Validity of the keratometric index: Large population-based study

PurposeTo determine the accuracy of the keratometric index of 1.3315 based on the Gullstrand model eye in predicting the power of the posterior cornea, Gullstrand's model was compared to a calculated keratometric index derived from actual measurements of the cornea.SettingEye Institute, Tan Tock Seng Hospital, Singapore.MethodsOne eye of 2429 subjects with a mean spherical equivalent of −5.32 diopters (D) ± 2.88 (SD) was measured with the Orbscan II (Bausch & Lomb). The following variables were analyzed: anterior radius of curvature (ranterior), posterior radius of curvature (rposterior), radius of keratometry (rsimK), and central pachymetry.ResultsThe ranterior, rposterior, and rsimK were normally distributed, with a mean of 7.87 ± 0.25 mm (95% confidence interval [CI], 7.38-8.36), 6.46 ± 0.26 mm (95% CI, 5.95-6.97), and 7.71 ± 0.27 mm (95% CI, 7.18-8.24), respectively. The mean ratio between the anterior corneal curvature and posterior corneal curvature was 1.22 ± 0.03 (95% CI, 1.16-1.28). Based on the measurements of each eye, the mean calculated keratometric index, Ncalc, was 1.3273 ± 0.0013 (95% CI, 1.3248-1.3298). Using Ncalc, the posterior corneal power was predicted to within ±0.50 D of the actual posterior power in 98.3% of eyes. The mean absolute error between the actual and calculated posterior power was 0.157 ± 0.123 D using Ncalc and 0.326 ± 0.133 D using the Gullstrand model.ConclusionModifying the keratometric index increased the accuracy of predicting the posterior corneal power.