Cornea: Structure, Function, and Role in Vision Correction

Five-Layer Corneal Structure

The cornea is a transparent, avascular tissue approximately 11.5 mm in diameter and 0.5–0.6 mm thick centrally. From front to back, it consists of five distinct layers:

1. Epithelium: 5–7 cell layers thick (~50 microns); rapid turnover (replaces itself every 7–10 days); provides a smooth optical surface; protective barrier against infection; rich in sensory nerve endings
2. Bowman's layer: 8–14 microns; acellular zone of densely packed collagen fibrils; does not regenerate if damaged; PRK ablation includes this layer; LASIK preserves it within the flap
3. Stroma: 90% of total corneal thickness (450–500 microns); composed of highly organized collagen lamellae that provide transparency and tensile strength; site of LASIK ablation; keratocytes (stromal cells) maintain the collagen structure
4. Descemet's membrane: 10–12 microns; basement membrane of the endothelium; thickens with age; strong elastic tissue
5. Endothelium: single cell layer (~5 microns); maintains corneal dehydration through active fluid pumps; non-regenerating (cell loss is permanent); endothelial cell density decreases with age

How the Cornea Refracts Light

The cornea contributes approximately 43 of the eye's total ~60 diopters of refractive power, making it the eye's primary refracting structure. This power arises because light changes speed (and therefore direction) when it crosses from air (refractive index 1.0) into the corneal epithelium (refractive index ~1.376) — a significant change that bends light toward the optical axis. The geometry of this refraction is determined by the corneal curvature: a steeper curve refracts light more strongly, contributing more optical power. A flatter curve provides less power. LASIK works by modifying this curvature to change the eye's refractive power precisely.

Why Corneal Thickness Is Critical for LASIK Safety

Corneal thickness directly determines how much laser ablation can be safely performed. The excimer laser removes approximately 12–15 microns of stroma per diopter of myopic correction. For a -6 D correction, approximately 75–90 microns of stroma are removed. Combined with the flap thickness (90–110 microns), the total tissue consumption can be substantial. Surgeons ensure that the remaining residual stromal bed is at least 250 microns to preserve structural integrity and prevent ectasia — the progressive forward bulging that can occur if the cornea is weakened excessively. See corneal thickness requirements.

How LASIK Reshapes the Corneal Stroma

In LASIK, a flap of epithelium, Bowman's layer, and anterior stroma is created and reflected back. The excimer laser (193 nm UV radiation) then performs photoablation — breaking molecular bonds in the stromal collagen with each ultrashort pulse, removing approximately 0.25 microns per pulse without generating heat or surrounding tissue damage. The ablation pattern is programmed based on the treatment plan: for myopia, more tissue is removed centrally to flatten the cornea; for hyperopia, more is removed peripherally to relatively steepen the center; for astigmatism, the ablation is asymmetric. See excimer laser technology.

Corneal Diseases That Affect LASIK Candidacy

• Keratoconus — progressive ectasia; absolute LASIK contraindication
• Corneal dystrophies — Fuchs endothelial dystrophy (endothelial failure risk with IOP elevation during flap creation); epithelial basement membrane dystrophy (EBMD/map-dot-fingerprint) increases epithelial ingrowth risk after LASIK, PRK preferred
• Herpes simplex keratitis — viral reactivation risk from surgical trauma; relative contraindication; prophylactic antivirals may be used in selected cases
• Corneal scarring — scars in the treatment zone reduce ablation predictability
• Pterygium — must be excised and cornea allowed to stabilize before LASIK

Corneal Wound Healing After LASIK

After LASIK, the corneal flap adheres through fibronectin deposition within hours. Keratocytes in the stromal bed undergo apoptosis then repopulate the ablated zone over weeks. The epithelium rapidly resurfaces the flap edge within 24 hours. The primary healing response is complete within 3–6 months, though subtle stromal remodeling continues for 12+ months. Dry eye results from corneal nerve disruption during flap creation — nerves regenerate over 6–12 months. Understanding this timeline explains why LASIK outcomes stabilize over months rather than immediately.

Corneal Nerves, Sensation, and Dry Eye

The cornea is the most densely innervated tissue in the body — approximately 7,000 nerve endings per mm², primarily from the nasociliary branch of the trigeminal nerve (V1). These sensory fibers enter the corneal stroma radially and arborize throughout the epithelium. They serve two functions: providing exquisite pain and touch sensitivity (the blink reflex and defensive tearing), and signaling the lacrimal glands via the trigeminal-lacrimal reflex to maintain baseline tear production. LASIK flap creation severs many of these stromal nerve fibers, reducing corneal sensitivity and lacrimal reflex tearing for 6–12 months until nerves regenerate — the basis of post-LASIK dry eye. SMILE, with its small incision, disrupts fewer nerve fibers and causes less dry eye. See dry eye after LASIK.