Introduction to Intra-stromal Corneal Ring Segments (ICRS)
Intra-stromal corneal ring segments (ICRS) are tiny, clear, ultrathin and precision-engineered crescents, made from polymethylmetacrylate. They can be used to treat refractive errors as well as manage certain corneal pathologies and offer an alternative method of surgical correction in such cases.
If a patient does not qualify for laser treatment, these ring segments provide a potential alternative. They are implanted into the periphery of the cornea to treat myopia of less than -4.25 dioptres, certain forms of keratoconus, and they are also indicated in patients who had prior corneal laser refractive surgery and now require an enhancement due to regression of the initial effect over time.
These segments reshape the cornea without removing tissue from the central optical zone of the cornea. It is one of the few corneal technologies that provides a reversible surgical option.
Intra-stromal Corneal Ring Segments
Intra-stromal corneal ring segments (ICRS) are tiny, clear, ultrathin and precision-engineered crescents, made from polymethylmetacrylate (PMMA), 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.
These segments are inserted into a circular tunnel, fashioned in the periphery of the cornea within the corneal substance by a femtosecond laser, as shown in the illustration on the right.
ICRS is a very useful addition to the refractive surgeon's array of methods to treat refractive errors, as well as manage certain corneal pathologies. Unlike refractive laser treatments, ICRS reshape the cornea without removing tissue from the central optical zone of the cornea, the critical area for clear vision.
Two versions of the technology are available in South Africa, namely the Keraring™ and INTACS™. The Keraring has a long track record and finds its application mainly in the management of keratoconus. INTACS, supplied by the company Keravision, was approved by the American FDA during April 1999 for the treatment of myopia of less than -4.25 diopter.
We will limit ourselves to INTACS for the purpose of this discussion of the refractive application of ICRS.
ICRS are currently available for the treatment of myopia less than -4.25 diopters and is one of the few corneal technologies to date that provides a reversible refractive surgical option. To obtain a sharp focus on the retina in the case of myopia, the corneal curvature needs to be flattened. This is achieved by the implantation of these specially-designed corneal ring segments into the periphery of the cornea. A further indication for the placement of these segments include their use in patients who had prior corneal laser refractive surgery and now require an enhancement due to regression of the initial effect over time, but do not have enough tissue to warrant a consecutive safe laser treatment. In these cases it is often not advisable to perform additional laser surgery in fear of destabilizing the cornea beyond the limits for long-term stability. Intra-stromal corneal rings offer an alternative method of surgical correction in these cases.
The amount of correction achieved is dependent on:
- The thickness of the segment.
- The arc length thereof.
- Their radius of curvature.
- The stromal depth at which it is placed.
Advantages of ICRS surgery
- It is a reversible technique (it is possible to remove the ICRS at any time).
- It may be performed on corneas which are not suitable for laser surgery.
- It provides good quality of vision, due to preservation of the corneal asphericity as well as the utilization of a large optical zone.
Disadvantages of ICRS surgery
- It is relatively expensive.
- Only low to moderate myopic refractive errors can be treated.
- For accurate placement (and predictable outcomes), an expensive femtosecond laser is required.
- It is advisable to treat only one eye at a time.
- The vision may fluctuate for up to 6 months after the surgery.
- ICRS do not correct presbyopia.
Indications for ICRS surgery
ICRS are recommended for myopic patients older than 20 years of age with a stable refractive error, who do not qualify for laser refractive surgery due to a thin cornea and/or a wide pupil. It may also be used in patients who had previous corneal laser refractive surgery and experienced a regression of the initial effect.
The ICRS are barely noticeable to other people with the naked eye and if noticed, may appear like a conventional contact lens in the eye. It is, however, impossible for the ICRS implant to become dry or smudged like a normal 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 patient's vision should start to improve the day after the procedure, but may fluctuate for up to 3 to 6 months. The corneal curvature usually stabilise within 6 to 9 months. Patients frequently report that they experience their vision to be clearer after the surgery compared to what they were used to with their glasses or contact lenses prior to the surgery. Sixty percent of patients actually see better than 20/20 uncorrected and report that colours are richer and outlines are sharper than before surgery. A patient's perspective, who underwent INTACS™ surgery, can be found elsewhere on this website.
Most patients return to work 2 to 5 days after the procedure.
Once the first eye has functional vision, one may continue to treat the fellow eye.
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, might be noticed. This will result in a halo effect around lights. Some patients learn to adapt to this effect, if present. When a patient has difficulty in adapting to this phenomenon, a special eye drop can be prescribed to use before night driving, which restricts pupil dilation and thus these visual distortions.
There is a small risk of infection in the period shortly after the surgery, like with most surgical procedures. In a very small percentage of cases, the segment may erode (extrude) through the anterior cornea. This is usually due to the segment not being placed initially at the correct corneal depth, as was sometimes seen with the manual placements thereof in the past. However, due to the availability of the femtosecond laser nowadays (by means of which the corneal incisions are made to an extremely high level of accuracy) this risk is very low. Should the body react to the prosthesis, it is possible to remove it in an atraumatic fashion.
Because patients respond and heal differently, it is possible that the entire myopic refractive error may not be corrected, or that there may even be a slight over correction.
In cases of under correction, the vision will be clearer without glasses compared to pre-operatively. However, less powerful glasses may still be required to refine the visual acuity to achieve the best visual potential, especially under visually demanding circumstances, like night driving.
For those who might be somewhat over corrected, especially those patients in the presbyopic category, close-up vision might be difficult without glasses, as is the case with all normal sighted persons within this age group. These presbyopic patients may therefore require reading glasses after the procedure.