Recently, the Scleral Shape Study Group (SSSG) studied the potential relationship between corneal and scleral elevation topography in patients who have ectasia and normal corneas (DeNaeyer et al, 2019). Specifically, this multi-site retrospective study, which included 115 eyes that had regular prolate corneal profiles and 227 eyes showing corneal ectasia, looked to determine whether corneal apex location had an effect on scleral asymmetry. Sagittal height values were measured along a meridian that bisected the corneal apex and the corneal geometric center at two points 180° apart along a 16mm chord (Figure 1). The difference between these sagittal values represents a quadrant-specific effect (QSE). The results revealed that ectatic eyes have a greater QSE along the same axis as the corneal apex compared to normal cases. The QSE effect was greater if the ectasia was ≥ 1.25mm from the corneal center. The scleral asymmetry in ectasia cases was further reinforced by the findings that 57% of ectatic eyes fit into the categories of asymmetric scleral groups previously defined by the SSSG (DeNaeyer et al, 2017), while only 25% of normal corneas fit into those categories.
Numerous studies have attempted to find relationships between corneal and scleral topographies (Kinoshita et al, 2016; Siebert and Jedlicka, 2016; Consejo and Rozema, 2018; Macedo-de-Araújo et al, 2019; and others. Full list available at www.clspectrum.com/references ). Piñero et al (2019) concluded that the corneo-scleral profiles for keratoconic eyes have higher degrees of asymmetry when compared to healthy eyes. The SSSG showed that ectatic eyes are not only more asymmetric compared to normal eyes, the asymmetry is highly correlated with the location of the apex. One question that arises is whether the sclera has an influence on the location of the corneal apex for keratoconus (Consejo and Rozema, 2018). Regardless, the findings suggest that keratoconic eyes are often best fit with either scleral designs that incorporate quadrant-specific haptics or free-form custom designs from measurement.
One 35-year-old keratoconus patient had previously been unsuccessful with scleral lenses due to discomfort. Corneo-scleral topography showed a highly asymmetric sclera that had a relative increased sagittal height along the same axis as the corneal apex (Figure 2). A 16mm spherical haptic scleral lens decentered inferior. A 16mm free-form customized scleral lens fit from corneo-scleral topography that exactly conforms to the asymmetric scleral shape centered on the eye and is successfully worn by the patient.
Patients who have keratoconus often have increased scleral asymmetry that can be predicted by corneal topography. These patients can benefit from customized back-surface scleral lens designs. CLS
For references, please visit www.clspectrum.com/references and click on document #286.