Evaluation of The Accuracy of Sub Bowman s Keratomileusis (SBK) Microkeratome in Flap Creation during Lasik Surgery

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1 The Egyptian Journal of Hospital Medicine (April 2018) Vol. 71 (3), Page Evaluation of The Accuracy of Sub Bowman s Keratomileusis (SBK) Microkeratome in Flap Creation during Lasik Surgery Hager K Elsaiid*, Tamer M El Mekkawi *, Mouamen M Seleet* and Rania S Elkitkat ** The department of Ophthalmology **, Faculty of Medicine, Ain Shams University Corresponding author: Hager Khaled Mohamed, mobile: , dr_gogo_ @hotmail.com ABSTRACT Purpose: To evaluate the accuracy of SBK microkeratome in flap creation during myopic LASIK surgery using anterior segment optical coherence tomography (AS-OCT). Design: A Prospective non-randomized noncomparative interventional clinical study. Methods: Flaps were created using the One Use-Plus SBK microkeratome (intended flap 90 µm) in 40 eyes of 20 patients. Flap thickness was measured using AS-OCT one month after surgery. Results: The mean achieved central corneal flap thickness in both eyes was 88.67±7.19, 1.33 µm thinner than the intended 90 µm thickness (difference from intended thickness -1.33±7.19) with a p value of which is statistically insignificant (p>0.05). The mean achieved central corneal thickness in the right eye was ± 7.95 μm (ranging from 70 μm to 112µm), 1.44 µm thinner than the intended 90 µm (difference from intended thickness ±7.95) thickness with a p value of which is statistically insignificant ( p>0.05). The mean achieved central corneal flap thickness in the left eye was ± 6.41 μm (ranging from 77µm to 105 µm) showing a difference from intended thickness of -1.22±6.41 µm deviation from the intended thickness that is statistically insignificant (p value of 0.185, p>0.05). Conclusion: Using the one use plus SBK microkeratome is a safe, accurate and effective modality for creating a thinner SBK flap, hence increasing the available residual stromal bed and preserving corneal tissue. It has excellent predictability and reproducibility when it comes to flap thickness with minimal intra operative complication. There was several limitations to our study. The tool of measurement (AS-OCT), is that the boundaries between the flap and stroma become ambiguous with time, increasing the examiner s subjectivity. Keywords: LASIK, SBK Microkeratome, Femtosecond laser, Ectasia, Myopia. INTRODUCTION Over the past decades, laser in situ keratomileusis (LASIK) has become the most common refractive procedure for the correction of refractive errors [1].The consistency and predictability of the corneal flap thickness is crucial in producing successful LASIK outcomes. The corneal flap thickness is directly related to LASIK safety; therefore, methods that improve the precision and minimize the degree of variation in corneal flap thickness are worthy of attention [2]. Today, a variety of microkeratomes, as well as femtosecond lasers, are in clinical use for flap creation [3].Good microkeratomes are associated with accurate cuts, less flap thickness variation, easy manipulation, and fewer complications [4]. One of the most feared complications is post LASIK corneal ectasia. Among many other factors, the residual corneal thickness after the ablation is crucial for decreasing the possibility of ectasia occurrence. Also it has been shown that the thinner the flap, the less risk for ectasia. Accuracy of the flap thickness produced using mechanical microkeratomes is mandatory in this calculation. Percentage of tissue altered (PTA) greater than 40% at the time of LASIK is significantly associated with the development of ectasia in eyes with normal preoperative topography [5]. Earlier studies using microkeratomes showed that there was a high deviation from the marked thickness for a given blade. Hence the development of SBK microkeratome with 90 µm thickness has shown high reproducible accuracy in different recent studies. This has made this type of mechanical microkeratome a competitor for femtolaser flap creation that has gained popularity in recent years, with the main advantage of creating accurate thin sub-bowamn flaps [6-8] Received:12 /3 /2018 DOI: / Accepted: 22/ 3/2018

2 Evaluation of The Accuracy of Sub Bowman s Keratomileusis Figure 1: showing a suction ring adherent to the globe. The figure also shows a pneumotonometer that measures IOP intraoperative [8]. PATIENTS AND METHODS Study design: A consecutive non comparative cross- sectional interventional clinical study was performed to evaluate the accuracy of the SBK microkeratome in flap creation during LASIK surgery. The LASIK procedure and AS-OCT examination was performed in El Hayah Hospital in Mohandsen, Cairo in the time interval between July 2017 and January The study was approved by the Ethics Board of Ain Shams University. Sample size: The study included 40 eyes of 20 patients, who underwent LASIK for myopic correction, using the One Use-Plus SBK microkeratome. Inclusion criteria: Patients included were: 1. Age > 20 years old. 2. Myopic and compound myopic astigmatic patients (-0.50 up to -9 Diopters, and/or astigmatism up to -4 D). 3. Central corneal thickness > 500 µm. 4. A stable refraction for at least 12 months before surgery, according to patients glasses prescriptions. 5. Contact lens abstinence for at least one week for soft lenses. Exclusion criteria: We excluded patients with any of the following: 1- Abnormal corneal topography. 2- Any ocular pathologies, mainly dry eye, corneal scars, corneal dystrophies, cataract, and retinal pathologies. 3- Single- eyed patients. 4- History of ocular trauma or previous surgery. 5- History of systemic diseases such as autoimmune diseases and diabetes mellitus. 6- Intraoperative complication, including free cap or incomplete cut. Preoperative preparation: A written informed consent was signed by all patients. Preoperative examination included: 1- A full review of the participant s general medical and ophthalmic histories. 2- Uncorrected visual acuity (UCVA) measurement using Snellen s chart. 3- Best corrected visual acuity (BCVA) using Snellen s chart. 4- Manifest and cycloplegic refraction. 5- Slit lamp examination of the anterior segment. 6- IOP measurement using Goldmann s applanation tonometry. 7- Fundus examination by indirect ophthalmoscopy and fundus biomicroscopy using the 90D lens. 8- Corneal topography, pachymetry and keratometry were assessed using the Sirius imaging System. 2637

3 Hager Elsaiid et al. Operative procedure: All eyes were targeted for emmetropia. The patient was draped and a speculum was used to open the eyelids. Corneal marking was done with a sterile marking pen at the temporal corneal quadrants. The Moria SBK (Moria Inc, Antony, France) microkeratome blade was placed with the suction ring applied to the eye to hold it in place and the motor was activated. This single-use, mechanical, automated, linear microkeratome places fixed forces on the whole cornea, producing a nasal hinged flap. The suction ring, nasal pedicle width and corneal flap diameter were set according to the manufacturer s nomogram, based on the preoperative corneal curvature. The same blade was used for both eyes starting with the right eye. The thickness of the corneal flap was set to 90 µm, using speed 1 of the motor. The flap was lifted, the corneal stroma was ablated by the Excimer laser (Wavelight Ex500, Wavelight Company, Germany) with active eye tracker. The flap was replaced using a special irrigating cannula with balanced saline solution. The flap alignment was checked by preoperative corneal marks alignment. The edges were then dried with a dry microsponge. Every patient was examined with the slit lamp after 15 minutes to ensure proper flap apposition and absence of wrinkles or folds. Contact lenses were used after flap reposition. Patients were instructed not to rub their eyes and to avoid any direct trauma. Moxifloxacin 2638 antibiotic eye drops 4 times/day for 5 days, prednisolone acetate 1% corticosteroid eye drops 4 times/day for 7days and preservative free artificial tears 5 times daily for 3 months were prescribed for patients as post-operative treatment. Postoperative follow up: For evaluating the performance of the microkeratome, flap thickness was measured after resolving of edema i.e. at one month after surgery using Topcon 3D OCT-2000 series, which also assessed flap architecture. Statistical analysis: Data was coded and entered using the statistical package SPSS (Statistical Package for the Social Science; SPSS Inc., Chicago, IL, USA) version 22. Descriptive statistics (mean, standard deviation, minimum, maximum, and range) were calculated for quantitative data and using frequency (count) and relative frequency (percentage) for categorical data. Comparison between central thickness and the intended thickness was done using one sample t test. Comparison between the 2 eyes was done using unpaired t test in normally distributed variables. P- values less than 0.05 were considered as statistically significant. RESULTS Forty eyes of twenty patients were enrolled in the study, 8 males and 12 females. Patients mean age was 29.3±6.52 SD (range of 22 to 40 years). The preoperative patients data are summarized in table 1.

4 Evaluation of The Accuracy of Sub Bowman s Keratomileusis Table 1 demonstrating descriptive baseline optical preoperative data of both eyes before LASIK surgery with Moria SBK microkeratome (Mean, SD, Minimum and Maximum) Descriptive preoperative data Mean SD Minimum Maximum Sphere (in diopters) Cylinder (in diopters) MRSE (in diopters) K1 (flat) (in diopters) K2 (steep) (in diopters) Mean K(in diopters) Pachymetry (CCT) in (µm) BCVA (Snellen) Corneal (White to white) Diameter CD in (mm) Flap accuracy and reproducibility: Postoperatively, the mean achieved central corneal flap thickness (at vertex) in both eyes was µm ±7.19, (ranging from 80 to 110 µm), with no statistically significant difference between the achieved central flap thickness and the intended thickness (difference of µm ±7.19). The post-operative measurements of the achieved flap thickness and its difference from the intended values are summarized in Tables 2. Furthermore, the difference in mean flap thickness between right and left eyes was not statistically significant (p value =0.879). (Table 3) Tables 2 showing descriptive data of achieved and intended central corneal flap thickness in both eyes and the correlation between them (P value) Mean Standard Minmum Maximum P value Deviation Flap thickness at vertex OD (µm) Flap thickness at vertex OS (µm) Flap thickness at vertex OU Difference (achieved-intended) at OD Difference (achieved-intended) at OS Table 3 showing descriptive data of flap reproducibility (difference between mean achieved central flap thicknesses in both eyes). Mean Standard Deviation Minimum Maximum P value difference between mean central flap in both eyes There was no statistically significant correlation between the mean achieved central flap thickness and each of the mean preoperative corneal thickness (CCT), mean preoperative K readings or the preoperative MRSE. Table 4 summarizes these correlations and states the actual P values. Table 4: Correlation between mean achieved central flap thickness and mean preoperative K readings, mean preoperative refractive spherical equivalent and mean preoperative central corneal thickness of patients who underwent LASIK (with Moria SBK Microkeratome). (P value) Preoperative Data P value (correlation with achieved central flap thickness) Mean K Mean Refractive Spherical Equivalent (MRSE) Central corneal thickness (CCT)

5 Hager Elsaiid et al. The flaps created with the SBK microkeratome were thinner in the central zone, with gradual increase in thickness towards the periphery. At 1 mm from the flap center, the nasal portions were thicker than the temporal portions, with a difference of 0.79 µm, while the inferior portions were thicker than the superior portions, with a difference of 0.47 µm. At 2 mm from the flap center, the temporal portions were thicker than the nasal portions with a difference of 1.32 µm; inferior portions are thicker than superior portions with a difference of 1.1 µm. At 3 mm from flap center, the temporal portions are thicker than the nasal portions 0.26 µm; the inferior portions are thicker than the superior portions by 2.36 µm. Details of variations of mean flap thickness at various points from the flap vertex are described in tables 5 and 6. Table 5: showing mean ±SD of mean flap thickness measurements at each of the 12 locations measured in both eyes (Variation in flap thickness from the center to the periphery) Region from vertex 1 mm from center 2 mm from center 3 mm from center Mean±SD µm Mean±SD µm Mean±SD µm Nasal 95.18± ± ±8.70 Temporal 94.39± ± ±9.73 Superior 95.04± ± ±7.15 Inferior 95.51± ± ±9.12 Table 6: showing mean deviation from mean achieved central flap thickness in both eyes, at each of the 12 measured points from the flap center. Region from vertex (flap center) 1 mm from center Mean deviation from central flap thickness in µm ± SD 2 mm from center Mean deviation from central flap thickness in µm ± SD 3 mm from center Mean deviation from central flap thickness in µm ± SD Nasal 5.18± ± ±8.70 Temporal 5.39± ± ±7.15 Superior 5.04± ± ±7.15 Inferior 5.51± ± ±9.12 DISCUSSION LASIK has become the technique of choice to correct moderate and some high levels of myopia, having a number of advantages over other refractive surgical procedures (e.g. photorefractive keratectomy (PRK), which include almost pain free healing, faster visual recovery, and no subepithelial haze formation [9]. SBK, as a modified LASIK surgery, can create a thinner corneal flap and thicker RSB. Its safety and efficacy are similar to those of LASIK [10]. A thin CCT below 500 μm is considered an important risk factor for post-lasik corneal ectasia. This could be reduced by using very thin flaps ( 100 μm) as in SBK, which can leave the patient with more residual stromal bed for the same refractive correction [11, 12]. Most flap research is mainly focused on the central portion of the flap and stromal bed [13]. In our study, AS-OCT (Spectral-domain OCT, Topcon 3D OCT-2000 series) was used to measure the flap thickness, being a non-invasive, noncontact, easy and fast tool for measurement, that 2640 provides a high resolution image quality. Since post-operative flap edema usually resolves by 1 week after surgery [14], it was used in our study to evaluate the accuracy and reproducibility using the measurements at this point of time. In the current study, it was found that using the One Use-Plus SBK microkeratome for flap creation was safe as evidenced by the absence of detected complications in the recruited cohorts. The mean flap thickness was 88.56± 7.95 µm in the right eyes and 88.78±6.41 µm in the left eyes. The deviation from intended thickness (90 um) in both eyes was statistically insignificant, indicating high accuracy of the achieved central flap thickness. Similarly, Zhai et al. [15] found that for an intended flap thickness of 110 µm, the flaps were more uniform, regular, and accurate with the One Use-Plus microkeratome, with a mean thickness of 114 ± 8 µm (range 98 to 130 µm). Furthermore, Chen et al. [16] analyzed the accuracy and consistency of the flaps created using the One Use-Plus microkeratome in 82 eyes (41 patients) and the M2 90-µm microkeratome (Moria,

6 Evaluation of The Accuracy of Sub Bowman s Keratomileusis Antony, France) in 54 eyes (27 patients), using the Visante AS-OCT (Visante, Carl Zeiss Meditec, Jena, Germany), at the center, 2 mm and 3.5 mm from the center both nasally and temporally. They found that the central flap thickness of cornea was ± 14.8 in the M2 90 µm microkeratome group, while it was dramatically thinner in the One Use-Plus SBK group ± In the One Use Plus SBK microkeratome group, the flap thickness at 2 mm nasally and temporally was µm ±11.6 and µm ±9.4 respectively, at 3.5 mm nasally and temporally 103.6±13 and 118.1±18.6 respectively, while in the M2 group, the mean flap thickness at 2 mm nasally and temporally was 155.6±14.8 and 157.9±12.8 respectively, and at 3.5 mm nasally and temporally 149.1±14.2 and 161.1±12.1 respectively. Chen et al. 2010, [16] concluded that the Moria One Use-Plus is a safe and effective mechanical microkeratome, which has better predictability and accuracy than the Moria M2. A study conducted by Zhai et al. [15] compared the flaps created by femtosecond laser, the One Use- Plus SBK microkeratome and the M2 90 microkeratome. They found that the IntraLase flaps and the SBK flaps have more uniform and regular thickness than the M2 90 flaps. They stated that the Moria One Use-Plus SBK can make more uniform thickness flaps compared to Moria M2 Single use 90 µm head, probably because the working mechanisms are different [15]. The created flap in the current study was found to be thinnest at the center with gradual increase in thickness towards the periphery, with a mean deviation from intended thickness of ± 9.57 µm in the nasal quadrant, ± 8.78 µm in the temporal quadrant, 8.25 µm in the superior quadrant and ±10.26 µm in the inferior quadrants. The mean achieved central flap thickness in the current study was not significantly affected by the mean preoperative central corneal thickness, K readings or mean refractive spherical equivalent. In contradiction Chen et al. [16] found that the flap thickness was affected by the corneal thickness and curvature. Multiple linear regression showed that for the One Use-Plus, the steeper the preoperative keratometry, the thicker the flap thickness, and for the M2, the thicker the preoperative pachymetry, the thicker the flap. They concluded that the flap created by the One Use-Plus was much thinner than the flap created with the M2; however, the One Use-Plus SBK cannot realize a fully planar-shaped flap [16]. On the other hand, the majority of studies on standard LASIK [17-18] found that the flap thickness did not correlate with the degree of myopia; however, in some LASIK studies, as the degree of myopia increased, the flaps tended to become thinner [19, 20]. Furthermore, Talal et al. s study [21], detected a statistically significant strong positive correlation between flap thickness and preoperative pachymetry, signifying that flaps tend to become thinner in corneas with low preoperative pachymetry and vice versa. Previous studies of the Moria M2 microkeratome and the One Use-Plus SBK microkeratome for standard LASIK and SBK reported similar findings [22,23,17,24]. However, the previous studies did not explain these findings, this may be different from our results due to our small sample size, less variations in K readings between the selected patients in our study and finally, the AS-OCT is a subjective method of measurement. In the current study, we used the Topcon 3D OCT-2000 series AS-OCT, which allows a field of image of 6 mm, so we could measure the center and at 1,2,3 mm nasal, temporal, superior and inferior to the flap center. The Visante AS OCT( Carl Zeiss, Meditec) used in other studies, gives wider field (from limbus to limbus), so they could obtain more measurement points with higher accuracy in measurements at the flap periphery [25]. Another limitation of the tool of measurement (AS-OCT), is that the boundaries between the flap and stroma become ambiguous with time, increasing the examiner s subjectivity [14]. CONCLUSION Using the One Use Plus SBK microkeratome is a safe, accurate and effective modality for creating a thinner SBK flap, hence increasing the available residual stromal bed and preserving corneal tissue. It has excellent predictability and reproducibility when it comes to flap thickness with minimal intra operative complication. There was several limitations to our study. The tool of measurement (AS-OCT), is that the boundaries between the flap and stroma become ambiguous with time, increasing the examiner s subjectivity. Another limitation is the small sample size leading to less variation. Also the short postoperative period at which we measured the flap thickness. 2641

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