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Comparison of the change in posterior corneal elevation and corneal biomechanical parameters after small incision lenticule extraction and femtosecond laser-assisted LASIK for high myopia correction

  • Author Footnotes
    1 Authors Bingjie Wang and Zhenyong Zhang contributed equally to the writing of the article.
    Bingjie Wang
    Footnotes
    1 Authors Bingjie Wang and Zhenyong Zhang contributed equally to the writing of the article.
    Affiliations
    Key Myopia Laboratory of Chinese Health Ministry, Department of Ophthalmology, Eye & ENT Hospital, Fudan University, No. 83, Fenyang Road, Shanghai 200031, China
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  • Author Footnotes
    1 Authors Bingjie Wang and Zhenyong Zhang contributed equally to the writing of the article.
    Zhenyong Zhang
    Footnotes
    1 Authors Bingjie Wang and Zhenyong Zhang contributed equally to the writing of the article.
    Affiliations
    Department of Ophthalmology, Putuo Hospital, Shanghai University of Traditional Chinese Medicine, Meiling North Road 409, Shanghai 200062, China
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  • Rajeev K Naidu
    Affiliations
    The University of Sydney, Camperdown, NSW 2006, Australia
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  • Renyuan Chu
    Affiliations
    Key Myopia Laboratory of Chinese Health Ministry, Department of Ophthalmology, Eye & ENT Hospital, Fudan University, No. 83, Fenyang Road, Shanghai 200031, China
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  • Jinhui Dai
    Affiliations
    Key Myopia Laboratory of Chinese Health Ministry, Department of Ophthalmology, Eye & ENT Hospital, Fudan University, No. 83, Fenyang Road, Shanghai 200031, China
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  • Xiaomei Qu
    Affiliations
    Key Myopia Laboratory of Chinese Health Ministry, Department of Ophthalmology, Eye & ENT Hospital, Fudan University, No. 83, Fenyang Road, Shanghai 200031, China
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  • Zhiqiang Yu
    Affiliations
    Key Myopia Laboratory of Chinese Health Ministry, Department of Ophthalmology, Eye & ENT Hospital, Fudan University, No. 83, Fenyang Road, Shanghai 200031, China
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  • Hao Zhou
    Correspondence
    Corresponding author.
    Affiliations
    Key Myopia Laboratory of Chinese Health Ministry, Department of Ophthalmology, Eye & ENT Hospital, Fudan University, No. 83, Fenyang Road, Shanghai 200031, China
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  • Author Footnotes
    1 Authors Bingjie Wang and Zhenyong Zhang contributed equally to the writing of the article.
Open AccessPublished:February 03, 2016DOI:https://doi.org/10.1016/j.clae.2016.01.007

      Highlights

      • Posterior corneal elevation (PCE) changes and corneal biomechanical parameters were evaluated for SMILE and FS-LASIK.
      • PCE change in FS-LASIK was greater than SMILE at 12 months post-operation.
      • CH and CRF were reduced for both procedures at 6 and 12 months, FS-LASIK demonstrated a greater reduction in CRF than SMILE.
      • PCE was moderately correlated with CH and CRF change, indicating an increase in PCE with greater reduction in CH and CRF.

      Abstract

      Purpose

      To compare posterior corneal elevation (PCE) changes and corneal biomechanical changes post fematosecond laser-assisted laser in situ keratomileusis (FS-LASIK) and small incision lenticule extraction (SMILE) procedures using the Pentacam system and the Ocular Response Analyzer (ORA).

      Design

      Retrospective observational case series study.

      Method

      106 patients with bilateral myopia who underwent either FS-LASIK (56 patients) or SMILE (50 patients) were reviewed, only the right eye was used in the analysis. Inclusion criteria include a spherical equivalent of −6.00DS and completion of 12 months follow up. The main outcome evaluated was change in PCE at 3 months, 6 months and 12 months post-operation using the Pentacam system. Corneal biomechanical parameters were evaluated at 6 and 12 months with the ORA.

      Results

      PCE change at 3 and 6 months were not significantly different between the two procedures (p = 0.064, p = 0.109 respectively). At 12 months, PCE change was greater in FS-LASIK than SMILE (p = 0.048). One-way ANOVA showed that for either procedure, the change in PCE did not differ at 3, 6 or 12 months post operation. CH and CRF values were reduced after both procedures, with FS-LASIK demonstrating a greater reduction in CRF than SMILE at 6 and 12 months (p = 0.037 and p = 0.001). Both CH and CRF reduction correlated with PCE increase at 6 and 12 months.

      Conclusion

      FS-LASIK demonstrated a greater increase in PCE than SMILE only at 12 months, as well as a greater reduction of CRF than SMILE. There were no significant differences in PCE change over time within either group.

      Keywords

      Laser in situ keratomileusis (LASIK), in which a corneal flap was initially created by a mechanical microkeratome but is now being increasingly replaced by the femtosecond laser, has gained world-wide popularity for myopia correction. Although the clinical results are accordingly enhanced, postoperative complications, amongst which LASIK-inflicted ectasia is the foremost concern, still occur. As most complications are believed to be associated with the corneal flap, people have been engaging in developing a new procedure that could avoid the flap creation. Recently, small-incision lenticule extraction (SMILE) has been established as a ‘flapless’ procedure in which an intrastromal lenticule is cut by a femtosecond laser and manually extracted through a peripheral corneal tunnel incision [
      • Sekundo W.
      • Kunert K.S.
      • 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.
      ,
      • Shah R.
      • Shah S.
      • Sengupta S.
      Results of small incision lenticule extraction: all-in-one femtosecond laser refractive surgery.
      ]. The refractive predictability, safety, and patient satisfaction of SMILE are comparable to femtosecond laser-assisted LASIK (FS-LASIK) [
      • Blum M.
      • Kunert K.
      • Schroder M.
      • Sekundo W.
      Femtosecond lenticule extraction for the correction of myopia: preliminary 6-month results.
      ,
      • Vestergaard A.
      • Ivarsen A.
      • Asp S.
      • Hjortdal J.O.
      Femtosecond (FS) laser vision correction procedure for moderate to high myopia: a prospective study of ReLEx((R)) flex and comparison with a retrospective study of FS-laser in situ keratomileusis.
      ,
      • Vestergaard A.
      • Ivarsen A.R.
      • Asp S.
      • Hjortdal J.O.
      Small-incision lenticule extraction for moderate to high myopia: predictability safety, and patient satisfaction.
      ]. SMILE may also have biomechanical benefits over LASIK in that it leaves the stroma overlying the lenticule intact. However, corneal hysteresis and corneal resistance factor, which are two metrics used to describe the biomechanical properties of the cornea, have been shown to have no significant difference between SMILE and FS-LASIK treatments in postoperative values [
      • Agca A.
      • Ozgurhan E.B.
      • Demirok A.
      • Bozkurt E.
      • Celik U.
      • Ozkaya A.
      • Cankaya I.
      • Yilmaz O.F.
      Comparison of corneal hysteresis and corneal resistance factor after small incision lenticule extraction and femtosecond laser-assisted LASIK: a prospective fellow eye study.
      ]. Because corneal biomechanical properties are largely unknown, it is not possible to theoretically predict the extent of the potential corneal deformation after refractive surgery [
      • Dupps W.J.
      • Wilson S.E.
      Biomechanics and wound healing in the cornea.
      ]. Posterior corneal changes are of great assistance in predicting the response of the cornea to ablation, with posterior elevation being the most effective parameter in the diagnosis of ectatic disorder, as demonstrated by Mihaltz [
      • Mihaltz K.
      • Kovacs I.
      • Takacs A.
      • Nagy Z.Z.
      Evaluation of keratometric pachymetric, and elevation parameters of keratoconic corneas with pentacam.
      ]. In this retrospective study, we aim to evaluate the corneal structural and biomechanical characteristics by comparing the changes in posterior corneal elevation (PCE) as well as corneal hysteresis (CH) and corneal resistance factor (CRF), after SMILE and FS-LASIK for high myopia correction, which is regarded as one of the risk factors for developing post-LASIK ectasia.

      1. Patients and methods

      1.1 Patients

      The records of patients who were referred to the Shanghai Eye & ENT Hospital for myopia correction between January 2012 and January 2014, were reviewed. For the present study, only patients who received FS-LASIK or SMILE for their high myopia corrections (pre-operative spherical equivalent (SE) more than −6.00 diopters (D)) and who completed 12 months of postoperative follow-up were included.
      In all cases, data was collected from the first eye (right eye) on which the procedure was performed. There were 106 patients (106 eyes) meeting these criteria.
      Of the 106 patients, 50 eyes underwent SMILE procedures (SMILE Group) while 56 eyes underwent FS-LASIK procedures (FS-LASIK Group). Minimum stromal residual bed thickness was 250 μm in both groups.
      Spherical equivalent, intraocular pressure (IOP), central corneal thickness (CCT), and posterior corneal elevation (PCE) in all patients pre-operation and 12 months post operation were retrieved. IOP was determined by a non-contact tensiometer (Topcon, Japan) and CCT and PCE were determined with a Pentacam system (Typ70700; Oculus; Wetzlar, Germany), which measures the anterior and posterior corneal surfaces using a rotating Scheimpflug camera. The Ocular Response Analyzer (ORA) (Reichert, Inc., Depew, NY) was used to measure the CH as well as CRF. An average of 4 measurements was used. Residual bed thickness (RBT) was estimated using the thinnest CCT reading and subtracting the non-nomogram-adjusted ablation depth and the flap thickness of 95 μm in FS-LASIK or the cap thickness of 100–120 μm in SMILE. Changes in PCE was obtained from the difference map and was determined by subtracting the postoperative elevation data from the preoperative elevation data based on the maximum difference in the central 4.0 mm zone. The reference best-fit sphere (BFS) was determined by the central 8.0 mm zone of the preoperative cornea, which was identical for both the preoperative and postoperative maps.

      1.2 Surgical procedures

      SMILE was performed using the VisuMax femtosecond laser system (Carl Zeiss Meditec) with a repetition rate of 500 kHz, pulse energy of 185–190 nJ, intended cap thickness of 100–120 μm, cap diameter of 7.5 mm, lenticule diameter of 6.1 to 6.6 mm (depending on the refractive error), and a 90°-angle side cut with a circumferential length of 2.1 mm at the superior position.
      FS-LASIK was performed with the VisuMax system for flap creation followed by Mel 80 excimer laser (Carl Zeiss Meditec) for stromal ablation, with an intended flap thickness of 95 μm, optical zone size of 5.75–6.50 mm and pulse energy of 185 nJ. The hinge was located at the superior position. All procedures were performed by one surgeon experienced in both types of procedures. (H.Z).
      A standard postoperative topical steroid (Flurometholone 0.1%) tapered over 30 days (or longer if deemed necessary), and topical antibiotic (Tobramycin 0.003%) QID for 7 days was given.

      1.3 Statistical analysis

      All statistical analyses were performed with a statistics program (SPSS 19.0 IBM Corp., Armonk, NY, USA). Independent-samples t-test was used to compare the differences between groups. One way repeated measures ANOVA test with the Bonferroni correction was used to compare PCE, CH and CRF change within groups at different postoperative time points. Pearson's correlation test was used to evaluate the relationship between variables. p<0.05 was considered significant.

      2. Results

      Table 1 demonstrates the clinical characteristics of the patients included in this study. There were no significant differences in the preoperative age, SE, CCT, and IOP between the two groups.
      Table 1Clinical characteristics of the patients included in this study.
      Mean ± standard deviation
      SMILE (n = 50)FS-LASIK (n = 56)P Value
      Age (y)25.26 ± 6.6424.75 ± 6.240.684
      Gender(F/M)27/2330/260.157
      Preop SE(D)−7.60 ± 1.12−7.68 ± 1.190.713
      Preop CCT(μm)542.96 ± 23.34548.00 ± 23.970.276
      Preop IOP(mmHg)14.68 ± 2.6514.94 ± 2.360.609
      SE = spherical equivalent, CCT = central corneal thickness, IOP = intraocular pressure.
      Lenticule thickness and ablation depth was not significantly different between the SMILE group and the FS-LASIK group (p = 0.921). Postoperative residual bed thickness was significantly less in the SMILE group compared to the FS-LASIK group (p < 0.001). (Table 2).
      Table 2Ablation depth and residual bed thickness post operation.
      Mean ± standard deviation
      SMILE (n = 50)FS-LASIK (n = 56)p value
      Lenticule thickness/

      Ablation Depth (μm)
      139.52 ± 8.13139.73 ± 12.940.921
      Residual bed thickness (μm)284.64 ± 22.99314.80 ± 27.30<0.001
      One-way repeated measure ANOVA and post hoc tests showed that post-operative CH values for both procedures were statistically significantly lower than preoperative values at 6 and 12 months, however between the two groups, there was no statistically significant difference (Table 3 and Fig. 1). The reduction in CH values at 6 and 12 months were also not significantly different between the two groups.
      Table 3CH comparison between SMILE and FS-LASIK and over time.
      Mean ± standard deviation
      SMILE (n = 50)FS-LASIK (n = 56)p value
      CH
      Preoperative10.52 ± 1.7110.85 ± 1.190.253
      Month 67.85 ± 1.818.43 ± 1.750.096
      Month 127.97 ± 2.058.31 ± 1.620.334
      Difference
      Month 6 − Preoperative CH−2.67 ± 1.29−2.42 ± 1.380.327
      pp < 0.001p < 0.001
      Month 12 − Preoperative CH−2.55 ± 1.44−2.53 ± 1.380.942
      pp < 0.001p < 0.001
      Month 12 − month 60.12 ± 1.03−0.11 ± 0.860.201
      pp = 1.000p = 0.499
      Figure thumbnail gr1
      Fig. 1Pre and Postop Corneal Hysteresis between SMILE and FS-LASIK. “x” statistically significantly less than preop values.
      Similarly, postoperative CRF values were significantly lower than preoperative values at 6 and 12 months for both groups, however between the two groups, there was no significant difference (Table 4 and Fig. 2). The reduction in CRF at 6 and 12 months was significantly greater in the FS-LASIK group than the SMILE group (p = 0.037 and p< 0.001, respectively, Table 4)
      Table 4CRF comparison between SMILE and FS-LASIK and over time.
      Mean ± standard deviation
      SMILE (n = 50)FS-LASIK (n = 56)p value
      CRF
      Preoperative10.07 ± 1.4910.62 ± 1.810.094
      Month 67.54 ± 1.667.53 ± 1.810.986
      Month 127.83 ± 1.647.29 ± 1.760.106
      Difference
      Month 6 − Preoperative CRF−2.53 ± 1.22−3.09 ± 1.460.037
      pp < 0.001p < 0.001
      Month12 − Preoperative CRF−2.24 ± 1.29−3.33 ± 1.34<0.001
      pp < 0.001p < 0.001
      Month 12 − month 60.29 ± 0.50−0.24 ± 0.100.001
      pp = 1.000p = 1.000
      Figure thumbnail gr2
      Fig. 2Pre and Postop Corneal resistance factor between SMILE and FS-LASIK. “x” statistically significantly less than preop values.
      A positive change in PCE indicate a forward shift of the posterior corneal surface. The change in PCE at 3, 6 and 12 months post operation for both FS-LASIK and SMILE are shown in Table 5. At 3 and 6 months post operation, the change in PCE was not significantly different between the two groups. By 12 months post operation, PCE change was statistically significantly greater in the FS-LASIK group than the SMILE group (p = 0.048). One-way repeated measures ANOVA test showed no significant difference in PCE change over the 3, 6 and 12 months postoperative period for either the FS-LASIK group (F(2,165) = 0.256, p = 0.774) or the SMILE group (F(2,147) = 0.140, p = 0.908).
      Table 5Change in PCE at 3, 6 and 12 months post operation.
      Mean change in PCE ± standard deviation(μm)
      SMILE (n = 50)FS-LASIK (n = 56)p value
      3 months0.96 ± 1.181.38 ± 1.100.064
      6 months1.06 ± 1.111.48 ± 1.510.109
      12 months0.98 ± 1.321.57 ± 1.680.048
      Pearson’s correlation test revealed a moderate negative correlation between the change in PCE at 6 and 12 months with a reduction in CH values at 6 and 12 months for both SMILE and FS-LASIK (Table 6). A moderate negative correlation was also found between changes in PCE at 6 and 12 months and changes in CRF values in SMILE and FS-LASIK. With the exception of FS-LASIK where a strong negative correlation was found between PCE change and CRH reduction at 6 months (Table 6). This implies that greater reduction in CH and CRF values is correlated with greater forward shift of the posterior corneal surface.
      Table 6Correlation between CH and CRF change with PCE at 6 and 12 months.
      SMILEFS-LASIK
      6 month
      CHCRFCHCRF
      PCE−0.667−0.431−0.523−0.752
      p<0.0010.002<0.001<0.001
      12 month
      PCE−0.613−0.465−0.427−0.527
      p<0.0010.0010.001<0.001
      PCE change at each post-operative time point was plotted against lenticule thickness/ablation depth and RBT in separate scatterplot graphs. Fig. 3 shows the relationship between ablation depth and change in PCE. The trend lines for both procedures are fairly flat at 3 months post-operation (Fig. 3A). At 6 and 12 months post-operation, both begin to demonstrate a positive trend where increasing ablation depth tended towards more forward displacement of the posterior corneal surface (Fig. 3B and C). Fig. 4 shows the relationship between RBT and change in PCE. The trend lines at 3 months post operation are also fairly flat for both procedures (Fig. 4A). At 6 and 12 months post operation, the trend line for FS-LASIK demonstrated a reverse trend, whereby a thicker RBT had less forward displacement of the posterior corneal surface; whereas a thinner RBT showed less forward protrusion of the posterior corneal surface in SMILE (Fig. 4B and C).
      Figure thumbnail gr3
      Fig. 3Relationship between PCE change and LT/AD at 3(A), 6(B) and 12(C) months post operation. LT = lenticule thickness, AD = ablation depth.
      Figure thumbnail gr4
      Fig. 4Relationship between PCE change and residual bed thickness at 3(A), 6(B) and 12(C) months post operation. RBT = residual stromal bed.

      3. Discussion

      In this retrospective study on 106 cases of high myopia that underwent SMILE and FS-LASIK, we presented data on changes of the posterior corneal surface and corneal biomechanical parameters post refractive surgery. The present study found that at 12 months post-operation, the FS-LASIK procedure resulted in a greater increase in PCE than the SMILE procedure, and that CH and CRF values were both reduced after surgery, with no significant difference between the two groups.
      Forward displacement of the posterior surface is similar in its characteristics to corneal ectasia and is believed to represent subclinical ectasia in otherwise normal LASIK [
      • Twa M.D.
      • Nichols J.J.
      • Joslin C.E.
      • Kollbaum P.S.
      • Edrington T.B.
      • Bullimore M.A.
      • Mitchell G.L.
      • Cruickshanks K.J.
      • Schanzlin D.J.
      Characteristics of corneal ectasia after LASIK for myopia.
      ,
      • Nishimura R.
      • Negishi K.
      • Saiki M.
      • Arai H.
      • Shimizu S.
      • Toda I.
      • Tsubota K.
      No forward shifting of posterior corneal surface in eyes undergoing LASIK.
      ]. Several factors, such as high myopia, low-residual stromal bed [
      • Randleman J.B.
      • Russell B.
      • Ward M.A.
      • Thompson K.P.
      • Stulting R.D.
      Risk factors and prognosis for corneal ectasia after LASIK.
      ,
      • Chang A.W.
      • Tsang A.C.
      • Contreras J.E.
      • Huynh P.D.
      • Calvano C.J.
      • Crnic-Rein T.C.
      • Thall E.H.
      Corneal tissue ablation depth and the Munnerlyn formula.
      ], and deep primary keratotomy resulting in a thick flap [
      • Haw W.W.
      • Manche E.E.
      Iatrogenic keratectasia after a deep primary keratotomy during laser in situ keratomileusis.
      ], have been shown to play a role in the pathogenesis of corneal ectasia after LASIK. In the past 10 years, the LASIK procedure has been optimized by the femtosecond laser, whereby a thinner, more uniform and accurate corneal flap could be cut. This allows comparatively more corneal tissue to be saved and in turn significantly reduces the possible risk of ectasia. However, complications relating to the corneal flap still occur in an otherwise perfect FS-LASIK, because a flap that could fracture the corneal biomechanical stability is needed before laser ablation. Dawson et al. found that the Bowman’s layer to be the strongest part of the cornea, followed by approximately 40% depth of the central anterior stroma (approx. 250 μm including Bowman’s layer and the epithelium), based on cohesive tensile strength measurements and histopathology of normal corneas [
      • Dawson D.G.
      • Grossniklaus H.E.
      • McCarey B.E.
      • Edelhauser H.F.
      Biomechanical and wound healing characteristics of corneas after excimer laser keratorefractive surgery: is there a difference between advanced surface ablation and sub-Bowman’s keratomileusis?.
      ]. As opposed to the LASIK procedure where the anterior stromal lamellae are severed by the creation of the flap and also by the excimer laser ablation, SMILE necessitates no creation of the corneal flap. Despite ending up with microdistortions in the Bowman’s layer [
      • Yao P.
      • Zhao J.
      • Li M.
      • Shen Y.
      • Dong Z.
      • Zhou X.
      Microdistortions in Bowman’s layer following femtosecond laser small incision lenticule extraction observed by Fourier-domain OCT.
      ], SMILE leaves the anterior-most stromal lamellae intact after the procedure (except for the region of the small incision), which could act as a factor in stabilizing the cornea after surgery. This stability may be advantageous long term, as seen in our study, where FS-LASIK showed a statistically significant greater change in PCE at 12 months post-operation, whereas in SMILE, PCE change remained stable.
      The ORA device provides biomechanical measures of CH and CRF of the cornea. CH represents corneal resistance to deformation, and thus the viscoelasticity of the corneal tissue, whilst CRF indicates the overall “resistance” of the cornea [
      • Terai N.
      • Raiskup F.
      • Haustein M.
      • Pillunat L.E.
      • Spoerl E.
      Identification of biomechanical properties of the cornea: the ocular response analyzer.
      ]. Several studies have found a reduction in CH and CRF post refractive surgery [
      • Agca A.
      • Ozgurhan E.B.
      • Demirok A.
      • Bozkurt E.
      • Celik U.
      • Ozkaya A.
      • Cankaya I.
      • Yilmaz O.F.
      Comparison of corneal hysteresis and corneal resistance factor after small incision lenticule extraction and femtosecond laser-assisted LASIK: a prospective fellow eye study.
      ,
      • Dou R.
      • Wang Y.
      • Xu L.
      • Wu D.
      • Wu W.
      • Li X.
      Comparison of corneal biomechanical characteristics after surface ablation refractive surgery and novel lamellar refractive surgery.
      ], as well as in keratoconic eyes [
      • Fontes B.M.
      • Ambrosio R.J.
      • Velarde G.C.
      • Nose W.
      Ocular response analyzer measurements in keratoconus with normal central corneal thickness compared with matched normal control eyes.
      ], suggesting that lower CH and CRF values may indicate a biomechanically weaker cornea. As some of the subjects required topical steroidal drops up to 3 months post-operation, CH and CRF values at 3 months post operation were omitted from analysis to minimise the effect of steroidal medication on IOP, and hence ORA measured values. Previous authors have reported similar results to our study, whereby both CH and CRF were reduced in a similar pattern post SMILE and FS-LASIK, with some reporting no significant difference between the two procedures [
      • Agca A.
      • Ozgurhan E.B.
      • Demirok A.
      • Bozkurt E.
      • Celik U.
      • Ozkaya A.
      • Cankaya I.
      • Yilmaz O.F.
      Comparison of corneal hysteresis and corneal resistance factor after small incision lenticule extraction and femtosecond laser-assisted LASIK: a prospective fellow eye study.
      ], and others reporting less change in biomechanical parameters in SMILE [
      • Wu D.
      • Wang Y.
      • Zhang L.
      • Wei S.
      • Tang X.
      Corneal biomechanical effects: small-incision lenticule extraction versus femtosecond laser-assisted laser in situ keratomileusis.
      ]. Our results suggest that post refractive surgery CH and CRF values remain reduced but stable at 12 months. Although no significant difference was found between the two procedures, we found that SMILE biomechanical parameters demonstrated a slight recovery at 12 months, whilst FS-LASIK biomechanical parameters continued a downward trend at 12 months. Overall, FS-LASIK demonstrated a greater reduction of CRF at 6 and 12 months post-operation. Therefore, although CH and CRF values are reduced to a similar value by the two procedures, biomechanically, FS-LASIK may still affect the cornea differently, owing to the differing nature of the two procedures.
      Further analysis revealed a moderate to significant correlation between PCE change at 6 and 12 months with CH and CRF reduction, i.e., the greater the reduction in CH and CRF values, the greater the forward shift of the posterior corneal surface. It is known that CH and CRF values are correlated with CCT, and in particular CH values are associated with corneal volume [
      • Hwang H.S.
      • Park S.K.
      • Kim M.S.
      The biomechanical properties of the cornea and anterior segment parameters.
      ]. It may be reasonable to assume that after removal of corneal tissue, the reduction in corneal volume and thickness leads to a reduced ability for the corneal to absorb energy, and hence reduced corneal resistance to deformation. An overall biomechanically weaker cornea could result in an increased forward shift of the posterior corneal surface. The exact mechanism of this relationship is unknown. Future studies with a larger sample size may be required to further evaluate the underlying mechanism for this finding.
      A residual stromal bed thickness of 250 μm is commonly used as the residual bed thickness necessary to prevent ectasia. It is generally accepted that the thinner the residual stromal bed thickness, the higher the risk of developing postoperative ectasia. However, a mathematical model that compares the relative tensile strength of the cornea after LASIK and SMILE has demonstrated that the thicker the cap (and therefore the deeper the lenticular tissue removal), and hence the lower the residual stromal bed thickness, the greater the residual tensile strength [
      • Reinstein D.Z.
      • Archer T.J.
      • Randleman J.B.
      Mathematical model to compare the relative tensile strength of the cornea after PRK LASIK, and small incision lenticule extraction.
      ]. As seen in Fig. 4B and C, the trend lines demonstrate a reverse relationship in SMILE, where a thinner residual bed has less forward protrusion of the posterior corneal surface. This lends strength to the idea that the cap, which includes the stronger anterior lamellae of the cornea, may be involved in stabilizing the corneal structure post-surgery. This may suggest that the traditional concept concerning the least thickness of residual bed is not true for SMILE; that is, SMILE does not follow the same criteria as LASIK for residual stromal bed thickness calculations.
      SMILE removes the corneal tissue in the deeper and relatively weaker stroma, it may be reasonable to speculate that for any given refractive correction, SMILE could leave the cornea with greater biomechanical strength than LASIK, as has been corroborated by the findings in our study. This may be particularly relevant to high myopia subjects, where more corneal tissue ablation is needed. Less forward shift of the posterior cornea, and less CRF reduction as seen with SMILE lend plausibility to the idea that it might be better suited to correcting high myopia, which itself is a risk factor for postoperative ectasia.
      The Pentacam system uses a rotating Scheimpflug camera to measure the anterior and posterior corneal surfaces. The overall repeatability of the Pentacam system has been found to be good [
      • Shankar H.
      • Taranath D.
      • Santhirathelagan C.T.
      • Pesudovs K.
      Anterior segment biometry with the Pentacam: Comprehensive assessment of repeatability of automated measurements.
      ]. Cioloni and Belin, who compared PCE changes after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) using the Pentacam system, found a mean PCE change of 2.64 ± 4.95 μm in the LASIK group and −0.88 ± 4.64 μm in the PRK group (p> 0.05) [
      • Ciolino J.B.
      • Belin M.W.
      Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy.
      ]. This reported value is similar in magnitude to the PCE changes in our study, suggesting that PCE is relatively small post refractive surgery, and that subclinical post-LASIK ectasia may not be as common as previously reported [
      • Ciolino J.B.
      • Belin M.W.
      Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy.
      ].
      One issue that may impact the analysis in our study was the difference in cap thickness (100–120 μm) and flap thickness (95 μm) of the two procedures. All FS-LASIK procedures aimed for this thickness as per the standard protocol of the Shanghai Eye and ENT Hospital of Fudan University. There is some evidence to suggest that flaps with a ±100 μm thickness lends the greatest corneal biomechanical stability, as deeper flaps may cause a greater disturbance of the corneal lamellae [
      • Slade S.G.
      The use of the femtosecond laser in the customization of corneal flaps in laser in situ keratomileusis.
      ]. Therefore the comparison between the two procedures demonstrate the typical clinical scenario at our hospital. Future studies may look at comparing different cap/flap thickness on corneal structural integrity and biomechanical parameters.
      FS-LASIK has been established for a long time and studied extensively in the literature. SMILE is a relatively new procedure for myopia correction and changes in PCE has not been reported previously. This study evaluated changes in PCE and biomechanical parameters post SMILE and FS-LASIK. Our study found that at 12 months post operation, PCE change became greater in the FS-LASIK group compared to the SMILE procedure, i.e. SMILE maintained posterior corneal surface stability better than FS-LASIK at 12 months post operation, however both PCE changes obtained through the Pentacam system were small and no subjects developed corneal ectasia in the 12 month follow up period. Cornea biomechanical changes were similar after the two procedures, although FS-LASIK demonstrated a greater reduction of CRF post-operation. This suggests that SMILE could be more advantageous biomechanically, particularly where high myopia correction is concerned. Further studies are warranted to determine if the larger PCE and CRF change in FS-LASIK is indicative of an increased risk of corneal ectasia than SMILE in the long term.

      Funding

      This study was supported by the Science and Technology Commission of Shanghai Municipality, grant no. 134119a5100.

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