Intrastromal Corneal Ring Segments (ICRS)

What does the ICRS procedure do?
How is the ICRS procedure preformed?

The implantation of intrastromal corneal ring segments (trade name Intacs) is the first approved nonlaser procedure to correct mild myopia, or nearsightedness. As in all surgical vision correction, the objective of the intrastromal corneal ring segments procedure is to reshape the cornea in order to correct for imperfections. Unlike most other refractive eye surgery procedures, however, the ICRS procedure corrects the vision problems without removing any eye tissue.

The basis of the ICRS procedure is the surgical implantation of two ultrathin arcs in the peripheral area of the cornea (See Anatomy of the Eye). These arcs (or segments) are manufactured from a polymer that has been safely used in cataract surgery for more than 40 years. When the arcs are implanted, they flatten the cornea to the degree required to correct the myopic condition. The procedure does not cut or remove any tissue, making this quite different from other refractive surgery procedures that permanently alter the cornea. They are designed to remain permanently in place, although the segments can later be removed or replaced to correct for possible sight changes as the eye ages.

The best candidates for the corneal ring procedure are usually those patients with mild myopia (-1.00 to -3.00) who have no more than 1.00 diopter of astigmatism. Patients should also be at least 21 years of age with vision that has been stable for at least one year and be free of eye disease.

What does the ICRS procedure do?

To understand how the ICRS procedure works, it is first necessary to understand the visual function of the eye. The eye works like a camera with two lenses. The first lens is the cornea, a clear membrane that covers the front of the eye. The second lens is the eye's natural crystalline lens, which is located behind the pupil. The cornea is responsible for about 70 percent of the eye's focusing power, while the natural lens fine-tunes the image before it is focused on the retina at the back of the eye. The retina works like the film in a camera, receiving light images and sending them through the optic nerve to the brain. If both lenses are working properly, the image is focused precisely on the surface of the retina, and the result is perfect 20/20 vision.

Just as people are born with different sizes and shapes of hands, their eyes also vary in form and proportion. A perfect eye has an evenly rounded cornea that allows light to fall exactly on the retina, resulting in perfect vision. If the cornea is too steep or if the eye is too long from front to back, light rays are focused in front of the retina resulting in myopia. The ICRS procedure flattens the cornea, bringing the light rays back in focus on the surface of the retina.

How is the ICRS procedure performed?

The ICRS surgical procedure takes about 15 minutes per eye and is performed on an outpatient basis. Patients are typically given a mild oral sedative along with topical anesthetic eyedrops to numb the eye.

To begin, the surgeon makes a tiny opening (less than 2 mm) near the upper edge of the cornea beneath the eyelid. The arcs are then placed in opposing positions on the outer edges of the cornea away from the central optical zone, the critical area for clear vision. The thickness of the arcs is determined by the degree of myopia that needs correcting. The higher the degree of myopia, the thicker the ring segments. When in place, they form a circle around the edges of the iris and alter the curvature of the eye. In essence, they stretch the cornea into a more flattened shape, thereby correcting the myopic condition. The patient does not feel the rings because they are placed beneath the nerve endings in the cornea.

Although the ring segments are designed to remain permanently in the corneal tissue, they can be replaced with thicker or thinner segments to achieve any adjustments that may be required, or they can be removed permanently. These adjustments are possible because there is no invasion of the visual axis or central optical zone of the cornea, and the procedure does not involve any removal of corneal tissue.

Most patients notice an improvement in vision within a day of surgery and are able to resume normal activities within two or three days.