Relationship Between Patient Age and Refractive Index of the Corneal Stroma During Refractive Surgery Assisted by Femtosecond Laser Flap Creation

PURPOSE: To measure the refractive index (RI) of the human corneal stroma in vivo using an objective Abbé refractometer (VCH-1) and to determine if RI of the stroma is related to age. METHODS: VCH-1 was used to measure RI at the central anterior stroma immediately after lifting the flap in neophyte patients preselected for laser in situ keratomileusis. Surgical procedures continued as preplanned after measuring the RI, and in binocular cases, measurements were taken from the right eye only. Corneal flaps were created using a femtosecond laser system. RESULTS: Mean RI (±SD, range) and age (±SD, range) values were 1.373 (±0.006, 1.358-1.385) and 36.55 (±12.26, 18-74). A significant linear correlation was found between age and RI. Least squares regression lines equating RI with age (x, years) were of the form: RI = 1.36911 + 0.000096x (r = +0.195; n = 115; P = 0.037). Mean RI (± SD) in the 2 age groups separated by the median age (≤34 and ≥35) were 1.371 (±0.007; n = 57) and 1.374 (±0.005; n = 58). The difference was statistically significant (P = 0.018). CONCLUSIONS: RI of the anterior stroma in vivo tends to be increased in older patients when the corneal flap is created using a femtosecond laser device.