Fitting a Non-Spherical Sclera
By Gregory W. DeNaeyer, OD, FAAO
Scleral GP contact lenses that have diameters between 12.5mm to 15mm share support from the cornea and the sclera, while scleral lenses that range from 15mm to 25mm should ideally place all bearing on the sclera. Of course, these scleral-type lenses not only fit the sclera, but they actually rest on the scleral conjunctiva. Research by van der Worp and colleagues (2010) has shown that the typical scleral shape is nonrotationally symmetrical. In other words, the sclera is not spherical or toric, but rather quadrant specific in its shape.
With that said, many practitioners are fitting scleral lenses that have spherical-back-surface haptics. In these cases, some mismatch between the spherical back surface of the lens and the sclera is offset by the spongy conjunctiva. Fitting spherical-back-surface haptic sclerals from a diagnostic lens set is a straightforward and successful approach for most patients. Occasionally, other alternatives may be necessary to improve a scleral contact lens fit in some patients. Design options are available that can improve a scleral lens fit when a spherical-back-surface haptic design is not providing a successful fitting relationship with the sclera.
Smaller Lens Design
The quadrant-specific differences in scleral topography increase with increasing distance away from the corneal limbus (van der Worp et al, 2010). Thus, switching to a smaller-diameter scleral lens should improve the fitting relationship when using a spherical-back-surface haptic designed scleral lens.
Toric-Back-Surface Haptic Designs
Oftentimes, a spherical-back-surface designed scleral lens will appear to have a with-the-rule fit on the sclera. In these cases, a back-surface-toric haptic may significantly improve fit and stability. Visser and colleagues (2006, 2007) have reported that scleral comfort and wearing time improves with a back-surface-toric haptic versus a spherical design.
Technology is available to produce quadrant-specific back-surface haptic designs, and it appears that more practitioners are taking advantage of this. Truform Optics, for instance, has reported manufacturing an increasing number of quadrant-specific designs for its DigiForm 18 scleral lens. Future advances in technology that will allow faster and more accurate sclera measurement may increase the utilization of such a design.
Producing a scleral lens from a mold of a patient's eye may be the most accurate way to conform a lens to the scleral surface. However, these lenses must be manufactured in PMMA, and they require increased fitting time.
Another consideration for patients who have a non-spherical sclera is switching to a hybrid lens design. The soft skirt of a hybrid contact lens has the advantage of being able to readily conform to an asymmetric sclera (Figures 1 and 2). CLS
Figure 1. Scleral contact lens exhibiting excessive superior edge lift.
Figure 2. Hybrid contact lens on the eye in Figure 1. The soft skirt improves the peripheral fit.
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #190.
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio. His primary interests include specialty contact lenses. He is also a consultant to Visionary Optics. Contact him at email@example.com.