4 All-in-One Femtosecond Refractive Laser Surgery

Marcus Blum and Walter Sekundo

Keywords: Femtosecond refractive lenticule extraction, small-incision lenticule extraction, surgical technique, clinical results, safety, long-term results, indications

4.1 Introduction

For many years now, a number of sophisticated excimer laser systems have been available to perform laser-assisted in situ keratomileusis (LASIK), a widely used refractive operation on the human cornea, with a very high accuracy. ¹ Shortly after the millennium, the femtosecond laser was introduced into refractive surgery. However, this technology was at first used solely to create the corneal flap and thus to take the place of the microkeratome. ^{2,​ 3} The refractive cut itself was still performed with the 193-nm excimer laser. ^{4,​ 5} A substantial part of the complications, for example, dry eyes and disturbance of corneal biomechanics, caused by or after LASIK surgery seems to be linked to the flap creation regardless of the method of flap cutting. The techniques described in this chapter have the potential to work without lifting a flap. After a series of experiments in the lab and in animal models as well as after some initial treatment of blind eyes, corneal refractive correction using exclusively a prototype of the VisuMax femtosecond laser (Carl Zeiss Meditec AG, Jena, Germany) became reality. During the 2006 annual meeting of the American Academy of Ophthalmology (AAO), we presented the first cases that underwent a new refractive procedure, independent of the excimer laser, a procedure since then known as femtosecond lenticule extraction (FLEx). When performing FLEx, both the flap and the refractive lenticule are cut in a “one-step” procedure by the femtosecond laser. ⁶ This technique is described later in the chapter. ^{7,​ 8,​ 9} FLEx turned out to be just a step toward developing a technique made possible by continuous improvements in surgical performance, energy settings, and laser technology. The flap is smaller, but similar to the one for the femtosecond LASIK. Lifting the flap is no longer necessary and this procedure can be performed through a small incision. This new technique is called small-incision lenticule extraction, or SMILE. The upper part of the cornea that does not have to be lifted anymore is called a cap (instead of a flap). The results of this minimally invasive procedure were first published by our group in 2011. ¹⁰ The potential advantages of this refined technique have encouraged a number of international groups to employ the newly developed 500-kHz femtosecond laser for refractive lenticule extraction (ReLEx). To avoid confusion, the femtosecond laser alone procedures have been patented by the manufacturer of the VisMax laser as a ReLEx with two possible techniques: the ReLEx FLEx and ReLEx SMILE. Meanwhile the SMILE became a well-known term, which in our opinion will remain so irrespective of the manufacturer of the laser. The rapid increase in available clinical data has led to an ongoing discussion about the advantages and disadvantages of ReLEx FLEx. We now describe this technique in its different stages of development and its current clinical applications.

4.2 Surgical Techniques of Refractive Lenticule Extraction

4.2.1 Femtosecond Lenticule Extraction

Surgical Technique

The ReLEx treatment is performed under topical anesthesia using three drops of preservative-free oxybuprocaine tetrachloride (e.g., Conjuncain EDO; Bausch & Lomb, Berlin, Germany) applied 2 to 3 minutes prior to surgery. After sterile draping and insertion of the lid speculum, the patient on his treatment bed is positioned under the VisuMax femtosecond laser. The eye is docked to the interface using mild suction. The patient fixates an internal target light for centration; he/she continues to observe the blinking target light even when the suction is being applied (▶ Fig. 4.1a). The surgeon observes the docking process through the operating microscope and controls the entire process using a joystick. The VisuMax femtosecond laser produces ultra-short pulses of light, at a repetition rate of 500 kHz with a typical pulse energy of 100 to 160 nJ that is focused at a precise depth in the corneal tissue. A plasma state develops with optical breakdown and a small gas bubble is formed in the cleavage plane. A series of bubbles are created in a spiral fashion with a typical spot distance of 3 to 5 µm resulting in cleaving of tissue planes. The femtosecond laser cuts the lenticule surfaces in the following order: posterior (centrifugal; ▶ Fig. 4.1b), anterior (centripetal), and sideways. The hinge can be created in any position; usually, it is done in a nasal or superior location (▶ Fig. 4.1c). The hinge is usually somewhat wider than in LASIK since there is no excimer laser ablation. A thin spatula is inserted under the flap near the hinge (▶ Fig. 4.1e); the flap is opened and reflected (▶ Fig. 4.1d). The refractive lenticule is subsequently grasped with a Blum forceps and extracted (▶ Fig. 4.1f). Next, the flap is repositioned and the interface irrigated (▶ Fig. 4.1g). The eye will be treated after surgery with the same steroid and antibiotic eye drops employed after LASIK.

The first eyes operated with this technique did not achieve the refractive accuracy of LASIK, and visual rehabilitation in general took longer. But after a few hundred treatments, similar precision like in LASIK was achieved due to a thorough analysis of the available data. Significant improvements were made due to the optimization of laser parameters as well as by altering the scan directions. Currently, 5-year results of the eyes initially treated with this technique have been published; the stability of the postoperative refraction is convincing ⁹ (▶ Fig. 4.2).

FLEx techniques have been applied not only in eyes with myopic astigmatism but also in refractive surgery to correct hyperopia. While in a first series of patients it could be proven that ReLEx is a feasible and effective procedure for treatment of hyperopia, the stability of the refractive results left much to be desired. The refraction in the group (47 eyes of 26 patients) was not stable, and on average the difference in spherical equivalent (ΔSEQ) was -0.08 D per month from month 3 to 6, and 0.02 D per month from month 6 to month 9 ¹¹. By modifying the lenticule geometry, a significantly better stability over a couple of months postoperatively became possible. ^{11,​ 12}

4.2.2 Small-Incision Lenticule Extraction

Surgical Technique

When we performed the first SMILE operations, we made two opposite incisions of an arc length of 80 degrees, thereby creating a potential tunnel. This was done to prevent inducing irregular astigmatism. After it became evident that the shallow incision depth would not lead to astigmatism, one of these incisions was no longer performed and the size of the incision continually decreased. Today most surgeons do only an arc-shaped incision of 2 or 3 mm, which serves as an access to the “pocket” containing the refractive lenticule to be removed. A thin spatula (e.g., Chansue’s dissector) is inserted through the side cut over the roof of the refractive lenticule, dissecting this plane followed by the bottom of the lenticule. The lenticule is subsequently grasped with a serrated McPherson forceps modified by Blum or a modified vitreoretinal peeling forceps designed by Shah and removed (▶ Fig. 4.3a–g). ¹³ After the removal of the lenticule, the intrastromal space is flushed using a standard LASIK irrigating cannula (▶ Fig. 4.3h). By exerting mild pressure on the initial incision, fluid is drained out (▶ Fig. 4.3i).

This is a rather challenging surgical technique that comes with the risk of perforation, leaving remnants of the lenticule behind or creating a via falsa. The postoperative regimen usually consists of preservative-free ofloxacin (Floxal EDO, Bausch & Lomb), dexamethasone (Dexa EDO, Bausch & Lomb), and hyaluronate lubricating drops (VisLube; Chemedica) four times per day each for 1 week. After this, only lubricating drops are used up to 3 months as needed. In cases where dissection turns out to be difficult, mild steroids might be given for a month.

Results

Utilizing the experience from initial studies, in subsequent case series, 93% of treated eyes were within ±0.5 D of the intended target refraction. ^{13,​ 14,​ 15} The quality of the laser dissection has been continually improved by refining laser parameters such as the energy setting and the distance between the laser spots. ^{16,​ 17,​ 18} Thanks to these improvements, the stability of the achieved correction with refractive lenticule extraction is better than with LASIK. The precision of the laser cuts has been demonstrated by a number of investigators, working independently of each other. ^{19,​ 20} The induction of higher-order aberrations (HOA) is within the range of about 0.1 µm (Malacara’s notation) and thus is equal to or better than excimer laser systems of the latest generation. This is to a large extent due to the low induction of spherical aberrations while coma occasionally might be somewhat higher. ²¹ Because of the insignificant increase in spherical aberrations, there are in general no problems with halo and glare following a ReLEx procedure, even in eyes with a relatively high preoperative refractive error.

Patient satisfaction generally is high after SMILE. Forty-eight patients were asked to fill out a questionnaire after 6 months of follow-up. On the scale of 0 to 100 (0 = very poor; 100 = best ever known vision), the mean quality of vision was graded 92.3. All patients reported full independence from spectacle correction. The questionnaire was asked in relation to each individual eye treated. Twenty-five patients (28.4%) reported a marked improvement of their vision and 60 patients (68.2%) reported an extreme improvement of their vision. The remaining eyes had “some improvement.” None complained of worsening. In general, the question “Would you have the surgery again?” was answered by 93.3% of patients with “Yes,” 6.7% of the patients answered “Not sure,” and there were no “No” answers. However, when asked specifically, 5.5% had some trouble with night vision and 6.6% with driving a car at night. There were no cases of glare, but seven cases (7.7%) felt dryness requiring lubrication compared with two cases (2.2%) preoperatively. ¹⁰

4.3 Safety

The technique’s safety record is good. In one of the largest study populations so far, Ivarsen et al reported the following complications ²²: 6% of epithelial defects, 1.8% of small tears at the incision edge, and one major tear in a single eye (0.06%). None of these patients complained about any problems later. While an adequate flow when flushing the interface can be assumed, we nevertheless recorded 8.8% of cases of intrastromal debris in our series. ²² In 14 eyes (0.8%), suction loss occurred during laser surgery.

A study by Wong et al described suction loss during refractive lenticule extraction (ReLEx FLEx and SMILE) as a relatively uncommon occurrence, with an overall cumulative incidence of 3.2%. ²³ The recommendations to cope with this kind of incident are as follows: stage 1 (posterior lenticule cut < 10%), restart; stage 2 (posterior lenticule cut > 10%), switch to LASIK; stage 3 (lenticule side cut), repeat the lenticule side cut and decrease the lenticule diameter by 0.2 to 0.4 mm; stage 4 (anterior lenticule cut), repeat the anterior lenticule cut; and stage 5 (anterior lenticule side cut), repeat the anterior lenticule side cut and decrease the lenticule diameter by 0.2 to 0.4 mm. ²³

Zhao et al reported cases of diffuse lamellar keratitis in 1.6% of 1,112 eyes that underwent SMILE. ²⁴ The onset usually occurs within 1 to 3 days postoperatively. Most common symptoms are photophobia and redness (88%), mild pain (44%), and tearing (33%). This incidence is lower than that reported after LASIK. One possible explanation is, according to Zhao et al, that the new-generation femtosecond laser systems use a higher pulse frequency and lower pulse energy for SMILE. ²⁴ It was previously reported that a high-laser-energy setting with the ensuing photodisruption induced tissue injury and accumulation of gas bubbles, resulting in increased cellular inflammatory responses and in diffuse lamellar keratitis. Mild to moderate inflammation typically is treated with intense topical steroids and sometimes with systemic steroids. More severe inflammation is usually treated by first irrigating the interface and then using topical steroids. The authors strongly recommend a careful slit-lamp examination on the first postoperative day, which is crucial for early diagnosis of and intervention in cases of diffuse lamellar keratitis. ²⁴

There was postoperative haze in 8% and minor infiltrations in the interface of 0.3% in our patients. Only in one of these cases did a complication affect visual acuity after 3 months. Microstriae were much less frequent compared to FLEx. However, some microstriae can still be observed after myopic SMILE due to the compression effect within the superior corneal layers after the removal of the underlying lenticule.

There is one case report in the literature reporting on a part of the lenticule remaining in the interface which subsequently led to astigmatism. ²³ The unwanted side effect most frequently reported by patients is temporary alterations of the tear film. It should be added, however, that both punctate corneal staining and subjective dry eye syndrome are less common in SMILE than in FLEx and other flap-based corneal refractive procedures. These findings support the hypothesis that SMILE reduces the amount of dissected nerve fibers.

4.4 Long-Term Results

Recent long-term studies focus less on the safety of this new surgical technique and more on the refractive stability over an extended time and on possible late complications. Stability so far has proven to be a major asset of ReLEx techniques: after FLEx as well as after SMILE, only minor regressions in the 5-year results were observed. The mean regression in those 41 eyes was only 0.07 D. The uncorrected distance visual acuity (UDVA) was 20/40 or better in all eyes ^{9,​ 24} (▶ Fig. 4.4). Today, after the establishment of nomograms, the stability of the achieved correction with refractive lenticule extraction is better than that with LASIK. In LASIK, the mean regression after 6 to 7 years is reported to range from 0.63 to 0.97 D—compared with 0.07-D regression in our study. It can be assumed that these findings reflect the higher internal structural stability in flapless SMILE-treated corneas. As refractive lenticule extraction gains acceptance with the introduction of the flap-free SMILE technique, new studies will ameliorate remaining concerns about stability and late complications.

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