This study analyzed the postoperative visual quality in patients who underwent consecutive SMILE surgery, comparing the first operated eyes with the second operated ones. While all patients displayed a marked enhancement in postoperative uncorrected visual acuity compared to preoperative metrics, the operation's aftermath was devoid of notable complications. Both eyes presented comparable safety and efficacy outcomes. Intriguingly, the degree of decentration was less pronounced in the first operated eyes than in the second. Moreover, horizontal coma and MTF values at spatial frequencies of 5, 10, 15, and 20 cycles/degree were notably better in the first operated eyes. Linear regression indicates that the differences in horizontal coma and MTF values might be attributed to variations in eccentric displacement.

Consistent with prior research, SMILE surgery has proven effective in enhancing the uncorrected visual acuity in both eyes, consistently demonstrating exceptional safety and efficacy [10, 11]. No discernible differences were observed in visual acuity, safety index, or efficacy index between the two eyes.

Precise centration during laser ablation is pivotal to achieve optimal post-surgical visual outcomes. Decentered ablation might elevate the risk of complications such as postoperative irregular astigmatism, halos, glare, diminished contrast sensitivity, monocular diplopia, and a decline in visual acuity [12]. A multitude of factors could explain why the first operated eyes registered a lower degree of postoperative decentration than the second. The position of the patient's head during surgery may affect the patient's posture on eye movement via Vestibular-ocular reflex (VOR), resulting in the deviation of the patient’s head position during the laser scanning [13, 14]. These movements might vary for each eye operated upon induced by ocular discomfort and stress. Past studies indicate that the first operated eyes experience less ocular discomfort [15]. This could be due to diminished anxiety and heightened awareness of pain during subsequent procedures, making individuals more cognizant of discomfort [16, 17]. However, certain literature about corneal refractive surgery has also documented no discernible disparity in photorefractive keratectomy( PRK) in terms of patients' cooperation and perceived pain during first and second eye [14]. No statistical differences in pain scores were observed for the postoperative period after laser in situ keratomileusis [18]. One possible guess: Once the first eye surgery completed, patients could expect similar series of process during the second eye surgery. This anticipation can potentially lead to increased anxiety and a decreased ability to tolerate pain.

Additional to the physiological factors, the order of surgical operations might influence short-term microdistortions of the Bowman's layer post-SMILE surgery, potentially impacting short-term results [15]. We also speculate the anesthesia factors involved. In order to reduce the impact of repeated eyelid opening on patient's psychological tension, we uniformly administered bilateral proparacaine eye drops for ocular anesthesia. The total duration of bilateral eye surgery is around 10 min, which within the effective time range of proparacaine eye drops [19]. However, the effectiveness may gradually decrease over time. Therefore, patients may experience more pronounced pain during the second eye surgery, leading to an increased decentered displacement. Although significant disparities in horizontal and overall eccentricity were observed between the groups, vertical eccentricity remained consistent. This observation could be linked to the higher prevalence of horizontal microsaccades during eye fixation [20].

The focus in refractive surgery has evolved from primarily prioritizing the procedure's safety and efficacy to enhancing the patients' visual quality. Evaluating advanced corneal visual quality typically employs wavefront aberrations, with particular attention to high-order aberrations (HOAs), while visual outcomes for eyes are generally represented by MTF values. Consistent with prior research, aberrations induced by decentration are primarily HOAs rather than lower-order aberrations such as astigmatism [21]. This study's findings align with this observation, noting minimal variance in lower-order aberrations, with the main difference in HOAs manifesting in horizontal coma.

MTF values denote the finest human eye resolution across varied spatial frequencies. Higher MTF values suggest superior optical quality [22]. In this analysis, the MTF values in the first surgical eyes at spatial frequencies of 5, 10, 15, and 20 cycles/degree surpassed those of the second surgical eyes postoperatively. Though LASIK surgeries can witness postoperative increases in low-order MTF due to factors like halo or interface refraction, the incidence in SMILE surgeries is reportedly minimal [23].

Past studies have highlighted a robust correlation between decentration and the alterations in wavefront aberration and contrast sensitivity post-SMILE surgery [24]. From this research, it can be inferred that HOAs and reduced MTF values may be associated with a significant rise in total eccentric displacement. While minor intraoperative decentered displacements after SMILE might not considerably influence postoperative visual acuity and refractive outcomes, they could diminish the effectiveness of corrected HOAs and even introduce corneal HOAs [25]. Emphasis on the relationship between horizontal decentration and resulting horizontal coma has been accentuated by multivariate correlation analyses [23], and the vertical coma prominence has also been reported [26]. The correlation between increased decentration and decreased vertical coma may be explained by no statistically difference shown in vertical decenter displacement between two eyes. High MTF values, often indicative of enhanced contrast sensitivity, are typically associated with optimal centration. Fluctuations in these values could be attributed to eccentric displacement, which signifies a deviation from the envisioned visual center, with more severe deviations correlating with reduced visual quality [27].

In clinical settings, it may be beneficial to prioritize the eye with a significant influence on overall visual quality as the first surgical target, such as the eye with superior visual acuity or the dominant one. For professionals like surgeons and athletes, where precise visual performance is paramount, determining the ideal eye for initial surgery is imperative.

This study encompasses data from 202 samples, all treated by a singular experienced physician. Multiple assessments of visual parameters, both pre- and post-surgery (2–3 repetitions), were conducted to ensure consistent, reliable results, highlighting the measures' reliability and repeatability.

However, this study is not without limitations. Despite rigorous quality control efforts to negate surgical bias, patient-specific factors like palpebral aperture size, anxiety levels, Bell's reflex strength, and corneal sensitivity might introduce unintended biases. Moreover, incorporating subjective questionnaires to gauge pain perception and other emotions during surgeries could offer deeper insights. Patient feedback post-operation might also furnish supportive information. It's also worth noting that this study's data scope extends only to three months post-operation. Given that MTF values and corneal aberrations might revert to their preoperative state six months post-surgery or later, investigations with extended postoperative timelines are essential.