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  5. FP411 : Comparison of Astigmatic Correction Between Small Incision Lenticule Extraction & Femtosecond LASIK

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Dr. Nitin Tiwari, T19511, Dr. Shreyas Ramamurthy, Dr. Gitansha Sachdev, Dr. Chitra Ramamurthy

 

After its introduction in 2001, there has been a revolutionary change the way Femtosecond flaps has been created for corneal refractive surgery.1 In order to enhance functional outcomes various ablation profiles has been designed and proven to be effective in terms safety and predictability. Wavefront-guided LASIK using aberrometry creates a customized ablation pattern to treat total higher order aberrations (HOAs) of the eye.2 Wavefront optimized (WFO) LASIK, on the other hand minimizes the amount of spherical aberration induced during LASIK surgery, so as to create an aspheric ablation profile.3 To have optimal astigmatism correction, it is important to minimize the induced astigmatism after refractive surgery.4

With the help of pupil tracking system and iris registration technique, WFO laser is able to obtain more accurate alignment and correction in refractive surgery. However, due to absence of above such software in SMILE the centration technique is subjective to surgeon.5

Several studies have been conducted to report the refractive outcomes after SMILE.6,7 However to find out the real values of astigmatism, application of the vector method is essential.8,9 To the best of our knowledge, no study to date has compared refractive outcomes using Alpin’s vector method after SMILE and WFO LASIK.

PATIENTS AND METHODS

200 eyes of 100 patients were enrolled in this prospective randomized interventional study at a tertiary eye care hospital in South India from May 2016 to May 2017. Patients were randomized using the Alternate Allocation method. All patients were giv­en a number (1 to 100). The odd-numbered patients un­derwent SMILE in both eyes and the even-numbered patients underwent WFO LASIK in both eyes. The study received approval from the Ethics Committee of our institute and was conducted in accordance with the tenets of the Declaration of Helsinki. A written informed consent was obtained from all patients prior to the surgical procedure. The inclusion criteria was age between 18 to 50 years with a documented refractive stability for a minimum period of one year (a change of 0.25 diopters [D] or less) and discontinuation of contact lenses for at least two weeks. Patients with a corrected distance visual acuity (CDVA) of 20/20 or better, a spherical equivalent refraction of up to -10.00 D with a high cylinder range from -2 D to -5.5 D were included in the study. Exclusion criteria included a central corneal thickness less than 490μm, a residual stromal bed lower than 290μm, topographic evidence of corneal ectasia, previous ocular surgery, history of herpetic eye disease, corneal scarring, collagen vascular disease, pregnancy and lactation.

All patients underwent a thorough evaluation including anterior and posterior segment examination, manifest refraction, assessment of uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA), dilated fundus, corneal topography using the Pen­tacam HR (Oculus GmbH, Wetzlar, Germa­ny) and noncontact tonometry (Topcon-CT80; Topcon, Tokyo, Japan) preoperatively and postoperatively. The manifest refraction data were converted to the corneal plane, and the vertex distance was set at 12 mm.

SURGICAL PROCEDURES

All surgeries were performed by single surgeon under topical anaesthesia and aseptic conditions. Small Incision Lenticule Extraction (SMILE) procedure was performed using the VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) with a 500 k-Hz repetition rate a pulse energy of 170 nJ. The intended cap thickness was 120 µ. The refractive lenticule diameter was set as 6.0 to 6.5 mm with 0.1 mm of transition zone. The patients were positioned under the curved contact glass and were asked to fixate at the target light, following which vacuum suction was activated. The posterior cut was fashioned first from the periphery to centre followed by the anterior cap cut in a spiral out fashion. This was followed by a superior side cut of 90-degree angulation at 12 o’clock. The separated refractive lenticule was extracted from a small incision using a blunt spatula.

LASIK flaps were fashioned using the VisuMax system with an intended thickness of 100 microns and a flap diameter of 8.5mm. The pulse energy was 140 nJ. The track and spot distance were set as 3.0 µ for flap creation and 1.5 µ for making the flap side cut.  Following blunt dissection and flap lift, the stromal bed was ablated with excimer laser (EX 500 Wavelight). The treatments for SMILE were dependent on the manifest refraction, and treatments for WFO LASIK were dependent on the wavefront refraction without nomogram adjustment.

Postoperatively, both groups received treatment with topical steroids (loteprednol etabonate 0.5% ophthalmic suspension) in tapering dose and lubricating drops (carboxymethylcellulose 0.5% ophthalmic drops). Follow-up visits included postoperative day one, six weeks and three months.

STATISTICAL ANALYSIS

Data analysis was done with the help of a computer using SPSS software (version 17.0; SPSS, Inc., Chicago, IL) for Windows. Using this software, range, frequencies, percentages, means, standard deviations, and P values were calculated. The Kolmogorov–Smirnov test was used to analyze the normality of parameters. The independent samples t test was performed for normally distributed data, whereas the nonparametric test was used for non normal distributions. The correlation analyses were performed using the Pearson method based on the normality of parameters. A P value of less than .05 denoted a significant relationship.

RESULTS

The mean age was 25.83 ± 6.68 years in the SMILE group and 26.42 ± 6.77 years in the WFO LASIK group respectively. The preoperative patient demographics are listed in Table 1 demonstrating age and spherical equivalent matched groups with no statistically significant difference in spherical and cylindrical diopters between 2 groups.

VECTOR ANALYSIS

Table 2 represents the vector analysis results comparison between the 2 groups. Using Alpin’s methods, Target induced astigmatism (TIA) was the astigmatic vector the surgery was intended to induce. Surgically induced astigmatism (SIA) was the vector the surgery actually induced. As the intended target refraction was emmetropia, the residual astigmatism was considered as difference vector (DV). The DV is an absolute measure of success and is preferably zero. There was no statistically significant difference in TIA, SIA and DV between both procedures. The correction index CI (the ratio of SIA to TIA) was higher in both SMILE (0.95) and WFO (0.98) suggesting an under correction of 5% and 2%, respectively. Also, it indicated that WFO LASIK was closer to achieving full correction than SMILE, however the difference being statistically not significant (P = 0.12). The magnitude of error (M of E) defined as arithmetic difference between magnitudes of the SIA and TIA was negative after SMILE and WFO LASIK, suggestive of under correction after both procedures. However, this difference was not statistically significant (P = 0.17). The angle of error (A of E) which is described by the vectors of the achieved correction (SIA) versus the intended correction (TIA), is useful in demonstrating the correct axis where the treatment is intended. This is demonstrated in Figure 1. In this study the mean A of E was negative, indicating that the achieved correction was located in the clockwise direction to the intended axis after both SMILE and WFO LASIK. The index of success (IOS) which is a relative measure of success was very closer to zero for both SMILE (0.04) and WFO LASIK (0.01). Significant positive correlation was found between the absolute values A of E and IOS after both procedures. The Pearson correlation coefficient (R) was 0.87 after SMILE and 0.90 after WFO LASIK, suggestive of very strong positive correlation (P > 0.05).  Also, the coefficient of determination (R2) between absolute values of A of E and IOS was 0.76 for SMILE and 0.82 for WFO LASIK (P > 0.05). The flattening index was used to describe the proportion of the achieved correction at the intended axis; the flattening index was lower after SMILE than after wavefront–guided LASIK but without statistical significance (P = 0.16).

The CI, M of E and FI indicated mild under correction after both SMILE and WFO LASIK in patients with high myopic astigmatism. Figure 2 presents the results on the magnitude of achieved correction versus attempted correction for astigmatism, and the under correction was observed after both procedures.

ASTIGMATISM CORRECTION ACCURACY

Residual manifest refraction of ≤ 0.50 D, representing the accuracy of cylinder correction was achieved in 90 % of eyes in the SMILE group and  94 % of the eyes in the WFO group (P = 0.61, Figure 3A and 3B)

UDVA OUTCOMES AND STABILITY

At 3 months postoperative visit, (Figure 4A and 4B) 74 % eyes of the SMILE group had UDVA of 20/20 or better and 88 % had UDVA of 20/25 or better. In WFO group, 79 % eyes had UDVA of 20/20 or better and 85 % had UDVA of 20/25 or better. The difference between the two groups was statistically not significant (P = 0.90).

GAIN AND LOSS OF LINES

The gain – loss data indicated that 12 % of eyes in the WFO group 5 % of the eyes in SMILE group gained 1 line (P < 0.02). This gain of 1 line data was statistically significant between 2 groups. 3 % of eyes in WFO group gained 2 lines whereas in SMILE group there was no gain of 2 lines (Figure 5).

DISCUSSION

Fewer studies have reported astigmatism correction outcomes using the Alpin’s vector method. This method has clinical benefits in the comparison of astigmatic correction between techniques, since it uses several parameters and provides a more realistic picture than simple numerical analysis of the treatment efficacy of a refractive procedure for astigmatism.10,11  Previous studies evaluating astigmatism correction showed superior results with femtosecond laser–assisted LASIK compared to SMILE.12,13 Our vector analysis results showed that the correction indices were less than 1.0, indicating an overall mild undercorrection of astigmatism, in both WFO(2%) and SMILE(5%) groups presenting with higher astigmatism  ≥ 2D. The unmodified nomogram might be one of the potential factors and could result in undercorrection. An adjustment of a specific nomogram for a particular procedure can further improve the astigmatism correction. However, a previous study by Zhang et al.14 reported severe under correction in Wavefront guided LASIK (13%) and SMILE (12%) in subgroup analysis of higher astigmatism. Ivarsen et al.15 demonstrated a significant under­correction of astigmatism and an increased error of treatment with higher attempted cylinder correction. The reported undercorrection was 13% per diopter of attempted cylinder correction in low astigmatic and 16% per diopter in highly astigmatic eyes. This was mainly attributed to noncompensation of errors of cy­clotorsion and nonapplication of nomograms during the procedure. Improved results for higher astigmatism treatment were achieved in our study in both SMILE and WFO as compared to previously published studies even though we didn’t apply any nomogram adjustment. Also, these results are consistent with those of Ivarsen and Hjortdal,16 whose outcomes only applied to the SMILE procedure in the absence of a control study. In a recent study were subgroup analysis was done between low cylinder and high cylinder for SMILE,17 one of the reason attributed for more undercorrection in the high cylinder group was a lower mean age compared to the low cylinder group; hence patients in this group were younger with different corneal biomechanics and epithelial healing patterns, which could have given rise to the undercorrection.18

It has been reported that a flap could induce the surgical astigmatism; therefore, there is minimal surgically induced astigmatism after SMILE because it is performed without a flap.19 Previous studies showed that the IntraLase flaps created with the femtosecond laser have better visual acuity and induce less astigmatism than the flaps created with the mechanical microkeratome in the LASIK procedure (0.22 D vs. 0.32 D), which indicates that the method of flap creation might dominate the surgically induced astigmatism, rather than the step of flap lifting.20,21 In our current study, we found no significant difference in the surgically induced astigmatism between the WFO LASIK and SMILE groups.

In the current study even though the CI was nearly close to 1 after both procedures, still there was mild deviation in the IOS from the ideal value, which should be 0. Possible reasons for a deviation in the IOS could be attributed to the axis rotation and decentration during the surgery.22 In the absence of eyetracking in SMILE technique, the direction of the astigmatism axis could be misaligned due to the shift of the pupil center. The pupil center shifted when the pupil diameter changed asymmetrically with different luminance.23 In addition, the cyclotorsion from the upright to supine position could also induce errors in the treatment of astigmatism.24 Alpin’s noted that if the treatment was misaligned by 30°, the effect of correction was reduced by half while another half was canceled by the torque effect. When it came to 45°, there was no flattening effect at all.4 In our study both SMILE and WFO achieved accurate correction within 20 degrees and 15 degrees respectively. On the other hand, error might be observed particularly in the eyes with a large angle kappa preoperatively, and the correlation between the angle kappa and decentration should be confirmed by further investigation.25

In the current study, Pearson correlation coefficient (R) was 0.87 after SMILE and 0.90 after WFO LASIK, suggestive of very strong positive correlation between absolute values of A of E and IOS after both procedures. This indicated that the accuracy of axis correction could influence astigmatism treatment.

The absolute values A of E value was higher after SMILE than after WFO LASIK because the former does not use a precise eye-tracking technique. The iris registration technique can be used to compensate for cyclotorsional misalignment from the seated position to the supine position. In the SMILE technique, the activated vacuum suction system acts as an adsorptive force to prevent the eye from rotating. Therefore, accurate centration is important for the correction of high astigmatism even if the minimal misalignment or cyclotorsion results in undercorrection and induces other astigmatism. In the current study the mean A of E was negative, indicating that the achieved correction was located in the clockwise direction to the intended axis after both SMILE and WFO LASIK for higher astigmatism, which was in contrast to the study published by Zhang et al.14 where results showed that the mean A of E value was negative after wavefront– guided LASIK, but positive after SMILE. In their study the achieved correction was located in the clockwise direction to the intended axis in wavefront–guided LASIK and counterclockwise to the intended axis in SMILE. In terms of flattening effect achieved, the current study showed better FI (0.95) for SMILE and (0.97) FOR WFO compared to results published by Kobashi et al. which showed insufficient flattening with both femtosecond lenticule extraction and SMILE (median flattening indices of 0.79 to 0.80).26

One of the limitation of this study was use of both right and left eye in each group, as this can sometimes lead to bias of orientation errors due to some degree of a mirror symmetric effect in the axes of astigmatism between the right and left eyes. Also, a contralateral eye-to-eye comparative study should be conducted to limit possible intersubject bias such as different healing properties, environmental, psychological and compliance issues. Nevertheless, to the best of our knowledge, this study is the first to compare the postoperative outcomes of astigmatism between wavefront optimized LASIK and SMILE using the vector method in higher astigmatism ≥ 2 D with such a large sample size. It is necessary to confirm these results with further studies and longer follow up period.

In conclusion, our results demonstrated that WFO and SMILE were essentially effective in correcting higher astigmatism ≥ 2 D in terms of cylindrical refraction, the predictability of astigmatic correction, and astigmatic vector analysis components without vision-threatening complications at the end of 3-month follow-up period. However, mild undercorrection was noted after both surgeries. Also, these results were better than all the previously published results. It is likely that, after gaining more experience with these techniques, a surgeon will be encouraged to correct the cylinders more accurately by using adjusted nomograms.

REFERENCES

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