Contact Lens Case Reports

Understanding Corneal and Scleral Shape

BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO

Historically, the terms base curve radius or back optical zone radius have described the primary curve on the posterior surface of a contact lens. Today, we are poised to make a paradigm shift away from this nomenclature to that of lens sagittal depth.

The sagittal depth of a contact lens is its absolute height (lower number = flatter, higher = steeper) in microns. Sagittal depth takes into account the effects of base curve radius, optical zone diameter, reverse curve radius, and peripheral lens design.

At Pacific University, we teach that five anatomical features contribute to anterior segment sagittal height: 1) The radius of curvature of the central (approximately 3mm) of the cornea, often referred to as “K”; 2) The midperipheral corneal eccentricity that extends from the apex of the cornea to a chord of approximately 10mm; 3) The corneal angle, which begins at the 10mm 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 (i.e., 14mm).

Eye Surface Profiler

Knowing the sagittal height of the anterior eye is a key component in fitting both soft and modern scleral lens designs. Measuring sagittal height (out to a chord of 15mm) had been limited to optical coherence tomography. Enter the Eye Surface Profiler (ESP; Eaglet-Eye, the Netherlands), which can measure the topography of the cornea and sclera up to a chord of 20mm. The instrument uses a dual light source system that projects two moiré patterns onto fluorescein that is instilled before map acquisition, allowing it to acquire 120,000 points of data for direct measurement of “true” anterior segment height (Figure 1).

Figure 1. The Eaglet-Eye Eye Surface Profiler. Note the moiré grid pattern projected onto the fluorescein and the resulting fringe pattern.

We have used the ESP primarily to measure overall sagittal height for traditional and custom soft lenses and to map scleral shape for fitting scleral lenses. Figure 2 shows how the instrument calculates the sagittal height of the bulbar conjunctiva/sclera 360 degrees around. This measurement is essential in quantifying the toricity and/or asymmetry of the scleral surface. The blue lines show the independent sagittal height of nasal/temporal and the superior/inferior portions of the cornea and sclera at a chord of 15mm. The red line shows the “average” sagittal height along the two principal meridians.

Figure 2. The surface area of the captured image as displayed as a height map. Note the pronounced asymmetry in height between the nasal and temporal sclera.

We believe that lens fitting by sagittal height will dominate the future. The ESP can further our understanding of anterior segment shape and, therefore, improve the fitting techniques for any contact lens that extends beyond the limbus. CLS

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 for CooperVision.