In SMILE the anterior-most stromal lamellae remain intact postoperatively. Here, the author discusses the biomechanical advantages of preserving the stronger anterior stroma and the resulting effects on spherical aberration control and optical quality.

Improved optical quality with SMILE

SMILE shows a lesser reduction in corneal tensile strength than LASIK and improves spherical aberration control

By Prof. Dan Z. Reinstein, MD, MA(Cantab), FRCSC, DABO, FRCOphth, FEBO

The introduction of small incision lenticule extraction (SMILE), a flapless form of keratomileusis, has changed the face of corneal refractive surgery in a number of ways, none more so than in corneal biomechanics and hence spherical aberration and optical quality.

In SMILE, the absence of a flap and the direct stromal lenticule extraction from within the body of the stroma through a 2-mm keyhole incision means that the anterior-most stromal lamellae remain intact postoperatively, except in the region of the small incision. This provides a biomechanical advantage as the anterior corneal stroma is known to be the strongest part of the stroma. For example, Randleman *et al.*^{1} measured the tensile strength of strips of stromal lamellae cut from different depths within the cornea and found a strong negative correlation between stromal depth and tensile strength. The anterior 40% of the central corneal stroma was the strongest region, whereas the posterior 60% of the stroma was weaker by at least 50%. Similar results for the nonlinear nature of stromal tensile strength have been reported by Scarcelli *et al.* using Brillouin microscopy.^{2}

In comparison, the anterior stromal lamellae are severed during laser in-situ keratomileusis (LASIK) by the creation of the flap and the excimer laser ablation, and by the excimer laser ablation during photorefractive keratectomy (PRK). Therefore, there is a lesser reduction in corneal tensile strength with SMILE than with LASIK and PRK.

Surgeons are accustomed to calculating the residual stromal thickness in LASIK as the amount of stromal tissue left under the flap; therefore, the first instinct is to apply this rule to SMILE. However, the actual residual stromal thickness in this procedure should be calculated as the total uncut stroma (i.e., the stroma above and below the lenticule). Given the decreasing strength of stroma with depth, a more crucial factor than residual stromal thickness is tensile strength. In order to consider this, we have developed a postoperative total tensile strength (PTTS) calculator^{3} based on Randleman’s data.^{1} In an example treatment removing 100 µm of stroma via ablation (LASIK or PRK) or as a lenticule (SMILE) from a 550-µm-thick cornea, the model calculated the PTTS to be 75% after SMILE performed with a 130-µm cap, 68% for PRK and 54% for a thin-flap (100 µm) LASIK procedure.

“...we can safely use SMILE to achieve an acceptable postoperative tensile strength.”

We can take advantage of this postoperative biomechanical difference by better controlling the induction of spherical aberration to improve optical quality. To investigate this, we analysed the PTTS after myopic correction with SMILE and LASIK in cohorts of 96 eyes each, matched by sphere (±0.25 D), cylinder (±0.25 D) and pachymetry (±20 microns). Mean values for spherical equivalence (SEQ), cylinder and pachymetry in both groups were approximately –4.83 D, –0.56 D and 540 µm, respectively.

The mean ± SD thickness of the cap in the SMILE group was 130 ± 6 µm and the LASIK group had a mean flap thickness of 96 ± 12 µm. Mean optical zone diameters were 6.7 ± 0.39 mm for SMILE and 6.08 ± 0.22 mm for LASIK. Mean lenticule thickness was 107 µm (range: 72–149 µm) for SMILE while the LASIK group had a mean ablation depth of 81 µm (range: 25–134 µm).

Mean PTTS was 73% (range: 65–82%) in the SMILE group and 57% (range: 45–72%) in the LASIK group. Across the entire range of myopia treated (up to –8.00 D), PTTS was about 16% greater on average in the SMILE eyes compared with the LASIK group.

Analyses of higher order aberration data found that SMILE induced significantly less spherical aberration than LASIK. Mean change from baseline spherical aberration was 0.11 ± 0.16 µm in the SMILE eyes and 0.31 ± 0.12 µm after LASIK (p<0.01).

(Figure 1) Predicted postoperative total tensile strength (PTTS) for a population of SMILE cases and a population of LASIK cases matched for sphere, cylinder and pachymetry.

Therefore, we found that spherical aberration could be controlled better in SMILE than in LASIK because we were able to increase the optical zone. Even though this meant greater tissue removal, the PTTS was still much higher in SMILE compared with LASIK. For example, in an eye with central pachymetry of 588 μm, a 7-mm zone was used to treat –10.00 D (203 μm tissue removal) with a 135-μm cap thickness. The stromal thickness under the lenticule was 250 μm, but the total uncut stromal thickness was 335 μm, which represents a postoperative tensile strength of 58%. In comparison, a –10.00 D LASIK treatment in which a 6-mm optical zone was used with a 100-μm flap left a residual stromal thickness of 298 μm, representing a postoperative tensile strength of 44%. The spherical aberration induced was 0.15 μm in the SMILE case and 0.75 μm in the LASIK case.

(Figure 2) Change in spherical aberration for the matched SMILE and LASIK populations.

This example demonstrates how we can safely use SMILE to achieve an acceptable postoperative tensile strength. For an eye with pachymetry of 588 μm, we would be able to treat –16.00 D in a 7-mm zone, leaving 250 μm of total uncut stroma (but only 168 μm under the lenticule), and a postoperative tensile strength of 44% - a level that has been considered safe in LASIK for two decades. The predicted spherical aberration induction for this correction would be only 0.39 μm, an improvement on the –10.00 D LASIK treatment.

Finally, there is the possibility of treating thinner corneas with SMILE. For example, the same –10.00 D correction in a 7-mm zone could be performed in a 490 μm cornea, leaving 236 μm total uncut stroma (154 μm under the lenticule) and retaining a postoperative tensile strength of 53%.

The superior tensile strength provided by preserving the stronger anterior stroma in SMILE allows larger optical zones, thus improving spherical aberration control and hence optical quality. This improvement is achieved while reducing the corneal tensile strength less than LASIK.

References

1. J.B. Randleman *et al., J. Refract. Surg.*, 2008;**24**:S85–S89.

2. G. Scarcelli, R. Pineda and S.H. Yun, *Invest. Ophthalmol. Vis. Sci.*, 2012;**53**:185–190.

3. D.Z. Reinstein, T.J. Archer and J.B. Randleman, *J. Refract. Surg.*, 2013;**29**:454–460.

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