The Ultimate Guide to SMILE Refractive Surgery for Optometrists

With the introduction of the VisuMax (Carl Zeiss Meditec) came the re-emergence of the intrastromal lenticule method which had first been described in 1996 using a picosecond laser by Ito et al.¹ The results of femtosecond lenticule extraction (FLEx) were first published in 2008,² followed by the first prospective trials for small incision lenticule extraction (SMILE) published in 2011.^3-5 As of the end of October 2018 there have been over 2,000,000 procedures performed worldwide with more than 2,000 surgeons trained to perform SMILE. United States Food and Drug Administration (FDA) approval was obtained in October 2016 for spherical myopia⁶ and in October 2018 for myopic astigmatism.⁷

The aim of this document is to provide an outline of the SMILE procedure and relevant perioperative care.

Watch Dr. Ryan Vida discuss SMILE surgery in this video!

Chapter 1: Preoperative assessment

Evaluating a patient for SMILE is essentially identical to screening a patient for LASIK. The main features are to perform a comprehensive ophthalmic examination, with a focus on obtaining an accurate refraction, screening for keratoconus, and diagnosis of pre-existing dry eye.

1.1 Ophthalmic examination

History
Ocular dominance
Pupils
Cover testing at distance and near, with and without glasses and ocular motility testing
Confrontational fields testing
Monocular and binocular uncorrected distance visual acuity (UDVA)
Monocular and binocular uncorrected intermediate visual acuity (UIVA)
Monocular and binocular uncorrected near visual acuity (UNVA)
Lensometry of current spectacles and corrected distance visual acuity (CDVA) and distance corrected near vision (DCNV) with these (the patient’s current vision with current correction)
Manifest distance refraction with CDVA
Contrast sensitivity testing (CSV-1000) through best manifest refraction correction
Night vision simulation for patients with high refractive error or larger than average scotopic pupil diameter (greater than 6.3 mm)
Slit-lamp examination including fluorescein staining
Tear break-up time (TBUT)
Schirmer's test
Goldmann applanation tonometry
Cycloplegic refraction and CDVA (after dilation with 1.0% Tropicamide)
Dilated fundus examination using slit-lamp and binocular indirect ophthalmoscopy

Importance of repeatability and accuracy of manifest refraction

The goal of manifest refraction in refractive surgery is to determine the optimal sphere and cylinder lenses that will maximize visual acuity for a particular eye. Refraction for glasses is often considered to be an art, whereas refraction for surgery should be considered a science. When refracting for spectacles, the artistic elements include pushing axes to the horizontal or vertical, minimizing the cylinder used, and keeping cylinder axis orthogonal between eyes, etc. Refraction for refractive surgery is quite different as the aim is to fully correct the existing refraction, so the maximum cylinder and exact axis of astigmatism should be determined irrespective of verticality and orthogonality to the opposite eye.

Manifest refraction is the most critical data entry component for refractive surgery planning and yet it is probably the least studied and researched. So much emphasis has been placed on measuring and correcting higher-order aberrations, when in fact for most eyes, these details are completely wiped-out by a sphere or cylinder (lower order) error of -0.25 D. The limited focus on perfecting the technique is evidenced by the relatively low number of published studies investigating the accuracy, repeatability or reproducibility of refraction. A review of the literature to date highlights the fact that the repeatability of manifest refraction is in the order of 0.25 D.⁸ Therefore, even under controlled conditions, there is an error associated with manifest refraction that likely cannot be eliminated. This is due to the fact that manifest refraction is commonly limited to be measured in 0.25 D increments since it is difficult for most patients to differentiate between the blur induced by smaller increments.

In a refractive surgery clinical setting, it is important that the reproducibility of manifest refraction between clinicians, both surgeons and co-managing optometrists, is known in order to compare measurements obtained at different time points and detect significant changes. This is particularly important in clinical settings where numerous different clinicians are performing refractions at different time-points. The accuracy of refractive surgery depends on nomogram adjustments, which in turn depend on the agreement between the measured refraction and the actual refraction. A nomogram can only be built with accurate manifest refraction measurements, both before and after surgery.

1.2 Refraction stability

As contact lenses can warp the front corneal surface, it is important that patients refrain from wearing contact lenses for a period before the preoperative consultation. One week is usually sufficient for soft contact lenses, two weeks for toric lenses and extended wear lenses, one month for each decade of wear for rigid gas permeable lenses. If a patient is initially seen within this time frame, further visits are required to monitor and confirm that refractive stability is reached before scheduling surgery.

1.3 Dry eye

Patients can still expect to experience dry eye symptoms after SMILE, but the symptoms are usually less than after LASIK and recover more quickly. Therefore, it is still important to diagnose and manage pre-existing dry eye before performing SMILE. Briefly, the elements of the preoperative dry eye assessment are:

Previous ocular history including detailed history for glasses and contact lens wear: period of wear, type of lens/glasses, wear modality, last worn, comfort or visual symptoms associated with wear, history of blepharitis
Ocular symptoms, especially dry eye symptoms with/without contact lenses and pattern, and use of lubricants
OSDI dry eye questionnaire⁹
Slit-lamp examination including fluorescein and lissamine green staining
Tear break-up time
Schirmer’s test
Strip meniscometry¹⁰
HD Analyzer optical quality and scatter¹¹

Additional tests that might be considered for patients with dry eye include corneal sensitivity (e.g. by Cochet-Bonnet aesthesiometry) and meibography (infrared, confocal, or OCT). The list below summarizes the types of dry eye that we aim to consider and manage:

Anterior blepharitis
Posterior blepharitis & meibomian gland dysfunction (MGD)
Exposure dry eye
Aqueous deficiency
Inflammatory dry eye
Anterior basement membrane dystrophy (ABMD)
Corneal hypoesthesia
Ocular surface toxicity

1.4 Contraindications for SMILE

In general, the contraindications for SMILE are the same as for LASIK including:

Previous herpes simplex virus (HSV) keratitis within 1 year (if more than 1 year, perform corneal sensitivity testing and use oral antiviral medication prophylaxis before and after surgery)
Active auto-immune disease – lupus, rheumatoid arthritis, etc.
Keratoconus or other corneal ectatic disease
Severe aqueous deficient dry eye
Unrealistic expectations

Relative contraindications include:

Diabetes with active retinopathy (higher risk of infection and slower healing response)
Fibromyalgia (FM) / myalgic encephalomyelitis (ME) / irritable bowel syndrome (IBS): referral to a corneal specialist needed, but SMILE can be performed with appropriate consent and management
Visually significant cataract: corneal refractive surgery is a good option for mild cataract as the cataract may not progress. Corneal surgery is also more accurate and avoids the increased risks associated with intraocular surgery
Glaucoma: can have surgery if IOP is well controlled with minimal visual field loss, and diurnal IOP curve is assessed before and after surgery
Vascular macular disease: note that PRK is conventionally safer than LASIK as no suction is applied, but the VisuMax affords the lowest increase in IOP of any femtosecond laser or microkeratome at levels that are negligible^12,13
Retinal detachment: clearance from a retinal specialist is recommended, beware scleral bands anteriorly
Epithelial or anterior stromal dystrophies: PRK may be a more appropriate treatment as it has a therapeutic effect,¹⁴ although the low suction applied by the VisuMax means SMILE can be performed even in patients with known ABMD
Fuch’s endothelial dystrophy: endothelial cell count is required before surgery, SMILE can be performed as long there are >1,200 cells/mm², consider treating one eye at a time
Corneal scar that could affect intrastromal cutting accuracy in the optically active zone
Depression or anxiety conditions if not stabilized
Non-availability for follow-up or co-management

If surgery is performed for patients with any of these conditions, an additional consent form is recommended to highlight the additional risk or reduced expectations.

Keratoconus

Just as for LASIK, keratoconus screening is the main focus of the suitability assessment for SMILE; keratoconus has conventionally been a contraindication for any elective refractive procedure that involves the removal of stromal tissue. Unfortunately, it seems that the attention given to the biomechanical advantages of SMILE has sometimes been misinterpreted as there being no risk of accelerating ectasia after SMILE, and therefore a reasonable treatment for keratoconus patients. However, this is not the case. SMILE, just as with any other type of corneal laser procedure, should not be performed in the presence of keratoconus.

Chapter 2: Informed consent

For any surgical procedure, the informed consent process is extremely important. This process should provide the patient with information on the risks and benefits for each of the alternative treatment options for their individual case. The full details of informed consent will not be discussed in detail here. However, the following sections set out some key areas to help communication with patients.

2.1 Lay explanation of SMILE

Given the similarities between LASIK and SMILE, patients often ask for an explanation of the difference and the reasons for preferring one procedure over the other. The method that we have evolved for describing this succinctly in layman’s terms is set out below—the script can be accompanied by a hand-drawn or manufactured image (Figure 1).

Here is a view of a cornea in cross-section [draw cross-sectional view diagram]. In LASIK, an infra-red laser is used to make a cut just below the surface of the cornea that becomes a flap. This flap is lifted and an ultra-violet laser is used to evaporate a lens of tissue. The flap is then put back in place, so at the end it looks like this from the outside [draw front view diagram].

In SMILE, the same tissue that was evaporated in LASIK is removed from slightly deeper in the center of the cornea [draw cross-sectional view diagram], with the lens of tissue delineated by the infra-red laser alone. Then a small tunnel is made to the surface and the tissue is removed through the tunnel. Therefore, it looks like this from the outside [draw front view diagram] with only a small incision visible on the surface, quite different to LASIK.

In SMILE, all of the tissue in front of the lens that was removed has not been cut, whereas in LASIK all the tissue in front of the lens has been cut. Because the front of the cornea is stronger than the back of the cornea, SMILE leaves the strongest part of the cornea intact rather than operating in the strongest part of the cornea as in LASIK. Similarly, most of the nerves in the cornea are in the front part, so SMILE disturbs the corneal nerves less than LASIK.

^{Figure 1: Simple cartoon diagram that is hand-drawn in front of the patient, together with the associated script, to help explain the difference between SMILE and LASIK.}

2.2 Lay explanation of expected outcome and safety

Prospective SMILE patients must also fully understand the possibility that the outcome may not be perfect, both in terms of refractive accuracy and visual acuity and quality. A standard script for describing these concepts also includes the use of a hand-drawn diagram.

While drawing the diagram shown in Figure 2, the explanation to the patient would be:

The overall aim of treatment is to go from here [draw UDVA line], what you currently see without glasses for both distance and near, to here [draw arrow from UDVA line to CVDA line], your best corrected level of vision at distance and near. This is the intended outcome based on all our findings and simulations so far [annotate CVDA line with current visual acuity].

However, there is a possibility that your eyes may heal slightly differently to how we expect, meaning that you don’t end up quite where we planned [CDVA line]. The eyes may settle either slightly over the target or slightly under the target [draw second arrow that ends short of the CDVA line]. Or one eye may be slightly over and the other slightly under the target. In these cases, the vision and visual comfort will not be as good as anticipated. In addition to this, if you get to here [CDVA line] initially, but then slowly drift back to the midway line, known as regression, again the vision will not be as expected. However, as long as glasses alone can still correct the blurring, then there is no damage to the vision, and we may be able to perform an enhancement procedure to maximize your vision to the intended target [draw a short arrow from the midway line to the CDVA line]. We do of course need to let everything settle and stabilize, during which time glasses can be used if needed and after repeating all the measurements, we can do a simple enhancement as long as it is safe to do so.

The other scenario is that if you end up just short of the perfect outcome [draw third arrow to end just short of the CDVA line], but glasses or contact lenses cannot correct your vision, then this would mean that some damage has been caused to the eyes by the surgery—i.e. you would have lost something you had before, some of your best spectacle corrected vision [shade the region between the midway line and CDVA line]. This equates to losing 2 lines on the vision chart [indicate on the ETDRS chart]. The risk of losing 2 lines in your case is <0.1% (<1% for higher prescriptions) [annotate the risk of losing 2 lines CDVA]. It ranges from <0.1% to <1% based on the prescription. However, it is usually possible to correct this last bit with further surgery if necessary.

For patients with high myopia, the risk of over/undercorrection and regression should be emphasized due to the high prescription. The possibility of not being able to perform a retreatment should also be stressed in patients with low predicted tissue reserves.

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^{Figure 2: Simple diagram that is hand-drawn in front of the patient, together with the associated script, to help explain the different possible scenarios of undercorrection, overcorrection, regression, retreatment, and the risk of loss of CDVA.}

2.3 Risks and alternatives discussed with the patient

In addition to the general explanation of the outcome and risks as described in the previous section, the following specific risks should be described in the information pack provided to the patient, and discussed with the patient:

Things that will happen:
- Dry eye symptoms expected to last on average 3-4 months, which will require the use of artificial tear drops or additional lubricants, and may be permanent in rare cases
- Fluctuations in vision due to tear film system instability and the gradual reduction in corneal swelling that causes random blurring as it dissipates, most noticeable in the first month, but may continue for 3-4 months
- Night glare and haloes around lights at night expected to last 3-4 months, more so for higher corrections, and may be permanent in some cases
Possible complications:
- Epithelial erosion
- Epithelial ingrowth / implantation
- Corneal haze
- Diffuse lamellar keratitis (DLK)
- Suction loss
Rare complications:
- Infection
- Retinal detachment
- Persistent corneal swelling
- Vascular occlusion
- Ectasia
- Equipment malfunction
Factors the surgery doesn’t change, but may affect the outcome:
- Cataract development
- Amblyopia (or lazy eye)
- Strabismus (squint)

Depending on the patient, there may be certain risks applicable to that individual, such as for high corrections and the relative contraindications described previously, or to highlight underlying conditions such as amblyopia and strabismus that SMILE surgery will not address. In these cases an additional consent form may be provided by the surgery center.

The alternatives to SMILE must also be discussed with the patient, including:

LASIK
Surface ablation
Phakic IOL
Clear refractive lens exchange

The discussion of alternatives depends largely on the correction to be treated, given the safety profile of intraocular surgery relative to corneal laser refractive surgery. Therefore, additional consent forms can be useful to provide further detail on the relative risks and benefits in patients with high ametropia, where the treatment decision is more balanced.

Chapter 3: SMILE procedure

During SMILE, four interfaces are created within the stroma that delineates a lenticule, as shown in Figure 3. The lenticule of tissue is then removed through a small incision to achieve the desired refractive correction. These interfaces are created from posterior to anterior because once an interface has been created it will obstruct the focus of the femtosecond laser.

Lenticule interface: this interface defines the refractive power of the lenticule and is created from the periphery and moves towards the center.
Lenticule side cut: this interface is created at the border of the lenticule to provide a clearly defined edge. This is also known, and programmed, as the minimum lenticule thickness.
Cap interface: this interface is created parallel to the corneal surface to delineate the upper surface of the lenticule, and extend further peripherally to provide access for the surgeon. The cap interface is created from the center towards the periphery.
Small incision(s): this interface creates a tunnel to link the cap interface to the corneal surface, created at the outside edge of the cap.

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^{Figure 3: Series of diagrams showing the femtosecond cutting sequence for a SMILE procedure; 1) lenticule interface from out-to-in, 2) lenticule side cut, 3) cap interface from in-to-out, and 4) the small incision(s). Reprinted with permission from Reinstein DZ, Archer TJ, Carp GI. The Surgeon’s Guide to Small Incision Lenticule Extraction (SMILE). Thorofare, New Jersey: SLACK Incorporated, 2018.}

3.1 Routine docking process

The docking process is an extremely important part of the procedure as this is what defines the treatment centration.

Centration in SMILE is based on the patient essentially auto-centrating by looking at the fixation light. During the docking process, the surgeon needs to verify that centration is on the visual axis, using the position of the reflex from the green fixation light as a reference in relation to the pupil. The aim is to center the treatment as close to the visual axis as possible.

The docking process is performed with the surgeon observing through the treatment microscope. Femtosecond cutting is started as soon as all final checks have been completed. If suction has been applied and consider unsatisfactory, you can simply turn off the suction and start the centration process again.

A matched-control study, using tangential topography difference maps, and found no statistically significant difference in centration between 100 SMILE eyes and 100 MEL 90 LASIK eyes (p=0.082); mean±SD centration offset was 0.20±0.11 mm (range: 0.00 to 0.50 mm) for the SMILE group and 0.17±0.10 mm (range: 0.00 to 0.53 mm) for the LASIK group.¹⁵ Similar results were found in other studies.^16-18

3.2 Accounting for cyclotorsion

The VisuMax does not currently include a software adjustment for cyclotorsion, but this can be done manually based on corneal marks obtained before surgery with the patient in the sitting position. Once suction has been applied and centration has been deemed satisfactory, the surgeon can then compare the location of the marks on the horizontal corneal diameter to the cross-hairs of the reticle on the planning screen. If a rotation is required, the surgeon can hold the contact glass and rotate it until the marks are aligned; the cornea will be rotated in turn due to the suction.

Once suction has been applied, the eye is immobilized so there is no requirement for dynamic cyclotorsion control as in LASIK. Results using this manual rotation technique have been published by Ganesh et al¹⁹ showing an improvement compared to other studies that have reported astigmatism outcomes for SMILE. A recent advance is to use three marks—two on the horizontal and a third on the vertical—as has been published by Jun et al.²⁰

3.3 Femtosecond Laser Cutting (photodisruption)

The femtosecond cutting is the most critical part of the procedure. While a suction loss rarely leads to the patient not receiving a complete treatment, it is obviously better if these can be avoided. The photodisruption process is monitored through the surgical microscope, with the microscope focused at the plane of the femtosecond bubble formation within the cornea.

3.4 Surgical Technique

After observing the completion of the femtosecond cutting, the cap and lenticule can be manually separated and the lenticule can be extracted. Many surgeons have personal preferences to technique.^21,22 The following protocol describes the bimanual surgical technique was developed at London Vision Clinic (Reinstein, Carp) and Tilganga Institute of Ophthalmology (Pradhan) through our collective experience of over 7,500 procedures. Essentially the same technique can be used irrespective of the location of the small incision, for example using the superior small incision as is required for SMILE in the US. The routine procedure will not be described in detail but can be seen in Figure 4.

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^{Figure 4: A series of images chronicling each step involved in a routine SMILE procedure. Reprinted with permission from Reinstein DZ, Archer TJ, Carp GI. The Surgeon’s Guide to Small Incision Lenticule Extraction (SMILE). Thorofare, New Jersey: SLACK Incorporated, 2018.}

Chapter 4: Postoperative Management and Complications

The postoperative management after SMILE is essentially the same as after LASIK. The patient should be started on a prophylactic medication regime that includes an antibiotic and steroid. After the day one appointment, our routine postoperative follow-up schedule is to see the patient at 1 month, 3 months and 1 year. Thereafter, patients should attend regular eye examinations every 1-2 years as normal.

Many of the postoperative complications are the same as found in LASIK and can be managed the same way and so will not be described here. These include epithelial defects, anterior basement membrane dystrophy, interface haze, interface debris, infection, and interface fluid syndrome. As with LASIK, dry eye symptoms are an expected side effect after SMILE albeit less severe and usually faster to recover. Additionally, epithelial ingrowth and diffuse lamellar keratitis (DLK) are also possible postoperative complications of SMILE. In many respects, the presentation and management of these two entities is equivalent between the two procedures. However, there are some differences to what is typically found after LASIK which are important to highlight. These are below.

4.1 Bowman’s microdistortions / cap folds

Management of the cap starts during the initial slit-lamp evaluation directly after the procedure. The cap should be smoothed using heavy fluorescein staining to aid visualization of even the subtlest annular or linear nanofolds by evaluation of the negative staining pattern. This slit-lamp technique can be repeated if necessary at the day one examination

Cap irregularities have been studied in detail by Prof Zhou’s group in Shanghai^23,24 who describe them as microdistortions in Bowman’s layer. In their first study, such microdistortions were observed on OCT in 88% of eyes the day after SMILE, although in the absence of clinically significant corneal striae.²³ The number of microdistortions was significantly reduced at 1 week and remained stable thereafter. The visual acuity was not affected by the presence of microdistortions. These results were supported by their second study which showed again that the number of microdistortions significantly reduced between 1 day and 1 month.²⁴ They also found the higher the myopia the greater the distortion, which fits with the hypothesis that microdistortions are due to the mismatch in length between the cap and bed.

In a study investigating the efficacy of performing intraoperative cap repositioning, Shetty et al²⁵ reported the incidence of microdistortions on Bowman’s layer was reduced from 60% if no cap repositioning was performed, down to 21% in the repositioned group. This was associated with a statistically significant difference in quality of vision measures by HD Analyzer.

4.2 Atypical DLK: Interface focal inflammatory keratitis after SMILE

DLK can occur after SMILE with the same classical presentation as after LASIK and the management is the same.²⁶ Sterile lamellar inflammatory infiltration can also present after SMILE with a unique appearance not typically seen after LASIK, appearing as focal infiltrates scattered throughout the interface with or without a diffuse component (Figure 5). This presentation is usually seen on day one after SMILE and can be alarming given the focal appearance, so the index of suspicion for infection needs to be very high and these cases need extremely close monitoring during the perioperative period. While the appearance is that of a multifocal infectious process, which if presenting 2-3 weeks later would need to include fungal and atypical mycobacteria in the differential, it is conceivable that staphylococcus and streptococcus could produce rapid multi-colony growth of this kind overnight, even though topical steroids have been used on the cornea. However, it is more likely to be a sterile inflammation such as staph marginal type from eyelid margin disease, from endotoxin or chemicals on the instruments, or due to autoimmune causes.

As described in a recent case report,²⁷ the first time we saw this, we had to rule out every possibility, so Gram stain and cultures of the infiltrates were performed and sent for analysis. A comprehensive interface washout and scraping of the infiltrates was also performed. All of the culture results were returned as negative—no growth—and the patient responded to the washout and topical treatment. Since this early experience we have seen 5 other cases with a similar presentation and now manage this similar to a classical DLK case. However, focal infiltrates are still more likely to cause localized melting, thus the threshold for performing an interface washout should be much lower compared with classical DLK.

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^{Figure 5: Slit-lamp photographs of focal DLK, or focal keratitis, one day after SMILE. The infiltrates were dense and scattered diffusely throughout the cornea. Reprinted with permission from Reinstein DZ, Archer TJ, Carp GI. The Surgeon’s Guide to Small Incision Lenticule Extraction (SMILE). Thorofare, New Jersey: SLACK Incorporated, 2018.}

4.3 Epithelial implant

Epithelial ingrowth after SMILE can sometimes present as an ‘implant’ rather than ‘ingrowth’, caused by stray epithelial cells being drawn into the interface by the surgical instrument. If the cells are affecting subjective or objective vision or affecting topography a washout by the surgeon may be needed.

Finally, there are a number of case reports in the literature discussing ectasia after SMILE.^28-31Just as for LASIK, keratoconus screening is the main focus of the suitability assessment for SMILE; keratoconus has conventionally been a contraindication for any elective refractive procedure that involves the removal of stromal tissue, including SMILE. It is important to remember that SMILE will not eliminate the risk of ectasia. Indeed, we should expect some cases of ectasia after SMILE, just as there are after PRK,^32,33 until we have a 100% fool-proof system for detecting keratoconus. For mild keratoconus, similar to combining PRK with CXL, some groups have been investigating combining SMILE and CXL. In the study by Graue-Hernandez et al,³⁴ refractive stability and safety were demonstrated over two-year follow-up in a group of 15 eyes treated with SMILE and CXL. Ganesh et al³⁵ have also reported promising results of SMILE with CXL in patients with thin corneas and borderline topography.

Conclusion

Refractive surgery patients have increasingly higher expectations, as they should, when it comes to their eyes. It is no longer enough to provide patients with great vision; we must also provide them with an experience. An experience that they will remember fondly and share with friends and family. That experience comes from a compilation of their preoperative, surgical, and postoperative journey. For the patient and all healthcare providers, from the optometrist to surgeon, that journey should be one of ease. SMILE helps each step of the way.

Preoperatively, having the discussion about an option that is flapless and bladeless and that takes less than 5 minutes to complete will relax patients and open doors for those that might have been too nervous to take the first step in the past.

Intraoperatively, the VisuMax femtosecond laser system is second to none and one of the key elements in creating a pleasant surgical experience. The primary reason is the complete lack of any sensation while the eye is immobilized under “suction.” The VisuMax uses a unique curved contact glass that requires very low energy to stabilize the eye during treatment. This gentle vacuum is much different from the high power systems that are used by the rest of the femtosecond lasers on the market. Of course, the VisuMax is also currently the only femtosecond laser that is approved to perform SMILE.

Postoperatively, one of biggest advantages of SMILE from a patient experience standpoint is the marked reduction in amount and duration of impaired corneal sensitivity.³⁶ Just as with LASIK, most patients also see very well the day after treatment. Another benefit for both the patient and co-managing doctor is the fact that there is no flap. No flap means no worries about eye rubbing, flap dislocations, and patients can return to normal activities more quickly.

SMILE, and its unique characteristics, combined with the accuracy and reproducibility of the VisuMax³⁷ makes it the ideal procedure for many patients.

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