Abstract:  Objective To predict the residual corneal depth more accurately by comparing the difference between practical and predicted ablated depth in different mypoic diopter eyes corrected by VisxStarS4 excimer laser. Design Retrospective case series. Participants 119 cases (234 eyes) with myopia were corrected by VisxStarS4 excimer laser in Tongren Hospital. The patients were divided into group I (low myopia, >-3 D 63 eyes),group II (medium myopia, -3~-6 D 93 eyes) and group III(high myopia, <-6 D 78 eyes).Methods Corneal flap was made with Zimmer femtosecond laser. VisxStarS4 excimer laser ablated the corneal stroma. The central corneal thickness (CCT) was measured by optical coherence tomography (OCT) before operation and 1 week after operation, to obtain the difference values between practical ablation depth and the predicted ablation depth. Main Outcome Measures CCT, difference value between practical and predicted ablation depth. Results The average CCT before operation of the group I, II, and III groups was 521.95±37.97 μm, 523.16±30.38 μm, and 524.77 ± 33.57 μm respectively (F=0.125, P=0.882). The predicted ablation depth (41.83±8.53 μm, 69.15±12.72 μm, 96.23±11.46 μm) was significantly different from practical ablation depth (46.13±9.78 μm, 77.05±16.72 μm, 107.78±14.27 μm) in each group (t=-2.63, -3.63, -5.57; P=0.001, ＜0.001, ＜0.001, respectively. The average difference between practical and predicted ablated depth in three groups was 6.40±4.44 μm, 8.89±6.84 μm, 11.81 ± 8.72 μm, respectively (F=10.497, P<0.001). The average difference values was significant different from each other when compared between any two of the three groups (P=0.030, 0.007, 0.000, respectively). Conclusion VisxStarS4 excimer laser can accurately predict the corneal ablation depth particularly in low and moderate myopia. The practical ablated value is more than the predicted ablated value.

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