History and evolution of thermokeratoplasty

ByRichard L. Lindstrom, MD

Multiple treatments for hyperopia currently exist, including LASIK, PRK, phakic IOL implantation, clear lens extraction and laser thermal keratoplasty. In my practice, I use each of these treatment options. I choose conductive keratoplasty for the treatment of emmetropic presbyopia and for astigmatism in patients who have had cataract surgery. Additional applications for CK include in combination with Intacs (KeraVision, Fremont, Calif.), in patients with keratoconus and for refractive enhancements of patients with overcorrection from previous laser refractive surgery.

History of thermokeratoplasty

Thermokeratoplasty has a long history. Leendert Jan Lans, MD, performed the original research on the topic in 1898 and defined the principles of keratotomy. His contribution to ophthalmology is evident in that one of the two major honorary awards and lectures of the International Society of Refractive Surgery of the American Academy of Ophthalmology is the Lans Award. The second major award of that society is the Barraquer Award, which is named after José Ignasi Barraquer, MD, who in 1949 developed the principles of lamellar surgery and investigated keratophakia and keratomileusis. More recently, in 1975, at the University of Florida, Anthony Gasset, MD, and Herbert Kaufman, MD, applied the first clinical use of thermokeratoplasty.¹

Svyatoslav Fyodorov, MD, one of the leaders in developing radial keratotomy, developed hyperopic thermokeratoplasty using a hot nickel-chromium probe. The probe was able to control thermal burns in the corneal stroma using a preset retractable cautery tip that was heated to temperatures as high as 600°C. This probe penetrated the cornea to a desired depth of up to 90% of corneal thickness.²

I choose conductive keratoplasty for the treatment of emmetropic presbyopia and for astigmatism in patients who have had cataract surgery. Additional applications for CK include in combination with Intacs, in patients with keratoconus and for refractive enhancements of patients with overcorrection from previous laser refractive surgery.

– Richard L. Lindstrom, MD

Another famous name in the history of thermokeratoplasty is Theo Seiler, MD, who in 1990 paved the way into the laser area with the first successful use of a holmium:YAG laser for contact laser thermal keratoplasty and the correction of hyperopia up to 5 D. Noncontact laser thermal keratoplasty performed with a holmium:YAG laser was proposed as an alternative for correction of low hyperopia.³

Thermokeratoplasty options

Other types of LTK are noncontact LTK and non-holmium LTK. Noncontact LTK involves the application of a circular pattern of eight spots, which is 800 µm in diameter, to the peripheral cornea with energy divided into 10 pulses, which elevates the stromal temperature. I participated in clinical trials showing that noncontact LTK performed with the Hyperion LTK laser system (Sunrise Technologies, Fremont, Calif.) provided an effective treatment for hyperopia. LTK was simple to perform and involved a laser beam delivered via a quartz fiberoptic handpiece. Eight spots were applied at the treatment zone with diameters of 6 mm, 7 mm and 8 mm. An almost perfect pattern was achieved each time, but this became problematic because adjustments in the spots could not be made. Surgeons were not able to add a spot or choose to initiate a different pattern to treat astigmatism. Although this technique caused an initial significant correction, and in some cases overcorrection, of hyperopia, it eventually led to a 100% regression. LTK with the Sunrise laser has a positive effect, but it is transient.^4,5

Non-holmium LTK involves the use of a CO₂laser and results in superficial retraction of corneal collagen and early regression of refractive effect. Another non-holmium option for LTK is that of diode laser thermal keratoplasty (DTK). This diode laser is not currently available in the United States but deserves to be mentioned because it seems to be similar to the holmium laser.

CK uses radiofrequency to create treatment spots with appropriate shape and depth. When performing CK, surgeons must strive to correctly apply treatment spots on the cornea to achieve effective results. A series of spots is placed in a circular pattern to create a band of circumferential tightening within the stroma, which secondarily steepens the central cornea.

CK histology evidence

The stroma consists of a lamellar lattice or 3-dimensional (3-D) grating of collagen fibrils. This 3-D grating begins to change as the collagen is shrunk at approximately 55°C. Denaturation, or permanent collagen shrinkage, occurs at an optimal temperature of 75°C. At higher temperatures, necrosis occurs and results in scarring.^6,7 However, if the collagen does not reach the optimal temperature for shrinkage, the tissue will rehydrate and regain its original configuration, resulting in refractive regression.

Optimal temperature can be reached and maintained with the use of the CK radiofrequency energy that is delivered deeply into the stroma (500 µm) via a Keratoplast tip that is 90 µm in width. The increased tissue temperature is induced by impedance in the flow of energy through collagen fibrils. These collagen fibrils reach approximately 75°C, which is the optimum shrinkage temperature for denaturation.

Figure 1 contains an example of the histology, or footprint, of a porcine cornea 1 week post-CK. The footprint is rather deep, although there is no significant endothelial damage and measures 250-µm wide and 509-µm deep. However, a normal human cornea’s mid-periphery, where surgeons apply these lesions, is typically 600-µm to 650-µm deep. I would also like to point out a cylindrical-shaped footprint, not a cone-shaped footprint, is achieved with CK.

Figure 1:
Example of a CK cylindrical footprint found in a pig cornea, 1 week after CK. Light transmission polarization is indicated by arrow.

Image courtesy of Refractec, Inc.

Figure 2 displays histologic evidence for stability, performed by Tatiana Naoumidi, MD, and Ioannis Pallikaris, MD, PhD, from Crete, Greece. According to this image, it appears that eyes will heal without significant scarring. Slight white scarring at Bowman’s membrane is evident. Therefore, it can be concluded that histologically, fibroblasts and keratocytes return and the patient is left with a relatively normally structured eye. However, the area in the center where the CK treatment spot was applied displays some collagen shrinkage and irregularity.

Figure 2:
Image displaying evidence for stability 6 months post-CK in corneal button. A mature fibroblast is indicated by the top arrow.

Image courtesy of T.L. Naoumidi, M.D., Crete, Greece.

Functional and biomechanical effects of CK

With CK, the treated cornea area creates an aspheric apical corneal elevation with slight steepening of 5.5 mm to 6 mm. The treatment belt zone is flattened as a result of a decreased circumference of 0.5 mm to 1.5 mm. The treatment footprint is somewhat larger than the diameter resulting from hyperopic LASIK or hyperopic surface ablation. In addition to a smooth peripheral blend, CK offers a higher quality of vision with less night vision symptoms. CK is not only a safe procedure, but it also provides the patient with a larger optical zone and a smooth blend. Research suggests that CK creates an aspheric cornea equipped with mild multifocality. However, biomechanical and optical effects are still being investigated and have not been fully defined. Data suggest that CK is a functional procedure rather than a standard refractive procedure, and patients appear to realize significant improvements in functional near vision with less loss of distance vision. These functional effects result in satisfied patients and allow surgeons to achieve blended vision vs. monovision, which is achieved by LASIK.

Surgeons should understand the functional difference between blended vision and monovision. The target with blended vision is approximately 1 D to 2 D of anisometropia. The majority of CK patients have no symptoms of stereopsis and relative amblyopia, and rarely experience monovision greater than 2 D.

References

Gasset AR, Kaufman HE. Thermokeratoplasty in the treatment of keratoconus. Am J Ophthalmol. 1975;79(2):226-232.

Neumann AC, Sanders D, Raanan M, De Luca M. Hyperopic thermokeratoplasty: clinical evaluation. J Cataract Refract Surg. 1991;17(6):830-838.

Seiler T, Matallana M, Bende T. Laser thermokeratoplasty by means of a pulsed holmium:YAG for hyperopic correction. Refract Corneal Surg. 1990;6(5):335-339.

Vinciguerra P, Kohnen T, Azzolini M, Radice P, Epstein D, Koch DD. Radial and staggered treatment patterns to correct hyperopia using noncontact holmium:YAG laser thermal keratoplasty. J Cataract Refract Surg. 1998;24(1):21-30.

Tassignon MJ, Trau R, Mathys B. Treatment of hypermetropia with the holmium:YAG laser—laser thermokeratoplasty (LTK) [in French]. Bull Soc Belge Ophthalmol. 1997;266:75-83.

Sporl E, Genth U, Smalfuss K, Seiler T. Thermomechanical behavior of the cornea. Ger J Opthalmol. 1996;5(6):322-327.

Pearce J, Thomsen S. Rate process analysis of thermal damage. In Welch AJ, van Gemert MJC, eds. Optical-Thermal response of laser-irridated tissue. New York, NY: Plenum:1995:561-605.