LASIK Eye Surgery

Learning about LASIK Surgery

LASIK is a surgical procedure intended to reduce a person's dependency on glasses or contact lenses. The goal of this Web site is to provide objective information to the public about LASIK surgery.

LASIK stands for Laser-Assisted In Situ Keratomileusis and is a procedure that permanently changes the shape of the cornea, the clear covering of the front of the eye, using an excimer laser. A knife, called a microkeratome, is used to cut a flap in the cornea. A hinge is left at one end of this flap. The flap is folded back revealing the stroma, the middlesection of the cornea. Pulses from a computer-controlled laser vaporize a portion of the stroma and the flap is replaced. There are other techniques and many new terms related to LASIK Surgery that you may hear about.

Laser in situ keratomileusis (ie, LASIK) is a surgical procedure designed to reshape the surface of the cornea with laser to focus visual images directly onto the retina, which improves visual acuity. The word keratomileusis is derived from the Greek root words keras (ie, hornlike cornea) and mileusis (ie, carving). It is the general term for surgical procedures that can improve or correct the eye's focus by permanently changing the shape of the cornea to bend light rays onto the retina and enhance the optical power of the eye. Refractive lamellar corneal surgery attempts to either remove, add, or modify the corneal stroma or structure so the anterior corneal curvature is altered as desired.

The cornea is the main refracting element of the optical system of the human eye, and small changes in the anterior curvature will produce relatively large changes in the patient's overall refraction. The term keratomileusis refers to refractive keratoplasty techniques that combine a lamellar (ie, horizontal, platelike) resection of the cornea performed to change the corneal radius of curvature with a microkeratome (ie, a knife that is used for incising the cornea) or other instrument and cryolathe, microkeratome excision, or excimer laser ablation. Refractive corneal surgery is a cornerstone for invasive vision correction from which subsequent surgical techniques and innovations are evolving.

The LASIK surgery procedure uses the excimer laser for photoablation of the corneal stromal bed to alter the corneal curvature, bending admitted light onto the retina and generating a refractive change. The reported benefits of the procedure are rapidly improved visual acuity and minimal postoperative recovery. The LASIK surgery procedure leaves the corneal surface nearly intact, resulting in minimal healing from the excimer laser light sculpting of the cornea. The LASIK surgery procedure is an effective outpatient procedure that is suitable not only for patients with strong lens prescriptions but also for patients with moderate to weaker prescriptions.

Approximately 62 million people in the United States are myopic (ie, nearsighted) and require eyeglasses or contact lenses for vision correction. There is a greater number of the population who are farsighted, have astigmatism, or have a combination of astigmatism with either farsightedness or nearsightednes. In 1998, approximately 420,000 LASIK procedures were performed in the United States. This number was almost double the number of LASIK procedures performed in 1997. Two million people throughout the world have benefited from the LASIK procedure.

An elective surgical procedure, the LASIK surgery procedure often is performed at the patient's expense, as many insurers will not reimburse the costs for an elective procedure. Consumer demand, generated by timely, satisfactory postoperative results, has made this procedure one of the fastest growing elective surgical procedures in the United States. As such, perioperative nurses should be familiar with LASIK and the implications of this innovative ophthalmic surgical procedure.

ANATOMY

A review of the refractive apparatus of the eye is necessary to better understand LASIK surgery. The eye is located within the orbital cavity of the skull between the cranium and the facial skeleton. This orbit is a bony depression where the eyeball, a bulbous, fluid-filled sac, rests within a cushion of adipose tissue. The orbit is a socket for the eyeball, muscles, nerves, and vessels that are necessary for proper functioning of the eye.

The eye is composed of three layers of tissue surrounding a fluid-filled center and occupies less than one-third of the orbit. The external corneal-scleral layer of the eye is fibrous and protects the other two layers, which are the middle and internal layers. The middle layer is a vascular, pigmented layer consisting of the iris, ciliary body, and choroid. The internal layer is called the sensory retina. The serous fluid content of the anterior chamber is called the aqueous humor, and the thicker, viscous fluid within the posterior chamber is the vitreous humor.

The eye consists of two spheres. The cornea composes the anterior one-sixth and has a radius curvature of approximately 8 mm. The posterior five-sixths, the sclera, has a radius curvature of approximately 12 mm. The limbus is the area where the cornea intersects with the sclera. The central portion of the bulbar conjunctiva is continuous at the limbus with the anterior epithelium of the cornea. The adult globe has an internal volume of 5.5 [cm.sup.3] and weighs approximately 7.5 g.

The sclera consists of densely packed collagen fibers that course in bundles and are arranged in a lamellar fashion (ie, layered). The randomness of the orientation of the collagen fibers and the hydration of the sclera contribute to its white appearance. The support for the posterior portion of the eye is provided by the sclera. At the external border between the cornea and the sclera is the external scleral sulcus depression.

The cornea is the anterior portion of the external layer of the globe. It is crescent-shaped (in cross-section), transparent, and avascular. The cornea comprises five layers: the epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium. The epithelium consists of five or six constantly renewing cell layers and nerve endings, which account for corneal sensitivity. Bowman's membrane comprises of connective tissue fibers and forms a barrier against trauma and infection but does not regenerate if damaged. The corneal stroma comprises dense connective tissue and accounts for 90% of corneal thickness that contains lamellar fibers in a regularly patterned configuration. Descemet's membrane is the thin layer between the stroma and the endothelium that may be inflamed or protrude when irritated. The endothelium is a single layer of cells that are similar to Bowman's membrane and do not regenerate if damaged. The endothelial cells are responsible for the proper hydration of the cornea. The cornea is smaller in vertical diameter (ie, 10.6 mm) than in horizontal diameter (ie, 11.7 mm), and it is thicker at the periphery than in the center. The cornea is 500 [Mu] to 600 [Mu] thick.

Corneal transparency gradually is attained before birth due to maturation of the superficial lamellae (ie, layered cells) and hydration activity of the endothelial cells. The lamellae are compared to a uniform stack of dishes. The terms lamellar and lamellae refer to the way the cells are configured and not to one particular cell. The cornea is clear because it has collagen fibers that are arranged in a systematic parallel manner, unlike the sclera, which has random arrangement of the fibers, making the sclera opaque. Corneal transparency after birth is due to a number of related factors, including regularity and smoothness of the covering epithelium, corneal avascularity, and the size and arrangement of the extracellular and cellular components in the stroma. Any alteration within this network affects the clarity of the cornea. The cornea, the pupillary opening within the iris, the lens, and the aqueous and vitreous humor combine to form the refractive media of the eye. The surface of the cornea and associated tear film is responsible for most of the refractive power of the eye.

The eye has been compared to a camera due to its ability to process images for interpretation by the occipital cortex of the brain. An image passes through the cornea in the form of light rays that refract (ie, bend) on its surface then converge through the aqueous humor to pass through the pupillary opening. The image passes through the lens, which further refracts the light through its anterior and posterior curvatures. The image proceeds to the posterior chamber of the eye, passing through vitreous humor to fall inverted and backwards onto the retina. The retina is a thin, transparent membrane of nerve cells and fibers that receives the image, converts it from light to electrical energy, and transfers it via the brain stem to the occipital cortex. Myopia causes a blurting of distant images because images fall short of the retina due to either elongation of the globe or excessive refractive surface area.

Eyes that are anteriorly or posteriorly shorter than the normal eye size with too little refractive power are hyperopic, or farsighted. The image appears to fall beyond or behind the retina, causing blurring of near images.