Small-incision lenticule extraction

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Small-incision lenticule extraction Majid Moshirfar, MD, Michael V. McCaughey, BS, Dan Z. Reinstein, MD, MA(Cantab), FRCOphth, Rupal Shah, MD, Luis Santiago-Caban, MD, Carlton R. Fenzl, MD

This review looks at the benefits, limitations, complications, and future applications of the smallincision lenticule extraction procedure. Using the search terms small incision lenticule extraction and femtosecond lenticule extraction, we obtained data from 56 articles (omitting German and Chinese articles) from the PubMed database. Small-incision lenticule extraction has shown efficacy, predictability, and safety that are proportionate to those of laser in situ keratomileusis (LASIK), with the additional benefit that it eliminates flap creation and the attendant risks. The potential advantages of the procedure related to improved biomechanical stability, postoperative inflammation, and dry-eye symptoms have not been fully established. Small-incision lenticule extraction–treated eyes have shown a reduced degree of postoperative corneal denervation and higher-order aberrations and an accelerated rate of corneal nerve convalescence relative to LASIK. Future possibilities related to long-term cryogenic storage of extracted lenticules with eventual reimplantation or donation have been investigated with encouraging preliminary results. Financial Disclosure: Drs. Reinstein and Shah are consultants to Carl Zeiss Meditec AG. No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2015; 41:652–665 Q 2015 ASCRS and ESCRS Online Video

In the field of refractive surgery, the femtosecond laser has gained favorable reception for use in the creation of corneal flaps, a process formerly performed by mechanical microkeratomes. Contemporaneous with the assimilation of the femtosecond laser in laser in situ keratomileusis (LASIK) procedures were investigations related to the laser's applicability to the performance of keratomileusis without flap creation.

Submitted: June 18, 2014. Final revision submitted: October 1, 2014. Accepted: October 2, 2014. From the Francis I. Proctor Foundation (Moshirfar), University of California San Francisco, San Francisco, California, the University of New Mexico School of Medicine (McCaughey), Albuquerque, New Mexico, and the John A. Moran Eye Center (Fenzl), University of Utah, Salt Lake City, Utah, USA; the London Vision Clinic (Reinstein), London, United Kingdom; the New Vision Laser Centers (Shah), Vadodara, India; the Ophthalmology Department (Santiago-Caban), University of Puerto Rico School of Medicine Ophthalmology Department, San Juan, Puerto Rico. Corresponding author: Majid Moshirfar, MD, Francis I. Proctor Foundation, University of California San Francisco, San Francisco, California 94143, USA. E-mail: [email protected].

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Q 2015 ASCRS and ESCRS Published by Elsevier Inc.

Despite the novelty and the limited investigative literature associated with the femtosecond lenticule extraction (FLEx, Carl Zeiss Meditec AG) and small-incision lenticule extraction (SMILE, Carl Zeiss Meditec AG), the 2 procedures have shown encouraging results in the treatment of myopia and myopia with mild to moderate astigmatic error.1,2 Small-incision lenticule extraction, the most recent development in femtosecond laser–based techniques, represents a less invasive alternative to femtosecond lenticule extraction as well as LASIK for the correction of myopic error. In this review, we discuss the applicability, pertinent patient considerations, efficacy, safety, and postoperative factors relative to the use of small-incision lenticule extraction. The features and analyses of this technique are the primary focus; however, because of the procedural homogeneity, femtosecond lenticule extraction–derived features are included for comparative evaluation. METHODOLOGY A literature search of PubMed from 1950 through 2014 was performed using terms such as small incision lenticule extraction and femtosecond lenticule extraction. Accessioned articles included 39 prospective studies, 5 retrospective studies, 7 case series studies, 3 case reports, and 2 computational http://dx.doi.org/10.1016/j.jcrs.2015.02.006 0886-3350

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modeling studies. Some articles3–10 were included for historical perspective. All articles written in the English language were included; articles written in German and Chinese were omitted. Because of the limited literature on the small-incision lenticule extraction and femtosecond lenticule extraction techniques, all relevant articles were included in the review. Figures 1 to 3 were developed from summation and percentage data in several articles on small-incision lenticule extraction2,11–25 and LASIK26–32; attempts were made to ensure uniformity in the preoperative refractive parameters and other potentially confounding variables. The significant disproportion of available published literature on each technique resulted in a significantly larger portion of LASIK patients (n Z 36 822) than small-incision lenticule extraction patients (n Z 1759). Direct juxtaposition of several smallincision lenticule extraction and LASIK parameters in this review is not intended to represent precise estimations because of the sample-size disparity and scarcity of randomized controlled trials evaluating these respective procedures. Nevertheless, we think the agglomerated information is worth mention.

HISTORICAL SYNOPSIS Interlamellar resection was the procedural archetype of modern-day refractive surgical techniques that use the principle of keratomileusis.3 Interlamellar resection encompassed 2 separate techniques for treatment of refractive error: extracorporeal optical shaping and in situ optical shaping. These techniques involved resecting corneal flap stroma or stroma beneath a previously raised or removed corneal flap, respectively. Extracorporeal optical shaping was accomplished through the use of a cryolathe, which enabled manipulation and alteration of removed corneal tissue to a desired refractive power.4 Impediments related to technical difficulties of the procedures and inconsistencies associated with surgical outcomes eventually led to development of an automated version of in situ optical shaping referred to synonymously as

Figure 1. Comparison of efficacy between LASIK and small-incision lenticule extraction. Numerical data were obtained from multiple small-incision lenticule extraction2,11–18,21,22,25 and LASIK26–31 studies. Sample sizes for each procedure are as follows: smallincision lenticule extraction, n Z 2002; LASIK, n Z 35 498 (LASIK Z laser in situ keratomileusis; SMILE Z small-incision lenticule extraction).

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Figure 2. Comparison of predictability between LASIK and smallincision lenticule extraction, showing spherical equivalent accuracy after 3, 6, and 12 months and percentage of eyes attaining specified differences in attempted versus achieved correction. Numerical data were obtained from multiple small-incision lenticule extraction2,11,12,14–18,21,23–25 and LASIK26–32 studies (LASIK Z laser in situ keratomileusis; SMILE Z small-incision lenticule extraction).

automated in situ keratomileusis or automated lamellar keratoplasty (ALK). This new technique replaced manual corneal lamellar resection with an automated approach, with the expectation of obtaining more reliable and precise surgical outcomes.5 Unfortunately, poor predictability of postsurgical outcomes and substandard safety of the procedure led to its eventual discontinuation. Implementation of excimer laser platforms for myopic correction initially showed a favorable potential6,7 as a result of the increased precision and reliability associated with use of the platforms. Eventually,

Figure 3. Safety: The percentage of eyes with a gain or loss of lines of CDVA 3,6, and 12 months after small-incision lenticule extraction. Information was extracted from several studies.2,11,12,14,16–21,23 Sample sizes for each time period are as follows: 1 month, n Z 162; 3 months, n Z 958; 6 months, n Z 217; 1 year, n Z 73 (CDVA Z corrected distance visual acuity).

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a combination of keratectomy with subsequent excimer laser ablation was successfully performed, further refining the keratomileusis procedure.8 A modification using a corneal flap in place of anterior keratectomy provided an additional refinement of the LASIK technique, eliminating the need for suture placement.9 The most recent alteration to LASIK has been introduction of the femtosecond laser in place of the microkeratome for creation of corneal flaps. The use of the femtosecond laser has been expanded to other procedures such as cataract surgery, penetrating keratoplasty, intrastromal corneal ring segment tunnel creation, and astigmatic keratotomy.10 Strategies for use of the femtosecond laser as a comprehensive method of correcting refractive error progressed to the development of the femtosecond lenticule extraction procedure, which effectively eliminated use of the excimer laser for refractive correction.1 Merits of femtosecond lenticule extraction were primarily related to perceived cost reduction, less laser energy, and increased surgical efficiency due to the use of a single laser platform instead of the 2-platform procedure in LASIK. The femtosecond lenticule extraction procedure involves intrastromal dissection of a refractive lenticule as well as corneal flap creation, performed exclusively by the femtosecond laser. An important capability of the femtosecond laser is the dissection of spherocylindrical and aspheric lenticule fragments instead of the planar sections formerly produced by ALK.5 The most current iteration in corneal refractive procedures is small-incision lenticule extraction, which circumvents creation of a corneal flap by producing a small peripheral corneal incision in its place. The first small-incision lenticule extraction treatments performed by Sekundo et al.2 as early as 2008 used 2 opposing 5.0 mm entry incisions at the 12 o'clock and 6 o'clock positions. Eventually, incisional quantity and magnitude were decreased, transitioning from a bi-incisional to a mono-incisional approach with a concurrent incision size decrease to 2.0 mm.33 The avoidance of corneal flap creation represents a less invasive technique with implications that potentially improve corneal biomechanical stability (ie, less risk for flap displacement over time) as well as corneal nerve integrity.10

org).11,34 The intrastromal lenticule is then extracted through the peripheral incision. Although up to 3 peripheral corneal incisions can be used, small-incision lenticule extraction is most commonly performed with a single 2.0 to 5.0 mm incision typically located superiorly or superotemporally. Small-incision lenticule extraction achieved Conformit
e Europ
eenne mark approval in 2009 and became clinically accessible in 2011 within Europe2 and Asia.11 It is currently awaiting U.S. Food and Drug Administration approval pending results in ongoing clinical trials. Specific sequential maneuvers during smallincision lenticule extraction involve initial docking using a disposable curved contact glass (treatment applanation interface), followed by patient fixation on a blinking green target. There are 3 discrete intraprocedural phases that the surgeon must recognize.

PROCEDURAL FEATURES Small-incision lenticule extraction is performed by using only the femtosecond laser to make 4 sequential photoablative incisions that create an intrastromal lenticule along with 1 corneal incision that extends to the anterior surface of the intrastromal lenticule (Figure 4) (Video 1, available at http://jcrsjournal.

Figure 4. Incision geometry of the small-incision lenticule extraction procedure. The lenticule cut (1) is performed (the underside of the lenticule), followed by the lenticule side cuts (2). Next, the cap interface (3) is created (the upper side of the lenticule), and finally a 2.0 to 3.0 mm small incision (4) is created superotemporally. The lenticule interfaces are dissected using a flap separator, and the lenticule is extracted manually, both via the small incision.

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The first is maintaining precise centration during docking with the contact glass; the second, verifying and maintaining suction until all femtosecond passes are completed; and the third, performing manual lenticular extraction without complications. During initial docking, it is important that proper head positioning is achieved by the patient tilting his/her head medially to avoid nasal contact with the cone of the contact glass interface. Correct centration can be verified by visualizing the displayed site of the first Purkinje reflex (as seen in the observation position of the Visumax laser [Carl Zeiss Meditec AG]) or in the center of the pupil. During docking, proper centration should be verified before corneal contact with the contact glass interface is started. Repeated docking without establishing suction or repeated commencement and discontinuation of suction will result in fluid accumulation in the glass interface. If this occurs, we recommend lowering the patient, eliminating residual fluid from the undersurface of the interface, and restarting the docking sequence. After centration is achieved, suction is initiated and followed by femtosecond laser–generated incisions occurring in a series of 4 steps: (1) creation of the posterior lenticule surface (from periphery to center) followed by a transition zone at the edge of the refractive zone (spherocylindrical treatments), (2) vertical edge incisions around the perimeter of the lenticule, (3) creation of the anterior lenticule surface (from center to periphery), and (4) peripheral corneal incision for lenticule access/extraction. Conventional parameters used during small-incision lenticule extraction are as follows: repetition rate 500 kHz, pulse energy 120 to 170 nJ, spot distance 2 to 5 mm, lenticule side-cut angle 70 degrees, lenticule diameter 5.75 to 7.00 mm, cap diameter 7.0 to 7.9 mm (typically 0.5 to 1.0 mm greater than the lenticule diameter), cap thickness 100 to 140 mm, side-cut circumferential length 3.0 to 5.0 mm, and minimum lenticule side-cut thickness 15 mm.12–15,33,35–39 The optical zone diameter is typically equivalent to the lenticule diameter when only spherical error is being corrected; a transitional zone is added for cylindrical corrections, altering the lenticular shape from ovoid to circular, with subsequent modification of the lenticule diameter.36 The total time for all femtosecond laser–generated incisions is between 20 seconds and 35 seconds regardless of the refractive error magnitude. Following lenticule creation, a manual spatula is inserted to separate residual lenticular appendages, first within the anterior lamellar plane and then within the posterior plane. Finally, a forceps is used to extract the intrastromal lenticule.

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Intraoperative topical antibiotics are generally used with or without administration of a steroidal or nonsteroidal antiinflammatory drug. Nocturnal use of a plastic shield may also be prescribed postoperatively, but a shield is not needed because there is no flap and therefore no risk for flap displacement. Typical postoperative medication consists of a topical steroid (dexamethasone, loteprednol, fluorometholone, or betamethasone) and a fluoroquinolone (moxifloxacin, ofloxacin, or levofloxacin) for 1 to 2 weeks in association with lubricating eyedrops. APPLICABILITY OF SMALL-INCISION LENTICULE EXTRACTION Small-incision lenticule extraction is currently limited to use in myopic patients (%
10.00 diopters [D]) with mild to moderate degrees of cylindrical error (%6.00 D). A recent study using femtosecond lenticule extraction to correct a hyperopic error showed potential feasibility in this subset of patients.40 Although the study did not specifically use smallincision lenticule extraction, the similarity of the 2 procedures suggests that the results may also be relevant to small-incision lenticule extraction. Preoperative parameters known to affect postoperative refractive correction include age and steepened corneal curvature.16 Despite reaching statistical significance, these parameters are associated with very minor degrees of refractive discrepancy and not thought to be of appreciable clinical significance by us. Current additional requirements for surgical candidacy correspond to those of LASIK (ie, residual stromal bed [RSB] thickness 250 to 275 mm, minimum corneal thickness 475 to 500 mm) in accordance with precautionary measures for decreasing the incidence of postoperative ectasia.10,41 These requirements may be modified in the future as they are currently based on our long-term experience with LASIKrelated surgical considerations. The lack of flap creation during small-incision lenticule extraction may provide a higher degree of corneal structural stability than LASIK, as described below. REFRACTIVE OUTCOMES Several studies have evaluated small-incision lenticule extraction by comparing the refractive outcomes with those of LASIK. In terms of postoperative refractive corollaries (Table 1), small-incision lenticule extraction has shown results that are nearly identical to those of femtosecond laser–assisted LASIK.2,11,12,17,18,42–44 An important consideration regarding reported results for small-incision lenticule extraction is that the majority of surgeons performing this procedure had a significant amount of prior experience with the technique.

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Table 1. Summary of efficacy and predictability results for small-incision lenticule extraction. Efficacy Study*

Eyes

Moderate myopia† Sekundo2 91 Shah11 39 60 Lin12 Kamiya14 52 Ivarsen15,z 106 51, 53 Sekundo18,x Ang25 17 High myopiajj Zhao13 50 669 Ivarsen15,z Hjortdal16 670 Vestergaard17 124 Vestergaard21 30 Yao22 43

Predictability

Patients

Follow-up

UDVA 20/20 or Better (%)

UDVA 20/40 or Better (%)

Mean Efficacy Index

G0.50 of Emmetropia (%)

G1.00 of Emmetropia (%)

48 39 31 26 d 27 17

6 mo 6 mo 3 mo 6 mo 3 mo 1y 1y

83.5 62 85 96 39 88 76.5

97.6 95 d 100 83 100 100

d d 1.04 G 0.20 0.99 0.92 G 0.19

80.2 91 d d 77 92 88.2

95.6 100 98.33 100 97 100 100

30 d 335 100 35 29

6 mo 3 mo 3 mo 3 mo 6 mo 1y

98.1 58 60 37 60 95

100 95 97 95 100 100

d d 0.90 G 0.25 d d d

77 80 77 88 d

98 95 94 95 97 d

UDVA Z uncorrected distance visual acuity *First author † Eyes with a preoperative spherical equivalent of !–6.00 D z Only high astigmatic corrections (R2.5 D) in the moderate myopia category; only low astigmatic corrections (!2.5 D) in the high myopia category x 51 and 53 denote the number of eyes included in each measured value for efficacy and predictability, respectively. jj Eyes with a preoperative spherical equivalent of R–6.00D

Small-incision lenticule extraction has been noted as a technically challenging procedure associated with a significant learning curve that precedes surgical competence.17,19,20 Efficacy A prospective comparative study by Lin et al.12 comprising 111 eyes treated with femtosecond laser– assisted LASIK or small-incision lenticule extraction analyzed the efficacy associated with each procedure 1 and 3 months postoperatively. The efficacy indices of small-incision lenticule extraction–treated eyes were 0.99 G 0.15 (SD) and 1.04 G 0.20 at 1 month and 3 months, respectively, and those of femtosecond laser–assisted LASIK-treated eyes were 1.02 G 0.17 and 1.10 G 0.23, respectively. Our literature review found 26% of patients with postoperative corrected distance visual acuity (CDVA) equal to or more than 20/16, 62% equal to or more than 20/20, and 93% equal to or more than 20/40 after small-incision lenticule extraction2,11–18,21,22 and 19%, 71%, and 95%, respectively, after LASIK (microkeratome and femtosecond laser assisted) (Figure 1).26–31 Predictability Aggregation of our data reflects predictability values (at 6 months and 12 months) of 98% and 99%, respectively, for small-incision lenticule

extraction2,11,12,14–18,21,23,24 and 97% and 99%, respectively, for LASIK (microkeratome and femtosecond laser assisted) (Figure 2).26–32 The predictability reported by Lin et al.12 revealed similar consonance between the 2 procedures at 1 month and 3 months: 100% and 98.33% within G1.0 D, respectively, of small-incision lenticule extraction–treated eyes and 94.12% and 100%, within G1.0 D, respectively, of femtosecond laser–assisted LASIK-treated eyes. Safety In the Lin et al. study,12 the safety indices of the 2 techniques at 1 month and 3 months were similar: 1.00 G 0.06 and 1.01 G 0.05, respectively, for smallincision lenticule extraction and 1.00 G 0.03 and 1.01 G 0.03, respectively, for femtosecond laser–assisted LASIK. The accumulated distribution of safety parameters (postoperative CDVA, lines gained or lost) for small-incision lenticule extraction were as follows: At 6 months, 0.50% lost more than 2 lines, 0.50% lost 2 lines, 8% lost 1 line, 60% were unchanged, 27% gained 1 line, and 4% gained 2 lines. At 12 months, 8% lost 1 line, 55% were unchanged, 33% gained 1 line, and 4% gained 2 lines, respectively (Figure 3 and Table 2).2,11,12,14,16–21,23 Preliminary assessment of the refractive stability of small-incision lenticule extraction appears to correspond to the regressive indices associated with

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Table 2. Summary of safety results for small-incision lenticule extraction.

Study*

Eyes

Moderate myopia† Sekundo2 91 Shah11 47 Lin12 60 Kamiya14 52 Sekundo18 53 Agca23 20 High myopiaz Hjortdal16 670 Vestergaard17 124 1574 Ivarsen19 Vestergaard21 30

Loss of 2 Lines (%)

Loss of 1 Line (%)

No Loss or Gain of Lines (%)

Gain of 1 Line (%)

Patients

Follow-up

Loss of R2 Lines (%)

Gain of R2 Lines (%)

48 39 31 26 27 20

6 mo 6 mo 3 mo 6 mo 1y 1y

1 0 0 0 0 0

1 0 1.67 0 0 0

9 4 1.67 15 11 0

54 70 96.67 77 45 80

32 21 0 8 38 20

3 4 0 0 6 0

d d 1.01 G 0.05 d 1.08 d

335 100 922 35

3 mo 3 mo 3 mo 6 mo

0 0 0 0

2.30 0 1 0

15 5 13 3

49 75 37 76

30 19 45 21

3.70 0.80 3 0

1.07 G 0.22 d d d

Mean Safety Index (%)

*First author † Eyes with a preoperative spherical equivalent of !–6.00 D z Eyes with a preoperative spherical equivalent of R–6.00 D

myopic LASIK. The annual regression of
0.08 D after small-incision lenticule extraction for myopic correction18 is similar to that after myopic LASIK (
0.10 G 0.18 D).45 However, the estimation of small-incision lenticule extraction–related regression is currently provisional because of the paucity of available literature and short-term and long-term results. After the intrastromal lenticule is removed during small-incision lenticule extraction, an anterior section of disconnected residual stromal tissue with overlapping Bowman and epithelial layers remains in place. This anterior segment of corneal tissue is referred to as a “cap” when performing small-incision lenticule extraction. Several analyses have studied the reliability and accuracy of cap thickness alone and in comparison with femtosecond laser–assisted LASIK corneal flaps to ensure proper procedural adequacy.13,33,35 Accuracy and reliability of smallincision lenticule extraction cap thickness paralleled those of femtosecond–assisted LASIK, further demonstrating the apparent interchangeability in reliability of the 2 techniques. Appraisal of postoperative outcomes between femtosecond lenticule extraction and small-incision lenticule extraction has also been evaluated. In a recent study by Kamiya et al.,14 no significant disparities were found in the predictability, efficacy, or refractive stability of the 2 procedures. Although more intricate aspects of postoperative refractive outcomes such as higher-order aberrations (HOAs) were not addressed in this study, it appears that general refractive outcomes in the 2 procedures were essentially equivalent. A possible association between postoperative stromal irregularity and poor refractive results has been

investigated indirectly through analysis of lenticules extracted during small-incision lenticule extraction.46 Scanning electron microscopy of the extracted lenticules showed consistent surface uniformity. However, the applicability of these findings is confined to purely myopic corrections because the lenticules did not have cylindrical correction.47 POSTOPERATIVE INFLAMMATION Postoperative inflammation is 1 of several important aspects to consider because of its association with corneal wound healing and associated visual consequences such as haze and regression of refractive correction.48–50 Comparative studies of postoperative corneal healing have shown significantly fewer inflammatory cells in corneas treated with femtosecond lenticule extraction/small-incision lenticule extraction than with femtosecond laser–assisted LASIK.51,52 In the study by Riau et al.,52 a statistically significant increase in inflammatory cells was observed when higher spherical corrections were attempted (
6.00 D to
9.00 D) with femtosecond laser–assisted LASIK than with femtosecond lenticule extraction/small-incision lenticule extraction. A greater proportion of ablated tissue during higher refractive corrections and increased energy absorption from the excimer laser were suggested as probably explanations for the differences. Both of the studies used rabbit eyes for their investigations of corneal inflammation. Conversely, a study evaluating interface backscatter observed a significant increase in postoperative values in small-incision lenticule extraction–treated eyes

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compared with femtosecond laser–assisted LASIKtreated eyes; the increase persisted for up to 3 months postoperatively.36 During the 6-month postoperative evaluations, there was no significant difference in corneal backscatter between the 2 groups; nevertheless, postoperative findings during the initial 3 months support previous observations of a comparatively delayed visual recovery following small-incision lenticule extraction.18,43 Although delayed postoperative visual recovery has been documented in numerous articles, it appears that subsequent improvements in software and laser parameters have significantly amended this disadvantage of smallincision lenticule extraction.34 HIGHER-ORDER ABERRATIONS Postoperative analysis of visual quality and additional objective evaluations of HOAs have been examined in femtosecond lenticule extraction/small-incision lenticule extraction procedures, and the results have been compared with those in femtosecond–assisted LASIK. A study comparing only femtosecond lenticule extraction and femtosecond laser–assisted LASIK concluded that femtosecond lenticule extraction induced significantly fewer 4th-order aberrations than femtosecond laser–assisted LASIK; however, there were no differences in CDVA or uncorrected distance visual acuity, 3rd-order aberrations, and total HOAs between the 2 procedures, leading the authors to conclude that both techniques ultimately produced nearly equivalent end results.53 Evaluation of visual quality and HOA measurements in small-incision lenticule extraction and femtosecond laser–assisted LASIK show equal patient satisfaction and a significant reduction in HOAs in the small-incision lenticule extraction– treated group.12 Although results in this study were based on 1-month and 3-month postoperative intervals, a previous study comparing HOAs between femtosecond lenticule extraction and femtosecond laser–assisted LASIK showed stability of results up to 1 year postoperatively.54 Given the relative similarity between femtosecond lenticule extraction and small-incision lenticule extraction, it appears likely that the differences in HOAs between small-incision lenticule extraction and femtosecond laser–assisted LASIK would remain uniform over a prolonged period. Specific assessment of induced HOAs and visual acuity outcomes in small-incision lenticule extraction and femtosecond lenticule extraction procedures were recently analyzed to differentiate the 2 procedures and potentially establish superiority. The 2 studies21,23 did not detect any significant differences in visual acuity, contrast sensitivity, or HOAs.

In addition, objective measurement of optical quality through use of a double-pass instrument was performed in eyes that had had small-incision lenticule extraction. Use of a double-pass instrument is considered a more accurate technique for retinal image quality assessment than use of wavefront-sensing devices.55 An increased amount of intraocular light scatter with minimal distortion of image quality was observed after small-incision lenticule extraction; it returned to preoperative values after 3 months.56 The magnitude of centration misalignment (performed during the early learning phase of smallincision lenticule extraction) has been investigated to determine the correlation with the postoperative development of HOAs. Notwithstanding lack of experience, observed calculated departures of centration relative to the corneal vertex were relatively minor (0.17 G 0.09 mm). The horizontal decentration was positively correlated with the quantity of induced horizontal trefoil; obversely, the vertical decentration did not result in HOA elevation.57 POSTOPERATIVE ENHANCEMENT A case report of inferior corneal steepening and resultant irregular astigmatism after femtosecond lenticule extraction due to a vestigial remnant of lenticular tissue58 underscores the importance of determining the optimum enhancement method. Successful enhancement of a previous femtosecond lenticule extraction procedure has been achieved in a manner similar to that of a LASIK enhancement; ie, by re-elevating the previously created flap and removing additional stromal tissue through ablation.59 Currently, there is no formally established method for treating patients who require enhancement after small-incision lenticule extraction. Proposed methods include photorefractive keratectomy (PRK) augmented by mitomycin-C (MMC), conversion to LASIK, thinflap LASIK in the cap, or another small-incision lenticule extraction procedure in or below the cap depending on the initial cap thickness.20,60 Topographyguided PRK in association with MMC has been successfully used to correct a residual refractive error following primary small-incision lenticule extraction.61 In 2 cases, PRK-derived enhancement resulted in corneal opacification; associated commonalities in both cases includes the absence of intraoperative MMC administration and an abbreviated postoperative time course relative to other studied patients. A recent adaptation of small-incision lenticule extraction software (Circle, Carl Zeiss Meditec AG) enables revision of the previously created cap by remodeling it into a larger diameter flap (with hinge) followed by excimer laser ablation. It has been shown that a

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significant proportion of postoperative small-incision lenticule extraction patients may experience impairment of CDVA up to 3 months postoperatively.19 For this reason, a sufficient time should be provided for recovery of visual function before further corrective procedures are attempted. Future studies addressing this issue are necessary and should receive more attention as the popularity of this procedure continues to grow. CORNEAL BIOMECHANICAL STRENGTH Investigative studies have compared other relevant aspects of small-incision lenticule extraction with those of both LASIK and femtosecond lenticule extraction procedures. Avoidance of corneal flap creation and consequent minimization of corneal nerve disruption is thought to confer a theoretical advantage to small-incision lenticule extraction over both LASIK and femtosecond lenticule extraction because of a greater degree of residual corneal stability and corneal sensation postoperatively. Corneal hysteresis (CH) and the corneal resistance factor (CRF) are parameters that have been used to measure and characterize relative corneal biomechanical strength.62 In a study by Agca et al.,37 comparison of postoperative parameters in small-incision lenticule extraction and femtosecond-assisted LASIK did not demonstrate significant differences in CH or CRF. In contrast, a study by Wu et al.63 found a statistically significant elevation in CH and CRF in small-incision lenticule extraction compared with femtosecond laser–assisted LASIK. Despite acquisition of equivalent or divergent results in CRF and CH, it should be kept in mind that CRF and CH are not entirely comprehensive evaluative techniques for analyzing corneal biomechanical stability. Previous investigations have shown a lack of variation in CRF and CH values after corneal collagen crosslinking procedures24,64,65 despite verified evidence of increased corneal rigidity in previously performed studies.66,67 Additional strategies for separate biomechanical quantitative analyses have used a noncontact tonometer combined with Scheimpflug imaging technology.68–70 Specific parameters of corneal deformation amplitude and associated inductive applanation time have been evaluated in femtosecond laser–assisted LASIK, laser-assisted subepithelial keratectomy, and smallincision lenticule extraction. There were no significant differences between the results in small-incision lenticule extraction and those in the other procedures.71 It is conceivable but currently inconclusive that smallincision lenticule extraction may offer increased postoperative corneal biomechanical stability compared with LASIK.

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Anatomically, it has been determined that the anterior segment of the cornea possesses greater biomechanical strength than posterior segments.72–75 Accordingly, it has been postulated that lenticule dissection in posterior segments of the stroma (ie, O120 mm) may result in greater corneal biomechanical stability. Measurement of total stromal tensile strength, a value derived from a predictive mathematical formula comparing LASIK, PRK, and small-incision lenticule extraction, theoretically anticipated a greater amount of postoperative tensile strength with small-incision lenticule extraction.76 According to additional calculations, a higher refractive correction could be performed with small-incision lenticule extraction than with LASIK or PRK and an equivalent amount of biomechanical stability achieved because small-incision lenticule extraction did not disrupt anterior corneal segments. Assessment of corneal stress differences between LASIK and small-incision lenticule extraction through use of an ancillary mathematical formula determined that small-incision lenticule extraction conferred a protective effect on the residual stromal bed (RSB) compared with LASIK.77 Diminished cumulative tension on the RSB is thought to potentially offer advantages such as avoiding refractive instability due to anterior corneal shift as well as the incidence of postoperative ectasia. Reinstein et al.78 evaluated postoperative stromal thickness variation after small-incision lenticule extraction and found a stromal augmentation of 8 mm compared with the lenticule depth measurement originally indicated by the femtosecond platform software. The authors proposed that the mechanism for the induced biomechanical change was postoperative peripheral lamellar expansion. Although small-incision lenticule extraction, unlike LASIK, does not disrupt the Bowman layer, microdistortions have been observed in the Bowman layer in patients who had myopic small-incision lenticule extraction.22 The distortions, which resulted from unavoidable tissue compression from a shortening of the cap's arc length, did not adversely affect vision; an increase in the quantity of distortions correlated with increasing lenticule thickness and lack of surgical experience with small-incision lenticule extraction. Further comparative studies assessing biomechanical stability in femtosecond lenticule extraction and small-incision lenticule extraction have shown no significant differences in CRF or CH between the techniques.38,79 A significant degree of attenuation in the stromal bed and thickening in the central corneal flap was demonstrated 1 week postoperatively in femtosecond lenticule extraction– treated eyes compared with small-incision lenticule extraction–treated eyes.80 Thereafter, no significant differences were observed between the groups over

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a follow-up period of 6 months. Small-incision lenticule extraction does not seem to confer any definitive advantages in biomechanical stability that can be evaluated with current methods of corneal structural quantification. However, avoidance of corneal-flap creation does appear to represent a preferable option in patients who are at higher risk for traumatic flap dislocation due to athletic involvement or other precarious situations. CORNEAL NERVE INTEGRITY Evaluation of postoperative corneal nerve damage suggests there is less severe denervation and accelerated neuronal healing with small-incision lenticule extraction than with LASIK and femtosecond lenticule extraction procedures.39,81,82 Postoperative corneal sensitivity in small-incision lenticule extraction–treated eyes appears to be superior to that in femtosecond laser–assisted LASIK eyes as well.83,84 The interrelation of corneal denervation and development of dry eye85 has prompted investigation of dry-eye symptoms associated with small-incision lenticule extraction. Despite improved corneal sensation relative to LASIK, small-incision lenticule extraction does not appear to significantly decrease postoperative dry-eye symptoms.86 Available studies indicate a lower propensity for corneal fluorescein staining and fewer subjective symptoms of dry eye with small-incision lenticule extraction than with LASIK.87–89 OTHER BENEFITS AND/OR LIMITATIONS Other potentially beneficial features of small-incision lenticule extraction compared with LASIK include a significantly shortened procedural time due to use of a single laser platform instead of the 2-platform procedure and exclusion of the refractive unpredictability of excimer laser–based techniques due to variance in corneal hydration.90 Eyes treated with small-incision lenticule extraction exhibit significantly fewer total HOAs than those treated with LASIK; in particular, the spherical aberration subset is significantly lower.12 Fewer spherical aberration defects may prove advantageous in patients with larger pupillary diameters in whom adverse effects related to spherical aberration are more pronounced.91 Currently, it is not clear whether small-incision lenticule extraction truly produces a more suppressed inflammatory response than LASIK. Manual detachment and separation of the lenticule during the procedure introduces a highly variable and confounding component, which will invariably lead to interindividual differences in postoperative results, especially during the initial learning phase of the procedure.

Currently, the small-incision lenticule extraction laser platform cannot use cyclotorsion control or eye-tracking technology, which may lead to some disadvantages for its use in the correction of higher astigmatic errors. A recent study15 showed efficacy in the correction of higher degrees of astigmatism (up to
5.75 D) with small-incision lenticule extraction, although there was a significant degree of undercorrection as well. Future technological modifications will likely improve the capacity to correct higher astigmatic errors with small-incision lenticule extraction. COMPLICATIONS Intraoperative complications during small-incision lenticule extraction can typically be attributed to loss of suction or quandaries regarding lenticule extraction. Loss of intraoperative suction can occur for many reasons. It most commonly results from patient eye contraction or sudden patient movement. Other reasons include fluid entry through suction ports or compressive forces against the contact glass resulting from intraocular gas-bubble transposition. In the event of suction loss during the primary pass of the femtosecond laser, the user will not ordinarily be allowed to complete the small-incision lenticule extraction procedure; the user will be prompted with the option to convert to a femtosecond laser–assisted LASIK procedure, at which time the eye must be recentered and redocked. Redocking can be challenging at this juncture due to potential pupillary obfuscation secondary to gas-bubble accumulation. In our experience, immediate retreatment is a convenient option that does not seem to adversely affect the results. Another potentially problematic intraoperative phase is lenticule extraction. During attempted lenticule delineation, incorrect tissue plane identification can result in primary separation of the posterior lenticule surface, resulting in its adherence to the stromal surface of the cap. In this situation, it is still possible to achieve lenticule separation, but it is more difficult. Postoperative complications include unintended abandonment of residual intrastromal lenticule fragments,58 incisional edge tearing, epithelial ingrowth, microstriae,2 epithelial abrasions, interface inflammation, and irregular topography.19 Table 3 shows complications reported in the reviewed articles.2,11,14,17–21,34 FUTURE POSSIBILITIES Use of small-incision lenticule extraction and femtosecond lenticule extraction for correction of hyperopia is currently under investigation. The first study evaluating femtosecond lenticule extraction for the correction of hyperopia obtained unsatisfactory results, with significant postoperative regression.40

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Table 3. Reported complications. Complication Peripheral corneal abrasions Corneal nontransparency Dry corneal surface, day 1 Lenticule extraction difficulties Minor tear at incision edge Suction loss Epithelial ingrowth Irregular corneal topography Visually insignificant microstriae Keratitis Cap perforation Fiber in interface Central corneal abrasions Mild edema of corneal cap, day 1 Monocular ghost images Interface inflammation Remaining lenticule remnant Diffuse lamellar keratitis Incomplete laser incision opening Superficial punctate corneal staining Major tear Lenticule extraction not possible

Incidence (%) 5.5 5.4 3.2 1.5 1.5 1.0 0.5 0.5 0.4 0.3 0.3 0.2 0.2 0.2 0.2 0.17 0.04 0.04 0.04 0.04 0.04 0.04

Total sample size: 2345 eyes. Information extracted from the following sources: 2,11,14,17–21,34

A second study with advanced lenticule profiles is currently underway in an ongoing trial in Germany.A Other experimental strategies for

treating hyperopia have been attempted in animal and human subjects. Autologous lenticule transplantation using small-incision lenticule extraction was performed successfully in rabbit eyes, showing nerve regeneration and acceptable corneal healing.92 Femtosecond lenticule extraction has also been used for lenticule extraction and reimplantation following lenticule cryopreservation in rabbits.93 The terms femtosecond laser corneal lenticule transplantation or endokeratophakia have been used to refer to this technique. Several human cadaveric stromal lenticular tissue samples obtained during femtosecond lenticule extraction have shown reasonably stable structural integrity and keratocyte sustenance after 1 month of cryopreservation.94 A case of successful allogeneic lenticule transplantation in a severely hyperopic patient has also been reported.95 Despite successful incorporation of the lenticule, a posterior corneal protuberance developed, contributing to a severe reduction in the refractive corrective ability of the implanted lenticule. Lenticule reimplantation, autologous or allogeneic, could prove useful in the treatment of ectatic conditions and spherical/cylindrical errors or for later restoration of corneal thickness in preparation for ablative procedures in presbyopic patients. A recent animal study by Lim et al.96 showed the feasibility of performing LASIK after previous lenticule extraction/reimplantation using smallincision lenticule extraction. The viability of long-term storage of extracted lenticules, feasibility of reimplantation, longevity, and reliability of implanted lenticules

Table 4. Comparison of advantages and disadvantages of refractive surgery procedures. Procedure

Advantages

Photorefractive keratectomy

Established, extensively investigated Ability to correct hyperopia/astigmatism

Femtosecond laser–assisted LASIK

Established, extensively investigated Rapid postoperative recovery Ability to correct hyperopia/astigmatism

Femtosecond lenticule extraction

Avoidance of intraoperative platform transition Potential for long-term lenticule storage/reimplantation High refractive long-term stability

Small-incision lenticule extraction

Less severe denervation Avoidance of flap creation and potential consequences Avoidance of intraoperative platform transition Decreased HOAs relative to LASIK Potential for long-term lenticule storage/reimplantation High refractive long-term stability Improved mechanical stability of postsurgical cornea

LASIK Z laser in situ keratomileusis

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Disadvantages Risk for postoperative haze Corneal denervation causing dry eye High regression rate Delayed recovery (several months) Flap-related complications Corneal denervation causing dry eye Intraoperative platform transitioning Risk for keratectasia Inability to correct hyperopia Corneal denervation causing dry-eye Flap-related complications Risk for keratectasia Delayed recovery (several weeks) Initial learning curve Uncertainty of enhancement strategy

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will be important in determining the practicality of the Barraquer revitalized technique.3

3. 4.

DISCUSSION Despite its relatively recent clinical introduction, small-incision lenticule extraction has become an important surgical procedure and may become a prominent component in the infrastructure of refractive surgery modalities. Expanded use of the femtosecond laser in refractive surgery appears to hold promise for improving surgical efficiency, eventually leading to new unprecedented techniques that are able to provide significant benefit to a substantial proportion of the population (Table 4). Postoperative efficacy, safety, and predictability have essentially paralleled those of LASIK, with the added benefit of HOA abatement, according to current evidence. The challenges in acquiring the surgical technique may result in a degree of postoperative outcome variability during the preliminary learning phase. In our experience, many surgeons have been able to master the learning curve quickly and achieve excellent results using the step-by-step training recommended by the manufacturer. This sequence consists of preliminary performance of femtosecond lenticule extraction followed by pseudo small-incision lenticule extraction, with eventual transition to small-incision lenticule extraction accompanied by gradual reduction of the entry incision length with surgical experience. Refinements of intraoperative- and platform-related parameters have led to a significant improvement in postoperative visual recovery, which should continue over time as additional alterations are implemented. The superiority of biomechanical stability with small-incision lenticule extraction has not been convincingly demonstrated; future analysis should clarify this aspect. Eventual establishment of hyperopic corrective ability and amelioration of astigmatic corrective capacity appear to be attainable developments as ongoing research addresses each matter. Prospective comparative studies accompanied by additional software iterations should facilitate the development of an optimum method of enhancement for small-incision lenticule extraction.

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REVIEW/UPDATE: SMALL-INCISION LENTICULE EXTRACTION

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OTHER CITED MATERIAL A. Sekundo W, Nubile M, “Future of Laser Vision,” presented at the Zeiss European Refractive Laser Group meeting, Istanbul, Turkey, June 2014

J CATARACT REFRACT SURG - VOL 41, MARCH 2015