The influence of age on the refractive index of the human corneal stroma resected using a mechanical microkeratome.

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

The VCH-1 was calibrated against a standard subjective Abbé refractometer using 7 randomly selected turbid semiliquid media. VCH-1 was used to measure RI at the central midstroma immediately after lifting the flap in neophyte patients preselected for laser-assisted in situ keratomileusis. Surgical procedures continued as preplanned after measuring the RI; in binocular cases, measurements were taken from the right eye only. Flaps were created using a mechanical microkeratome.

RESULTS

The VCH-1 was capable of measuring the RIs of all 7 turbid semiliquid media. The average RMS difference between RI estimates according to the VCH-1 and standard subjective Abbé refractometer was 0.003. The mean RI (± SD) of the stroma was 1.369 (± 0.008; range, 1.356-1.390), and the mean age (± SD) of the subjects was 33.7 years (± 8.86; range, 18-56 years). A significant linear correlation was found between age and RI. Least squares regression lines equating RI with age (x, years) was of the form: RI = 1.35711 + 0.00034x (r = +0.383, n = 36, P = 0.011).

CONCLUSIONS

RI of the stroma in vivo has a tendency to be increased in older patients when the stroma is resected using a mechanical microkeratome.