Role of the Axial Length/Corneal Radius Ratio in Determining the Refractive State of the Eye

The purpose of our study was to investigate the extent to which the ratio between axial length and corneal radius (the AL/CR ratio) determines the refractive state of the human eye. Subjects for the study were 194 young adults between the ages of 18 and 30 years, whose spherical equivalent refraction ranged from +7.91 to -13.32 D. For each subject, manifest objective refraction, corneal radii, and axial ocular distances were measured. Lens power was determined using a formula proposed by Bennett and Rabbetts. The AL/CR ratio was found to be approximately 3.00 for emmetropic eyes, ranging from 2.60 for the most highly hyperopic eye to 4.10 for the most highly myopic eye. The coefficient of determination of linear regression for the AL/CR ratio as a function of spherical equivalent refraction was 0.84, suggesting that 84% of the variance in refraction could be accounted for by variance of the AL/CR ratio. The results of the study suggest that for a given amount of ametropia, an eye having a relatively high AL/CR ratio would tend to have a low-powered lens (indicating that the lens had "emmetropized"), whereas an eye having a relatively low AL/CR ratio would tend to have a high-powered lens. We conclude that the AL/CR ratio can provide information concerning the extent to which the lens has emmetropized by reducing its power concurrent with axial elongation.