Excimer laser refractive surgery : corneal wound ... - Helda - Helsinki.fi

2 REVIEW OF THE LITERATURE
2.1 GROSS CORNEAL ANATOMY
The cornea is an avascular and transparent structure situated in front of the eye. The
structural and physiological properties of the cornea determine its optical performance to
refract light. The cornea is the most powerful refractive lens of the eye comprising on
average 45 D of the approx. 60-70 D total refractive power of the eye. The central
thickness of the cornea is between 500 to 550 µm and 600 to 700 µm at the corneal
periphery (Doughty and Zaman 2000, Ehlers and Hjortdal 2004, Salvetat et al. 2010). This
difference in thickness between the periphery and central corneal generates a disparity in
curvature creating an aspheric optical system. The cornea has an elliptical shape when
viewed frontally; this configuration arises from an extension of opaque sclera tissue that
covers the cornea superiorly and inferiorly. In the adult cornea, the horizontal and vertical
average diameters are 12 mm (range 11 to 12.5 mm) and 11 mm (range, 10 to 11.5 mm),
respectively (Rufer et al. 2005).
2.1.1 Tear film
The pre-corneal tear film supports and maintains the ocular surface. It lubricates the
epithelium, protects the cornea from external agents, modulates wound healing through its
components and, secondary to the air-tear interface, creates the first refractive surface.
Three different layers constitute the tear film (Prydal and Campbell 1992, Prydal et al.
1992); a. The inner hydrophilic mucous layer derived from the conjunctival goblet cells
and corneal epithelial cells (Chao et al. 1980). This layer is attached to the superficial cells
of the corneal epithelium and its essential role is to allow spreading of the tear film, and to
prevent the adhesion of foreign pathogens and debris onto the ocular surface. b. The
aqueous layer is derived from the main and accessory lacrimal glands. It contains proteins,
electrolytes, cytokines and growth factors that modulate the ocular surface and the
response to damage. c. The external lipid layer derived from the meibomian glands
prevents the evaporation of tears (Mishima and Maurice 1961, Rolando and Refojo 1983)
and stabilizes the tear film.
2.1.2 Corneal structure
The cornea is built of around 200 collagen sheets, which are laid at approximately 90
degrees to each other. This collagen structure provides mechanical strength and
transparency and allows for undisturbed image formation on the retina. It is composed
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