Subjects
Thirteen patients with hyperopic (13 eyes) were enrolled prospectively between March 2017 and June 2018 at the Eye and ENT Hospital of Fudan University (Table 1). Approval was obtained from the institutional ethics committee, and all patients signed informed consent. All procedures adhered to the tenets of the Declaration of Helsinki.
Inclusion criteria were as follows: age ≥ 18 years; sphere +2 to + 6.0 diopters (D), with astigmatism up to 3.0 D; the difference between manifest and cycloplegic refraction was no more than 1.0 D.
Patients with abnormal topography, dilated pupil size less than 7 mm, history of intraocular surgery, and glaucoma were excluded.
Preoperative examinations included slit lamp examination, objective and subjective refraction assessments, uncorrected distance visual acuity (UDVA) measurement, corrected distance visual acuity (CDVA) measurement, corneal tomography with a rotating Scheimpflug camera (Pentacam, Oculus, Wetzlar, Germany), and fundus examination.
IOP Measurement
IOPNCT (non-contact IOP): The non-contact tonometer (TX-20, Canon, Tokyo, Japan) was used. One average value was automatically calculated from 3 measurements. Reproducibility of measurements was identified previously.[6]
bIOP (biomechanical corrected IOP): The Corvis ST (Corneal Visualization Scheimpflug Technology instrument; Oculus, Wetzlar, Germany) is a Scheimpflug-based dynamic corneal tonometer, which incorporates the corneal biomechanics and IOP parameters. The system uses an algorithm to calculate biomechanically corrected IOP (bIOP) and compensates for changes in corneal thickness and stiffness.[7]
IOPg (Goldmann-correlated IOP), IOCcc (cornea compensated IOP), CRF (corneal resistance factor), and CH (corneal hysteresis): These four values were derived from ocular response analyzer (ORA; Reichert Ophthalmic Instruments, Depew, NY, USA). ORA uses an air puff to deform the cornea. Because of its viscoelastic nature, the cornea resists the air puff, resulting in different values for the inward and outward flexing, which is termed as corneal hysteresis (CH). The corneal resistance factor (CRF) represents the resistance of cornea.[8] Each eye was measured 4 times, and only measurements with a waveform score greater than 5 were used for further analysis.
Two corneal biomechanical parameters derived from Corvis ST were analyzed. A1 Time (first applanation time), and HC DA (deformation amplitude, the largest anterior- posterior displacement of the cornea apex at the highest concavity phase) were the most repeatable and reproducible parameters.[9] SP-A1 (resultant pressure [adjusted pressure at A1 (adj AP1) – biomechanically compensated IOP (Biop)] divided by deflection amplitude at A1) was a new parameter which was used to describe the cornea stiffness, higher values of which meant a more stiff cornea.[2]
All measurements were performed by the same examiner (FD) to decrease inter-observer variability and were taken at approximately the same time of day.
Surgical Techniques
All surgeries were performed by the same surgeon (ZXT). After standard sterile draping, all patients were treated with the VisuMax laser (Carl Zeiss Meditec AG, Jena, Germany, version 3.1) with repetition rate of 500 kHz and pulse energy of 30 nJ. The following settings were used for hyperopic SMILE: the cap diameter was 8.8 mm and the thickness 120 μm; the optical zones ranged between 5.3–6.3 mm, with a 2-mm transition zone; a single 2.0-mm side cut was made at the 12 o’clock position with an angle of 90°.
The detailed steps of SMILE have been described previously.[10] Total suction time was approximately 35 seconds. After lenticule scanning, the surgeon used a splitter to separate the upper interface, following the lower lenticule interface separation. The lenticule was then removed by superior incision. Thereafter, the surgeon examined the cornea with a built-in slit lamp to detect whether parts of the lenticule remained. One drop of prednisolone and levofloxacin was instilled at the end of the surgery.
All surgeries were performed successfully, with no intraoperative or postoperative complications.
Postoperatively, the patients were instructed to use fluorometholone eye drops 8 times a day, and to reduce the usage frequency by 1 every 3 days (totally 24 days). Artificial tears were prescribed for 3 to 4 weeks, for use as needed.
Follow-up
Patients were examined at 1 week, 1 month, and 3 months postoperatively. At each follow-up visit, the visual acuity, subjective refraction, corneal topography, and IOP measurements were performed using three devices.
Statistical Analyses
All data were recorded and analyzed using SPSS (version 22, IBM Corp, USA). First, the Kolmogorov–Smirnov test was used to check the normality of data. Linear mixed-model analysis of variance with post hoc least significant difference multiple comparisons were used to compare the postoperative IOP measurements between different visits and different methods at the same visit. The Spearman rank correlation was used to assess the corneal biomechanical parameters obtained from the Corvis ST and to determine potential postoperative factors affecting the postoperative IOP measurements. P< 0.05 was considered statistically significant.