Having been available in Australia for five years, what is the role of Small Incision Lenticule Extraction – known as SMILE – in corneal refractive surgery, compared to LASIK and photorefractive keratectomy procedures?
Although laser in situ keratomileusis (LASIK) continues to represent the predominant corneal refractive surgery technique throughout the world, Small Incision Lenticule Extraction (SMILE) is becoming more popular and for good reason.1 LASIK requires the use of two separate lasers; the femtosecond laser for flap creation and an excimer laser to reshape the corneal bed. The ideal procedure would involve only one laser platform and still retain the ability to successfully treat a range of refractive error. This has largely been the space occupied by SMILE, which has now been available in Australia for approximately five years, and through Europe for a full decade.2 The SMILE procedure involves the use of the femtosecond laser to create an intrastromal lenticule that is then manually extracted through a small, peripheral incision.3 In this article we will examine the SMILE procedure in 2020, in particular the reported safety and efficacy outcomes, and how the procedure compares to existing procedures such as LASIK and photorefractive keratectomy (PRK).
patient selection favours higher myopes although the treatment range has decreased to include low myopia also
SMILE is the end result of a long-term evolution of pre-existing refractive surgery techniques. The direct history of the procedure dates back to 2002 when a German government taskforce was appointed to find novel uses for a recently developed technology; the femtosecond laser. Proposing the possibility of an intrastromal lenticule for refractive correction, Carl Zeiss Meditec began work in 2004 to bring the idea to market.4 The first clinical studies commenced in 2006 before the first SMILE treatment occurred in April 2007. Six-month outcomes of these studies were published in 2008 with the initial international cohorts following several years later in 2011.5,6
Since the introduction of SMILE for myopia to our practice in late 2015, there has been a significant uptake of the technology across Australia and New Zealand, with 17 local units joining over 1,000 units worldwide. The SMILE technique accounted for approximately 17% of total global laser vision correction procedures in 2019.7
The optimal refractive outcome begins with appropriate patient selection which remains a critical component of the refractive consultation. It is essential that the surgeon considers the relative advantages of all available refractive techniques and recommends surgery accordingly. The modern refractive surgeon, if they are to be successful, must offer the full range of corneal and lenticular refractive procedures in order to provide the best results for his or her patients.
the SMILE procedure has been shown to have a less pronounced impact upon the ocular surface and corneal innervation compared with LASIK
One of the specific proposed benefits of SMILE over LASIK has been the maintenance of corneal biomechanical properties following surgery.8-13 This suggests that deeper corrections in the stroma may be possible in SMILE patients without increasing the relative risk of corneal ectasia following surgery. Residual bed thickness (RSB) for most LASIK surgeons remains limited to 300 microns, which has additional implications for the level of refractive error treated.14 Our extended experience with SMILE would suggest that an RSB of 250 microns is acceptable. More recently, Ong et al suggest that the percentage of tissue altered (PTA) can be extended in SMILE without impact on safety.14 Previously, a PTA of >40% was considered an independent risk factor for ectasia following LASIK. The authors found a maximum PTA of 47.9% amongst their SMILE cohort without further incident, confirming the potential extension of myopic refractive treatment ranges.15 The published literature offers a variety of possible reasons for this difference.12,13,16 Cohesive tensile strength appears greater in the anterior stroma lamellae than the posterior stromal lamellae.12 Sinha Roy and co-authors suggest that the LASIK flap essentially decouples the anterior cornea from its anchoring boundaries prior to exposing the stromal bed unlike SMILE which mostly preserves anterior stromal structure.12
Tangential tensile strength and shear strength, that is the stiffness of the lamellae and the resistance to torsional forces, have also been shown to be depth dependent.16 Limiting changes to the anterior anatomy and structure would therefore be expected to continue to contribute and maintain strength postoperatively. Khamar and co-authors, in a prospective comparison study, found biomechanical weakening in 13.3% vs. 36.7% for SMILE and LASIK eyes respectively. Alternatively, the authors found 40% and 13.3% of eyes with no detectable biomechanical changes. These results were recorded prior to flap lifting or lenticule removal although on postoperative assessment, these intraoperative values did not appear to make a difference at both one week or one month follow up, suggesting minimal correlation. Sample size was small and long-term follow up not available, which reduces the potential clinical impact of the research, suggesting additional studies is required.17 The variation seen within the literature is likely to reflect the evolving diagnostic technology available to assess corneal biomechanics and the relative lack of a gold standard. The proposed retention of biomechanical strength following SMILE suggests the risk of corneal ectasia following surgery, particularly in subtle cases of irregularity, will be reduced, however, keratoconus remains an absolute contraindication for both LASIK and SMILE procedures.18 Positively, the initial cohort data at 10 years described no late complications albeit in a small population.2 It is important, however, to point out that keratectasia can occur after SMILE. Appropriate preoperative screening and excluding patients from surgery who are at risk is mandatory.
Additionally, the SMILE procedure has been shown to have a less pronounced impact upon the ocular surface and corneal innervation compared with LASIK, thereby impacting dry eyerelated complications.19 Reductions in postoperative tear production and tear film break up time (TFBUT) were identified in both LASIK and SMILE cohorts in a recent large meta-analysis, however only LASIK changes were found to be statistically significant.19 These results are replicated across a number of studies and provide consistent evidence that both the corneal flap and subsequent excimer laser ablation represent significant contributing factors to dry eye in laser refractive surgery patients.20-22 These findings are not universal however, with several studies failing to show a statistical difference across dry eye parameters such as tear film osmolarity and tear break-up time or Schirmer’s tests.22,23 Although moderate to severe dry eye remains a contraindication to all corneal laser refractive surgery, with patients at risk of poor visual and safety outcomes, evidence would suggest that patients with mild dry eye may present as more appropriate candidates for SMILE surgery compared to LASIK.24 This reflects our practical experience, but we aggressively manage any signs of ocular surface disease before any corneal refractive procedure.
Separately, many of the standard parameters for SMILE surgery continue to reflect the required considerations for other laser refractive techniques. A list of relative and absolute contraindications for SMILE, in comparison to standard contraindications for both LASIK and PRK surgery, are listed in Table 1.
It remains essential that all potential patients undergo a comprehensive visual and ocular assessment prior to surgery. Refractive stability representing a change of less than 0.5D over 12 months is mandatory. Allowing for appropriate corneal thickness, the available range of clinical treatment for SMILE is from -1.00D to -10.00D and astigmatism up to 5.00D (Table 2).
Although SMILE represents a more technical refractive procedure for the surgeon, evidence indicates that the procedure is safe, with minimal complications reported during surgery.3,20 Our personal experience over five years supports this.
Suction loss has been indicated to occur in between 0.5% to 4.4% of patients in cohort studies, although the upper threshold represents earlier experience.25,26 Suction loss has variably been attributed to Bell’s reflex, fixation light tracking, nocioreceptive reflex, false suction and patient anxiety.25 Management is dependent on the timing of the suction loss itself. Re-engagement is possible in the majority of cases without further incident, unlike in LASIK where the procedure may be delayed, or in some cases transferred to surface ablation. If suction loss occurs prior to the laser ablation, the patient can simply be re-engaged and the procedure continued immediately. Epithelial disturbances may occur due to the docking procedure, or at the entry site for the lenticule removal through the laser-created channel, however these remain rare and clinically non-significant in most cases. A bandage contact lens is rarely required in our experience. It is possible that the lenticule may be partially torn during the procedure. Further attempts to remove the remaining lenticule should be undertaken as intra-stromal residual lenticules have been identified as contributing to refractive surprises and irregular astigmatism if left in situ.27,28
It remains intuitive that in the absence of a corneal flap, as required with the LASIK procedure, the risk of related concerns such as flap displacement or striae are removed. This represents a small but significant relative advantage of the SMILE procedure.
With respect to the LASIK procedure, SMILE patients lag behind across the early visual recovery period (24 hours)
Reported postoperative complications include epithelial ingrowth, diffuse lamellar keratitis (DLK), corneal haze, irregular astigmatism, increased aberrations, ectasia, punctate corneal staining and mild dry eye.3,4,19,29
Ingrowth may be caused by epithelial cells displaced at the time of the procedure or through cells invading the SMILE incision site. Both the laser settings and my technique have evolved over time which has meant less manipulation. Anecdotally, this has led to even fewer aberrant cells within the intrastromal space on biomicroscopy examination post procedure.
A review of safety incorporating existing literature revealed an accumulative risk of epithelial ingrowth of 0.5% in SMILE patients.30 Mild cases continue to require observation only and rarely lead to further concerns. As expected, the risk of irregular astigmatism or corneal melt increases significantly if the ingrowth is considered severe and washout procedures may be required in a small selection of cases. My current experience reflects this with only two cases requiring secondary treatment. The patient was returned to surgery and the cells removed. Visual acuity was 6/6 at day one with no pain or photophobic symptoms.
Diffuse Lamellar Keratitis (DLK)
DLK represents an acute inflammatory response leading to increasing discomfort, photophobia and a reduction in visual acuity, typically presenting during the immediate postoperative period. Severe cases may lead to stromal necrosis without further treatment although most cases resolve relatively quickly with topical corticosteroids. A definitive cause of DLK remains unclear however the condition has variously been attributed to the introduction of bacteria, surgical debris, meibomian gland secretions and immune infiltrates.31 The existence of epithelial defects has been shown to significantly increase the risk of DLK. We have had no cases of DLK in our SMILE cohort.
The incidence of DLK following SMILE has been reported as between 0.04 to 1.6%.30,31 It has been suggested that the proximity of the SMILE lenticule in relation to the limbus and limbal vasculature may increase inflammatory cells in relation to other laser refractive procedures, however again, in our experience this has not provided a practical impact.
Corneal Haze and Oedema
The evolution of surgery parameters and laser energy have anecdotally reduced the incidence of mild to moderate haze in our practice. This is reinforced by the findings of Moshifar et al, which found 0.2% of SMILE patients in their cohort experienced mild corneal oedema.30 With respect to the LASIK procedure, SMILE patients lag behind across the early visual recovery period (24 hours). Lui and co-authors suggest a comparative increase in interface haze may represent a contributing factor, albeit resolving with standard topical treatment protocol.32
Micro distortions have been listed within the literature, albeit on optical coherence tomography (OCT) rather than on biomicroscopy.33 It is difficult to identify the clinical impact of micro distortions on vision and visual quality as generally there is minimal evidence of additional signs, such as corneal striae on biomicroscopy.
Irregular Astigmatism and Increased Higher Order Aberrations
Significant irregular astigmatism is rare across all current laser refractive surgery techniques but can represent a significant concern. The impact of the broader functional optical zone of SMILE may have additional benefits. A recent comparative study showed a significant increase in both regular and irregular astigmatism at 8mm in a LASIK cohort. An increase in regular astigmatism and asymmetry was seen in the corresponding SMILE cohort albeit this did not reach statistical significance. Positively, there was a significant decrease in irregularities in these patients.34 Retained lenticules or lenticule fragments have been shown to induce irregular astigmatism and loss of visual acuity, although prompt removal reduces any impact on vision.35 This complication remains exceedingly rare in an experienced refractive practice.
The ideal placement of the refractive lenticule, or laser ablation remains a broader conversation, however surgical decentration can represent a clinically significant concern. Decentration can lead to an immediate functional impact on visual quality through the induction of higher order aberrations such as coma.36 The current SMILE surgical technique does place the onus on the surgeon to lead centration, as opposed to laser ablative procedures, which can be supported by laser tracking capabilities. The impact with experienced surgeons is likely to be minimal between procedures, as seen by Chan and co-authors who found no difference in measured decentration between SMILE and LASIK cohorts.37
Although SMILE represents a more technical refractive procedure for the surgeon, evidence indicates that the procedure is safe, with minimal complications reported during surgery
Undoubtedly, corneal ectasia remains the most significant long-term concern for refractive surgeons. As discussed previously, one of the proposed benefits of SMILE over LASIK in particular, is the minimisation of biomechanical changes following surgery which would suggest a reduction in the risk for ectatic changes following the surgical procedure. The majority of reported ectasia case reports within the literature do suggest the preexistence of corneal irregularity, such as keratoconus, which would indicate that this remains a contraindication for all types of refractive surgery.
The majority of literature relates the presence of significant dry eye symptoms with the transection of corneal nerves during surgery.19 As described earlier within the article, results suggest that SMILE patients recover more quickly and have less significant dry eye symptoms than patients undergoing the LASIK procedure. These findings have clinical significance.19 Subjective patient satisfaction remains increased in SMILE patients in comparison to LASIK groups.21,22 Anecdotally this is what we have experienced in our clinical practice; despite the slightly longer visual recovery of SMILE patients compared to LASIK, the patients themselves report being more comfortable and happier during the postoperative recovery period.
VISUAL AND REFRACTIVE OUTCOMES
Blum and co-authors recently published a re-evaluation of the original SMILE cohort at 10 years following surgery.2 Fifty-six eyes of the original 91 eyes were available for assessment and the authors found a postoperative spherical equivalent (SE) -0.35 ± 0.66D (mean preop SE -4.82D) with total regression of refractive effect recorded as -0.35 ± 0.66D over the same period. No patients lost lines of best corrected visual acuity. This represents the longest available follow up with patients, albeit the sample size remains limited.
Recent analysis of US FDA premarket approval studies indicated that SMILE was comparable to toric phakic intraocular lenses in high myopia.38 Using similar data, Schallhorn and co-authors found no difference between SMILE, wavefrontguided (WFG) and topography-guided LASIK cohorts, albeit the WFG cohort appeared to experience a significantly greater regression of refractive effect.39 There was no difference in residual astigmatism between cohorts, which has represented a potential disadvantage of the SMILE technique due to perceived concerns around cyclotorsion and patient positioning, opening incision and lenticule extraction methods amongst others.40 Perhaps as a reflection of this, Chen et al found statistically less residual astigmatism in a comparative cohort of cyclotorsioncorrected and non-corrected eyes, although both outcomes remained clinically insignificant (-0.07 ± 0.07D vs 0.02 ± 0.13D respectively). Both groups had equivalent spherical residual error.41
It has been proposed that SMILE may be of greater benefit in patients with higher refractive corrections. In a comparative study at three years post-procedure, Xia et al found a statistically significant change in refraction between LASIK and SMILE cohorts (0.89D vs 0.14D respectively), suggesting greater stability over time.42 The original SE were similar (-8.05 ± 1.12D vs. -8.11 ± 1.09D for LASIK and SMILE cohorts). Final higher order aberrations and spherical aberration was less in the SMILE group, however coma was increased in the SMILE cohort.41 The reduction in induced aberrometry has been identified in previous studies, suggesting that long-term visual quality, particularly in higher corrections may remain at, or close to preoperative levels.43,44
A true measure of safety of any refractive procedure remains the percentage of eyes that may have reduced by two or more lines of corrected distance visual acuity (CDVA). The published findings report a high value of 2.4% in an early study with a reasonable cohort (n = 790 at three months) by Hjortdal and co-authors.45 Multiple linear regression analyses did not reveal clinically significant contributing factors and, with minimal follow up, it was unknown if corrected visual acuity improved with time. A 2016 meta-analysis found no difference in loss of (CDVA) greater than one line in SMILE and LASIK cohorts, suggesting broader equivalence.46
Our practice received the Visumax femtosecond laser in late 2014 and surgeons have since completed over 3,000 SMILE procedures, representing the largest local cohort. Currently, SMILE accounts for approximately 50% of laser refractive procedures although this figure differs between surgeons. As highlighted earlier, patient selection favours higher myopes although the treatment range has decreased to include low myopia also (5.2% low myopia, 58.7% moderate myopia and 36.1% high myopia). Refractive analysis indicated that the mean SE of our treated SMILE cohort was -5.49 ± 2.01D (maximum treatment -12.13D). Including all postoperative SE targets, the mean post-surgery SE was -0.16 ± 0.46D. The mean absolute difference from target was 0.25 ± 0.28D, representing equivalence to best practice. Ninety-two (92.4%) of eyes achieved 6/6 or better and 99.4% 6/9 or better (in patients with plano target). Although revision of our energy settings and improvement of technique has improved the immediate visual recovery for our patients, SMILE patients still lag minimally behind LASIK patients as previously mentioned. This does not appear to represent a significant issue for patients who generally achieve ~ 6/9 at the day one assessment. Enhancement rates for SMILE patients are approximately 2% of all cases, with surface ablation the retreatment procedure of choice. Safety remains a priority and intraoperative complications requiring follow-up treatment were less than 0.1%. No patient lost corrected acuity in our cohort. One patient has been diagnosed with corneal ectasia, reiterating a need for constant vigilance at the consultation and following the procedure.
SMILE IN THE FUTURE
A limitation of SMILE is the absence of hyperopic correction although preliminary studies are promising. Pradhan and co-authors recently published 12-month hyperopic SMILE outcomes (preop SE + 5.61 ± 1.21D, range +1.00 to +6.01D). Fifty three (53%) of eyes were within ± 0.50D and the mean arithmetic difference from target -0.22 ± 0.88D (range -2.20 to +3.00D).47 Although these outcomes remain promising, additional research and nomogram refinement is required.
A range of therapeutic indications for SMILE lenticules remain under investigation and provide significant promise for the treatment of keratoconus and corneal disease.48-50
Five years on, SMILE represents a safe and effective refractive option for the correction of myopia that now has become the dominant refractive procedure at our clinic. We look forward to further improvements in the technology to facilitate centration and astigmatic alignment.
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Associate Professor Michael Lawless, Associate Professor Colin Chan, Dr Tess Huynh and Dr Patrick Versace contributed patients to the analysis.
Professor Gerard Sutton MBBS MMed FRANZCO is internationally recognised as an expert in cataract surgery, laser vision correction and corneal transplantation. He has performed over 20,000 surgical procedures, and is the Sydney Medical School Foundation Professor of Clinical Ophthalmology and Eye Health. Prof Sutton consults at Vision Eye Institute Chatswood and is the Medical Director of the Lions NSW Eye Bank. He remains actively involved in research.
Chris Hodge PhD BAppSc is clinical research coordinator at Vision Eye Institute with over 100 published articles in peer-reviewed scientific journals. He is also a clinical senior lecturer with The University of Sydney, Save Sight Institute, Sydney Medical School and Honorary Associate of the Graduate School of Health, University of Technology Sydney.
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