The Keravision Intrastromal Corneal "Ring" or Intacs™
Introduction
In April of 1999, the FDA approved a new form of vision correction surgery which can treat up to -3.50 diopters of nearsightedness (myopia) using a non-laser technique. The procedure involves implanting plastic (polymethylmethacrylate - PMMA), semi-circular segments into the periphery of the cornea. The thickness of the plastic segments determines the amount of correction. Currently, there are three thicknesses available to treat nearsightedness from -1.00 diopters to -3.50 diopters.
Procedure
The intrastromal corneal ring (ICR) or Intacs™ achieve their result by flattening the front corneal curvature, thereby diminishing and/or eliminating the nearsightedness. Specifically, surgery is performed by making a small 2mm incision in the upper part of the cornea (the clear window of the eye) approximately 70% corneal depth. Two pockets are then formed in the periphery of the cornea. The plastic segments are then inserted into these pockets away from the central portion of the vision.
The surgical procedure is performed as an outpatient using powerful numbing eyedrops for anesthesia, is painless, and takes about 15 to 20 minutes for both eyes. The post-operative course consists of very mild discomfort similar to an eyelash in the eye for the first 24 to 36 hours after the procedure. Patients begin to see significant improvement of vision on the first day after the surgery and may return to work the first day after surgery. Antibiotic eye drops containing cortisone are used 4 times a day for one week.
RESULTS
Accuracy
The results of the FDA studies on 410 eyes ranging in myopia from -1.00 diopter to -3.50 diopter demonstrated that 98% of patients achieved 20/40 vision or better, 78% of patients achieved
20/20 or better, and 56% achieved 20/16 or better. For low degrees of myopia, these results are as good or slightly better than achievable using LASIK or PRK. (Table 1)
TABLE 1
Predictability
With respect to predictability, at 12 months 68% of patients were within 0.50 diopter (half of a unit of nearsightedness) of intended correction, and 90% of patients were within 1.00 diopter.
Enhanced Visual Performance
19.5% of patients demonstrated a statistically significant improvement in best spectacle corrected visual acuity (BSCVA) compared to their pre-operative vision of one or more lines on the visual chart. However, we cannot predict which patients will fall into this 19.5%. Therefore, no patient can "expect" to have this result.
Astigmatism
The ring segments (Intacs™) do not correct for astigmatism per se. However, patients with low amounts of astigmatism (less than 1.00 diopter) may have the Intacs™ and still achieve excellent post-operative vision without glasses or contact lenses. As in any form of refractive surgery, rarely the Intacs™ will actually induce a small amount of astigmatism. This is generally not a clinically significant problem.
Risks
Although no procedure is risk free, the risks of serious damage to the eye from ICR surgery are minimal and include the possibility of infection introduced at the time of surgery, surface cells of the eye (epithelium) growing into the tracks where the rings have been implanted requiring removal of the ring, and the remote possibility of permanent damage to the cornea or the inside of the eye. Any surgery on the eye can result in a permanent decrease or loss of vision. Fortunately, severe complications are exceedingly low. Side effects of the ICR include glare, halos, difficulty with vision at night, and fluctuation of vision throughout the day.
Even if a complication does not occur, as with all surgical refractive procedures, patients may not be 100% satisfied with the result and/or may require the use of glasses to "fine tune" the vision for distance tasks such as driving by day or night. Additionally, if monovision is not performed, patients over 40 having both eyes corrected for distance will require reading glasses just as they would if they had not had surgery but continued to wear contact lenses.
Reversibility
If for any reason a patient is not satisfied with the vision following implantation of the ring segments, they can be removed. In almost all cases the eye is returned to (or very close to) its original refraction (amount of nearsightedness). In some cases following removal, the amount of nearsightedness and/or astigmatism is not identical to pre-operative levels and a new spectacle (or contact lens) prescription is required. In some cases following removal, the amount of nearsightedness and/or astigmatism is slightly more or slightly less than pre-operative levels. In some cases following removal of the segments there will be a small amount of astigmatism (curvature of the cornea) actually induced by the surgery where none had existed before the surgery.
How to Choose A Surgeon
As in all surgery, it is important to carefully choose the surgeon. We have specialized in cornea and refractive surgery for over 20 years. Other than the FDA ICR trial sites, we have more experience in performing ICR surgery than any surgeon in Northern California.
INTACS™ vs. LASIK
Although LASIK and PRK for low amounts of nearsightedness is safe and extremely effective, the ICR offers an alternative for those patients concerned about having laser surgery performed over the central visual portion of the eye. The advantage of the ICR over PRK or LASIK for patients with low myopia is that no surgery is performed in the central visual axis. Another advantage is that if the patient is not satisfied with the vision and/or there are any problems associated with the ICR, it is easy to remove the ICR segments and in almost all cases, return the eye to, or very close to, its pre-surgical state. Additionally, as the eye changes its near focus with age, the rings can be replaced to accommodate for this change.
How does a patient choose one surgical modality over another? Fortunately, the ICR, LASIK, and PRK all have very excellent results in patients with low amounts of myopia. It is certainly a very personal decision as to which procedure to choose. The good news is that barring a severe complication, which is very rare with experienced surgeons, the surgical results are comparable and are outstanding. The choice primarily relates to the reversibility of the ICR procedure if results are not satisfactory and/or a problem occurs, and whether a patient is concerned about having surgery on the central visual axis. The relative advantages of the ICR versus LASIK are enumerated below in Table 2.
TABLE 2
