مهندسی پزشکی

Process

The planning and analysis of corneal reshaping techniques such as LASIK have been standardized by the American National Standards Institute, an approach based on the Alpins method of astigmatism analysis. The FDA website on LASIK states,

"Before undergoing a refractive procedure, you should carefully weigh the risks and benefits based on your own personal value system, and try to avoid being influenced by friends that have had the procedure or doctors encouraging you to do so."^[51]

The procedure involves creating a thin flap on the eye, folding it to enable remodeling of the tissue beneath with a laser and repositioning the flap.

Preoperative procedures

Contact lenses

Patients wearing soft contact lenses are instructed to stop wearing them 5 to 21 days before surgery. One industry body recommends that patients wearing hard contact lenses should stop wearing them for a minimum of six weeks plus another six weeks for every three years the hard contacts have been worn. The cornea is avascular because it must be transparent to function normally. Its cells absorb oxygen from the tear film. Thus, low-oxygen-permeable contact lenses reduce the cornea"s oxygen absorption, sometimes resulting in corneal neovascularization—the growth of blood vessels into the cornea. This causes a slight lengthening of inflammation duration and healing time and some pain during surgery, because of greater bleeding. Although some contact lenses (notably modern RGP and soft silicone hydrogel lenses) are made of materials with greater oxygen permeability that help reduce the risk of corneal neovascularization, patients considering LASIK are warned to avoid over-wearing their contact lenses.

Pre-operative examination and education

In the United States, the FDA has approved LASIK for age 18 and over. More importantly the patient"s eye prescription should be stable for at least one year prior to surgery. The patient may be examined with pupillary dilation and education given prior to the procedure. Before the surgery, the patient"s corneas are examined with a pachymeter to determine their thickness, and with a topographer, or corneal topography machine,^[1] to measure their surface contour. Using low-power lasers, a topographer creates a topographic map of the cornea. The procedure is contraindicated if the topographer finds difficulties such as keratoconus^[1] The preparatory process also detects astigmatism and other irregularities in the shape of the cornea. Using this information, the surgeon calculates the amount and the location of corneal tissue to be removed. The patient is prescribed and self-administers an antibiotic beforehand to minimize the risk of infection after the procedure and is sometimes offered a short acting oral sedative medication as a pre-medication. Prior to the procedure, anaesthetic eye drops are instilled. Factors that may rule out LASIK for some patients include large pupils, thin corneas and extremely dry eyes.^[52]