We conducted a literature search for SMILE research from 2011 to 2022 using the Science Citation Index-Expanded (SCIE) of the Web of Science Core Collection (WoSCC) to identify SMILE-related publications, limited to “article” and “review”, over the past 12 years (from 2011 to 2022) with no language restriction. Our search strategy was as follows: Topic = (“small incision lenticule extraction” OR “small incision lenticule extractions” OR SMILE*). All retrieved records were downloaded on May 14, 2022.

The annual number of publications, type of documents, and languages on SMILE studies were analyzed using CiteSpace 6.2.1 (Drexel University, Philadelphia, PA, United States). The impact factors of the journals were provided by the 2021 Journal Citation Reports (Clarivate Analytics, Philadelphia, PA, United States). Elements of SMILE research, including countries/regions, keywords, journals, and main co-cited journals, were identified via VOS viewer 1.6.15 (Leiden University, Leiden, Netherlands). A publication was assigned equally to all participating countries/regions or institutions when it was completed by collaborations between more than one country/region or institution. Network maps for countries/regions, institutions, journals, and the main co-cited journals were generated by the VOS viewer in addition to cluster analysis and density maps for high-frequency keywords. On the bibliometric maps generated by the VOS viewer, different nodes represent elements, and the larger the size of the node, the higher the number or frequency of elements is. A line, which connects two nodes, reflects the relationship between different elements, and its thickness indicates the strength of the relationship. Nodes of different colors represent different clusters. Parameters of the VOS viewer were set as follows: fractional counting at the counting method, ignoring documents with too many authors (maximum number of authors per document: 25). Microsoft Office Excel 2019 (Redmond, Washington, United States) was used to manage data. The correlation between the year and the number of articles was expressed by the linear correlation coefficient (R²).

Outputs of the annual publication with an upward trend are shown in Figure 2. There were two and four papers published in 2011 and 2012, respectively. The number of publications was 61 in 2015, which increased to more than 80 in 2017, and reached the highest in 2020 (n = 120). A significant correlation between the number of studies and the year of publication was found with a high coefficient of determination (R² = 0.62).

The top 10 countries or regions, and institutions among the 604 institutions in 45 countries are shown in Table 1, according to the number of publications.

A total of 45 countries/regions contributed to SMILE research. China published the highest number of papers (n = 326), followed by the United States (n = 140), Germany (n = 81), India (n = 68), and France (n = 58). As shown in Figure 3A, the annual output of most countries showed an upward trend. All countries/regions were used to construct a country/region network map (Figure 3B).

The top 10 institutions were distributed in five countries/regions, four of which were in China (Table 1). As Figure 3C shows, institutions (70/604, 11.59%) with six or more (T = 6) publications were used to construct the co-authorship network. The institutions were then divided into six clusters of different colors.

Seventy-seven scholarly journals published papers on SMILE research. Over 250 papers were published in the top two journals, both of which were published in the United States (Table 2). The Journal of Refractive Surgery published the most papers (n = 166), followed by the Journal of Cataract and Refractive Surgery (n = 98), and BMC Ophthalmology (n = 48). Among 1144 co-cited academic journals, five had more than 1000 citations, and all of them were from the United States. The Journal of Refractive Surgery had the most co-citations (n = 4427), followed by the Journal of Cataract and Refractive Surgery (n = 3928), Investigative Ophthalmology & Visual Science (n = 1252), Cornea (n = 1217), and Ophthalmology (n = 1155).

Journals (36/77, 46.75%) with a publication number greater than or equal to three (T = 3) were used to construct the citation network map, which can be divided into five clusters with different colors (Figure 4A).

Journals (65/1144, 5.68%) with co-citations greater than or equal to 25 (T = 25) were used to construct the co-citation network (Figure 4B).

A total of 686 hotspot keywords for SMILE research were extracted with a frequency of occurrence of 1720. Subsequently, 65 keywords that appeared more than four times were included and classified into six clusters on the map (Figure 5), including cluster 1 (biomechanics, collagen cross-linking, in red), cluster 2 (ectasia and corneal topography, in green), cluster 3 (dry eye and corneal sensation, in blue), cluster 4 (visual quality, glare, and astigmatism, in yellow), cluster 5 (complication and femtosecond laser, in purple), and cluster 6 (hyperopia and intraocular pressure, in bright blue).

The top 10 co-cited references in SMILE research are listed in Table 3. Each reference was co-cited at least 93 times.

The top 25 citation bursts pertaining to the development of SMILE were identified from 2011 to 2022 (Table 4). The increasing number of citations of these papers in a certain period indicated rapid dissemination. Among them, the first citation burst appeared in 2011, while the last eight citation bursts began after 2020.

Studies on SMILE surgery peaked in 2020. The number of articles published in various countries is mostly on the rise, indicating the rapid development of SMILE. Among them, China has the largest number of papers. This may be due to an early start of SMILE surgery in China. Currently, SMILE has become the most common corneal surgical method for myopia correction in China, and more than three million SMILE surgeries have been completed in China. Therefore, it is not surprising to find that Chinese research institutions occupy the top four of 10 seats with good international collaboration. The United States is potent enough to mention here, as SMILE surgery was approved by the United States Food and Drug Administration (FDA) in 2016 (30). In addition, as shown in Figure 3B, a tight collaboration was observed between the United States and China. The Journal of Refractive Surgery and Journal of Cataract and Refractive Surgery ranked first and second in the lists of the most productive and co-cited journals, respectively.

Owing to the development of femtosecond lasers, corneal laser surgery has entered the femtosecond era. The femtosecond laser replaced the keratome, which was the preferred tool for laser-assisted in situ keratomileusis (LASIK) flap making. Thereafter, the FLEx (2) and SMILE surgery (3, 4) came into being, followed by gradual development. The most cited literature in SMILE-related research was published between 2008 and 2014 (Table 3). During this period, the surgical technique for SMILE was still in its infancy, and the most important discussion was associated with the predictability and safety of SMILE surgery since they were the focuses of most of the top 10 co-cited studies (3, 4, 6, 8–10, 31).

Furthermore, predictability has remained a hotspot. Initial research focused on early postoperative predictability, but current researchers are more concerned about long-term predictability since five of the 11 top references with the strongest citation bursts in 2022 were focused on this aspect (Table 4). Several studies have shown exciting long-term predictivity, stability, safety, and high patient satisfaction (24, 32). This provides clinicians with confidence in SMILE surgery. However, factors related to corneal regression and comparisons between SMILE and other refractive surgeries are worth investigating.

Since the incision of SMILE is small, another interest is whether fewer changes in corneal biomechanics occur during SMILE surgery. Researchers have compared biomechanical results of SMILE and LASIK using dynamic Scheimpflug imaging (Corvis ST) (20, 33), Ocular Response Analyzer (ORA) (34), mathematical analyses (7, 35), as well as meta-analysis (36), and demonstrated that SMILE seems to be superior to LASIK in terms of preserving corneal biomechanics. However, common problems of corneal refractive surgery, such as refractive regression and postoperative corneal ectasia, still cannot be avoided in SMILE surgery. Nineteen cases of corneal ectasia after SMILE surgery have been reported since 2017, although the previous tomography findings were not normal in some of these cases. Based on the literature review of ectasia, the incidence of post-refractive ectasia in eyes without identifiable preoperative risk factors is 20 per 100,000 eyes for photorefractive keratectomy (PRK), 90 per 100,000 eyes for LASIK, and 11 per 100,000 eyes for SMILE (37). In addition, it is believed that the combination of Corvis ST and Pentacam can fill the void in preoperative risk prediction and early diagnosis of corneal ectasia and keratoconus; however, the data output by Corvis ST still warrants further discussion.

The visual quality of SMILE surgery has been widely studied. Aberration is the most commonly used method for evaluating visual quality. Many studies have shown that a smaller spherical aberration was induced by SMILE compared to LASIK (24, 38, 39), which might be due to the larger optical zone after SMILE (24). With the emergence of new detection methods, results of the visual quality of SMILE surgery using methods such as optical quality assessment system (OQAS) (40, 41), Oculus Cataract Quantifier (C-Quant) (42), disk halo sizes (43), and corneal densitometry (44) have been reported successively. The overall trend indicates that SMILE surgery has no significant impact on visual quality three months postoperatively. No significant difference in corneal transparency has been shown in the first postoperative week (44).

Astigmatism is another important research topic. Unlike LASIK, in which an infrared-guided pupil tracking system is used during the surgical process, SMILE mainly depends on the surgeon’s judgment. Consequently, the comparison of astigmatism vector analysis between the two surgeries has attracted much attention. Although there is no unified conclusion at present, it is reported that there is slight inferiority and more under-correction during SMILE than during LASIK when treating low-to-moderate astigmatism, and a comparable rate of under-correction when treating high astigmatism (45, 46). Comparing outcomes of astigmatism correction of SMILE surgery with those of other surgical methods, as well as different types of astigmatism correction, need to be studied. In addition, software that enhances eye tracking or cyclotorsion compensation is also being developed and will soon be available (1).

Complications of SMILE are a constant concern, especially postoperative complications such as corneal ectasia. Other common postoperative complications include dry eyes and diffuse lamellar keratitis (47).

Initially, intraoperative complications gained more attention than postoperative ones, as a certain learning curve is required for surgeons performing SMILE. In 2014, Ivarsen et al. (5) published a study based on clinical results and surgical complications of more than 1,500 SMILE operations. This article is also the third in the top co-cited references in SMILE-related research and the first clinical practice result for SMILE with a large sample. In this study, tearing at the incision (114/1800) and difficulties in lenticule separation (34/1800) are common causes of intraoperative complications. Other common intraoperative complications include suction loss, opaque bubble layer (OBL), and black spots (47). In the initial period of the learning curve for SMILE, some patients may experience delayed vision recovery, although the phenomenon may be improved by lowering the femtosecond laser energy and advancing the surgical experience (48). In general, SMILE is associated with rapid visual recovery, and most patients can achieve 20/20 visual acuity within one day after surgery according to our and other surgeons’ experience (49).

The main highlight of SMILE is the characteristic small incision. Compared to the approximately 20 mm incision in LASIK, a small incision design guarantees a reduction in flap-related complications, such as flap loss and flap displacement. In addition, it also reduces damage to the corneal nerves. Compared with LASIK, the recovery time of corneal sensation and dry eye symptoms after SMILE has been proved to be shortened through clinical research and experimental research (50, 51).

Treatment of hyperopia with SMILE is not easy. The result of the earliest attempt using FLEx was not satisfactory. By enlarging the transition zone in SMILE, its stability is improved, and the refractive outcomes are similar to those of LASIK (52, 53). At present, global clinical observation of hyperopia SMILE has led to preliminary results, and it is believed that hyperopia SMILE will be progressing in the next few years (54).

For correction of hyperopia, an important and attractive surgical correction is the transplantation of lenticules obtained from the SMILE procedure (55). Lenticule keratophakia and epikeratophakia are reversible in SMILE, and the visual quality offers unique advantages (56, 57). Moreover, the discarded tissue was also reused as a bio-scaffold for stromal engineering, and an ocular drug delivery system of active molecules (58, 59).