Contact Lens Case Reports
Comparing Ways to Measure Sagittal Height
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Our November 2012 column titled “New Thoughts on Anterior Segment Sagittal Height” described the 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 “Central K”; 2) the midperipheral corneal eccentricity that extends from the apex of the cornea to a chord of approximately 8.0mm to 10.0mm; 3) the corneal angle, which begins at the 10.0mm chord and extends to the limbus; 4) the overall corneal diameter; and 5) the scleral angle that begins at the limbus and extends out to any given chord (Figure 1).
Figure 1. The five anatomical features that contribute to the sagittal height of the anterior segment.
We have identified that one of the more consistent indicators of peripheral corneal and scleral height is the corneal angle that forms at a chord of 10.0mm. The lower the angle, the lower the scleral height, and vice versa (Figure 2).
Figure 2. The corneal angle that forms at a chord of 10.0mm.
Calculating Sagittal Height
Engineers at Medmont have recently developed a new algorithm that calculates the corneal sagittal height and corneal angle at a chord of 10.0mm. The algorithm extends the corneal angle out across the peripheral cornea, the limbus, and onto the sclera. An extrapolation of the sagittal height can then be calculated at any desired chord from 12.0mm to 20.0mm (Figure 3).
Figure 3. The algorithm extends the corneal angle out across the peripheral cornea, the limbus, and onto the sclera.
In a poster presented at the 2015 American Academy of Optometry meeting, Dr. Brooke Harkness compared the sagittal height results calculated by the Medmont Extrapolated Sagittal Height (ESH) module to those achieved by the Carl Zeiss Meditec Visante optical coherence tomographer (OCT) and the Eaglet-Eye Eye Surface Profiler (ESP) (Figure 4).
Figure 4. Sagittal height measurements at a chord of 15.0mm with the Visante OCT, the ESP, and the Medmont ESH.
Today, it is well understood that the sagittal height of the anterior eye is a key component in fitting custom soft and modern scleral lens designs. This study indicates that at a 15.0mm chord, the Medmont ESH module provides sagittal height measurements in agreement with both the Visante OCT and the ESP. CLS
For references, please visit www.clspectrum.com/references and click on document #247.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Contamac. Mark André is an associate professor of optometry at Pacific University. He is also a consultant to CooperVision.