Comparison of the effective optical zone after small incision lenticule extraction measured by corneal topography and modulation transfer function in myopia

Objective

To measure the effective optical zone (EOZ) after small incision lenticule extraction (SMILE) by using corneal topographic map and modulation transfer function (MTF), and to analyze the related factors affecting the EOZ.

Methods

A retrospective observational case series study was performed.Sixty-two myopic patients (62 eyes) who underwent SMILE between December 2015 and July 2017 in Jinan Mingshui Eye Hospital were enrolled.The EOZ was measured by using tangential corneal curvature topographic map and MTF pre- and 6-month post-operatively.The repeatability of data was determined by intraclass correlation coefficient (ICC) and Cronbach Alpha coefficients; the agreement of data was identified by using Bland-Altman plot; the correlations between EOZ and surgical parameters were analyzed by utilizing Pearson correlation coefficient.This study protocol was approved by the Ethics Committee of Jinan Mingshui Eye Hospital(No.2015[013]). Written informed consent was obtained from each patient before surgery.

Results

The ICC and Cronbach Alpha coefficients of EOZ measured by corneal topographic map were greater than 0.9, which showed high intraobserver repeatability.The Bland-Altman plots displayed relatively good agreement between the two methods.The 95% limits of agreement was -0.49 to 0.89 μm.Six months after SMILE, the EOZ measured by tangential corneal curvature gradient topographic map was (5.32±0.25)mm, which was (1.18±0.25)mm smaller than predicted optical zone, and the EOZ measured by MTF method was (5.07±0.32)mm, which was (0.20±0.35)mm smaller than corneal tomographic EOZ, and the difference was significant (t=-4.487, P<0.01). A negative correlation was found between the EOZ and attempted refractive correction (r=-0.364, P=0.004). The positive correlations were found between the EOZ and ΔKm or ΔQ 6 months after SMILE (r=0.367, 0.514; both at P<0.01).

Conclusions

The EOZ measured by tangential corneal curvature topographic map after SMILE is of high repeatability and is consistent with the result calculated by MTF method.The EOZ is significantly reduced after SMILE.

Myopia; Optical zone; Femtosecond laser; Small incision lenticule extraction; Corneal topography; Modulation transfer function