Association of Treatment Decentration with Early Outcomes Following SMILE Pro: A Secondary Analysis

Abstract

Purpose

To evaluate the association of system-recorded treatment decentration with 3-month visual, refractive, and higher-order aberration (HOA) outcomes after SMILE Pro (VisuMax 800 femtosecond laser).

Patients and Methods

This secondary exploratory analysis of prospectively collected data included 186 eyes of 94 consecutive patients. Treatment decentration—the system-recorded scalar distance between the corneal vertex and the achieved lenticule center—was categorized as <0.10 mm (n = 94), 0.10–0.20 mm (n = 73), or ≥0.20 mm (n = 19). Primary outcomes were 3-month uncorrected distance visual acuity (UDVA), residual spherical equivalent (SE), and anterior corneal wavefront HOA (6-mm pupil) with its change from baseline (ΔHOA). Generalized estimating equations (GEE) clustered by patient constituted the primary analysis; rank-based tests were secondary.

Results

Median decentration was 0.098 mm (IQR 0.067–0.150 mm; maximum 0.345 mm). At 3 months, 98.4% of eyes achieved UDVA ≥20/20, 96.2% were within ±0.50 D of target, and no eye lost corrected distance visual acuity. UDVA, residual SE, and postoperative HOA were not significantly associated with decentration group (all GEE Wald P > 0.26). ΔHOA was greater in the 0.10–0.20 mm group than in the <0.10 mm group (GEE β = +0.048 μm, 95% CI 0.013–0.082, P = 0.007), whereas rank-based tests were marginal (Kruskal–Wallis P = 0.065), the ≥0.20 mm group did not differ, and the continuous Spearman correlation was not significant (ρ = 0.116, P = 0.114).

Conclusion

Within the narrow decentration range achieved with CentraLign-guided SMILE Pro, decentration magnitude was not associated with 3-month visual or refractive outcomes. Smaller higher-order aberration induction was associated with decentration below 0.10 mm than with decentration of 0.10 to <0.20 mm; this association was not observed in the ≥0.20 mm stratum.

Introduction

Small incision lenticule extraction (SMILE) is an established modality of corneal refractive surgery, offering preservation of corneal biomechanical integrity, a lower incidence of dry eye, and elimination of the flap required in laser in situ keratomileusis (LASIK).^1,2

SMILE Pro is performed using the VisuMax 800 femtosecond laser platform (Carl Zeiss Meditec AG, Jena, Germany). This system employs a laser repetition rate of 2 MHz, compared with 500 kHz for the VisuMax 500. The higher repetition rate reduces the time required for lenticule creation (laser scanning) to approximately 10 seconds. Total corneal suction time, which additionally includes docking and centration, is correspondingly shortened. Nevertheless, suction time remains longer than the lenticule-scanning time alone.^3,4 The platform also integrates CentraLign, a centration guidance system that displays the position of the treatment cone relative to the corneal vertex in real time. Comparative studies have confirmed that the SMILE Pro is associated with statistically significantly less treatment-zone decentration and induced vertical coma than conventional VisuMax 500 SMILE.^5–7

Treatment decentration—the displacement between the intended and the system-recorded achieved lenticule center—is a recognized correlate of refractive and aberrometric outcomes in corneal refractive surgery. Decentrations exceeding 0.3–0.5 mm have been associated with increased coma, reduced contrast sensitivity, and symptoms such as glare and halos.^8,9 For SMILE performed with the VisuMax 500, published mean decentration values range from 0.17 mm to 0.36 mm, and decentration has been consistently associated with induced higher-order aberrations (HOAs), particularly coma.^10–13 Notably, most previous studies measured postoperative decentration using corneal topography or tomography, while this study adopted intraoperative system-recorded decentration based on corneal topography. Measurements from these two methods may not be directly comparable.^14,15

Despite growing adoption of the VisuMax 800, it remains unclear whether decentration below 0.20 mm—a commonly cited upper limit of acceptable decentration in the SMILE literature^13,16 and the range encompassing the vast majority of SMILE Pro cases in experienced centers—is associated with measurable differences in visual, refractive, or aberrometric outcomes. Such evidence would inform intraoperative quality assessment, patient counselling, and further refinement of centration technology. The purpose of this study was therefore to evaluate the association of the prespecified study variable of system-recorded treatment decentration, anal, sed both continuously and using exploratory post hoc decentration strata, with 3-month visual, refractive, and aberrometric outcomes in a consecutive SMILE Pro cohort.

Patients and Methods

Study Design and Participants

The underlying investigation was a prospectively designed observational study conducted at the Eye and ENT Hospital of Fudan University, with prospective enrollment, treatment, follow-up, and clinical data collection between November 2024 and January 2025. The present manuscript is a secondary analysis of these prospectively collected data. It was conducted in compliance with the Declaration of Helsinki and received approval from the Ethics Committee of the Eye and ENT Hospital of Fudan University (approval No. 2024050-2). All patients gave their written informed consent prior to surgery.

Consecutive patients undergoing bilateral or unilateral SMILE Pro surgery who completed the 3-month postoperative visit were eligible. Inclusion criteria were: age ≥18 years; myopia with or without astigmatism (spherical equivalent [SE] up to −10.00 D; cylinder up to −4.00 D); best-corrected distance visual acuity (CDVA) ≥0.8 decimal; and stable refraction (change ≤0.50 D per year for ≥12 months). Exclusion criteria included irregular corneal topography, keratoconus or keratoconus suspect, concurrent systemic or ocular disease, and prior ocular surgery.

Among 193 eyes from 97 consecutive screened patients, 7 eyes were excluded: one eye (right eye of one patient) for an extreme preoperative HOA value of 8.036 μm (next-highest value in the cohort: 0.742 μm), inconsistent with the clinical presentation and most likely a measurement artifact, per a pre-specified artifact criterion (any preoperative HOA measurement >5× the cohort interquartile range above the 75^th percentile); two eyes of one patient lost to the 3-month visit; and four eyes of two patients for whom the system decentration record could not be retrieved from the laser platform (one of these patients additionally lacked 3-month aberrometry). Missing data were handled by complete-case analysis; baseline characteristics of excluded patients were unremarkable. The final analytic sample comprised 186 eyes of 94 patients (92 bilateral, 2 unilateral). The participant flow is shown in Figure 1.

Figure 1.

Study flowchart. Of 193 eyes of 97 consecutive patients screened, 7 eyes were excluded (1 eye with an artifactual preoperative higher-order aberration measurement; 2 eyes of 1 patient lost to 3-month follow-up; 4 eyes of 2 patients without a retrievable system decentration record, of which 2 eyes additionally lacked 3-month aberrometry), yielding 186 eyes of 94 patients stratified by treatment decentration into three groups.

SMILE Pro Surgical Procedure

All procedures were performed by a single experienced surgeon using the VisuMax 800 femtosecond laser (software version 1.2.4.1008). Fixed surgical parameters included a cap thickness of 120 μm, a cap diameter of 7.6 mm, 90° side-cut angles for both the cap and lenticule, and a laser repetition rate of 2 MHz with the use of CentraLign. Standard postoperative drugs included topical fluoroquinolone and corticosteroid drops tapered over 2 weeks.

Outcome Measures

Primary outcomes at 3 months were: monocular uncorrected distance visual acuity (UDVA; decimal notation, converted to logMAR for analysis); residual manifest SE (D); and anterior corneal wavefront HOA root mean square (RMS) over a 6-mm pupil diameter, together with its change from baseline (ΔHOA = 3-month value minus preoperative value). Secondary outcomes were coma and trefoil RMS and their changes from baseline; the proportions of eyes achieving UDVA ≥1.0 decimal (20/20) and ≥1.2 decimal (20/16); the proportions within ±0.50 D and ±0.25 D of target SE; the change in lines of CDVA; and the efficacy index (mean of eye-level ratios of postoperative UDVA to preoperative CDVA) and safety index (mean of eye-level ratios of postoperative CDVA to preoperative CDVA). Visual acuity line changes were calculated on the logMAR scale (one line = 0.1 logMAR) with full manifest correction at both visits.

Corneal wavefront aberrations (anterior corneal surface, derived from Placido-disc topography) were measured with the OPD-Scan III (NIDEK Co., Ltd, Gamagori, Japan) and expanded into Zernike polynomials over a 6-mm analysis diameter. Measurements were obtained under dim room lighting without pharmacologic dilation; patients were instructed to blink immediately before acquisition to stabilize the tear film, and only acquisitions meeting the instrument’s quality criteria (adequate Placido mire coverage without lid or tear-film artifacts) were retained, with repeat captures obtained as needed.

Treatment decentration was defined as the scalar (Euclidean) distance, in millimeters, between the corneal vertex—the reference center of the VisuMax 800 coordinate system, established at docking under CentraLign guidance—and the achieved lenticule center recorded by the laser platform in the surgical report at completion of lenticule creation. The horizontal (x) and vertical (y) offset components are recorded by the system; the scalar distance analyzed here is their Euclidean norm and does not retain directional information. This is a device-derived value and differs from postoperative topography-based estimates of achieved optical-zone decentration.

Decentration Grouping

For exploratory stratification, eyes were categorized by treatment decentration into Group 1 (<0.10 mm), Group 2 (0.10–0.20 mm), and Group 3 (≥0.20 mm). These cutoffs were defined post hoc for the present secondary analysis rather than prespecified as clinical thresholds. The 0.20-mm cutoff was selected because it represents a commonly cited upper limit in the SMILE literature.^13,16 Given the small size of Group 3 (n = 19, 10.2% of the cohort), all findings for this stratum should be regarded as exploratory and hypothesis-generating.

Statistical Analysis

Normality was assessed for each variable individually with the Shapiro–Wilk test. Normally distributed variables (age, preoperative SE) are summarized as mean ± standard deviation (SD) and non-normal variables as median (interquartile range, IQR); the summary statistic used is indicated in each table.

Because 92 of 94 patients contributed bilateral data, generalized estimating equations (GEE; Gaussian family, exchangeable correlation structure, clustered by patient) constituted the primary inferential analysis for all between-group comparisons of continuous outcomes; effect estimates (β) with 95% confidence intervals (CI) and Wald tests are reported. Rank-based analyses (Kruskal–Wallis H-test; when significant, pairwise Mann–Whitney U-tests with Bonferroni correction, k = 3) were performed as distribution-free secondary analyses, and one-way ANOVA was used as a sensitivity check for normally distributed baseline variables. Categorical variables were compared with the chi-squared test or Fisher exact test when any expected cell count was <5. Spearman rank correlation evaluated associations between continuous decentration and each outcome. All tests were two-tailed with α = 0.05. Analyses were performed using Python 3.12 (pandas, scipy, statsmodels).

Results

Patient Demographics and Decentration Distribution

The final sample comprised 186 eyes of 94 patients (92 bilateral, 2 unilateral). Mean patient age was 32.2 ± 5.6 years (range 19–46), and 52 of 94 patients (55.3%) were female. Mean preoperative SE was −4.51 ± 1.66 D (range −8.62 to −0.88 D), and median preoperative cylinder was −0.75 D (IQR −1.19 to −0.25 D). The achieved lenticule thickness was 103.3 ± 25.0 μm and the programmed optical zone was 6.73 ± 0.15 mm. Median treatment decentration was 0.098 mm (IQR 0.067–0.150 mm; range 0.000–0.345 mm), and 89.8% of eyes were below 0.20 mm. Group 1 comprised 94 eyes (50.5%), Group 2 comprised 73 eyes (39.2%), and Group 3 comprised 19 eyes (10.2%). Baseline characteristics did not differ significantly among groups (Table 1).

Table 1.

Preoperative Baseline Characteristics by Decentration Group

Characteristic	Group 1 (<0.10 mm) (n = 94)	Group 2 (0.10–0.20 mm) (n = 73)	Group 3 (≥0.20 mm) (n = 19)	P
Age, years, mean ± SD	32.4 ± 5.5	32.0 ± 5.8	32.5 ± 5.6	0.905a
Female, n (%)	58 (61.7)	35 (47.9)	11 (57.9)	0.203b
Preoperative SE, D, mean ± SD	−4.59 ± 1.57	−4.45 ± 1.80	−4.38 ± 1.59	0.807a
Preoperative cylinder, D	−0.50 (−1.00 to −0.25)	−0.75 (−1.00 to −0.50)	−1.00 (−1.25 to −0.50)	0.485c
Preoperative CDVA, decimal	1.00 (1.00–1.00)	1.00 (1.00–1.00)	1.00 (1.00–1.00)	1.000c
Preoperative HOA, μm	0.235 (0.188–0.309)	0.231 (0.165–0.281)	0.251 (0.211–0.281)	0.291c
Preoperative coma, μm	0.098 (0.068–0.141)	0.088 (0.052–0.131)	0.091 (0.061–0.130)	0.237c
Preoperative trefoil, μm	0.160 (0.102–0.222)	0.150 (0.090–0.211)	0.192 (0.104–0.232)	0.633c
Lenticule thickness, μm	106.5 (89.0–124.5)	96.0 (79.0–127.0)	102.0 (83.0–111.5)	0.581c
Optical zone, mm	6.80 (6.80–6.80)	6.80 (6.80–6.80)	6.80 (6.75–6.80)	0.772c

Notes: Values are median (interquartile range) unless otherwise stated; normally distributed variables (age, preoperative SE; Shapiro–Wilk P > 0.05) are shown as mean ± SD. ^aOne-way ANOVA. ^bChi-squared test. ^cKruskal–Wallis test.

Abbreviations: SE, spherical equivalent; CDVA, corrected distance visual acuity; HOA, higher-order aberrations; SD, standard deviation.

Three-Month Visual and Refractive Outcomes

At 3 months, the overall UDVA was 1.2 decimal (IQR 1.0–1.2; median logMAR −0.079). Overall, 183 of 186 eyes (98.4%) achieved UDVA ≥1.0 (20/20) and 119 of 186 (64.0%) achieved UDVA ≥1.2 (20/16). Mean residual SE was +0.11 ± 0.28 D; 179 of 186 eyes (96.2%) were within ±0.50 D and 133 (71.5%) within ±0.25 D of target. None of these outcomes was significantly associated with decentration group (GEE Wald P = 0.269 for UDVA and P = 0.535 for residual SE; all rank-based and categorical P > 0.29; Table 2). Even in Group 3 (≥0.20 mm), 18 of 19 eyes (94.7%) achieved UDVA ≥1.0 and 18 of 19 (94.7%) were within ±0.50 D. The overall refractive outcomes are summarized in Figure 2, and outcomes stratified by decentration group in Figure 3.

Table 2.

Three-Month Postoperative Outcomes by Decentration Group

Outcome	Group 1 (<0.10 mm) (n = 94)	Group 2 (0.10–0.20 mm) (n = 73)	Group 3 (≥0.20 mm) (n = 19)	P value (rank/categorical)	GEE Wald P
UDVA, decimal	1.20 (1.00–1.20)	1.20 (1.00–1.20)	1.00 (1.00–1.20)	0.368a	0.269
UDVA ≥1.0 (20/20), n (%)	93 (98.9)	72 (98.6)	18 (94.7)	0.336b	—
UDVA ≥1.2 (20/16), n (%)	62 (66.0)	48 (65.8)	9 (47.4)	0.291b	—
Residual SE, D	0.125 (0.000–0.250)	0.125 (0.000–0.250)	0.000 (−0.250 to 0.250)	0.327a	0.535
Absolute residual SE, D	0.250 (0.125–0.375)	0.250 (0.125–0.375)	0.250 (0.188–0.375)	0.404a	—
Within ±0.50 D, n (%)	92 (97.9)	69 (94.5)	18 (94.7)	0.420b	—
Within ±0.25 D, n (%)	69 (73.4)	52 (71.2)	12 (63.2)	0.648b	—
HOA, μm	0.279 (0.216–0.344)	0.279 (0.236–0.381)	0.272 (0.203–0.328)	0.466a	0.430
Coma, μm	0.143 (0.101–0.199)	0.157 (0.109–0.204)	0.144 (0.096–0.189)	0.518a	—
Trefoil, μm	0.183 (0.108–0.240)	0.191 (0.132–0.267)	0.174 (0.130–0.241)	0.628a	—

Notes: Values are median (interquartile range) for continuous variables. ^aKruskal–Wallis test. ^bFisher exact test (applied when any expected cell count was <5).

Abbreviations: UDVA, uncorrected distance visual acuity; SE, spherical equivalent; HOA, higher-order aberrations; GEE, generalized estimating equations.

Figure 2.

Standard six-panel refractive outcomes report for 186 SMILE Pro eyes at 3 months. (A) Cumulative uncorrected distance visual acuity (UDVA) with preoperative corrected distance visual acuity (CDVA) overlay. (B) Change in logMAR lines of CDVA. (C) Attempted vs achieved spherical equivalent refraction; the solid line is the identity line and the dashed line the least-squares fit. (D) Distribution of residual spherical equivalent refraction. (E) Refractive astigmatism before and after surgery. (F) Change in mean spherical equivalent refraction from baseline to 3 months; error bars indicate one standard deviation, and the dotted line indicates emmetropia.

Figure 3.

Three-month outcomes stratified by treatment decentration group (<0.10 mm; 0.10–0.20 mm; ≥0.20 mm). (A) UDVA (logMAR). (B) Absolute residual spherical equivalent. (C) Higher-order aberration (HOA) root mean square. (D) Coma. (E) Trefoil. (F) Proportions of eyes achieving key efficacy and accuracy thresholds. Box plots show the median and interquartile range with individual data points overlaid. Continuous outcomes were compared with generalized estimating equations (primary) and Kruskal–Wallis tests (secondary); no comparison was statistically significant.

No eye lost one or more lines of CDVA at 3 months; 164 of 186 eyes (88.2%) gained one or more lines and 8 (4.3%) gained two or more lines. The high proportion of line gains is expected in young myopes after keratorefractive surgery, because correction at the corneal plane eliminates spectacle minification, so postoperative CDVA frequently exceeds preoperative spectacle-corrected CDVA. The safety index was 1.19 and the efficacy index 1.13. No intraoperative complications (suction loss, black spots, incomplete lenticule cut, cap tear, or conversion to another procedure) and no postoperative complications (diffuse lamellar keratitis, epithelial ingrowth, infection, or retreatment) occurred during follow-up.

Three-Month Higher-Order Aberration Outcomes

Mean HOA RMS increased from 0.25 ± 0.10 μm preoperatively to 0.30 ± 0.12 μm at 3 months. Absolute postoperative HOA, coma, and trefoil did not differ significantly across decentration groups (GEE Wald P = 0.430 for HOA; Kruskal–Wallis P = 0.466, 0.518, and 0.628, respectively; Table 2).

Aberration Change from Baseline

ΔHOA differed across decentration groups in the primary GEE analysis (Wald P = 0.017): Group 2 showed a greater HOA increment than Group 1 (β = +0.048 μm, 95% CI +0.013 to +0.082, P = 0.007; median 0.079 μm [IQR 0.008–0.141] vs 0.041 μm [IQR −0.036 to 0.106]), whereas Group 3 did not differ from Group 1 (β = −0.000 μm, 95% CI −0.063 to +0.062, P = 0.994). The secondary rank-based analysis was marginal (Kruskal–Wallis H = 5.456, P = 0.065; Bonferroni-corrected Group 1 vs Group 2 P = 0.066), and ΔHOA in Group 3 (median 0.047 μm) was intermediate between Groups 1 and 2—that is, the pattern across groups was non-monotonic. ΔComa and Δtrefoil showed no significant group differences (GEE Wald P = 0.258 and P = 0.535; Table 3). The distribution of aberration changes by group and the continuous association between decentration and ΔHOA are shown in Figure 4.

Table 3.

Aberration Change from Baseline (Δ = 3-Month Minus Preoperative) by Decentration Group

Outcome	Group 1 (<0.10 mm) (n = 94)	Group 2 (0.10–0.20 mm) (n = 73)	Group 3 (≥0.20 mm) (n = 19)	Kruskal–Wallis P	GEE Wald P
ΔHOA, μm	0.041 (−0.036 to 0.106)	0.079 (0.008–0.141)	0.047 (−0.051 to 0.094)	0.065	0.017
ΔComa, μm	0.054 (−0.025 to 0.100)	0.065 (−0.004 to 0.118)	0.066 (0.003–0.094)	0.401	0.258
ΔTrefoil, μm	0.020 (−0.048 to 0.093)	0.055 (−0.051 to 0.119)	0.028 (−0.048 to 0.101)	0.514	0.535

Notes: Values are median (interquartile range). Primary GEE analysis of ΔHOA: Group 2 vs Group 1, β = +0.048 μm (95% CI +0.013 to +0.082), P = 0.007; Group 3 vs Group 1, β = −0.000 μm (95% CI −0.063 to +0.062), P = 0.994. Bonferroni-corrected pairwise Mann–Whitney comparisons for ΔHOA: Group 1 vs Group 2, P = 0.066; Group 1 vs Group 3, P = 1.000; Group 2 vs Group 3, P = 0.678. The ≥0.20 mm group is exploratory; non-significant results for this stratum should not be interpreted as evidence of equivalence given limited statistical power.

Abbreviations: HOA, higher-order aberrations; GEE, generalized estimating equations.

Figure 4.

Aberration change from baseline (Δ= 3-month minus preoperative value) by treatment decentration. (A) ΔHOA. (B) ΔComa. (C) ΔTrefoil. (D) Scatter plot of ΔHOA against continuous treatment decentration (Spearman ρ = 0.116, P = 0.114); no fit line is shown because the reported analysis is a rank correlation. In panel A, the bracket indicates the primary generalized estimating equation comparison between the <0.10 mm and 0.10–0.20 mm groups (β = +0.048 μm, P = 0.007; Bonferroni-corrected Mann–Whitney P = 0.066). The asterisk indicates P < 0.05 in the primary GEE analysis.

Continuous Correlation and Sensitivity Analysis

Treating decentration as a continuous variable, the Spearman correlation with ΔHOA was positive but not statistically significant (ρ = 0.116, P = 0.114), and the continuous GEE estimate was likewise non-significant (β = +0.14 μm per mm, 95% CI −0.17 to +0.46, P = 0.365). No significant correlations were detected between decentration and UDVA (ρ = 0.046, P = 0.534), absolute residual SE (ρ = 0.021, P = 0.781), or postoperative HOA (ρ = 0.016, P = 0.824).

In the pre-specified sensitivity analysis re-including the eye excluded for the artifactual preoperative HOA measurement, the rank-based result was essentially unchanged (Kruskal–Wallis P = 0.053), while the GEE point estimates retained their direction but lost precision, reflecting the −7.67 μm artifactual ΔHOA contributed by that eye; this supports the pre-specified exclusion. One-way ANOVA for the normally distributed baseline variables confirmed the absence of group differences (age, P = 0.905; preoperative SE, P = 0.807).

Discussion

This exploratory analysis of 186 eyes used prospectively collected data to evaluate the association between system-recorded treatment decentration and 3-month visual, refractive, and aberrometric outcomes. The principal findings are: (i) no significant association of decentration—up to the observed maximum of 0.345 mm—with UDVA, residual SE, or absolute postoperative HOA, and no loss of corrected visual acuity in any eye; and (ii) a small difference in HOA induction in the 0.10–0.20 mm stratum relative to the <0.10 mm stratum in the cluster-adjusted primary analysis, which was not mirrored by the ≥0.20 mm stratum and was not supported by a significant continuous correlation. Because the strata were defined post hoc, these findings are exploratory and should not be interpreted as evidence for validated decentration thresholds.

The mechanism by which decentration can induce HOA is well established: displacement of the lenticule removal zone from the visual axis breaks the rotational symmetry of the postoperative corneal surface and generates coma-like aberrations whose magnitude scales with the displacement.^8,9 For VisuMax 500 SMILE, Li et al¹⁰ found decentration (mean 0.17 ± 0.09 mm) significantly associated with wavefront aberrations in 100 eyes; Liu et al¹¹ reported that greater distance from the corneal vertex correlated with induced HOA in 101 eyes; Chan et al¹² found subclinical decentration correlated with induced coma (r = 0.384) and total HOA (r = 0.498) in 78 eyes; and Lee et al¹³ in the largest series (360 eyes; mean decentration 0.36 ± 0.22 mm), identified decentration >0.335 mm as the level associated with significantly greater induction of total HOA, coma, and spherical aberration—while UDVA and manifest refraction remained satisfactory even above that level.

The markedly lower decentration in the present cohort (median 0.098 mm; 89.8% below 0.20 mm; maximum 0.345 mm) relative to the VisuMax 500 literature is consistent with the effect of CentraLign guidance combined with the shorter suction time of the VisuMax 800, as demonstrated in matched comparisons by Kim and Chung⁵ and by Yang et al.⁶ This has two consequences. First, the whole cohort operates on the lower, shallower portion of the decentration–HOA dose-response function described for the VisuMax 500, which reduces the aberrometric contrast between adjacent strata. Second, comparisons with earlier topography-based decentration studies must be made cautiously, because the system-recorded metric used here reflects the intraoperative offset of the treatment center from the corneal vertex, not the postoperatively measured optical-zone displacement.^14,15

The visual and refractive results align with the growing VisuMax 800 literature.^3–7,17 Reinstein et al³ reported that, of 128 eyes treated, 86% of the 118 eyes with 3-month data were within ± 0.50 D of target. Saad et al⁴ reported 99% within ± 1.00 D in the first European series, and the prospective multicenter post-market study by Sekundo et al¹⁷ confirmed excellent predictability and safety in 473 eyes of 237 patients at 6 months, with no case of ≥2 lines of CDVA loss. The present data (98.4% UDVA ≥20/20; 96.2% within ±0.50 D; safety index 1.19; no CDVA loss) are consistent with these benchmarks. Even the exploratory ≥0.20 mm group achieved 94.7% UDVA ≥20/20 and 94.7% within ±0.50 D. This agrees with Huang et al¹⁶ who found that decentration >0.20 mm did not significantly impair visual acuity outcomes after VisuMax 500 SMILE, with aberrometric penalties concentrated in their high-astigmatism (cylinder ≥2.50 D) group rather than the low-astigmatism comparison group.

The aberrometric signal identified here—greater ΔHOA in the 0.10–0.20 mm stratum than in the <0.10 mm stratum in the cluster-adjusted analysis—should be interpreted with restraint, for three reasons. First, the effect size is small: the between-group difference in medians was 0.038 μm, well below the 0.1–0.2 μm HOA RMS change that Applegate et al¹⁸ suggested is generally required before measurable effects on high-contrast visual acuity are expected; consistent with this, no visual penalty accompanied the signal. Second, the pattern across strata was non-monotonic—the ≥0.20 mm group showed an intermediate median change—and the continuous correlation was not significant, so these data do not demonstrate a dose-response relationship. Third, the ≥0.20 mm stratum comprised only 19 eyes with an estimated power below 25% for the observed effect sizes, and its median decentration (0.220 mm) lies far below the 0.335 mm level identified by Lee et al¹³—so the absence of a significant difference in this stratum must not be interpreted as evidence of equivalence, nor does this study establish any acceptable upper limit for decentration. The low median preoperative cylinder of the cohort (−0.75 D) may further attenuate aberrometric sensitivity to moderate decentration, given that decentration-related aberration induction is amplified in high-astigmatism eyes.¹⁶

A companion prospective study from our center (Wei et al)¹⁹ evaluated early objective and subjective visual quality after keratorefractive lenticule extraction with the VisuMax 800 in an overlapping patient population and enrollment period. Both reports therefore derive from prospectively collected data, but they address different research questions and use different analytical frameworks. The companion study characterized longitudinal within-cohort trajectories of visual quality, analyzed aberrations at a 5-mm pupil diameter, and did not stratify by decentration. The present study is a secondary exploratory analysis that examines between-stratum associations of system-recorded decentration with 3-month outcomes at a 6-mm pupil, using decentration as a prespecified study variable but defining the specific strata post hoc. Several limitations warrant consideration. First, the ≥0.20 mm group was small (n = 19), conferring very limited statistical power; all conclusions for this stratum are exploratory. Second, follow-up was limited to 3 months; whether the observed aberrometric differences persist, widen, or attenuate at later time points remains to be determined. Third, subjective quality-of-vision measures, contrast sensitivity, and mesopic function—outcomes more sensitive to decentration-related aberrations than high-contrast acuity—were not assessed, so a small ΔHOA difference could carry perceptual consequences that UDVA cannot detect. Fourth, the scalar decentration metric does not preserve directional information, precluding axis-specific analysis of horizontal and vertical decentration components in relation to directional coma. Fifth, the system-derived decentration metric differs from the postoperative topographic difference-map estimates used in much of the prior literature, limiting direct numerical comparability. Sixth, the single-center, single-surgeon secondary exploratory design limits generalizability, although procedural standardization enhances internal consistency. Seventh, although the primary analysis accounted for bilateral correlation by GEE, the rank-based secondary analyses treated eyes as independent. Finally, four eyes of two patients were excluded because the system decentration record could not be retrieved, and missing data were handled by complete-case analysis.

Future studies should address these limitations directly: larger prospective multicenter cohorts enriched for decentration ≥0.20 mm; vector analysis of decentration direction relative to the coma axis; follow-up of 12 months or longer; and—most importantly—visual-quality endpoints beyond high-contrast acuity, including contrast sensitivity, mesopic visual acuity, glare and halo assessment, and validated patient-reported outcome measures.

Conclusion

In this secondary exploratory analysis of prospectively collected data, system-recorded treatment decentration was not associated with 3-month visual acuity, refractive accuracy, or safety outcomes within the observed range. A small, non-monotonic between-stratum difference in HOA induction was observed in the 0.10–0.20 mm stratum, without a consistent dose-response or accompanying visual penalty. These exploratory associations require confirmation in larger prospective studies incorporating visual-quality measures, and the post hoc strata should not be interpreted as validated centration benchmarks.

Funding Statement

National Key Research and Development Program of China (2024YFC2510805).

Ethical Approval

The study was approved by the Ethics Committee of the Eye & ENT Hospital of Fudan University (approval No. 2024050-2) and adhered to the contents of the Declaration of Helsinki. All patients provided written informed consent.

Disclosure

The authors report no conflicts of interest in this work.