Lack of stereopsis does not reduce surgical performance but prolongs the learning curve of vitreoretinal surgery

PURPOSE

To investigate whether individuals with long-term reduced stereopsis were able to obtain the same level of surgical skills in simulated vitreoretinal surgery on the Eyesi Surgical Simulator as individuals with normal stereopsis.

METHODS

Twenty-four medical students were recruited and divided into two groups according to their degree of stereopsis: Group 1 (n = 12) included subjects with normal stereopsis (60 arcsec or lower) and Group 2 (n = 12) included subjects with reduced stereopsis (120 arcsec or higher). Stereopsis was tested using the TNO test (Lameris Ootech BV, Nieuwegein, the Netherlands). The participants were trained in virtual reality-simulated vitreoretinal surgery and continuously measured using a test with solid validity evidence and a pre-defined pass-fail score. All data were analysed using the Wilcoxon rank sum test.

RESULTS

We observed no differences in overall performance scores for any of the four modules. The participants with reduced stereopsis used 5.8 more attempts in bimanual training (p = 0.04), 8.8 more attempts in removal of posterior hyaloid (p = 0.04), 9.1 more attempts in navigation training (p = 0.20) and 0.3 fewer attempts in removal of internal limiting membrane (p = 0.69).

CONCLUSION

The final performance scores on the Eyesi Surgical Simulator were independent of the degree of stereopsis. However, the number of attempts to achieve the pre-defined pass-fail score increased significantly with reduced stereopsis in two of four modules. These results indicate that a high degree of stereopsis is not necessary to become proficient in microsurgery but may prolong the learning curve.

Bidirectional 5-year risks of diabetic retinopathy, glaucoma and/or ocular hypertension: Results from a national screening programme

PURPOSE

We aimed to investigate if diabetic retinopathy (DR), glaucoma and/or ocular hypertension (OHT) are prospectively linked, as previous studies have proposed cross-sectional associations, but longitudinal data from larger cohorts are lacking.

METHODS

We performed a bidirectional 5 years prospective, registry-based cohort study. We extracted data from national registers, including the Danish Registry of Diabetic Retinopathy, the Danish Civil Registration System, the Danish National Patient Register and the Danish National Prescription Registry. DR level was defined by the highest level of the two eyes. Glaucoma and/or OHT was defined by diagnostic codes (H40*) or at least three redeemed prescriptions of glaucoma medication (S01E*) within 1 year. We included 205 970 persons with diabetes and 1 003 170 age- and gender-matched non-diabetes controls. Exposures were level-specific DR (i) and glaucoma and/or OHT (ii), and outcomes were hazard ratios (HRs) for 5 years incident glaucoma and/or OHT (i) and DR (ii).

RESULTS

Persons with diabetes were more likely to develop glaucoma and/or OHT (multivariable adjusted HR 1.11, 95% CI 1.06-1.15), but this did not depend on the level of DR. In persons with diabetes, those with glaucoma and/or OHT were more likely to develop DR (multivariable adjusted HR 1.12, 95% CI 1.03-1.23) within 5 years.

CONCLUSION

In a national cohort, diabetes associated with a little higher risk of upcoming glaucoma and/or OHT, and, inversely, the presence of the latter predicted a higher risk of incident DR. Nevertheless, our data do not seem to justify including glaucoma evaluation in the national Danish DR-screening programme.

Relationship between Diabetic Retinopathy and Primary Open-Angle Glaucoma: A Systematic Review and Meta-Analysis

BACKGROUND

Pathophysiological overlaps exist between diabetes and primary open-angle glaucoma (POAG) and presence of diabetes increases risk of POAG. Considering that diabetic retinopathy (DR) is an ocular complication of diabetes, one could speculate that DR as a severity measure may associate with or even predict POAG. Given that POAG is asymptomatic in early stages, an association to DR may prove clinically important and facilitate an earlier diagnosis of POAG.

OBJECTIVES

To investigate if DR is associated with and predictive of POAG.

METHOD

We systematically searched 11 literature databases on May 12th, 2021. We screened a total of 1,535 records and found six studies eligible for qualitative and quantitative analysis. Two independent authors reviewed the studies, extracted data, and evaluated risk of bias within individual studies. Studies were reviewed qualitatively, and meta-analyses were made based on the odds ratios (ORs) with 95 % confidence intervals of the association between DR and POAG using the random-effects model. Subgroup analyses were made on the association between subtypes of DR and POAG.

RESULTS

Six studies (two longitudinal and four cross-sectional) were eligible for review with a total of 255,614 patients with diabetes, of which 20,483 patients had any degree of DR and 5,258 had POAG. All studies were based on patients with type 2 diabetes except one with both type 1 and type 2 patients. Any DR was not associated with POAG (OR 1.17; 95% confidence interval (CI95%): 0.58-2.35; P = 0.65). Further stratification revealed that neither cross-sectional (OR 1.00; CI 95%: 0.56 to 1.81, P = 0.99) nor longitudinal studies (OR 1.47; CI 95%: 0.57 to 3.78, P = 0.43) demonstrated an association between DR and POAG.

CONCLUSIONS

We did not find convincing evidence of an associations between DR and prevalent or incident POAG.

Pretraining of basic skills on a virtual reality vitreoretinal simulator: A waste of time

PURPOSE

To investigate whether pretraining of basic skills in virtual vitreoretinal surgery affected the performance curve when proceeding to procedure-specific modules.

METHODS

This study was a prospective, randomized, controlled, two-centre study. Medical students were randomized into two groups: Group 1 pretrained basic psycho-motor skills (Navigation Training level 2 and Bimanual Training level 3) until they reached their performance curve plateau. Hereafter, both groups trained on the procedure-specific modules (Posterior Hyaloid level 3 and ILM Peeling level 3) until they reached their performance curve plateau. Plateau was defined as three consecutive sessions with the same score with an acceptable variation. The primary outcome was time used to reach performance curve plateau in the procedure-specific modules.

RESULTS

A total of 68 medical students were included, and equally randomized into two groups. The participants in Group 1 used a median time of 88 minutes to reach plateau in the basic skills modules but did not differ from Group 2 in time to reach plateau on the procedure-specific modules (183 min versus 210 min, p = 0.40) or in the amplitude of plateau. Group 1 and 2 differed significantly in the starting score of ILM peeling level 3 (0 (0-0) versus 3.5 (0-75), p = 0.03).

CONCLUSION

We were not able to show positive skill transfer from basic skills training to the procedure-specific modules in time, starting score or amplitude of plateau. Thus, we recommend that aspiring vitreoretinal surgeons proceed directly to simulation-based training of procedures instead of spending valuable training time on basic skills training.

[Pathological and therapeutic possibilities in inhibiting ocular angionesis]

In a variety of corneal and retinal diseases, ocular ischaemia mediates vascular endothelial growth factor (VEGF) expression, which in turn causes angiogenesis and increased vascular permeability. Consequently, VEGF-inhibitory treatment has been introduced in different ocular diseases. Hypoxia-inducible factor (HIF)-1α is an upstream regulator of VEGF-A, and increased HIF-1α expression has been established in ocular animal models. Upcoming translational studies are expected to increase our understanding of the complex balance between pro- and anti-angiogenic factors in the eye.

Virtual vitreoretinal surgery: effect of distracting factors on surgical performance in medical students

PURPOSE

We aimed to investigate the effect of four distracting factors on surgical performance in virtual vitreoretinal surgery.

METHODS

Nineteen novice surgeons completed a validated training programme on the Eyesi surgical simulator (VRmagic GmbH, Manheim, Germany; software version 2.9.2) until a calculated target score was reached. The training programme consisted of four modules: navigation level 2 (Nav2), bimanual training level 3 (BimT3), posterior hyaloid level 3 (PostH3) and internal limiting membrane peeling level 3 (ILMP3). When properly trained, the participants completed the training programme once without distraction to generate reference scores and times. Next, they conducted the training programme under the influence of each of the following: auditory distraction, 12 hr of fasting, interrupted sleep and 24 hr of sleep deprivation. Wilcoxon signed-rank test was used to compare the distraction-induced results to the participants' reference scores and times.

RESULTS

As compared to reference score (328), a lower performance was found for all four distractions: auditory distraction (289, p = 0.0012), fasting (302, p = 0.02), sleep interruption (304, p = 0.02) and sleep deprivation (300, p = 0.0006). In particular, PostH3 performance was influenced by all four interventions. (86 versus 50, p = 0,0012, 65, p = 0.05, 72, p = 0.05, 54, p = 0.0007 respectively).

CONCLUSIONS

Virtual vitreoretinal surgery is an important tool for practicing complex surgical skills without compromising patient safety. In this study, deleterious effects on surgical performance were induced by four independent distracting factors. This knowledge is useful to optimize surgeons' work conditions and ensuring the best possible treatment of patients.

[Simulation-based training in ophthalmologic surgery]

Over the past few decades, simulation-based training has become an increasingly widespread learning tool for training in medical procedures. This type of training provides the opportunity to learn in a safe simulation-based environment, where both technical and diagnostic skills can be developed. In Denmark, there has been developed evidence for simulation-based training in ophthalmology in cataract and vitreoretinal surgery. This form of evidence-based training is meaningful for real clinical setting, and in this review, we argue, that is important to apply for all medical fields.

Prophylactic treatment of intraocular pressure elevation after uncomplicated cataract surgery in nonglaucomatous eyes - a systematic review

The purpose of this systematic review was to evaluate the literature regarding prophylactic treatment of intraocular pressure (IOP) elevation after uncomplicated cataract surgery to provide an evidence-based guideline for cataract surgeons. The relevant literature was identified in EMBASE and PubMed. The risk of bias was assessed according to the 'Cochrane Handbook for Systematic Reviews of Interventions' and the ROBINS-I tool. The Grading of Recommendation, Assessment, Development and Evaluation (GRADE) criteria were used to rate the quality of evidence, and relevant data were systematically extracted to evaluate the pressure-lowering effect of the active substances. The primary outcomes for this systematic review were the absolute and relative pressure-lowering effect of the different drugs after 3-8 hr and 1 day after surgery. In total, 23 randomized controlled trials and one nonrandomized controlled study consisting of 45 treatment arms with 14 different active substances were included in the qualitative synthesis. According to the GRADE criteria, nine trials were graded as 'high' quality of evidence, 12 trials as 'moderate', while three trials were given the grade 'low' quality of evidence. The primary outcomes showed most consistency between the trials, which studied the effect of timolol, and presented a relative effect from 18.6% to 29.6% at 3-8 hr and 9.8% to 23.6% at day 1. This systematic review indicates that timolol, latanoprost and travoprost alone or medications containing timolol as an additive active substance, such as dorzolamide + timolol, brinzolamide + timolol and brimonidine + timolol, are characterized by a good relative IOP-lowering effect, which can be gained by a single dose postoperatively.

Comparison of corneal biomechanical changes after refractive surgery by noncontact tonometry: small-incision lenticule extraction versus flap-based refractive surgery - a systematic review

Corneal refractive surgery disrupts corneal integrity and reduces biomechanical stability in consequence of the beneficial refractive alteration. The minimal invasive cap-based refractive procedure, small-incision lenticule extraction (SMILE), has been proposed to affect corneal integrity less than flap-based procedures, due to the fibre-sparing incision of the strong anterior corneal lamellae. Flap-based procedures include laser-assisted in situ keratomileusis (LASIK), femtosecond laser-assisted in situ keratomileusis (FS-LASIK) and femtosecond lenticule extraction (FLEx). The purpose of this systematic review was to evaluate corneal biomechanical changes by noncontact air pulse tonometry after treatment of myopia/myopic astigmatism with SMILE compared to flap-based refractive surgery. A total of 220 publications were identified through a systematic search in PubMed and Embase. Two levels of screening identified nine studies (three randomised controlled trials (RCT) and six nonrandomised clinical trials) eligible for the review. All the nonrandomised clinical trials were graded to have an overall serious risk of bias. Measurements with the Corvis ST were not included in any of the eligible studies. The RCTs found no statistical significant differences between SMILE or flap-based procedures concerning corneal hysteresis (CH) or corneal resistance factor (CRF), as measured with the Ocular Response Analyzer. However, a greater reduction in CRF and CH was found in the flap-based group in five and two of the nonrandomised studies, respectively. The findings in this review illustrate that the presumed biomechanical advantages of a cap-based small incision could not be demonstrated in existing studies and by commercially available technology. However, studies with lower levels of evidence suggest less affection of corneal viscoelastic properties after SMILE when evaluating corneal stability by noncontact tonometry.

Choroidal thickness and myopia in relation to physical activity - the CHAMPS Eye Study

PURPOSE

To describe the relationship between choroidal thickness (CT) and myopia in relation to physical activity (PA) in a population-based child cohort.

METHODS

In a prospective study of 307 children from the CHAMPS Study Denmark, we used objective data from GT3X accelerometer worn at four periods between 2009 and 2015 to determine the amount and intensity of PA. Intensity was estimated as counts/minutes, and cut-off points were defined at four intensity levels. Eye examinations were performed in 2015 and included autorefraction in cycloplegia, axial length (AL) by biometric and fovea-centred enhanced depth imaging optical coherence tomography. By a semi-automated method, we measured the CT at 17 targets per eye representing anatomically different locations (subfoveal, 1 and 3 millimetre in each direction of fovea).

RESULTS

Mean age at the eye examination was 15.4 ± 0.7 years. The mean AL was 23.5 ± 0.9 mm, and the mean subfoveal CT was 369 ± 87 μm. Choroidal thickness (CT) was 331 ± 68 μm for the overall macula, 355 ± 78 μm for the 1-mm zone and 304 ± 60 μm for the 3-mm zone. All CT measurements were thinner in myopic eyes (p < 0.0001) and in boys (p < 0.05). We found no association between total PA and the CT by either mixed model analysis (p = 0.074) or linear regression by any intensity levels (p = 0.22, p = 0.15 and p = 0.43).

CONCLUSION

Among adolescents aged 14-17 years, there was no association between objective PA exposures and the CT, AL or refractive error.

Retinal vascular diameters in relation to physical activity in Danish children - The CHAMPS Eye Study

Our objective was to determine associations between retinal vascular caliber and physical activity (PA) in a school-based child cohort. In a prospective study, we created a childhood cumulative average PA-index using objectively measured PA (accelerometry) assessed at four periods between 2009 and 2015. Cumulative exposure to PA intensities was estimated. Cross-sectional examinations on biomarkers, anthropometry, and ophthalmological data including retinal fundus photographs were performed in 2015. Semi-automated measurements of retinal vascular diameters were performed and summarized into central retinal arteriolar and venular equivalents (CRAE, CRVE). We included 307 participants. Mean age in 2015 was 15.4 years (0.7). The mean CRAE and CRVE were 156.5 μm (2.8) and 217.6 μm (7.7), respectively. After adjusting for age, gender, and axial length, more time in PA was independently related to thinner retinal venules (β-coefficient = -1.25 μm/%, 95% confidence interval = -2.20, -0.30, P < .01). Sedentary time was associated with wider venules (P < .01). Furthermore, birthweight (β-coefficient = 0.56 μm/%, 95% confidence interval = 0.18, 0.95, P < .01) was associated with CRVE. Blood pressure was associated with thinner retinal arterioles (β-coefficient = -0.19 μm/mmHg, 95% confidence interval = -0.36, -0.01, P = .04). We concluded that children with higher PA in childhood had thinner retinal venular caliber. Our results suggest that PA during childhood positively impacts the retinal microcirculation and that retinal vascular analysis may be a possible assessment to detect microvascular impairments in children with an increased risk of future cardiovascular disease.

Physical activity and myopia in Danish children-The CHAMPS Eye Study

PURPOSE

To determine associations between physical activity (PA) and myopia in Danish school children and investigate the prevalence of myopia.

METHODS

This is a prospective study with longitudinal data on PA in a Danish child cohort. Physical activity (PA) was measured objectively by repeated ActiGraph accelerometer measurement four times with different intervals (1-2.5 years) at the mean ages 9.7, 11.0, 12.9 and 15.4 years. Mean intensity of PA was estimated as counts/minutes, and time spent in sedentary, light, moderate and vigorous PA was summed using defined cut-off points. The ophthalmologic examination was conducted at the mean age of 15.4 ± 0.7 years and included cycloplegic autorefraction and biometry.

RESULTS

A total of 307 children participated in the Childhood Health, Activity, and Motor Performance School (CHAMPS) Eye Study. The cycloplegic spherical equivalent (SE) was 0.30 ± 1.46 dioptres. The prevalence of myopia was 17.9% (SE ≤-0.5 dioptres). Mean axial length (AL) was 23.5 ± 0.9 mm. For all participants, the overall mean daily distribution of PA was 67.2% in sedentary, 25.6% in light, 4.4% in moderate and 2.9% in vigorous PA. Age- and sex-adjusted linear regression showed no association between PA and SE or AL. In a prospective slope analysis, there was no association between accumulated PA during the 7 years and AL or SE.

CONCLUSION

The prevalence of myopia among Danish children was 17.9%. By logistic regression and slope analysis, we found no association between PA and myopia, in this first of its kind study based on objective and repeated PA data.

Noninvasive Retinal Markers in Diabetic Retinopathy: Advancing from Bench towards Bedside

The retinal vascular system is the only part of the human body available for direct, in vivo inspection. Noninvasive retinal markers are important to identity patients in risk of sight-threatening diabetic retinopathy. Studies have correlated structural features like retinal vascular caliber and fractals with micro- and macrovascular dysfunction in diabetes. Likewise, the retinal metabolism can be evaluated by retinal oximetry, and higher retinal venular oxygen saturation has been demonstrated in patients with diabetic retinopathy. So far, most studies have been cross-sectional, but these can only disclose associations and are not able to separate cause from effect or to establish the predictive value of retinal vascular dysfunction with respect to long-term complications. Likewise, retinal markers have not been investigated as markers of treatment outcome in patients with proliferative diabetic retinopathy and diabetic macular edema. The Department of Ophthalmology at Odense University Hospital, Denmark, has a strong tradition of studying the retinal microvasculature in diabetic retinopathy. In the present paper, we demonstrate the importance of the retinal vasculature not only as predictors of long-term microvasculopathy but also as markers of treatment outcome in sight-threatening diabetic retinopathy in well-established population-based cohorts of patients with diabetes.

Virtual vitreoretinal surgery: validation of a training programme

PURPOSE

To test the validity of the eyesi surgical simulator as an assessment tool in a virtual reality vitreoretinal training programme.

METHODS

In collaboration with an experienced vitreoretinal surgeon, a virtual vitreoretinal training programme was composed on the eyesi surgical simulator, software version 2.9.2 (VRmagic GmbH, Manheim, Germany). It was completed twice by three groups: 20 medical students, ten residents of ophthalmology and five trained vitreoretinal surgeons. The programme contained six training modules: navigation level 2 (Nav2), forceps training level 5 (ForT5), bimanual training level 3 (BimT3), laser coagulation level 3 (LasC3), posterior hyaloid level 3 (PostH3) and internal limiting membrane peeling level 3 (ILMP3). The scores in each module were assessed from two to five different factors (tissue treatment, efficiency, target achievement, instrument handling and microscope handling), and it was possible to achieve 100 points in each module.

RESULTS

At the final training session, the highest overall median score was found for the vitreoretinal surgeons (vitreoretinal surgeons: 434 points, residents: 394.5 points, medical students: 272.5 points, p < 0.01). This was also found in four of the six modules. These were Nav2 (p = 0.03), BimT3 (p < 0.01), PostH3 (p < 0.01) and ILMP3 (p < 0.01). On the other hand, the three groups did not differ regarding ForT5 (p = 0.16) or LasC3 (p = 0.75).

CONCLUSIONS

We developed a training programme with validity for the eyesi surgical simulator as an assessment tool for overall score and for four of six vitreoretinal modules. These findings could potentially make the programme a useful tool in the training of future vitreoretinal surgeons.

Fixation stability and implication for multifocal electroretinography in patients with neovascular age-related macular degeneration after anti-VEGF treatment

PURPOSE

To quantify fixation stability in patients with neovascular age-related macular degeneration (nAMD) at baseline, 3 and 6 months after anti-vascular endothelial growth factor (anti-VEGF) treatment and furthermore asses the implications of an unsteady fixation for multifocal electroretinography (mfERG) measurements.

METHODS

Fifty eyes of 50 nAMD patients receiving intravitreal anti-VEGF treatment with either bevacizumab or ranibizumab and eight eyes of eight control subjects were included. Fixation stability measurements were performed with the Eye-Link eyetracking system and the retinal area in degrees² (deg²) containing the 68 % most frequently used fixation points (RAF68) was calculated. MfERG P1 amplitude and implicit time were analyzed in six concentric rings and as a summed response. Patients were examined at baseline, 3 and 6 months. Four different mfERG recordings were performed for the control subjects to mimic an involuntary unstable fixation: normal central fixation, 2.4°, 4.8°, and 7.1° fixation instability.

RESULTS

For control subjects, a fixation instability of 2.4° (corresponding to the central hexagon) did not reduce mfERG ring amplitudes significantly, whereas 4.8° and 7.1° fixation instability reduced the amplitudes significantly in rings 1 and 2 (p < 0.001) as well as in the peripheral rings in the 7.1° instability condition (p < 0.001). Fixation stability improved non-significantly for patients at 3 and 6 months. The size of the retinal area of fixation was at baseline, 3 and 6 months negatively correlated to visual acuity (VA) (r_baseline = -0.65, r_{3 months} = -0.60, and r_{6 months} = -0.66 respectively, p < 0.001) and mfERG amplitudes of the three innermost rings (r_baseline = -0.29, p = 0.042, r_3 months = -0.43, p = 0.003 and r_6 months = -0.31, p = 0.042). The VA cutoff for a fixation area less than 5 deg² (approximately the central hexagon) was 65, 77, and 68 ETDRS letters (corresponding a maximal Snellen equivalent of 0.31) at baseline, 3 and 6 months, respectively.

CONCLUSIONS

MfERG amplitudes in recordings of nAMD patients are at substantial risk of being reduced due to poor fixation as a large number of patients may use a fixation area of more than 5 deg². Fixation monitoring during recording as well as interpretation of results should be performed with care, especially in patients with poor visual acuity.

[Sudden visual loss after long-lasting spinal surgery]

Visual loss after anaesthesia and surgery is a rare, unexpected and devastating complication which particularly occurs after long-lasting spinal and cardiac surgery. There are several suggested risk factors but most of them are unsupported. Usually both eyes are involved and there is a painless profound loss of vision. The pupils are not reacting to light and at the beginning normal fundoscopy is found. The prognosis is very bad and no documented treatment is known. In this case report a 61-year-old man experienced complete loss of vision after complicated spinal surgery.

Past and present of corneal refractive surgery: a retrospective study of long-term results after photorefractive keratectomy and a prospective study of refractive lenticule extraction

Surgical correction of refractive errors is becoming increasingly popular. In the 1990s, the excimer laser revolutionized the field of corneal refractive surgery with PRK and LASIK, and lately refractive lenticule extraction (ReLEx) of intracorneal tissue, using only a femtosecond laser, has become possible. Two new procedures were developed, ReLEx flex (FLEX) and ReLEx smile (SMILE). Until this thesis, only a few long-term studies of PRK with a relatively limited number of patients had been published; therefore, this thesis intended to retrospectively evaluate long-term outcomes after PRK for all degrees of myopia for a large number of patients. Furthermore, a prospective contralateral eye study comparing FLEX and SMILE, when treating high to moderate degrees of myopia, had not been performed prior to this study. This was the second aim of this thesis. In the first study, results from 160 PRK patients (289 eyes) were presented. Preoperative spherical equivalent ranged from -1.25 to -20.25 D, with 78% having low myopia (<-6 D). Average follow-up time was 16 years (range 13-19 years), making this the longest published follow-up study on PRK patients. Outcomes from eyes with low myopia were generally superior to outcomes from eyes with high myopia, at final follow-up. Seventy-two percent were within ± 1.00 D of target refraction, as compared to 47% of eyes with high myopia. However, results from a subgroup of unilateral treated PRK patients indicated that refraction at final follow-up was affected by myopic progression. Fifty percent of eyes with low myopia had uncorrected 20/20 distance visual acuity or better, as compared to 22% of eyes with high myopia. Haze did not occur if attempted corrections were <-4 D, and only eyes with high myopia lost two lines or more of CDVA (corrected distance visual acuity). Eighty-one per cent were satisfied or very satisfied with their surgery.

CONCLUSION

The results support the continued use of the excimer laser for corneal surface ablation as a treatment option for correction of low degrees of myopia, and as the treatment of choice for subgroups of refractive patients (thin corneas, etc.). The results also highlight that treatment of higher degrees of myopia with standard PRK should only be done today under special circumstances, due to low refractive predictability, and high risk of corneal haze. Technological advances since then should be taken into account when comparing these results with contemporary techniques. In the second study, 35 patients were randomized to receive FLEX in one eye and SMILE in the other. Preoperative spherical equivalent refraction ranged from -6 to -10 D with low degrees of astigmatism. A total of 34 patients completed the 6 month follow-up period. Refractive and visual outcomes were very similar for the two methods, as well as tear film measurements and changes in corneal biomechanics. Ninety-seven percent were within ± 1.00 D of target refraction, no eyes lost two lines or more of CDVA, and contrast sensitivity was unaffected after both procedures. The changes in higher-order aberrations were also very similar. There were also no differences in tear film parameters 6 months after surgery, although less postoperative foreign body sensation was reported within the first week after surgery in SMILE eyes. Corneal sublayer pachymetry measurements demonstrated equally increased epithelial thickness 6 months after surgery. Contrary to expectations, it was not possible to measure the theoretical biomechanical advantages of a small corneal incision in SMILE as compared to a corneal flap in FLEX. The main differences between FLEX and SMILE were found when the corneal nerves and intraoperative complications were evaluated. Thus, corneal sensitivity was better preserved and corneal nerve morphology was less affected after SMILE, but intraoperative complications occurred more frequently, although without visual sequela. Finally, 97% were satisfied or very satisfied with both their surgeries.

CONCLUSION

The results support the continued use of both FLEX and SMILE for treatment of up to high degrees of myopia. Overall, refractive and visual results for both procedures were good and similar, but from a biological point of view, the less invasive SMILE technique is more attractive, as demonstrated in this study, despite being slightly more surgically demanding than FLEX.