Refractive surgeries are one of the most commonly performed elective ophthalmic procedures worldwide. The journey of refractive surgery has been a continual evolution from the now archaic corneal incisional surgeries to excimer laser-based corneal ablative procedures, to the present-day minimally invasive femtosecond laser-based techniques such as small incision lenticule extraction (SMILE). Till date, more than 40 million laser-assisted in situ keratomileusis (LASIK) and 2 million SMILE procedures have been performed worldwide.1
Advancements in corneal diagnostics and laser technology have paved way for the development of customized corneal ablation profiles with a revival of interest in surface ablative procedures. Despite the excellent outcomes observed with the current procedures in treating myopic errors, their results in presbyopia and hyperopia are not as predictable. Ongoing research in this ever-expanding field may help us to improve our understanding of these conditions and optimize their management in the foreseeable future. We herein trace the evolution of refractive surgeries in ophthalmology, the current trends and the future frontiers (Figs. 1 and 2).
The concept of reshaping the cornea to correct refractive errors was first proposed by Hjalmar Schiotz in 1885, who employed limbal relaxing incisions to correct postcataract surgery astigmatism. Subsequently, Leendert Jan Hans studied the utility of corneal incisions for treating astigmatism and proposed the concept of corneal flattening occurring in the meridian perpendicular to the incision. The introduction of radial keratotomy (RK) by Sato in 1939 heralded the era of incisional corneal refractive surgeries. He performed anterior and posterior radial corneal incisions to flatten the central cornea and correct myopic refractive errors; however, the posterior corneal incisions were associated with endothelial cell damage and bullous keratopathy in up to 70% of patients.2 Fyodorov, a corneal surgeon from Russia modified the technique to perform only anterior corneal keratotomy incisions and employed various multifactorial nomograms to improve predictability of outcomes.3 Meanwhile, hexagonal keratotomy was introduced by Dr Antonio Méndez for treating hyperopic refractive errors, wherein, corneal incisions were placed circumferentially in a hexagonal configuration to cause steepening of the central cornea.4
Fig. 1: A timeline describing the evolution of refractive surgery. (FS: femtosecond; LASIK: laser-assisted in situ keratomileusis; SMILE: small incision lenticule extraction; FDA: Food and Drug Administration; PRK: photorefractive keratectomy; ICL: implantable collamer lens)
Fig. 2: The past, the present, and the future of refractive surgeries. (LASIK: laser-assisted in situ keratomileusis; PRK: photorefractive keratectomy; SMILE: small incision lenticule extraction)
RK enjoyed widespread popularity through the 1970s and 80s; however, it was eventually discontinued due to unpredictable and fluctuating visual outcomes and the introduction of more accurate excimer laser technology for corneal ablation.5,6
The concept of keratomileusis or “corneal carving” was pioneered by José Ignacio Barraquer in 1964. Epikeratoplasty was described in 1980s based on Barraquer’s technique wherein a lathed donor stromal lenticule was sutured onto the cornea.3 The 1970s and 80s saw the emergence of corneal reshaping techniques including radial thermal keratoplasty and conductive keratoplasty, which were less invasive and technically simpler. Fyodorov introduced radial thermal keratoplasty to treat hyperopia and astigmatism. The shrinkage of collagen fibrils gave rise to a midperipheral purse string effect with corresponding steepening of central cornea. Conductive keratoplasty was developed in 1990s for low-moderate hyperopia and presbyopia, wherein central corneal steepening was achieved with the controlled delivery of high-frequency and low-energy electric current to the mid-peripheral stroma. The corneal reshaping techniques were discontinued due to associated complications, lack of long-term stability, and significant regression.7–10
The introduction of argon fluoride (ArF) lasers in 1983 for corneal ablation marked a paradigm shift in the field of corneal refractive surgery. It enabled removal of precise amounts of stromal tissue within a fraction of microns while causing negligible damage to the adjacent tissue.11 Photorefractive keratectomy (PRK) was the first procedure to employ excimer laser-mediated corneal ablation for reshaping the cornea in order to correct myopic refractive errors, and received FDA approval in 1995.12 Over the years, innovations in excimer laser technology such as the use of higher frequency lasers, introduction of flying spot lasers, advanced eye-tracking systems, and the introduction of Gaussian beam profile have enhanced the safety and efficacy of visual outcomes. Postoperative pain and corneal haze associated with epithelial removal in PRK led to the development of flap-based corneal ablative procedures. In 1990, Pallikaris described LASIK wherein he created a corneal flap with a guarded microkeratome and ablated the underlying stromal bed with excimer laser; the procedure received FDA approval in 1999.13
Advancements in corneal diagnostics, including corneal imaging and aberrometry, facilitated the development of customized laser vision correction. Customized corneal ablation aimed to treat the pre-existing ocular aberrations or minimize their induction during the procedure. Wavefront optimized LASIK minimizes the induction of new spherical aberrations during LASIK and continues to remain the most commonly used ablation profile.14 Wavefront-guided ablation aims to objectively correct the total ocular aberrations measured preoperatively by an aberrometer or wavefront sensor.15 Corneal topography-guided ablation was essentially introduced as a modality to treat irregular corneas including keratoconus, postkeratoplasty astigmatism, and healed keratitis.16
The advent of femtosecond laser (FS) technology in early 2000s revolutionized the field of refractive surgery.17 The use of a highly focused photodisruptive laser, employing ultra-short pulses, heralded the era of high precision flap-based and flapless procedures. Femtosecond laser-assisted flaps were associated with better precision, reproducibility, faster visual recovery, and lesser incidence of postoperative dry eyes as compared with microkeratome flaps.18
Femtosecond lenticule extraction (FLEx), introduced in 2006, was the first procedure to utilize a single FS laser platform (VisuMax, Carl Zeiss Meditec AG, Jena, Germany) for creating the corneal flap and an intrastromal lenticule, thus eliminating the need for excimer laser-mediated stromal ablation.19 The technique was further modified to extract the intrastromal lenticule via a small side cut incision instead of a flap, known as SMILE.20
The history of lens-based procedures for treating refractive errors dates back to the late 18th century when Abbé Desmonceaux of France proposed the removal of crystalline lens to treat high myopic errors. Removal of crystalline lens with intraocular lens (IOL) implantation for treating refractive errors or refractive lens exchange gained popularity following advancements in the field of cataract surgery.21
Phakic IOLs for the correction of myopia were first introduced in 1953 by Benedetto Strampelli. Initial phakic IOLs were meant to be implanted in the anterior chamber and fell out of favor due to associated endothelial decompensation and glaucoma. The 1980s saw a revival of phakic IOL surgery with advancements in IOL design and material. Iris fixated lens and posterior chamber phakic IOLs were developed with a favorable safety profile.22 The Artisan (Ophtec) iris claw lens received FDA approval in 2004; subsequently, its modification, Artiflex (Ophtec), made of flexible silicone with a larger optic size was introduced. Implantable Collamer lens (ICL), a posterior chamber phakic IOL, was first introduced in 1993. The enhanced biocompatibility and superior optics of ICL afforded a favorable safety profile while providing excellent visual outcomes.5 The earlier models of ICL were associated with a significant incidence of lenticular opacities caused by the intermittent contact between the ICL and crystalline lens. This was reduced in the subsequent models due to improvements in the ICL design and material, furthering the widespread acceptance of these lenses.23
Cornea-based refractive surgeries are the current standard of care, with LASIK and SMILE being the most widely performed corneal refractive surgeries. Femtosecond laser for creation of corneal flaps prior to ablation has further enhanced the safety and predictability of LASIK; however, it is associated with its unique set of complications related to suction loss and cavitation bubbles breakthrough. The advent of newer femtosecond lasers employing a higher frequency, lower energy, and tighter spot and line separation has led to a decrease in the laser-related complications. The prospect of a flapless corneal refractive surgery, which was at par with LASIK in terms of precision, efficacy, and safety profile, became a reality with the introduction of SMILE. Though associated with a considerably steeper learning curve, the procedure has distinct advantages over LASIK including a better corneal biomechanical profile, less dry eyes, lesser induced higher-order aberrations, and absence of flap-related complications. SMILE has recently received FDA approval for the treatment of astigmatism up to 3D in addition to myopia, further enhancing its scope for refractive correction.24,25 The technique has also shown promising results for correction of hyperopia in various clinical trials.26
A resurgence in surface ablation techniques has been witnessed in the recent times owing to the advent of customized corneal ablative treatments, transepithelial ablations, and its utility in performing retreatments following SMILE or LASIK. Topography-guided ablation was first introduced for treatment of irregular corneas; however, it has recently shown promising results in virgin eyes, with studies implying its significant potential for superior visual outcomes, both in terms of visual acuity and quality. Topography-guided ablation for treating myopia with or without astigmatism received US FDA approval in 2016.27
Among the lens-based refractive procedures, posterior chamber phakic IOL implantation remains the most commonly performed surgery today. The newer fourth-generation ICL (V4c or EVO Visian ICL) with a central 360 microns hole at the center of its optic (KS-AquaPORT) was introduced in 2011 with the aim of preventing secondary cataract and eliminating the need for peripheral iridotomy. Subsequently, the V5 model (EVO+ Visian ICL) with a large diameter optic was introduced in 2016 to alleviate the night vision symptoms in patients with larger mesopic pupil. The long-term efficacy and safety of ICL implantation has been demonstrated in patients with high-refractive errors unsuitable for corneal procedures as well as low and moderate myopia, making it a feasible alternative to corneal refractive surgeries.28,29
The unparalleled safety and efficacy of FS-LASIK and SMILE has led to their soaring popularity over the years; however, certain issues such as iatrogenic ectasia and postoperative dry eye disease remain unresolved. Recent advances in corneal imaging have allowed us to look beyond placido disk technology for preoperative screening of patients at risk to develop postoperative ectasia. While newer technologies such as Scheimpflug technology, corneal biomechanical assessment, and high-resolution optical coherence tomography (OCT) imaging are more sensitive at detecting subclinical keratoconus, the ideal tool that can identify the patients predisposed to postoperative ectasia with optimal sensitivity and specificity continues to elude us. The use of techniques such as machine learning, which rely on artificial intelligence to improve the diagnostic accuracy of subclinical keratoconus in refractive surgery patients, may help us to eliminate iatrogenic ectasia following laser vision correction.30
Postoperative dry eye remains another common side effect observed after LASIK and SMILE, though the incidence is lower with latter. Agents that promote corneal re-innervation may help to mitigate this postoperative adverse effect in the future.31
SMILE is a less invasive alternative to LASIK for laser vision correction while being comparable in terms of efficacy and safety. The need for retreatment following SMILE is considerable and has been reported to be around 2–4.4% in various studies. Retreatment options following SMILE include surface ablation, thin flap LASIK, and converting the cap to flap using the CIRCLE software. Re-SMILE is being investigated as an enhancement option owing to its benefit in preserving the biomechanical strength of the eyes while ensuring patient comfort.32
Corneal laser treatment for correction of hyperopia remains a challenge. The outcomes of LASIK for hyperopic refractive error correction remain confounded with issues such as increased regression due to epithelial remodeling and visual disturbances resulting from the hyperprolate corneal shape, especially in patients with higher magnitudes of error. Hyperopic SMILE is a relatively new procedure that entails the creation of a negative meniscus lenticule, which is then extracted. The lenticular profile for hyperopic SMILE is still evolving and its efficacy and safety are still being investigated in clinical studies.26 Stromal tissue additive procedures for correction of hyperopia may be more suitable in patients with higher refractive errors or those unsuitable for corneal ablative procedures.6 These investigational procedures entail the intrastromal implantation of a SMILE lenticule beneath a flap (lenticule intrastromal keratoplasty) or in a stromal pocket (small incision lenticule intrastromal keratoplasty) for hyperopic correction.33
Presbyopia is another frontier yet to be conquered by refractive surgeons. Presbyopia correction is presently the most dynamic domain in laser vision correction owing to increasing demands of spectacle independence among older population. Presbyopic corneal ablation relies on the creation of a multifocal corneal profile or an increased depth of focus; however, a satisfactory approximation of the dynamic physiological accommodation necessary for countering the presbyopic symptoms is not provided. Furthermore, presbyopic corneal ablation performed in phakic patients predisposes them to unpredictable refractive outcomes following cataract surgery, which may be required subsequently.34 Newer generation multifocal intraocular lenses are a viable alternative for patients requiring concomitant cataract surgery. In addition, phakic presbyopic IOLs are a promising addition to the armamentarium of presbyopic surgeries.
Rapid strides have been made in the field of refractive surgery over the past few decades, propelled by the increasing demands for precision and safety by surgeons and patients alike. Femtosecond laser-assisted corneal ablative and lenticule extraction procedures continue to enjoy overwhelming popularity for myopic correction. Posterior chamber phakic IOLs have excellent efficacy and safety and are a feasible alternative to LASIK or SMILE across the spectrum of myopic refractive correction.
Today, the field of refractive surgery offers us endless opportunities to not just rid the patients of their spectacles but also elevate their quality of vision and life. While newer technology and scientific advances continue to emerge in the field of presbyopia correction, a therapeutic intervention, which can successfully simulate the dynamic features of the natural accommodative process, will be the future of refractive surgery. The evolution of refractive surgery will require to keep pace with the growing demands for superlative vision while ensuring long-term stability, optimal patient comfort, and a favorable side effect profile.
- Aristeidou A, Taniguchi EV, Tsatsos M, Muller R, McAlinden C, Pineda R, et al. The evolution of corneal and refractive surgery with the femtosecond laser. Eye Vis (Lond). 2015;2:12.
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- Pallikaris IG, Naoumidi TL, Astyrakakis NI. Long-term results of conductive keratoplasty for low to moderate hyperopia. J Cataract Refract Surg. 2005;31(8):1520–9.
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- Seiler T, Dastjerdi MH. Customized corneal ablation. Curr Opin Ophthalmol. 2002;13(4):256–60.
- Kim A, Chuck RS. Wavefront-guided customized corneal ablation. Current Opinion Ophthalmol. 2008;19(4):314–20.
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- Ratkay-Traub I, Ferincz IE, Juhasz T, Kurtz RM, Krueger RR. First clinical results with the femtosecond neodymium-glass laser in refractive surgery. J Refract Surg. 2003;19(2):94–103.
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- Sekundo W, Kunert KS, Blum M. Small incision corneal refractive surgery using the small incision lenticule extraction (SMILE) procedure for the correction of myopia and myopic astigmatism: results of a 6 month prospective study. Br J Ophthalmol. 2011;95(3):335–9.
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- Dishler JG, Slade S, Seifert S, Schallhorn SC. Small-Incision Lenticule Extraction (SMILE) for the Correction of Myopia with Astigmatism: Outcomes of the United States Food and Drug Administration Premarket Approval Clinical Trial. Ophthalmology. 2020;127(8):1020–34.
- Moshirfar M, McCaughey MV, Reinstein DZ, Shah R, Santiago-Caban L, Fenzl CR. Small-incision lenticule extraction. J Cataract Refract Surg. 2015;41(3):652–65.
- Wang Y, Ma J. Future Developments in SMILE: Higher Degree of Myopia and Hyperopia. Asia Pac J Ophthalmol (Phila). 2019;8(5):412–6.
- Stulting RD, Fant BS, T-CAT Study Group, Bond W, Chotiner B, Durrie D, et al. Results of topography-guided laser in situ keratomileusis custom ablation treatment with a refractive excimer laser. J Cataract Refract Surg. 2016;42(1):11–8.
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- Siedlecki J, Luft N, Priglinger SG, Dirisamer M. Enhancement Options After Myopic Small-Incision Lenticule Extraction (SMILE): A Review. Asia Pac J Ophthalmol (Phila). 2019;8(5):406–11.
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