SMILE (Small Incision Lenticule Extraction): Biomechanical Strength & Nerve Sparing

In human corneal biomechanics (proven by Dr. John Marshall and Dr. Cynthia Roberts), the anterior 40% of the corneal stroma possesses significantly higher cohesive tensile strength than the posterior 60%. Collagen lamellae in the anterior stroma are tightly interwoven and cross-linked.

In LASIK:

$$\text{LASIK Flap Cutting: } \mathbf{\text{Severs 100\% of Anterior Lamellae Across } 300^\circ \text{ Circumference}}$$

The flap contributes virtually zero biomechanical load-bearing strength permanently. In SMILE, the lenticule is dissected beneath an intact anterior stromal cap ($120\text{--}140\ \mu\text{m}$ thick). The strong anterior lamellae remain continuous, preserving postoperative corneal tensile strength by up to 25% more than LASIK.

The long ciliary sensory nerves enter the mid-stroma in the periphery and branch perpendicularly into the anterior cornea, forming the sub-basal nerve plexus.

• LASIK: A 20mm circumferential flap incision severs nearly 90% of sub-basal nerve trunks, causing profound neurotrophic dry eye lasting 6 to 12 months.
• SMILE: The surgeon enters through a tiny $2.0\text{ to } 3.0\ \text{mm}$ keyhole incision (an 80% reduction in incision length). The vast majority of corneal sensory nerves remain completely intact, reducing postoperative dry eye symptoms to baseline within weeks.

The latest generation (Zeiss VisuMax 800 / SMILE Pro) fires at 2 MHz (2 million pulses per second):

$$\text{Lenticule Creation Time Collapses from } 25\ \text{seconds down to } \mathbf{8\text{ to } 10\ \text{seconds!}}$$

Incorporating automated cyclotorsion alignment (OcuLign) and centration aid (CentraLign), astigmatism correction achieves parity with wavefront-guided excimer treatments.