Corneal Diameter

The impact on sagittal height and soft lens fitting 



Jason Jedlicka OD FAAO FSLS FCLSA

Clinical Associate Professor& Chief of Cornea and Contact Lens Service

Indiana University School of Optometry, Bloomington, Indiana (USA)

Introduction

Fitting soft lenses has traditionally been based on central keratometry readings and a base curve selection. Changing the mindset to consider corneal diameter and, ultimately, sagittal height will provide better outcomes in many instances. 

Sclerals Lead the Way

Scleral lenses have been rapidly growing in popularity among contact lens fitters for several years. When the first commercially available scleral lenses were released nearly 20 years ago, the fitting approach was like that of all other contact lenses, using base curve to drive the initial lens selection. Over time, practitioners came to realize that thinking in terms of sagittal height was a more useful approach to fitting scleral lenses compared to base curve, and this mindset has taken hold in most lens designs now. In many ways, this way of thinking about lens fitting could and should influence how we fit soft contact lenses as well.

Soft lenses cover the entire cornea and a portion of sclera. The cornea nearly always has more depth than the sclera does due to its shape. When we fit a contact lens to a larger cornea, the lens will travel over more cornea and less sclera. Conversely, when we fit a lens of equal diameter on an eye with a smaller cornea, a greater percentage of the lens will cover the sclera. Again, keep in mind that the cornea has more depth to it than the sclera does. Therefore, an eye with a larger cornea will have more depth to it over, say, a 14 mm chord than would an eye with a smaller cornea. 

(In) Depth

Central corneal curvature provides some information about the depth of the eye. A steeper central cornea generally indicates that the eye should have greater depth to it.  However, the cornea flattens between the central 3 mm and the limbus, and this rate of flattening is variable. In most instances, corneas that are steeper tend to flatten more rapidly. Corneas that are flatter tend to not flatten as rapidly. Therefore, the sagittal depth of two eyes with noticeably different keratometry readings may not be significantly different. 

Figure 1 – 10.8 mm cornea with central K’s of 43.00D.  Note the mean SAG at 10.0 mm is 1708 microns, while the mean SAG at 14.0 mm is 3420 microns



Consider the impact of corneal diameter on the sagittal height of the eye under a soft contact lens. If we consider a typical soft contact lens to have a diameter of 14.0 mm, how does a larger cornea affect the fit? The following figures demonstrate how the corneal region has more depth compared to the scleral region. Both of the following corneas have a central keratometry reading of 43.00D. In figure 1, the HVID is 10.8 mm.  In figure 2, the HVID is 12.6 mm. The central 10 mm chord difference between the two eyes is only 38 microns. However, the difference in SAG in the zone between 10 mm and 14 mm is 122 microns, for a total difference of 160 microns.

Figure 2 – 12.6 mm cornea with central K’s of 43.00D.  Note the mean SAG at 10.0 mm is 1737 microns, while the mean SAG at 14.0 mm is 3580 microns



What is the significance of a SAG difference of 160 microns? Roughly speaking, it is about 0.2mm in base curve, or the difference between an 8.7 BC and an 8.5 BC. So, if we were to fit both eyes in 14.0 mm lenses, the larger cornea should be fit in a steeper base curve lens despite the fact that the eyes have the same keratometric readings. Alternatively, fitting the larger cornea in a larger lens with the same base curve as the smaller cornea, in which the added lens diameter extends over flatter / shallower sclera, also provides a better fit. 

Closing Remarks

When it comes to soft lens fitting, thinking in terms of the total depth of the cornea as a product of its curvature and the diameter, as opposed to just its curvature alone, will result in better-fitting lenses. When corneal diameters are significantly larger than average (11.8 mm), choosing a lens with a larger diameter or steeper base curve will improve the fitting relationship. Conversely, a smaller cornea may be better fitted with a smaller-diameter lens when possible or with a flatter base curve than expected when not possible.

Dr. Jedlicka is an Associate Professor at Indiana University School of Optometry and is the Director of the Cornea and Contact Lens Service. He is a Diplomate in the American Academy of Optometry's Section on Cornea, Contact Lenses and Refractive Technologies, a past President of the Scleral Lens Education Society, and a Fellow and board member of the Contact Lens Society of America. He has contributed to multiple textbooks as an author on the topic of scleral lenses. He is the developer of the Zenlens and ZenRC scleral lenses and is aconsultant to Bausch + Lomb.