Advantages and Disadvantages of Different Cap Thicknesses

Fig. 12.1
Anterior segment photography of an eye 3 months post 130 μm SMILE procedure. Anterior segment optical coherence tomography and Optical Quality Analysis System (OQAS) (Visometrics SL, Terrassa, Barcelona) of the same eye


Fig. 12.2
Orbscan (Orbtek Inc./Bausch & Lomb, Tampa, FL, USA) corneal topography and OQAS of one left eye 3 months post 140 μm SMILE procedure


Fig. 12.3
Orbscan (Orbtek Inc./Bausch & Lomb, Tampa, FL, USA) corneal topography of one right eye pre- and 3 months post 160 μm procedure

In some cases, we had more difficulties in dissecting the stroma as deeper was the cut in some cases but not in all of them. That is why we cannot make a significant difference also in this point.

No complications were observed during the surgery and the immediate postoperative period.

12.6 Discussion

SMILE has good refractive outcomes and other benefits such as reduction in scatter. This procedure induces fewer higher-order aberrations and exerts a minimal negative impact on the quality of the retinal image leading to good vision [36]. Moreover, it is less invasive and there is a reduction in postoperative irritation, less loss of corneal sensitivity, less inflammation, and less impact on tear production, because the short incision cuts fewer corneal nerves (see Chaps. 2 and 3). In addition, tissue extraction is more precise and repeatable regardless of the treatment prescription. As no flap is created, there is a potential better postoperative biomechanical stability and a lesser risk of secondary corneal ectasia, due to the preservation of the anterior corneal stromal layer, which in turn preserves corneal resistance (see Chap. 13) [79].

Dry eye is a common complaint among patients who have undergone refractive surgery, including laser in situ keratomileusis (LASIK), photorefractive keratectomy, and femtosecond LASIK whereby and the incidence of dry eye varies among these different patients [711]. It has been reported that patients who develop dry eye after refractive surgery also have elevated risks of developing subsequent refractive regression and ocular surface damage. In addition, refractive surgery procedures also interrupt the normal organization and regeneration of the corneal nerves, which in turn lead to a prolonged reduction in corneal sensation (see Chap. 3).

A recent study added an important issue that surgery could result in dry eye symptoms, tear film instability, and decreased corneal sensitivity. Furthermore, SMILE has superiority over Femto-LASIK in lower risk of postoperative corneal fluorescein staining and less reduction of corneal sensation [12].

As no flap is created with the SMILE technique, there is theoretically a higher postoperative biomechanical stability and a lower risk of secondary corneal ectasia due to the anatomical preservation of the anterior corneal stromal layer. The treatment is carried out under Bowman’s layer, which creates greater corneal resistance. Several authors argue that – from the biomechanical point of view – in a cornea operated on by LASIK surgery, the useful corneal thickness is that of the residual stromal bed, as the flap thickness is not important when it comes to maintaining the corneal structure: corneal weakening is greater when the residual stromal thickness is thinner (the recommended residual stromal thickness is 250–300 μm) [1214]. With the SMILE technique, this concept changes: because of the preservation of Bowman’s layer, a greater lenticule depth (= a thicker cap) shell go along with more corneal resistance [13, 14].

Conversely, it is possible that being deeper leads to a lower precision of the femtosecond laser cut diminishing the refractive predictability of the procedure. To overcome this possible energy loss and following discussions with the company, we decided to increase 3 % of the spherical equivalent correction for every 10 μm of higher depth in relation to the 130 μm cap. That regularly meant that for all of our 160 μm depth SMILE we were correcting 10 % higher.

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May 26, 2017 | Posted by in OPHTHALMOLOGY | Comments Off on Advantages and Disadvantages of Different Cap Thicknesses

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