### Results

#### Visual Acuity

Visual acuity and refraction conditions postoperatively are noted in Table 2 . The mean safety index after aspheric LASIK and aspheric LASEK were 1.03 ± 0.14 and 1.03 ± 0.11, respectively, with no statistically significant difference between the groups (P = 0.83). The mean efficacy index after aspheric LASIK and aspheric LASEK were 0.98 ± 0.14 and 1.00 ± 0.15 respectively, with no statistically significant changes between two groups (P = 0.59).

#### Refraction Errors (D)

The mean refraction errors after aspheric LASIK and aspheric LASEK were +0.62 ± 0.38 (D) and +0.74 ± 0.36 (D), with no statistically significant difference (P = 0.16) (Figure 1).

Figure 1.

**The mean refraction errors after aspheric LASIK and LASEK.** Attempted spherical equivalent refraction versus achieved manifest refraction spherical equivalent (MRSE) in the Q-value guided LASIK group and LASEK group 3 years after surgery. LASIK: y = 1.07x + 0.15, R^{2} = 0.81; LASEK: y = 1.00x + 0.6, R^{2} = 0.91.

#### Q Value Changes

The mean Q value for the LASIK group and the LASEK group were noted in Figure 2. There were no differences in Q values postoperatively between the two groups (P = 0.838 and P = 0.759, respectively).

Figure 2.

**The mean Q value for the two groups.** The mean Q value increased from -0.35 ± 0.15 to 0.79 ± 0.33 in LASIK and from -0.35 ± 0.15 to 0.79 ± 0.30 in LASEK group.

#### Topographic Maps Analysis

The AST index was calculated as AST = simK_{2}-simK_{1}. It indicates astigmatism from the anterior surface of the cornea. Before surgery, the mean AST was 1.29 ± 0.56 in the LASIK group and 1.15 ± 0.71 in the LASEK group (P = 0.992). At year 3 after surgery, the mean AST was noted in Figure 3.

Figure 3.

**The mean AST at year 3 after surgery.** The postoperative mean AST was 1.041 ± 0.38 in the LASIK group and 0.78 ± 0.45 in the LASEK group, with a statistically significant difference (P < 0.005). But there were no significant differences in changes of AST between the two groups.

Topographic maps showed various indices calculated with topographic map data that could reflect the regularity of the corneal anterior surface, including ISV, IVA and IHA. These indices were achieved from the Allegro topolyzer using the indices mode. In this study, statistically significant (P < 0.001) changes in all indices following surgery (ISV, IVA, IHA) were noted. There were no statistically significant differences (P > 0.05) between the two Q value guided operations ( Table 3 ).

#### Topography-derived Wavefront Errors

Before surgery, the mean spherical aberration (Z_{4} ^{0}) was 0.23 ± 0.08 μm in the LASIK group and 0.22 ± 0.08 μm in the LASEK group (P = 0.129). Up to 3 years after surgery for both procedures, the mean spherical aberration differed significantly between the two groups (P = 0.029). (Figure 4) The mean spherical aberration increased in both groups but was greater in the Q value guided LASEK group compared to the LASIK group. The mean coma increased in both groups and there were no differences between aspheric LASIK and aspheric LASEK procedures (P = 0.315).

Figure 4.

**The mean spherical aberration between the two groups.** Before surgery, the mean spherical aberration (Z_{4} ^{0}) was 0.23 ± 0.08 μm in the LASIK group and 0.22 ± 0.08 μm in the LASEK group (P = 0.129), without difference. Up to 3 years after surgery, the mean spherical aberration was 0.50 ± 0.19 μm in LASIK and 0.61 ± 0.19 μm in LASEK, differed significantly between the two groups (P = 0.029).

#### Relevant Factors Analysis

Pearson correlation coefficients of postoperative spherical aberration(Z40) and coma(Z31) were noted in Table 4 and Table 5 respectively. According to correlation coefficients, we analyzed impact factors of spherical aberration and coma using stepwise multiple regression analysis. The multivariate linear model of spherical aberration (Z_{4} ^{0}) and coma (Z_{3} ^{1}) were noted in Table 6 and Table 7 respectively.

BMC Ophthalmol. 2012;12(15) © 2012 BioMed Central, Ltd.