When considering scleral lens (SL) design, is the amount of corneal toricity a predictor for toricity out onto the bulbar conjunctiva/scleral surface? Numerous—mostly small, pilot-size—studies have concluded that there is not a definitive correlation between the cornea and the scleral surface (Kinoshita et al, 2016; Siebert and Jedlicka, 2017; Lopez-Alcon and Castejon, 2018).
Contrary to these results, several recent publications did find a correlation between the cornea and sclera, particularly when the corneal astigmatism was > 2.00D (Macedo-de-Araújo et al, 2019; Consejo and Rozema, 2018; Ritzmann et al, 2018). This has been my experience, especially when evaluating toric corneal elevation changes located in the paralimbal region (Figure 1). However, there are exceptions.
Consider This Case
A 17-year-old patient had previously been evaluated, and keratoconus was ruled out. The patient was unsuccessful with soft lenses secondary to poor vision and with corneal GP lenses because of discomfort. Power maps from placido corneal topography revealed symmetric with-the-rule corneal astigmatism, with simulated keratometry values of OD 41.51/44.84 @ 098 and OS 41.37/44.70 @ 087. Manifest refraction was OD –2.50 –3.00 x 177, 20/30 and OS –3.50 –2.75 x 172, 20/25. Corneoscleral topography measured with-the-rule paralimbal elevation steepening of the cornea in both eyes but spherical scleras without toricity or significant asymmetry (Figure 2).
Using the Scleral Shape Study Group classification, this patient would be classified as having spherical bulbar conjunctiva/scleras (DeNaeyer et al, 2017). SLs with spherical landing zones were dispensed with the following parameters: OD 16.5mm diameter, 40.97D base curve (BC), –1.47D power, 20/25; and OS 16.5mm diameter, 40.17D BC, –1.17D power, 20/25. The patient was thrilled with his vision and satisfied with comfort.
Profilometry, such as corneoscleral topography, is the most efficient and accurate way of determining the need for toricity in the landing zone of a SL for patients who have high astigmatism. Analysis of elevation maps will tell you the orientation of scleral toricity, which allows for the proper placement of front-surface toricity when needed. Don’t have access to a profilometer? Paralimbal elevation data from placido-based corneal topography can give an indication of scleral toricity. You can verify potential scleral toricity with diagnostic lenses having sphere and toric landing zones; however, as in this case, the corneal and scleral shape might not match. CLS
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