Introduction
Small-incision lenticule extraction (SMILE) is highly appealing to patients and surgeons alike because of its minimally invasive and virtually painless nature. However, it is not free from potential complications.
In this chapter, we discuss advantages and limitations of SMILE to provide an understanding of the genesis of possible complications. We then propose some strategies to avoid complications in the first place and to manage them if encountered despite due diligence.
Theoretical Advantages and Limitations of SMILE
The basic principle of SMILE is to reshape the anterior corneal surface by subtracting (i.e., removing) stromal tissue from the center of the cornea. The same applies to laser in situ keratomileusis (LASIK); therefore most complications of SMILE are similar to those of LASIK in signs, symptoms, and treatment. In SMILE, as opposed to LASIK, a corneal flap is not created. Instead, a small incision is made in the mid-periphery of the cornea with a femtosecond (FS) laser, and the lenticule is removed through this self-sealing incision.
This “keyhole procedure” is certainly more demanding in terms of surgical dexterity than performing photorefractive keratectomy (PRK) or LASIK and a steep learning curve is to be expected when starting SMILE. Therefore we strongly advise not performing the SMILE procedure before getting enough experience doing LASIK with the VisuMax II laser (Carl Zeiss Meditec). The minimally invasive technique of SMILE makes the treatment of some complications more challenging than after LASIK for the inexperienced surgeon. We therefore also recommend getting comfortable with the management of LASIK complications first. On the other hand, due to the same minimally invasive technique of SMILE, some of these complications are expected to be less frequent in the hands of an experienced surgeon.
Advantages
The most obvious advantage of SMILE compared to LASIK is the absence of a flap. As early as on the first day after uncomplicated SMILE, patients can resume every one of their daily activities, including water sports and putting on make-up, without the risk of traumatic flap dislocation—which may occur even several years after LASIK. When in SMILE a side cut of a few millimeters of length is created, fewer corneal nerves are severed than with a LASIK flap, leading to less impaired corneal sensitivity. This is thought to be the main reason why dry-eye symptoms after SMILE are less pronounced and shorter in duration than after LASIK.
The SMILE procedure has additional potential advantages, because no corneal flap is created: SMILE may pose less risk for postsurgical ectasia than refractive lenticule extraction (ReLEx) or LASIK. Several theoretical models postulate a greater tectonic stability after SMILE than after flap procedures. However, these mathematical models are based on certain assumptions that are difficult to verify at present; the real advantage of SMILE may be smaller than calculated based on these assumptions. Only time will tell the real incidence of ectasia after SMILE. To date, there are few reports of ectasia—all of these cases were also not suitable for LASIK (mostly owing to irregular topography). At present, at least for the novel SMILE surgeon, it seems prudent to apply the same inclusion and exclusion criteria as for LASIK. In the future, these criteria may change once the following question is satisfactorily answered: At what depth should the lenticule be extracted? It is generally accepted that the anterior stroma is biomechanically stronger than the posterior stroma and that the LASIK flap does not contribute significantly to the tectonic stability, making a thinner flap more desirable than a thicker one. The opposite is being discussed in SMILE: extracting the lenticule at a deeper level may be beneficial in terms of ectasia risk although a thinner residual stromal bed is left behind.
In SMILE, the crucial step for the refractive result, the laser application, takes place inside the cornea, that is, in a closed system. Therefore in contrast to excimer-laser ablative surgery, SMILE is not influenced by external factors, such as room temperature and humidity or the water content of the cornea. In other words, in LASIK, dehydration of the flap may lead to striae formation and dehydration of the stroma may cause an inconstant ablation rate in higher corrections or prolonged flap-open time, all affecting the visual outcome. In addition, in LASIK, eye movements may change the angle of incidence of the laser beam on the cornea during laser ablation, also contributing to the induction of higher-order aberrations. In contrast, in SMILE, the eye is fixated and the cornea is normally hydrated during laser application. These principal differences may explain why, according to the published literature, SMILE provides equal or better results compared to excimer-laser ablations for the correction of myopia regarding predictability and long-term stability of the attempted correction and postoperative visual quality.
These conceptual benefits and the superior results that we actually yielded in our clinical application have made SMILE our method of choice for the corneal correction of myopia and myopic astigmatism.
Limitations
SMILE for hyperopic and mixed astigmatic treatments has been performed in animal models and clinical studies but currently is not routinely available. The geometry of the tissue to be removed in a hyperopic treatment differs substantially from the lenticule extracted in a myopic SMILE. In hyperopia, the subtracted tissue is thinner in the center and the rim but thicker in the mid-periphery, resembling a donut. This requires even more surgical skill than removing a myopic lenticule, which is thickest in the center. Hence, some complications are expected to occur more frequently, including suction loss, tearing of the tissue, dry-eye issues, and more.
Furthermore, compared to excimer-laser procedures, which have been refined for almost 3 decades, SMILE is still evolving. At present, there is no iris recognition confirming the correct matching of treatment plan and eye before starting the treatment. Thus the potentially very severe complication of treating an eye with data belonging to another eye, thus aggravating the refractive error beyond the limits that can be corrected with laser surgery, must be ruled out by careful checking. We recommend doing a team time-out. Currently, there is no rotational eye-tracking compensating for less than optimal cyclotorsional alignment before fixating the eye. Last, it is not possible to adjust centration after docking. However, as these technologies already exist in other lasers, their implementation into a SMILE laser should be possible.
How to Avoid Complications
The best complications are those avoided. Therefore the treatment procedure is described step by step, including some pearls to avoid typical caveats. These recommendations are given to the best of our current knowledge and abilities and represent one, but certainly not the only, way to perform SMILE. With ongoing technical improvements and a growing body of scientific evidence, changes in these recommendations seem inevitable.
- 1.
In high astigmatic corrections, you may want to mark the cornea in a sitting position to manually compensate any cyclotorsional eye movements before docking to avoid an astigmatic undercorrection.
- 2.
Make sure to enter the correct treatment data and select the correct patient/eye/procedure (preferably with a team time-out to rule out an inappropriate treatment.
- 3.
Select the desired mode (Standard, Fast, Expert). Choosing Expert mode allows for determining spot and track spacing separately for each cut type (e.g., flap cut, side cut), pulse energy, and more. Another bonus of the Expert mode is that a more detailed report of the procedure is created and saved by the laser.
- 4.
Carefully instruct patients as to what they are going to experience during the procedure and what they are expected to do before actually starting in order to ensure best cooperation:
- a.
“Look at the blinking green light” (allowing us to center on the line of sight).
- b.
“There will be no pain but you will feel some pressure while your eye is fixated and the laser is applied.”
- c.
“Your vision will get foggy starting from the periphery, the green light may appear to move; don’t try to follow it.”
- d.
“Don’t move and don’t speak while suction is on.”
- a.
- 5.
Apply anesthetic eye drops right before the next steps. It is imperative that they are unpreserved because preservatives damage the epithelium, thus facilitating abrasions and epithelial nests ( Fig. 17.1 , ) in the interface.
Fig. 17.1
Epithelial cell nest within interface (1.5 years postoperatively).
- 6.
We then drape the eye in a sterile fashion; other surgeons prefer not to bother the patient in order not to compromise cooperation. These surgeons often apply the laser in both eyes before draping the eye and removing the lenticule later.
- 7.
We insert a speculum with suction and rinse the ocular surface with a sterile balanced salt solution; others use a wet sponge to clean the cornea. We check through the microscope that the ocular surface is free from eyelashes, other particles, and tear-film residues to avoid problems obtaining and maintaining suction as well as obstruction of the laser beam, leading to less than optimal tissue separation.
- 8.
The docking procedure should be performed swiftly with the patient fixating on the green target. If centration is not perfect, suction should be released. If need be, the ocular surface may be cleaned or moistened before suction is applied again.
Difficulties in Obtaining Suction
In deep-set eyes or patients with a big nose or small palpebral fissure, it may be difficult to sufficiently applanate the cornea with the conical patient interface to build up proper suction. In these cases, the cooperation of the patient is needed to exactly tilt the head to an optimal position while the fixation of the blinking target must not be compromised.
It is mandatory to obtain proper centration first before starting the laser to avoid a decentered treatment. It may occur with a patient unable to fixate the green blinking target, usually because of anxiety. It is crucial to make the patient as comfortable as possible before and during surgery to obtain the patient’s cooperation.
Preparation for Lenticule Extraction
Start the laser and observe the progress of the bubble layer formation through the microscope. Very dense (opaque) areas may be due to a pulse energy setting that is too high; “black” areas without noticeable stain may be due to a pulse energy that is too low ( Fig. 17.2 ). Both may be indicative of difficult tissue separation. Therefore these areas should be attacked first during manual separation while the surrounding tissue is still providing stability to avoid lenticule tears and other problems.
We prefer to extract the lenticule directly after the laser treatment because the visibility of the cutting planes quickly deteriorates as the gas bubble layers dissolve within minutes, theoretically increasing the risk of a via falsa preparation.
Experienced surgeons may be able to perform a lenticule separation with a wide range of spatula-like instruments. However, for the surgeon starting with SMILE, we recommend a dedicated SMILE instrument ( Fig. 17.3 , ; 6-836-1 SMILE Double Ended Dissector with Taneri spoon tip, Duckworth & Kent). This instrument has a semi-sharp spoon-shaped tip that facilitates lenticule dissection in every direction, including reverse movements.
Lenticule Dissection and Extraction
This part of the procedure may be the most demanding for the surgeon and the least comfortable for the patient. Luckily, there is no need to rush as the cornea may still be considered closed and the refractive result will not be compromised as long as the lenticule is completely extracted in the end.
- •
Open the side cut gently with the blunt rounded tip of the SMILE instrument. Make sure that the opening is long enough to minimize the risk of traumatic enlargements (cap tears). We recommend 5 mm for the first cases.
- •
Identify the anterior and posterior plane of the lenticule with the same tip. We recommend opening the entrance to the anterior plane over the entire length of the side cut. Then, point the tip slightly downward to enter the lower plane about half the size of the side cut. Some resistance together with a visible lenticule edge above the tip are signs that the instrument has entered the posterior plane. This step is easier if the cap diameter exceeds the lenticule diameter by at least 1 mm.
- •
Dissecting the anterior (upper) plane first is much easier than peeling the lenticule from the relatively mobile cap and avoids many complications, such as cap tears, cap perforation, and ruptured lenticules.
- •
Dissect the posterior plane. We prefer to stabilize the globe during dissection using suitable forceps. Some surgeons simply instruct the patient to look into the bright light of the microscope, thus avoiding the discomfort of grasping the eye with forceps.
- •
Extract the lenticule using suitable forceps. Only attempt to extract the lenticule after full separation to avoid a torn lenticule. To this end, it is advisable to make a far-reaching circular movement with the dissecting instrument anterior and posterior of the lenticule to ensure complete separation.
Flushing the Interface
The interface may be flushed after lenticule extraction to minimize the risk of remaining debris, thus reducing the risk of diffuse lamellar keratitis (DLK) and potentially also that of microdistorsions of the Bowman layer. On the other hand, flushing may lead to more postoperative edema, causing a slower visual recovery. We are currently involved in a multicenter study to better evaluate both options.
Difficulties in Obtaining Suction
In deep-set eyes or patients with a big nose or small palpebral fissure, it may be difficult to sufficiently applanate the cornea with the conical patient interface to build up proper suction. In these cases, the cooperation of the patient is needed to exactly tilt the head to an optimal position while the fixation of the blinking target must not be compromised.
It is mandatory to obtain proper centration first before starting the laser to avoid a decentered treatment. It may occur with a patient unable to fixate the green blinking target, usually because of anxiety. It is crucial to make the patient as comfortable as possible before and during surgery to obtain the patient’s cooperation.
Preparation for Lenticule Extraction
Start the laser and observe the progress of the bubble layer formation through the microscope. Very dense (opaque) areas may be due to a pulse energy setting that is too high; “black” areas without noticeable stain may be due to a pulse energy that is too low ( Fig. 17.2 ). Both may be indicative of difficult tissue separation. Therefore these areas should be attacked first during manual separation while the surrounding tissue is still providing stability to avoid lenticule tears and other problems.
