Thermal keratoplasty techniques face off at ISRS meeting

Top doctors discussed the pros and cons of three thermal keratoplasty techniques for hyperopia.

NEW ORLEANS — Thermokeratoplasty procedures are receiving increasing attention as options for correcting hyperopia and possibly presbyopia. Clinical trials are reporting greater stability of correction with refined treatment protocols. During the annual meeting of the International Society of Refractive Surgery here, Edward E. Manche, MD, George O. Waring III, MD, FACS, FRCOphth, and James Rowsey, MD, discussed the latest developments in thermokeratoplasty in a “face-off” comparing three techniques currently available or in development.

One of the surgeons still urges skepticism, based on his experiences with earlier techniques. But even he is enthusiastic about modern results with one technique.

The technologies represented in the face-off included conductive keratoplasty (CK) using the Refractec ViewPoint CK system, diode thermal keratoplasty (DTK) using the Rodenstock DTK from ProLaser Medical Systems, and laser thermal keratoplasty (LTK) using the Sunrise Hyperion laser.

Conductive keratoplasty

“So far, CK appears to be more stable than LTK,” said Dr. Manche, of Stanford, Calif., taking the side of CK. “CK achieves stability after approximately 3 months. LTK starts at a lower attempted correction of about +2.5 D and by 2 years has progressed back to a +1 D.”

Dr. Manche said the difference in stability is due to the differing methods of heating the cornea.

“Radio frequency energy is laserless. Heat results from the tissue’s resistance to the current,” he said.

He added that the advantage of this is a consistent temperature gradient along the probe for an optimal time/temperature profile for collagen shrinkage.

Another difference is that CK uses a 400-µm probe to uniformly heat the area of the cornea to 65°C, which creates a cylindrical footprint. LTK, in contrast, penetrates the cornea to about 300 µm and creates a conical footprint.

“LTK affects the cornea more superiorly and less deeply,” he said.

According to Dr. Manche, 9-month data on CK is comparable to the Summit Autonomous premarket approval data submitted to the Food and Drug Administration for treatment of hyperopia with LASIK.

“The [treatment of hyperopia with LASIK] data did have a little bit higher manifest, up to a +2 D on mean, and the CK was a +1.86 D. But the results are quite similar, with 95% of patients achieving 20/40 or better and 50% achieving 20/20 or better,” he said.

Dr. Manche said that predictability was also similar between both procedures, 88% of patients being within 1 D and 65% being within 0.5 D of target correction. He said the safety profile for CK is good, with no patients losing greater than two lines of vision, and that CK holds benefits over LTK.

“[One advantage] is that you achieve a larger optical zone. With a larger optical zone there is less potential for glare and halos and possibly improved contrast sensitivity,” he said.

Dr. Manche concluded by stating that CK has an excellent safety profile, with no loss of best corrected visual acuity, and few reported complications as seen in LASIK.

“CK provides outcomes equal to H-LASIK, has an excellent safety profile and appears to have greater stability than previous thermokeratoplasty procedures, especially LTK,” he said.

The FDA approved CK with conditions late in November (see "FDA panel says CK is approvable for temporary reduction of hyperopia" for more on the approval).

Diode thermal keratoplasty

Dr. Waring said that DTK using the Prolaser Rodenstock DTK is in evolution, “as I think all thermal keratoplasty is. I think we need to work out the variables of age and burn depth as well as the technical variables with the instruments. I think we should continue research along these lines,” he said.

“I’m afraid that the thermokeratoplasty approach up till now has been one-size-fits-all, more or less, by just changing the location and number of the spots,” he continued.

According to Dr. Waring, of Emory Vision Center in Atlanta, DTK is performed using a continuous wave diode laser that has the ability to adjust the amount of energy delivered to the cornea at each spot.

“Therefore, you should be able to get more predictable depth of burns,” he said.

He said that DTK uses spot-by-spot, manually applied treatments at 6-mm and 7-mm zones following radial marks, whereas the Sunrise Ho:YAG laser for LTK applies all eight spots at once by splitting the beam.

“This means the diode should give more flexibility in terms of spot depth and placement and maybe more accuracy as well,” he said.

Dr. Waring also commented on an early trial series done at Tübingen University, Germany. He said the trial results cannot be used to judge the final efficacy of DTK because the results were grouped generally, placing all participants together, before it was appreciated that age and corneal thickness have an effect on outcomes.

“You see a large spread of age from about 20 to 77 years and you see that we are talking in round numbers from about 2.5 D up to 5 D average correction,” he said.

Dr. Waring said that an average of 3 D of hyperopia can be corrected using a 6-mm zone.

“But there is an initial overshoot and then a change over 6 to 9 months for stability,” he said. “Similarly, in the 7-mm zone you can correct an average of about 2 D of change. If you look at the visual acuities on the European decimal scale, you see what you would expect. But the visual acuity seems to stabilize a little quicker than the refraction does.”

Dr. Waring said they have had similar outcomes in multicenter trials and that age has a big effect on the results. There was much more regression seen in younger patients as compared to older patients.

“You get your initial change and then quite a regression, with an average of 1 D of effect in young people age 21 to 24. So age is very important, not only for stability but for calculating the outcome,” he said.

Dr. Waring said that 100% of the eyes treated for 0.75 D or 1 D are stable by 6 months or between 6 and 9 months.

“I think thermal keratoplasty is the same [as RK]. You have to change the corneal structure. I don’t think it’s really any different than RK. Different mechanism, same basic biomechanics,” he said.

He added that measuring the thickness of the cornea on an individual eye for DTK is as important as measuring the thickness of the eye for RK.

“Once RK matured, none of us would have done RK without a pachymeter measuring the thickness, without setting the depth of the knife blade. Similarly, we shouldn’t be doing thermal keratoplasty without measuring the thickness of the cornea and being able to adjust the depth of the burn,” he said.

Laser thermal keratoplasty

Dr. Rowsey, of Tarpon Springs, Fla., said the main concern in thermokeratoplasty using any of these modalities is regression.

“The normal cornea is type 1 collagen. When heating the cornea, it shrinks at about 60°C, maximum at about 90°C. In that range, collagen denaturation begins. The collagen is unwound from its triple helix into random alpha chains and loosens the tensile strength in the cornea,” he said.

He referred to his own work in 1976 at the Los Alamos scientific laboratories on radio frequency heating of the surface of the cornea.

“In that period between 1976 and 1981, we treated the cornea in keratoconus patients. But after approximately 3 years, we noticed there were some areas of thinning of the cornea and gradual corneal hysteresis back to — in some cases — irregular astigmatism,” he said.

“Subsequently, we had an update and retraction. The instability of the corneal stroma that produced the regression required that we retract our recommendation for LTK in keratoconus patients.

“As William Safire said, the trick to eating crow is convincing others how good it tastes. I’ve eaten crow on heating the cornea 20 years ago and so I’m more hesitant to eat more crow on heating the cornea again.”

According to Dr. Rowsey, the most recent studies of LTK conducted by Sunrise demonstrate that judging by the amount of regression at 18 months, continuation at that rate of regression would mean relative corneal stability for 10 years.

“But most of my patients that are coming for hyperopic treatment have already been convinced they are going to see well not a month later or 2 months later, but an hour later. So the chair time it takes them to understand that it sometimes takes a week, sometimes a month or sometimes even 3 to 6 months and that a subsequent treatment may be required decreases their enthusiasm,” he said.

Overall early results with LTK were disappointing, with 50% regression by the 2-year follow-up. However, most patients have excellent visual recovery after about 3 months, he said.

“So you have to continue to incorporate that degree of regression into your algorithm,” he said.

Dr. Rowsey referred to a study conducted by Dan Durrie, MD. In the first 29 patients, a total of 43% of the eyes lost one line of vision. However, that has been overcome by modifications of the nomogram and refinement of the technique.

“So that now we don’t expect anywhere near this degree of loss of vision postoperatively. Of 612 patients in the Sunrise protocol, only four eyes lost best corrected vision, two of which were because of cataracts. Also, two of the reported decreases in vision returned to normal vision,” he said.

“The thing I found amazing is that the doctors who had done [procedures] in the thousands range have diminished their enthusiasm because of the requirements for the both the time of explanation and the enhancement rate that’s required.

“Several good surgeons have given it up, observing that most of the patients, 100% of them, are happy prior to LTK, but some of them require a lot of hand-holding time, explaining to them on the day after surgery and the week after, the month after that their vision will return. Then at 2 to 3 months all of the sudden they are becoming happy again as their vision stabilizes. Three months of hand-holding tires the patient and the doctor.

“Some of the newer procedures may heat the cornea in a more controlled fashion, and we may have longer stability, but I want you to consider that unless you have 3 years of follow-up in this procedure and the data that confirms that stability, it can continue to remodel,” Dr. Rowsey said.

For Your Information:

• Edward E. Manche, MD, can be reached at Stanford University Medical Center, Room A175, 300 Pasteur Dr., Stanford, CA, 94305, (650) 725-5765; fax: (650) 723-7918; email: edward.manche@stanford.edu. Dr. Manche does not have a direct financial interest in any of the products mentioned in this article nor is he a paid consultant for any companies mentioned.
• George O. Waring III, MD, FACS, FRCOphth, can be reached at Emory Vision Correction Center, 4170 Ashford Dunwoody Rd., Ste. 300, Atlanta, GA 30319; (404) 250-9700; fax: (404) 250-9006; e-mail: georgewaring@emoryvision.com. Dr. Waring does not have a direct financial interest in any of the products mentioned in this article. He is a paid consultant for ProLaser.
• James Rowsey, MD, can be reached at St. Luke’s Cataract and Laser Institute, 43309 U.S. Highway 19 N, Tarpon Springs, FL 34689; (800) 282-9905; fax: (727) 938-5606. Dr. Rowsey has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Refractec Inc., manufacturers of the ViewPoint CK system, can be reached at 3 Jenner, Suite 140, Irvine, CA 92618; (949) 784-2600; fax: (949) 784-2601.
• ProLaser Medical Systems, distributors of the Rodenstock DTK, can be reached at P.O. Box 31341, London, Great Britain, SW18 4FP; (44) 20-8875-2900; fax: (44) 20-8875-2909; e-mail: info@prolasermedical.co.uk; Web site: www.dtklaser.com.
• Sunrise Technologies, makers of the Hyperion laser, can be reached at 3400 W. Warren Ave., Fremont, CA 94538; (510) 771-2389; fax: (510) 771-2292; Web site: www.sunrise.md.