Insights into the sagittal height (sag) of the anterior eye can be a helpful tool in the fitting of both custom soft and scleral GP lens designs. Today, we believe that five anatomical features contribute to the sagittal height of the anterior eye (Figure 1):
- The central radius of curvature (approximately 3.0mm) of the central cornea
- The midperipheral corneal eccentricity, which extends from the apex of the cornea to a chord of approximately 10.0mm
- The overall corneal diameter
- The corneal angle that begins at the 10.0mm chord and extends to the limbus.
- The scleral sag, which begins at the corneal diameter and extends out to any given chord (i.e., 13.0mm to 20.0mm).
Sag and Soft Lens Fitting
For us, all soft lens fitting begins with a measurement of the corneal diameter, to which we add 2.50mm for “scleral drape” on each side of the cornea (Figure 2 Top). In this example, a corneal diameter of 12.1mm would require a soft lens with a diameter of approximately 14.6mm.
Anterior segment optical coherence tomography (OCT) of this patient’s eye shows a sagittal height (at the 14.6mm chord) of 3,650 microns (Figure 2 Bottom). Figure 3 indicates that the lens parameters closest to this sag would have a base curve of 8.9mm and a diameter of 14.5mm; however, this lens would have an inadequate sagittal height for this eye.
Our clinical experience has shown that when fitting a soft contact lens, it is necessary to select a lens with a sagittal height that is approximately 400 microns higher compared to the sagittal height of the eye. This is most likely related to the modulus and elasticity of the soft lens material. We call this the Pacific Rule.
Therefore, in this example, the base curve of a soft contact lens can be determined by first calculating the sagittal height of the eye (at 14.6mm), which in this case is 3,650µm, then adding 400µm (the Pacific Rule) for a total lens sag required of 4,050µm.
According to the Figure 3 sag chart, it is possible to match the sagittal height required (4,050µm) with the sagittal height of the contact lens. In this example, a contact lens with a base curve of 8.4mm, a diameter of 14.5mm, and a sagittal height of 4,059µm matches very closely with the calculated corneal/scleral sagittal height. CLS