Mathematical Model to Compare the Relative Tensile Strength of the Cornea After PRK, LASIK, and Small Incision Lenticule Extraction

Reinstein, Dan Z.; Archer, Timothy J; Randleman, J. Bradley

doi:10.3928/1081597x-20130617-03

Cited by 290 publications

(206 citation statements)

References 20 publications

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“…3 The intrastromal lenticule can be created in the desired corneal depth and leaves the anterior stromal layer (the cap) intact. Mathematical 4 and computational modelling analyses 5,6 suggest that SMILE may be better at preserving the biomechanical integrity than other flapbased laser refractive procedures such as laser-assisted in situ keratomileusis (LASIK) and femtosecond lenticule extraction (FLEX). In vivo biomechanical studies evaluating the corneal deformation response with noncontact tonometry have been inconsistent, and SMILE has been equal or superior to LASIK.…”

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confidence: 99%

Refractive Correction and Biomechanical Strength Following SMILE With a 110- or 160-μm Cap Thickness, Evaluated Ex Vivo by Inflation Test

Damgaard

Ivarsen

Hjortdal

2018

Invest. Ophthalmol. Vis. Sci.

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PURPOSE. To examine the refractive correction and corneal biomechanical strength after small incision lenticule extraction (SMILE) by using a 110-or 160-lm cap thickness.METHODS. Thirty-two human donor corneas were allocated into 4 groups, combining one of two cap thicknesses (110 and 160 lm) with one of two spherical corrections (À4 D and À8 D). Each cornea was mounted on an artificial anterior chamber. The chamber pressure was adjusted by an attached 8% dextran media column.The anterior and posterior sagittal 3-mm-diameter curvature (r sag3mm ) and the total corneal refractive power (TCRP 4mm,apex,zone ) were obtained before and after SMILE at a chamber pressure of 15 or 40 mm Hg. The average changes after surgery (D ¼ postoperative À preoperative) and at increased chamber pressure (d ¼ 40 mm Hg À 15 mm Hg) were compared. RESULTS. A 110-lm cap thickness caused more anterior flattening (Dr 15,À8D , 1.02 6 0.08 mm versus 0.60 6 0.17 mm), less posterior steepening (Dr 15,À8D , À0.19 6 0.11 mm versus À0.45 6 0.20 mm), and more myopic correction (DTCRP 15,À8D , À6.30 6 0.96 D versus À4.55 6 1.66 D) than a 160-lm cap thickness for À8 D SMILE (P < 0.034), but not for À4 D SMILE (DTCRP 15,À4D,110lm , À3.86 6 1.31 D versus DTCRP 15,À4D,160lm , À3.57 6 1.27 D, P ¼ 0.718). After SMILE, increased chamber pressure caused anterior steepening (P < 0.014), which was similar at cap thicknesses of 110 and 160 lm (dr À4D , À0.13 6 0.14 mm versus À0.09 6 0.05mm, P ¼ 0.431).CONCLUSIONS. For high myopic corrections, a 160-lm cap caused less anterior curvature flattening and more posterior steepening than a 110-lm cap, and consequently less myopic correction. The inflation test revealed a reduction in the biomechanical strength after SMILE; this was similar when using a 110-or 160-lm cap thickness.

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confidence: 99%

Refractive Correction and Biomechanical Strength Following SMILE With a 110- or 160-μm Cap Thickness, Evaluated Ex Vivo by Inflation Test

Damgaard

Ivarsen

Hjortdal

2018

Invest. Ophthalmol. Vis. Sci.

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“…In recent years, greater attention has been paid to the topic of corneal biomechanics. [1][2][3][4] In vitro experimental studies such as stress-strain tensile tests on corneal strips and inflation tests on the whole cornea have shown the viscoelastic, hyperelastic, and anisotropic features of both animal and human corneas. [5][6][7][8] Electromagnetic radiation scanning approaches are able to demonstrate how the collagen fibrils are spatially arranged within the stroma tissue, which have a great impact on the corneal biomechanical performance.…”

Section: Discussionmentioning

confidence: 99%

Air Puff Induced Corneal Vibrations: Theoretical Simulations and Clinical Observations

Han¹,

Chen²,

Zhou³

et al. 2014

J Refract Surg

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PURPOSE: To investigate air puff induced corneal vibrations and their relationship to the intraocular pressure (IOP), viscoelasticity, mass, and elasticity of the cornea based on theoretical simulations and preliminary clinical observations. METHODS:To simulate the corneal movement during air puff deformation, a kinematic viscoelastic corneal model was developed involving the factors of corneal mass, damping coefficient, elasticity, and IOP. Different parameter values were taken to investigate how factors would affect the corneal movements. Two clinical ocular instruments, CorVis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) and the Ocular Response Analyzer (ORA; Reichert, Inc., Buffalo, NY), were employed to observe the corneal dynamical behaviors. RESULTS:Numerical results showed that during the air puff deformation, there would be vibrations along with the corneal deformation, and the damping viscoelastic response of the cornea had the potential to reduce the vibration amplitude. With consistent IOP, the overall vibration amplitude and inward motion depths were smaller with a stiffer cornea.CONCLUSIONS: A kinematic viscoelastic model of the cornea is presented to illustrate how the vibrations are associated with factors such as corneal mass, viscoelasticity, and IOP. Also, the predicted corneal vibrations during air puff deformation were confirmed by clinical observation.[J Refract Surg. 2014;30(3):208-213.]

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“…This is without considering the further benefit that SMILE conserves the stronger anterior stromal lamellae by performing the refractive correction intrastromally. 6 This factor also sets SMILE apart from photorefractive keratectomy (PRK); we have previously modeled that there is less reduction in total tensile strength after SMILE compared to PRK for equivalent tissue removal due to the fact that the PRK ablation occurs within the strongest anterior stroma. 6 The difference in how the structural integrity of the stroma is affected enables a larger optical zone to be used in SMILE.…”

confidence: 99%

SMILE and LASIK in Low Myopia

Teus¹,

Benito-Llopis²

2015

J Refract Surg

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We read with great interest the article by Reinstein et al. about the clinical outcomes of small incision lenticule extraction (SMILE) in low myopia published in the December 2014 issue. 1 We have believed for a long time that refractive surgery to correct small refractive errors, such as low myopia, is extremely demanding. Usually preoperative uncorrected visual acuity is reasonably good; therefore, patients with these small defects need a surgical technique that is not only precise (ie, low percentage of eyes with significant postoperative refractive error), but also safe, because even a small decrease in visual acuity will be noticed by the patient.In fact, we published some years ago an article that analyzed the visual outcomes of both LASIK and surface ablation in patients with low myopia and low astigmatism.2 It is interesting that only two studies (ours and Reinstein et al.'s) have been published in peer-reviewed journals about this interesting subject, as the authors correctly point out in their article.Unfortunately, we strongly disagree with the main conclusion of the article by Reinstein et al.: that SMILE for low myopia was found to be safe and effective with outcomes similar to those previously reported for LASIK. We believe that this conclusion does not follow the results of the study and the comparison with the peer-reviewed literature. The percentage of eyes within 0.5 diopter (D) of emmetropia postoperatively was, in our study, 96.16% after LASIK and 98.7% after surface ablation, not 95% and 91%, as Reinstein et al. incorrectly quote (original data shown in Figure 4, page 143 of our article). In their article, only 84% of eyes after SMILE were within 0.5 D of emmetropia. Furthermore, in our study only 1.26% of eyes had a postoperative spherical equivalent higher than -0.5 D after LASIK, compared to 10% of eyes after SMILE in their article. In our opinion, such significant differences in predictability between excimer laser and SMILE procedures make excimer laser clearly superior in this population.However, it is not only the predictability that matters. The speed of visual recovery is clearly faster in LASIK, and the amount of tissue removed to correct these small refractive defects was reported by Reinstein et al. as 79 µm on average and ranging from 51 to 108 µm with SMILE, compared to the average of 25 to 30 µm in excimer laser ablation for low myopia. In our opinion, all of these differences make the potential (clinically still unproven) advantage of reduced risk of postoperative ectasia after SMILE clearly not reason enough to currently choose this technique over excimer laser, at least for the correction of low myopia. Reply:We would like to thank Prof. Teus and Dr. de Benitos-Llopis for their letter and for raising these issues, and we apologize for misquoting the results from their previous study 1 -we had misinterpreted the labels on the predictability histogram. However, we do not agree with the comments made in relation to comparing small incision lenticule extraction (SMILE) and L...

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Mathematical Model to Compare the Relative Tensile Strength of the Cornea After PRK, LASIK, and Small Incision Lenticule Extraction

Abstract: 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.

Cited by 290 publications

References 20 publications

Refractive Correction and Biomechanical Strength Following SMILE With a 110- or 160-μm Cap Thickness, Evaluated Ex Vivo by Inflation Test

Refractive Correction and Biomechanical Strength Following SMILE With a 110- or 160-μm Cap Thickness, Evaluated Ex Vivo by Inflation Test

Air Puff Induced Corneal Vibrations: Theoretical Simulations and Clinical Observations

SMILE and LASIK in Low Myopia

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