Intrastromal corneal rings (or intracorneal rings) are small devices implanted in the eye to correct vision. A typical vision correction using corneal rings would involve an ophthalmologist making a small incision in the cornea of the eye, and inserting two crescent or semi-circular shaped ring segments between the layers of the corneal stroma, one on each side of the pupil. The embedding of the rings in the cornea has the effect of flattening the cornea and changing the refraction of light passing through the cornea on its way into the eye.
Corneal rings are typically used for patients with mild to moderate myopia (-1.0 to -3.0 diopters of correction and 1.0 diopter or less of astigmatism). An advantage over other surgical vision correction procedures like PRK or LASIK is that the implants are removable, which could allow for reversal or partial reversal of the correction, or replacement with different rings to change the amount of correction. Additional advantages are a lower risk of side effects, and a greater success rate of achieving the desired correction.
Potential complications are similar to other surgical vision correction procedures, and include infection and vision distortions, particularly problems with night vision.
The ring segments themselves are made of PMMA (an acrylic glass more commonly known under the tradenames Perspex or Plexiglas). The leading manufacturer is Addition Technology, which markets the rings under the trade name Intacs.
With the advent of wavefront LASIK and Implantable Collamer Lenses, Intacs or Implantable Corneal ring segments are rarely used for the correction of myopia. Intacs is the only FDA approved intracorneal ring segment. Today they are mainly used for the treatment of Keratoconus. In June 2004 FDA approved them for the management of keratoconus. In April 2010 the FDA approved the expanded range of Intacs. The intacs have to be placed in a channel in the cornea. This is best accomplished by a corneal surgeon. The channel can be created mechanically by an automated instrument or by femtosecond laser like intralase. The ring segments are then implanted in these channels. The ring segments come in different thickness. Intacs can vary in thickness from 210 to 450 micrometres. They are placed around 7 mm from the optical center. A suture is placed at the entrance of the channels after the implantation of the Intacs.
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