Dr.PAVITRA PATEL,Dr.Rohit Shetty,Dr.Pooja Khamar
Abstract
Purpose: To study the biomechanical implications of flap and cap cut during LASIK and SMILE procedures, respectively, on contralateral in vivo human cornea
Methods: 9 subjects, with bilaterally matched refraction, pachymetry and intraocular pressure, underwent contralateral LASIK and SMILE. We evaluated using peak deformation amplitude (DA) and corneal stiffness (kc) before surgery, after cut, after refractive correction and at 1-month follow-up.
Results: kc of LASIK eyes was107.6±5.4, 99.8±5, 88.5±3.9 and 88.4±4.2N/m, respectively. kc of SMILE eyes was 97.9±3.2, 92.3±2.5, 84.7±3.6 and 84.3±3.6N/m, respectively. Difference in kc between before and after cut for LASIK and SMILE were -7.8±2.5 and -5.6±2.2N/m (p=0.2). Difference in DA were 0.06±0.02 and 0.05±0.02mm, respectively.
Conclusion: SMILE cap(without lenticule removal) causes less biomechanical weakness compared to only flap. These are novel discoveries, which attempts to understand how much weakness is contributed by individual components of modern refractive surgery. The implications of this study helps in deciding the type of refractive surgery & its impact on post op outcomes.
Introduction
Laser in situ Keratomileusis (LASIK) has delivered safe and efficacious outcomes for correction of refractive error.1 Despite superior screening methods and biomechanical analyses, post-LASIK ectasia remains an unwanted complication.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 cuts of smaller span (not a near 360 degree 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., NY, USA) and dynamic Scheimpflug analyzer (Corvis-ST, Oculus Optikgerate Gmbh Wetzlar, Germany) reported mostly similar biomechanical changes after SMILE and LASIK.7-19 Therefore, theoretical models and patient measurements weren’t 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 since post-operative 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 SMILE and LASIK. Corvis-ST measurements were preformed preoperatively, 1 week (cut and tissue removal) and 1 month (cut and tissue removal) after the procedures. We added an additional measurement, which was performed intra-operatively before the completion of LASIK and SMILE procedure. In this additional step, measurement was made after flap cut in LASIK eyes and cap cut (along with the side incision) in SMILE eyes. To our knowledge, this would be the first report of true biomechanical changes induced by just the flap or the cap alone in patient corneas undergoing myopic refractive surgery. Further comparisons were performed with the follow-up measurements and earlier studies.
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 intra-operative measurements with the Corvis-ST. The study followed the tenets of the Declaration of Helsinki.Twenty-four 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 diopter [D] equivalent refraction with astigmatism not more than −3 D) for a period of 1 year (change less than 0.25 D) and a calculated residual stromal bed thickness of 250 μm. Patients with less than 480-μm central corneal thickness (CCT) or history of keratoconus, diabetes, collagen vascular disease, pregnancy, breastfeeding, and any prior ocular surgery or trauma were excluded from the study. If the patient was contact lens user, then 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. The flap wasn’t lifted and the patient waited in the surgical area for 3 hours since the area was moderated for both temperature and humidity. Then, the Corvis-ST measurement was repeated. 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.
At follow-up, Corvis-ST measurement was again repeated 1 week and 1 month after LASIK. In SMILE eyes, only the side-cut incision and 3-D geometry of the lenticule was cut but not separated from the surrounding stroma. Then, the patient was made to wait for 3 hours in the surgical area. After the wait period in the surgical area, Corvis-ST measurement was repeated. Then, the SMILE procedure was completed by separating the lenticule from the stroma and extracting it through the side-cut. Again, Corvis-ST measurement was repeated at 1 week and 1 month. In LASIK eyes, Corvis-ST measurements weren’t repeated after flap lifting due to possible challenges in centering the patient cornea for the LASIK surgery and risk of infections/inflammations. Intra-operative use of Corvis-ST also had the added risk of flap dislocation, if it was performed after completion of LASIK. Similar risks of infection/inflammation were also possible in SMILE eyes. Therefore, no measurements were performed either after flap lifting or after lenticule separation from the surrounding tissue or immediately after completion of surgery.
A single experienced surgeon performed all the 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, USA) cut the flap and ablated the tissue in one eye. The flap had a 9.0-mm diameter, 110-μm thickness, a side cut angle of 70°, canal width of 1.5 mm, and hinge position at 90°. Optical zone diameter was 6.0 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 6.0 and 7.7 mm, respectively. After creation of the refractive lenticule,
it was dissected and extracted manually through a superior 3-mm side cut. Cornea was remoistened with a wet Merocel sponge 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. Routine postoperative regimen was followed for both eyes. This included moxifloxacin hydrochloride 0.5% eye drops (Vigamox; Alcon Laboratories, Inc.) 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, USA) four times a day for 3 months.
A total of 30 Corvis-ST variables were analyzed. The variables were either machine derived or determined from waveform analyses of the entire deformation amplitude signal. The analyzed primary variables were as follows:
arc length of the cornea (Arc length), time (Time), velocity (Velocity), deformation amplitude (DA), deflection amplitude, horizontal length (Deflection length) of cornea between the two peripheral corneal bends of 1st applanation (A1), 2nd applanation (A2) and highest concavity (HC)maximum deformation amplitude (DA Max), deflection amplitude (Deflection amplitude Max) and its time (Deflection amplitude Max Time), and arc length (Arc length Max)
ratio of DA between center and periphery (1 mm and 2 mm) designated as DA Ratio Max 1mm and DA Ratio Max 2mm, respectively.
Integrated radius and maximum inverse of concave radius of curvature (Max Inverse radius)
Stiffness parameter at A1 (SP-A1)Corneal stiffnesses [Kc (constant) and Kc (mean)] derived from waveform analyses of deformation amplitude signal with a biomechanical model19
Maximum whole eye movementand its time
Two other parameters, ARTh (Ambrosio relational thickness) and Corvis biomechanical index (CBI) were also assessed. Since the surgery caused a reduction in thickness and these two indices were a function of corneal thickness, they were biased towards it. Therefore, ARTh and CBI were analyzed as a secondary set of parameters.
Statistical analysis
All continuous variables were assessed for normality of distribution. Since some variables were non-parametric in distribution, Friedman test for repeated measures was used. Median with 95% confidence interval was calculated for each variable. Repeated measure was used since each variable was measured at different time points. The “N-1” chi-squared test was used to compare proportions. MedCalc v18.7 (MedCalc Inc, Ostend, Belgium) was used for statistical analyses. The software adjusted the p-value for multiple comparisons. These multiple comparisons were preoperative (1), flap/cap cut (2), 1 week (3) and 1 month (4). A p-value less than 0.05 was considered statistically significant.
Results
Table 1 lists the Corvis-ST parameters of the LASIK eyes. The last column describes the results of the statistical comparisons. Some of the parameters indicated reduction in corneal strength, e.g., lower stiffness, shorter lengths, earlier A1 and later A2 times, greater deformation and deflection amplitudes, lower inverse radius and greater integrated radius (Table 1). Among the 31 variables, 4 (13.3% of the total numberof variables) were similar between (1) and (2) but differed (p<0.01) from (3) and (4) [(1),(2) vs. (3),(4) in Table 1]. Eleven variables (36.7%) were such that both (1) and (2) differed significantly (p<0.01) from each other as well as from (3) and (4) [(1) vs. rest, (2) vs. rest in Table 1]. Nine variables (30.0%) were similar among all time points (p>0.05) while five (13.3%) variables were similar among (2), (3) and (4) but differed significantly (p<0.01) from (1) [(1) vs. rest in Table 1].
Table 2 lists the Corvis-ST parameters of the SMILE eyes. The last column describes the results of the statistical comparisons. Similar to LASIK eyes, some of the variables indicated decrease in corneal strength after SMILE. However, the proportion of variables was different. For (1),(2) vs. (3),(4), 12 (40.0%) met the significance criteria. For (1) vs. rest, (2) vs. rest, only 4 (13.3%) met the criteria. Five variables (16.7%) weren’t significant among all time points while 7 (20.0%) met the criteria of (1) vs. rest.For (1),(2) vs. (3),(4), the proportion of variables was significantly different (p=0.02) between the LASIK and SMILE eyes. For (1) vs. rest, (2) vs. rest criteria, the proportion of variables also was significantly different (p=0.02) between the LASIK and SMILE eyes (p=0.03). Overall, corneal stiffness parameters decreased after creation of flap/cap and decrease further after completion of LASIK/SMILE procedure. These decreases in magnitudes of stiffnesses were similar between the two procedures (p>0.2). In LASIK eyes, median ARTh was 442.8, 368.6, 201.4 and 193.1 at (1), (2), (3) and (4), respectively [p<0.01; (1),(2) vs. (3)(4)]. The CBI was 0.016, 0.22, 0.99 and 0.98, respectively [p<0.0;(1) vs. rest, (2) vs. rest]. In SMILE eyes, ARTh was 388.7, 409.1,190.9 and 193.9, respectively [p<0.01; (1),(2) vs. (3)(4)]. The corresponding CBI was 0.029, 0.027, 0.99 and 0.99, respectively [p<0.01; (1) vs. rest, (2) vs. rest]. At later follow-ups [(3) and (4)], no significant differences were observed between LASIK and SMILE eyes with respect to change in the parameters (p>0.05).
Table 1: Biomechanical parameters of LASIK patients. The 4th column indicates significant differences between the time-points [(1),(2),(3),(4)]. For example, (1)(2) vs. (3),(4) indicates that (1) and (2) were similar but differed significantly from both (3) and (4). A1, A2 and HC are 1st applanation, 2nd applanation and highest concavity, respectively. DA is deformation amplitude. SP-A1 is the stiffness parameters at A1 time. NS implies not significant.
| Preop (1) | Flap cut (2) | 1 week (3) | 1 month (4) | p-value (<0.01) | ||
| A1 | Arc length (mm) | -0.017 (-0.02,0.013) | -0.015 (-0.022,-0.014) | -0.013 (-0.014,-0.01) | -0.011 (-0.013,-0.007) | All |
| Deflection amplitude (mm) | 0.095 (0.086,0.10) | 0.093 (0.086,0.107) | 0.083 (0.070,0.088) | 0.079 (0.067,0.084) | (1),(2) vs. (3),(4) | |
| Deflection length (mm) | 2.25 (2.14,2.38) | 2.23 (2.11,2.37) | 2.02 (1.75,2.09) | 1.90 (1.54,2.06) | (1),(2) vs. (3),(4) | |
| DA (mm) | 0.135 (0.12,0.14) | 0.13 (0.12,0.15) | 0.12 (0.10,0.13) | 0.11 (0.10,0.12) | (1),(2) vs. (3),(4) | |
| Time (ms) | 7.40 (7.23,7.61) | 7.30 (7.20,7.5) | 7.06 (6.95,7.25) | 7.12 (7.02,7.24) | (1) vs. rest, (2) vs. rest | |
| Velocity (m/s) | 0.15 (0.138,0.158) | 0.15 (0.143,0.162) | 0.16 (0.153,0.166) | 0.157 (0.147,0.164) | NS | |
| A2 | Arc length (mm) | -0.025 (-0.029,0.022) | -0.024 (-0.029,-0.017) | -0.015 (-0.019,-0.013) | -0.014 (-0.017,0.011) | (1),(2) vs. (3),(4) |
| Deflection amplitude (mm) | 0.11 (0.10,0.12) | 0.12 (0.11,0.13) | 0.092 (0.08,0.1) | 0.088 (0.081,0.095) | (1),(2) vs. (3),(4) | |
| Deflection length (mm) | 3.0 (2.66,3.16) | 2.93 (2.71,3.12) | 2.31 (2.02,3.49) | 2.21 (1.67,3.36) | NS | |
| DA (mm) | 0.36 (0.35,0.40) | 0.35 (0.33,0.42) | 0.36 (0.32,0.38) | 0.36 (0.32,0.38) | NS | |
| Time (ms) | 21.50 (21.34,21.67) | 21.54 (21.24,21.70) | 21.74 (21.60,21.87) | 21.77 (21.54,21.86) | (1) vs. rest, (2) vs. rest | |
| Velocity (m/s) | -0.29 (-0.30,-0.27) | -0.30 (-0.32,-0.28) | -0.30 (-0.32,-0.29) | -0.29 (-0.31,-0.28) | NS | |
| DA Ratio Max 1mm | 1.60 (1.58,1.62) | 1.58 (1.55,1.61) | 1.69 (1.65,1.71) | 1.70 (1.67,1.75) | (1) vs. rest, (2) vs. rest | |
| DA Ratio Max 2mm | 4.34 (4.15,4.50) | 4.45 (4.17,4.82) | 5.24 (4.82,5.53) | 5.19 (4.88,5.39) | (1) vs. rest, (2) vs. rest | |
| Arc length Max (mm) | -0.19 -0.20,0.17) | -0.17 (-0.19,-0.16) | -0.13 (-0.15,-0.10) | -0.12 (-0.15,-0.095) | (1) vs. rest, (2) vs. rest | |
| DA Max (mm) | 1.14 (1.04,1.16) | 1.17 (1.07,1.25) | 1.16 (1.12,1.22) | 1.19 (1.12,1.28) | (1) vs. rest | |
| Deflection amplitude Max (mm) | 0.98 (0.94,1.03) | 1.02 (0.96,1.08) | 1.06 (1.01,1.10) | 1.05 (1.02,1.15) | (1) vs. rest, (2) vs. (4) | |
| Deflection amplitude Max Time (ms) | 16.11 (16.06,16.27) | 15.91 (15.70,16.08) | 16.14 (15.94,16.31) | 16.17 (15.76,16.50) | NS | |
| HC | Arc length (mm) | -0.153 (-0.163,-0.147) | -0.141 (-0.15,-0.12) | -0.105 (-0.130,-0.089) | -0.092 (-0.108,-0.069) | (1) vs. rest, (2) vs. rest |
| Deflection amplitude (mm) | 0.98 (0.93,1.02) | 1.01 (0.95,1.05) | 1.04 (1.0,1.1) | 1.04 (1.0,1.11) | (1) vs. rest, (2) vs. rest | |
| Deflection length (mm) | 6.76 (6.42,6.83) | 6.72 (6.54,6.89) | 6.64 (6.53,6.89) | 6.71 (6.55,6.74 | NS | |
| DA (mm) | 1.14 (1.04,1.16) | 1.17 (1.07,1.25) | 1.16 (1.12,1.22) | 1.19 (1.12,1.28) | (1) vs. rest | |
| Time (ms) | 15.86 (15.21,16.49) | 15.55 (15.02,16.63) | 15.29 (15.09,16.63) | 15.42 (15.12,16.78) | NS | |
| Integrated Radius (mm) | 7.69 (7.28,7.94) | 8.18 (7.40,8.58) | 9.76 (9.08,10.53) | 9.88 (9.10,10.60) | (1) vs. rest, (2) vs. rest | |
| Kc (constant) [N/m] | 105.6 (99.8,108.9) | 101.68 (95.75, 106.59) | 95.34 (92.40,100.83) | 98.94 (88.97,102.08) | (1) vs. rest, (2) vs. rest | |
| Kc (mean) [N/m] | 96.1 (91.3,103.1) | 90.09 (86.45,97.26) | 81.78 (77.03,89.91) | 85.74 (77.07,90.92) | (1) vs. rest, (2) vs. rest | |
| Max Inverse radius (mm-1) | 0.167 (0.158,0.176) | 0.186 (0.165,0.207) | 0.195 (0.183,0.204) | 0.194 (0.186,0.205) | (1) vs. rest, (2) vs. rest | |
| SP_A1 | 102.9 (91.5,112.4) | 94.69 (87.18,103.31) | 95.06 (72.93,103.74) | 98.93 (78.92,103.63) | (1) vs. rest | |
| Whole Eye Movement Max (mm) | 0.258 (0.243,0.293) | 0.265 (0.236,0.319) | 0.245 (0.206,0.277) | 0.277 (0.227,0.295) | NS | |
| Whole Eye Movement Max Time (ms) | 21.78 (21.53,22.47) | 21.61 (21.31,22.49) | 21.46 (21.17,21.84) | 21.62 (21.02,22.02) | NS |
Table 2: Biomechanical parameters of SMILE patients. The 4th column indicates significant differences between the time-points [(1),(2),(3),(4)]. For example, (1)(2) vs. (3),(4) indicates that (1) and (2) were similar but differed significantly from both (3) and (4). A1, A2 and HC are 1st applanation, 2nd applanation and highest concavity, respectively. DA is deformation amplitude. SP-A1 is the stiffness parameters at A1 time. NS implies not significant.
| Preop (1) | Cap cut (2) | 1 week (3) | 1 month (4) | p-value (p<0.01) | ||
| A1 | Arc length (mm) | -0.015 (-0.019,0.012) | -0.017 (-0.019,-0.014) | -0.012 (-0.014,-0.007) | -0.010 (-0.012,-0.008) | (1),(2) vs. (3),(4) |
| Deflection amplitude (mm) | 0.090 (0.084,0.099) | 0.099 (0.089,0.101) | 0.076 (0.073,0.086) | 0.077 (0.071,0.084) | (1),(2) vs. (3),(4) | |
| Deflection length (mm) | 2.17 (2.07,2.29) | 2.20 (2.09,2.36) | 1.91 (1.65,2.04) | 1.92 (1.79,2.01) | (1),(2) vs. (3),(4) | |
| DA (mm) | 0.13 (0.12,0.14) | 0.13 (0.125,0.139) | 0.11 (0.10,0.13) | 0.11 (0.10,0.12) | (1),(2) vs. (3),(4) | |
| Time (ms) | 7.41 (7.25,7.51) | 7.25 (7.09,7.40) | 7.04 (6.96,7.17) | 7.08 (6.90,7.17) | (1) vs. rest, (2) vs. rest | |
| Velocity (m/s) | 0.151 (0.145,0.156) | 0.16 (0.155,0.165) | 0.161 (0.155,0.164) | 0.159 (0.151,0.166) | (1) vs. rest | |
| A2 | Arc length (mm) | -0.025 (-0.027,0.020) | -0.022 (-0.025,-0.014) | -0.014 (-0.018,-0.008) | -0.013 (-0.017,-0.006) | (1),(2) vs. (3),(4) |
| Deflection amplitude (mm) | 0.116 (0.109,0.119) | 0.116 (0.106,0.129) | 0.093 (0.086,0.108) | 0.089 (0.075,0.100) | (1),(2) vs. (3),(4) | |
| Deflection length (mm) | 3.48 (3.12,3.75) | 3.80 (2.98,3.96) | 2.97 (2.15,3.60) | 3.08 (2.65,3.62) | (1),(2) vs. (3),(4) | |
| DA (mm) | 0.36 (0.35,0.39) | 0.40 (0.37,0.45) | 0.35 (0.30,0.39) | 0.37 (0.30,0.38) | NS | |
| Time (ms) | 21.55 (21.41,21.67) | 21.72 (21.59,21.81) | 21.78 (21.63,21.85) | 21.80 (21.67,21.96) | (1) vs. rest, (2) vs. (4) | |
| Velocity (m/s) | -0.292 (-0.312,0.288) | -0.311 (-0.317,-0.302) | -0.305 (-0.326,-0.294) | -0.300 (-0.311,-0.283) | (1) vs. (2),(3) | |
| DA Ratio Max 1mm | 1.60 (1.56,1.62) | 1.60 (1.56,1.63) | 1.70 (1.66,1.74) | 1.70 (1.69,1.75) | (1),(2) vs. (3),(4) | |
| DA Ratio Max 2mm | 4.29 (4.15,4.57) | 4.44 (4.30,4.76) | 5.13 (4.98,5.68) | 5.44 (5.10,5.80) | (1) vs. rest, (2) vs. rest | |
| Arc length Max (mm) | -0.18 (-0.19,-0.16) | -0.18 (-0.19,-0.16) | -0.11 (-0.15,-0.10) | -0.11 (-0.15,-0.10) | (1),(2) vs. (3),(4) | |
| DA Max (mm) | 1.12 (1.09,1.18) | 1.18 (1.12,1.23) | 1.19 (1.12,1.23) | 1.22 (1.11,1.27) | (1) vs. rest | |
| Deflection amplitude Max (mm) | 0.99 (0.96,1.06) | 1.02 (0.99,1.06) | 1.04 (1.00,1.14) | 1.07 (1.01,1.16) | (1),(2) vs. (3),(4) | |
| Deflection amplitude Max Time (ms) | 15.96 (15.67,16.18) | 16.03 (15.75,16.15) | 16.0 (15.90,16.14) | 15.8 (15.4,16.14) | NS | |
| HC | Arc length (mm) | -0.148 (-0.159,-0.137) | -0.141 (-0.155,-0.107) | -0.092 (-0.100,-0.080 | -0.089 (-0.099,-0.065) | (1),(2) vs. (3),(4) |
| Deflection amplitude (mm) | 0.98 (0.94,1.04) | 1.0 (0.98,1.05) | 1.03 (0.99,1.13) | 1.06 (0.99,1.12) | (1) vs. rest, (2) vs. (3) | |
| Deflection length (mm) | 6.66 (6.48,6.72) | 6.80 (6.67,6.94) | 6.65 (6.52,6.91) | 6.64 (6.37,6.80) | NS | |
| DA (mm) | 1.12 (1.09,1.18) | 1.18 (1.12, 1.23) | 1.19 (1.12,1.23) | 1.22 (1.11,1.27) | (1) vs. rest | |
| Time (ms) | 15.59 (15.35,16.76) | 16.0 (15.32,16.65) | 15.67 (15.25,16.43) | 15.94 (15.48,16.51) | NS | |
| Integrated Radius (mm) | 7.99 (7.07,8.37) | 8.46 (7.82,9.15) | 9.61 (9.17,10.50) | 10.05 (9.45,10.76) | (1) vs. rest, (2) vs. rest | |
| Kc (constant) [N/m] | 102.4 (97.7,106.9) | 96.9 (94.7,101.9) | 96.3 (90.8,100.8) | 93.3 (89.3,98.2) | (1) vs. rest, (2) vs. (4) | |
| Kc (mean) [N/m] | 94.0 (85.9,97.6) | 86.6 (82.8,89.4) | 82.2 (76.2,88.3) | 80.9 (75.8,86.6) | (1) vs. rest, (2) vs. rest | |
| Max Inverse radius (mm-1) | 0.174 (0.162,0.184) | 0.185 (0.168,0.195) | 0.195 (0.182,0.203) | 0.202 (0.193,0.214) | (1),(2) vs. (3),(4) | |
| SP_A1 | 104.5 (101.5,111.1) | 96.9 (83.7,103.1) | 85.5 (76.2,100.1) | 86.1 (74.1,98.1) | (1) vs. rest, (2) vs. (4) | |
| Whole Eye Movement Max (mm) | 0.264 (0.24,0.29) | 0.308 (0.269,0.332) | 0.262 (0.220,0.300) | 0.29 (0.222,0.307) | (2) vs. rest | |
| Whole Eye Movement Max Time (ms) | 21.85 (21.31,22.56) | 21.83 (21.33,22.60) | 21.66 (21.28,22.04) | 21.48 (21.30,22.00) | NS |
Discussion
A recent study tested the difference in corneal elastic modulus in human corneal samples ex vivo (2-D 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 also implied reduced Brillouin modulus after flap creation in the anterior (1/3rd region) stroma of porcine eyes.22 Thus, severing of the fibers either by 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 intra-operatively. 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 Corvis-ST. In Tables 1 and 2, two statistical inferences were key. Firstly, (1),(2) vs. (3),(4) indicated significant difference between the 1st two and the last two time points but (1) and (2) were similar. Secondly, (1) vs. rest, (2) vs. rest indicated that significant difference existed between preoperative and flap/cap. Using the above definitions, the salient findings of the study were as follows:
As expected, some of the deformation parameters indicated biomechanical weakening after flap and cap creation, e.g., decrease in stiffness, earlier 1st applanation.
Temporal assessment of these parameters also showed progressive weakening of the cornea after tissue removal (ablation and lenticule extraction), e.g., Kc (constant) and Kc (mean).
In LASIK, 36.7% of the parameters belonged to (1) vs. rest, (2) vs. restindicating significant biomechanical changes after flap creation. This changed to 13.3% in SMILE eyes (p=0.03) indicating that the LASIK flap caused a greater biomechanical change in the cornea than SMILE cap.
The above observation was also supported by the % number of parameters in the (1),(2) vs. (3),(4) significance groups.
Interestingly, the above differences between flap and cap cuts did not influence the outcomes at 1 week and 1 month. This indicated that the biomechanical effect of tissue removal was primary determinant of deformation parameters in the long term.
Thus, acute biomechanical benefits of cap over flap were lost after tissue removal. The benefits may not be the only determinant of long term corneal biomechanical changes.
The link between the intraoperative and the follow-up measurements were analyzed in this study for the first time. It was possible that some acute edema in the cornea intraoperatively may had 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 wasn’t observed clinically. ARTh is representative of corneal thickness23 and since no tissue was removed in (2), significant changes from (1) to (2) were probably artifactual. The CBI included ARTh and its results were also affected.23Interestingly, the increase in CBI from preoperative to cap cut was lower in SMILE eyes than LASIK eyes but complete absence of some intraoperative edema couldn’t be ruled out.Increase in CBI was equivalent to biomechanical degradation, which was representative of keratoconus.23Among the ORA studies, 6 reported no difference between LASIK and SMILE eyes while 4 reported a better biomechanical outcome after SMILE than LASIK.7-16Among the Corvis-ST studies, 3 reported that some biomechanical parameters reported better outcome after SMILE than LASIK.7,10,19The other studies (2 in number) reported no biomechanical differences between LASIK and SMILE eyes.17,18Thus, the overall summary from all the studies indicated similar biomechanical outcomes after LASIK and SMILE in the long term. These findings were similar to the 1 week (3) and 1 month (4) outcomes (Tables 2 and 3).
Other than biomechanical outcomes, SMILE and LASIK have differences in temporal wound healing and visual recovery.24,25 It would be interesting to correlate the acute biomechanical differences between SMILE cap and LASIK flap eyes to differential wound healing between them. 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.23 In summary, 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 removal reduced or eliminated the biomechanical differences induced in the acute phase by the flap or cap cuts. Thus, safety criteria established for recommending LASIK to patients should be followed for recommending SMILE to patients as well. This requires further evaluation in future studies.
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