Biomechanical corneal changes induced by different flap thickness created by femtosecond laser

Corneal stress-strain index in myopic Indian population

AIM

The purpose is to study the corneal stress-strain index (SSI) in myopic refractive error among Indian subjects.

METHODS

A retrospective study where young myopic subjects aged between 11 and 35 years who had undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2021 were enrolled. Subjects with central corneal thickness (CCT) <500 μ, intraocular pressure (IOP) >21 mmHg, history of any systemic and ocular disease or any previous ocular surgery, high astigmatism, corneal disease such as keratoconus were excluded. Subjects with missing data or having poor quality scan were excluded. Corneal biomechanical properties and corneal SSI were assessed using Corvis ST. For statistical purposes, eyes were divided into four different groups and were analyzed using one-way ANOVA.

RESULTS

Nine hundred and sixty-six myopic eyes with mean ± standard deviation age, IOP, and CCT of 26.89 ± 4.92 years, 16.94 ± 2.00 mmHg, and 540.18 ± 25.23 microns, respectively, were included. There were 311, 388, 172, and 95 eyes that were low, moderate, severe, and extreme myopic. Deformation amplitude ratio at 1 mm and 2 mm were similar across different myopic groups. A significant increase in max inverse radius, ambrosia relational thickness, biomechanically corrected IOP, integrated radius was noted with an increase in myopic refractive error. Corvis biomechanical index, corneal SSI was found to be decreased significantly with an increase in myopic refractive error. We noted a significant positive association between myopic refractive error and SSI (P < 0.001).

CONCLUSION

Corneal SSI was found to be reduced in extreme myopic eyes.

Age-related variations in corneal stress-strain index in the Indian population

Purpose

To report age-related variations in corneal stress-strain index (SSI) in healthy Indians.

Methods

It was a retrospective study where healthy Indian individuals aged between 11 and 70 years who had undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2021 were enrolled. Composite corneal biomechanical parameters and corneal SSI were abstracted from Corvis ST and compared across different age groups using one-way analysis of variance (ANOVA). Also, Pearson's correlation was used to evaluate the association between age and SSI.

Results

Nine hundred and thirty-six eyes of 936 patients with ages between 11 and 77 years with mean ± SD intraocular pressure (IOP) and pachymetry of 16.52 ± 2.10 mmHg and 541.13 ± 26.39 μs, respectively. Composite corneal biomechanical parameters such as deformation amplitude ratio max at 1 mm (P < 0.001) and 2 mm (P < 0.001), biomechanically corrected IOP (P = 0.004), stiffness parameter at A1 (P < 0.001, Corvis biomechanical index (P < 0.018), and SSI (P < 0.001) were found to be significantly different as a function of age group. We noted a statistically significant positive association of SSI with age (P < 0.001), spherical equivalent refractive error (P < 0.001), and IOP (P < 0.001) and a significant negative association with anterior corneal astigmatism (P < 0.001) and Anterior chamber depth (ACD) (P < 0.001). Also, SSI was positively associated with SPA1 and bIOP, whereas negatively associated with integrated radius, max inverse radius, and Max Deformation amplitude (DA) ratio at 1 mm and 2 mm.

Conclusion

We noted a positive association of corneal SSI with age in normal healthy Indian eyes. This information could be helpful for future corneal biomechanical research.

Clinical Evaluation of Corneal Biomechanics following Laser Refractive Surgery in Myopic Eyes: A Review of the Literature

The role of corneal biomechanics in laser vision correction (LVC) is currently being raised in the assessment of postoperative corneal ectasia risk. The aim of the paper was to evaluate the changes in corneal biomechanics after LVC procedures based on a systematic review of current studies. The results of a search of the literature in the PubMed, Science Direct, Google Scholar, and Web of Science databases were selected for final consideration according to the PRISMA 2020 flow diagram. Included in our review were 17 prospective clinical studies, with at least 6 months of follow-up time. Corneal biomechanical properties were assessed by Ocular Response Analyzer (ORA), or Corvis ST. The results of the study revealed the highest corneal biomechanics reduction after laser in situ keratomileusis (LASIK) followed by small incision lenticule extraction (SMILE) and surface procedures, such as photorefractive keratectomy (PRK) or laser-assisted sub-epithelial keratectomy (LASEK). In SMILE procedure treatment planning, the use of thicker caps preserves the corneal biomechanics. Similarly, reduction of flap thickness in LASIK surgery maintains the corneal biomechanical strength. Future prospective clinical trials with standardization of the study groups and surgical parameters are needed to confirm the results of the current review.

Short term changes in corneal stress-strain index and other corneal biomechanical parameters post-laser in situ keratomileusis

Purpose:

To report the short-term changes in a corneal stress-strain index (SSI) and other corneal biomechanical parameters post-laser in situ keratomileusis (LASIK) surgery.

Methods:

A retrospective study was conducted at a tertiary eye care center wherein patients who had undergone LASIK (microkeratome blade and femtosecond bladeless LASIK) between July and December 2019 were enrolled. Patients of age group 20–40 years, best-corrected visual acuity of 20/20, intraocular pressure (IOP) <22 mmHg, pre-LASIK pachymetry >500 microns, and corneal astigmatism ≤3.00 D were included. Subjects with a prior history of refractive surgery, any other ocular or systemic disease, poor-quality scans, intraoperative complications, and missing data were excluded. Corneal biomechanical properties including SSI were analyzed using Corvis ST and compared using the Paired T-test for each group separately at pre-LASIK, and 1-month post-operatively.

Results:

Overall, 202 eyes were reviewed, and 79 eyes fulfilled the inclusion criteria. Forty-three and 36 eyes had undergone Microkeratome Blade LASIK (Group I) and Femto LASIK (Group II), respectively. Overall, 29 and 26 corneal biomechanical parameters out of 33 changed significantly post-Microkeratome Blade LASIK and Femto LASIK, respectively. Statistically significant changes were noted in all the parameters at A1, maximum and Vinciguerra screening parameters (P < 0.001), however, no changes were noted in SSI in both the groups when compared with the pre-surgery data.

Conclusion:

Though the reduction in SSI was not statistically significant, other biomechanical parameters showed significant biomechanical changes pre- and post-LASIK surgeries in both the groups. However, a long-term study with a larger sample size would be required to understand the changes and stability in SSI post-refractive surgery.

Eighty-micron flap femtosecond-assisted LASIK for the correction of myopia and myopic astigmatism

PURPOSE

To evaluate the safety and efficacy of 80-μm flap femtosecond laser-assisted LASIK and the early clinical and refractive outcomes in the correction of myopia and myopic astigmatism.

SETTING

Private practice, outpatient.

DESIGN

Prospective study.

METHODS

Patients who underwent femtosecond-assisted LASIK between February and April 2018 were included. Inclusion criteria were myopia from -1.00 to -8.00 diopters (D) and astigmatism up to -3.00 D and no previous surgeries. All patients were tested preoperatively and on day 1 and month 3 for uncorrected distance visual acuity (UDVA), manifest refraction, corrected distance visual acuity (CDVA), intraocular pressure (IOP), slitlamp and dilated fundus examination, Schirmer I test with anesthesia, and ocular surface disease index questionnaire. The FEMTO LDV Z8 was used for flap construction and the Wavelight Allegretto 400 excimer for refractive treatment. Flap thickness was measured at week 1 with anterior segment optical coherence tomography (AS-OCT).

RESULTS

Eighty-two eyes were included. Logarithm of the minimum angle of resolution UDVA was 1.28 ± 0.53 preoperatively, 0.02 ± 0.05 at day 1, and 0.14 ± 0.127 at month 3. There was no loss of CDVA lines. The mean flap thickness measured at 1 week with AS-OCT was 73 ± 6.7 µm.

CONCLUSIONS

The use of ultrathin flaps, just below Bowman's layer, with the Ziemer LDV Z8 femtosecond laser was possible, safe, reliable, and reproducible. Eighty-micron flaps allowed for excellent vision on 1 day post-LASIK and might be a good alternative to maintain an appropriate percentage of tissue altered, especially when attempting greater corrections or larger treatment zones.

Study on change in corneal biomechanics and effect of percent tissue altered in myopic laser-assisted in situ keratomileusis

Purpose

To evaluate corneal biomechanical changes and their correlation with the percentage of tissue altered (PTA) in myopic femtosecond (FS)-flap LASIK.

Methods

Prospective longitudinal observational study of 80 eyes of FS LASIK. Demographic details, LASIK parameters, preoperative and postoperative (day 1, month 1, 3, and 6), UCVA, BCVA, refraction, corneal topography, corneal hysteresis (CH), and a corneal resistance factor (CRF) were noted. Change in CH and CRF and its correlation with PTA were analyzed. Data were analyzed in three subgroups [subgroup 1: PTA 23 to <27%; subgroup 2: 27 to <33%; subgroup 3: 33 to <40%].

Results

FS LASIK for MRSE -3.5D ± 1.6D with mean PTA of 31.6 ± 4.4% (range 23.8-39.8%), showed statistically significant decrease in CH and CRF. Mean CH decreased from a preoperative value of 10.4 ± 1.9 to 8.1 ± 1.1; mean CRF from 10.5 ± 1.6 to 7.5 ± 1.3 at 6-months postoperative period, respectively. Mean preoperative CH decreased by 25%, 24%, 23%, and 21% and mean preoperative CRF decreased by 34%, 28%, 28%, and 28% at postoperative day 1, month 1, 3, and 6 follow-ups. Mean CH and CRF showed a significant negative correlation with PTA (CH: r = - 0.33 [P = <0.0001], CRF: r = -0.34 [P = <0.001]. Subgroup analysis noted greater decrease in CRF and CH in eyes with higher PTA (subgroup 3).

Conclusion

Myopic FS LASIK causes a decrease in corneal biomechanics with a significant negative correlation with PTA indicating a greater decrease in corneal biomechanics with higher PTA.

Age-related variation in corneal biomechanical parameters in healthy Indians

Purpose:

To report age-related variations in corneal biomechanical parameters in healthy Indians.

Methods:

A retrospective study where healthy Indian individuals aged between 5 and 70 years having undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2018 and having best corrected visual acuity of 20/20 were enrolled. Subjects with central corneal thickness <500 microns, intra-ocular pressure (IOP) ≥ 22 mmHg, refractive error ≥ 6.00D, history of any systemic and ocular disease, previous ocular surgery, poor scans quality, and subjects with any missing data were also excluded. Corneal biomechanical parameters were noted and compared across different age groups.

Results:

Total of 3125 eyes had undergone the Corvis ST analysis. After applying exclusion criteria, 718 right eyes of 718 patients were included for the analysis and were further divided into different age groups as per each decade (sample size), such as 5-10 (37), 11-20 (113), 21-30 (396), 31-40 (116), 41-50 (39), 50 and above (17). All the subjects were matched for IOP and central corneal thickness (p > 0.05). A total of 19 out of 26 corneal biomechanical parameters were significantly different across age groups (p < 0.05). Vinciguerra screening parameters, such as deformation amplitude ratio max, biomechanically corrected IOP, and stiffness parameter A1 were significantly different across different age groups (p < 0.05).

Conclusion:

Corneal biomechanical parameters are affected by age as cornea becomes progressively stiffer. The information reported here would serve as a reference for future corneal biomechanical researches and would help in differentiating the abnormal eyes from normal healthy eyes.

Laser-assisted in-situ keratomileusis (LASIK) with a mechanical microkeratome compared to LASIK with a femtosecond laser for LASIK in adults with myopia or myopic astigmatism

BACKGROUND

Laser-assisted in-situ keratomileusis (LASIK) is a surgical procedure that corrects refractive errors. This technique creates a flap of the outermost parts of the cornea (epithelium, bowman layer, and anterior stroma) to expose the middle part of the cornea (stromal bed) and reshape it with excimer laser using photoablation. The flaps can be created by a mechanical microkeratome or a femtosecond laser.

OBJECTIVES

To compare the effectiveness and safety of mechanical microkeratome versus femtosecond laser in LASIK for adults with myopia.

SEARCH METHODS

We searched CENTRAL (which contains the Cochrane Eyes and Vision Trials Register) (2019, Issue 2); Ovid MEDLINE; Embase; PubMed; LILACS; ClinicalTrials.gov and the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP). We used no date or language restrictions. We searched the reference lists of included trials. We searched the electronic databases on 22 February 2019.

SELECTION CRITERIA

We included randomized controlled trials (RCTs) of LASIK with a mechanical microkeratome compared to a femtosecond laser in people aged 18 years or older with more than 0.5 diopters of myopia or myopic astigmatism.

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures expected by Cochrane.

MAIN RESULTS

We included 16 records from 11 trials enrolling 943 adults (1691 eyes) with spherical or spherocylindrical myopia, who were suitable candidates for LASIK. Five hundred and forty-seven participants (824 eyes) received LASIK with a mechanical microkeratome and 588 participants (867 eyes) with a femtosecond laser. Each trial included between nine and 360 participants. In six trials, the same participants received both interventions. Overall, the trials were at an uncertain risk of bias for most domains. At 12 months, data from one trial (42 eyes) indicates no difference in the mean uncorrected visual acuity (logMAR scale) between LASIK with a mechanical microkeratome and LASIK with a femtosecond laser (mean difference (MD) -0.01, 95% confidence interval (CI) -0.06 to 0.04; low-certainty evidence). Similar findings were observed at 12 months after surgery, regarding participants achieving 0.5 diopters within target refraction (risk ratio (RR) 0.97, 95% CI 0.85 to 1.11; 1 trial, 79 eyes; low-certainty evidence) as well as mean spherical equivalent of the refractive error 12 months after surgery (MD 0.09, 95% CI -0.01 to 0.19; 3 trials, 168 eyes [92 participants]; low-certainty evidence). Based on data from three trials (134 eyes, 66 participants), mechanical microkeratome was associated with lower risk of diffuse lamellar keratitis compared with femtosecond laser (RR 0.27, 95% CI 0.10 to 0.78; low-certainty evidence). Thus, diffuse lamellar keratitis was a more common adverse event with femtosecond laser than with mechanical microkeratome, decreasing from an assumed rate of 209 per 1000 people in the femtosecond laser group to 56 per 1000 people in the mechanical microkeratome group. Data from one trial (183 eyes, 183 participants) indicates that dry eye as an adverse event may be more common with mechanical microkeratome than with femtosecond laser, increasing from an assumed rate of 80 per 1000 people in the femtosecond laser group to 457 per 1000 people in the mechanical microkeratome group (RR 5.74, 95% CI 2.92 to 11.29; low-certainty evidence). There was no evidence of a difference between the two groups for corneal haze (RR 0.33, 95% CI 0.01 to 7.96; 1 trial, 43 eyes) and epithelial ingrowth (RR 1.04, 95% CI 0.11 to 9.42; 2 trials, 102 eyes [50 participants]). The certainty of evidence for both outcomes was very low.

AUTHORS' CONCLUSIONS

Regarding the visual acuity outcomes, there may be no difference between LASIK with mechanical microkeratome and LASIK with femtosecond laser. Dry eye and diffuse lamellar keratitis are likely adverse events with mechanical microkeratome and femtosecond laser, respectively. The evidence is uncertain regarding corneal haze and epithelial ingrowth as adverse events of each intervention. The limited number of outcomes reported in the included trials, some with potentially significant risk of bias, makes it difficult to draw a firm conclusion regarding the effectiveness and safety of the interventions investigated in this review.

Effects of the LASIK flap thickness on corneal biomechanical behavior: a finite element analysis

Background

It is well known that the biomechanical properties change after LASIK refractive surgery. One reason is the impact of flap creation on the residual stroma. The results have revealed that the change is closely related with the flap thickness in several studies. However, the quantitative relationships between the distributions of displacement and stress on the corneal surface and flap thickness have not been studied. The aim of the study was to quantify evaluate the biomechanical change caused by the LASIK flap.

Methods

By building a finite element model of the cornea, the displacement, the stress and the strain on the corneal surface were analyzed.

Results

The results showed that the corneal flap could obviously cause the deformation of the anterior corneal surface. For example, the displacement of the corneal vertex achieved 15 μm more than that without corneal flap, when the thickness of corneal flap was 120 μm thick. This displacement was enough to cause the change of aberrations in the human eyes. In the central part of the cornea, the stress on the anterior corneal surface increased with flap thickness. But the change in the stress on the posterior corneal surface was significantly less than that on the anterior surface. In addition, the stress in the central part of the anterior corneal surface increased significantly as the intra-ocular pressure (IOP) increase. Furthermore the increase of IOP had a clearly less effect on stress distribution at the edge of the cornea. Distributions of strain on the corneal surface were similar to those of stress.

Conclusions

The changes in the biomechanical properties of cornea after refractive surgery should not be ignored.

The effect of flap thickness on corneal biomechanics after myopic laser in situ keratomileusis using the M-2 microkeratome

Purpose

The purpose of this study was to determine the effect of flap thickness on corneal biomechanics after myopic laser in situ keratomileusis (LASIK).

Methods

This is a prospective controlled non-randomized, institutional study. Patients underwent either epi-LASIK with mitomycin (advanced surface ablation [ASA]), thin-flap LASIK (90 µm head), or thick-flap LASIK (130 µm head). In ASA, the Moria Epi-K hydroseparator was used. LASIK flaps were created using the Moria M-2 mechanical microkeratome. The corneal hysteresis (CH) and corneal resistance factor (CRF) were measured preoperatively and 3 months after surgery, using the Ocular Response Analyzer^®.

Results

Ten patients (19 eyes) underwent ASA, 11 patients (16 eyes) underwent thin-flap LASIK, and 11 patients (16 eyes) underwent thick-flap LASIK. The mean preoperative CH was 10.47±0.88, 10.52±1.4, and 11.28±1.4 mmHg (p=0.043), respectively, decreasing after surgery by 1.75±1.02, 1.66±1.00, and 2.62±1.03 mmHg (p=0.017). The mean reduction of CH per micron of central corneal ablation was 0.031, 0.023, and 0.049 mmHg/µm (p=0.005). Mean preoperative CRF was 10.11±1.28, 10.34±1.87, and 10.62±1.76 mmHg (p=0.66), decreasing after surgery by 2.33±1.35, 2.77±1.03, and 2.92±1.10 mmHg (p=0.308). The mean reduction of CRF per micron of central corneal ablation was 0.039, 0.040, and 0.051 mmHg/µm (p=0.112).

Conclusion

Thick-flap LASIK caused a greater reduction of CH and CRF than thin-flap LASIK and ASA, although this was statistically significant only for CH. ASA and thin-flap LASIK were found to be biomechanically similar.

Considerations for refractive surgery in the glaucoma patient

PURPOSE OF REVIEW

Given the popularity of keratorefractive surgery, and an aging populous of patients who have undergone these procedures, there is an increasing need for updated management protocols. This is particularly relevant for patients with chronic progressive diseases such as glaucoma, due to the variety of related diagnostic and management challenges inherent to these diseases. Here, we will review the current literature to provide an update on the management of patients with glaucoma who are undergoing, or have had laser ablative refractive surgery. Preoperative testing and eligibility considerations, intraoperative factors, and postoperative observation and follow-up will be discussed.

RECENT FINDINGS

Intraoperative intraocular pressure (IOP) rise during flap creation is associated with low risk of acute complications, and furthermore do not appear to have significant long term effects. Modern technologies have improved our ability to determine accurate IOP after refractive surgery despite postoperative changes in corneal architecture. Furthermore, advances in structural imaging allow for earlier detection of even subtle glaucomatous nerve damage.

SUMMARY

Although glaucoma remains a relative contraindication to refractive surgery, it is a safe procedure for many patients with appropriate perioperative management and follow-up. Advancements in diagnostic modalities have allowed for earlier detection of glaucomatous disease, and subsequent earlier intervention when appropriate. Standardized diagnostic algorithms and rigorous perioperative assessment are critical to safe management of glaucoma patients undergoing refractive corneal surgery.

Laser-assisted subepithelial keratectomy (LASEK) versus laser-assisted in-situ keratomileusis (LASIK) for correcting myopia

BACKGROUND

Near-sightedness, or myopia, is a condition in which light rays entering the eye along the visual axis focus in front of the retina, resulting in blurred vision. Myopia can be treated with spectacles, contact lenses, or refractive surgery. Options for refractive surgery include laser-assisted subepithelial keratectomy (LASEK) and laser-assisted in-situ keratomileusis (LASIK). Both procedures utilize a laser to shape the corneal tissue (front of the eye) to correct refractive error, and both create flaps before laser treatment of corneal stromal tissue. Whereas the flap in LASEK is more superficial and epithelial, in LASIK it is thicker and also includes some anterior stromal tissue. LASEK is considered a surface ablation procedure, much like its predecessor, photorefractive keratectomy (PRK). LASEK was developed as an alternative to PRK to address the issue of pain associated with epithelial debridement used for PRK. Assessing the relative benefits and risks/side effects of LASEK and LASIK warrants a systematic review.

OBJECTIVES

To assess the effects of LASEK versus LASIK for correcting myopia.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL), which contains the Cochrane Eyes and Vision Trials Register (2016, Issue 10); MEDLINE Ovid (1946 to 24 October 2016); Embase.com (1947 to 24 October 2016); PubMed (1948 to 24 October 2016); LILACS (Latin American and Caribbean Health Sciences Literature Database; 1982 to 24 October 2016); the metaRegister of Controlled Trials (mRCT) (www.controlled-trials.com), last searched 20 June 2014; ClinicalTrials.gov (www.clinicaltrials.gov); searched 24 October 2016; and the WHO International Clinical Trials Registry Platform (ICTRP) (www.who.int/ictrp/search/en); searched 24 October 2016. We did not use any date or language restrictions in the electronic searches for trials.

SELECTION CRITERIA

We considered only randomized controlled trials (RCTs) for the purposes of this review. Eligible RCTs were those in which myopic participants were assigned randomly to receive either LASEK or LASIK in one or both eyes. We also included paired-eye studies in which investigators randomly selected which of the participant's eyes would receive LASEK or LASIK and assigned the other eye to the other procedure. Participants were men or women between the ages of 18 and 60 years with myopia up to 12 diopters (D) and/or myopic astigmatism of severity up to 3 D, who did not have a history of prior refractive surgery.

DATA COLLECTION AND ANALYSIS

Two review authors independently screened all reports and assessed the risk of bias in trials included in this review. We extracted data and summarized findings using risk ratios (RRs) for dichotomous outcomes and mean differences (MDs) for continuous outcomes. In the absence of clinical and methodological heterogeneity across trials, we used a random-effects model to calculate summary effect estimates. We used a fixed-effect model when including fewer than three trials in a meta-analysis. When clinical, methodological, or statistical heterogeneity was observed across trials, we reported our findings in a narrative synthesis.

MAIN RESULTS

We identified four eligible trials with 538 eyes of 392 participants for the review, but only three trials (154 participants) provided outcome data for analysis. We found no ongoing trials. Two of four trials were from China, one trial was from Turkey, and the location of one trial was not reported. The risk of bias for most domains was unclear due to poor reporting of trial methods; no trial had a protocol or trial registry record. Three trials enrolled participants with mild to moderate myopia (less than -6.50 D); one trial included only participants with severe myopia (more than -6.00 D).The evidence showed uncertainty in whether there is a difference between LASEK and LASIK in uncorrected visual acuity (UCVA) at 12 months, the primary outcome in our review. The RR and 95% confidence interval (CI) at 12 months after surgery was 0.96 (95% CI 0.82 to 1.13) for UCVA of 20/20 or better and 0.90 (95% CI 0.67 to 1.21) for UCVA of 20/40 or better based on data from one trial with 57 eyes (very low-certainty evidence). People receiving LASEK were less likely to achieve a refractive error within 0.5 diopters of the target at 12 months follow-up (RR 0.69, 95% CI 0.48 to 0.99; 57 eyes; very low-certainty evidence). One trial reported mild corneal haze at six months in one eye in the LASEK group and none in the LASIK group (RR 2.11, 95% CI 0.57 to 7.82; 76 eyes; very low-certainty evidence). None of the included trials reported postoperative pain score or loss of visual acuity, spherical equivalent of the refractive error, or quality of life at 12 months.Refractive regression, an adverse event, was reported only in the LASEK group (8 of 37 eyes) compared with none of 39 eyes in the LASIK group in one trial (low-certainty evidence). Other adverse events, such as corneal flap striae and refractive over-correction, were reported only in the LASIK group (5 of 39 eyes) compared with none of 37 eyes in the LASEK group in one trial (low-certainty evidence).

AUTHORS' CONCLUSIONS

Overall, from the available RCTs, there is uncertainty in how LASEK compares with LASIK in achieving better refractive and visual results in mildly to moderately myopic participants. Large, well-designed RCTs would be required to estimate the magnitude of any difference in efficacy or adverse effects between LASEK and LASIK for treating myopia or myopic astigmatism.

Corneal biomechanics after small-incision lenticule extraction versus Q-value-guided femtosecond laser-assisted in situ keratomileusis

Purpose

The aim of this study was to compare the changes in corneal biomechanical properties following small-incision lenticule extraction (SMILE) versus Q-value–guided femtosecond laser-assisted in situ keratomileusis (Q-FS-LASIK).

Methods

In this prospective comparative study, patients with a sphere plus cylinder measurement of less than −10.00 D and cylinder measurement of less than −5.00 D were included in the study. A total of 160 patients (160 eyes) with myopia and myopic astigmatism were divided into the two groups, with 80 patients (80 eyes) allocated to SMILE and 80 patients (80 eyes) allocated to Q-FS-LASIK. Corneal hysteresis (CH) and the corneal resistance factor (CRF) were quantitatively assessed using the Ocular Response Analyzer (ORA) preoperatively and at 1 day, 2 weeks, and 1 and 3 months postoperatively.

Results

Both types of surgery were associated with statistically significant decreases in CH and the CRF at postoperative day 1 (both P < 0.01). In both groups, the decreases subsequently stabilized with no further deteriorations compared to postoperative day 1 (P > 0.05). Both groups showed similar biomechanical changes at each time point (all P > 0.05).

Conclusions

Both SMILE and Q-FS-LASIK resulted in a decrease in CH and the CRF at postoperative 1 day, with the decreases stabilizing after this point. There were no significant differences between the short term effects of SMILE and Q-FS-LASIK on corneal biomechanical properties.

Corneal biomechanical changes in eyes with small incision lenticule extraction and laser assisted in situ keratomileusis

Background

Evaluating the corneal biomechanical changes using the Ocular Response Analyzer and the Corvis ST in eyes with incision lenticule extraction (SMILE) and laser assisted in situ keratomileusis (LASIK).

Methods

This is a retrospective study that included 50 eyes equally divided into two groups. The first group included eyes that underwent SMILE procedure using VisuMax® 500 kHz laser system (Carl Zeiss Meditec, Jena, Germany) and the second group included eyes that underwent LASIK procedure using the EX500 Allegretto excimer laser platform (Wavelight GmbH, Erlangen, Germany). The Ocular Response Analyzer (ORA) and the Corvis ST (CST) measured the corneal biomechanical changes before and after the procedures.

Results

The ORA showed significant decrease of corneal hysteresis (CH) and corneal resistance factor (CRF) in both groups postoperatively. The percentage of change of CH and CRF were found to be significantly higher in group II. There was no significant difference in the IOP with the ORA and the CST pre and postoperatively in either group. Using CST, the deformation amplitude and HC peak distances increased significantly in both groups. It was also noted that the mean percentage of change of the deformation amplitude was nearly five times higher in group II than group I.

Conclusion

Both LASIK and SMILE substantially decreased the corneal biomechanical properties with greater reduction in the LASIK group.

Comparison of performances of femtosecond laser and microkeratome for thin-flap laser in situ keratomileusis

OBJECTIVE

To compare the clinical outcomes of femtosecond laser (FS) and microkeratome (MK) for thin-flap in situ keratomileusis (LASIK).

MATERIALS AND METHODS

Data from patients with moderate to high myopia (spherical equivalent, >-4 diopters [D]) who underwent thin-flap LASIK using FS (199 eyes/110 patients) and MK (157 eyes/86 patients) were analyzed in this retrospective study. Preoperative and postoperative data were recorded from day 1 and months 1 and 3, postoperatively. Visual and refractive outcomes were compared for efficacy, safety, predictability, stability, and the efficacy and safety indices.

RESULTS

Three months postoperatively, more eyes in the FS group had an uncorrected distance visual acuity (UCVA) of 20/40 or better compared to MK group (relative risk [RR] 1.01, 95% confidence interval [CI], 0.97-1.05, P = 0.58); significantly more eyes in FS group had an UCVA of 20/20 or better (RR, 1.26, 95%CI, 1.08-1.48, P = 0.003). Intraoperative bleeding occurred in 5% and 36.7%, respectively, of FS and MK groups. There were no significant differences in the refractive predictability within spherical equivalents of 0.5 D (FS, 72%; MK, 63%) and 1.0 D (FS, 90%; MK, 86%). Both groups showed good stability 3 months, postoperatively. The efficacy index in FS group was 113.4%; that in MK group was 102.5% at 3 months postoperatively (P < 0.05). The safety indices for FS and MK groups at 3 months postoperatively were 116.4% and 108.2%, respectively (P < 0.05).

CONCLUSIONS

Both methods of thin-flap created LASIK were effective, safe, predicable, and stable. FS group had significant improvements in efficacy and safety, confirmed by the efficacy and safety indices, compared to MK group. Lasers Surg. Med. 48:596-601, 2016. © 2016 Wiley Periodicals, Inc.

Topographic Findings of the Porcine Cornea

The porcine eye is often used as an ex vivo animal model in ophthalmological research. It is well suited for investigations concerning refractive surgery; however, corneal topography data are scarce. This study investigated the corneal topography and pachymetry of the porcine eye to provide further reproducible data. We evaluated freshly enucleated porcine eyes (n = 16) by performing computerized corneal topographies (Orbscan® IIz, Bausch and Lomb, Rochester, NY, USA). We assessed the steepest and flattest keratometric powers (K1 and K2, units in diopters (D)), astigmatism (D), white-to-white (WTW) diameter (mm), thinnest point pachymetry (µm), anterior and posterior best-fit sphere (BFS) (D), refractive power of the anterior and posterior curvatures, and total refractive power of the cornea (D). The mean keratometric powers were 39.6 ± 0.89 D (K1) and 38.5 ± 0.92 D (K2), and the mean astigmatism was 1.1 ± 0.78 D. The mean WTW diameter was 13.81 ± 0.83 mm, and the mean corneal thickness was 832.6 ± 40.18 µm. The BFSs were 38.14 ± 0.73 D (anterior) and 42.56 ± 1.15 D (posterior), and the mean refractive powers were 43.27 ± 1.08 D (anterior) and -5.15 ± 0.20 D (posterior); therefore, the mean of the total refractive power was 38.16 ± 1.00 D. The topography and pachymetry of the porcine cornea showed a specific configuration differing from the human cornea. When using animal ex vivo models such as porcine corneas for experimental corneal surgery, findings such as these should be considered.

Changes in biomechanical properties of the cornea and intraocular pressure after myopic laser in situ keratomileusis using a femtosecond laser for flap creation determined using ocular response analyzer and Goldmann applanation tonometry

PURPOSE

To compare intraocular pressure (IOP) measurements before and after laser in situ keratomileusis (LASIK) using a femtosecond laser for flap creation, and to identify factors that may influence the preoperative and postoperative IOP, and the change in IOP after LASIK.

METHODS

Forty eyes from 20 patients who underwent treatment for myopia using a femtosecond laser for flap creation were enrolled in this study. The IOP and corneal biomechanical markers were prospectively measured preoperatively and 1 month after LASIK with Goldmann applanation tonometry (GAT) and the ocular response analyzer (ORA). Manifest refraction spherical equivalent, central corneal thickness, ablation depth, and mean corneal curvature (K reading) were also recorded.

RESULTS

After LASIK, there was a significant reduction in the IOP measurement with respect to the corneal-compensated intraocular pressure (IOPcc) (Δ=-0.67±2.07 mm Hg), Goldmann-correlated pressure (IOPg) (Δ=-3.92±2.19 mm Hg), and GAT (Δ=-2.6±2.51). Multiple regression analysis revealed that the corneal hysteresis (CH) and corneal resistance factor (CRF) are statistically significant predictors of IOPcc, IOPg, and GAT (P<0.000). In this analysis, the preoperative manifest refraction spherical equivalent, CH, and CRF were significant predictors of ΔIOPcc (adjusted R²=0.401) and ΔIOPg (adjusted R²=0.386). The preoperative SE and central corneal thickness significantly predicted ΔGAT (adjusted R²=0.464).

CONCLUSIONS

ORA provides a more complete measurement of IOP after LASIK with a femtosecond laser than GAT because ORA provides greater knowledge of the corneal biomechanics in terms of CH and CRF.

Consecutive laser in situ keratomileusis and accelerated corneal crosslinking in highly myopic patients: preliminary results

PURPOSE

To report the preliminary results of an evaluation of the safety and predictability of Lasik Xtra, a technique combining laser in situ keratomileusis (LASIK) and accelerated corneal crosslinking, in highly myopic patients.

METHODS

In this consecutive comparative case series, 70 consecutive eyes undergoing LASIK for correction of high myopia (-8.00 D to -19.00 D manifest refractive spherical equivalent) were prospectively recruited and treated with Lasik Xtra and compared with a retrospective consecutive control group of 64 eyes who had undergone LASIK alone for correction of high myopia. The follow-up was 3 months. Outcome measures included uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), and refraction.

RESULTS

A total of 61% of LASIK only eyes achieved UDVA of 20/25 or better, compared to 98% of Lasik Xtra eyes (p<0.001) at 3 months. A greater percentage of eyes were within ±0.50 of the intended correction in the Lasik Xtra group (88%) than in the LASIK only group (65%) at 3 months (p = 0.005). Linear regression of the scatterplot of attempted versus achieved correction reveals a coefficient of determination of 0.83 in the LASIK only group vs 0.99 in the Lasik Xtra group. A trend (p = 0.051) towards greater refractive drift in the LASIK group (-0.13 D) vs the Lasik Xtra group (-0.04 D) was observed. No adverse events were observed in either group.

CONCLUSIONS

Lasik Xtra did not reduce the refractive accuracy of the LASIK procedure. The addition of crosslinking may induce early stabilization of the cornea after LASIK, improving the predictability of refractive outcomes in highly myopic subjects.

Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: finite-element analysis

PURPOSE

To theoretically compare the corneal stress distribution of laser in situ keratomileusis (LASIK) with the stress distribution of small-incision lenticule extraction.

SETTING

Cleveland Clinic Cole Institute, Cleveland, and The Ohio State University, Columbus, Ohio, USA.

DESIGN

Computational modeling study.

METHODS

A finite-element anisotropic collagen fiber-dependent model of myopic surgery using patient-specific corneal geometry was constructed for LASIK, small-incision lenticule extraction, and a geometry analog model with unaltered material properties from preoperative but with postoperative geometry including thickness. Surgical parameters, magnitude of myopic correction, LASIK flap thickness, and lenticule depth in small-incision lenticule extraction were varied. Two sets of models, 1 with uniform and 1 with depth-dependent material properties, were constructed.

RESULTS

Stress distribution between small-incision lenticule extraction simulations and the geometry analog model were similar. In contrast, LASIK consistently reduced stress in the flap and increased stress in the residual stromal bed (RSB) compared with the geometry analog model. An increase in flap thickness or lenticule depth resulted in a greater increase in RSB stress in the LASIK model than in the small-incision lenticule extraction model.

CONCLUSIONS

Small-incision lenticule extraction may present less biomechanical risk to the residual bed of susceptible corneas than comparable corrections involving LASIK flaps. Deeper corrections in the stroma may be possible in small-incision lenticule extraction without added risk for ectasia.

FINANCIAL DISCLOSURES

Proprietary or commercial disclosures are listed after the references.

Small incision lenticule extraction (SMILE) history, fundamentals of a new refractive surgery technique and clinical outcomes

This review summarizes the current status of the small incision lenticule extraction (SMILE) procedure. Following the early work by Sekundo et al. and Shah et al., SMILE has become increasingly popular. The accuracy of the creation of the lenticule with the VisuMax femtosecond laser (Carl Zeiss Meditec) has been verified using very high-frequency (VHF) digital ultrasound and optical coherence tomography (OCT). Visual and refractive outcomes have been shown to be similar to those achieved with laser in situ keratomileusis (LASIK), notably in a large population reported by Hjortdal, Vestergaard et al. Safety in terms of the change in corrected distance visual acuity (CDVA) has also been shown to be similar to LASIK. It was expected that there would be less postoperative dry eye after SMILE compared to LASIK because the anterior stroma is disturbed only by the small incision, meaning that the anterior corneal nerves should be less affected. A number of studies have demonstrated a lower reduction and faster recovery of corneal sensation after SMILE than LASIK. Some studies have also used confocal microscopy to demonstrate a lower decrease in subbasal nerve fiber density after SMILE than LASIK. The potential biomechanical advantages of SMILE have been modeled by Reinstein et al. based on the non-linearity of tensile strength through the stroma. Studies have reported a similar change in Ocular Response Analyzer (Reichert) parameters after SMILE and LASIK, however, these have previously been shown to be unreliable as a representation of corneal biomechanics. Retreatment options after SMILE are discussed. Tissue addition applications of the SMILE procedure are also discussed including the potential for cryo-preservation of the lenticule for later reimplantation (Mohamed-Noriega, Angunawela, Lim et al.), and a new procedure referred to as endokeratophakia in which a myopic SMILE lenticule is implanted into a hyperopic patient (Pradhan et al.). Finally, studies reporting microdistortions in Bowman's layer and corneal wound healing responses are also described.

Biomechanical efficacy of collagen crosslinking in porcine cornea using a femtosecond laser pocket

PURPOSE

The aim of this study was to assess the biomechanical efficacy of transepithelial collagen crosslinking using the femtosecond laser pocket technique compared with that using the standard crosslinking (CXL) technique.

METHODS

Forty ex vivo porcine eyes were divided into 4 groups with 10 samples each. Group 1 comprised the untreated controls. Group 2 was the standard CXL group with debridement, instillation of 0.1% riboflavin-dextran solution for 15 minutes before and every 5 minutes during the 30 minutes of irradiation with ultraviolet A (UVA) light of 370 nm and an irradiance of 3 mW/cm². Group 3 pertained to the femtolaser pocket control with an intrastromal pocket but without riboflavin/UVA. Group 4 was the femtolaser pocket CXL group with an intrastromal pocket of an 8-mm diameter at a 180-μm depth, riboflavin/dextran application for 15 minutes and subsequent exposure to UVA light for 30 minutes. Postoperatively, biomechanical stress-strain measurements were performed.

RESULTS

In the standard CXL group, the stress at 10% strain was 207.8 ± 64.1 × 10 Pa (+79.45% vs. controls; P = 0.021) compared with 115.8 ± 20.8 × 10 Pa in the untreated control group; in the crosslinked femtolaser pocket group, it was 159.5 ± 30.4 × 10 Pa (+37.74%; P = 0.049), in the non-cross-linked femtolaser pocket group, it was 103.5 ± 17.3 × 10 Pa (-10.62%; P = 0.103). The Young modulus was 5.4 MPa (+100% vs. controls) in the standard CXL group, 3.7 MPa (+37.04%) in the crosslinked femtolaser pocket group, and 2.4 MPa (-11.12%) in the non-cross-linked femtolaser pocket group compared with 2.7 MPa in the untreated control group.

CONCLUSIONS

The biomechanical effect of CXL using the femtolaser pocket technique is about 50% less pronounced than that after standard CXL. Future studies will show whether the efficacy of the technique can still be improved and whether the clinical effect is sufficient for stabilizing ectatic corneas.