Application of small-incision lenticule extraction in myopic astigmatic correction

Abstract

Myopia and myopic astigmatism are highly prevalent in our locality and is almost generalized in the young adult population. In recent decades, corneal laser refractive surgery evolved rapidly and has become increasingly accepted worldwide as a safe and effective method for refractive error correction. Laser insitu keratomileusis (LASIK) is the most commonly performed laser refractive surgery mainly due to a rapid visual recovery and, superior safety and efficacy profiles. Small-incision lenticule extraction (SMILE) is a newly developed refractive surgery in which an intrastromal lenticule is created with femtosecond laser and is removed through a small arcuate incision. It has demonstrated promising visual and refractive outcomes and safety profiles comparable to LASIK in correction of myopia. Some concerns regarding the precision of astigmatic correction have been raised because of the surgeon-dependent centration technique and the lack of cyclotorsion control on the current SMILE platform. Currently, SMILE is approved by the U.S. Food and Drug Administration for the correction of myopia only, but not myopic astigmatism. The aim of this thesis is to evaluate the application of SMILE in correcting myopia and myopic astigmatism. Refractive cylinders were evaluated using vector analysis, which is the standard method to determine the effectiveness of astigmatic correction. Firstly, comparative clinical studies between SMILE and two other commonly performed laser refractive surgical techniques, LASIK and photorefractive keratectomy (PRK) are presented. SMILE demonstrated high capability in correcting a wide range of myopic astigmatism. As compared to PRK, SMILE demonstrated better predictability in highly myopic eyes. However, inferior results were observed with SMILE in correcting mild to moderate astigmatism when compared to LASIK. As for the correction of high myopic astigmatism, SMILE and LASIK offered comparable astigmatic correction. More precise correction was found with higher preoperative astigmatism. Contrary to common belief, SMILE did not show significant undercorrection in high astigmatic correction. Secondly, several factors that contribute to increase variance in astigmatic correction are investigated. These include surgeon’s experience along the learning curve, patient’s ocular residual astigmatism and variations in the location of incision and treatment zone. The results illustrated the importance of surgical technique and patient selection in attaining satisfactory treatment outcome in SMILE. On the other hand, location of the small incision and subclinical treatment decentration did not affect astigmatic correction. A small case series of retained lenticule fragments, which is a rare complication of SMILE, is presented. Residual astigmatism resolved after subsequent removal of the lenticule fragments. Lastly, long-term stable astigmatic outcome is achievable after SMILE in an evaluation of clinical outcomes over 5 years. In conclusion, SMILE has a high capacity in myopic astigmatic correction. It shows comparable predictability to conventional laser refractive surgery techniques. Because most of the surgical steps in SMILE rely on the operating surgeons, a significant learning curve exists. Patient selection is also crucial in attaining satisfactory treatment outcomes.