Contact Lens Case Reports
New Thoughts on Anterior Segment Sagittal Height
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
The sagittal height of the anterior eye is a key component when fitting both custom soft contact lenses and modern scleral lens designs. We have identified five anatomical features that contribute to the sagittal height of the anterior segment:
1. The radius of curvature of the central (approximately 3.0mm) cornea, often referred to as “K.”
2. The midperipheral corneal eccentricity, which extends from the apex of the cornea to a chord of approximately 10.0mm.
3. The corneal angle, which begins at the 10.0mm chord and extends to the limbus.
Figure 1. Corneal/scleral data, 11.0mm cornea.
Figure 2. Corneal/scleral data, 11.8mm cornea.
4. Overall corneal diameter.
5. The scleral angle, which begins at the limbus and extends out to any given chord, i.e., 14.0mm.
Topographical studies at Pacific University have shown that in normal (non-diseased) eyes, the sagittal height across a chord of 10.0mm is surprisingly consistent and contributes approximately 1,650μm to 1,750μm to the overall sagittal height. Beyond the 10.0mm chord, the eye tends to lose its curvature and forms a tangent (straight) line that extends from 10.0mm out to the limbus and sclera.
The Role of Corneal Diameter
As a general rule, smaller-diameter corneas exhibit less sagittal height and, conversely, larger-diameter corneas exhibit greater sagittal height (Figures 1 through 3, which profile corneas that have small, medium, and large corneal diameters). However, this is not always the case.
Our research indicates that one of the more consistent indicators of sagittal height is the corneal angle that forms at a chord of 10.0mm to the limbus and, ultimately, the sclera. The lower the angle, the less the sagittal height; the higher the angle, the greater the sagittal height. In Figure 4, the sagittal height of two eyes from 10.0mm to the apex is almost identical, yet the overall sagittal height is 400 microns greater in the cornea that has the steeper corneal/scleral angle. CLS
Figure 3. Corneal/scleral data, 12.5mm cornea.
Figure 4. Low corneal angle and high corneal angle and its contribution to sagittal height.
For references, please visit www.clspectrum.com/references.asp and click on document #204.
|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.|