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 Continuing Medical Education Laser Thermokeratoplasty for Hyperopia and Presbyopia
Daniel S. Durrie, MD Recent approval of the Hyperion laser thermokeratoplasty (LTK) device allows patients with low hyperopia or who desire surgically induced monovision the potential of a quick, simple procedure to achieve refractive correction. Approximately 80 percent of adult hyperopes are “low hyperopes,” requiring corrections of 3.00 D or less.1 This indicates a tremendous potential for the technology to help many patients. The Principle and How It Works Thermal keratoplasty has had a long, albeit checkered, history, beginning with Lans’ classic cautery experiments in 1898 and extending to modern pre-clinical and clinical laser thermokeratoplasty studies.2 Thermal modification of  corneal collagen occurs when the tissue temperature is elevated by
23-40°C to reach a critical range of 58-75°C. Collagen exists in the
cornea in a triple helix formation, and as heat is applied this formation is
altered to a partly coiled arrangement. This new arrangement results in
contraction of the collagen fibrils similar to tightening purse strings. Since
the LTK works in the periphery, the result is a peripheral constriction and a
steepening of the central cornea. Typical one-day postoperative observation.The spots (sloughing epithelial tissue) are much less apparent at 24 to 48 hours. Characteristics The Hyperion laser system is a holmium YAG laser with a 2120-nm wavelength. The aiming beam is a 635-nm diode laser. The Hyperion system delivers eight simultaneous laser spots in a ring-like fashion at a 6-mm optical zone then eight more laser spots at a 7mm OZ. Each spot is approximately 0.6mm in diameter. Each delivery requires only 1.4 seconds for a total surgical time of 2.8 seconds. The pattern can be rotated if necessary. The greater the amount of laser energy applied, the greater the shrinkage of the collagen fibrils and the greater the steepening of the central cornea. The procedure is performed under topical anesthesia and could be performed in any exam room. The patient simply places his forehead into a head-rest similar to that on a slit lamp, and, with a lid speculum inserted, the patient focuses on an LED target. The eye-tracker control is used to monitor pupil movement and can terminate the  treatment if the eye moves substantially. LTK has the advantage of being considered non-invasive. No instruments touch the eye, and the procedure is extremely rapid. Because this treatment does not involve the central optical zone of the cornea, there is subsequently less risk of loss of best-corrected visual acuity. Another advantage of LTK is that the structural integrity of the cornea is not compromised. Spot size, location and pattern. LTK has disadvantages as well. First, the procedure is a relatively new technology with limited long-term follow-up. Secondly, it is limited to low levels of hyperopia, less than +2.50 D. Astigmatic corrections are still under investigation. Clinical Study Results Pre-market Approval (PMA) Cohort results met all of the U.S. Food and Drug Administration guidelines for safety and efficacy. The study involved 612 eyes, which were followed over a two-year period. Ages ranged from 40-78, and refractive errors ranged from +0.75 to +2.50 D spherical equivalent. No more than 1.00 D of cylinder was permitted in the study. All patients were targeted for plano. At 12 months, 85 percent were 20/40 or better uncorrected. At 24 months, 68 percent were 20/40 or better uncorrected. At 24 months, only 37 percent of the patients had residual refractive errors greater than +1.00 D. Only one patient had induced astigmatism greater than 2 D. There were no reported adverse events during the study and no loss of best corrected visual acuity (BCVA) for any patients at 24 months. At three months 1.2 percent of the patients had a loss of greater than two lines of best corrected visual acuity. At 12 months this dropped to 0.2 percent, and by 24 months, there were no patients with a loss of more than two lines of best corrected visual acuity. The loss in best corrected visual acuity was most likely due to irregular astigmatism, which is often seen in collagen-shrinkage procedures. As a side note, there were no patients with recurrent corneal erosion after LTK. The majority of study patients were satisfied, with the greatest satisfaction among those patients 50-59 years of age. The primary reason for dissatisfaction was undercorrection. From our clinical trial experience, some hyperopic patients were dissatisfied due to anisometropia, as the study did not allow for bilateral simultaneous treatments. Before LTK, 92 percent of the patients required correction of some kind. After LTK, only 21 percent of the patients used correction for certain tasks.  Preoperatively 24 percent of the patients experienced problems
seeing at night. At 24 months, this percentage dropped to 17 percent.
One of the characteristics of collagen shrinkage procedures is the potential for regression of effect. Sunrise Technologies has found that the effect of the LTK lasts at least three to five years. The effect of the procedure lasts longer in some patients. An LTK corneal elevation map. Compared to published results of PRK and
LASIK, long-term refractive stability of LTK is similar.3 Due to the
regression of patients within the study, the FDA has stipulated that this is a
“non-permanent” procedure. Re-treatments with laser
thermokeratoplasty are possible and have been performed, however, long-term
data on the stability of these re-treatments is limited. All hyperopic corneal
reshaping procedures seem to result in more regression than myopic procedures.
One theory suggests it may be caused by epithelial “fill-in” of the
hyperopic “gutter.” Another may be attributed to the simple fact that
humans tend to become more hyperopic during later years. There has been
evidence of an increase in hyperopia in middle-aged and older adults, due to
changes in thickness and refractive indices of the crystalline
lens,4 and, possibly, to a decrease in axial length.5
Therefore the regression may partially be a result of normal aging tendencies,
and studies should be compared to control groups of hyperopic patients within
the same age ranges. Because of these findings it may be possible that older
hyperopic patients may simply gain an additional diopter of hyperopia over a
few years if left untreated.
Sunrise system software calculates a resulting variable pulse energy depending on patient age and intended correction. The energy emitted varies between 226 and 258 millijoules (mJ) per pulse. Studies demonstrated that refractive outcome depended on three variables: age, spot number and applied laser energy. One study indicated the amount of regression for patients under 20 was more than double that of patients over 20.9 The theory behind this was that it was due to the decreased water content of the older corneal stroma. Study results also demonstrate that for similarly aged patients 240 mJ/pulse would result in 0.81 D of effect, while 300 mJ/pulse would result in 2.16 D of effect.10 Postop Medications Immediate post-treatment medications include one drop tobramycin and one drop of diclofenac. The patient is also given acetaminophen with codeine, one or two tablets every four hours for the indication of discomfort. The patient is instructed to continue with Tobradex gtts q.i.d. for one week and preservative-free artificial tears when necessary to prevent dryness, promote healing and help comfort. They may also be given diclofenac for 24 hours if needed for discomfort. Postoperative Care and Expectations Immediately after the procedure, as the anesthetic ceases, the patient may experience mild to moderate foreign body sensation. He may also become photophobic. The patient will immediately notice enhanced near vision and blurry distance vision due to the myopic overshoot. The follow-up visits are similar to other refractive surgery procedures, in that the schedule of follow-up is one day, one week, one month and three months.
At one day, the patient may still be experiencing mild to moderate photophobia and scratchiness. This varies from patient to patient and some may be quite uncomfortable. These patients should maintain their oral analgesic and diclofenac q.i.d. for one day only. The symptoms should dissipate over a few days. On slit-lamp exam, the eye should be relatively quiet. Laser spots are quite visible with overlying necrotic epithelium. Some of these necrotic spots may have already been released and only remain for the first 48 hours. All eyes at this point show deep corneal striae connecting treatment sites; these persist up to three weeks. The eight-spot ring pattern should be well-centered. The holmium spots usually show a penetration depth of 50-80 percent of corneal thickness. The central cornea should be clear, and the anterior chamber should be quiet. The initial refractive error at one day can be anywhere from -1.00 to -3.00 D. This effect immediately begins to recede and within the first week the majority of this myopic overshoot will subside. (See chart, p. 78) Studies have shown the most dramatic period of change occurs in the first three months. From three months to 24 months, the regression is quite minimal. Patients should be reminded that they will be experiencing temporary dryness for approximately one week and may notice halos and glare around lights at night during this time. Instructions should also include an explanation of fluctuating vision, especially in the first two weeks and of lesser degree in the first few months. Depending on treatment and target, help the patient understand the normalcy of the myopic overshoot and its importance for an optimal outcome. By the week-one visit, the foreign body sensation and photophobia have ended, the mild fluctuating vision begins to subside, and the patient has begun to adapt to the surgically induced monovision. Spots are markedly visible with no overlying necrotic tissue. Deep striae still remain. By one month, there is little fluctuation, and the patient is now close to the intended target although there still may be a slight overshoot of about 0.50D. At this point the treatment spots are no longer visible with the naked eye. Unlike LASIK, there is no effect on intraocular pressures, because there is limited alteration of corneal thickness. The three-month visit is simply to confirm the final prescription. You will also notice that the holmium spots are barely visible. Other Options for Hyperopia Hyperopic LASIK. Perhaps the best option for larger refractive errors is hyperopic LASIK. Unlike LASIK for myopia, which directs most of the ablation in the central cornea, hyperopic LASIK involves laser ablation in the peripheral cornea. Most experts would agree that hyperopic LASIK is effective up to about 4.00-5 .00 D of hyperopia, yet some lasers are been approved to 6.00 D. The procedure is similar to that of myopic LASIK but usually uses a larger flap, such as 9.0 mm. Most patients will initially experience a myopic overcorrection resulting in good near vision but poor distance correction and need to be educated about this prior to surgery. This myopic “overshoot” often lasts two to four weeks depending on the original refractive error. It is also important to monitor the preoperative topography and elevation more closely in hyperopic LASIK candidates. Clear Lens Exchange. Most patients above 4.00-5 .00 D will be better candidates for a clear lens exchange, particularly if they are older than 45 years of age. For younger patients with large hyperopic refractive errors, phakic intraocular lenses, such as the Staar ICL (implantable contact lens) or Artisan lens, may be options, since they will not affect accommodation. Clear lens exchange is the same as a cataract procedure, but is elective, and involves the removal of the clear lens and substitution of an IOL to achieve the desired correction. Many times a clear lens exchange is accompanied by a limbal relaxing incision to correct concurrent astigmatism. Toric IOLs are also an option in patients with significant astigmatism. Radio Frequency Wave Technology. Another future technology, currently in clinical trials, uses radio frequency waves to shrink collagen fibrils. Based on the use of a controlled release of radio frequency energy introduced into the stroma, in conjunction with the eye’s natural conductive properties, the temperature of the corneal tissue is increased, resulting in collagen shrinkage. A column of tissue (about 100-µm wide by 500-µm deep) reaches the optimal tissue temperature of 65°C. This technology involves 24 individual placements of radio frequency spots.  The optical zone marks of 6, 7 and 8mm act as a template for the
treatment application. Once the mark is applied, the surgeon then begins
applying treatment superiorly and continues until all of the rings of treatment
are complete. Striae then begin to form between the treatment spots, creating a
band of tightening. It is the tightening of the tissue that results in a
steepening of the central cornea. Clinical trials show very promising results,
though the procedure is in early trials and has limited long-term data.
Holmium: YAG pulses delivered simultaneously to eight spots on a 6.0-mm optical zone and eight spots on a 7.0-mm OZ, 1.4 second exposure per ring. Other Applications of Holmium LTK Astigmatism. Holmium laser thermokeratoplasty may be used in the future for primary astigmatism correction or for the residual or induced astigmatism following cataract or other refractive procedures. Spot patterns and nomograms are under development. Presbyopia. Perhaps the most exciting application of this technology will be in the emmetropic presbyopic population. Since studies have shown that HLTK is more effective for patients above age 40, and there is less regression of effect for lower prescriptions, these two characteristics may make the procedure ideal for presbyopes. For example, a +2.50 D hyperope may regress over time to +1.00 D, yet this 1.50 D of correction would be ideal for a patient starting at plano, to allow the non-dominant eye to see at near. Refractive surgery treatment for hyperopia is still in its infancy, with many new technologies being developed. Although LASIK will continue to dominate as the procedure of choice for the hyperope, LTK adds a new procedure to the arsenal of refractive surgical options. Compared to LASIK, this procedure will likely be less expensive, less invasive, with a quicker surgical time and will likely be performed in an office setting. Unlike LASIK, it will not likely be the primary procedure for moderate to higher levels of hyperopia due to the possibility of regression. LTK has been shown to be safe, predictable and stable for lower levels of hyperopia. The procedure is non-invasive and there is less potential for loss of BCVA. Performing this procedure outside of the operating room is a great benefit and is more convenient for the patient and the surgeon. LTK is a potential tool for overcorrected LASIK and PRK. The greatest potential for this technology may be in the presbyopic population.
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