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INTRACORNEAL implants may be a useful option in select patients with high hyperopia, although concerns remain about the mysterious deposits that appear on the lenses in virtually all patients who receive them, according to German ophthalmologist Michael C. Knorz MD. Hyperopia of more than +3.0 D is beyond the scope of LASIK, PRK and thermokeratoplasty procedures. While phakic IOLs can be an effective option in such individuals, performing such invasive surgery in healthy eyes without progressive disease remains controversial. Meanwhile, intracorneal implants have a similar safety profile to LASIK but also have the advantage of being more or less reversible, Dr Knorz told the XXI Congress of the ESCRS. Various types of intracorneal implants have been investigated over the past 30 years. They have included the use of re-shaped stromal tissues from the patient's own cornea, and implants made from a variety of artificial materials. However, most of these techniques have been abandoned because of safety concerns. More recently, a new, more biocompatible design of intracorneal implant, the Permavision implant from Anamed Inc., has become commercially available in Europe .The implant is composed of a microporous hydrogel material with a 78% water content and a refractive index very close (1.376 ± 0.005) to that of the corneal stroma. In addition, the optical clarity of the material means that it transmits 99% of light. Furthermore, the material is also more permeable than the stroma as regards both glucose and oxygen, which should ensure adequate nutrition of the cornea. Dr Knorz described the results obtained in a multicentre trial in which he participated and which involved 109 eyes with a mean spherical equivalent of +3.51 D (range +3.0 D to + 8.0 D) underwent implantation of the intracorneal inlays.At three months follow-up, the mean spherical equivalent was 0.09 D and roughly two-thirds within 0.5 D of intended refraction, he said, adding: "Predictability with the implants is pretty good but not great. Two-thirds are where we want them. This compares favourably with conductive keratoplasty, which brings 50% of lower hyperopes within half a dioptre of intended correction. On the other hand, it is inferior to what we expect from myopic LASIK where you usually see 90-92 % within half a dioptre." Dr Knorz noted that among the remaining third of patients there were several eyes that were more than 2.0 D outside of intended correction. He pointed out, however, that this was partly due to the fact that several patients had refractive errors beyond that of the study protocol (that is, more than +6.0 D). Moreover, it is a relatively simple matter to replace an implant of incorrect power with one of the correct power. However, over one-third of patients lost at least one line of BSCVA, and 15% lost two or more lines. Again this may have been due to the inclusion of patients who did not conform to the study protocol, he said. Complications included decentration, "deposits", haze glare and halos. Of those, decentration is the most easily avoided and treated, Dr Knorz noted, adding: "If you have a de-centred ablation with LASIK you really have problems but with the implants you simply lift the flap, re-centre the implant and all is well. What we have learned from experience is that decentrations can be avoided by using small-hinged inferior flaps. In any case it's not critical, as it is easy to treat." The "deposits" are another matter, however, as it is not yet at all clear what causes them or even what they really are. They occur in virtually every patient, beginning on the edge of the implant and gradually progressing towards the centre during the first six postoperative months. In some patients, the deposits completely encapsulate the implant. "Even with confocal microscopy studies, we weren't able to determine what the deposits were. Maybe it's similar to what we saw with INTACS because there is a little space at the very edge of the implant and that provides room for debris or cells to accumulate. The implant itself remains crystal clear: there are no deposits within the implant. Instead it's a like an interface response although it is not inflammatory and does not respond to steroids and it is not something you can really remove." What really makes the deposits a cause for concern is that they cause forward light scattering, which in turn causes glare and other photic phenomena. Furthermore, a fairly high proportion of patients have been troubled enough by their visual symptoms to request explantation of the intracorneal implants. In Dr Knorz's own series of 21 patients, six have requested explantation because of visual symptoms apparently caused by the deposits. In addition, similar deposits have occurred with another intracorneal implant from AMO, and in his series of seven eyes receiving the implants, one patient has requested explantation because of deposit-related symptoms, he noted. Removing the implants is the only course of action available to the surgeon when a patient finds the symptoms intolerable. While that appears to solve most of the problems, the appearance of the patient's cornea remains slightly abnormal. "To say that the procedure is reversible is not entirely accurate. The symptoms get much better and most of the glare disappears. However, if you look at the cornea, and the longest post-explantation follow-up is about a year, you still see some faint haziness adjacent to the former position of the implant." The question of whether the deposits can be prevented remains unresolved. It may be that deeper implantation of the implants could reduce the occurrence of the phenomenon. However, that would require the use of special microkeratomes that have yet to be designed. "The data I presented is probably limited by the fact that the implants are not in the position they should be. Research with previous intracorneal implant designs carried out independently by both Barraquer and Choyce suggests that they should be implanted much more deeply in the cornea although not so deep as to cause ectasia, considering the unsutured flap."Dr Knorz said that intracorneal lens implants seem to be a logical option in hyperopia, having the advantages of being reversible and able to correct a fairly high degree of hyperopia. However, questions remain about the ideal surgical technique to be used with the implants. Furthermore, they are useful only in a small target population as most high hyperopes have cylinder, which the implants will not correct. "If intracorneal implants are to have a renaissance it will be as a niche technology. Their mainstream use seems unlikely based on current results." Michael C. Knorz, MD |
