Surgical instruments for small incision lenticule extraction (SMILE)

KEYWORDS:

• Surgical instruments
• small incision lenticule extraction

During the last 4 years, small incision lenticule extraction (SMILE) has become clinically available in Europe and Asia as an alternative to laser-assisted in situ keratomileusis (LASIK) for the correction of myopia [1]. In the SMILE procedure, a femtosecond laser (Visumax, Carl Zeiss Meditec) is used to create an intrastromal lenticule with a 2–4 mm peripheral arcuate incision. Following lenticule creation, removal of the lenticule is performed by blunt separation of the anterior surface of the lenticule followed by the posterior surface. The lenticule is then grasped and removed directly through a small peripheral incision (https://youtu.be/8DG-1x3dC08) [2]. It has been shown that the visual and refractive efficacy, safety and predictability profiles of SMILE for myopia or myopic astigmatism are good, and patient satisfaction is high [3].

Compared to LASIK, SMILE is more technically demanding and with a different learning curve, as the procedure has to be performed through a small incision. It can potentially therefore be more technically challenging in patients with low myopia corrections, as the lenticule thickness to be extracted is thinner than in high refractive corrections. Animal studies have shown that the post-operative inflammation following SMILE mainly comes from the surgical manipulation during the procedure, rather than from the femtosecond laser energy [4,5], and surgical experience of the surgeon can also effects the post-operative inflammation [2]. Hence, optimal instrumentation for SMILE should minimize the surgical manipulation in the interface and provide a smooth dissection plane.

Following lenticule removal, studies have documented a delay in the visual recovery and subjective symptoms of visual fluctuations and episodes of blurring of vision in the early post-operative period after SMILE compared to LASIK [6–8]. This might be due to: (i) suboptimal laser photo-disruption of the stromal fibers that subsequently causes interface scattering due to traumatic dissection [9] – this can be rectified by optimization of the laser settings, (ii) micro-distortions in Bowman’s layer following lenticule removal [10], and (iii) transient increased stromal keratocyte activity at the interface resulting from the stromal pocket irrigation [11]. Hence, smooth, atraumatic dissection, of the stromal lenticule and sweeping of the anterior cap both to disperse the microdistortions to the periphery, and also to aid in drainage of the pocket fluid, can enhance the postoperative visual recovery.

My instrument of choice to perform the dissection is the ASICO SMILE dissector (ASICO, catalog number AE-2403, http://www.asico.com/products /marker/corneal/mehta-smile-pocket-dissector.html#). The instrument is double ended with an elongated Sinskey-style hook on one side that allows delineation of the anterior and posterior planes of the lenticule. This will make subsequent dissection and removal easier. The hook is 0.75 mm in length to compensate for a smaller optical zone size of 6 mm if used instead of the standard 6.5 mm. Following delineation, one must perform lenticule dissection. A dissector is required to break the stromal microbridges between the lenticule and the stromal cap and those between the lenticule and stromal bed, to make the lenticule freely extractable. The importance of a smooth dissection cannot be over emphasized as the interface quality is associated with immediate postoperative visual acuity, contrast sensitivity and higher order aberration induction [12]. We have previously shown that different dissectors can cause a wide variation in smoothness of the interface [13]. My instrument of choice (ASICO, catalog number AE-2403) has a thin shaft to reduce risk of incisional extension especially in small incision surgery (e.g. 2 mm), appropriate shaft angulation to fit corneal curvature, and a blunt, highly polished dissecting tip and edge to allow smooth separation of the correct plane. Following dissection of lenticule, a microforceps is used to extract the intrastromal lenticule from the pocket. Some surgeons use a microforceps that has a squeeze handle, like Shah forceps (Segal Optics, catalog number 1286.C305), or Tan Endoglide forceps (AngioTech/Network Medical Products, catalog number 53–951), to improve fixation of the tissue. My preferred forceps of choice is the latter as the forceps are a smaller gauge so there is a reduced risk of peripheral incisional tear, especially if the patient unintentionally moves.

However, in cases where the surgeon inadvertently dissects the posterior surface first (more common in cases of low myopia correction), identification of the anterior surface and subsequent removal becomes difficult as the anterior surface of the lenticule is compacted against the anterior stromal surface [14] and the edge of the lenticule is not visualized easily, to be grasped by forceps. In such cases a lenticule stripper can be inserted into the plane between the posterior surface of the lenticule and underlying stroma, and then can ‘hook’ or pull out the lenticule directly (ASICO catalog number AE-2404, and http://journals.plos.org/plosone/article?id =10.1371/journal.pone. 0113774) [14]. This will allow complete removal of the lenticule which is important, as any remnant of the intrastromal lenticule may lead to visual sequelae, such as induced irregular astigmatism [15]. When using the lenticule stripper, surgeons should pay attention to the angle between the stripper tip and the posterior surface of the lenticule to avoid inadvertent damage of the interface.

Reported perioperative complications include; epithelial abrasions at the incision, difficulty in removing the lenticule, tears at the margins of the incision, suction loss, central abrasion, and cap perforation [1]. The management of suction loss depends on the surgical step at which it occurs [16] but can be associated with delayed visual recovery and increased inflammation [1]. However, the commonest perioperative complication is epithelial damage at the incision site. The risk is, of accidental epithelial implantation, into the pocket, that occurs in approximately 0.7% of cases [17]. In cases where the epithelium is advancing toward the visual axis, it can be associated with irregular astigmatism, or trigger corneal melting, hence the epithelium should be removed. My instrument of choice is the epithelial remover (ASICO, catalog number AE-2404) which can facilitate scraping of the epithelium from the underside of the anterior cap as well as from the stromal bed. To use the instrument correctly, surgeons must pass the instrument beyond the area of epithelial implantation and also avoid expanding the epithelium into the pocket, and then sweep the epithelium toward the main incision (https://youtu.be/d7xw2F1QYnQ).

As SMILE is a relatively new procedure, it is expected that new instruments will be required that can help surgeons overcome the different surgical skills required to perform this procedure and manage intra-operative and post-operative complications.

Declaration of interests

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.