Small Incision Lenticule Extraction for Hyperopia: 3-Month Refractive and Visual Outcomes

Patients

Ethics approval was obtained from the Nepal Health Research Council for this prospective case series of consecutive hyperopic SMILE procedures by three experienced surgeons (KRP, DZR, and GIC) using the 500-kHz VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) at the Tilganga Institute of Ophthalmology, Kathmandu, Nepal. Inclusion criteria were attempted correction in the maximum hyperopic meridian between +1.00 and +7.00 diopters (D), astigmatism up to 6.00 D, medically suitable for SMILE, normal corneal topography, and no ocular disease. Inclusion criteria were corrected distance visual acuity (CDVA) of 20/200 or worse for phase I, 20/100 and 20/200 for phase II, 20/40 and 20/80 for phase III, and 20/40 or better for phase IV of the study. Phases were advanced once safety had been demonstrated for the previous group. Patients treated as part of phase IV between May 2014 and February 2017 were included in the current study.

A complete ophthalmologic examination was performed prior to surgery by an in-house optometrist, as has been described previously.^5–7 The preoperative examination also included corneal Placido topography by ATLAS (Carl Zeiss Meditec), corneal tomography by Pentacam (Oculus Optikgeräte, Wetzlar, Germany), corneal tomography and epithelial thickness mapping by RTVue optical coherence tomography (Optovue, Inc., Fremont, CA), pupillometry (VIPTM-200; Neuroptics, Irvine, CA), and handheld ultrasound pachymetry (SP-3000; Tomey Corporation, Nagoya, Japan). Refractions were done by the in-house optometrist at the first visit, including both a manifest and a cycloplegic refraction, performed according to the standardized London Vision Clinic protocol to push maximum plus and maximum cylinder.⁸ The manifest refraction was repeated by the operating surgeon at a second visit on a separate day with the aim of pushing maximum plus with the knowledge of the cycloplegic refraction. This manifest refraction was used to plan the treatment. Contrast sensitivity was measured using the Functional Acuity Contrast Test (FACT; Vision Sciences Research Corporation, San Ramon, CA).

Planning

The following criteria were applied for planning the procedure. First, the predicted postoperative residual stromal thickness under the lenticule must be greater than 250 µm at the location of maximum tissue removal. Second, the attempted correction was limited such that the predicted postoperative keratometry was less than 51.00 D, calculated using the manifest refraction at the corneal plane added to the preoperative keratometry at the axis of treatment by vector analysis. Finally, an arbitrary maximum attempted correction of +7.00 D was applied. Therefore, some young patients were treated as a partial correction to debulk a high hyperopic refraction.

Surgical Protocol

All SMILE treatments were performed using the 500-kHz VisuMax femtosecond laser as described previously.^5,9 In all eyes, two incisions were created: a 2-mm incision superonasally and a 2-mm incision superotemporally. Intended cap thickness was 120 µm in all eyes. Cap diameter used was 8.8 mm in all eyes, using a medium (M) contact glass. Optical treatment zone diameter was between 6.3 and 6.7 mm, with a 2-mm transition zone. The minimum lenticule thickness was 30 µm in all eyes. The spot and track distance were 4.6 µm for the cap and lenticule interfaces, 1.5 µm for the lenticule side cut, and 2 µm for the small incision. The energy level was set to 34 (170 nJ) in the VisuMax software.

The surgical technique was essentially identical to that used for myopic SMILE, as has been described previously.^5–7,9 The treatment was centered on the coaxially sighted corneal light reflex, confirmed by the surgeon by comparing the relative positions of the corneal reflex and pupil center to the Placido eye image obtained by the ATLAS topography scan. The femtosecond laser cutting for the hyperopic lenticule was performed as previously described.^2,3,5,6 The lenticule dissection was performed as described previously^5–7,9–11 using the MMSU1297 Reinstein lenticule separator instrument (Malosa Medical, Halifax, United Kingdom). The lenticule was then hydrated with balanced salt solution on the corneal surface to distend it for visual inspection for completeness and edge smoothness.

On completion, the patient was brought to the operating room slit lamp, fluorescein was instilled, and full central distention of the cap was achieved by centrifugal stroking using a dry micro-spear sponge to ensure that any redundant cap (due to mismatch of cap vs bed length) was redistributed to the periphery to minimize the appearance of cap folds or microdistortions on Bowman's layer.¹²

Postoperative Course and Evaluation

Patients were instructed to instill ofloxacin 0.1% together with prednisolone acetate (1%) four times a day for 1 week, as a standard protocol for broad-spectrum prophylaxis, and wear plastic shields for sleeping during the first week. The surgeon evaluated the patient 1 day postoperatively and an optometrist examined the patient at 1 and 3 months as described previously.^5–7

Statistical Analysis

Outcome analysis was performed according to the Standard Graphs for Reporting Refractive Surgery.¹³ Eyes where the intended postoperative refraction was not emmetropia (partial correction patients) were excluded in the efficacy analysis. Vector analysis was performed for refractive cylinder as described by Alpins,¹⁴ with the cylinder axes reflected in the vertical meridian for left eyes. Student's t tests were used to calculate the statistical significance of any changes in log contrast sensitivity. Microsoft Excel 2010 software (Microsoft Corporation, Seattle, WA) was used for data entry and statistical analysis. A P value of less than .05 was defined as statistically significant.

Population

A total of 93 eyes of 62 patients underwent hyperopic SMILE between April 2014 and February 2017, of which 3-month data were available in 82 eyes of 53 patients (88% follow-up). Table 1 shows the demographic data for the study population. The population was skewed toward young patients with high hyperopia due to the nature of the refractive error distribution in Nepal, where there are few patients with low to moderate hyperopia. The optical zone was 6.3 mm for 35 eyes (43%), 6.5 mm for 46 eyes (56%), and 6.7 mm for 1 eye (1.2%). The maximum lenticule thickness was 153 ± 26 µm (range: 54 to 180 µm).

Table 1: Study Demographics

Outcomes

Figure 1 shows the Standard Graphs for Reporting Refractive Surgery. Uncorrected distance visual acuity (UDVA) was 20/32 or better in 58% of eyes, relative to 64% with preoperative CDVA of 20/32 or better. Spherical equivalent refraction was within ±0.50 D in 59% and within ±1.00 D in 76% of eyes. There was one line loss of CDVA in 17% of eyes, and one eye (1.2%) lost three lines but recovered to one line lost at the subsequent 9-month visit.

Figure 1. Nine standard graphs for reporting refractive surgery showing the visual and refractive outcomes for 82 hyperopic small incision lenticule extraction treatments. UDVA = uncorrected distance visual acuity; CDVA = corrected distance visual acuity; D = diopters; Postop = postoperative; Preop = preoperative; SEQ = spherical equivalent refraction; TIA = target induced astigmatism; SIA = surgically induced astigmatism

Figure 2 shows the vector analysis for refractive cylinder and the main outcome measures are shown in Table A (available in the online version of this article). The scatter plot for surgically induced astigmatism vector (SIA) versus target induced astigmatism vector (TIA) shows that the refractive cylinder correction was on target in terms of magnitude. The angle of error histogram shows that the refractive correction was placed accurately on the intended meridian for the majority of eyes, with 86% within ±15°. There was one eye where the angle of error was 76°, indicating an induction of refractive cylinder from −1.00 × 168 to −2.00 × 175. On reviewing this case, the preoperative corneal astigmatism was 2.64 D @ 79°, so the apparent induced refractive cylinder may have been an underestimation of cylinder in the preoperative manifest refraction, which was subsequently measured after surgery. There was one outlier of 2.96 for the correction index, but this was for a cylinder change from −0.25 × 50 to −0.50 × 130.

Figure 2. Vector analysis of refractive cylinder displayed as polar plots for target induced astigmatism vector (TIA), surgically induced astigmatism vector (SIA), difference vector (DV), and correction index (CI). D = diopters

Table A: Vector Analysis of Refractive Cylinder

ATLAS average keratometry changed from 42.77 ± 1.87 D (range: 38.50 to 46.50 D) before surgery to 47.45 ± 2.15 D (range: 41.00 to 52.00 D) at 3 months after surgery. ATLAS average corneal astigmatism changed from 1.19 ± 0.74 D (range: 0.04 to 3.90 D) before surgery to 1.31 ± 0.64 D (range: 0.28 to 3.26 D) at 3 months after surgery. Table 2 includes the mesopic contrast sensitivity data before and after surgery showing that there was no change at 3, 12, and 18 cycles per degree (cpd), but there was a statistically significant decrease at 6 cpd.

Table 2: Change in Contrast Sensitivity (Functional Acuity Contrast Test) in Log Units