Corneal biomechanical properties after LASIK, ReLEx flex, and ReLEx smile by Scheimpflug-based dynamic tonometry

Comparison of different corneal residual bed thickness after small incision lenticule extraction (SMILE)

PURPOSE

To evaluate the corneal biological parameters stability between the different corneal residual bed thickness (RBT) after Small Incision Lenticule Extraction (SMILE).

METHODS

In this prospective clinical trial, 127 eyes of 64 patients underwent SMILE. According to the corneal RBT, the patients were divided into the 250-270 µm, 270-290 µm and 290-310 µm groups. Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) and Scheimpflug camera (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany) measurements were performed preoperatively, 1 day, 1week, 1month and 3 months after surgery.

RESULTS

The keratometer values among the three groups were no significant differences in postoperative periods (each P > 0.05), except the corneal thickness values (each P < 0.05). In the 250-270 µm and 270-290 µm groups, the keratometer and corneal thickness values were decreased at postoperative 1 week and increased at 1 and 3 months. The 290-310 µm group significantly higher posterior maximum elevation (PME) than the 250-270 µm group at 1 and 3 months (P = 0.022, 0.022, respectively), and higher preoperative thinnest point (PTE) at 1 week and 1 month (P = 0.013, 0.035, respectively). The PME of the 290-310 µm group was higher than the 270-290 µm group at 3 months (P = 0.045), and higher PTE at 1 week and 3 months (P = 0.022, 0.02, respectively). In all three groups, the maximal deformation amplitude (DA) was significantly higher at 1 and 3 months compared to postoperative 1 day and 1 week, and the IOP was decreased at 1 month then recovered at 3 months (each P < 0.05).The DA of the 250-270 µm group was significantly higher than the 290-310 µm group at postoperative 1 week, 1 and 3 months (P = 0.001, 0.01, 0.02, respectively). The change of the posterior corneal elevation and biomechanical parameters values were no significant differences among the three groups in postoperative periods (each P > 0.05).

CONCLUSIONS

The range of 250-310 µm RBT was safe and stable at the early postoperative of SMILE. The RBT may be positively correlated with the posterior corneal elevation.

Corneal Biomechanics After SMILE, Femtosecond-Assisted LASIK, and Photorefractive Keratectomy: A Matched Comparison Study

Purpose

To evaluate the change in corneal stiffness after small incision lenticule extraction (SMILE), femtosecond laser-assisted in situ keratomileusis (FS-LASIK), and photorefractive keratectomy (PRK).

Methods

Age, gender, spherical equivalent, and central corneal thickness (CCT)-matched cases undergoing SMILE with a 120-µ cap, FS-LASIK with a 110-µ flap, and PRK were enrolled. One-year change in the stress-strain index, stiffness parameter at first applanation, integrated inverse radius, deformation amplitude ratio at 2 mm, and deformation amplitude ratio at 1 mm were compared between the surgical groups by linear mixed-effect models.

Results

Within each surgical group, 120 eyes completed 1 year of follow-up. The residual stromal bed (RSB) thickness and (RSB/CCTpostop) were 348.1 ± 35.0 (0.74), 375.4 ± 31.0 (0.77) and 426.7 ± 2 µm (0.88) after SMILE, FS-LASIK, and PRK, respectively. The 1-year change in all biomechanical indices was significant, except the stress-strain index with PRK (P = 0.884). The change in all indices with SMILE were significantly greater than with FS-LASIK and with PRK (all P < 0.01), except the deformation amplitude ratio at 1 mm change between SMILE and FS-LASIK (P = 0.075). The changes in all indices with FS-LASIK were significantly greater than with PRK (all P < 0.05).

Conclusions

Although SMILE preserves the greatest amount of anterior cornea with a cap thickness of 120 µ, this also produces the smallest RSB and the greatest decrease in stiffness. Thus, the RSB is shown to be the predominant determinant of stiffness decreases, rather than the preserved anterior cornea. We recommend using a thinner cap to achieve a thicker RSB and a lesser decrease in the corneal stiffness in the SMILE procedure.

Translational Relevance

After refractive surgery, RSB is predominant determinant of stiffness decreases, rather than the preserved anterior cornea.

Corneal Biomechanics Losses Caused by Refractive Surgery

Recent advances, specifically in the understanding of the biomechanical properties of the cornea and its response to diseases and surgical interventions, have significantly improved the safety and surgical outcomes of corneal refractive surgery, whose popularity and demand continue to grow worldwide. However, iatrogenic keratectasia resulting from the deterioration in corneal biomechanics caused by surgical interventions, although rare, remains a global concern. On one hand, in vivo biomechanical evaluation, enabled by clinical imaging systems such as the ORA and the Corvis ST, has significantly improved the risk profiling of patients for iatrogenic keratectasia. That is despite the fact the biomechanical metrics provided by these systems are considered indicators of the cornea's overall stiffness rather than its intrinsic material properties. On the other hand, new surgical modalities including SMILE were introduced to offer superior biomechanical performance to LASIK, but this superiority could not be proven clinically, creating more myths than answers. The literature also includes sound evidence that tPRK provided the highest preservation of corneal biomechanics when compared to both LASIK and SMILE. The aim of this review is twofold; to discuss the importance of corneal biomechanical evaluation prior to refractive surgery, and to assess the current understanding of cornea's biomechanical deterioration caused by mainstream corneal refractive surgeries. The review has led to an observation that new imaging techniques, parameters and evaluation systems may be needed to reflect the true advantages of specific refractive techniques and when these advantages are significant enough to offer better protection against post-surgery complications.

In Vivo Evaluation of the Effects of SMILE with Different Amounts of Stromal Ablation on Corneal Biomechanics by Optical Coherence Elastography

This work aims to depth-resolved quantitatively analyze the effect of different stromal ablation amounts on the corneal biomechanical properties during small incision lenticule extraction (SMILE) using optical coherence elastography (OCE). A 4.5-MHz ultrasonic transducer was used to excite elastic waves in the corneal tissue. The OCE system combined with the antisymmetric Lamb wave model was employed to achieve a high-resolution, high-sensitivity, and depth-resolved quantitative detection of the corneal Young's modulus. Eighteen rabbits were randomly divided into three groups; each group had six rabbits. The first and second groups underwent -3D and -6D SMILE surgeries, and the third group was the control group, respectively. Young's modulus of the corneal cap and residual stromal bed (RSB) were both increased after SMILE, which shared the stress under intraocular pressure (IOP). Furthermore, the Young's modulus of both the corneal cap and RSB after 3D SMILE group were significantly lower than that in the -6D group, which indicated that the increases in the post-operative corneal Young's modulus were positively correlated with the amount of stromal ablation. The OCE system for quantitative spatial characterization of corneal biomechanical properties can provide useful information on the extent of safe ablation for SMILE procedures.

Correlations among Corneal Biomechanical Parameters, Stiffness, and Thickness Measured Using Corvis ST and Pentacam in Patients with Ocular Hypertension

Background

To preliminary explore the correlations among corneal biomechanical parameters, stiffness, and thickness in patients with ocular hypertension (OHT) before and after treatment with topical antiglaucoma medications.

Methods

This was a retrospective study that included 35 eyes with newly diagnosed OHT. Axial length (AL), apical corneal thickness, and minimum corneal thickness were measured using Pentacam. The lengths, velocities, and times of the first and second corneal applanations (A1L, A1V, A1T, A2L, A2V, and A2T, respectively); the highest concavity radius; highest concavity peak distance (PDHC); highest concavity deformation amplitude (DAHC); highest concavity time (HCT); pachymetry (PACH); stress-strain index (SSI); stiffness parameter-A1 (SP-A1); deformation amplitude ratio (DA ratio); integrated radius (IR); Ambrosio's relational thickness horizontal (ARTh); corneal biomechanical index; noncorrected intraocular pressure (IOPnct); and biomechanically corrected IOP (bIOP) values were measured using the corneal visualization Scheimpflug technology (Corvis ST/CST).

Results

After 5 weeks of treatment, Goldman applanation tonometer-IOP, IOPnct, bIOP, PACH, A1T, A2V, SSI, SP-A1, and ARTh decreased, but A1V, A2T, PDHC, DAHC, DA ratio, and IR increased significantly (all p < 0.05). SP-A1 and A1T were positively associated with premedication IOP and IOP changes, whereas A1V, A2T, PDHC, and IR were negatively associated (all p < 0.05). DAHC and DA ratio had significantly negative correlations with IOP variations. PDHC was found to be positively correlated with AL (p < 0.05). A positive relationship was noted between SP-A1 and HCT before medication (p < 0.05).

Conclusions

SP-A1 was significantly and consistently associated with IOP. HCT might be correlated with SP-A1. SP-A1 and CST parameters could serve as potential biomarkers for evaluating OHT treatment efficacy.

Changes in Corneal Biomechanical Properties After Small-Incision Lenticule Extraction and Photorefractive Keratectomy, Using a Noncontact Tonometer

PURPOSE

The aim of this study was to evaluate and compare early corneal biomechanical changes after small-incision lenticule extraction (SMILE) and photorefractive keratectomy (PRK).

METHODS

The study comprised 74 patients eligible for refractive surgery, equally allocated to PRK (37 patients) and SMILE (37 patients). Corneal biomechanical properties were recorded and compared between the 2 groups at preoperatively and 3 months after surgery using a dynamic ultra-high-speed Scheimpflug camera equipped with a noncontact tonometer.

RESULTS

Both procedures significantly affected corneal biomechanical properties at 3 months after surgery. Patients in the PRK group showed significantly better results for deformation amplitude ratio (DA ratio) ( P = 0.03), maximum inverse radius (InvRadMax) ( P = 0.02), and A2 time ( P = 0.03). The mean changes in DA ratio, HC radius, InvRadMax, and Ambrosio relational thickness were significantly higher in the SMILE group in comparison with those of the PRK group (all, P < 0.05). In both groups, change in CCT was significantly correlated with changes in DA ratio and InvRadMax ( P < 0.05).

CONCLUSIONSS

Both SMILE and PRK refractive surgeries significantly altered corneal biomechanical properties but the changes were more prominent after SMILE.

A stochastic approach to estimate intraocular pressure and dynamic corneal responses of the cornea

IntraOcular Pressure (IOP) is one of the most informative factors for monitoring the eye-health. This is usually measured by tonometers. However, the outputs of the tonometers depend on the physical and geometrical properties of the cornea. Therefore, the common practice is to develop a numerical model to generate some correction factors. The main challenge here is the accuracy and efficiency of a numerical model in predicting the IOP and Dynamic Corneal Response (DCR) of each patient. This study addresses this issue by developing a two-step surrogate model based on adaptive sparse Polynomial Chaos Expansion (PCE) for fast and accurate prediction of the IOP. In this regard, first, an FE model of the cornea has been developed to predict the DCR parameters. This FE model has been replaced with a PCE-based surrogate model to speed up the simulation step. The uncertainties in the geometry and material model of the cornea have been propagated through the surrogate model to estimate the distributions of the DCR parameters. In the second step, the combination of DCR parameters and the input parameters provide a proper parameter space for developing an efficient data-driven PCE model to predict the IOP. Moreover, sensitivity analysis by using PCE-based Sobol indices has been performed. The results demonstrate the accuracy and efficiency of the proposed method in predicting the IOP. Sensitivity analysis revealed that IOP measurement was influenced mostly by deflection amplitude and applanation time. The analysis indicates the importance of the interactions between the parameters.

The Shape of Corneal Deformation Alters Air Puff–Induced Loading

Purpose: To determine the dynamic modification of the load exerted on the eye during air-puff testing by accounting for the deformation of the cornea.

Methods: The effect of corneal load alteration with surface shape (CLASS) was characterized as an additional component of the load produced during the concave phase where the fluid outflow tangential to the corneal surface creates backward pressure. Concave phase duration (t_CD), maximum CLASS value (CLASS_max), and the area under CLASS-time curve (CLASS_int) are calculated for 26 keratoconic (KCN), 102 normal (NRL), and 29 ocular hypertensive (OHT) subjects. Tukey’s HSD tests were performed to compare the three subject groups. A p-value less than 0.05 was considered statistically significant.

Results: Accounting for CLASS increased the load by 34.6% ± 7.7% at maximum concavity; these differences were greater in KCN subjects (p < 0.0001) and lower in OHT subjects (p = 0.0028) than in NRL subjects. t_CD and CLASS_int were significantly longer and larger, respectively, for KCN subjects than those in the NRL and OHT groups (p < 0.0001).

Conclusion: Load characterization is an essential step in assessing the cornea’s biomechanical response to air-puff–induced deformation. The dynamic changes in the corneal surface shape significantly alter the load experienced by the corneal apex. This implies a subject-specific loading dynamic even if the air puff itself is identical. This is important when comparing the same eye after a surgical procedure or topical medication that alters corneal properties. Stiffer corneas are least sensitive to a change in load, while more compliant corneas show higher sensitivity.

Comparison of visual outcomes between 120-µm and 140-µm cap thicknesses 12 months after small incision lenticule extraction

To compare visual and refractive outcomes as well as changes in high-order aberrations in patients with 120- versus 140-µm cap thicknesses 12 months after small incision lenticule extraction. Ninety-four patients were randomized to receive small incision lenticule extraction with either a 120-µm cap thickness (n = 47) or a 140-µm cap thickness (n = 47) to treat myopia or myopic astigmatism, if not both. In an analysis of right eyes only during the 12-month follow-up period, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), refractive outcomes, and high-order aberrations were evaluated. The distribution of patients by age and sex between the groups did not differ significantly (P = .803 and P = .680, respectively). CDVA, spherical and cylindrical refraction, and changes in total high-order aberration, spherical aberration, coma, and trefoil were similar between the groups at 6 and 12 months, postoperatively. However, UDVA was statistically significantly higher in patients with 140-μm cap thicknesses than with 120-μm cap thicknesses at 6 and 12 months postoperatively (P < .001 and P < .001, respectively). Patients with 140-µm cap thickness showed greater improvement in UDVA than ones with 120-µm cap thickness at 12-month follow-up (P = .005). Both 120- and 140-μm cap thicknesses in small incision lenticule extraction were safe and effective thicknesses for correcting myopia or myopic astigmatism. The patients with 140-μm cap thickness had better improvement in UDVA after 12-month follow-up compared to patients with 120-μm cap thickness.

Intraocular pressure after myopic laser refractive surgery measured with a new Goldmann convex prism: correlations with GAT and ORA

Background

The purpose of this study is to describe measurements using a newly developed modified Goldmann convex tonometer (CT) 1 year after myopic laser refractive surgery. Intraocular pressure (IOP) measurements were compared with IOP values obtained by Goldmann applanation tonometer (GAT), and Ocular Response Analyzer (ORA).

Methods

Prospective double-masked study performed on thirty eyes of thirty patients that underwent laser in situ keratomileusis (LASIK; n = 19) or photorefractive keratectomy (PRK; n = 11). IOP was measured before and 3 and 12 months after surgery. Intraclass correlation coefficient (ICC) and Bland-Altman plot were calculated to assess the agreement between GAT, CT, IOPg (Goldmann-correlated IOP) and IOPcc (corneal-compensated IOP) from ORA.

Results

Twelve months after LASIK, IOP measured with CT showed the best correlation with IOP measured with GAT before surgery (GATpre) (ICC = 0.886, 95% CI: 0.703–0.956) (15.60 ± 3.27 vs 15.80 ± 3.22; p < 0.000). However, a moderate correlation was found for IOP measured with IOPcc and CT 12 months after LASIK (ICC = 0.568, 95% CI: − 0.185 – 0.843) (15.80 ± 3.22 vs 12.87 ± 2.77; p < 0.004). Twelve months after PRK, CT showed a weak correlation (ICC = − 0.266, 95% CI: − 3.896 – 0.663), compared to GATpre (17.30 ± 3.47 vs 16.01 ± 1.45; p < 0.642), as well as poor correlation (ICC = 0.256, 95% CI: − 0.332 – 0.719) with IOPcc (17.30 ± 3.47 vs 13.38 ± 1.65; p < 0.182).

Conclusions

Twelve months after LASIK, IOP measured with CT strongly correlated with GAT before surgery and could therefore provide an alternative method for measuring IOP after this surgery. More studies regarding this new convex prism are needed to assess its accuracy.

[Experimental studies of the biomechanical properties of the cornea]

The review presents the results of experimental studies of the biomechanical properties of the cornea. Selective evaluation of the individual corneal structures (for example, limiting membranes) using classical mechanical tests is to a certain extent limited due to the rather small thickness of these structures and the related difficulties in sample fixation. In real practice, the use of a method better adapted for conducting such studies - atomic force microscopy (AFM) - remains promising, since on the one hand it eliminates the need for mechanical capture and retention of the sample, and on the other - provides the capability for studying its segments separately.

Comparison of changes in corneal volume and corneal thickness after myopia correction between LASIK and SMILE

Myopia is the most common refractive error. Surgical correction with laser is possible. LASIK and SMILE are the techniques currently most used. Aim of the study was to compare changes in corneal volume and thickness after the respective laser treatment. 104 eyes of 52 patients were matched based on refractive error into two equally sized groups, either treated with LASIK or SMILE. Measurements were obtained from the Scheimpflug camera (Pentacam) preoperatively and at 3 and 12 months postoperatively. 3 months postoperatively, the flapless SMILE procedure resulted in a significant overall greater loss of corneal volume (P < 0.01) and corneal thickness (P < 0.01) compared to LASIK. No significant difference was found when comparing the 3 to 12-months values in each group. Within the currently used ranges of refractive error correction, loss in central corneal thickness and corneal volume with SMILE is higher in comparison to LASIK. As greater loss in corneal volume and thickness might contribute to higher level of corneal instability maximum ranges of refractive error correction with SMILE should not supersede those set currently for LASIK until more long-term results on corneal ectasia are available for SMILE.

Small Incision Lenticule Extraction (SMILE): Myths and Realities

The emergence of SMILE in the last decade has provided an alternative to LASIK for patients considering cornea laser refractive surgery. SMILE offers a novel approach using the femtosecond laser to create an intrastromal lenticule that can be removed through a small three to four millimeter incision.The purpose of this study is to review the recent literature on popular SMILE claims - reduced iatrogenic dry eye, better recovery of corneal sensation, and a biomechanically stronger cornea - summarize the published outcomes, and determine which claims are myths versus realities.SMILE is still in its infancy as a refractive technique in the US after recent USFDA approval for its treatment of myopia astigmatism in October 2018. Future randomized controlled studies are needed to compare its outcomes to LASIK, which has well-documented good visual outcomes, rapid postoperative recovery, and good safety profile.

Excimer laser tissue interactions in the cornea

Excimer lasers induces significant changes to corneal structure and corneal biomechanics. The aim of this paper is to describe all laser-tissue interactions which are relevant for clinical practice, particularly, we will focus on laser ablations profiles, causes of regression and haze and prevention of those. At last the manuscript will describe the impact on corneal biomechanics of different Laser Vision Corrections techniques.

Clinical outcomes after small-incision lenticule extraction versus femtosecond laser-assisted LASIK for high myopia: A meta-analysis

AIM

To compare postoperative clinical outcomes of high myopia after small-incision lenticule extraction (SMILE) and femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK).

METHODS

From March 2018 to July 2020, PubMed, MEDLINE, Embase, the Cochrane Library, and several Chinese databases were comprehensively searched. The studies meeting the criteria were selected and included; the data were extracted by 2 independent authors. The clinical outcome parameters were analyzed with RevMan 5.3.

RESULTS

This meta-analysis included twelve studies involving 766 patients (1400 eyes: 748 receiving SMILE and 652 receiving FS-LASIK). Pooled results revealed no significant differences in the following outcomes: the logarithm of the mean angle of resolution (logMAR) of postoperative uncorrected distance visual acuity (weighted mean difference (WMD) = -0.01, 95% confidence interval (CI): -0.02 to 0.00, I2 = 0%, P = 0.07 at 1 mo; WMD = -0.00, 95% CI: -0.01 to 0.01, I2 = 0%, P = 0.83 at 3 mo; WMD = -0.00, 95% CI: -0.01 to 0.00, I2 = 32%, P = 0.33 in the long term), and the postoperative mean refractive spherical equivalent (WMD = -0.03, 95% CI: -0.09 to 0.03, I2 = 13%, P = 0.30). However, the SMILE group had significantly better postoperative corrected distance visual acuity (CDVA) than the FS-LASIK group (WMD = -0.04, 95% CI, -0.05 to -0.02, I2 = 0%, P<0.00001). In the long term, postoperative total higher-order aberration (WMD = -0.09, 95% CI: -0.10 to -0.07, I2 = 7%, P<0.00001) and postoperative spherical aberration (WMD = -0.15, 95% CI: -0.19 to -0.11, I2 = 29%, P<0.00001) were lower in the SMILE group than in the FS-LASIK group; a significant difference was also found in postoperative coma (WMD = -0.05, 95% CI: -0.06 to -0.03, I2 = 30%, P<0.00001).

CONCLUSION

For patients with high myopia, both SMILE and FS-LASIK are safe, efficacious and predictable. However, the SMILE group demonstrated advantages over the FS-LASIK group in terms of postoperative CDVA, while SMILE induced less aberration than FS-LASIK. It remains to be seen whether SMILE can provide better visual quality than FS-LASIK; further comparative studies focused on high myopia are necessary.

Study of corneal biomechanical properties in patients with childhood glaucoma

AIM

To study of corneal biomechanical properties and intraocular pressure (IOP) measured with Corvis Scheimpflug Technology (ST) in patients with childhood glaucoma (CG).

METHODS

Cross-sectional study in which 89 eyes were included 56 of them with CG. Only one eye per patient was included. The following variables were obtained from the clinical history and the ophthalmological examination: age, sex, IOP, number of surgeries, and the cup/disc ratio (CDR). The following parameters were recorded using Corvis ST: corrected by biomechanics IOP (bIOP), not corrected IOP (nctIOP), central corneal thickness (CCT), maximum concavity [radius, peak distance (PD) and deformation amplitude], applanation 1 and 2 (length and velocity). The mean age was 23±14.55 and 33±19.5 years old for the control group and CG group, respectively. Totally 36 were males and 53 were females. In the CG group, 7 patients were controlled only with medical treatment. Sixteen had at least one previous goniotomy, 19 had at least one trabeculectomy, and 11 had an Ahmed implant.

RESULTS

A significant and positive intraclass correlation coefficient was found between Goldman IOP and the IOP measured by Corvis in both groups. No differences were found between the IOP measured with Corvis and Goldman using a student t-test. Regarding biomechanical parameters, there were differences in the applanation length 2 (A-L2), in the applanation velocity 2 (A-V2) and in the PD. By sex, only the applanation length 1 (A-L1) was found to be different in control group. A positive and significant Pearson correlation was found between CDR and the A-L1.

CONCLUSION

Corneal biomechanical properties have shown differences between CG and healthy subjects and also between men and women.

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.

Complications of small incision lenticule extraction

The procedure of small incision lenticule extraction (SMILE) was introduced in 2011, and since then there has been an increase in the number of cases undergoing this procedure worldwide. The surgery has a learning curve and may be associated with problems in the intraoperative and postoperative periods. The intraoperative problems during SMILE surgery include the loss of suction, the occurrence of altered or irregular opaque bubble layer and black spots, difficulty in lenticular dissection and extraction, cap perforation, incision-related problems, and decentered ablation. Most of the postoperative problems are similar as in other laser refractive procedures, but with decreased incidence. The identification of risk factors, clinical features, and management of complications of SMILE help to obtain optimum refractive outcomes.

A Review on Different Tonometers for Intraocular Pressure Measurement After Photorefractive Keratectomy or Small Incision Lenticule Extraction

This paper aims to review different tonometers for intraocular pressure measurement in eyes after photorefractive keratectomy or small incision lenticule extraction. An Entrez Pubmed search was performed on July 19th, 2020. There were 32 eligible articles investigated in the use of tonometers or biomechanical properties of cornea in post photorefractive keratectomy eyes and 11 articles investigated in post small incision lenticule extraction eyes. Corvis ST and dynamic contour tonometry were found to be accurate for intraocular pressure measurement in post photorefractive keratectomy eyes. For post small incision lenticule extraction eyes, Corvis ST was found to be more accurate than other tonometers. Other tonometers such as Goldmann applanation tonometer, noncontact tonometry, Tonopen, ocular response analyzer can also be used in post photorefractive keratectomy or small incision lenticule extraction eyes in clinical practice; however, it is important for ophthalmologists to take note of the chances of discrepancies.

The best optical zone for small-incision lenticule extraction in high myopic patients

Small-incision lenticule extraction (SMILE) is an effective and safe procedure for the correction of myopia due to minimally invasive and noncorneal flap surgery. However, the SMILE procedure has certain requirements for corneal cap thickness, attempted refractive correction, residual stromal bed thickness, and optical zone diameter, which sometimes make surgeons hesitant to choose SMILE or other refractive surgeries. The requirements limit its use in patients with high myopia. The purpose of this review was to find the optimal parameters of SMILE through discussing the best optical zone for high myopic patients, the visual quality of different optical zones, the choice of corneal cap thickness, and their effects on corneal biomechanical parameters, so surgeons can provide reference recommendations for patients with high myopia in choosing a reasonable and safe procedure.

Effect of accelerated corneal crosslinking combined with transepithelial photorefractive keratectomy on dynamic corneal response parameters and biomechanically corrected intraocular pressure measured with a dynamic Scheimpflug analyzer in healthy myopic patients

PURPOSE

To evaluate the effect of accelerated corneal crosslinking (CXL) combined with transepithelial photorefractive keratectomy (PRK) on changes in new dynamic corneal response parameters and the biomechanically corrected intraocular pressure (IOP) measured using a dynamic Scheimpflug analyzer (Corvis ST).

SETTING

Yonsei University College of Medicine and Eyereum Eye Clinic, Seoul, South Korea.

DESIGN

Retrospective case series.

METHODS

Medical records of eyes of healthy myopic patients having transepithelial PRK or transepithelial PRK with CXL were examined. Main outcome variables were the biomechanically corrected IOP and new dynamic corneal response parameters including the deformation amplitude ratio at 1.0 mm (DAR1) and at 2.0 mm (DAR2), stiffness at first applanation and at highest concavity, and the integrated inverse radius preoperatively and 6 months postoperatively.

RESULTS

The study comprised 69 eyes (69 patients); 35 had transepithelial PRK and 34, transepithelial PRK with CXL. The DAR1, DAR2, and integrated inverse radius significantly increased, while stiffness at first applanation and at highest concavity decreased postoperatively in both groups. Changes in the DAR2 and integrated inverse radius in the transepithelial PRK group were significantly larger than in the transepithelial PRK with CXL group without and with analysis of covariance with the spherical equivalent change or corneal thickness change as a covariate. No significant differences in the biomechanically corrected IOP occurred preoperatively or postoperatively in either group.

CONCLUSIONS

Results indicate that prophylactic CXL combined with transepithelial PRK has a role in reducing the change in corneal biomechanical properties. The dynamic Scheimpflug analyzer showed stable biomechanically corrected IOP measurements preoperatively and postoperatively.

New parameters for evaluating corneal biomechanics and intraocular pressure after small-incision lenticule extraction by Scheimpflug-based dynamic tonometry

PURPOSE

To evaluate parameters and dynamic corneal densitometry with a new dynamic Scheimpflug analyzer (Corvis ST) in eyes having small-incision lenticule extraction (SMILE).

SETTING

Qvision, Vithas Virgen del Mar Hospital, Almería, Spain.

DESIGN

Retrospective case series.

METHODS

The study comprised eyes from the same institution having small-incision lenticule extraction surgery. Preoperative and 1-month postoperative measurements were taken.

RESULTS

Forty-three eyes were evaluated. The mean difference in intraocular pressure (IOP) and biomechanically corrected IOP before and after surgery was 2.24 mm Hg ± 1.26 mm (SD) (P = .001) and 0.57 ± 1.77 mm Hg (P = .04), respectively. All dynamic Scheimpflug analyzer parameters changed significantly after surgery (P < .05). The variation in each parameter was correlated with the removed corneal thickness (P < .05), except the stiffness parameter at the first applanation (P = .15). None of the 4 dynamic corneal densitometry parameters changed significantly as a result of surgery (P ≥ .29). A new sign, described as an inclined brightness fringe moving through the corneal periphery, appeared preoperatively in eyes with higher dynamic corneal densitometry. This sign was more prevalent postoperatively (48.8% versus 72.1%) (P = .04).

CONCLUSIONS

The biomechanically corrected IOP measured after surgery with the dynamic Scheimpflug analyzer showed better agreement with the preoperative values than IOP. The stiffness parameter was not dependent on the amount of removed corneal thickness. A new sign correlated with dynamic corneal densitometry was found and might be related to changes in corneal hydration and biomechanics.

Changes in biomechanically corrected intraocular pressure and dynamic corneal response parameters before and after transepithelial photorefractive keratectomy and femtosecond laser-assisted laser in situ keratomileusis

PURPOSE

To evaluate the changes in biomechanically corrected intraocular pressure (IOP) and new dynamic corneal response parameters measured by a dynamic Scheimpflug analyzer before and after transepithelial photorefractive keratectomy (PRK) and femtosecond laser-assisted laser in situ keratomileusis (LASIK).

SETTING

Yonsei University College of Medicine and Eyereum Eye Clinic, Seoul, South Korea.

DESIGN

Retrospective case series.

METHODS

Medical records of patients having transepithelial PRK or femtosecond-assisted LASIK were examined. The primary outcome variables were biomechanically corrected IOP and dynamic corneal response parameters, including deformation amplitude ratio 2.0 mm, stiffness parameter at first applanation, Ambrósio relational thickness through the horizontal meridian, and integrated inverse radius before the procedure and 6 months postoperatively.

RESULTS

Of the 129 patients (129 eyes) in the study, 65 had transepithelial PRK and 64 had femtosecond-assisted LASIK. No significant differences in biomechanically corrected IOP were noted before and after surgery. The deformation amplitude ratio 2.0 mm and integrated inverse radius increased, whereas the stiffness parameter at first applanation and the Ambrósio relational thickness through the horizontal meridian decreased after surgery (P < .001). The changes in deformation amplitude ratio 2.0 mm and integrated inverse radius were smaller in transepithelial PRK than femtosecond-assisted LASIK (P < .001). Using analysis of covariance, with refractive error change or corneal thickness change as a covariate, the changes in deformation amplitude ratio 2.0 mm and integrated inverse radius were smaller in transepithelial PRK than femtosecond-assisted LASIK (P < .001).

CONCLUSIONS

The dynamic Scheimpflug analyzer showed stable biomechanically corrected IOP measurement before and after surgery. The changes in dynamic corneal response parameters were smaller with transepithelial PRK than with femtosecond-assisted LASIK, indicating less of a biomechanical effect with transepithelial PRK.

Biomechanics of LASIK Flap and SMILE Cap: A Prospective, Clinical Study

PURPOSE

To analyze the acute effect of flap cut in laser in situ keratomileusis (LASIK) eyes and cap cut in small incision lenticule extraction (SMILE) eyes on corneal biomechanical properties of patients undergoing surgery.

METHODS

This was a prospective, interventional, longitudinal case series. Forty-eight eyes of 24 patients underwent contralateral LASIK and SMILE. Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) measurements were performed preoperatively, intraoperatively, and 1 week and 1 month after surgery. In LASIK eyes, the flap was cut but not lifted before intraoperative measurements. In SMILE eyes, the cap and side cut incision were made before intraoperative measurement. Thirty biomechanical variables were analyzed, assuming multiple comparisons.

RESULTS

In LASIK and SMILE eyes, 36.7% and 13.3% of the total number of variables detected biomechanical weakening after flap and cap cuts (P = .02), respectively. Further, 13.3% and 40% of the total variables detected no biomechanical changes after flap and cap cut, respectively (P = .03). These acute biomechanical effects of flap and cap cuts did not influence 1-week and 1-month measurements (P > .05) because both LASIK and SMILE eyes showed similar biomechanical weakening.

CONCLUSIONS

Flap and cap cuts induced biomechanical weakening in patient corneas. The flap caused more weakening than the cap intraoperatively. However, biomechanical differences between LASIK and SMILE eyes were similar after removal of tissue and ongoing wound healing. [J Refract Surg. 2019;35(5):324-332.].

Corneal Ectasia Risk And Percentage Tissue Altered In Myopic Patients Presenting For Refractive Surgery

Purpose

A percentage tissue altered (PTA) score of ≥40% has been advocated as an independent indicator of post-operative ectasia risk following laser in-situ keratomileusis (LASIK). This study was performed to test the hypothesis that refractive procedures, such as laser-assisted sub-epithelial keratectomy (LASEK) or small incision lenticule extraction (SMILE), may alter the range of PTA, within which refractive corneal surgery can be safely performed.

Setting

Refractive department, tertiary ophthalmic hospital.

Design

Retrospective observational study.

Methods

Review of case notes was performed for patients who presented for refractive surgeries, other than LASIK. To determine the risk of corneal ectasia for each patient prior to refractive surgery, we estimated what each patient’s PTA would have been if they had undergone LASIK. The Randleman Ectasia Risk Score System (ERSS) was also calculated.

Results

114 eyes (66 patients) were included. 94 eyes underwent SMILE. 20 eyes underwent LASEK. A significant proportion of eyes had PTA ≥40% – SMILE eyes: up to 31.9%, LASEK eyes: up to 60.0% (at presumed LASIK flap of 120 μm). The maximum calculated PTA was up to 47.9% in the SMILE group and up to 51.5% in the LASEK group. Using ERSS, 12.8–16% of SMILE eyes and 15.0–80.0% of LASEK eyes would have been considered to have moderate-to-high ectasia risk. No post-surgical ectasia was observed at 3 years.

Conclusion

SMILE and LASEK alter the range of PTA, within which corneal refractive surgery may be performed with a lower risk of developing post-operative corneal ectasia; a safe PTA threshold needs to be determined for these procedures before recommendations for clinical practice can be made.

Corneal refractive surgery combined with simultaneous corneal cross-linking: Indications, protocols and clinical outcomes-A review

Corneal refractive surgery is one of the most common approaches for correction of refractive errors. Combined corneal refractive surgery and corneal cross-linking (CXL) has been proposed as a method to achieve better refractive stability and to prevent iatrogenic corneal ectasia. However, there are concerns regarding its indications, surgical safety, standardization of protocols and long-term effect on corneal tissue. This review article aims to discuss the current knowledge and recent updates on combination of CXL and refractive surgery.

Corneal biomechanical properties after SMILE versus FLEX, LASIK, LASEK, or PRK: a systematic review and meta-analysis

BACKGROUND

The aim of this study was to compare the postoperative corneal biomechanical properties between small incision lenticule extraction (SMILE) and other corneal refractive surgeries.

METHODS

A systematic review and meta-analysis were conducted. Articles from January 2005, to April 2019, were identified searching PubMed, EMBASE, Web of Science, and International Clinical Trials Registry Platform. Studies that compared SMILE with other corneal refractive surgeries on adult myopia patients and evaluated corneal biomechanics were included. Multiple effect sizes in each study were combined. Random-effects model was conducted in the meta-analysis.

RESULTS

Twenty-two studies were included: 5 randomized controlled trials (RCTs), 9 prospective and 6 retrospective cohort studies, and 2 cross-sectional studies. Using the combined effect of corneal hysteresis (CH) and corneal resistance factor (CRF), which were obtained from ocular response analyzer (ORA), the pooled Hedges' g of SMILE versus femtosecond laser-assisted in situ keratomileusis (FS-LASIK) was 0.41 (95% CI, 0.00 to 0.81; p = 0.049; I² = 78%), versus LASIK was 1.31 (95% CI, 0.54 to 2.08; p < 0.001; I² = 77%), versus femtosecond lenticule extraction (FLEX) was - 0.01 (95% CI, - 0.31 to 0.30; p = 0.972; I² = 20%), and versus the group of photorefractive keratectomy (PRK) and laser-assisted sub-epithelial keratectomy (LASEK) was - 0.26 (95% CI, - 0.67 to 0.16; p = 0.230; I² = 54%). The summary score of Corvis ST (CST) after SMILE was comparable to FS-LASIK/LASIK with the pooled Hedges' g = - 0.05 (95% CI, - 0.24 to 0.14; p = 0.612, I² = 55%).

CONCLUSIONS

In terms of preserving corneal biomechanical strength after surgeries, SMILE was superior to either FS-LASIK or LASIK, while comparable to FLEX or PRK/LASEK group based on the results from ORA. More studies are needed to apply CST on evaluating corneal biomechanics after refractive surgeries.

High-speed OCT-based ocular biometer combined with an air-puff system for determination of induced retraction-free eye dynamics

We demonstrate a swept source OCT-based ocular biometer integrated with an air-puff stimulus to study the reaction of the eye to mechanical stimulation in vivo. The system enables simultaneous measurement of the stimulus strength and high-speed imaging of the eye dynamics along the visual axis. We characterize the stimulus and perform optimization of the data acquisition for a proper interpretation of the results. Access to the dynamics of axial eye length allows for a determination of the eye retraction, which is used to correct the air-puff induced displacement of ocular structures. We define the parameters to quantify the reaction of the eye to the air puff and determine their reproducibility in a group of healthy subjects. We observe the corneal deformation process and axial wobbling of the crystalline lens. OCT biometer combined with the air puff is the first instrument with the potential to provide comprehensive information on the biomechanics of ocular components.

A novel method for quantifying the biomechanical parameters of orbital soft tissue using a corneal dynamic scheimpflug analyser: a retrospective study

BACKGROUND

To demonstrate that the Corvis ST could be used to quantify the biomechanical parameters of the orbital soft tissues by measuring and comparing whole eye movement (WEM) using the Corvis in normal eyes and in eyes of patients with Graves ophthalmopathy.

METHODS

Forty four eyes of 44 ophthalmologically normal subjects and 28 eyes of 28 patients with Graves ophthalmopathy were included in the study. After Corvis test, the examiners recorded WEM by air puff. In the patients with Graves ophthalmopathy, the partial correlation coefficient adjusted for age and gender was calculated to analyze the correlation between exopthalmometry and WEM. Same analysis was repeated for the correlation between and the cross sectional area (%) of the extraocular muscles in the orbit CT and WEM.

RESULTS

WEM was 0.314 ± 0.083 mm in the normal subjects and 0.227 ± 0.079 mm in the Graves ophthalmopathy group (p = 0.000). The exophthalmometry was not significantly correlated with WEM after adjusting for age and gender (R = 0.083, p = 0.688). In the 21 Graves ophthalmopathy patients examined by orbit CT, after adjusting for age and gender, WEM significantly decreased as the cross sectional area (%) of the extraocular muscles in the orbit increased (R = - 0.461, p = 0.047).

CONCLUSIONS

WEM by Corvis could be used to quantify the biomechanical parameters of the orbital soft tissue. However, it is unclear whether WEM effectively represents the orbital biomechanical parameters, because WEM is only 0.6% of the orbital depth.

Microdistortions in Bowman's Layer 3 Years After SMILE for Myopia

PURPOSE

To investigate the characteristics and changes in microdistortions in Bowman's layer 3 years after small incision lenticule extraction (SMILE).

METHODS

In this non-randomized, prospective study, 50 eyes of 28 patients who received SMILE with a mean spherical equivalent of -6.72 ± 1.41 diopters (D) were divided into a thin lenticule group (lenticule thickness ⩽ 135 µm) and a thick lenticule group (lenticule thickness > 135 µm). Examinations were performed preoperatively and 1 day, 1 month, and 3 years after SMILE. Microdistortions in Bowman's layer determined via optical coherence tomography were analyzed digitally.

RESULTS

All patients completed the follow-up with no complications. Three years after SMILE, no eyes exhibited loss of corrected distance visual acuity. The safety index was 1.20 ± 0.15 and the efficacy index was 1.01 ± 0.10. The respective mean lengths of microdistortions at 1 day, 1 month, and 3 years after SMILE were 2,397.47 ± 1,777.22, 3,444.87 ± 2,145.82, and 1,283.81 ± 1,184.43 μm. The respective mean microdistortion lengths at each time-point were 1,781.29 ± 17,78.15, 2,617.78 ± 1,949.67, and 1,112.95 ± 1,245.88 μm in the thin lenticule group and 2,692.10 ± 1,825.20, 3,739.20 ± 2,072.60, and 1,332.44 ± 1,137.33 μm in the thick lenticule group.

CONCLUSIONS

Microdistortions in Bowman's layer can remain 3 years after SMILE, but their lengths decrease over time. Thicker lenticules were associated with the early occurrence of microdistortions, but this effect decreased in the long term. [J Refract Surg. 2019;35(2):96-101.].

Biomechanical changes in the cornea following cataract surgery: A prospective assessment with the Corneal Visualisation Scheimpflug Technology

IMPORTANCE

Intraocular pressure (IOP) is often reduced following cataract surgery. Postoperative changes in corneal stiffness are likely to be at least partly responsible for any reduction in IOP measured with applanation tonometry.

BACKGROUND

To determine the effect of cataract surgery and corneal incision size on corneal biomechanics.

DESIGN

Prospective randomized trial.

PARTICIPANTS

One hundred prospectively enrolled patients qualifying for cataract surgery.

METHODS

Participants were randomized to clear corneal incisions with a 2.20 or 2.85 mm keratome. Corneal Visualisation Scheimpflug Technology (Corvis-ST) tonometry and dynamic corneal response measurements were obtained preoperatively, and 3 mo postoperatively. Multiple regression analysis was completed using R software.

MAIN OUTCOME MEASURES

Corvis-ST biomechanical parameters.

RESULTS

Ninety-three eyes of 93 patients were included in the final analysis. Mean Corvis-ST biomechanically corrected IOP decreased by 3.63 mmHg postoperatively (95% confidence interval = 2.97-4.35, P ≤ 0.01), and central pachymetry increased by 6.96 μm (4.33-9.59, P ≤ 0.01). Independent of IOP and pachymetry changes, mean (±SE) corneal first applanation stiffness parameter reduced by 9.761 ± 3.729 (P = 0.01) postoperatively. First applanation velocity increased by 0.007 ± 0.002 ms, second applanation velocity increased by 0.012 ± 0.004 ms (P ≤ 0.01), the first applanation deformation amplitude increased by 0.008 ± 0.002 mm (P ≤ 0.01) and the deflection amplitude at highest concavity increased by 0.030 ± 0.069 (P ≤ 0.01). There were no significant differences between different incision size groups.

CONCLUSIONS AND RELEVANCE

Corneal stiffness is reduced 3 mo following cataract surgery and is associated with falsely low IOP measurements. This finding may be important for glaucoma patients and in particular when assessing the effectivity of minimally invasive glaucoma surgery devices.

Osteogenesis imperfecta and the teeth, eyes, and ears-a study of non-skeletal phenotypes in adults

Osteogenesis imperfecta (OI) is a disease causing bone fragility; however, it potentially affects all organs with a high content of collagen, including ears, teeth, and eyes. The study is cross-sectional and compares non-skeletal characteristics in adults with OI that clinicians should be aware of when caring for patients with OI.

INTRODUCTION

Osteogenesis imperfecta (OI) is a hereditary connective tissue disorder. The skeletal fragility is pronounced; however, OI leads to a number of extra-skeletal symptoms related to the ubiquity of collagen type 1 throughout the human body. The vast majority of knowledge is derived from studies performed in the pediatric population. Thus, we aimed to investigate the nature and prevalence of ophthalmologic, odontologic, and otologic phenotypes in an adult population with OI.

METHODS

The study population comprises 85 Danish OI patients (age 44.9 ± 15.9 years). Fifty-eight patients had OI type I, 12 OI type III, and 15 OI type IV according to the classification by Sillence. Audiometric evaluations and dental examinations were performed in 62 and 73 patients, respectively. Ophthalmologic investigations were performed in 64 patients, including measurements of the central corneal thickness.

RESULTS

All patients, except two, had corneal thickness below the normal reference value. Patients with OI type I and patients with a quantitative collagen defect had thinner corneas compared to patients with OI type III and other patients with a qualitative collagen defect. One patient in this cohort was diagnosed with and treated for acute glaucoma. Dentinogenesis imperfecta was diagnosed in one fourth of the patients, based on clinical and radiographic findings. This condition was predominately seen in patients with moderate to severe OI. Hearing loss requiring treatment was found in 15 of 62 patients, of whom three were untreated. The most prevalent type of hearing loss (HL) was sensorineural hearing loss, whereas conductive HL was solely seen in patients with OI type III. The patients with the most severe degrees of HL were patients with mild forms of OI. Age was associated with increased HL.

CONCLUSION

Although significant health problems outside the skeleton are frequent in adult patients with OI, the patients are not consistently monitored and treated for their symptoms. Clinicians treating adult patients with OI should be aware of non-skeletal health issues and consider including regular interdisciplinary check-ups in the management plan for adult OI patients.

Comparison of corneal biomechanical properties of patients with dry eye secondary to Sjögren's syndrome and healthy subjects

PURPOSE

The goal of this study is to determine whether any difference in corneal biomechanical properties exists between Sjögren's syndrome dry eye patients and healthy subjects.

METHODS

Thirty-one patients diagnosed with Sjögren's syndrome and associated dry eye manifestations and 44 healthy individuals were included in the study. Ultrasonic pachymetry (UP) was used to measure central corneal thickness (CCT). Corneal biomechanical parameters were obtained using ocular response analyzer (ORA). The main parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann correlated intraocular pressure (IOPg) and corneal compensated IOP (IOPcc). A Student's t-test for independent groups was performed to compare the mean of these variables between both groups.

RESULTS

Mean CH values in Sjögren's syndrome and healthy subject eyes were 10.1mmHg and 11.18mmHg respectively, representing a statistically significant difference (P=0.003). No other variable measured differed between cases and controls (P>0.05). Mean CRF values were 9.51mmHg and 10.37mmHg respectively, and mean CCT measured by UP in cases and controls was 527.41μm and 552.51μm respectively.

CONCLUSIONS

Sjögren's syndrome can influence corneal biomechanical properties, specifically CH. ORA measurements should be considered of interest in the evaluation of Sjögren syndrome subjects.

Clinical outcomes of small-incision lenticule extraction and femtosecond laser-assisted wavefront-guided laser in situ keratomileusis

Small-incision lenticule extraction and wavefront-guided femtosecond-assisted laser in situ keratomileusis (LASIK) provide good visual outcomes and an efficacious correction of myopia or myopic astigmatism with similar levels of safety. Some differences in predictability and the percentage of eyes gaining lines of corrected distance visual acuity are present. A higher level of coma is present after small-incision lenticule extraction, with no clear differences between techniques in spherical aberration. Better contrast sensitivity is achieved in the early postoperative period after wavefront-guided femtosecond laser-assisted LASIK, possibly because of less intrastromal light backscattering and Bowman layer microdistortions. Corneal sensitivity decreased less after small-incision lenticule extraction because of less severe decrease in subbasal nerve density, which has a significant effect on symptomatology and dry-eye tests (P < .05). Significant corneal biomechanical changes occurred after both techniques (P < .05), with no scientific evidence supporting the superiority of 1 technique over the other.

FINANCIAL DISCLOSURE

Dr. Piñero received an unrestricted educational grant from Abbott Medical Optics, Inc. Neither author has a financial or proprietary interest in any material or method mentioned.

Biomechanical Properties of Human Cornea Tested by Two-Dimensional Extensiometry Ex Vivo in Fellow Eyes: Femtosecond Laser-Assisted LASIK Versus SMILE

PURPOSE

To investigate the biomechanical properties of the ex vivo human cornea after flap-based versus cap-based laser refractive surgery in the same donor.

METHODS

In this experimental study, 11 pairs of human corneas unsuitable for transplantation were equally divided into two groups. Corneas from the right eye were treated with femtosecond laser-assisted LASIK (FSLASIK) and corneas from the left eye with small incision lenticule extraction (SMILE). Pachymetry was measured in each eye directly before laser refractive surgery. All corneas were subjected to a refractive correction of -10.00 diopters (D) sphere and -0.75 D cylinder at 0° with a 7-mm zone, using either a 110-μm flap (FS-LASIK) or 130-μm cap (SMILE). For two-dimensional biomechanical measurements, corneoscleral buttons underwent two testing cycles (preconditioning stress-strain curve from 0.03 to 9.0 N and stress-relaxation at 9.0 N during 120 sec) to analyze the elastic and viscoelastic material properties. The effective elastic modulus was calculated. Statistical analysis was performed with a confidence interval of 95%.

RESULTS

In stress-strain measurements, the effective elastic modulus was 1.47 times higher (P = .003) after SMILE (median = 8.22 [interquartile range = 4.76] MPa) compared to FS-LASIK (median = 5.59 [inter-quartile range = 2.77] MPa). The effect size was large (r = 0.83). No significant differences (P = .658) were observed among stress-relaxation measurements, with a mean remaining stress of 181 ± 31 kPa after SMILE and 177 ± 26 kPa after FS-LASIK after relaxation.

CONCLUSIONS

Compared to a flap-based procedure such as FS-LASIK, the SMILE technique can be considered superior in terms of biomechanical stability, when measured experimentally in ex vivo human fellow eye corneas. [J Refract Surg. 2018;34(6):419-423.].

Predicting Refractive Outcome of Small Incision Lenticule Extraction for Myopia Using Corneal Properties

Purpose

To investigate whether preoperative corneal topographic and biomechanical parameters (CTBPs) predict postoperative residual refractive error (RRE).

Methods

We retrospectively included 151 eyes from 151 patients of small-incision lenticule extraction (SMILE) with target RRE of plano and 3-month measurements of refractive error from Tianjin Eye Hospital. Multivariate linear/logistic regressions were performed to associate age, gender, preoperative refractive error, lenticule thickness, and CTBPs with postoperative RRE/the occurrence of myopic RRE ≤ −0.25 diopter (D). Stepwise regression was used for feature selection. Leave-one-cross-validation was used for model evaluation by the area under the receiver operating characteristic curve (AUC).

Results

From linear regression, more myopic RRE was associated with higher preoperative myopia, intraocular pressure (IOP), flattest curvature of anterior cornea (AC), and highest concavity deformation (HCD), and was associated with lower anterior elevation, anterior asphericity, steepest curvature of AC, and second applanation velocity. The occurrence of ≤ −0.25 D RRE was associated with higher myopia, IOP, posterior elevation and asphericity, flattest curvature of AC, first applanation velocity and HCD, and was associated with lower first applanation stiffness parameter, central corneal thickness, anterior elevation and asphericity, steepest curvature of AC, and second applanation velocity as well as thinner lenticule thickness. Compared to the baseline model using age, gender, and preoperative refractive error, adding CTBPs significantly (P < 0.001) improved the AUC performance to 0.771 from 0.615.

Conclusions

Postoperative outcomes of SMILE can be predicted by individual CTBPs.

Translational Relevance

Our findings could be used to customize a refractive nomogram based on individual corneal properties improving outcomes and patient satisfaction.

Review of Corneal Biomechanical Properties Following LASIK and SMILE for Myopia and Myopic Astigmatism

Worldwide, femtosecond Laser Assisted In-situ Keratomileusis (LASIK) is a well known and commonly used refractive technique, although Small Incision Lenticule Extraction (SMILE) has become increasingly popular since it was introduced in 2011. In LASIK, a corneal flap is cut with a microkeratome or femtosecond laser, followed by thinning of the stromal bed with excimer laser ablation. In SMILE, a minor intrastromal lenticule is cut with a femtosecond laser and subsequently removed through a small incision, leaving the anterior and strongest part of the cornea almost intact. Both LASIK and SMILE require cutting of corneal lamellae that may reduce the biomechanical stability of the cornea, with the potential risk of corneal iatrogenic ectasia as a severe complication. However, SMILE preserves the anterior corneal integrity and may, in theory, better preserve the corneal biomechanical strength than LASIK after surgery. A review aimed to examine the current literature that describes and compares the corneal biomechanical properties after Laser Assisted In-situ Keratomileusis (LASIK) and Small Incision Lenticule Extraction (SMILE). A comprehensive search was performed in Pubmed.gov using the following search queries: Corneal biomechanical properties, corneal biomechanics, ocular response analyser, ocular response analyzer, ORA, ex vivo, in vitro, Corvis, Corvis ST, LASIK, and SMILE.

Brillouin microscopy: assessing ocular tissue biomechanics

PURPOSE OF REVIEW

Assessment of corneal biomechanics has been an unmet clinical need in ophthalmology for many years. Many researchers and clinicians have identified corneal biomechanics as source of variability in refractive procedures and one of the main factors in keratoconus. However, it has been difficult to accurately characterize corneal biomechanics in patients. The recent development of Brillouin light scattering microscopy heightens the promise of bringing biomechanics into the clinic. The aim of this review is to overview the progress and discuss prospective applications of this new technology.

RECENT FINDINGS

Brillouin microscopy uses a low-power near-infrared laser beam to determine longitudinal modulus or mechanical compressibility of tissue by analyzing the return signal spectrum. Human clinical studies have demonstrated significant difference in the elastic properties of normal corneas versus corneas diagnosed with mild and severe keratoconus. Clinical data have also shown biomechanical changes after corneal cross-linking treatment of keratoconus patients. Brillouin measurements of the crystalline lens and sclera have also been demonstrated.

SUMMARY

Brillouin microscopy is a promising technology under commercial development at present. The technique enables physicians to characterize the biomechanical properties of ocular tissues.

Contralateral Eye Comparison of SMILE and Flap-Based Corneal Refractive Surgery: Computational Analysis of Biomechanical Impact

PURPOSE

Computational analyses were performed to quantify and directly compare the biomechanical impact of flapless and flap-based procedures in a series of patients undergoing small incision lenticule extraction (SMILE) in one eye and flap-based femtosecond lenticule extraction in the other.

METHODS

Tomographic data from 10 eyes of 5 patients undergoing femtosecond laser refractive lenticule extraction for myopic astigmatism with or without a stromal flap (femtosecond lenticule extraction in one eye, SMILE in the contralateral eye) were used to generate computational models. Inverse finite element analyses were performed at physiologic intraocular pressure followed by forward analyses at elevated intraocular pressure to assess corneal displacement and stress under differential loading. Case-specific treatment settings were incorporated. Preoperative material constants were obtained through inverse finite element analyses, and the surgically induced change in fiber stiffness within each flap was determined by minimization of the error between the simulated and actual 6-month topographic outcomes.

RESULTS

Flap-based procedures produced a 49% (range: 2% to 87%) greater mean reduction in effective stromal collagen fiber stiffness within the flap region than contralateral SMILE cases. Lower stresses and deformations were observed within the residual stromal bed in SMILE cases than in flap-based cases. Stromal bed displacements and stresses were more affected by a loading increase in flap-based eyes than flapless eyes.

CONCLUSIONS

Intrastromal flapless procedures had less impact on anterior stromal collagen mechanics and resulted in lower stromal bed displacements and stresses than flap-based procedures in contralateral eyes. However, biomechanical impact varied widely between individuals and this reinforces the need for individualized assessment of ectasia risk. [J Refract Surg. 2017;33(7):444-453].

Corneal biomechanics after laser refractive surgery: Unmasking differences between techniques

The hypothesis that small-incision lenticule extraction provides better preservation of corneal biomechanics than previous laser refractive techniques has led to a growth in the interest in clinical and experimental research in this field. This hypothesis is based on the fact that corneal layers with greater stiffness are preserved with this new technique. However, this hypothesis is controversial because clinical research has shown a great disparity in the outcomes. In this review, we performed an in-depth analysis of the factors that might affect corneal biomechanics in laser refractive surgery procedures from a macrostructural to a microstructural viewpoint. New advances in algorithms with current devices or the introduction of new devices might help unmask the possible advantages of small-incision lenticule extraction in corneal biomechanics.

Evaluation of Corneal Biomechanics After Excimer Laser Corneal Refractive Surgery in High Myopic Patients Using Dynamic Scheimpflug Technology

OBJECTIVE

To compare the effect of femtosecond-assisted thin flap laser-assisted in situ keratomileusis (FS-LASIK) and photorefractive keratectomy with mitomycin-C (PRK-MMC) in highly myopic patients (>7.0 D [D]) on corneal biomechanical parameters.

METHODS

In this prospective comparative interventional case series, 60 patients (30 patients in each group) with a manifest refractive spherical equivalent (MRSE) greater than 7.0 D were enrolled. Corvis ST parameters were measured before and at 3 and 6 months postoperatively.

RESULTS

Preoperatively, mean MRSE was -8.65±1.51 D in FS-LASIK and -8.04±1.70 D in PRK-MMC groups (P=0.149), and corneal thickness was 570.67±36.79 μm and 507.12±32.55 μm, respectively (P<0.001). At 6 months, both groups showed significantly higher applanation (A) 2 time and A2-velocity (P<0.05 in both), whereas intraocular pressure (IOP), corrected IOP, A1 time, A1-velocity, and radius at highest concavity were significantly reduced (P<0.05 in all). In the FS-LASIK group, there was a significant increase in deformation amplitude (DA) (P=0.001), and significant decreases in A2-length (P=0.004). Peak distance increased in the PRK-MMC group (P=0.029). At 6 months, after controlling for fellow eye correlations and preoperative corneal thickness between the two groups, decreases in IOP, A1-time, A2-length and radius, and the increase in DA was greater in FS-LASIK.

CONCLUSION

This study demonstrated significant changes in Corvis ST ocular biomechanical metrics after both PRK-MMC and FS-LASIK in high myopic patients, indicating the significant effect of excimer laser refractive surgery on corneal biomechanical properties. However, changes that occur with FS-LASIK are more significant than with PRK-MMC. Further randomized studies are needed to better characterize the pattern of biomechanical changes associated with each type of surgery.

Characteristics of corneal biomechanical responses detected by a non-contact scheimpflug-based tonometer in eyes with glaucoma

PURPOSE

To determine the corneal biomechanical properties in eyes with glaucoma using a non-contact Scheimpflug-based tonometer.

METHODS

Corneal biomechanical responses were examined using a non-contact Scheimpflug-based tonometer. The tonometer parameters of the normal control group (n = 75) were compared with those of the glaucoma group (n = 136), including an analysis of glaucoma subgroups categorized by visual field loss.

RESULTS

After adjusting for potential confounding factors, including the intraocular pressure (IOP), central corneal thickness (CCT), age and axial length, the deformation amplitude was smaller in the glaucoma group (1.09 ± 0.02 mm) than in the normal control group (1.12 ± 0.02 mm; p value = 0.031). The deformation amplitude and the deflection amplitude of the severe glaucoma group (1.12 ± 0.02 mm and 0.92 ± 0.01 mm) were significantly greater than that of the early glaucoma group (1.07 ± 0.01  mm and 0.88 ± 0.11 mm, p = 0.006 and p = 0.031), whereas that of the moderate glaucoma group (1.09 ± 0.02 mm and 0.90 ± 0.02 mm) was greater than that of the early glaucoma group, but this difference was not statistically significant. The deformation amplitude showed a negative correlation with the CCT in the normal control group (r = -0.235), with a weaker negative relationship observed in the early glaucoma group (r = -0.099). However, in the moderate and severe glaucoma groups, the deformation amplitude showed a positive relationship with the CCT, showing an inverse relationship. The duration and number of antiglaucomatous eyedrops used had negative correlations with the CCT in eyes with moderate and severe glaucoma.

CONCLUSION

Overall, the glaucoma group showed significantly less deformable corneas than did the normal controls, even after adjusting for the IOP, CCT, age and axial length. However, there were also differences according to the severity of glaucoma, where the corneal deformation amplitude was greater in the severe glaucoma group compared to the early glaucoma group. The combined effects of stiffening due to glaucoma and increased viscoelastic properties caused by the chronic use of antiglaucomatous eyedrops may have resulted in the present findings.