SMILE, surgical selection and complications

4. Small Incision Lenticule Extraction (SMILE)

Small Incision Lenticule Extraction (SMILE) is an all-femtosecond intrastromal refractive procedure that eliminates both excimer photoablation and corneal flap creation.

Surgical Technique

Using a low-energy, high-frequency femtosecond laser (e.g., VisuMax, $500\text{ kHz}$), four sequential photodisruptive cuts are performed within the stroma:

1. Refractive Lenticule Posterior Interface: The deeper cutting plane created from the periphery towards the centre, forming the posterior refractive surface.
2. Lenticule Edge: A vertical cylindrical cut ($10–15\,\mu\text{m}$ thick) defining the outer boundary of the lenticule.
3. Refractive Lenticule Anterior Interface (Cap): The superficial lamellar plane created from the centre outwards, situated at a fixed depth of $100–120\,\mu\text{m}$ beneath the anterior epithelial surface.
4. Side-Cut Entry Tunnel: A small, arcuate $2–4\text{ mm}$ incision (typically at the 12 o'clock or superotemporal position).
5. Manual dissection is performed using a dedicated pocket-separator spatula to free the anterior and posterior planes, followed by extraction of the intrastromal lenticule through the side cut with micro-forceps.

Biomechanical & Neurological Advantages

• Biomechanics: Anterior stromal lamellae contribute strongly to corneal strength. A small-incision lenticule procedure preserves more anterior continuity than a flap, but comparative clinical biomechanics are complex. Neither a fixed 60% strength attribution nor a guaranteed 40–50% LASIK loss applies to every eye. SMILE can still cause ectasia.
• Reduced Denervation Dry Eye: The sub-basal nerve plexus arises from long ciliary nerves entering mid-stroma and branching vertically into the sub-epithelial space. In LASIK, flap creation severs these nerves around $300^\circ$ of the circumference. SMILE generally causes less early nerve disruption than flap creation in comparative studies, but dry-eye outcomes vary. Pre-existing surface disease, technique and follow-up matter; dry eye and reduced sensation remain possible.

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5. Preoperative Patient Selection & Ectasia Risk Stratification

Assess refractive stability, realistic goals, dry eye, corneal sensation, lid disease, pupil, binocular status and ocular comorbidity. Review topography and tomography, pachymetry distribution, contact-lens warpage, family history, atopy and rubbing. A normal anterior map alone does not establish safety.

The historical Randleman Ectasia Risk Score System considers topography, residual stromal bed, age, corneal thickness and refractive error. Its older scoring model does not replace modern tomography or establish universal safety for all procedures.

For LASIK, residual stromal bed (RSB) and percentage of tissue altered (PTA) are:

$$\mathrm{RSB}=\mathrm{CCT}-(\mathrm{flap}+\mathrm{ablation}),\qquad \mathrm{PTA}=\frac{\mathrm{flap}+\mathrm{ablation}}{\mathrm{CCT}}\times100\%$$

For CCT 510 micrometres, flap 110 and ablation 100, RSB is 300 micrometres and PTA is 41.2%. A PTA around or above 40% was associated with ectasia in particular LASIK studies. It is a risk marker, not a biological cliff or a validated identical threshold for PRK and SMILE. An RSB of 300 micrometres does not guarantee safety. In a concerning case, defer LASIK and reassess alternatives; PRK is not automatically safe in an ectatic cornea, and a phakic lens requires its own anatomical eligibility assessment.

Explain infection, haze, dry eye, residual error, regression, dysphotopsia and rare loss of best-corrected vision. Surgery changes optical correction but does not remove high-myopia retinal risk. Contact lenses, spectacles and no surgery remain legitimate choices.

6. Complications of Keratorefractive Surgery

Intraoperative Complications

1. Suction Loss: If suction breaks during femtosecond flap creation, immediately stop laser delivery. If the cut has not crossed into the visual axis, suction can be re-established immediately. If suction breaks during a mechanical microkeratome pass, abort the procedure immediately, reposition the partial flap without laser ablation, and allow 3 to 6 months of healing before re-attempting.
2. Free cap: Stop and assess tissue integrity and orientation. Replace the viable cap correctly and decide whether to defer ablation; completing ablation is not automatically safe after this complication.
3. Buttonhole Flap: A central flap defect can occur during mechanical flap creation, with risk influenced by corneal shape and instrument factors. Abort the planned ablation and carefully reposition the flap. Reassess healing, topography and visual quality before considering further surgery. Later surface ablation may be considered in selected eyes, but neither a single keratometry cutoff nor a compulsory six-month PRK-with-MMC pathway applies to every defect.
4. Opaque Bubble Layer (OBL): Femtosecond cavitation gas pockets escape anteriorly into the subepithelial or stromal space. Can interfere with excimer eye-tracking systems; surgery should be paused for 15–30 minutes until gas reabsorbs.

Postoperative Complications: Diffuse Lamellar Keratitis (DLK)

Diffuse Lamellar Keratitis ("Sands of the Sahara") is a sterile, non-infectious, diffuse inflammatory infiltrate of polymorphonuclear neutrophils (PMNs) confined strictly to the flap-stromal interface, appearing 1 to 5 days postoperatively. Etiologies include bacterial endotoxins on surgical instruments, microkeratome blade oils, or povidone-iodine entry.

Other Postoperative Complications

• Epithelial Ingrowth: Proliferation of epithelial cells into the interface (1–2% of microkeratome LASIK, higher in re-treatments). Staged as mild (peripheral, non-progressive; observe) versus clinically significant (advancing within 2 mm of visual axis, flap melting, foreign body sensation; requires surgical flap lift, mechanical scraping of bed and flap underside, alcohol application, and secure repositioning with sutures or fibrin glue).
• Infectious Keratitis: Early onset (days 1–3) is typically staphylococcal or streptococcal; delayed/atypical onset (weeks 2–4) is classically Mycobacterium chelonae, Nocardia, or fungi. Requires urgent sampling and intensive organism-directed treatment. A flap/interface infection often needs flap lifting and interface sampling or irrigation; select antimicrobial coverage with the corneal team, timing and culture findings rather than one compulsory combination.