Axial Length to Corneal Radius of Curvature Ratio and Refractive Errors

Purpose: To determine the distribution of axial length (AL) to corneal radius of curvature (CRC) ratio and to evaluate its association with refractive errors in an Iranian population.

Methods: In this cross sectional study, multistage cluster sampling was used to select subjects 40-64 years of age residing in Shahroud, northern Iran. All subjects underwent manifest and cycloplegic refraction, and biometry using the Allegro Biograph (WaveLight AG, Erlangen, Germany). Individuals with a history of intraocular surgery, extensive pterygia and ocular trauma were excluded.

Results: Of a total of 6,311 patients, 5190 (82.2%) participated in the study. We excluded 247 patients to adhere with study criteria and 132 patients due to missing data. Mean AL/CRC was 3.034 [95% confidence interval (CI), 3.031-3.037]. AL/CRC was 3.028 and 3.042 in female and male subjects, respectively (P<0.001). The R2 coefficients between spherical equivalent (SE) refractive error and AL/CRC, AL, CRC, lens thickness, and anterior chamber depth were 0.607, 0.351, 0.012, 0.038, and 0.091, respectively. Linear regression showed a 12.1 diopter (D) shift towards myopia with every 1 unit increase in AL/CRC (P<0.001). Mean AL/CRC was 3.472 among myopes with SE less than-5.0D; this value decreased linearly and was as low as 2.690 among hyperopes with SE more than 5.0D. R2 coefficients for AL/CRC with spherical and cylindrical power were 0.560 and 0.071, respectively.

Conclusions: Minimal changes in AL/CRC lead to large changes in refractive error. The correlation between refractive errors was significantly stronger with the AL/CRC ratio than with AL and CRC alone.