The Open Ophthalmology Journal




ISSN: 1874-3641 ― Volume 12, 2018
REVIEW ARTICLE

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



Iben Bach Damgaard1, *, Mohamed Reffat2, Jesper Hjortdal1
1 Department of Ophthalmology, Aarhus University Hospital, Aarhus, Denmark.
2 Mansoura Ophthalmic Hospital, Mansoura, Egypt

Abstract

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.

Keywords: Myopia, SMILE, LASIK, Myopia astigmatism, Corvis, Corvist ST.


Article Information


Identifiers and Pagination:

Year: 2018
Volume: 12
Issue: Suppl-1, M5
First Page: 164
Last Page: 174
Publisher Id: TOOPHTJ-12-164
DOI: 10.2174/1874364101812010164

Article History:

Received Date: 19/10/2017
Revision Received Date: 08/01/2018
Acceptance Date: 20/01/2018
Electronic publication date: 27/07/2018
Collection year: 2018

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© 2018 Damgaard et al.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: (https://creativecommons.org/licenses/by/4.0/legalcode). This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


* Address correspondence to this author at the Department of Ophthalmology, Aarhus University Hospital, Noerrebrogade 44, Building 10, 2nd floor, 8000 Aarhus C, Denmark, Tel: +45 25522506, Fax: + 45 86121653; E-mail: iben.b.pedersen@gmail.com




1. INTRODUCTION

During the last few decades, laser refractive surgery has gained extensive interest for correction of refractive errors such as myopia, hyperopia, astigmatism, and presbyopia. Worldwide, femtosecond laser-assisted in-situ keratomileusis (LASIK) has been widely implemented in clinical practice [1Solomon KD, Fernández de Castro LE, Sandoval HP, et al. LASIK world literature review: Quality of life and patient satisfaction. Ophthalmology 2009; 116(4): 691-701.[http://dx.doi.org/10.1016/j.ophtha.2008.12.037] [PMID: 19344821] ] In LASIK, a reproducible flap of a predetermined thickness is made with a femtosecond laser, and a stromal bed shaped with excimer laser according to the amount of refractive correction is needed. In 2011, Small Incision Lenticule Extraction (SMILE) was introduced as a refinement of the LASIK technique [2Sekundo W, Kunert KS, Blum M. Small incision corneal refractive surgery using the Small Incision Lenticule Extraction (SMILE) procedure for the correction of myopia and myopic astigmatism: Results of a 6 month prospective study. Br J Ophthalmol 2011; 95(3): 335-9.[http://dx.doi.org/10.1136/bjo.2009.174284] [PMID: 20601657] , 3Shah R, Shah S, Sengupta S. Results of small incision lenticule extraction: All-in-one femtosecond laser refractive surgery. J Cataract Refract Surg 2011; 37(1): 127-37.[http://dx.doi.org/10.1016/j.jcrs.2010.07.033] [PMID: 21183108] ]. In SMILE, an intrastromal lenticule is cut with a femtosecond laser, and subsequently removed through a minor incision. Both SMILE and LASIK have shown high efficacy, predictability, and safety [4Ivarsen A, Asp S, Hjortdal J. Safety and complications of more than 1500 small-incision lenticule extraction procedures. Ophthalmology 2014; 121(4): 822-8.[http://dx.doi.org/10.1016/j.ophtha.2013.11.006] [PMID: 24365175] -9Pedersen IB, Ivarsen A, Hjortdal J. Three-year results of small incision lenticule extraction for high myopia: Refractive outcomes and aberrations. J Refract Surg 2015; 31(11): 719-24.[http://dx.doi.org/10.3928/1081597X-20150923-11] [PMID: 26444906] ], but SMILE may have an advantage of being a flap-free procedure preserving the corneal biomechanical strength better than LASIK [10Sinha Roy A, Dupps WJ Jr, Roberts CJ. Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: Finite-element analysis. J Cataract Refract Surg 2014; 40(6): 971-80.[http://dx.doi.org/10.1016/j.jcrs.2013.08.065] [PMID: 24857440] , 11Reinstein DZ, Archer TJ, Randleman JB. Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction. J Refract Surg 2013; 29(7): 454-60.[http://dx.doi.org/10.3928/1081597X-20130617-03] [PMID: 23820227] ].

The cornea consists of approximately 200 collagen lamellae containing collagen fibres that are crucial to withstand the intraocular pressure and maintain the corneal shape [12Aghamohammadzadeh H, Newton RH, Meek KM. X-ray scattering used to map the preferred collagen orientation in the human cornea and limbus. Structure 2004; 12(2): 249-56.[http://dx.doi.org/10.1016/j.str.2004.01.002] [PMID: 14962385] , 13Bergmanson JP, Horne J, Doughty MJ, Garcia M, Gondo M. Assessment of the number of lamellae in the central region of the normal human corneal stroma at the resolution of the transmission electron microscope. Eye Contact Lens 2005; 31(6): 281-7.[http://dx.doi.org/10.1097/01.ICL.0000165280.94927.0D] [PMID: 16284509] ]. The lamellar interweaving and corneal cross-linking ensure the biomechanical strength of the cornea, the anterior 1/3 being the strongest part [14Randleman JB, Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF. Depth-dependent cohesive tensile strength in human donor corneas: implications for refractive surgery. J Refract Surg 2008; 24(1): S85-9.[PMID: 18269156] ]. The tensile strength in the corneal lamellae decreases after incision and does not contribute to the overall corneal resistance to the intraocular pressure. Due to the orientation of the lamellae, a vertical cut causes greater reduction in the corneal biomechanical strength than a cut parallel to the corneal surface. [15Knox Cartwright NE, Tyrer JR, Jaycock PD, Marshall J. Effects of variation in depth and side cut angulations in LASIK and thin-flap LASIK using a femtosecond laser: a biomechanical study. J Refract Surg 2012; 28(6): 419-25.[http://dx.doi.org/10.3928/1081597X-20120518-07] [PMID: 22692524] ]. The irreversible corneal alterations after refractive surgery affect the biomechanical properties by reducing the biomechanical strength. Thus, iatrogenic ectasia is one of the severe complications caused by a biomechanical weakening after laser refractive surgery, seen by corneal thinning, protrusion, increased myopia, irregular astigmatism, and decreased visual acuity [16Randleman JB, Russell B, Ward MA, Thompson KP, Stulting RD. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology 2003; 110(2): 267-75.[http://dx.doi.org/10.1016/S0161-6420(02)01727-X] [PMID: 12578766] ]. Iatrogenic ectasia has been reported in a few cases after both LASIK [17Moshirfar M, Smedley JG, Muthappan V, Jarsted A, Ostler EM. Rate of ectasia and incidence of irregular topography in patients with unidentified preoperative risk factors undergoing femtosecond laser-assisted LASIK. Clin Ophthalmol 2014; 8: 35-42.[PMID: 24363553] ] and SMILE [18El-Naggar MT. Bilateral ectasia after femtosecond laser-assisted small-incision lenticule extraction. J Cataract Refract Surg 2015; 41(4): 884-8.[http://dx.doi.org/10.1016/j.jcrs.2015.02.008] [PMID: 25747164] -21Wang Y, Cui C, Li Z, et al. Corneal ectasia 6.5 months after small-incision lenticule extraction. J Cataract Refract Surg 2015; 41(5): 1100-6.[http://dx.doi.org/10.1016/j.jcrs.2015.04.001] [PMID: 25953471] ], although the comprehensive evaluation of the SMILE cases showed preoperative abnormal topographic patterns in almost all cases.

LASIK may cause a greater reduction of the biomehcanical strength due to the almost circumferential cut during flap creation, compared with only a 2-3 mm incision length in SMILE. Several studies have tried to answer this question both in vivo and ex vivo, and by mathematical or finite-element models of the cornea. This review aimed to describe current literature regarding corneal biomechanical properties after LASIK and SMILE. Our search was performed in pubmed.gov, using the following research terms: Corneal biomechanical properties, corneal biomechanics, ocular response analyser, ocular response analyzer, ORA, ex vivo, in vitro, Corvis, Corvis ST, LASIK, and SMILE. A total of 57 studies were included in the initial search, where six studies were excluded due to non-English language. Based on abstract reading, 14 studies described and compared the biomechanical properties following LASIK and SMILE. Comparisons with other laser refractive techniques such as photorefractive keratectomy (PRK), FLEX, and LASEK as well as the dependency of IOP, CCT, refractive status, cap/flap thickness, and age were outside the topic of this review.

1.1. In vivo Corneal Biomechanical Assessment

Several approaches to in vivo assessment of the biomechanical properties have been proposed, including Brillouin microscopy [22Scarcelli G, Kling S, Quijano E, Pineda R, Marcos S, Yun SH. Brillouin microscopy of collagen crosslinking: Noncontact depth-dependent analysis of corneal elastic modulus. Invest Ophthalmol Vis Sci 2013; 54(2): 1418-25.[http://dx.doi.org/10.1167/iovs.12-11387] [PMID: 23361513] ], optical coherence elastrography [23Ford MR, Dupps WJ Jr, Rollins AM, Sinha RA, Hu Z. Method for optical coherence elastography of the cornea. J Biomed Opt 2011; 16(1): 016005.[http://dx.doi.org/10.1117/1.3526701] [PMID: 21280911] ], and supersonic shear wave elastrography [24Touboul D, Gennisson JL, Nguyen TM, et al. Supersonic shear wave elastography for the in vivo evaluation of transepithelial corneal collagen cross-linking. Invest Ophthalmol Vis Sci 2014; 55(3): 1976-84.[http://dx.doi.org/10.1167/iovs.13-13445] [PMID: 24519426] ]. To date, there are only two commercially available devices to analyse some corneal biomechanical properties in vivo; the Ocular Response Analyser (ORA) [25Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg 2005; 31(1): 156-62.[http://dx.doi.org/10.1016/j.jcrs.2004.10.044] [PMID: 15721708] ] and the Corvis ST [26Pedersen IB, Bak-Nielsen S, Vestergaard AH, Ivarsen A, Hjortdal J. Corneal biomechanical properties after LASIK, ReLEx flex, and ReLEx smile by Scheimpflug-based dynamic tonometry. Graefes Arch Clin Exp Ophthalmol 2014; 252(8): 1329-35.[http://dx.doi.org/10.1007/s00417-014-2667-6] [PMID: 24862302] ].

Fig. (1)
Screenshot from the Ocular Response Analyser showing the applanation signal at inward and outward applanation (red peaks). Corneal Hysteresis (CH) is defined as the difference in the applied pressure during the first and second applanation. CRF: Corneal Resistance Factor. IOPg: Goldman Correlated Intraocular Pressure. IOPcc: Corneal Compensated Intraocular Pressure. CCT: Central Corneal Thickness.


1.1.1. The Ocular Response Analyser

The Ocular Response Analyser (ORA, Reichert Inc., Dephew, NY) is a non-contact differential tonometer evaluating the in vivo corneal viscoelasticity and intraocular pressure during a collimated air-pulse pressurizing the corneal apex [25Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg 2005; 31(1): 156-62.[http://dx.doi.org/10.1016/j.jcrs.2004.10.044] [PMID: 15721708] ]. The corneal inward (P1) and outward (P2) applanation points are registered with an electro-optical infrared system Fig. (1) and translated into IOP values based on the applied pressure. Corneal Hysteresis (CH) is defined as the difference between the applied pressure during inward and outward applanation (CH=P1-P2), and describes cornea ability to dissipate energy due to viscous damping. Corneal hysteresis reflects the combined change in the viscosity and elasticity, previously shown in an experimental study with the ORA [27Glass DH, Roberts CJ, Litsky AS, Weber PA. A viscoelastic biomechanical model of the cornea describing the effect of viscosity and elasticity on hysteresis. Invest Ophthalmol Vis Sci 2008; 49(9): 3919-26.[http://dx.doi.org/10.1167/iovs.07-1321] [PMID: 18539936] ]. Corneal Resistance Factor (CRF) is determined by an empirical formula, based on the correlation between P1, P2, and CCT, reflecting the overall corneal resistance. Both values have been shown to be affected by CCT [28Shah S, Laiquzzaman M, Cunliffe I, Mantry S. The use of the Reichert ocular response analyser to establish the relationship between ocular hysteresis, corneal resistance factor and central corneal thickness in normal eyes. Cont Lens Anterior Eye 2006; 29(5): 257-62.[http://dx.doi.org/10.1016/j.clae.2006.09.006] [PMID: 17085066] -31Alhamad TA, Meek KM. Comparison of factors that influence the measurement of corneal hysteresis in vivo and in vitro. Acta Ophthalmol 2011; 89(5): e443-50.[http://dx.doi.org/10.1111/j.1755-3768.2011.02150.x] [PMID: 21457485] ], IOP [30Kamiya K, Hagishima M, Fujimura F, Shimizu K. Factors affecting corneal hysteresis in normal eyes. Graefes Arch Clin Exp Ophthalmol 2008; 246(10): 1491-4.[http://dx.doi.org/10.1007/s00417-008-0864-x] [PMID: 18546008] -32Oncel B, Dinc U, Orge F, Yalvac B. Comparison of IOP measurement by ocular response analyzer, dynamic contour, Goldmann applanation, and noncontact tonometry. Eur J Ophthalmol 2009; 19(6): 936-41.[http://dx.doi.org/10.1177/112067210901900607] [PMID: 19882585] ], and age [33Foster PJ, Broadway DC, Garway-Heath DF, et al. Intraocular pressure and corneal biomechanics in an adult British population: The EPIC-Norfolk eye study. Invest Ophthalmol Vis Sci 2011; 52(11): 8179-85.[http://dx.doi.org/10.1167/iovs.11-7853] [PMID: 21931129] -35Kotecha A, Elsheikh A, Roberts CR, Zhu H, Garway-Heath DF. Corneal thickness- and age-related biomechanical properties of the cornea measured with the ocular response analyzer. Invest Ophthalmol Vis Sci 2006; 47(12): 5337-47.[http://dx.doi.org/10.1167/iovs.06-0557] [PMID: 17122122] ], and must be taken into consideration when interpreting ORA outcomes.

1.1.2. The Corvis ST

(Oculus, Wetzlar, Germany) was later presented as an alternative device for in vivo acquisition of the corneal biomechanical properties. It combines non-contact tonometry with high-speed Scheimpflug visualization of the corneal deformation during the symmetrically metered air pulse Fig. (2). With 4330 frames per second, the Corvis ST records the dynamic deformation and determines velocity, length, and time lapse during applanation and highest concavity Table 1. The initial Corvis ST software version presented a limited number of parameters; later new variables followed describing the inward and outward applanation in further detail.

Fig. (2)
Screenshot of corneal deformation during an air-pulse from the Corvis ST with first inward applanation and maximal deformation.


Table 1
Description of Corvis ST parameters. *: Parameters avaliable with the first software version.


2. THE ORA: LASIK VERSUS SMILE

Several studies have assessed the ORA parameters after laser refractive procedures to describe corneal biomechanical properties after both LASIK and SMILE, whereas only a few studies have compared the biomechanical alterations after surgery Table 2 [26Pedersen IB, Bak-Nielsen S, Vestergaard AH, Ivarsen A, Hjortdal J. Corneal biomechanical properties after LASIK, ReLEx flex, and ReLEx smile by Scheimpflug-based dynamic tonometry. Graefes Arch Clin Exp Ophthalmol 2014; 252(8): 1329-35.[http://dx.doi.org/10.1007/s00417-014-2667-6] [PMID: 24862302] , 36Xia L, Zhang J, Wu J, Yu K. Comparison of corneal biological healing after femtosecond LASIK and small incision lenticule extraction procedure. Curr Eye Res 2016; 41(9): 1202-8.[http://dx.doi.org/10.3109/02713683.2015.1107590] [PMID: 26833247] -44Agca A, Ozgurhan EB, Demirok A, et al. Comparison of corneal hysteresis and corneal resistance factor after small incision lenticule extraction and femtosecond laser-assisted LASIK: A prospective fellow eye study. Cont Lens Anterior Eye 2014; 37(2): 77-80.[http://dx.doi.org/10.1016/j.clae.2013.05.003] [PMID: 23830802] ]. A retrospective study by Osman et al. [38Osman IM, Helaly HA, Abdalla M, Shousha MA. Corneal biomechanical changes in eyes with small incision lenticule extraction and laser assisted in situ keratomileusis. BMC Ophthalmol 2016; 16: 123.[http://dx.doi.org/10.1186/s12886-016-0304-3] [PMID: 27457241] ] examined 25 LASIK- and 25 SMILE-treated patients one month after surgery, and found a more profound reduction in the CH and CRF parameters after LASIK than SMILE (preop SE: LASIK −5.16±1.42D, SMILE −5.43±1.17D). These findings were in agreement with Wang et al. [42Wang D, Liu M, Chen Y, et al. Differences in the corneal biomechanical changes after SMILE and LASIK. J Refract Surg 2014; 30(10): 702-7.[http://dx.doi.org/10.3928/1081597X-20140903-09] [PMID: 25291754] ] where CH and CRF decreased more in LASIK-treated patients with myopia more than -6D at one week, and one and three-month follow-up. However, the authors did not find any difference in the CH or CRF reduction in patients treated for less than -6D. Wang et al. [42Wang D, Liu M, Chen Y, et al. Differences in the corneal biomechanical changes after SMILE and LASIK. J Refract Surg 2014; 30(10): 702-7.[http://dx.doi.org/10.3928/1081597X-20140903-09] [PMID: 25291754] ] also examined the additional p1 and p2 areas, describing the area of the amplitude during inward and outward applanation. In their comparative study of 79 LASIK- and 187 SMILE- treated patients, the p1 and p2 areas decreased more after LASIK than after SMILE in patients needing more than -6D correction, suggesting a generally softer cornea after LASIK than after SMILE [45Kerautret J, Colin J, Touboul D, Roberts C. Biomechanical characteristics of the ectatic cornea. J Cataract Refract Surg 2008; 34(3): 510-3.[http://dx.doi.org/10.1016/j.jcrs.2007.11.018] [PMID: 18299080] ]. However, the difference was non-significant in their study group of low myopic patients.

Table 2
Studies comparing ORA measurements following LASIK and SMILE. Re com: Retrospective comparable study. Pro com: prospective comparable study. RCT paired: Randomized, controlled, paired-eyed study. * graphical illustration, but values not reported. ** Standard derivations not reported in text. n: number of eyes. CH: Corneal Hysteresis. CRF: Corneal Resistance Factor. Δ: postoperative – preoperative.


The individual variation in corneal biomechanical properties may cause an in-between group difference when comparing the ORA parameters. Hence, a paired-eyed study design provides more strength when evaluating the biomechanical properties after laser refractive surgery. Agca et al. [44Agca A, Ozgurhan EB, Demirok A, et al. Comparison of corneal hysteresis and corneal resistance factor after small incision lenticule extraction and femtosecond laser-assisted LASIK: A prospective fellow eye study. Cont Lens Anterior Eye 2014; 37(2): 77-80.[http://dx.doi.org/10.1016/j.clae.2013.05.003] [PMID: 23830802] ] examined in a prospective paired-eyed study patients treated with LASIK or SMILE in each of the two eyes (preop SE: LASIK −3.71±1.83D, SMILE −3.62±1.79D), and found a similar reduction in viscoelasticity at six months. However, this may be caused by correction of moderate to low myopia with only a minor reduction in the corneal stiffness beyond the sensitivity of the device [46Yang E, Roberts CJ, Mehta JS. A review of corneal biomechanics after LASIK and SMILE and the current methods of corneal biomechanical analysis. J Clin Exp Ophthalmol 2015; 6.]. High myopic correction requires removal or ablation of more stromal tissue than low myopic correction. Thus, Li et al. [37Li H, Wang Y, Dou R, et al. Intraocular pressure changes and relationship with corneal biomechanics after SMILE and FS-LASIK. Invest Ophthalmol Vis Sci 2016; 57(10): 4180-6.[http://dx.doi.org/10.1167/iovs.16-19615] [PMID: 27548888] ] examined the average decrease of CH and CRF per amount of removed or ablated tissue, and found a greater reduction after LASIK than SMILE (preop SE: LASIK -5.95±1.78D, SMILE -5.60±1.43D), which may be attributed to the flap creation during LASIK.

A major limitation of a few of the comparative studies was the lack of corneal-compensated IOP and CCT comparisons between groups, as these parameters in some studies were correlated with both CH and CRF [28Shah S, Laiquzzaman M, Cunliffe I, Mantry S. The use of the Reichert ocular response analyser to establish the relationship between ocular hysteresis, corneal resistance factor and central corneal thickness in normal eyes. Cont Lens Anterior Eye 2006; 29(5): 257-62.[http://dx.doi.org/10.1016/j.clae.2006.09.006] [PMID: 17085066] , 30Kamiya K, Hagishima M, Fujimura F, Shimizu K. Factors affecting corneal hysteresis in normal eyes. Graefes Arch Clin Exp Ophthalmol 2008; 246(10): 1491-4.[http://dx.doi.org/10.1007/s00417-008-0864-x] [PMID: 18546008] ]. Pedersen et al. [26Pedersen IB, Bak-Nielsen S, Vestergaard AH, Ivarsen A, Hjortdal J. Corneal biomechanical properties after LASIK, ReLEx flex, and ReLEx smile by Scheimpflug-based dynamic tonometry. Graefes Arch Clin Exp Ophthalmol 2014; 252(8): 1329-35.[http://dx.doi.org/10.1007/s00417-014-2667-6] [PMID: 24862302] ] evaluated estimated marginal means in 35 LASIK- and 29 SMILE-treated patients and took into consideration the dependency of IOP and postoperative CCT. No significant differences were found in CH and CRF up to one year after surgery, although the preoperative values were not reported in this study. However, both CH and CRF describe the viscoelasticity of the corneal tissue, where an increase in elasticity may equalize the decrease in viscosity and thereby mask an actual impact of the surgical intervention on the elasticity [27Glass DH, Roberts CJ, Litsky AS, Weber PA. A viscoelastic biomechanical model of the cornea describing the effect of viscosity and elasticity on hysteresis. Invest Ophthalmol Vis Sci 2008; 49(9): 3919-26.[http://dx.doi.org/10.1167/iovs.07-1321] [PMID: 18539936] ].

3. THE CORVIS ST: LASIK VERSUS SMILE

Shen et al. [47Shen Y, Chen Z, Knorz MC, Li M, Zhao J, Zhou X. Comparison of corneal deformation parameters after SMILE, LASEK, and femtosecond laser-assisted LASIK. J Refract Surg 2014; 30(5): 310-8.[http://dx.doi.org/10.3928/1081597X-20140422-01] [PMID: 24904933] ] were the first to retrospectively report the biomechanical outcomes after LASIK and SMILE using the Corvis ST. They found no significant differences in any of the evaluated parameters three months after surgery Table 3. However, only the postoperative values were described, whereas a comparison of the average change due to surgery would provide more information about the biomechanical impact following LASIK and SMILE. Sefat et al. [48Sefat SM, Wiltfang R, Bechmann M, Mayer WJ, Kampik A, Kook D. Evaluation of changes in human corneas after femtosecond laser-assisted LASIK and Small-Incision Lenticule Extraction (SMILE) using non-contact tonometry and ultra-high-speed camera (corvis ST). Curr Eye Res 2015; 3683: 1-6.[PMID: 26554611] ] also reported similar biomechanical responses after LASIK and SMILE with the Corvis ST in a subgroup matched for age, preoperative CCT, IOP, preoperative spherical equivalent, and ΔCCT. Osman et al. [38Osman IM, Helaly HA, Abdalla M, Shousha MA. Corneal biomechanical changes in eyes with small incision lenticule extraction and laser assisted in situ keratomileusis. BMC Ophthalmol 2016; 16: 123.[http://dx.doi.org/10.1186/s12886-016-0304-3] [PMID: 27457241] ] calculated and compared the percentage of change in preoperative and postoperative measurements in a comparative study of 25 LASIK- and 25 SMILE-treated patients. The authors found significant less reduction in A1 time, HC time, and A2 time after SMILE than LASIK, which may reflect a less compliant cornea after the flap-free procedure. Furthermore, the percentage of increase in deformation amplitude during highest concavity was significantly larger in LASIK than SMILE, suggesting a more severe inward deformation during the air pulse after LASIK, possibly due to a more compliant cornea.

It has previously been questioned if the repeatability and reproducibility of the Corvis ST parameters available with the first software version were acceptable (Table 2) [49Bak-Nielsen S, Pedersen IB, Ivarsen A, Hjortdal J. Repeatability, reproducibility, and age dependency of dynamic Scheimpflug-based pneumotonometer and its correlation with a dynamic bidirectional pneumotonometry device. Cornea 2015; 34(1): 71-7.[http://dx.doi.org/10.1097/ICO.0000000000000293] [PMID: 25393092] -51Nemeth G, Hassan Z, Csutak A, Szalai E, Berta A, Modis L Jr. Repeatability of ocular biomechanical data measurements with a Scheimpflug-based noncontact device on normal corneas. J Refract Surg 2013; 29(8): 558-63.[http://dx.doi.org/10.3928/1081597X-20130719-06] [PMID: 23909783] ]. Hence, a retrospective study by Pedersen et al. [26Pedersen IB, Bak-Nielsen S, Vestergaard AH, Ivarsen A, Hjortdal J. Corneal biomechanical properties after LASIK, ReLEx flex, and ReLEx smile by Scheimpflug-based dynamic tonometry. Graefes Arch Clin Exp Ophthalmol 2014; 252(8): 1329-35.[http://dx.doi.org/10.1007/s00417-014-2667-6] [PMID: 24862302] ] examined only the variables with a coefficient of variation <10% [49Bak-Nielsen S, Pedersen IB, Ivarsen A, Hjortdal J. Repeatability, reproducibility, and age dependency of dynamic Scheimpflug-based pneumotonometer and its correlation with a dynamic bidirectional pneumotonometry device. Cornea 2015; 34(1): 71-7.[http://dx.doi.org/10.1097/ICO.0000000000000293] [PMID: 25393092] ] including A1 deflection length and HC deflection length, which were not standard parameters in the initial Corvis ST software. After adjusting for postoperative CCT, IOP, and age, only HC Time was significantly shorter in LASIK than SMILE, suggesting that a LASIK- treated corneas reached their highest concavity at an earlier stage. However, the shorter HC time was not seen in an additional group of FLEX-treated patients, a refractive technique also requiring the creation of a corneal flap (Femtosecond Lenticule Extraction). Furthermore, none of the remaining Corvis ST parameters supported the hypothesis of a more compliant cornea after LASIK compared with SMILE [26Pedersen IB, Bak-Nielsen S, Vestergaard AH, Ivarsen A, Hjortdal J. Corneal biomechanical properties after LASIK, ReLEx flex, and ReLEx smile by Scheimpflug-based dynamic tonometry. Graefes Arch Clin Exp Ophthalmol 2014; 252(8): 1329-35.[http://dx.doi.org/10.1007/s00417-014-2667-6] [PMID: 24862302] ].

Table 3
Studies comparing Corvis ST parameters following LASIK and SMILE. Only the original Corvis ST values are included in the table. * Estimated marginal means at following values: Age at examination 36.7 years, CCT 472 mm, IOPcc 13.0mmHg. HC PD: Highest concavity peak distance. HC DA: Highest concavity deformation amplitude.


3.1. Ex vivo Corneal Biomechanical Assessment

Only limited peer-reviewed ex vivo studies have assessed and compared the biomechanical weakening after cap and flap creation. A study by Cartwright et al. [15Knox Cartwright NE, Tyrer JR, Jaycock PD, Marshall J. Effects of variation in depth and side cut angulations in LASIK and thin-flap LASIK using a femtosecond laser: a biomechanical study. J Refract Surg 2012; 28(6): 419-25.[http://dx.doi.org/10.3928/1081597X-20120518-07] [PMID: 22692524] ] examined the corneal compliance after femtosecond lamellar and side cut in human donor corneas mounted on artificial anterior chambers. With radial shearing speckle pattern interferometry, the authors calculated the corneal apical displacement during increased chamber pressure (inflation test). They found a higher corneal compliance after side cut incision than after in-plane delamination, due to more severe damaging of the collagen fibres. Thus, the average percentage increase in apical displacement during inflation was 5% for delamination in 160μm depth and 33% after 90° side cut in 160μm depth. Although these results suggest more severe weakening after flap creation than after pocket creation, the authors did not examine the effect of a minor incision created using SMILE.

Traditional strip extensiometry was also used to examine the biomechanical properties after LASIK and SMILE presented by Kanellopoulos et al. (ePoster PA049, AAO, 13th November 2015, Las Vegas). The biomechanical tensile strength was examined in four laser refractive groups treated with LASIK and SMILE for -3D and -8D, respectively. Using biaxial in-plane tensile tests on laser-treated corneas, the authors found a similar reduction in tensile strength in LASIK and SMILE for higher myopic corrections (-8D), but less tensile strength reduction in LASIK than SMILE for the low myopic group (-3D). One possible explanation may be the surgical approach, as SMILE requires removal of more tissue than ablated in LASIK to reach equivalent correction. However, these results should be interpreted with caution as the orientation of the collagen fibrils and the pressure induction in strip extensiometry are not similar to the intact eye. Furthermore, another and similar ex vivo strip extensiometry study of porcine eyes treated with SMILE and FLEX suggests that SMILE may be superior in terms of biomechanical stability after surgery [52Spiru B, Kling S, Hafezi F, Sekundo W. Biomechanical differences between femtosecond Lenticule Extraction (FLEx) and Small Incision Lenticule Extraction (SmILE) tested by 2D-extensometry in ex vivo Porcine eyes. Invest Ophthalmol Vis Sci 2017; 58(5): 2591-5.[http://dx.doi.org/10.1167/iovs.16-20211] [PMID: 28494492] ].

4. FINITE-ELEMENT 3D MODELS OF STRESS DISTRIBUTION

Corneal biomechanical alterations after LASIK and SMILE have previously been compared in a computer modelling study using a finite-element anisotopic collagen fibre-dependent model [10Sinha Roy A, Dupps WJ Jr, Roberts CJ. Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: Finite-element analysis. J Cataract Refract Surg 2014; 40(6): 971-80.[http://dx.doi.org/10.1016/j.jcrs.2013.08.065] [PMID: 24857440] ]. By tomographic measurements from a normal, a LASIK-treated, and a SMILE-treated cornea, the authors managed to compare the stress distribution after SMILE and LASIK with a geometric analogue model using an untreated control cornea. The stress distribution in the SMILE simulation was comparable to the analogue model with a maximal stress in the superficial layer of the cornea. For the LASIK simulation, the stress was greater in the residual stromal bed after LASIK compared with the corresponding geometry analogue. A simulated thicker flap in LASIK caused greater increase in the corneal stress values than removal of a SMILE lenticule in the deeper layer. Thus, the study suggests that the stromal residual bed after LASIK is exposed to increased stress due to the flap creation, while SMILE preserves the biomechanical strength almost similar to what is seen in an untreated cornea [10Sinha Roy A, Dupps WJ Jr, Roberts CJ. Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: Finite-element analysis. J Cataract Refract Surg 2014; 40(6): 971-80.[http://dx.doi.org/10.1016/j.jcrs.2013.08.065] [PMID: 24857440] ].

5. MATHEMATICAL MODEL OF TENSILE STRENGTH DISTRIBUTION

Cohesive strength [14Randleman JB, Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF. Depth-dependent cohesive tensile strength in human donor corneas: implications for refractive surgery. J Refract Surg 2008; 24(1): S85-9.[PMID: 18269156] ], tangential tensile strength [53Scarcelli G, Pineda R, Yun SH. Brillouin optical microscopy for corneal biomechanics. Invest Ophthalmol Vis Sci 2012; 53(1): 185-90.[http://dx.doi.org/10.1167/iovs.11-8281] [PMID: 22159012] ], and shear strength [54Petsche SJ, Chernyak D, Martiz J, Levenston ME, Pinsky PM. Depth-dependent transverse shear properties of the human corneal stroma. Invest Ophthalmol Vis Sci 2012; 53(2): 873-80.[http://dx.doi.org/10.1167/iovs.11-8611] [PMID: 22205608] ] have been shown to decrease through the cornea with the anterior 40% being the strongest part. Consequently, Reinstein et al. [11Reinstein DZ, Archer TJ, Randleman JB. Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction. J Refract Surg 2013; 29(7): 454-60.[http://dx.doi.org/10.3928/1081597X-20130617-03] [PMID: 23820227] ] developed a mathematical model of the non-linear tensile strength distribution in the cornea to evaluate the biomechanical weakening after LASIK and SMILE. Data was retrieved from a previous in vitro strip extensiometry study on evaluation of the corneal stromal tensile strength as a function of depth [14Randleman JB, Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF. Depth-dependent cohesive tensile strength in human donor corneas: implications for refractive surgery. J Refract Surg 2008; 24(1): S85-9.[PMID: 18269156] ]. The model predicted that the postoperative total stromal strength was better preserved after SMILE than after LASIK due to the creation of a corneal cap rather than a flap. Hence, if 100-μm tissue removal was performed in a 550-μm thick cornea, the postoperative relative total tensile strength would be 75% after SMILE (130-μm cap) and 54% after LASIK (110-μm flap). Furthermore, the model predicted that an extracted SMILE lenticule approximately 100 μm thicker than the ablation depth in LASIK would provide the same postoperative reduction in the total tensile strength (130-μm cap and 110-μm flap). Depending on the ablation and lenticule profile, this would correspond to a possibility for 7.75D more myopic correction in SMILE than LASIK with equivalent weakening of the postoperative total strength. It may be questioned, if it is actually possible to correct higher degrees of myopia with SMILE than LASIK, using a residual bed thickness below the generally recommended 250 μm limit [11Reinstein DZ, Archer TJ, Randleman JB. Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction. J Refract Surg 2013; 29(7): 454-60.[http://dx.doi.org/10.3928/1081597X-20130617-03] [PMID: 23820227] ].

CONCLUSION

Corneal biomechanical properties are of major importance in laser refractive surgery and must be taken into consideration to reduce the risk for iatrogenic ectasia. Some in vivo corneal biomechanical alterations are possible to quantify with ORA and Corvist ST but are difficult to interpret and use in clinical practice due to their dependence of IOP, CCT, refractive status, and age. Development and refinement are needed if ORA and Corvis ST should be implemented in a screening procedure of the biomechanical strength before refractive surgery. Mathematical analysis and finite-element models suggest that SMILE may preserve corneal biomechanical properties better than LASIK. Furthermore, the current in vivo studies performed with ORA and Corvis ST indicate that SMILE is equal or superior to LASIK in terms of preservation of the postoperative biomechanical strength. However, future paired-eyed studies comparing LASIK and SMILE are needed to support this conclusion and to eliminate the in-between group variability in corneal biomechanical properties. As iatrogenic ectasia has been reported in very few patients, it is recommended to follow the same contraindications as used in LASIK, especially when planning SMILE in borderline or corneas at risk.

CONSENT FOR PUBLICATION

Not applicable.

CONFLICT OF INTEREST

The authors declare no conflict of interest, financial or otherwise.

ACKNOWLEDGEMENTS

Declared none.

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"Open access journals are probably one of the most important contributions to promote and diffuse science worldwide."


Jaime Sampaio
(University of Trás-os-Montes e Alto Douro, Portugal)

"Open access journals make up a new and rather revolutionary way to scientific publication. This option opens several quite interesting possibilities to disseminate openly and freely new knowledge and even to facilitate interpersonal communication among scientists."


Eduardo A. Castro
(INIFTA, Argentina)

"Open access journals are freely available online throughout the world, for you to read, download, copy, distribute, and use. The articles published in the open access journals are high quality and cover a wide range of fields."


Kenji Hashimoto
(Chiba University, Japan)

"Open Access journals offer an innovative and efficient way of publication for academics and professionals in a wide range of disciplines. The papers published are of high quality after rigorous peer review and they are Indexed in: major international databases. I read Open Access journals to keep abreast of the recent development in my field of study."


Daniel Shek
(Chinese University of Hong Kong, Hong Kong)

"It is a modern trend for publishers to establish open access journals. Researchers, faculty members, and students will be greatly benefited by the new journals of Bentham Science Publishers Ltd. in this category."


Jih Ru Hwu
(National Central University, Taiwan)


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