Contact Lens Case Reports
Effects of Corneal Eccentricity
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Fitting soft lenses is best accomplished by matching the sagittal depth of the lens with that of the eye's anterior segment. In his 1992 landmark paper, "Ocular sagittal height and soft contact lens fitting," Graeme Young, PhD, MPhil, FCOptom, DipCL, FAAO, identified the five anatomical features that contribute to anterior segment height: central corneal radius of curvature, corneal eccentricity, corneal diameter, and scleral radius and eccentricity. Scleral radius and shape contribute little to overall sag. Predominantly the corneal parameters of radius, eccentricity and diameter dictate how high or low the anterior segment may be.
In ranking order of contribution to sagittal height, corneal diameter dominates followed by corneal eccentricity. Surprisingly, influencing the sag least is central radius of curvature. For years practitioners have been told that the central radius of curvature had the greatest influence on corneal sagittal height, however, its position is third on the list. That leaves corneal diameter and eccentricity for us to consider.
In previous case reports we have discussed the contribution of corneal diameter, and it is clear that the larger the corneal diameter the greater the corneal sag and conversely, the smaller the corneal diameter the lesser the sag. However, a less understood parameter is that of corneal eccentricity and how it contributes to overall corneal sag. Figure 1 illustrates six different corneal shapes all with the same central radius of curvature, 43.00D (7.85 mm), and the same diameter (11.8mm). As the corneal eccentricity (e-value) increases from 0 (green line) to 100 (blue line), the corneal sag decreases.
Figure 1. Corneal eccentricity and sagittal depth.
To further illustrate the effect of corneal eccentricity, we selected three corneas with identical apical radii of 42.50D (7.85mm) and measured their corneal sag over a cord of 8.0mm (Figures 2 and 3). Table 1 clearly shows that as the eccentricity increases, the sagittal depth decreases.
Figure 2. Corneal sag of 1080 resulting from an apical radius 42.50D and an eccentricity of 28.
Figure 3. Corneal sag of 1041 resulting from an apical radius 42.50D and an eccentricity of 84.
Therefore, in fitting soft lenses, it helps to remember three rules: the larger the corneal diameter the greater the sagittal depth; the lower the corneal eccentricity the greater the sag; and the steeper the central radius of curvature the greater the sag. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #180.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.