Small Incision Lenticule Extraction (SMILE): Anterior Stromal Preservation, Corneal Biomechanics & Optical Quality

Biomechanical studies demonstrate that tensile strength is not uniform across corneal depth. The anterior $40\%$ of the stroma ($150\text{--}200\ \mu\text{m}$) contains densely interwoven, branching collagen lamellae that contribute over $70\%$ of total corneal structural rigidity:

$$\sigma_{\text{tensile}}(z) \propto e^{-k \cdot z} \implies \text{Anterior Stroma is } \mathbf{2\times \text{ Stronger than Posterior Stroma}}$$

In LASIK, cutting a $110\ \mu\text{m}$ flap severs these crucial load-bearing anterior lamellae around $300^\circ$ of circumference, causing substantial biomechanical destabilization. SMILE leaves the anterior stromal bridge and Bowman's layer untouched.

The thickness of the extracted stromal lenticule ($t_{\text{lenticule}}$) is calculated based on Munnerlyn's parabolic relation across the optical zone diameter ($S$):

$$t_{\text{lenticule}} = \frac{S^2 \cdot D}{3} + t_{\text{edge}} \quad (\text{where } t_{\text{edge}} \approx 10\text{--}15\ \mu\text{m for ease of dissection})$$

For a $-6.00\ \text{D}$ correction across a $6.5\ \text{mm}$ optical zone, the lenticule center thickness is approximately $\frac{(6.5)^2 \times 6}{3} + 15 \approx \mathbf{100\ \mu\text{m}}$. The femtosecond laser cuts the posterior surface first, followed by the anterior lenticule cap and the $2\ \text{mm}$ side-pocket incision.

Because SMILE uses a $2\text{--}3\ \text{mm}$ incision rather than a $20\ \text{mm}$ LASIK flap circumference, over $80\%$ of the subbasal corneal nerve plexus is preserved. Corneal sensation recovers within 4 to 8 weeks (compared to 6 to 12 months in LASIK), slashing chronic post-surgical dry eye rates.