George is a truck driver who has keratoconus (KC) and is a long-time GP contact lens wearer. He recently agreed to explore other options and finally accepted being refitted into scleral lenses.

For me, George’s eyes were not a challenge to fit based on his KC profile. As expected, at delivery, everything was textbook. However, when I put the slit lamp away, George’s face told a different story. While he admitted that the lenses were unbelievably comfortable, he did report difficulty seeing clearly.

Over-refraction showed with-the-rule residual astigmatism (RA) in both eyes. Despite the improvement when adding toric power, there was no “wow” effect. Keratometry over the lenses gave a low 0.75D. I thought it may be due to flexure and re-ordered thicker lenses. With these new ones, the same complaints remained despite stable, centered, front-toric scleral lenses.

Lens Flexure Explained Lens flexure, a known phenomenon in the GP world (Collins et al, 2001), occurs when lenses are fitted steeper than the corneal curvature or when an empty space, not filled with tears, exists between the lens and the eye (Corzine and Klein, 1997). Scleral lenses vault over the corneal surface, generating a reservoir that is filled entirely with fluid and helps to support the lens. In such cases, flexure is almost impossible to consider. A lens thicker than 0.25mm is also not considered as flexible (Phillips and Stone, 1989).

Assessing Flexure Over-keratometry/topography—over the lens on the eye—is suggested to assess lens flexure (Sorbara et al, 1992). Flexure is defined as the change in lens power on bending and occurs during blinking (Holden et al, 1976), not under static assessment. Keratometry/topography cannot then assess flexure properly. Flexure must be measured by applying an incremental load on the lens and monitoring its deformation until breakage (British Standards Institution, 2000). Obviously, this is not applicable when the lens is worn. Over-keratometry/topography evaluates the lens through the tear film, mimicking lens shape. Consequently, wetting and rewetting of the lens may be the cause of the “curvature” variation that is attributed to lens flexure.

Higher-Order Aberrations (HOAs) Optically, a scleral lens induces refractive index variation between the reservoir fluid and the corneal surface. These variations generate anterior corneal surface aberrations (Choi et al, 2007), which are then added to those from the irregular posterior cornea (Chen and Yoon, 2008). In fact, KC patients experience coma and not RA (Hashemi et al, 2016). These HOAs increase with lens decentration (Sabesan et al, 2013), as the reservoir thickness increases, and if the lens is not well aligned with the conjunctiva (Barnett and Fadel, 2017).

Differentiating HOAs Practitioners can easily differentiate HOAs from RA in practice. Holding a loose lens in front of the scleral lens and not getting a “wow” effect translates into the presence of HOAs rather than RA. This is confirmed when a second pair of front-toric lenses is tried, but they lead to the same deceiving visual acuity. I have found that this may be more likely to happen when KC patients are able to be refracted to 20/30 or better in glasses. In such cases, they will do poorly, visually speaking, in scleral lenses.

Other Options Increasing the lens thickness will not compensate for HOAs. The only way to fix these visual issues is to design scleral lenses with wavefront-guided front-surface optics (Hastings et al, 2019). Unfortunately, they are not currently widely available. In the meantime, it is also possible to switch to hybrid lenses or to go back to corneal GPs.

For George, hybrid lenses provided him with vision as crisp as he experienced with his old lenses, but his comfort and the lens stability were highly improved. Both of us were finally able to smile again.

Clinical Tips

Scleral lenses may be associated with HOAs, but this is easily confounded with RA. In such cases, practitioners have to be flexible and consider other options. CLS

For references, please visit www.clspectrum.com/references and click on document #293.