New Developments in the Lenticule Extraction Procedure

US Ophthalmic Review, 2014;7(1):20–5 DOI: http://doi.org/10.17925/USOR.2014.07.01.20

Abstract:


For the last 20 years controlled excimer laser ablation of corneal tissue, either directly from the corneal stromal surface or from the corneal interior after creation of a superficial corneal flap, has become widely used to correct myopia, hyperopia, and astigmatism. Recently, an intrastromal refractive procedure whereby a tissue lenticule is cut free in the corneal stroma by a femtosecond laser and removed through a small peripheral incision has been introduced. The procedure avoids creation of a corneal flap and the potential associated risks while avoiding the slow visual recovery of surface ablation procedures. The all-femtosecond-based flap-free intracorneal refractive procedure has been documented to be a predictable, efficient, and safe procedure for correction of myopia and astigmatism. Technologic developments related to further improved cutting quality, hyperopic, and individualized treatments are desirable.
Keywords: Corneal refractive surgery, femtosecond laser, small incision lenticule extraction, myopia, astigmatism
Disclosure: Jesper Hjortdal, MD, PhD, has received travel support from Carl Zeiss Meditec, Jena, Germany. Anders Ivarsen, MD, PhD, has no conflicts of interest to declare.
Received: January 07, 2014 Accepted March 01, 2014
Correspondence: Jesper Hjortdal, MD, PhD, Department of Ophthalmology, Aarhus University Hospital, 8000 Aarhus C, Denmark. E: jesper.hjortdal@dadlnet.dk
An erratum to this article can be found below.

Over the last few years, surgical extraction of a refractive lenticule, or ReLEx®, has evolved as a new treatment in the field of keratorefractive surgery. Currently, the VisuMax® femtosecond (FS) laser (Carl Zeiss Meditec, Jena, Germany) is the only platform to offer this treatment. The 500 kHz VisuMax laser generates very fast pulses (10-15 s range) in the near-infrared spectrum. Depending on the specific laser settings, each pulse conveys approximately 150 nJ, which causes localized photodisruption at the focal point. The generated plasma expands, creating a cavitation bubble, and, as individual cavitation bubbles fuse, the stroma is cut with a minimum of collateral damage. The VisuMax FS laser uses a high numerical aperture and a concave contact glass to focus the laser pulses with very high precision. Thus, laser spots of approximately 1 μm diameter are placed with a defined distance of 2–5 μm in a spiral pattern. To ensure centration on the visual axis, the patient fixates on a blinking light, and suction is applied at the limbus to maintain stability of the eye. Initially, the posterior refractive surface of the lenticule is cut, followed by creation of the plano anterior surface, which is slightly enlarged in diameter to facilitate surgical manipulation.

This review article reviews the current state of the technique, updated clinical results,1 experimental studies, and, finally, presents some of the challenges that need to be addressed by new technologies.

Depending on the method used to access the lenticule, ReLEx can be split into FLEx, in which a laser-assisted in situ keratomileusis (LASIK)-like flap allows surgical removal of the lenticule, and small incision lenticule extraction (SMILE) in which a small incision (approximately 2–4 mm in length) is created for manual lenticule extraction. A blunt spatula is used to break any remaining tissue bridges after the laser treatment, and thelenticule is removed with a pair of forceps (see Figure 1). For further details on the surgical approach, please refer to Sekundo et al.,2 Shah et al.,3 and Vestergaard et al.4

In contrast to LASIK, ReLEx is a one-laser approach, where the critical laser treatment is performed on the intact cornea rather than on exposed corneal stroma. Consequently, the potential variability associated with the excimer laser photoablation is avoided. In addition, the minimally invasive SMILE treatment has several theoretical advantages over flap-based treatments, including little trauma to the corneal surface, less corneal denervation, and better biomechanical strength due to an almost intact anterior stroma. Since the first introduction of ReLEx, the repetition rate of the VisuMax laser has been increased from 200 to 500 kHz, and the settings for laser spot size, energy, and distance have been optimized, changes that may have had a significant impact on the clinical outcome after surgery. Furthermore, the flap-based FLEx represents an evolutionary step before SMILE and is today primarily used as an introductory step for new ReLEx surgeons. Due to these changes, this review focuses primarily on studies concerning SMILE.

Currently, the VisuMax allows myopic corrections up to –10 diopters (D) spherical equivalent (SE) correction, with an astigmatic component of up to 5 D. Hyperopic treatments are not available at the moment, although one study has reported on the outcome of hyperopic FLEx.5 The VisuMax laser is Conformité Européenne (CE) marked and is currently being evaluated in clinical studies for the approval of SMILE by the US Food and Drug Administration (FDA).

Myopia
Refractive Outcome
Overall, ReLEx has been reported to have high refractive predictability in moderate and high myopia. In the largest report to date on SMILE in 670 myopic eyes 3 months after surgery (preoperative SE refraction was –7.2 D) the mean error in SE refraction was –0.25 ± 0.44 D, with 80 % of eyes within ± 0.50 D and 94 % within ± 1.0 D.6 We recently extended this evaluation to the first 1,574 eyes 3 months after SMILE and found a similar mean error of –0.15 ± 0.50 D with 77 % of eyes within ± 0.50 D and 95 % within ± 1.0 D.7 Other reports on SMILE3,4,8,9 and 500 kHz FLEx10–14 have found similar refractive outcomes in smaller numbers of patients. Most studies have included patients with moderate and high myopia, while the refractive predictability in treatment of low myopia (less than 2 D) has not been evaluated thoroughly or compared with results after FS-LASIK or photorefractive keratectomy (PRK).

The refractive stability after SMILE has not been extensively investigated. However, in one study on 279 eyes with high myopia, refraction was found to be stable from 1 to 3 months after surgery, although a minor regression of –0.15 D was observed during the first month.4 Another study on 54 eyes found no regression during the first 6 months after surgery.8 Similarly, no regression has been found during the first 3–6 months after 500 kHz FLEx10,11,13,14 or for 1 year after 200 kHz FLEx.15,16 A prospective, randomized, paired-eye study comparing SMILE and FLEx documented no significant regression between 1 week and 6 months. The procedures were similar in terms of safety, efficacy, predictability, and stability, suggesting that the presence or absence of lifting the flap does not significantly affect these visual and refractive outcomes.17

Interestingly, the refractive predictability after SMILE has been found to be unrelated to the degree of the attempted myopic correction.6 This is in contrast to excimer-based treatments, which show decreasing precision with increasing myopic correction.18 Furthermore, other parameters including preoperative corneal power, patient age, and gender have beenfound to have limited impact on the refractive outcome after SMILE.6

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Keywords: Corneal refractive surgery, femtosecond laser, small incision lenticule extraction, myopia, astigmatism