Analysis of Corneal Refractive Surgery in Soldiers from a Specific Region, China


 Background: Increasing numbers of young people of the appropriate age are joining the army after vision improvement by refractive surgery. However, there is little research on the long-term influence of this population and the potential impact on mission capability. Methods: A cluster sampling survey was conducted. The respondents were soldiers who planned to apply for military academies in a specific region of China in 2020. According to the Physical Examination Standards for Citizens Eligible for Enlistment and the Physical Examination Law for Citizens Eligible for Enlistment, a medical history inquiry, vision examination, color vision examination, slit-lamp examination, and personal interview were performed to obtain information regarding the patients' preoperative myopia, corneal refractive surgery method used, operation time, and symptoms of postoperative discomfort. Results: Among the 1263 soldiers who underwent the physical examination, 435 soldiers (862 eyes) had a history of corneal refractive surgery, accounting for 34.44%. Among the soldiers with a history of corneal refractive surgery, the treatment of preoperative low-grade myopia accounted for 41.61%, moderate-grade myopia accounted for 43.68%, and high-grade myopia accounted for 14.71%. Regarding the surgical methods used, small incision lenticule extraction (SMILE) accounted for 23.44%, excimer laser in situ keratomileusis (LASIK) accounted for 66.67%, and trans-epithelial photorefractive keratectomy (PRK) accounted for 9.89%. The postoperative time was 2.76 ± 1.01 years. Soldiers with uncorrected visual acuity (UCVA) ≤ 4.9 accounted for 3.94%, and soldiers with UCVA > 4.9 accounted for 96.06%. Slit-lamp examination found no surgical complications affecting visual function, such as dry eye syndrome, corneal infection, corneal haze (above grade 3), and corneal epithelial implantation. Conclusion: The proportion of soldiers who have undergone corneal refractive surgery has significantly increased, and LASIK is the main surgical method used. The impact of complications related to corneal refractive surgery on the army should be emphasized, and health education and reasonable training should be strengthened to avoid eye-related diseases.

96.06%. Slit-lamp examination found no surgical complications affecting visual function, such as dry eye syndrome, corneal infection, corneal haze (above grade 3), and corneal epithelial implantation.
Conclusion: The proportion of soldiers who have undergone corneal refractive surgery has signi cantly increased, and LASIK is the main surgical method used. The impact of complications related to corneal refractive surgery on the army should be emphasized, and health education and reasonable training should be strengthened to avoid eye-related diseases.

Background
With the increasing proportion of myopia among young people in China and the development of corneal refractive surgery, increasing numbers of people are choosing to undergo corneal refractive surgery to improve their vision. Many young people with poor vision join the army after undergoing vision improvement through surgery. Previous studies reported on the complications of soldiers in a short period of time after corneal refractive surgery. However, there are few reports on the long-term visual function and complication of soldiers. Soldiers especially for who are about to hold special positions in the army and receive more professional training, comprise a special professional group that requires high visual ability, and they face a signi cantly higher probability of eye injuries than other people. Research on the status of corneal refractive surgery in these soldiers and targeted eye health education and differentiated management are of practical signi cance for reducing surgery-related training injuries. In this paper, a survey of corneal refractive surgery was conducted among soldiers in a speci c region of China, and the analysis results are reported. to obtain information on the participants' medical history and ocular surgery history. Spherical equivalent (SE) was calculated as follows: SE = diopter of spherical power (DS) + 1/2 diopter of cylindrical power (DC). According to the surgical mechanism and potential complications, this study divided corneal refractive surgeries into three groups: 1. small incision lenticule extraction (SMILE); 2. laser keratomileusis, including excimer laser in situ keratomileusis (LASIK) and femtosecond excimer laser in situ keratomileusis (Fs-LASIK); 3. epithelial photorefractive keratectomy (PRK), including trans-epithelial photorefractive keratectomy (Trans-PRK). The study mainly used slit-lamp examination to observe postoperative changes in corneal characteristics and the medical history inquiry method to determine the surgery method.

Results
The proportion of respondents who had undergone corneal refractive surgery and their vision information are shown in Table 1 4.9, 6 eyes had low diopter power (SE > -3.0D), 15 eyes had moderate diopter power (-6.0D < SE ≤ -3.0D), and 13 eyes had high diopter power (SE ≤ -6.0D) before surgery.

Discussion
The proportion of soldiers who have undergone corneal refractive surgery has increased signi cantly.
According to the 2018 national survey of myopia in children and adolescents in China, the overall myopia rate in adolescents is 53.6%; 81% of those affected are high school students, and 21.9% of senior high school students have hypermyopia (SE ≤ -6.0D), indicating that myopia has become a critical issue affecting eye health. To meet the vision requirements in the Physical Examination Standards for Citizens Eligible for Enlistment, many young people choose to undergo corneal refractive surgery before joining the army. In 2008, a re-examination of eye diseases in 20320 recruits in the eld forces found that 3.24% of recruits underwent refractive surgery before enlistment [1] . A survey of 4579 recruits who enlisted in the army in 2013 found that 12.3% underwent refractive surgery [2] . In 2015, the proportion of recruits who had undergone corneal refractive surgery was 15.32% [3] . Another survey of 526 recruits in 2015 found that 13.1% of them underwent corneal refractive surgery before enlistment, and those with low-and moderate-grade myopia accounted for 61.6% [4] . This study found that among the soldiers who had an average time since enlistment of 3.20 years and had undergone corneal refractive surgery, 34.40% were male, and 34.75% were female. The shortest postoperative time was 1 year, and the longest was 6 years, with an average of 2.76 years. An analysis of recruits' physical examinations over the years showed that the proportion of soldiers who had undergone corneal refractive surgery increased gradually (Fig. 1). Although corneal refractive surgery can signi cantly improve the UCVA, it is also associated with visual regression and other complications. This study found that among the 862 eyes that underwent corneal refractive surgery, 34 (3.94%) showed visual deterioration, with UCVA < 5.0; furthermore, among these 34 eyes, 82.35% had moderate and high diopters preoperatively. The increased proportion of people undergoing corneal refractive surgery has led to an increase in common complications, such as xerophthalmia, glare, poor contrast sensitivity, and night vision disorders [3,5,6] . Among soldiers who have undergone corneal refractive surgery, 28% have experienced xerophthalmia compared to 6% of soldiers who have not undergone refractive surgery, indicating that the surgery group has more than 4 times the rate of xeropthalmia that the non-surgery group [7] . Soldiers often work in special environments for long periods, and their irregular rest times both during the day and at night can easily worsen xerophthalmia and asthenopia. The medical team should strengthen health education, pay attention to the healthy use of eyes, and arrange for reasonable tasks. Arti cial tears medicine and other reserved drugs can be appropriately used if there is a high incidence of such complications.
With the emergence of new surgical techniques, such as SMILE and Trans-PRK, the methods of corneal refractive surgery have become diversi ed in recent years. This study found that among 435 surgeries, LASIK was the main surgical method for low-, moderate-, and high-grade myopia, accounting for 66.67%; SMILE was a widely used method for low-and moderate-grade myopia, accounting for 23.44%; PRK accounted for only 9.9%, and although it was mainly used for low-and moderate-grade myopia, a small number of soldiers with high-grade myopia did use it. Different surgical methods have different complications and postoperative risk factors. Studies have found that the proportion of glare and night vision disorders after corneal refractive surgery is 23%; LASIK results in the highest rate of these disorders, at 33% [8] . Dennis et al. [9] found that after PRK, slight haze can cause decreased visual acuity under complex light interference and can decrease contrast sensitivity under normal conditions and complex light interference, which directly in uences performance capacity. This study did not nd haze (above grade 3), which can negatively impact vision, in the soldiers; however, haze (grade 1-2) was common among the soldiers on slit-lamp examination. Soldiers who have undergone corneal refractive surgery often have a high risk for potential ocular trauma, especially those who have undergone LASIK. An investigation of 1131 hospitalized soldiers with ocular trauma at 27 military hospitals from January 2006 to December 2010 found that among 3 soldiers who had undergone radial keratotomy (RK), 2 soldiers had eyeball ruptures; among 72 eyes that had undergone LASIK, 69 eyes had mechanical ocular trauma, and 29 eyes had corneal ap dislocation [2] . LASIK is a surgical method that uses excimer lasers to perform refractive cutting of the stromal layer of the corneal ap to correct refractive errors. It has advantages of safety, reliability, predictable vision, and strong stability [10] . However, this surgery affects the corneal stroma ap. Even after many years, the tissue healing of the corneal ap and corneal stroma might not achieve a normal state since the postoperative healing reaction of the two is negligible; therefore, corneal ap displacement and wrinkles are more likely to occur [11] . Soldiers are often involved in vigorous exercises and ghts, so the eye injury rate in soldiers is signi cantly higher than that normal people, making a weakly healed corneal ap more vulnerable to injury [12] . Corneal ap displacement and wrinkles are more likely to occur after trauma and impacts and can result in impaired vision due to complications such as keratitis and epithelial implantation. In addition, the treatment effect is poor because the lack of obvious early symptoms means these complications cannot be diagnosed and treated in a timely manner [13] . This study found that a soldier had accidentally injured his eyes during training 1 year after undergoing LASIK and had suffered corneal epithelial implantation (Fig. 2). Because of the negligible impact on the UCVA in the early stage, the patient did not seek medical care until 1 year after the onset, which caused great di culty in treatment. SMILE can avoid the complication of corneal ap displacement, has faster postoperative vision recovery and less incidence of xerophthalmia than LASIK, and has become very popular in recent years [14,15] . The complications are different for different surgical methods, and their in uences on performance capacity are different due to different surgical mechanisms. Therefore, hierarchical management should be considered for different corneal refractive surgeries to effectively reduce the in uence of such surgeries on military operations and complications related to corneal refractive surgery, thereby avoiding military training injuries.
For patients with hypermyopia, corneal refractive surgery cannot improve fundus lesions. At present, there are no clear requirements regarding the degree of myopia required in soldiers before corneal refractive surgery during military physical examination. In this study, preoperative high-grade myopia (SE ≤ -6.0D) was present in 14.71% of the soldiers who had undergone refractive surgery. Some studies pointed out that there was a high probability of retinal degeneration, lattice degeneration, and retinal detachment after surgery [16] . A study reported that 658 soldiers who had undergone refractive surgery experienced a signi cant increase in visual deterioration, retinal detachment, and retinal tears after 2 years of enlistment [1] . Therefore, hierarchical management should be performed in response to the different corneal refractive surgery methods and the collected information regarding the preoperative diopter and eye disease history. Especially for those with a high degree of myopia before surgery, special attention should be paid to the fundus; regular dilatation examinations should be performed for the early detection and intervention of retinopathy so that serious complications, such as retinal tear and retinal detachment, can be avoided.
Although implantable contact lenses (ICLs) can signi cantly improve vision and have obvious advantages in the treatment of high-grade myopia, there are relatively high risks of postoperative cataracts, glaucoma, corneal endothelial decompensation, lens dislocation, and decreased visual quality [17] . Therefore, applicants who have undergone ICL implantation are considered "unquali ed" in the physical examination for citizens eligible for enlistment, and aspiring soldiers should think twice before choosing this refractive surgery method. This study found that 5 soldiers had hypochromatopsia with normal single-color recognition ability, and they all had a history of corneal refractive surgery. Li et al.
reported that intraocular scattering can be changed after LASIK, causing symptoms such as halo, starburst, decreased contrast sensitivity and impaired color vision recognition ability [18] . Whether corneal refractive surgery can impair color vision and cause other changes in binocular visual function must be clari ed in further systematic studies.

Conclusion
Corneal refractive surgery is an essential treatment for myopia, and refractive surgery methods are continuously being developed. Soldiers who have undergone corneal refractive surgery accounted a considerable proportion in the army. We surveyed the corneal situation of soldiers who postoperative time was about 3 years and found some interesting results, while which need to be further studied and longerterm observation. Individualized requirements for corneal refractive surgery should be de ned in the physical examination for citizens eligible for enlistment and standardized inspection items to ensure that the army recruits high-quality soldiers. At the same time, differentiated and re ned management and training in the army is also a topic that needs further discussion and attention.

Declarations
Ethics approval and consent to participate The study was approved by the Ethics Committee of the 6th Medical Centre of Chinese People's Liberation Army General Hospital. All patients gave informed consent and agreed to participate in the study.

Consent for publication
Not applicable.

Availability of data and materials
The data and materials used in the current study are all available from the corresponding author upon reasonable request.

Con icts of Interest
All authors declare no nancial con ict of interests.

Authors' contributions
Authors' Contributions: WYS, CYT contributed to data analysis, manuscripts preparation and revised version. WYS, HZJ and BC contributed the data colleting. LQC, LWQ, GMZ given some good advices in project design and manuscripts revision.