Simple formula for determining the location of the equator.

The determination of the position of the equator is very important during strabismus surgery for safe maximum recession. The purpose of this study is to present a simple formula for estimating the location of the equator (E = axial length x pi/4 - corneal diameter/2). A previous formula (E = 2(pi)r[beta/360]), although well presented, proved relatively complex to understand and apply to the clinical situation. There was no significant difference between the values calculated by the new formula and the previous formula when the horizontal corneal diameter was between 10.0 and 11.5 mm (P = 0.23, r = 0.998). In this range, the mean difference between the values calculated by the two formulae was only 0.1 +/- 0.08 mm (range 0-0.35 mm). Another advantage of the new formula is that the actual measured values of corneal diameter and axial length can be used. It is hoped that this simple formula can be used as an alternative for determining the location of the equator.