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

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          Abstract

          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/CCT postop) 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.

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          Most cited references29

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          Introduction of Two Novel Stiffness Parameters and Interpretation of Air Puff-Induced Biomechanical Deformation Parameters With a Dynamic Scheimpflug Analyzer.

          Ashraf Mahmoud, Jeffrey Bons, Cynthia Roberts … (2017)
          To investigate two new stiffness parameters and their relationships with the dynamic corneal response (DCR) parameters and compare normal and keratoconic eyes.
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            Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction.

            J Bradley Randleman, T. Archer, D Reinstein (2013)
            To develop a mathematical model to estimate the relative differences in postoperative stromal tensile strength following photorefractive keratectomy (PRK), LASIK, and small incision lenticule extraction (SMILE). Using previously published data where in vitro corneal stromal tensile strength was determined as a function of depth, a mathematical model was built to calculate the relative remaining tensile strength by fitting the data with a fourth order polynomial function yielding a high correlation coefficient (R(2) = 0.930). Calculating the area under this function provided a measure of total stromal tensile strength (TTS), based only on the residual stromal layer for PRK or LASIK and the residual stromal layers above and below the lenticule interface for SMILE. Postoperative TTS was greatest after SMILE, followed by PRK, then LASIK; for example, in a 550-μm cornea after 100-μm tissue removal, postoperative TTS was 75% for SMILE (130-μm cap), 68% for PRK, and 54% for LASIK (110-μm flap). The postoperative TTS decreased for thinner corneal pachymetry for all treatment types. In LASIK, the postoperative TTS decreased with increasing flap thickness by 0.22%/μm, but increased by 0.08%/μm for greater cap thickness in SMILE. The model predicted that SMILE lenticule thickness could be approximately 100 μm greater than the LASIK ablation depth and still have equivalent corneal strength (equivalent to approximately 7.75 diopters). This mathematical model predicts that the postoperative TTS is considerably higher after SMILE than both PRK and LASIK, as expected given that the strongest anterior lamellae remain intact. Consequently, SMILE should be able to correct higher levels of myopia. Copyright 2013, SLACK Incorporated.
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              Determination of Corneal Biomechanical Behavior in-vivo for Healthy Eyes Using CorVis ST Tonometry: Stress-Strain Index

              Ashkan Eliasy, Kai-Jung Chen, Riccardo Vinciguerra … (2019)
              Purpose: This study aims to introduce and clinically validate a new algorithm that can determine the biomechanical properties of the human cornea in vivo. Methods: A parametric study was conducted involving representative finite element models of human ocular globes with wide ranges of geometries and material biomechanical behavior. The models were subjected to different levels of intraocular pressure (IOP) and the action of external air puff produced by a non-contact tonometer. Predictions of dynamic corneal response under air pressure were analyzed to develop an algorithm that can predict the cornea's material behavior. The algorithm was assessed using clinical data obtained from 480 healthy participants where its predictions of material behavior were tested against variations in central corneal thickness (CCT), IOP and age, and compared against those obtained in earlier studies on ex-vivo human ocular tissue. Results: The algorithm produced a material stiffness parameter (Stress-Strain Index or SSI) that showed no significant correlation with both CCT (p > 0.05) and IOP (p > 0.05), but was significantly correlated with age (p < 0.01). The stiffness estimates and their variation with age were also significantly correlated (p < 0.01) with stiffness estimates obtained earlier in studies on ex-vivo human tissue. Conclusions: The study introduced and validated a new method for estimating the in vivo biomechanical behavior of healthy corneal tissue. The method can aid optimization of procedures that interfere mechanically with the cornea such as refractive surgeries and introduction of corneal implants.
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                Author and article information

                Journal
                Journal ID (nlm-ta): Transl Vis Sci Technol
                Journal ID (iso-abbrev): Transl Vis Sci Technol
                Journal ID (publisher-id): TVST
                Title: Translational Vision Science & Technology
                Publisher: The Association for Research in Vision and Ophthalmology
                ISSN (Electronic): 2164-2591
                Publication date (Electronic): 16 March 2023
                Publication date Collection: March 2023
                Volume: 12
                Issue: 3
                Electronic Location Identifier: 12
                Affiliations
                [1 ]Noor Ophthalmology Research Center, Noor Eye Hospital, Tehran, Iran
                [2 ]Department of Ophthalmology & Visual Sciences, Department of Biomedical Engineering, The Ohio State University, Columbus, OH, USA
                [3 ]School of Engineering, University of Liverpool, Liverpool, UK
                [4 ]National Institute for Health Research (NIHR) Biomedical Research Centre for Ophthalmology, Moorfields Eye Hospital NHS Foundation Trust and UCL Institute of Ophthalmology, London, UK
                [5 ]Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, China
                [6 ]School of Computer, Mathematical, and Natural Sciences, Morgan State University, Baltimore, MD, USA
                [7 ]Noor Research Center for Ophthalmic Epidemiology, Noor Eye Hospital, Tehran, Iran
                Author notes
                [* ] Correspondence: Soheila Asgari, Noor Ophthalmology Research Center, Noor Eye Hospital, No. 96 Esfandiar Blvd., Vali'asr Ave., Po Box: 3475-19395, Tehran, Iran. e-mail: soheilaasgari@ 123456gmail.com
                Article
                Publisher ID: TVST-22-5169
                DOI: 10.1167/tvst.12.3.12
                PMC ID: 10029763
                PubMed ID: 36928130
                SO-VID: 25e5ba3b-422a-4887-ad11-898e4fcbbb31
                Copyright © Copyright 2023 The Authors
                License:

                This work is licensed under a Creative Commons Attribution 4.0 International License.

                History
                Date accepted : 26 January 2023
                Date received : 08 November 2022
                Page count
                Pages: 9
                Categories
                Subject: Refractive Intervention
                Subject: Refractive Intervention

                Keywords: corneal stiffness,small incision lenticule extraction,femtosecond laser-assisted in situ keratomileusis,photorefractive keratectomy,matched comparison study
                Data availability:
                Keywords: corneal stiffness, small incision lenticule extraction, femtosecond laser-assisted in situ keratomileusis, photorefractive keratectomy, matched comparison study

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