INTRA-STROMAL CORNEAL RING SEGMENTS
Apart from their application in refractive surgery, Intra-stromal corneal ring segments (ICRS) are very useful to the corneal surgeon's array of methods to manage certain corneal pathologies. A discussion of the refractive application of these devices is available elsewhere on this website.
ICRS are tiny, clear, ultrathin and precision-engineered crescents, made from polymethylmetacrylate (PMMA). This is the same material used in the manufacturing of certain hard contact lenses. Two of these crescents usually combine to achieve the desired correction, although depending on the corneal curvature, it is possible to only use one segment. The first picture on the right shows what the crescents look like and the second picture shows their position inside a cornea.
2 versions of the technology are available in South Africa, namely the Keraring™ and INTACS™. The Keraring has a long track record, and finds it application mainly in the management of keratoconus and other ectatic corneal conditions. INTACS™ were popularized by the company Keravision for the treatment of low to moderate myopia and was approved by the American FDA during April 1999 for the treatment thereof.
The management of keratoconus and other ectatic corneal pathologies underwent significant change since 2000 with the advent of corneal cross-linking. In order to avoid (or at least postpone) a corneal transplant, the ICRS technology was introduced to improve the distorted corneal shape in these eyes. It is possible to modify and optimize the cornea's shape through the placement of these rings. The Kerarings are especially popular for achieving this result, since they are available in different thicknesses, arc lengths and radius of curvature.
The amount of change of the corneal profile is dependent on:
- The thickness of the segment.
- The arc length thereof.
- Their radius of curvature.
- The stromal depth at which it is placed.
These segments are inserted into a circular tunnel, fashioned in the periphery of the cornea within the corneal substance by a special surgical instrument, or more accurately by a femtosecond laser.
ADVANTAGES OF ICRS SURGERY IN THERAPEUTIC APPLICATIONS
- It is a relatively low risk procedure compared to a corneal transplant.
- It is a minimally invasive and controlled procedure.
- It is a reversible technique, since the ICRS may be removed or exchanged at any time.
- It is possible to adjust the effect of the rings.
- It provides the option to return to glasses or soft contact lenses in eyes that became unsuitable to the wearing thereof due to the extent of progression of the disease.
- It may assist in the stabilization of the disease process.
- It may 'buy time' for patients who cannot afford the long and drawn out management of a corneal transplant.
- It may avoid the need for a corneal transplant.
DISADVANTAGES OF ICRS SURGERY
- It is relatively expensive.
- For accurate placement and predictable outcomes, a femtosecond laser is required.
- In spite of an initial success, a corneal transplant may eventually be unavoidable, depending on the progression of the disease.
INDICATIONS FOR ICRS SURGERY
The placement of Keraring segment(s) are indicated in:
- Irregular astigmatism.
- Early forms of keratoconus.
- Pellucid marginal degeneration.
- Ectasia due to previous failed refractive surgery.
The ICRS are barely noticeable with the naked eye and their appearance in the eye may be compared to that of a contact lens. It is, however, impossible for the ICRS implant to become dry or smudged like a contact lens, since it lies within the corneal stroma. Patients are therefore not aware of the ICRS implants, as they are placed into the corneal substance, beneath the sensory nerve endings. The picture on the left shows the implants that are barely noticeable to an external observer, even on close inspection. Although many patients experience immediate improvement in their vision, others find that their vision starts to improve approximately a week after the procedure. The vision, however, often fluctuate for several months but most patients return to work 2 to 5 days after the procedure.
The placement of ICRS segment(s) does not necessarily negate the use of glasses or contact lenses – it is only intended to optimize the surface shape of the cornea in these conditions. Therefore it is often required to still wear soft contact lenses or glasses, or to proceed with the placement of a phakic intra-ocular lens to deal with the remaining refractive error, should the patient wish to be less dependent on optical aids. The goal with such patients is rather to avoid an invasive procedure like a corneal transplant.
The picture on the left shows a cross section of the cornea in the area of the green arrow and the picture on the right demonstrates how the external corneal profile is bent into a more regular shape.
There are some side effects, which may potentially occur after ICRS implants. The ring segments are only available in certain sizes. Therefore, if the pupil dilates wider than the ring size in dark conditions, the edge of the ring will be noticed. This will result in a halo effect around lights. Some patients learn to adapt to this effect. If, however, the patient cannot adapt, a special eye drop can be prescribed to use before night driving to restrict pupil dilation.
There is a risk that the body will reject the implant or that the segment will erode through the anterior cornea. However, due to the fact that the segment is placed deep within the corneal stroma, this risk is very low. Should the body react to the prosthesis, it can be removed.