Biomechanical Properties of Human Cornea Tested by Two-Dimensional Extensiometry Ex Vivo in Fellow Eyes: PRK Versus SMILE

Biaxial hyperelastic and anisotropic behaviors of the corneal anterior central stroma along the preferential fibril orientations. Part I: Measurement and calibration of personalized stress-strain curves

Lacking specimens is the biggest limitation of studying the mechanical behaviors of human corneal. Extracting stress-strain curves is the crucial step in investigating hyperelastic and anisotropic properties of human cornea. 15 human corneal specimens extracted from the small incision lenticule extraction (SMILE) surgery were applied in this study. To accurately measure the personalized true stress-strain curve using corneal lenticules, the digital image correlation (DIC) method and finite element method were used to calibrate the stress and the strain of the biaxial extension test. The hyperelastic load-displacement curves obtained from the biaxial extension test were performed in preferential fibril orientations, which are arranged along the nasal-temporal (NT) and the superior-inferior (SI) directions within the anterior central stroma. The displacement and strain fields were accurately calibrated and calculated using the digital image correlation (DIC) method. A conversion equation was given to convert the effective engineering strain to the true strain. The stress field distribution, which was simulated using the finite element method, was verified. Based on this, the effective nominal stress with personalized characteristics was calibrated. The personalized stress-strain curves containing individual characteristic, like diopter and anterior surface curvature, was accurately measured in this study. These results provide an experimental method using biaxial tensile test with corneal lenticules. It is the foundation for investigating the hyperelasticity and anisotropy of the central anterior stroma of human cornea.

Motion-Tracking Brillouin Microscopy Evaluation of Normal, Keratoconic, and Post-Laser Vision Correction Corneas

PURPOSE

To characterize focal biomechanical differences between normal, keratoconic, and post-laser vision correction (LVC) corneas using motion-tracking Brillouin microscopy.

DESIGN

Prospective cross-sectional study.

METHODS

Thirty eyes from 30 patients (10 normal controls [Controls], 10 post-LVC, and 10 stage I or II keratoconus [KC]) had Scheimpflug and motion-tracking Brillouin microscopy imaging using a custom-built device. Mean, maximum (max) and minimum (min) Brillouin shift, spatial standard deviation, and max-min values were compared. Min values were correlated with local Brillouin values at multiple Scheimpflug imaging locations.

RESULTS

Mean (P < .0003), min (P < .00001), spatial standard deviation (P < .01), and max-min (P < .001) were significantly different between the groups. In post hoc pairwise comparisons, the best differentiators for group comparisons were mean (P = .0004) and min (P = .000002) for Controls vs KC, min (P = .0022) and max-min (P = .002) for Controls vs LVC, and mean (P = .0037) and min (P = .0043) for LVC vs KC. Min (area under the receiver operating characteristic = 1.0) and mean (area under the receiver operating characteristic =  0.96) performed well in differentiating Control and KC eyes. Min values correlated best with Brillouin shift values at the thinnest corneal point (r2 = 0.871, P = .001) and maximum keratometry value identified in the tangential curvature map (r2 = 0.840, P = .002).

CONCLUSIONS

Motion-tracking Brillouin microscopy effectively characterized focal corneal biomechanical alterations in LVC and KC and clearly differentiated these groups from Controls. Primary motion-tracking Brillouin metrics performed well in differentiating groups as compared with basic Scheimpflug metrics, in contrast to previous Brillouin studies, and identified focal changes after LVC where prior Brillouin studies did not.

Iatrogenic Keratectasia after Refractive Surgery - Causes, Prophylaxis, Therapy

Iatrogenic keratectasia is induced thinning and protrusion of the cornea after laser refractive surgery. Known risk factors include an excessively thin postoperative residual stromal bed, a thicker flap, or preoperatively undetected evidence of preexisting subclinical keratoconus. The rate of post-refractive ectasia in eyes without identifiable preoperative risk factors is 20 per 100 000 eyes for photorefractive keratectomy, 90 per 100 000 eyes for laser in situ keratomileusis, and 11 per 100 000 eyes for small incision lenticule extraction. Traditional screening tools for preoperative risk include the ectasia risk score system and percentage of tissue alteration. More recent methods include corneal elastography and epithelial mapping, in addition to Artificial Intelligence methods for data analysis. Therapy includes contact lenses, cross-linking, implantation of intracorneal ring segments, penetrating or lamellar keratoplasty, and, in early studies, implantation of corneal lenticules.

Visual Outcomes of Small-Incision Lenticule Extraction (SMILE) in Thin Corneas

We aimed to find out whether thin (≤500 μm) or normal (>500 μm, control) corneal thickness would impact efficacy and safety outcomes of small-incision lenticule extraction (SMILE). We retrospectively analyzed medical records of adult patients who had undergone SMILE. A total of 57 eyes were included in the “thin corneas” group and 180 eyes in the “control” group. At one month after surgery, rates of patients with uncorrected distance visual activity (UDVA) ≥ 0.8 were significantly higher in patients from the control group compared to the “thin corneas” group (87 vs. 71%, respectively p < 0.01), though rates were comparable at 3 months (87 vs. 76%, respectively, p > 0.05). SMILE had comparable safety in patients with thin and normal corneas. Procedure result predictability was comparable between groups. Regression analysis demonstrated that cap thickness impacted posterior corneal biomechanics, and the volume of removed tissue had a higher influence in patients with thin corneas. Moreover, an increase in cap thickness was associated with better final BCVA. Further study is needed for the evaluation of the impact of thin corneas on SMILE outcomes and planning. Our study also indicates that patients with thin corneas might require a different approach to nomogram calculation.

Determining the Utility of Epithelial Thickness Mapping in Refractive Surgery Evaluations

PURPOSE

To determine the impact of corneal epithelial thickness maps on screening for refractive surgery candidacy in a single refractive surgical practice.

DESIGN

Comparison of screening methods.

METHODS

A total of 100 consecutive patients who presented for refractive surgery screening were evaluated. For each patient, screening based on Scheimpflug tomography, clinical data, and patient history was performed and a decision on eligibility for laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and small incision lenticule extraction (SMILE)was independently made by 2 masked examiners. Examiners were then shown patients' epithelial thickness maps derived from optical coherence tomography (OCT). The percentage of screenings that changed after evaluating the epithelial thickness maps, with regard to candidacy for surgery, and ranking of surgical procedures from most to least favorable was determined.

RESULTS

Candidacy for corneal refractive surgery changed in 16% of patients after evaluation of the epithelial thickness maps, with 10% of patients screened in and 6% screened out. Surgery of choice changed for 16% of patients, and the ranking of surgical procedures from most to least favorable changed for 25% of patients. A total of 11% of patients gained eligibility for LASIK, whereas 8% lost eligibility for LASIK. No significant difference was found between the evaluations of the 2 examiners.

CONCLUSIONS

Epithelial thickness mapping derived from optical coherence tomography imaging of the cornea altered candidacy for corneal refractive surgery, as well as choice of surgery, in a substantial percentage of patients in our practice, and was thus a valuable tool for screening evaluations. Overall, the use of epithelial thickness maps resulted in screening in a slightly larger percentage of patients for corneal refractive surgery.

Repeatability of Corvis ST to Measure Biomechanical Parameters Before and After Myopic Refractive Surgery

PURPOSE

To assess the repeatability of several corneal biomechanical parameters with a Scheimpflug tonometer (Corvis ST) in myopic eyes and eyes that underwent Transepithelial Photorefractive Keratectomy (TransPRK), Small-Incision Lenticule Extraction (SMILE), or Femtosecond Laser-Assisted In Situ Keratomileusis (FS-LASIK) surgery.

SETTING

Eye Hospital of Wenzhou Medical University, Wenzhou, China.

DESIGN

Prospective, randomized controlled study.

METHODS

This study included a total of 315 eyes from 315 patients (135 myopes, 58 post-TransPRK, 52 post-SMILE, and 70 post-FS-LASIK). Three consecutive scans were performed to evaluate the repeatability of the 40 parameters examined.

RESULTS

In all eyes, the coefficient of variation (CoV) for intraocular pressure (IOP) and biomechanical corrected IOP (bIOP) ranged from 7.29% to 9.47% and 6.11% to 7.75%, respectively; the CoV of pachymetry was <0.8%. The intraclass correlation coefficient (ICC) of Corvis Biomechanical Index-Laser Vision Correction (CBI-LVC) was 0.680 for post-TransPRK, 0.978 for post-SMILE, and 0.911 for post-FS-LASIK. The CoV of Stress-Strain Index (SSI) was 204.93% for post-TransPRK, 91.92% for post-SMILE, and 171.72% for post-FS-LASIK. The CoV of the six clinically important dynamic corneal response (DCR) parameters ranged from 2.0% to 7.8% for myopia, 1.8% to 11.1% for post-TransPRK, 2.1% to 8.7% for post-SMILE, and 1.8% to 8.8% for post-FS-LASIK.

CONCLUSIONS

Excellent intra-measurement repeatability of IOP, bIOP and pachymetry was observed in all groups; SSI measurement in post-laser vision correction (LVC) corneas displayed more variation. Caution is warranted when assessing SSI in post-LVC corneas for the purpose of diagnosing iatrogenic ectasia or evaluating biomechanical remodeling of postoperative refractive corneas.

Changes in Corneal Biomechanical Properties in PRK Followed by Two Accelerated CXL Energy Doses in Rabbit Eyes

PURPOSE

To evaluate whether photorefractive keratectomy (PRK) combined with the two commonly delivered energy doses in accelerated corneal cross-linking (A-CXL) could help the cornea maintain its preoperative stiffness level.

METHODS

A total of 72 corneas of 36 healthy white Japanese rabbits were randomly divided into four equal groups. The groups included an untreated control group and three that had undergone PRK. After tissue ablation, one of the latter three groups (PRK group) was left untreated, whereas the other two were exposed to riboflavin (0.22% concentration by volume) and ultraviolet-A (370 nm) with the same irradiation (30 mW/cm2) but different CXL energy doses of 1.8 J/cm2 (PXL group) and 2.7 J/cm2 (PXH group). Dynamic Scheimpflug analyzer (Corvis ST; Oculus Optikgeräte GmbH) measurements of stiffness parameter at first applanation (SP-A1), Stress-Strain Index (SSI), and other dynamic corneal response parameters were taken 3 days preoperatively and 1 month postoperatively. Subsequently, ex vivo inflation testing was performed and the tangent modulus of each specimen was estimated using an inverse analysis process.

RESULTS

In comparison to the control group, the tangent modulus at a stress of 10 kPa decreased by 8.9% in the PRK group and increased by 10.6% and 22.4% in the PXL and PXH groups, respectively. SP-A1 decreased postoperatively in the PRK group (P < .05), indicating an overall stiffness reduction of -7.4, -3.5, and -5.3 mm Hg/mm in PRK, PXL, and PXH groups, respectively. The material stiffness parameter SSI remained almost unchanged in the PRK group (P = .989), increased slightly in the PXL group (8.3%, P = .077), and increased significantly in the PXH group (11.1%) (P < .05).

CONCLUSIONS

Biomechanical deterioration following PRK was significant and could not be fully compensated for by ACXL with either 1.8 or 2.7 J/cm2 doses. The increased value of corneal overall stiffness was higher in A-CXL with 2.7 J/cm2 energy than with 1.8 J/cm2 energy. [J Refract Surg. 2021;37(12):853-860.].

The Percent Volume Altered in Correction of Myopia and Myopic Astigmatism With PRK, LASIK, and SMILE

PURPOSE

To determine the volumes altered during photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), and small incision lenticule extraction (SMILE) correction of myopia and myopic astigmatism.

METHODS

The volumes of stromal tissue affected by surgical correction of pure and astigmatic myopia were calculated in this theoretical study for PRK, LASIK, and SMILE treatments. These results were then used to calculate representative values for percent volume altered (PVA) restricted to stromal corneal tissue.

RESULTS

For the same spherical equivalent, there were remarkable differences between the amount of tissue removed between pure and compound myopic corrections. The PVA of LASIK procedures was the largest, whereas the PVA was comparable between PRK and SMILE. PRK had the lowest PVA for low myopic and compound astigmatic corrections and SMILE had the lowest PVA for high myopic and compound myopic astigmatism procedures.

CONCLUSIONS

The comparison of volumes of tissue removed and PVA values of different treatment modalities provided in this study may give clinicians an improved understanding about the potential influence of geometrical parameters of those methods on the biomechanics of the cornea. [J Refract Surg. 2020;36(12):844-850.].

Contribution of Bowman layer to corneal biomechanics

PURPOSE

To compare the elastic modulus of thin corneal lamellas using 2D stress-strain extensometry in healthy ex vivo human corneal lamellas with or without the presence of Bowman layer.

SETTING

Center for Applied Biotechnology and Molecular Medicine, University of Zurich, Switzerland; ELZA Institute, Dietikon, Switzerland; Department of Ophthalmology, Philipps University of Marburg, Germany.

DESIGN

Prospective experimental laboratory study.

METHODS

Healthy human corneas were stripped of Descemet membrane and the endothelium for Descemet membrane endothelial keratoplasty. After epithelium removal, corneas were divided into 2 groups. In Group 1, Bowman layer was ablated with an excimer laser (20 μm thick, 10 mm). In Group 2, Bowman layer was left intact. Then, a lamella was cut from the anterior cornea with an automated microkeratome. Elastic and viscoelastic material properties were analyzed by 2D stress-strain extensometry between 0.03 and 0.70 N.

RESULTS

Twenty-six human corneas were analyzed. The mean lamella thickness was 160 ± 37 μm in corneas with Bowman layer and 155 ± 22 μm in corneas without. No statistically significant differences between flaps with and without Bowman layer were observed in the tangential elastic modulus between 5% and 20% strain (11.5 ± 2.9 kPa vs 10.8 ± 3.7 kPa, P > .278).

CONCLUSIONS

The presence or absence of Bowman layer did not reveal a measurable difference in corneal stiffness. This may indicate that the removal of Bowman layer during photorefractive keratectomy does not represent a disadvantage to corneal biomechanics.

Is it Possible to Derive the Dresdner Correction Formula Using a Finite Element Program?

INTRODUCTION

The aim was to construct a model cornea by CAD and finite element software to find out how the intraocular pressure compares to the forces for applanation at the outside of the model cornea. These data were to be compared to the Dresdner correction formula. Thereby, it was possible to find out whether the model was plausible and to find hints as to why a correction for how the intraocular pressure depends on the corneal thickness is necessary at all.

METHODS

Using the open-source software FreeCad and geometrical data for the cornea of the literature, an average cornea was constructed. On this average cornea, a finite element analysis was performed using the free software z88aurora. The intraocular pressure was measured by applanation of the outer cornea. The necessary forces were analysed.

RESULTS

In this model, the intraocular pressure had to be corrected depending on the corneal thickness. The correction factor was k_{mean; finite elements} = 19.17 - 0.0334*corneal thickness. The necessary correction did not exclusively depend on the relation between the endothelial area and the area of the outer cornea: for this relation alone the correction would have been k_{area-relation} = 1.0361 - 0.0006*corneal thickness.

DISCUSSION

The model correction formula was close to the Dresdner formula. The relation between endothelial area and the area of the outer cornea could only explain about half of the necessary correction.

Quasi-Static Optical Coherence Elastography to Characterize Human Corneal Biomechanical Properties

Purpose

Quasi-static optical coherence elastography (OCE) is an emerging technology to investigate corneal biomechanical behavior in situations similar to physiological stress conditions. Herein OCE was applied to evaluate previously inaccessible biomechanical characteristics of human corneal tissue and to study the role of Bowman's layer in corneal biomechanics.

Methods

Human corneal donor buttons (n = 23) were obtained and Descemet's membrane and endothelium were removed. In 11 corneas, Bowman's layer was ablated by a 20 µm stromal excimer laser ablation. Buttons were mounted on an artificial anterior chamber and subjected to a pressure modulation from 10 to 30 mm Hg, and back to 10 mm Hg, in steps of 1 mm Hg. At each step, a spectral-domain optical coherence tomography scan was obtained. Displacements were analyzed by optical flow tracking, and strain over the entire stromal depth was retrieved from the phase gradient of the complex interference signal.

Results

During pressure increase, corneal tissue moved upward (486–585 nm/mm Hg) but did not fully recover (Δ= 2.63 to 8.64 µm) after pressure decrease. Vertical corneal strain distribution was negative in the anterior and positive in the posterior cornea, indicating simultaneous corneal compression and expansion, respectively. Bowman's layer caused minor localized differences in corneal strain distribution.

Conclusions

Corneal strain distribution is more complex than previously assumed, with a fundamental difference in mechanical response between the anterior and posterior stroma. Clinically, OCE technology might be used to monitor the progression of corneal ectatic diseases and to determine the success of corneal cross-linking.

Similar Biomechanical Cross-linking Effect After SMILE and PRK in Human Corneas in an Ex Vivo Model for Postoperative Ectasia

PURPOSE

To evaluate the biomechanical effect of corneal cross-linking (CXL) in paired human corneas following small incision lenticule extraction (SMILE) or photorefractive keratectomy (PRK) in an ex vivo model for postoperative ectasia.

METHODS

Twenty-six paired human corneas preserved in tissue culture medium were equally divided into two groups: right and left corneas were treated with PRK and SMILE, respectively. Corneal thickness was measured in all eyes before surgery. Corneas were stretched using an extensometer with two cycles of up to 9 N (570 kPA stress), followed by accelerated CXL with irradiance of 9 mW/cm² for 10 minutes (fluence 5.4 J/cm²) in both groups. The elastic modulus was evaluated using two-dimensional stress-strain extensometry.

RESULTS

Following accelerated CXL, the ectatic cornea model showed a mean effective elastic modulus of 17.2 ± 5.3 MPa after PRK and 14.1 ± 5.0 MPa after SMILE. Although the elastic modulus in corneas previously subjected to PRK was higher, there was no significant biomechanical difference between the two groups (P = .093).

CONCLUSIONS

Under similar conditions, both experimental groups (PRK followed by CXL and SMILE followed by CXL) were characterized by similar biomechanical stability as measured experimentally on ex vivo human fellow corneas. The data suggest that, in the event of postoperative ectasia, the biomechanical improvement achieved by CXL may be similar after PRK and SMILE. [J Refract Surg. 2020;36(1):49-54].

SMILE – Small Incision Lenticule Extraction

Die SMILE (small incision lenticule extraction) zählt zu den Verfahren der refraktiven Lentikel-Extraktion und hat sich im letzten Jahrzehnt zu einem etablierten Bestandteil des modernen refraktivchirurgischen Spektrums entwickelt. Dieser Beitrag gibt einen Überblick über Patientenselektion, Operationsmethode, mögliche Komplikationen und klinische Ergebnisse dieser Methode.

[SMILE - Small Incision Lenticule Extraction]

Refractive lenticule extraction is a corneal surgical technique that uses a femtosecond laser exclusively to create an intrastromal refractive lenticule for the correction of myopia and myopic astigmatism. In small incision lenticule extraction (SMILE) the generated refractive lenticule is subsequently extracted through a small incision. The reported efficacy, predictability and safety of the flap-less SMILE procedure is similar to those of femtosecond laser in situ keratomileusis (LASIK). Advantages of SMILE over LASIK include less iatrogenic dry eye, fewer induced higher-order aberrations, and potentially less biomechanical weakening of the cornea. However, there is a steeper surgeon learning curve for SMILE as the procedure is technically more challenging than LASIK. Furthermore, the current SMILE laser platform cannot use cyclotorsion control or eye-tracking technology and retreatment options are more complex compared to LASIK. This review looks at patient selection, surgical method, possible complications, retreatment options, and postoperative outcome of the SMILE technique.