This study is a single-center, single-surgeon, prospective, comparative study of the medical records of healthy patients who underwent FS-LASIK at an ambulatory surgicenter. A total of 66 eyes from 66 patients were included in this study. The research protocols were approved by the Institutional Review Board at Guangdong Provincial People’s Hospital (No. GDREC2017206H(R2)). The study followed the tenets of the Declaration of Helsinki, and all subjects were thoroughly informed of the procedure and provided written informed consent.
The inclusion criteria for the surgery were as follows: patients were willing to receive the surgery and had good compliance and cognitive ability, age of 18 or older, stable refraction for at least 2 years, refractive error not greater than − 8.0 diopters (D) sphere or -4.0D of astigmatism. Exclusion criteria included any history of ocular trauma, corneal disease, corneal scarification, uveitis or retinal disease, any history of corneal laser treatment or ocular/intraocular surgery, less than 3 months of discontinued use of rigid contact lenses, less than 1 month of discontinued use of soft contact lenses, any topical corticosteroid use, pregnancy, systemic immunologic disease, long-term use of psychoactive drugs or diagnosed psychiatric disorder, and inability to cooperate with ophthalmic examinations. Eyes with possible keratoconus were excluded by using the keratoconus screening test of Pentacam HR (Oculus, Germany). Central cornea thickness (CCT) was measured by ultrasonic pachymetry (Tomey Sp-3000, Japan), and corneal radius (mean R) was measured by an autorefractometer (NIDEK ARK-1 s, Japan).
CID measurements
The CID was prepared as reported elsewhere.14 In short, the CID was put on a slit lamp unit and consisted of a 2 mm circular flat indenter, a digital screen display and a foot switch connected to it (Fig. 1).
The corneal indentation device is placed on a slit lamp unit. CID consists of a 2 mm circular flat indenter, a digital screen display, and a foot switch.
After the cornea was anaesthetized with one drop of 0.5% proparacaine, the indenter was moved toward the center of the cornea until the low pitch signal was sound, which indicated a stable preload of 0.001 to 0.003 N. When the foot switch was pressed, the indenter moved forward and compressed the cornea to a depth of 1 mm at 12 mm/s and immediately retracted. The entire indentation process was completed in approximately 0.2 seconds. Effective measurement results showed a smooth and linear load‒displacement curve on its screen. The CID measured corneal stiffness, which was the slope of the corneal displacement from 0.3 to 0.6 mm. Three valid results were measured. Since corneal biomechanics depend on IOP, Goldmann applanation tonometer (GAT) measurements were performed 15 minutes after CID measurements to avoid any potential massaging effect. Three readings (GAT-IOP) were taken, and the averaged results were used for analysis.
Determining the corneal tangent modulus (EIOP) at a particular IOP involves substituting raw data, including CCT(t), mean R (r), and corneal stiffness (SIOP), into a generalized Eq. 13:
$${\text{E}}_{IOP}=\frac{a\left(\text{r}-\text{t}/2\right)\sqrt{1-{{\nu }}^{2}}}{{\text{t}}^{2}}\times {S}_{IOP}$$
\({\nu }\) is Poisson’s ratio of the cornea, and \(a\) is a geometrical constant highly correlated with µ, where µ is determined by the equation below16:
$${\mu }={r}_{0}{\left[\frac{12(1-{\nu }^{2})}{{(r-t/2)}^{2}{t}^{2}}\right]}^{1/4}$$
The radius of the circular flat indenter that is in full contact with the cornea is denoted as \({r}_{0}\).
Surgical procedures
All surgeries were performed by the same experienced surgeon (J.Z.). A suction ring was applied to the anterior segment of the eye to immobilize the eyeball, after which an 8.5 mm flap of 100 µm thickness was cut with a femtosecond laser (IntraLase iFS 150, Abbott Medical Optics Inc., Santa Ana, California). The flap was separated and lifted, followed by ablation in a 6.0 mm optical zone using a wavefront-guided excimer laser (Technolas 217z100 excimer laser platform, Bausch & Lomb, Rochester, NY). After repositioning the flap, postoperative topical medications were given. The postoperative medication regimen for FS-LASIK included the use of a transparent eye shield or goggles to protect the eyes after surgery. Antibiotic eye drops were applied for 14 consecutive days, 4 times a day, with 1 drop each time. Corticosteroid eye drops were applied for 1 week, 4 times a day, with 1 drop each time, and the dosage was adjusted as needed. Artificial tears or eye surface repair agents were applied in the form of eye gel. Follow-up examinations were recommended at postoperative day 1, week 1, month 1, month 3, month 6, and year 1. The CCT, mean R, corneal stiffness and intraocular pressure (GAT-IOP) measurements were repeated 6 months after surgery. All examinations were conducted by a single experienced examiner, with the patient in a sitting position and in a single clinic visit, in the same half-day session (morning 08:30–11:30) to minimize diurnal effects.
Statistical analysis
Data analysis was performed by using SPSS for Mac software (Version 25.0; SPSS, Chicago, IL). The Kolmogorov‒Smirnov test verified that the data were normally distributed. Correlation between corneal parameters and intraocular pressure changes before and after surgery was performed by paired t test; Pearson test was performed to analyse the correlation between changes in corneal tangent modulus and other biological parameters (CCT, corneal curvature, intraocular pressure, etc.). A predictive model was established to estimate the amount of change in the corneal tangent modulus. For all tests, P < 0.05 was statistically significant.