Journal of Refractive Surgery

Biomechanics 

Biomechanics of LASIK Flap and SMILE Cap: A Prospective, Clinical Study

Pooja Khamar, MD; Rohit Shetty, MD, PhD, FCRS; Ravish Vaishnav, MD; Mathew Francis, MTech; Rudy M.M.A. Nuijts, MD, PhD; Abhijit Sinha Roy, PhD

Abstract

PURPOSE:

To analyze the acute effect of flap cut in laser in situ keratomileusis (LASIK) eyes and cap cut in small incision lenticule extraction (SMILE) eyes on corneal biomechanical properties of patients undergoing surgery.

METHODS:

This was a prospective, interventional, longitudinal case series. Forty-eight eyes of 24 patients underwent contralateral LASIK and SMILE. Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) measurements were performed preoperatively, intraoperatively, and 1 week and 1 month after surgery. In LASIK eyes, the flap was cut but not lifted before intraoperative measurements. In SMILE eyes, the cap and side cut incision were made before intraoperative measurement. Thirty biomechanical variables were analyzed, assuming multiple comparisons.

RESULTS:

In LASIK and SMILE eyes, 36.7% and 13.3% of the total number of variables detected biomechanical weakening after flap and cap cuts (P = .02), respectively. Further, 13.3% and 40% of the total variables detected no biomechanical changes after flap and cap cut, respectively (P = .03). These acute biomechanical effects of flap and cap cuts did not influence 1-week and 1-month measurements (P > .05) because both LASIK and SMILE eyes showed similar biomechanical weakening.

CONCLUSIONS:

Flap and cap cuts induced biomechanical weakening in patient corneas. The flap caused more weakening than the cap intraoperatively. However, biomechanical differences between LASIK and SMILE eyes were similar after removal of tissue and ongoing wound healing.

[J Refract Surg. 2019;35(5):324–332.]

Abstract

PURPOSE:

To analyze the acute effect of flap cut in laser in situ keratomileusis (LASIK) eyes and cap cut in small incision lenticule extraction (SMILE) eyes on corneal biomechanical properties of patients undergoing surgery.

METHODS:

This was a prospective, interventional, longitudinal case series. Forty-eight eyes of 24 patients underwent contralateral LASIK and SMILE. Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) measurements were performed preoperatively, intraoperatively, and 1 week and 1 month after surgery. In LASIK eyes, the flap was cut but not lifted before intraoperative measurements. In SMILE eyes, the cap and side cut incision were made before intraoperative measurement. Thirty biomechanical variables were analyzed, assuming multiple comparisons.

RESULTS:

In LASIK and SMILE eyes, 36.7% and 13.3% of the total number of variables detected biomechanical weakening after flap and cap cuts (P = .02), respectively. Further, 13.3% and 40% of the total variables detected no biomechanical changes after flap and cap cut, respectively (P = .03). These acute biomechanical effects of flap and cap cuts did not influence 1-week and 1-month measurements (P > .05) because both LASIK and SMILE eyes showed similar biomechanical weakening.

CONCLUSIONS:

Flap and cap cuts induced biomechanical weakening in patient corneas. The flap caused more weakening than the cap intraoperatively. However, biomechanical differences between LASIK and SMILE eyes were similar after removal of tissue and ongoing wound healing.

[J Refract Surg. 2019;35(5):324–332.]

Laser in situ keratomileusis (LASIK) has delivered safe and efficacious outcomes for correction of refractive error.1 Despite superior screening methods and biomechanical analyses, ectasia remains an unwanted complication after LASIK.2,3 The flap cut and tissue ablation in LASIK can cause ectasia in biomechanically compromised or suspect corneas, even in patients with low refractive error.4 On the other hand, the cap cut in small incision lenticule extraction (SMILE) requires a smaller cut (not a near 360° flap) in the anterior stroma of the cornea. Therefore, theoretical models suggested that SMILE would have a biomechanical advantage over LASIK.5,6 However, clinical investigations with the Ocular Response Analyzer (ORA, Reichert Inc., Depew, NY) and dynamic Scheimpflug analyzer (Corvis ST; Oculus Optikgeräte GmbH, Wetzlar, Germany) reported mostly similar biomechanical changes after SMILE and LASIK.7–19 Therefore, theoretical models and patient measurements were not in complete agreement.

Unfortunately, none of the above biomechanical studies investigated the fundamental biomechanical differences between flap cut in LASIK and cap cut in SMILE because postoperative measurements were performed after the cuts and tissue removal were completed.7–19 This would require an alternate study design. In this study, we conducted a contralateral biomechanical comparison of LASIK and SMILE. Corvis ST measurements were performed preoperatively and 1 week and 1 month postoperatively. We added an additional measurement after flap cut in LASIK eyes and cap cut (along with the side incision) in SMILE eyes, which was performed intraoperatively before the completion of LASIK and SMILE. To our knowledge, this would be the first report of “true” biomechanical changes induced by the flap or the cap alone in corneas undergoing myopic refractive surgery. This study attempted to establish the biomechanical differences between SMILE cap and LASIK flap cut before the cornea underwent structural change caused by tissue removal. Further comparisons were performed with follow-up measurements and earlier studies.7–19

Patients and Methods

This was a prospective, interventional, longitudinal case series. The study was approved by the Narayana Nethralaya Ethics Committee, Bangalore, India. Written informed consent was obtained from the patients after detailed explanation of the intraoperative measurements with the Corvis ST. The study followed the tenets of the Declaration of Helsinki. Forty-eight eyes of 24 patients underwent LASIK in one eye and SMILE in the fellow eye. Each eye was assigned to either LASIK or SMILE by a coin toss. Inclusion criteria were stable refraction (less than −10.00 diopters [D] equivalent refraction with astigmatism of not more than −3.00 D) for a period of 1 year (change less than 0.25 D) and a calculated minimum residual stromal bed thickness of 250 µm. Patients with central corneal thickness of less than 480 µm or a history of keratoconus, diabetes mellitus, collagen vascular disease, pregnancy, breast-feeding, and any prior ocular surgery or trauma were excluded from the study. Contact lens use was discontinued for at least 2 weeks before measurements.

Corvis ST measurements were performed before surgery, after flap/cap cut, and after surgery. In LASIK eyes, the flap was cut with a femtosecond laser but the flap was not lifted. In the fellow eye undergoing SMILE, only the side cut incision and three-dimensional geometry of the lenticule was cut but not separated from the surrounding stroma. The patient waited in the surgical area for 3 hours because the area had controlled temperature and humidity for surgical procedures such as LASIK (as recommended by the manufacturer). The Corvis ST measurement was repeated in both eyes. After the measurement, the patient's eye was redocked to the excimer laser and LASIK was completed by lifting the flap and ablating the underlying stroma. SMILE was completed by separating the lenticule from the stroma and extracting it through the side cut in the fellow eye. Corvis ST measurements were repeated at 1 week and 1 month postoperatively. In LASIK eyes, Corvis ST measurements were not repeated after flap lifting due to possible challenges in centering the patient cornea for LASIK and the risk of infections/inflammations. Intraoperative use of the Corvis ST also had the added risk of flap dislocation if it was performed immediately after completion of LASIK. Similar risks of infection/inflammation were also possible in SMILE eyes. Therefore, no measurements were performed after either flap lifting or lenticule separation from the surrounding tissue or immediately after completion of surgeries.

A single experienced surgeon (RS) performed all surgeries under topical anesthesia using 0.5% proparacaine hydrochloride (Paracain; Sunways Pvt. Ltd., Mumbai, India) instilled two or three times. The WaveLight FS200 femtosecond laser and WaveLight EX500 excimer laser platform (Alcon Laboratories, Inc., Fort Worth, TX) cut the flap and ablated the tissue in one eye. The flap had a 9-mm diameter, 110-µm thickness, side cut angle of 70°, canal width of 1.5 mm, and hinge position at 90°. The optical zone diameter was 6 mm. The VisuMax femtosecond laser system (Carl Zeiss Meditec AG, Jena, Germany) cut the cap and lenticule in the fellow eye. Cap thickness was 110 µm. Lenticule and cap diameter was 6 and 7.7 mm, respectively. After creation of the refractive lenticule, it was dissected and extracted manually through a superior 3-mm side cut. The cornea was remoistened with a wet Merocel sponge (Beaver-Vistec International, Waltham, MA) at the end of the procedure. After the surgery, one drop of moxifloxacin hydrochloride 0.5% (Vigamox; Alcon Laboratories, Inc.) was applied to both eyes. The routine postoperative regimen was followed for both eyes. This included moxifloxacin hydrochloride 0.5% eye drops (Vigamox) four times a day for 1-week, tapering doses of topical 1% fluorometholone eye drops (Flarex; Alcon Laboratories, Inc.), and topical lubricants (Optive; Allergan, Inc., Parsippany, NJ) four times a day for 3 months.

Thirty Corvis ST variables were analyzed. The variables were either machine derived or determined from waveform analyses of the entire deformation amplitude signal.19,20 The analyzed primary variables were as follows:

  1. Arc length of the cornea (Arc length), time (Time), velocity (Velocity), deformation amplitude (DA), deflection amplitude, and horizontal length (Deflection length) of the cornea between the two peripheral corneal bends of 1st applanation (A1), 2nd applanation (A2), and highest concavity (HC);

  2. Maximum deformation amplitude (DA Max), deflection amplitude (Deflection amplitude Max) and its time (Deflection amplitude Max Time), and arc length (Arc length Max);

  3. Ratio of DA between the center and periphery (1 mm and 2 mm) designated as DA Ratio Max 1mm and DA Ratio Max 2mm, respectively

  4. Integrated radius and maximum inverse of concave radius of curvature (Max Inverse radius);

  5. Stiffness parameter at A1 (SP-A1);

  6. Corneal stiffnesses [Kc (constant) and Kc (mean)] derived from waveform analyses of deformation amplitude signal with a biomechanical model19,20;

  7. Maximum whole eye movement and its time.

Two other variables, ARTh (Ambrósio relational thickness) and the Corvis Biomechanical Index (CBI), were also assessed. ARTh described the distribution of corneal thickness relative to its minimum value in a given cornea. The CBI included ARTh in its derivation. Therefore, ARTh and the CBI were analyzed as a secondary set of variables because reduced thickness artificially affected their measurements. Also, it was assumed that the surgery, having a greater proportion of variables indicating biomechanical change (weakening) after flap/cap cut, caused a greater biomechanical weakening of the cornea overall. Thus, the proportion of variables indicating biomechanical change versus no biomechanical change after flap/cap cut was statistically compared between the LASIK and SMILE eyes.

Statistical Analyses

All continuous variables were assessed for normality of distribution with the Kolmogorov–Smirnov test. Because some variables were non-parametric in distribution, the Friedman test for repeated measures was used. For a non-parametric distribution, the median with 95% confidence interval (CI) was calculated for each variable. Repeated measures analyzed each variable (in a paired manner) between time points simultaneously for a given eye. The “N-1” chi-square test was used to compare the proportions. MedCalc software (version 18.7; MedCalc Inc., Ostend, Belgium) was used for statistical analyses. The software adjusted the P value for repeated measures. These repeated measures were preoperative (1), flap/cap cut (2), 1 week (3), and 1 month (4). A P value of less than .05 was considered statistically significant.

Results

Table 1 lists the preoperative features of LASIK and SMILE eyes. All features were similar between the two groups (P > .05). The median corrected distance visual acuity was 0.0 logMAR (95% CI: 0.0 to 0.0 logMAR) preoperatively. At 1 month postoperatively, the median uncorrected distance visual acuity was 0.0 logMAR (95% CI: −0.13 to 0.13 logMAR). Table 2 lists the Corvis ST variables of the LASIK eyes. The last column describes the results of the statistical comparisons. Some of the variables indicated reduction in corneal strength (eg, lower stiffness, shorter lengths, earlier A1 and later A2 times, greater deformation and deflection amplitudes, and lower inverse radius and greater integrated radius). Among the 31 variables, 4 (13.3% of the total number of variables) were similar between preoperative and flap cut but differed (P < .01) from 1 week and 1 month [(1),(2) versus (3),(4) in Table 2]. Eleven (36.7%) variables were such that both preoperative and flap cut differed significantly (P < .01) from each other and from 1 week and 1 month [(1) versus rest, (2) versus rest in Table 2]. Nine (30.0%) variables were similar among all time points (P > .05, not significant in Table 2). Five (13.3%) variables were similar among flap cut, 1 week, and 1 month time-points but differed significantly (P < .01) from preoperative [(1) versus rest in Table 2].

Preoperative Demographics (Median [95% CI])

Table 1:

Preoperative Demographics (Median [95% CI])

Biomechanical Parameters of LASIK Patientsa

Table 2:

Biomechanical Parameters of LASIK Patients

Table 3 lists the Corvis ST variables of the SMILE eyes. The last column describes the results of the statistical comparisons. Similar to LASIK eyes, some of the variables indicated a decrease in corneal strength after SMILE. However, the proportion of variables was different. For preoperative, cap cut versus 1 week and 1 month [(1),(2) versus (3),(4) in Table 3], 12 (40.0%) met the significance criteria. For preoperative versus rest, cap cut versus rest [(1) versus rest, (2) versus rest in Table 3], only 4 (13.3%) met the criteria. Five variables (16.7%) were not significant among all time-points (P > .05, not significant). Seven (20.0%) met the criteria of preoperative versus rest [(1) versus rest in Table 3]. For preoperative cap cut versus 1 week and 1 month [(1),(2) versus (3),(4)], the proportion of variables was significantly different (P = .02) between the LASIK and SMILE eyes. For preoperative versus rest, cap cut versus rest criteria [(1) versus rest, (2) versus rest], the proportion of variables also was significantly different (P = .02) between the LASIK and SMILE eyes (P = .03). Overall, corneal stiffness parameters decreased after creation of flap/cap and reduced further after completion of LASIK/SMILE procedures. These decreases in magnitudes of stiffnesses up to 1 week and 1 month were similar between the two procedures (P > .20).

Biomechanical Parameters of SMILE Patientsa

Table 3:

Biomechanical Parameters of SMILE Patients

In LASIK eyes, median ARTh was 442.8, 368.6, 201.4, and 193.1 at preoperative (1), flap cut (2), 1 week (3), and 1 month (4), respectively [(1),(2) versus (3),(4), P < .01]. The CBI was 0.016, 0.22, 0.99, and 0.98, respectively [(1) versus rest, (2) versus rest, P < .01]. In SMILE eyes, ARTh was 388.7, 409.1, 190.9, and 193.9, respectively [(1),(2) versus (3),(4), P < .01]. The corresponding CBI was 0.029, 0.027, 0.99, and 0.99, respectively [P < .01; (1) versus rest, (2) versus rest]. At later follow-up visits (1 week and 1 month), no significant differences were observed between LASIK and SMILE eyes with respect to change in the variables (P > .05). Tables 45 show the change in the indices: 1 week minus preoperative and 1 month minus preoperative, respectively. At 1 month, median intraocular pressure (bIOP in Corvis ST) was 16.1 mm Hg (95% CI: 14.2 to 16.6 mm Hg) and 15 mm Hg (95% CI: 14.1 to 16.1 mm Hg) in LASIK and SMILE eyes, respectively (P = .52). Also, median central corneal thickness was 459.5 µm (95% CI: 442.2 to 508.3 µm) and 457.7 µm (95% CI: 437.7 to 499 µm) in LASIK and SMILE eyes, respectively (P = .45). Thus, intraocular pressure and central corneal thickness were not confounders affecting the variables differentially between the groups postoperatively.

Change in Biomechanical Parameters (1 Week Minus Preoperative)a

Table 4:

Change in Biomechanical Parameters (1 Week Minus Preoperative)

Change in Biomechanical Parameters (1 Month Minus Preoperative)a

Table 5:

Change in Biomechanical Parameters (1 Month Minus Preoperative)

Discussion

A recent study tested the difference in corneal elastic modulus of human corneal samples ex vivo (two-dimensional stretch testing) after LASIK and SMILE.21 The study showed that the modulus of SMILE corneas was 1.47 times that of LASIK corneas.21 Another ex vivo study on LASIK flap with Brillouin scattering implied reduced Brillouin modulus after flap creation in the anterior (one-third region) stroma of porcine eyes.22 Thus, severing of the fibers by either flap or cap should lead to some biomechanical weakening. However, no clinical study on patients had quantified exclusively the biomechanical effect of flap and cap in patients undergoing refractive surgery. The novel aspect of this study was the exclusive assessment of flap- and cap-induced deformation changes in the patient corneas intraoperatively. The Corvis ST allowed exclusive assessment of deformation of the cornea in response to air-puff applanation. A salient finding of this study was that flap and cap cut differences were actually detected by the Corvis ST. In Tables 23, two statistical inferences were key. First, (1),(2) versus (3),(4) indicated a significant difference between the first two and the last two time-points but (1) and (2) were similar (Tables 23). Second, (1) versus rest, (2) versus rest indicated that significant differences existed between preoperative and flap/cap cut but changes at 1 week and 1 month were similar. Using the above definitions, the salient findings of the study were as follows:

  1. As expected, some of the deformation variables indicated biomechanical weakening after flap and cap creation (eg, decrease in stiffness, earlier 1st applanation).

  2. Temporal assessment of these variables also showed increased weakening of the cornea after tissue removal (ablation and lenticule extraction) (eg, Kc [constant] and Kc [mean]).

  3. In LASIK, 36.7% of the variables belonged to preoperative versus rest [(1) versus rest], flap cut versus rest [(2) versus rest], indicating significant biomechanical changes after flap creation. This changed to 13.3% in SMILE eyes (P = .02), indicating that the LASIK flap caused a greater biomechanical change in the cornea than the SMILE cap.

  4. The above observation was also supported by the percentage of variables in the preoperative, flap/cap cut versus 1 week, 1 month [(1),(2) versus (3),(4)] significance group.

  5. In LASIK and SMILE eyes, 50% and 46.7% of the variables were unchanged after surgery at all time-points, respectively. Transient corneal deformation by applanation is three-dimensional, but only two-dimensional variables were either reported by the device or calculated by waveform analyses. Thus, not all variables were affected by the surgery and this proportion was nearly the same in both LASIK and SMILE eyes. Only those variables, altered due to surgery, were of interest.

  6. Interestingly, the variables reported similar magnitude of change between LASIK and SMILE eyes up to 1 week and 1 month. This indicated that the biomechanical effect of tissue removal was the primary determinant of the change in deformation variables 1 week and 1 month after surgery despite the differences seen after flap/cap cut.

The link between the intraoperative and the follow-up measurements was analyzed in this study for the first time. It was possible that some acute edema in the cornea intraoperatively may have led to inaccuracies in the detection of the posterior edge and corneal thickness. Therefore, a sharp decrease in ARTh was noted from preoperative (1) to flap cut (2), which was not observed clinically. This new effect was not reported earlier. ARTh is representative of corneal thickness distribution from the center to the periphery of a cornea.23 Because no tissue was removed after flap/cap cut, significant changes in ARTh from preoperative to flap/cap cut were probably artifactual. The CBI included ARTh and its results were also affected.23 Neither quality assessment of posterior edge detection nor occurrence of flap/cap interface edema was possible because optically distorted corrected Scheimpflug images were not available to us. Thus, the CBI and ARTh may not be useful to assess the flap/cap effects on corneal deformation relative to the preoperative state. However, ARTh and the CBI may still be useful to detect progressive onset of ectasia in the long term after surgery.23 The variables in Tables 23 were derived exclusively from the anterior edge of the cornea (interface of air and epithelium) and did not suffer from either the limitations of posterior edge detection or edema.

Among the ORA studies, 6 reported no difference between LASIK and SMILE eyes and 4 reported a better biomechanical outcome after SMILE than LASIK.7–16 Among the Corvis ST studies, 3 reported that some biomechanical variables reported better outcomes after SMILE than LASIK.7,10,19 The other studies (2) reported no biomechanical differences between LASIK and SMILE eyes.17,18 Thus, similar biomechanical outcomes after LASIK and SMILE in the long term may be the logical conclusion because definitive trends were obtained. These findings were similar to the 1-week (3) and 1-month (4) outcomes (Tables 23). Our earlier study also showed similar biomechanical changes after LASIK and SMILE up to 6 months of follow-up with the Corvis ST.19 Thus, extending this study to a longer follow-up beyond 1 month was not essential.

Other than biomechanical outcomes, SMILE and LASIK have differences in temporal wound healing and visual recovery.24,25 Currently, no clinical device directly quantifies viscoelastic relaxation,26 collagen crimping,19 and tissue hydration26 in patients. This limits the scope of the analyses that could be performed by us or in future studies. However, our results indicated that temporal wound healing of the cornea minimized the acute biomechanical differences between cap and flap to an extent that no significant biomechanical differences between LASIK and SMILE eyes were detected at 1 week and 1 month. Refined techniques such as inverse finite element modeling of patient corneal biomechanical properties with applanation may shed more light on the finer differences between LASIK and SMILE eyes.27,28 SMILE cap appeared to cause less biomechanical change in the cornea than LASIK flap in patient corneas. This is a unique finding. Further, temporal healing of the cornea and tissue appeared to dominate the biomechanical differences induced in the acute phase by the flap and cap cuts. Thus, safety criteria established for recommending LASIK to patients should also be followed for recommending SMILE. This requires further evaluation in future studies.

References

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  28. Francis M, Khamar P, Shetty R, et al. In vivo prediction of air-puff induced corneal deformation using LASIK, SMILE, and PRK finite element simulations. Invest Ophthalmol Vis Sci. 2018;59:5320–5328. doi:10.1167/iovs.18-2470 [CrossRef]

Preoperative Demographics (Median [95% CI])

ParameterLASIK (n = 24)SMILE (n = 24)P
Intraocular pressure (bIOP, mm Hg)16 (14 to 17.7)16.1 (14.4 to 17.1).69
Central corneal thickness (µm)528.4 (509.2 to 546.3)521.0 (503.4 to 542).12
Sphere (D)−4.25 (−5.50 to −3.00)−4.00 (−5.31 to −3.00).83
Cylinder (D)−0.88 (−1.06 to −0.44)−0.50 (−0.81 to −0.25).29
Spherical equivalent (D)−4.56 (−6.13 to −3.69)−4.44 (−5.47 to −3.47).57

Biomechanical Parameters of LASIK Patientsa

ParameterPreoperative (1)Flap Cut (2)1 Week (3)1 Month (4)Pb
A1
  Arc length (mm)−0.017 (−0.02 to 0.013)−0.015 (−0.022 to −0.014)−0.013 (−0.014 to −0.01)−0.011 (−0.013 to −0.007)All
  Deflection amplitude (mm)0.095 (0.086 to 0.10)0.093 (0.086 to 0.107)0.083 (0.070 to 0.088)0.079 (0.067 to 0.084)(1) to (2) vs (3) to (4)
  Deflection length (mm)2.25 (2.14 to 2.38)2.23 (2.11 to 2.37)2.02 (1.75 to 2.09)1.90 (1.54 to 2.06)(1),(2) vs (3),(4)
  DA (mm)0.135 (0.12 to 0.14)0.13 (0.12 to 0.15)0.12 (0.10 to 0.13)0.11 (0.10 to 0.12)(1),(2) vs (3),(4)
  Time (ms)7.40 (7.23 to 7.61)7.30 (7.20 to 7.5)7.06 (6.95 to 7.25)7.12 (7.02 to 7.24)(1) vs rest, (2) vs rest
  Velocity (m/s)0.15 (0.138 to 0.158)0.15 (0.143 to 0.162)0.16 (0.153 to 0.166)0.157 (0.147 to 0.164)NS
A2
  Arc length (mm)−0.025 (−0.029 to 0.022)−0.024 (−0.029 to −0.017)−0.015 (−0.019 to −0.013)−0.014 (−0.017 to 0.011)(1) to (2) vs (3) to (4)
  Deflection amplitude (mm)0.11 (0.10 to 0.12)0.12 (0.11 to 0.13)0.092 (0.08 to 0.1)0.088 (0.081 to 0.095)(1),(2) vs (3),(4)
  Deflection length (mm)3.0 (2.66 to 3.16)2.93 (2.71 to 3.12)2.31 (2.02 to 3.49)2.21 (1.67 to 3.36)NS
  DA (mm)0.36 (0.35 to 0.40)0.35 (0.33 to 0.42)0.36 (0.32 to 0.38)0.36 (0.32 to 0.38)NS
  Time (ms)21.50 (21.34 to 21.67)21.54 (21.24 to 21.70)21.74 (21.60 to 21.87)21.77 (21.54 to 21.86)(1) vs rest, (2) vs rest
  Velocity (m/s)−0.29 (−0.30 to −0.27)−0.30 (−0.32 to −0.28)−0.30 (−0.32 to −0.29)−0.29 (−0.31 to −0.28)NS
  DA ratio max 1mm1.60 (1.58 to 1.62)1.58 (1.55 to 1.61)1.69 (1.65 to 1.71)1.70 (1.67 to 1.75)(1) vs rest, (2) vs rest
  DA ratio max 2mm4.34 (4.15 to 4.50)4.45 (4.17 to 4.82)5.24 (4.82 to 5.53)5.19 (4.88 to 5.39)(1) vs rest, (2) vs rest
  Arc length max (mm)−0.19 [−0.20 to 0.17)−0.17 (−0.19 to −0.16)−0.13 (−0.15 to −0.10)−0.12 (−0.15 to −0.095)(1) vs rest to (2) vs rest
  DA max (mm)1.14 (1.04 to 1.16)1.17 (1.07 to 1.25)1.16 (1.12 to 1.22)1.19 (1.12 to 1.28)(1) vs rest
  Deflection amplitude max (mm)0.98 (0.94 to 1.03)1.02 (0.96 to 1.08)1.06 (1.01 to 1.10)1.05 (1.02 to 1.15)(1) vs rest, (2) vs (4)
  Deflection amplitude max time (ms)16.11 (16.06 to 16.27)15.91 (15.70 to 16.08)16.14 (15.94 to 16.31)16.17 (15.76 to 16.50)NS
HC
  Arc length (mm)−0.153 (−0.163 to −0.147)−0.141 (−0.15 to −0.12)−0.105 (−0.130 to −0.089)−0.092 (−0.108 to −0.069)(1) vs rest, (2) vs rest
  Deflection amplitude (mm)0.98 (0.93 to 1.02)1.01 (0.95 to 1.05)1.04 (1.0 to 1.1)1.04 (1.0 to 1.11)(1) vs rest, (2) vs rest
  Deflection length (mm)6.76 (6.42 to 6.83)6.72 (6.54 to 6.89)6.64 (6.53 to 6.89)6.71 (6.55 to 6.74)NS
  DA (mm)1.14 (1.04 to 1.16)1.17 (1.07 to 1.25)1.16 (1.12 to 1.22)1.19 (1.12,1.28)(1) vs rest
  Time (ms)15.86 (15.21 to 16.49)15.55 (15.02 to 16.63)15.29 (15.09 to 16.63)15.42 (15.12 to 16.78)NS
  Integrated radius (mm)7.69 (7.28 to 7.94)8.18 (7.40 to 8.58)9.76 (9.08 to 10.53)9.88 (9.10 to 10.60)(1) vs rest, (2) vs rest
  Kc (constant) [N/m]105.6 (99.8 to 108.9)101.68 (95.75 to 106.59)95.34 (92.40 to 100.83)98.94 (88.97 to 102.08)(1) vs rest, (2) vs rest
  Kc (mean) [N/m]96.1 (91.3 to 103.1)90.09 (86.45 to 97.26)81.78 (77.03 to 89.91)85.74 (77.07 to 90.92)(1) vs rest, (2) vs rest
  Max inverse radius (mm-1)0.167 (0.158 to 0.176)0.186 (0.165 to 0.207)0.195 (0.183 to 0.204)0.194 (0.186 to 0.205)(1) vs rest, (2) vs rest
  SP_A1102.9 (91.5 to 112.4)94.69 (87.18 to 103.31)95.06 (72.93 to 103.74)98.93 (78.92 to 103.63)(1) vs rest
  Whole eye movement max (mm)0.258 (0.243 to 0.293)0.265 (0.236 to 0.319)0.245 (0.206 to 0.277)0.277 (0.227 to 0.295)NS
  Whole eye movement max time (ms)21.78 (21.53 to 22.47)21.61 (21.31 to 22.49)21.46 (21.17 to 21.84)21.62 (21.02 to 22.02)NS

Biomechanical Parameters of SMILE Patientsa

ParameterPreoperative (1)Cap Cut (2)1 Week (3)1 Month (4)Pb
A1
  Arc length (mm)−0.015 (−0.019 to 0.012)−0.017 (−0.019 to −0.014)−0.012 (−0.014 to −0.007)−0.010 (−0.012 to −0.008)(1),(2) vs (3),(4)
  Deflection amplitude (mm)0.090 (0.084 to 0.099)0.099 (0.089 to 0.101)0.076 (0.073 to 0.086)0.077 (0.071 to 0.084)(1),(2) vs (3),(4)
  Deflection length (mm)2.17 (2.07 to 2.29)2.20 (2.09 to 2.36)1.91 (1.65 to 2.04)1.92 (1.79 to 2.01)(1),(2) vs (3),(4)
  DA (mm)0.13 (0.12 to 0.14)0.13 (0.125 to 0.139)0.11 (0.10 to 0.13)0.11 (0.10 to 0.12)(1),(2) vs (3),(4)
  Time (ms)7.41 (7.25 to 7.51)7.25 (7.09 to 7.40)7.04 (6.96 to 7.17)7.08 (6.90 to 7.17)(1) vs rest, (2) vs rest
  Velocity (m/s)0.151 (0.145 to 0.156)0.16 (0.155 to 0.165)0.161 (0.155 to 0.164)0.159 (0.151 to 0.166)(1) vs rest
A2
  Arc length (mm)−0.025 (−0.027 to 0.020)−0.022 (−0.025 to −0.014)−0.014 (−0.018 to −0.008)−0.013 (−0.017 to −0.006)(1),(2) vs (3),(4)
  Deflection amplitude (mm)0.116 (0.109 to 0.119)0.116 (0.106 to 0.129)0.093 (0.086 to 0.108)0.089 (0.075 to 0.100)(1),(2) vs (3),(4)
  Deflection length (mm)3.48 (3.12 to 3.75)3.80 (2.98 to 3.96)2.97 (2.15 to 3.60)3.08 (2.65 to 3.62)(1),(2) vs (3),(4)
  DA (mm)0.36 (0.35 to 0.39)0.40 (0.37 to 0.45)0.35 (0.30 to 0.39)0.37 (0.30 to 0.38)NS
  Time (ms)21.55 (21.41 to 21.67)21.72 (21.59 to 21.81)21.78 (21.63 to 21.85)21.80 (21.67 to 21.96)(1) vs rest, (2) vs (4)
  Velocity (m/s)−0.292 (−0.312 to 0.288)−0.311 (−0.317 to −0.302)−0.305 (−0.326 to −0.294)−0.300 (−0.311 to −0.283)(1) vs (2),(3)
  DA ratio max 1mm1.60 (1.56 to 1.62)1.60 (1.56 to 1.63)1.70 (1.66 to 1.74)1.70 (1.69 to 1.75)(1),(2) vs (3),(4)
  DA ratio max 2mm4.29 (4.15 to 4.57)4.44 (4.30 to 4.76)5.13 (4.98 to 5.68)5.44 (5.10 to 5.80)(1) vs rest, (2) vs rest
  Arc length max (mm)−0.18 (−0.19 to −0.16)−0.18 (−0.19 to −0.16)−0.11 (−0.15 to −0.10)−0.11 (−0.15 to −0.10)(1),(2) vs (3),(4)
  DA max (mm)1.12 (1.09 to 1.18)1.18 (1.12 to 1.23)1.19 (1.12 to 1.23)1.22 (1.11 to 1.27)(1) vs rest
  Deflection amplitude max (mm)0.99 (0.96 to 1.06)1.02 (0.99 to 1.06)1.04 (1.00 to 1.14)1.07 (1.01 to 1.16)(1),(2) vs (3),(4)
  Deflection amplitude max time (ms)15.96 (15.67 to 16.18)16.03 (15.75 to 16.15)16.0 (15.90 to 16.14)15.8 (15.4 to 16.14)NS
HC
  Arc length (mm)−0.148 (−0.159 to −0.137)−0.141 (−0.155 to −0.107)−0.092 (−0.100 to −0.080)−0.089 (−0.099 to −0.065)(1),(2) vs (3),(4)
  Deflection amplitude (mm)0.98 (0.94 to 1.04)1.0 (0.98 to 1.05)1.03 (0.99 to 1.13)1.06 (0.99 to 1.12)(1) vs rest, (2) vs (3)
  Deflection length (mm)6.66 (6.48 to 6.72)6.80 (6.67 to 6.94)6.65 (6.52 to 6.91)6.64 (6.37 to 6.80)NS
  DA (mm)1.12 (1.09 to 1.18)1.18 (1.12 to 1.23)1.19 (1.12 to 1.23)1.22 (1.11 to 1.27)(1) vs rest
  Time (ms)15.59 (15.35 to 16.76)16.0 (15.32 to 16.65)15.67 (15.25 to 16.43)15.94 (15.48 to 16.51)NS
  Integrated radius (mm)7.99 (7.07 to 8.37)8.46 (7.82 to 9.15)9.61 (9.17 to 10.50)10.05 (9.45 to 10.76)(1) vs rest, (2) vs rest
  Kc (constant) [N/m]102.4 (97.7 to 106.9)96.9 (94.7 to 101.9)96.3 (90.8 to 100.8)93.3 (89.3 to 98.2)(1) vs rest, (2) vs (4)
  Kc (mean) [N/m]94.0 (85.9 to 97.6)86.6 (82.8 to 89.4)82.2 (76.2 to 88.3)80.9 (75.8 to 86.6)(1) vs rest, (2) vs rest
  Max inverse radius (mm−1)0.174 (0.162 to 0.184)0.185 (0.168 to 0.195)0.195 (0.182 to 0.203)0.202 (0.193 to 0.214)(1),(2) vs (3),(4)
  SP_A1104.5 (101.5 to 111.1)96.9 (83.7 to 103.1)85.5 (76.2 to 100.1)86.1 (74.1 to 98.1)(1) vs rest, (2) vs (4)
  Whole eye movement max (mm)0.264 (0.24 to 0.29)0.308 (0.269 to 0.332)0.262 (0.220 to 0.300)0.29 (0.222 to 0.307)(2) vs rest
Whole eye movement max time (ms)21.85 (21.31 to 22.56)21.83 (21.33 to 22.60)21.66 (21.28 to 22.04)21.48 (21.30 to 22.00)NS

Change in Biomechanical Parameters (1 Week Minus Preoperative)a

ParameterLASIKSMILE
A1
  Arc length (mm)0.004 (0.001 to 0.007)0.005 (0.002 to 0.007)
  Deflection amplitude (mm)−0.013 (−0.021 to −0.006)−0.013 (−0.017 to −0.007)
  Deflection length (mm)−0.32 (−0.45 to −0.16)−0.21 (−0.44 to −0.073)
  DA (mm)−0.016 (−0.021 to −0.01)−0.012 (−0.023 to −0.007)
  Time (ms)−0.33 (−0.41 to −0.26)−0.35 (−0.38 to −0.22)
  Velocity (m/s)0.01 (0.0 to 0.022)0.012 (0.0 to 0.017)
A2
  Arc length (mm)0.01 (0.005 to 0.014)0.01 (0.006 to 0.015)
  Deflection amplitude (mm)−0.02 (−0.033 to −0.007)−0.021 (−0.031 to −0.013)
  Deflection length (mm)−0.42 (−0.65 to 0.08)−0.55 (−0.74 to −0.03)
  DA (mm)−0.041 (−0.08 to 0.01)−0.025 (−0.054 to 0.022)
  Time (ms)0.27 (0.19 to 0.33)0.24 (0.14 to 0.32)
  Velocity (m/s)−0.020 (−0.028 to 0.0)−0.013 (−0.023 to 0.0)
  DA ratio max 1mm0.09 (0.06 to 0.13)0.11 (0.07 to 0.14)
  DA ratio max 2mm0.83 (0.67 to 1.05)1.0 (0.77 to 1.22)
  Arc length max (mm)0.059 (0.039 to 0.075)0.053 (0.036 to 0.064)
  DA max (mm)0.065 (0.02 to 0.09)0.058 (0.044 to 0.091)
  Deflection amplitude max (mm)0.072 (0.05 to 0.11)0.055 (0.040 to 0.078)
  Deflection amplitude max time (ms)0.020 (−0.16 to 0.22)−0.033 (−0.19 to 0.24)
HC
  Arc length (mm)0.05 (0.025 to 0.065)0.059 (0.033 to 0.088)
  Deflection amplitude (mm)0.077 (0.049 to 0.106)0.067 (0.046 to 0.081)
  Deflection length (mm)0.056 (−0.089 to 0.155)0.073 (−0.109 to 0.323)
  DA (mm)0.065 (0.021 to 0.09)0.058 (0.044 to 0.091)
  Time (ms)0.039 (−0.20 to 0.25)−0.154 (−0.35 to 0.09)
  Integrated radius (mm)1.79 (1.64 to 2.41)2.04 (1.66 to 2.28)
  Kc (constant) [N/m]−7.55 (−11.48 to −5.61)−6.36 (−9.09 to −2.86)
  Kc (mean) [N/m]−14.06 (−18.16 to −8.78)−10.15 (−12.91 to −7.98)
  Max inverse radius (mm−1)0.024 (0.015 to 0.03)0.020 (0.016 to 0.024)
  SP_A1−13.56 (−22.81 to −7.71)−16.857 (−25.7 to −11.44)
  Whole eye movement max (mm)−0.032 (−0.054 to −0.0)0.0 (−0.029 to 0.027)
  Whole eye movement max time (ms)−0.203 (−0.484 to 0.179)−0.180 (−0.883 to 0.64)

Change in Biomechanical Parameters (1 Month Minus Preoperative)a

ParameterLASIKSMILE
A1
  Arc length (mm)0.006 (0.003 to 0.009)0.005 (0.003 to 0.008)
  Deflection amplitude (mm)−0.016 (−0.025 to −0.009)−0.009 (−0.019 to−0.004)
  Deflection length (mm)−0.44 (−0.64 to −0.23)−0.22 (−0.35 to−0.12)
  DA (mm)−0.017 (−0.024 to −0.009)−0.018 (−0.027 to−0.009)
  Time (ms)−0.36 (−0.44 to −0.23)−0.34 (−0.42 to−0.28)
  Velocity (m/s)0.01 (0.0 to 0.019)0.006 (0.0 to 0.011)
A2
  Arc length (mm)0.01 (0.007 to 0.013)0.013 (0.008 to 0.015)
  Deflection amplitude (mm)−0.02 (−0.033 to−0.019)−0.027 (−0.040 to −0.019)
  Deflection length (mm)−0.33 (−0.83 to 0.29)−0.32 (−0.80 to 0.22)
  DA (mm)−0.020 (−0.038 to 0.007)−0.027 (−0.040 to 0.019)
  Time (ms)0.29 (0.15 to 0.40)0.29 (0.12 to 0.40)
  Velocity (m/s)0.0 (−0.025 to 0.01)−0.006 (−0.019 to 0.006)
  DA ratio max 1mm0.10 (0.07 to 0.14)0.12 (0.07 to 0.15)
  DA ratio max 2mm0.96 (0.64 to 1.08)1.05 (0.89 to 1.24)
  Arc length max (mm)0.063 (0.041 to 0.074)0.059 (0.033 to 0.078)
  DA max (mm)0.088 (0.05 to 0.12)0.070 (0.030 to 0.094)
  Deflection amplitude max (mm)0.085 (0.055 to 0.11)0.081 (0.05 to 0.10)
  Deflection amplitude max time (ms)0.058 (−0.41 to 0.31)−0.087 (−0.47 to 0.24)
HC
  Arc length (mm)0.06 (0.053 to 0.074)0.059 (0.036 to 0.084)
  Deflection amplitude (mm)0.067 (0.032 to 0.086)0.058 (0.008 to 0.12)
  Deflection length (mm)0.058 (−0.188 to 0.14)0.068 (−0.162 to 0.157)
  DA (mm)0.088 (0.049 to 0.118)0.067 (0.030 to 0.094)
  Time (ms)0.27 (−0.11 to 0.38)0.17 (−0.23 to 0.44)
  Integrated radius (mm)2.27 (1.56 to 2.76)2.29 (1.62 to 2.50)
  Kc (constant) [N/m]−7.73 (−11.13 to−4.65)−6.99 (−10.07 to−3.64)
  Kc (mean) [N/m]−11.87 (−16.02 to−11.11)−10.47 (−13.74 to−7.16)
  Max inverse radius (mm−1)0.021 (0.018 to 0.027)0.032 (0.019 to 0.040)
  SP_A1−9.641 (−24.77 to −7.45)−11.251 (−25.7 to−8.44)
  Whole eye movement max (mm)0.021 (−0.018 to 0.04)0.0 (−0.015 to 0.018)
  Whole eye movement max time (ms)−0.21 (−0.846 to 0.01)−0.249 (−0.831 to 0.237)
Authors

From the Department of Cornea and Refractive Surgery, Narayana Nethralaya, Bangalore, India (PK, RS, RV); Imaging, Biomechanics and Mathematical Modeling Solutions Lab, Narayana Nethralaya Foundation, Bangalore, India (MF, ASR); and University Eye Clinic Maastricht, Maastricht University Medical Center, Maastricht, the Netherlands (RMMAN).

Supported in part by the Indo-German Science and Technology Center, India.

Dr. Nuijts serves as a consultant for Alcon, Asico, Chiesi, and Theapharma and a speaker for Abbott, Alcon, Bausch & Lomb, Carl Zeiss, Chiesi, HumanOptics, Ophtec, Oculentis, and Gebauer. Dr. Sinha Roy has intellectual property on biomechanical modeling of the eye through Cleveland Clinic Innovations. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (RS, RMMAN, ASR); data collection (RS); analysis and interpretation of data (PK, RV, MF, ASR); writing the manuscript (PK, RV, MF, ASR); critical revision of the manuscript (RS, RMMAN); statistical expertise (ASR)

Correspondence: Abhijit Sinha Roy, PhD, Narayana Nethralaya, #258A Hosur Road, Bommasandra, Bangalore 560099, India. E-mail: asroy27@yahoo.com

Received: November 08, 2018
Accepted: March 19, 2019

10.3928/1081597X-20190319-01

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