thus, it is unknown whether thickness differences as compared to the edges on standard soft SiHy lenses have an impact on lens performance on the eye.
What about decentration? Especially on irregular corneas as in Figure 2, it is not surprising to see hybrid lenses decenter. Steepening the hybrid skirt curve or the GP base curve can help to center the lens on the eye and alleviate the observed edge fluting.
Is it dehydration/deformation? When removing a hybrid lens from a lens vial or a case, the GP portion of the lens can adhere to the surface of the container and cause the soft skirt to bend and lose its shape. Alternatively, a portion or all of the soft skirt can become dry and stiff. If the hybrid skirt is not adequately hydrated or shaped, a poor edge fit can ensue. Rehydrating and digitally reshaping the soft skirt can help.
Perhaps it’s just too flat! Lastly, there is the possibility that even with the GP portion of the lens properly centered, the SiHy skirt of the hybrid is too flat compared to the underlying cornea/conjunctival surface, resulting in excessive movement and edge fluting. In this case, steepening the hybrid skirt can help improve the edge fit. CLS
For references, please visit www.clspectrum.com/references and click on document #271.