Q-value customized versus wavefront-optimized ablation in femtosecond laser-assisted LASIK for myopia and myopic astigmatism: a prospective contralateral comparative study

This contralateral eye study comprised 80 eyes of 40 patients, 16 males (40%) and 24 females (60%), with a mean age of 28.12 ± 7.55 years (range, 20 to 47 years). All patients completed their 6-month follow-up schedule.

Preoperative data

The preoperative values of the spherical error and MRSE were significantly higher in the custom-Q group than the WFO group, while the UDVA and CDVA were significantly lower. Other parameters showed no significant preoperative differences between the two treatment groups (Table 1). On further analysis of the refractive error into mild, moderate, and high myopia, there were significantly more myopic spherical errors and MRSEs (P = 0.014 and P = 0.019, respectively) in the custom-Q than the WFO group in the high myopia subgroup, but non-significant differences were observed in the mild and moderate myopia subgroups.

Table 1 Preoperative data of both treatment groupsVisual outcome

The UDVA significantly improved, while the CDVA showed non-significant change in each of the treatment groups even through myopia subgrouping. However, both the UDVA and CDVA were significantly better in the WFO than in the custom-Q group in preoperative and postoperative follow-up (Table 2). In each of myopia subgroup, there were non-significant differences between the two treatment groups in UDVA and CDVA regarding the preoperative, postoperative and the amount of change.

Table 2 Visual and refractive outcomes in both treatment groups

Seven eyes in the custom-Q group (17.5%) achieved 20/20 UDVA (Fig. 1a); Four eyes in the mild myopia (66.7%) and three eyes in the moderate myopia subgroup (33.3%). For the WFO group, 11 eyes (27.5%) achieved 20/20 UDVA (Fig. 2a); Seven eyes in the mild myopia (58.3%) and four eyes in the moderate myopia subgroup (26.7%). None of the eyes achieved 20/20 in high myopia in either group. Collectively, for myopia up to − 6.00 D, 46.7% of custom-Q group and 40.7% of the WFO group achieved 20/20.

Fig. 1figure 1

Visual and refractive outcomes in the custom-Q group 6 months postoperatively. a Cumulative postoperative Snellen UDVA as compared to the cumulative preoperative Snellen CDVA. b Difference between preoperative and postoperative CDVA in terms of lost or gained lines. c Proximity of the achieved to the intended spherical equivalent. d Preoperative and postoperative refractive cylinder. CDVA, corrected distance visual acuity; UDVA, uncorrected distance visual acuity

Fig. 2figure 2

Visual and refractive outcomes in the wavefront-optimized group 6 months postoperatively. a Cumulative postoperative Snellen UDVA as compared to the cumulative preoperative Snellen CDVA. b Difference between preoperative and postoperative CDVA in terms of lost or gained lines. c Proximity of the achieved to the intended spherical equivalent. d Preoperative and postoperative refractive cylinder. CDVA, corrected distance visual acuity; UDVA, uncorrected distance visual acuity

In the custom-Q group, one eye (2.5%) gained two lines and two eyes (5%) lost one line in the CDVA logMAR (in the high myopia subgroup), while 37 eyes (92.5%) had their preoperative CDVA preserved (Fig. 1b). The two eyes in the custom-Q group were in the high myopia subgroup and were treated using an optical zone of 6 mm; they had an oblate shift of 1.00 and 1.36, and an increase in the RMSh by 0.71 and 1.44 µm. In the WFO group, three eyes (7.5%) lost one line (two in the high myopia and one in the moderate myopia subgroup) while 37 eyes (92.5%) had their preoperative CDVA preserved (Fig. 2b). Two of the three eyes that lost one line in the WFO group were in the high myopia subgroup and were treated using an optical zone of 6 mm. They had an oblate shift of 1.12 and 0.30, and an increase in the RMSh by 0.93 and 0.42 µm, while the third eye had an oblate shift of 0.98 and an increase in the RMSh by 0.17 µm.

Seventeen eyes in the custom-Q group (42.5%) had anisometropia of ≥ 3.00 D (15 eyes in the high myopia subgroup and two eyes in the moderate myopia subgroup). The magnitude of anisometropia ranged from − 3.00 to − 6.25 D (mean ± SE: − 4.1 ± 0.3 D; 95% CI: − 3.50 to − 4.70 D). Out of these 17 eyes, there were 12 eyes (30% of the custom-Q group, 11 eyes in the high myopia subgroup and one eye in the moderate myopia subgroup) with suggested amblyopia as their preoperative CDVA were ≤ 20/32 (≥ 0.2 logMAR). They were worse than contralateral eyes of the WFO group by at least two lines with anisometropia of ≥ 3.00 D. The depth of amblyopia ranged from 0.2 to 0.5 logMAR lines, and it was two lines in five eyes, three lines in two eyes, four lines in four eyes and five lines in one eye. There was a weak and non-significant correlation between the magnitudes of anisometropia and the depths of amblyopia (Spearman coefficient r = 0.22, P = 0.499).

Spearman’s rank-order correlation was used to determine the relationship between the inter-ocular difference in MRSE (magnitude of anisometropia) and the difference in preoperative CDVA, which showed a strong positive correlation (r = 0.838, P < 0.001).

Refractive outcome and optical zone

Significant improvement of spherical and cylindrical errors and MRSE was noted in both treatment groups, but a significantly smaller optical zone had to be used in the custom-Q group (Table 2). The preoperative and postoperative MRSE were significantly more myopic, and the amount of error corrected was significantly greater in the custom-Q group (Table 2).

In the mild and moderate myopia subgroups, there was no significant difference in preoperative, postoperative or the amount of error corrected, between the custom-Q and the WFO groups. In the high myopia subgroup, the MRSE was significantly more myopic in the custom-Q group, both in the preoperative visit (P = 0.019) and postoperative visit (P = 0.009), while the amount of error corrected was not significantly different between the two treatment groups (P = 0.065).

In all eyes of the mild and moderate myopia subgroups, the optical zone was 6.5 mm. In the high myopia subgroup, the optical zone had to be decreased to 6 mm in 21 out of 25 eyes (84%) in the custom-Q group and in 9 out of 13 eyes (69%) in the WFO group. Even while decreasing the optical zone to 6 mm, six eyes in the high myopia subgroup of the custom-Q treatment group had to be undercorrected with intended postoperative MRSE of − 0.50 to − 0.75 D.

Fifteen eyes (37.5%) in the custom-Q group and 28 eyes (70%) in the WFO group were within 0.25 D of the intended target MRSE, and 21 eyes (52.5%) in the custom-Q group (Fig. 1c) and 35 eyes (87.5%) in the WFO group (Fig. 2) were within 0.50 D of the intended target MRSE. On analyzing the mild and moderate myopia subgroups together, 12 eyes (80%) in the custom-Q and 22 eyes (81.5%) in the WFO group were within 0.25 D of the intended target MRSE. In addition, 14 eyes (93.3%) in the custom-Q and 27 eyes (100%) in the WFO group were within 0.50 D of the intended target MRSE.

The cylindrical error showed statistically significant postoperative improvement in each treatment group, but non-significant difference was noted between the two groups in preoperative, postoperative and the amount of error corrected (Table 2, Figs. 1d and 2d).

Pachymetry change at pupillary center and mid-periphery

No significant difference between the two treatment groups was noted in the mean mid-peripheral pachymetry, while pachymetry at the pupillary center showed significantly thinner postoperative values in the custom-Q than WFO group (Table 3).

Table 3 Pachymetry changes in both treatment groups

The mean mid-peripheral pachymetry showed non-significant differences between custom-Q and WFO groups in all myopia subgroups for the preoperative, postoperative and amount of change. In mild myopia, the P values were 0.454, 0.888 and 0.512, respectively. In moderate myopia, the P values were 0.788, 0.270 and 0.152, respectively. In high myopia, the P values were 0.866, 0.723 and 0.890, respectively.

In the mild myopia subgroup, the mean postoperative mid-peripheral ablation depth was non-significantly greater in the custom-Q (23.8 ± 5.7) than the WFO group (18.5 ± 1.7). In the moderate and high myopia subgroups, the mean postoperative mid-peripheral ablation depth was non-significantly greater in the WFO group, 38.0 ± 5.8 for the custom-Q group and 51.5 ± 6.4 for the WFO group in the moderate myopia subgroup and 65.8 ± 3.1 for the custom-Q group and 67.7 ± 6.0 for the WFO group in the high myopia subgroup.

Keratometry readings

Both the custom-Q and the WFO ablation profiles led to a statistically significant flattening-effect on K1, K2 and Km (P < 0.001 for each). The preoperative K1, K2 and Km readings were non-significantly different in the two treatment groups, but the postoperative readings showed a statistically significantly greater flattening in the custom-Q treatment group (Table 4).

Table 4 Keratometry, Q-value and root mean square changes in both treatment groupsCorneal asphericity

The Q-value showed a statistically significant oblate shift (P < 0.001) in each treatment group with non-significant difference between the mean postoperative Q-values of both groups (P = 0.089; Table 4, Fig. 3).

Fig. 3figure 3

Proximity of the achieved to the preoperative Q-value in both treatment groups (taking the preoperative Q-value as the target asphericity for the custom-Q group). a All manifest refractive spherical equivalent (MRSE) subgroups; b Low myopia subgroup; c Moderate myopia subgroup; d High myopia subgroup

In the mild, moderate, and high myopia subgroups, both treatment profiles led to a statistically significant postoperative oblate shift, a P value of 0.027, 0.018 and less than 0.001, respectively in the custom-Q group, and a P value of 0.002, 0.001 and 0.002, respectively in the WFO group. However, on comparing both treatment profiles in each myopia subgroup, there were non-significant differences in preoperative, postoperative Q-value or the amount of oblate shift in mild myopia (P = 0.963, 0.925 and 0.605, respectively), in moderate myopia (P = 0.928, 0.233 and 0.270, respectively) and in the high myopia subgroup (P = 0.295, 0.415 and 0.242, respectively). The mean change in postoperative corneal asphericity showed statistically non-significant (P = 0.094) greater oblate shift in the custom-Q group (0.8 ± 0.1) than the WFO group (0.60 ± 0.08). On subgroup analysis, both treatment profiles had similar amounts of oblate shift in the mild myopia subgroup (0.20 ± 0.05 for the custom-Q and 0.20 ± 0.04 for the WFO group, P = 0.6). In the moderate myopia subgroup, the mean oblate shift was non-significantly greater in the WFO group than the custom-Q group (0.5 ± 0.1 and 0.3 ± 0.1, respectively, P = 0.27). In the high myopia group, the mean oblate shift was non-significantly greater in the custom-Q group than the WFO group (1.2 ± 0.1 and 1.0 ± 0.2, respectively, P = 0.24).

Root mean square of corneal HOAs (RMSh) at 6 mm zone

There was a statistically significant increase in the postoperative RMSh in both treatment groups (P < 0.001, Table 4). There was no significant difference between the two treatment groups regarding the preoperative (P = 0.658), postoperative (P = 0.133) and the amount of increase in the RMSh (P = 0.06).

The amount of increase in the RMSh, although non-significant, was greater in the custom-Q group than the WFO group (0.50 ± 0.06 and 0.31 ± 0.04 μm, respectively, P = 0.06). In the mild myopia subgroup, we found a non-significant greater increase in RMSh in the WFO than the custom-Q group (0.16 ± 0.02 and 0.10 ± 0.01 μm, respectively, P = 0.08). The RMSh also showed a non-significant greater increase in the WFO group than the custom-Q group (0.29 ± 0.06 and 0.17 ± 0.08 μm, respectively, P = 0.055) in the moderate myopia subgroup. In the high myopia subgroup, the custom-Q group had a non-significantly greater increase than the WFO group (0.73 ± 0.06 and 0.50 ± 0.10 μm, respectively, P = 0.069).

Subgroup analysis excluding eyes with amblyopia

A subgroup analysis was conducted and involved only eyes without amblyopia, where both treatment groups were compared. The results showed no significant difference in preoperative, postoperative and the amount of change in the UDVA, CDVA, optical zone, MRSE, pachymetry at pupillary center, mid-peripheral pachymetry, keratometry readings, corneal asphericity and RMSh. For corneal asphericity, the amount of oblate shift was almost equal in both groups as it was 0.6 ± 0.1 in the custom-Q and 0.60 ± 0.08 in WFO group with P = 0.901.

Correlations

A Spearman’s rank-order correlation was performed to assess the relationship between the preoperative MRSE and the postoperative Q-value in both custom-Q and WFO treatment groups. A strong, negative, statistically significant correlation in both treatment groups was found (r = − 0.74, r = − 0.6, respectively, P < 0.001). The same correlation was also noticed between the preoperative MRSE and the depth of ablation at the pupillary center and the mid-periphery in both treatment groups (r = − 0.8, P < 0.001). The amount of increase of RMSh showed a strong, negative, statistically significant correlation to the preoperative MRSE in the custom-Q group (r = − 0.87, P < 0.001), and in the WFO group (r = − 0.6, P < 0.001). Likewise, the optical zone showed a strong, negative, statistically significant correlation to the amount of increase in RMSh in the custom-Q group (r = − 0.74, P < 0.001), but there was a weak, negative, statistically significant correlation to RMSh change in the WFO group (r = − 0.3, P = 0.036).

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