Excimer Laser (Refractive Surgery)

The excimer laser is an argon-fluoride (ArF) gas excited dimer laser emitting 193 nm ultraviolet light, used since 1990 in corneal refractive surgery and corneal therapy. Mechanism is cold photochemical: the high-energy UV photons break carbon-carbon bonds in stromal proteoglycans without significant heat damage to surrounding tissue, achieving sub-micron precision ablation (~0.25 μm per pulse) and permitting precise reshaping of cornea to alter refractive power. Standard ablation rate is 1 micron per 6-7% of corneal volume, with myopic correction flattening central cornea, hyperopic correction steepening central cornea, and astigmatism correction differentially ablating along axis.

Surgical procedures using excimer laser: Photorefractive keratectomy (PRK, original technique 1987-present, surface ablation after epithelial removal mechanically/alcohol/laser, mitomycin C for haze prevention, longer healing 4-7 days, slight discomfort, slower visual recovery, no flap-related complications, preferred in thin corneas, irregular topography, athletes, military), LASIK (laser-assisted in situ keratomileusis, gold standard for many surgeons since 1990, microkeratome-created flap or femtosecond laser flap [iLASIK, all-laser LASIK], stromal ablation under flap, flap repositioning without sutures, rapid recovery 24-48 hours, less discomfort, possible flap complications), Femto-LASIK (femtosecond laser flap is now standard, more precise/predictable), LASEK/Epi-LASIK (variations of surface ablation), PTK (phototherapeutic keratectomy for superficial corneal scars, dystrophies, recurrent erosion). Modern ablation profiles: conventional (theoretical ideal), wavefront-guided (treats higher-order aberrations measured by aberrometer, useful for irregular astigmatism, post-LASIK glare), topography-guided (Contoura Vision, treats corneal surface irregularities, CXL-PLUS for keratoconus), aspheric Q-adjusted (preserves prolate corneal shape, reduces spherical aberration), wavefront-optimized (compensates for induced aberrations).

Preoperative evaluation: stable refraction for ≥1 year, age usually >18-21, manifest and cycloplegic refraction, corneal topography (Placido disc, Scheimpflug Pentacam/Galilei, OCT for posterior elevation), pachymetry ultrasound and Scheimpflug (residual stromal bed RSB ≥250-300 μm post-ablation; total cornea >480-500 μm minimum; rule out forme fruste keratoconus), pupil size (mesopic >6 mm risks halos with smaller optical zones), tear film evaluation (Schirmer, TBUT, MGD), dry eye treatment before surgery, ocular surface optimization. Contraindications: keratoconus, forme fruste keratoconus, post-LASIK ectasia, autoimmune disease (controlled okay), pregnancy/lactation, unstable refraction, severe dry eye, glaucoma, immunocompromise, inadequate corneal thickness. Correction range: myopia up to -10 D, hyperopia up to +5-6 D, astigmatism up to 6 D (combinations may further limit). Outcomes: 95-99% achieve uncorrected visual acuity 20/40 or better, 80-90% 20/20 or better. Complications: dry eye most common (resolves usually within 6 months), night vision symptoms (halos, glare), under/over-correction, regression, infection (1/2000), keratitis, ectasia (1/2500-5000), epithelial ingrowth (LASIK), DLK (LASIK), flap dislocation/wrinkles, haze (PRK, prevented by mitomycin C). Adjuncts and future: SMILE (small incision lenticule extraction, femtosecond only without flap, alternative to LASIK), corneal cross-linking (CXL) for keratoconus and post-LASIK ectasia, topo-guided CXL, intracorneal ring segments. Postoperative care: topical antibiotic, steroid taper, NSAID, lubrication intensive, follow-up day 1, week 1, month 1, 3, 6, 12, photoprotection.