Commentary: Evolution and practice patterns of thin flap sub-Bowman laser in situ keratomileusis in refractive surgery

The primary aim of any refractive surgical procedure is to reduce dependence on spectacles and contact lenses for performing daily activities and provide cosmesis to the patient. Since its inception, refractive surgical techniques have rapidly evolved, and various surgical procedures are available based on patient indication and needs.[1] The term keratomileusis has been derived from the Greek words “cornea” (kerato) and “to carve” (mileusis). In laser in situ keratomileusis (LASIK), the principle of keratomileusis is combined with excimer laser stromal bed ablation to obtain a perfect postoperative outcome. In today’s era, LASIK is the most commonly performed keratorefractive procedure worldwide due to proven benefits such as safety, efficacy, and early visual recovery with minimal patient discomfort.[2] The conventional step in LASIK is flap creation either by microkeratome or a femtosecond laser. The thickness of the flap has evolved from 180 to 90 um in current clinical practice. Sub-Bowman Keratomileusis (SBK), also known as thin flap LASIK, is a refractive surgical procedure where a thinner flap of 90–110 um is created using a microkeratome.[3] Durrie and Houston introduced the concept of SBK in 2008. The primary benefit is that it provides an excellent stromal bed for excimer laser ablation in patients with moderate to high myopia. This, in turn, safeguards the cornea from the risk of corneal ectasia and vision loss in future.[4] Few previous studies reported that the ideal flap thickness should be 130 um or more to prevent the risk of irregular astigmatism, epithelial ingrowth, folds, and flap striae. SBK has changed the practice patterns with two advantages. It provides a good Residual stromal bed (RSB) for excimer laser ablation, and microkeratome SBK obviates the need for a femtosecond laser for flap creation and preserves the biomechanical stability of the cornea. With SBK, LASIK can be performed in thin corneas as RSB thickness is thicker as compared to that of what is achieved by conventional LASIK.[5] As per previous studies, the incidence of the thin flap after LASIK has been reported to be 0.3–0.75%. Prandi et al.[6] assessed the impact of flap thickness on 421 eyes of myopic LASIK patients. The flap was created using Moria microkeratome, and the laser used was Chiron Technolas 217C. The patients were divided into three groups (group 1, flap thickness 100 um and <130 um; and group 3, flap thickness >or = 130 pm). The uncorrected (UCVA) and best-corrected visual acuity (BCVA), residual spherical refraction, keratometry, complication, retreatment, and correlation between flap thickness and preoperative corneal thickness were assessed as day 1, 30, and 180 days postoperatively. They concluded that patients with thinner flaps had better functional results than those with thicker flaps. Eleftheriadis et al.[7] retrospectively studied the effect of flap thickness on visual and refractive outcomes in 196 myopic LASIK eyes. The UCVA, BCVA, and manifest refraction were assessed for 6 months postoperatively. They concluded that thinner flaps were associated with the rapid visual recovery and there was less myopic spherical equivalent, but UCVA and final BCVA were not correlated with flap thickness. The postoperative complications were unrelated, and the flap thickness did not affect the efficacy, safety, and predictability or the rate of enhancement procedure. Cobo-Soriano et al.[8] analyzed contrast sensitivity and visual and refractive outcomes in 280 consecutive eyes of myopic patients who underwent LASIK with thin flaps compared to those with thicker flaps. The authors assessed the efficacy, predictability, and contrast sensitivity between three groups of flap thickness thin (<100 um, n = 105), medium (100–129 um, n = 122), and thick (>130 um, n = 53). The authors found that refractive outcomes were comparable between the three groups, but visual outcomes were better in thinner flap groups. There are approximately eight refractive surgical procedures in a surgeon’s armamentarium. The list includes photorefractive keratectomy (PRK), LASIK, small incision lenticule extraction (SMILE), implantable collamer lens, clear lens exchange, cataract surgery, and corneal collagen cross-linking, and SBK is one among them.[9] In patients with marginal corneal thickness or persons involved in the army and sports, PRK and SMILE are better options compared to SBK. So, in a nutshell, every surgeon has to weigh the risk versus benefit ratio for patients who are suitable candidates for refractive surgery. SBK is one such potential option which is being widely practiced these days. The authors’ current study[10] attempts to understand the long-term visual quality after femtosecond LASIK versus SBK. The study is a potential addition to the literature as long-term comparative outcomes of the two techniques are lacking in the literature. The results are interesting as there was no significant difference in the BCVA, total higher-order aberrations, autumn tea, and total clover aberrations between the two procedures except the total coma aberrations, which were higher in the Femto LASIK group. These results will help refractive surgeons decide on the type of procedure. Another important finding from the study is that the total higher-order aberration, spherical aberrations, total coma aberrations, and clover aberrations are increased after surgery compared to the preoperative value by both techniques. Although the sample size is small in this study, the results are an eye-opener for the clinicians as SBK is a potential option for patients who cannot afford femto LASIK and at places where Femto is not available. Still, large-scale studies or randomized controlled trials are needed to give better insights regarding the preference of thin flap over thicker flap and SBK over femto LASIK.