In recent years, there has been much written about the use of sagittal height techniques to measure the height of the anterior segment of the eye and sagittal depth techniques to measure the depth of both rigid scleral and soft contact lenses.
Today, there are a number of optical coherence tomography (OCT)- and profilometry-based techniques that can provide accurate sagittal height data out to chords of up to 20.0mm. While these systems have shown some wonderful promise, they still remain predominantly research tools in university practices.
Alternative for Practice Use
When it comes to estimating the sagittal height of the cornea and sclera, a more practical tool is an arc length calculator. Such calculators estimate the sagittal height of the eye at any desired diameter based on central keratometric reading and corneal diameter.
The benefit of using an arc length calculator is demonstrated in the following case.
A 32-year-old male who had a longstanding history of myopia was recently fitted in both eyes with daily disposable soft contact lenses with a base curve (BC) of 8.4mm, overall diameter (OAD) of 14.0mm, and a lens sagittal height of 3,742 microns. The patient’s simulated keratometric readings were right eye 42.00/42.62 and left eye 41.87/42.50. The patient’s most striking anatomical feature was his large corneal diameters, which measured 12.8mm in both eyes.
Figure 1 shows the patient’s right and left eyes following his maximum contact lens wearing time of 8 hours per day. We can see that the 14.0mm lenses are far too small for his large 12.8mm corneas. Additionally, the daily disposable lenses have inadequate sagittal depth, resulting in lens decentration and conjunctival injection.
To refit the patient, we entered his simulated keratometric readings, corneal diameter, and spectacle Rx into the arc length calculator, which came up with the following custom soft lenses: right eye 8.4mm BC, –6.25D, 15.4mm OAD; left eye 8.4mm BC, –6.75D, 15.4mm OAD (Figure 2).
Figure 3 shows these lenses on the patient’s eyes. The new custom soft lenses had a sagittal depth of 4,920 microns, which is 1,178 microns deeper than his daily disposable lenses had been. The custom lenses provided the patient with all-day comfort and completely eliminated his conjunctival injection.
This case shows how a simple arc length calculator can be used as a guide for determining the base curve, power, and diameter of custom soft lenses.
We believe that in the future, new, easy-to-use instruments will provide an even more accurate understanding of the shape of the anterior segment and, therefore, improve the fitting techniques of all contact lenses (rigid or soft) that extend beyond the limbus. CLS