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WHILE laser and non-laser thermal keratoplasty techniques are achieving increasing success in the treatment of moderate hyperopia, they are likely to be superseded by LASIK or phakic IOLs or other refractive techniques, according to Prof. Thomas Kohnen MD. "Current thermal keratoplasty techniques such as LTK and CK can provide a temporary and safe correction of hyperopia for +0.75 D to +3.0 D, but these types of procedures will become obsolete unless the problems with regression can be solved," Prof Kohnen told the XXI Congress of the ESCRS. Thermal keratoplasty treatments for hyperopia involve applying heat in the range of 55° - 75° to the peripheral corneal stroma. In this way the techniques induce collagen shrinkage and thereby increase the corneal curvature. Most current techniques involve the use of a laser. The one exception is conductive keratoplasty, which uses radiofrequency probes. A common feature of all the thermal keratoplasty techniques is that they require an initial overcorrection, after which regression occurs and the treated eye achieves the desired correction. The regression is greatest in the early postoperative months and the aim is that once target refraction is achieved the cornea will stabilise. However, with all of the current techniques regression appears to continue after the target refraction is achieved, although to a much lesser extent. In fact, the regression appears to persist for as long as patients are followed, diminishing both the safety and predictability of the procedures. The first contact Holmium laser thermokeratoplasty system had such high degrees of regression and unpredictability (43% loss of 1 line of BCVA) and additional problems with induced irregular astigmatism that the system used was abandoned in the US . Better results were achieved with the non-contact Sun 1000 Holmium laser system from Sunrise . However, in the FDA Phase IIa clinical trial the refraction changed by over half a dioptre among those undergoing higher corrections up to 4.0 D between six and 12 months postoperatively. "The main regression occurs during the first six months but there is still continuous regression. In their summary of the results, the FDA said that with the one ring treatment there was a change in the cornea of 0.5 D and with the two ring treatment up to 1.5 D of hyperopic change." Newer laser thermal keratoplasty systems such as the Hyperion™ (Sunrise Technologies) have included several enhancements to the original device such as consecutive ring-treatment, an eyetracker and individual control of energy delivered to each spot. The upgrades appear to have yielded improved outcomes. Based on results of a clinical trial involving 612 eyes with a follow-up of three months to two years, the FDA granted approval for the Hyperion system to be used for the "temporary reduction" of +0.75 D to +2.5 D of hyperopia and less than 0.75 D of astigmatism. However, several investigators in Europe and the United States have reported difficulties with induced astigmatism when using the system, and questions remain about its long-term stability. Further refinements to LTK using the Sunrise system are now under study. They include the use of three rings and lower energies, to make the procedure more predictable and with less regression, and pre-wetting the cornea to reduce the amount of induced astigmatism. Wavefront technologies are also being introduced to LTK and may make possible the correction of lower- and higher-order aberrations. Diode lasers can provide another means of achieving collagen shrinkage in the cornea. These are employed in the DTK Prolaser system from Rodenstock. As with the earlier Holmium laser systems, the laser probe contacts the cornea and focuses heat at a certain depth within the cornea. The published research with the laser is not very extensive. However, a study carried out at the University of Tuebingen , where the laser system was first developed, showed that the laser achieved a 2.0 D reduction in hyperopia in 45 hyperopic patients after one year's follow-up. Furthermore, refraction had greatly stabilised within the first postoperative month, during which time there was only 1.0 D of regression. Between three and six months follow-up there was less than 0.20 D of regression after which refraction appeared to stabilise. "The results are not bad but still the question of long term stability is the main issue," Prof Kohnen said. Conductive keratoplasty (Viewpoint™ CK REFRACTEC Inc, USA ) is the one non-laser thermal keratoplasty technique currently available. More than 300 surgeons are certified to perform conductive keratoplasty and nearly 20,000 procedures have been performed worldwide. The conductive keratoplasty system uses radiofrequency probes inserted into the stroma to induce collagen shrinkage. As with other thermal keratoplasty systems the "spots" are positioned around the corneal periphery in one or two rings so that that the collagen will shrink to produce a "purse-string" effect which increases corneal curvature. However, as with the laser systems, the effects of CK are both somewhat imprecise and temporary and regression appears to continue indefinitely. In the USFDA trial, which involved 318 eyes that underwent conductive keratoplasty for the correction of low to moderate hyperopia, only 54% were within 0.5 D of emmetropia at 24 months' follow-up while 82% were within 1.0 D. Furthermore, while after 24 months there was a mean refractive change of less than half a dioptre in 75% of eyes (versus the FDA target of 50%) and of 1.0 D or less in 96% of eyes (versus the FDA target of 95%), there was a continuous change in refraction averaging at 0.14 D per month between nine and 12 months, and 0.03 D per month between the 12th and 24th months. Prof Kohnen noted that while the results with CK and some of the latest versions of LTK provided fairly good results in the short to medium term, they compare unfavourably in terms of regression to other alternatives available for hyperopia such as LASIK and phakic IOLs. "These procedures will only continue to be a part of clinical practice if problems with regression can be solved, and at the moment I am a little bit sceptical that there will be a way to do this." Thomas Kohnen
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