Five-year follow-up of corneal morphology and corneal refractive power changes after uneventful DMEK.

PURPOSE: To investigate changes of corneal thickness spatial profile (CTSP), corneal volume (CV) distribution, and total corneal refractive power (TCRP) over a course of 60 months after uneventful Descemet membrane endothelial keratoplasty (DMEK). METHODS: In our prospective, comparative study, sixty DMEK cases without intraoperative and postoperative complications and with complete 60-month follow-up were included (group 1). CTSP at corneal apex (CCT) and at 2 mm, 4 mm, 6 mm, and 8 mm rings, CV in 3 mm, 5 mm, 7 mm, and 10 mm zones, and TCRF in 2 mm, 4 mm 6 mm, and 8 mm zones were evaluated preoperatively and at 3, 6, 12, 24, and 60 months postoperatively. The 60-month results were compared to an age-matched group of uncomplicated pseudophakic eyes (group 2; n = 20). RESULTS: The CCT and CTSP at 2, 4, and 6 mm increased significantly at 60 months compared to 3-month outcomes (P < 0.001). Similarly, CV increased significantly in 3 mm, 5 mm, and 7 mm zones at 60 months compared to 3 months outcomes (P < 0.001). The TCRP showed in all zones a significant decrease at 3 months (P < 0.001) followed by a continuous and significant increase at 60 months (P < 0.001). The 60-month CCT and CTSP at 2 mm were similar to group 2 (P ≥ 0.094). CONCLUSION: Sixty months after uneventful DMEK, CT within the central 2 mm zone and CV at 3 mm zone were similar to uncomplicated pseudophakic eyes. A continuous and statistically significant increase of TCRP was observed in all measured zones after the 3-month examination.

Corneal Remodeling After Myopic SMILE Versus FS-LASIK: A Spatial Analysis of Short- and Mid-Term Corneal Thickness, Volume, and Shape Changes.

PURPOSE: To evaluate the short- and mid-term changes of corneal thickness spatial profile (CTSP), corneal volume distribution (CVD), and corneal asphericity after small-incision lenticule extraction (SMILE) for correction of myopia and astigmatism and compare the results with femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK). METHOD: Thirty eyes of 18 patients who underwent SMILE were compared with a group of 30 eyes of 16 patients who underwent FS-LASIK. The groups were matched for preoperative central corneal thickness and lenticule thickness/ablation depth. Scheimpflug corneal tomography was performed preoperatively and postoperatively at 2 months and 3 years. The CTSP was evaluated on 4 concentric rings (2, 4, 6, and 8 mm). The CVD was evaluated at 3 concentric zones (3, 5, and 7 mm). Changes in the anterior and posterior asphericity at a 6-mm zone were also evaluated. RESULTS: Between the 2-month and 3-year examination, the CTSP showed a similar increase for both groups at all measured points and rings ( P ≥ 0.168). The CVD also showed a similar increase for both groups at all measured zones ( P ≥ 0.278). The anterior corneal asphericity remained stable after SMILE (2-mo Q = 0.46 ± 0.27; 3-yr Q = 0.45 ± 0.27; P = 0.711) but decreased significantly after FS-LASIK (2-mo Q = 0.52 ± 0.47; 3-yr Q = 0.47 ± 0.44; P = 0.028). Similarly, the posterior corneal asphericity remained stable after SMILE (2-mo Q = -0.11 ± 0.15; 3-yr Q = -0.11 ± 0.13; P = 0.902) but decreased significantly after FS-LASIK (2-mo Q = -0.13 ± 0.14; 3-yr Q = -0.16 ± 0.15; P = 0.034). CONCLUSIONS: CTSP and CVD between the 2-month and 3-year examination showed a similar increase after SMILE and FS-LASIK. During the postoperative course, the anterior and posterior corneal asphericity remained more stable after SMILE compared with FS-LASIK.

Hyperopic SMILE Versus FS-LASIK: A Biomechanical Comparison in Human Fellow Corneas.

PURPOSE: To investigate the biomechanical properties of ex vivo human paired corneas after hyperopic correction via cap-based versus flap-based laser-assisted refractive surgery. METHODS: In this prospective experimental study, 13 pairs of human corneas unsuitable for transplantation were equally divided into two groups. The pachymetry was performed in each eye just before the laser procedure. Corneas from the right eye were treated with small incision lenticule extraction (SMILE), whereas corneas from the left eye of the same donor were treated with femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK). All corneas were subjected to a refractive correction of +6.00 diopters (D) sphere with a 6.5-mm zone under a 120-µm cap (SMILE) or a 7-mm zone under a 110-µm flap (FS-LASIK). For two-dimensional biomechanical measurements, the 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 seconds) 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 not significantly different (P > .311) between SMILE (13.5 ± 12.8 MPa) and FS-LASIK (7.56 ± 17.9 MPa). In stress-relaxation measurements, the remaining stress was not significantly different (P = .841) between SMILE (124 ± 20 kPa) and FS-LASIK (126 ± 21 kPa). CONCLUSIONS: Unlike myopic correction, after hyperopic correction the cap-based procedure (SMILE) and the flap-based technique (FS-LASIK) may be considered equivalent in terms of biomechanical stability when measured experimentally in ex vivo human fellow eye corneas. [J Refract Surg. 2021;37(12):810-815.].

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.

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].

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

PURPOSE: To investigate the biomechanical properties of the ex vivo human paired corneas after completion of photorefractive keratectomy (PRK) versus small incision lenticule extraction (SMILE) in the same donor. METHODS: In this experimental study, 13 pairs of human corneas unsuitable for transplantation were equally divided into two groups. Corneal thickness was measured in each eye directly before laser refractive surgery. Corneas from the right eye were treated with PRK and corneas from the left eye with SMILE. 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 surface ablation (PRK) 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 seconds) 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 not significantly different (P = .081) between SMILE (9.58 ± 4.26 MPa) and PRK (11.9 ± 4.90 MPa). The effect size was medium (Cohen's d = 0.58). In stress-relaxation measurements, the remaining stress was not significantly different (P = .878) between SMILE (122 ± 33 kPa) and PRK (123 ± 30 kPa). CONCLUSIONS: The lenticule extraction procedure (SMILE) and the surface ablation technique (PRK) may be considered equivalent in terms of biomechanical stability when measured experimentally in ex vivo human fellow eye corneas. [J Refract Surg. 2019;35(8):501-505.].

Spontaneous breakage of intracorneal ring segments (ICRS) 8 years after implantation.

We present a case of spontaneous in situ breakage of intracorneal ring segments (ICRSs) 8 years after their implantation in a patient with keratoconus. The patient presented to our clinic with a red and painful right eye that had not improved despite topical steroids and antibiotics. The decision was made to explant the broken ICRSs from the cornea and send them for laboratory analyses, by which manufacturer defects were excluded. It is noteworthy that corneal curvature re-steepening was observed 4 months after ICRS removal despite the performance of crosslinking (CXL) 1 year prior to ICRS implantation.

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.].

Biomechanical Differences Between Femtosecond Lenticule Extraction (FLEx) and Small Incision Lenticule Extraction (SmILE) Tested by 2D-Extensometry in Ex Vivo Porcine Eyes.

Purpose: To evaluate the biomechanical stability of ex vivo porcine corneas after femtosecond lenticule extraction (FLEx) and small incision lenticule extraction (SmILE) refractive surgeries. Methods: Forty-five porcine eyes were equally divided into three groups: Groups 1 and 2 were treated with FLEx and SmILE procedure, respectively. Group 3 served as control. A refractive correction of -14 diopters (D) with a 7-mm zone using either a 160-µm flap (FLEx) or a 160-µm cap (SmILE) was performed. For two-dimensional (2D) elastic and viscoelastic biomechanical characterization, two testing cycles (preconditioning stress-strain curve from 1.27 to 12.5 N, stress-relaxation at 12.5 N during 120 seconds) were conducted. Young's modulus and Prony constants were calculated. Results: At 0.8% of strain, FLEx (370 ± 36 kPa) could resist a significantly lower stress than SmILE (392 ± 19 kPa, P = 0.046) and the control group (402 ± 30 kPa, P = 0.013). Also, FLEx (46.1 ± 4.5 MPa) had a significantly lower Young's modulus than the control group (50.2 ± 3.4 MPa, P = 0.008). The Young's modulus of SmILE (48.6 ± 2.5 MPa) had values situated between untreated corneas and FLEx-treated corneas. When compared to untreated controls, the stress resistance decreased by 8.0% with FLEx and 2.5% with SmILE; Young's modulus decreased by 5.1% with FLEx and 1.04% with SmILE. With a cap-based procedure, both anterior cap and stromal bed carry the intraocular pressure, while in a flap-based procedure, only the stromal bed does. Conclusions: Compared to flap-based procedures like FLEx, the cap-based technique SmILE can be considered superior in terms of biomechanical stability, when measured experimentally in ex vivo porcine corneas.

Biomechanical Weakening of Different Re-treatment Options After Small Incision Lenticule Extraction (SMILE).

PURPOSE: To determine the corneal weakening induced by different re-treatment options after small incision lenticule extraction (SMILE) and investigate the potential of corneal cross-linking (CXL) to reestablish the original corneal stress resistance. METHODS: A total of 96 freshly enucleated porcine corneas were used. The initial refractive correction was defined to be -11.00 diopters (D) and the required enhancement to be -3.00 D. Three different re-treatment options were analyzed: -3D Re-SMILE, -3D photorefractive keratectomy (PRK) on top of the SMILE cap, and cap-to-flap conversion and -3D excimer ablation on the stromal bed (LASIK). The control condition did not receive any treatment. Subsequently, accelerated CXL (9 mW/cm2, 10 min) was performed in two groups with currently common enhancement techniques: following cap-to-flap conversion (-3D LASIK enhancement) and in controls. Biomechanical properties were measured with stress-strain extensometry ranging from 1.27 to 12.5 N. RESULTS: The Re-SMILE and PRK enhancement did not significantly reduce the overall elastic modulus of the cornea compared to controls (24.7 ± 2.23 and 22.7 ± 2.61 versus 23.8 ± 3.35 MPa, P ≥ .176), whereas LASIK enhancement did (22.2 ± 3.37 MPa, P = .048). CXL treatment significantly increased the elastic modulus compared to all non-cross-linked conditions (P ≤.001). Refractive surgery decreased the overall elastic modulus by 7%, whereas CXL increased it by 20%. CONCLUSIONS: In enhancement, the corneal biomechanical integrity is less affected with both Re-SMILE and PRK enhancement. Corneal weakening through laser refractive surgery is small compared to the stiffening effect after CXL. [J Refract Surg. 2017;33(3):193-198.].