【摘要】 目的：探讨准分子激光原位角膜磨镶术(LASIK)术中负压吸引时间对角膜瓣厚度和直径的影响。方法：根据负压吸引时间的不同将60只猪眼随机分为3组：组1(10s)、组2(20s)和组3(30s)，使用法国产Moria M2型角膜板层刀分别吸引角膜10s，20s和30s后切削角膜瓣，利用角膜超声测厚仪测量角膜瓣厚度，应用镀铬游标卡尺测量角膜瓣直径。结果：组1、 2和3角膜瓣厚度分别为146.05±13.46，157.35±18.95和169.25±21.02μm，各组间比较有显著性差异(P=0.001)。各组的平均角膜瓣直径分别为8.63±0.19mm(组1，10s)，8.89±0.24mm(组2，20s)和9.06±0.18mm(组3，30s)。各组间比较有显著性差异(P<0.01)。结论：LASIK术中随着负压吸引时间的延长，角膜瓣厚度和直径均增加。
【关键词】 负压吸引;角膜瓣厚度;LASIK
Effect of microkeratome suction duration on corneal flap thickness and diameter in pigs
XiaoLi Ma, JianGang Xu, HanQiang Liu
Foundation item: Natural Science Foundation of Liaoning Province, China (No.20042081)
Department of Ophthalmology, the First Affiliated Hospital of China Medical University, Shenyang 110001, Liaoning Province, China
Abstract AIM: To determine the effect of suction duration on thickness and diameter of corneal flap created by microkeratome in porcine eyes in laser in situ keratomileusis (LASIK).METHODS: Sixty porcine eyes were randomly assigned to three groups according to different suction durations: group 1 (10 seconds), group 2 (20 seconds), and group 3 (30 seconds). A Moria M2 microkeratome (Moria, France) with a 160μm head was used to create a corneal flap. Corneal flap thickness was measured by automated ultrasonic pachymetry, and the flap diameter was measured by a vernier caliper.RESULTS: The flap thickness of group 1, group 2 and group 3 was 146.05±13.46μm, 157.35±18.95μm and 169.25±21.02μm, respectively. There was a statistically significant difference among three groups (P=0.001). The mean flap diameter in groups 1, 2 and 3 was 8.63±0.19mm, 8.89±0.24mm and 9.06±0.18mm, respectively. A statistically significant difference was found among groups (P<0.01).CONCLUSION: In LASIK in porcine eyes, an increase in suction duration resulted in a thicker and greater flap.
KEYWORDS: suction duration; lamellar corneal flap thickness; LASIK
INTRODUCTION
Since its introduction in 1990[1], laser in situ keratomileusis (LASIK) has become currently the most frequently used surgical treatment to correct refractive errors worldwide[2,3]. LASIK involves a primary procedure of creating a corneal flap by microkeratome. The flap thickness based on the distance between the fixed microkeratome plate and the edge of the metal blade[4]. Some studies found variables that affect LASIK flap thickness include corneal curvature, corneal thickness, the amount of vacuum obtained during the procedure, microkeratome blade translation and oscillation rates, and blade reuse[510]. Our study explored the effects of different suction durations on flap thickness in LASIK.
MATERIALS AND METHODS
Materials Porcine eyes were obtained from a local abattoir. The eyes were kept at 4℃ in moist chambers and used for experiments within 12 hours of enucleation. The status of the epithelium in each eye was checked, and eyes with epithelial defects were excluded to avoid any bias in corneal thickness and incision angle measurements. Porcine eyes were randomly assigned to three groups, which were exposed to different suction durations: group 1 (10 seconds, 20 eyes), group 2 (20 seconds, 20 eyes), and group 3 (30 seconds, 20 eyes).
Methods
Preoperative examinations The intraocular pressure(IOP) of porcine eyes was controlled by the injection of normal saline through the optic nerve and maintained within 20mmHg to 25mmHg. IOP was measured with CT60 noncontact tonometer (TOPCON, Japan). The IOP of each porcine eye was measured three times and then averaged.
Measurements of central corneal thickness (CCT) and keratometry were performed using Micropach 200 automated ultrasonic pachymeter (Sonomed, Japan) and RK3 automatic keratometer (Canon, Japan). The CCT and keratometry of each porcine eye were measured three times and then averaged.
Microkeratome and surgical techniques A Moria M2 microkeratome (Moria, France) with a 160μm head was used to create a corneal flap. The Size 1 suction ring was chosen according to the manufacturer’s instructions. The porcine eyes were placed on a stand with sufficient support for the surgical procedure. The same surgeon performed all procedures. A drop of normal saline was used to moisten and lubricate the cornea to facilitate the flap resection. The incision using the microkeratome was made from the midpoint of the temporal and inferior side to the nasal side (superior hinge) after the vacuum pressure of the Moria 2 control unit reached 108mmHg. Using different suction durations, the corneal flaps were made with microkeratome. The suction ring was removed and the corneal flap was retracted, exposing the underlying corneal stroma.
Measurement of Flap Thickness and Diameter After resection, the remaining corneal stromal thickness was measured centrally three times by the automated pachymeter. The difference in the corneal thickness and the remaining stromal thickness was defined as the flap thickness[11,12]. The diameter of the flap were measured three times using vernier caliper (Fushun measurement equipment factory, China).
Statistical Analysis Oneway analysis of variance and Tukey’s multiple comparison tests were used to evaluate the differences after various suction durations. P≤0.05 was considered statistically significant.
RESULTS
Preoperative CCT and Keratometry There were no differences in corneal thickness and keratometric values among the three groups (CCT: F=0.035, P=0.996;keratometry: F=0.032, P=0.969; Table 1).
Flap Thickness Values for the flap thickness varied with the suction duration. The thickness of group 1, group 2 and group 3 was 146.05± 13.46μm (10 seconds), 157.35± 18.95μm (20 seconds), and 169.25±21.02μm (30 seconds) respectively. There was a statistically significant difference among 3 groups (P=0.001, Table 1). The flaps of group 1 (10 seconds) were statistically thinner than those of group 3 (30 seconds, P<0.01). However，there was no statistical significance between neither groups 1 and 2 (P=0.128), nor group 2 and 3 (P=0.103).
Flap Diameter The flap diameter increased along with suction duration (Table 1). The flaps of group 1 (10 seconds) were statistically larger than those of group 2 (20 seconds, P<0.01) and group 3 (30 seconds, P<0.01). Moreover，there was also statistical significance between groups 2 and 3 (P=0.028).
DISCUSSION
Although LASIK is safe, there are some complications, including iatrogenic keratectasia[3,13], which is the most serious late complication following LASIK. Altered biomechanics of the cornea following LASIK predispose it to ectasia despite normal IOP. The strength of cornea following LASIK is determined by the residual stromal bed thickness. If the corneal flap is unexpectedly thicker than intended, inadequate residual stromal thickness may occur, and then keratectasia may occur. Furthermore, variability in flap thickness can also have a direct effect on refractive correction because the depth of keratectomy relates to the amount of intraoperative bioelastic corneal change, which in turn affects the accuracy of the desired curvature change[14,15]. Therefore, the desired thickness of an ideal corneal flap should be consistent, predictable and accurate.
LASIK microkeratomes are designed to cut a predetermined flap thickness based on the distance between the fixed microkeratome plate and the edge of the metal blade[4]. Other variables that affect LASIK flap thickness include corneal curvature, corneal thickness, the amount of vacuum obtained during the procedure, microkeratome blade translation and oscillation rates, and blade reuse[510]. Because the eyeball is bioelastic, continuous suction delivered by a suction ring can change the shape of the cornea, causing it to bulge above the cutting plane, resulting in inconsistent flap thickness. Therefore, we focused only on the effect of suction duration on flap thickness and tried to control the other factors as precisely as possible in our study.
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