In order to understand refractive eye surgery, it is essential to understand how the eye works. Think of the eye as a camera. Light rays that come into a camera must be focused on the film. So it is with the eye. The cornea, like the lens of the camera, focuses the incoming light rays. The human crystaline lens is behind the iris and adjusts the focus for distance or near vision. The light rays are sharply focused on the retina, like the film of the camera. Approximately 60% of the focusing power occurs on the cornea.
In people with myopia, we can think of the cornea as being too powerful for the length of the eye and light rays are focused in front of the retina.
In order to correct myopia, we use the laser to alter the shape of the cornea, reducing its power, and allowing the light rays to be focused on the retina.
This is done by removing a precise amount of tissue with the laser. The pattern of tissue removal is such that it "flattens" the cornea and reduces the effective power of the cornea.
In order for us to have a stable post-operative optical system, the laser-created changes must be permanent or longlasting. This can occur if the underlying cornea stroma is treated since this tissue does not remodel itself.
The cornea works like a simple lens. Light rays strike the surface. Light rays strike the surface and the change of index of refraction causes the bright rays to be bent toward the surface normal (a line perpendicular to the corneal surface).
This principle is a law of physics and is known as Snell's Law . The shape of the cornea determines its focusing power. The corneal surface is composed of a layer of surface epithelial cells, their underlying basement membrane (Bowman's layer), a relatively thick stromal layer made up of sheets of collagen, and a single cell layer of endothelial cells.
The epithelial cells make up the smooth optical surface and are very active in replicating and moving to constantly remodel and provide a smooth surface. Alterations of the epithelium are not permanent or long standing since the tissue will quickly remodel and heal to resume its smooth contour. The epithelial shape will, however, reflect the surface curvature of the underlying cornea stroma.
The cornea stroma is made up of collagen fibrils that are arranged as overlapping sheets of tissue. Each sheet is separated by approximately half the wavelength of light and, thus, remains optically clear, transmitting and not scattering light.
The underlying endothelial cells control the relative water content of the overlying cornea stroma and maintain approximately 78% water content. This assures the appropriate separation of the collagen sheets so they transmit light rather than scatter the light (as seen in a cornea scar).
The strategy for laser vision correction is to remove or displace the surface epithelium and expose the underlying cornea stroma so that the laser treatment can cause a permanent alteration to the corneal shape. Since the cornea stroma will not remodel itself in most cases, the alteration in its shape or contour will be permanent. When the epithelium grows back or the flap with epithelium is repositioned, it will "take on" the shape of the underlying stroma as it remodels and allow a new "optically correct" corneal power to occur following the laser treatment.