Chapter 24 – Laser Thermal Keratoplasty and Conductive Keratoplasty
DIMITRI T. AZAR
DOUGLAS D. KOCH
• Laser thermal keratoplasty (LTK) is a procedure using laser energy to heat the cornea and increase its curvature.
• Conductive keratoplasty (CK) steepens the cornea using high radiofrequency currents.
• LTK and CK steepen the central cornea by shrinking peripheral collagen.
• The corneal time-space heat distributions differ between LTK and CK, which may explain the greater stability of CK.
• Renewed interest in LTK has emerged because of the future potential of wave front-guided treatments.
• Despite the instability of corrections, LTK and CK have a major advantage of untouched central cornea.
Laser thermal keratoplasty (LTK) and conductive keratoplasty (CK) are procedures aimed at altering the corneal curvature using laser energy to heat the peripheral corneal collagen. This results in shrinking of the peripheral and paracentral stromal collagen, flattening of the peripheral cornea, and steepening of the central cornea, the last of which offers a means of treating hyperopia and hyperopic astigmatism ( Fig. 24-1 ). LTK has been plagued, however, by regression of the refractive effect. Improved understanding of the response of corneal collagen to heat and the availability of state-of-the-art heat delivery systems may result in a promising future for LTK and related thermal refractive surgical procedures.
Conductive keratoplasty (CK) delivers low-energy, high-frequency (radiofrequency) current to heat the peripheral collagen and shrink it, which results in seemingly more stable central steepening of the cornea ( Fig. 24-2 ).
The use of heat to alter the curvature of the cornea can be traced to 1879, when Gayet used cautery to treat keratoconus. That procedure remained popular until penetrating keratoplasty was popularized by Castroviejo in 1936. In 1898, Lans described a method to reduce astigmatism by changing the corneal curvature in rabbit eyes by applying electrocautery to the peripheral cornea. By 1933, three reports of successful correction of severe astigmatism using corneal cautery had been published.   
The discovery by Stringer and Parr in 1964 of the shrinkage temperature of corneal collagen (55–58°C) led to renewed interest
Figure 24-1 Side view of the location of a 16-spot laser thermal keratoplasty application with the resultant corneal steepening.
Figure 24-2 The ViewPoint CK System from Refractec Inc.
in thermal keratoplasty. Over the past three decades, several nonlaser and laser devices have been tested.
In the early 1970s, a thermostatically controlled electric probe was used to flatten the central cornea in keratoconus patients by Gasset et al., who demonstrated excellent 1-year results
in five patients. Other investigators reported a high incidence of regression and a case of total regression. Reported complications ranged from mild surface problems to corneal melting and stromal scarring.    Nowadays, this method is used only as a surgical adjunct during keratoplasty to flatten steep cones and improve the quality of the host trephination. In a later attempt to minimize the surface problems of the electric probe, a 1.6?MHz radiofrequency probe (the Los Alamos probe) was designed to deliver thermal energy localized to a zone of 200–400?µm inside the stroma, thus sparing the epithelium and endothelium.    Poor predictability and regression of the effect led to withdrawal of this probe in 1987.  In 1984, Fyodorov used a retractable wire probe heated to 600° C for 0.3?sec, preset to penetrate the cornea at 95% depth, to perform deep coagulations in a radial pattern (radial keratoplasty) for the correction of hyperopia. A retrospective review of 159 of Fyodorov’s patients revealed unacceptably high unpredictability, regression, and serious complications (stromal necrosis, endothelial damage, and corneal decompensation). 
Fyodorov proposed a less invasive approach of laser coagulation of the cornea to overcome some of these problems. This was followed by the development and testing of CO2 lasers for corneal thermal shrinkage. CO2 lasers produced 10.6?µm radiation with very shallow penetration in the corneal tissue (99% was absorbed in the first 50?µm of the stroma).  A 1.54?µm yttrium-erbium-glass laser was also tested. It created gray coagulation cones that extended to Descemet’s membrane, but its penetration depth of at least 1?mm risked endothelial and iris damage. Efforts to find a laser with a penetration depth that approximates the corneal thickness led to the holmium:yttrium-aluminum-garnet (Ho:YAG) solid-state laser. Such a laser would have to emit light in the wavelength range of 1.9–2.3?µm, and to achieve homogeneous coagulation throughout the corneal stroma, the laser beam would have to be focused so that the maximal energy occurs in the central stroma.  In contrast to the conical footprints of LTK, the CK approach using radiofrequency current results in a cylindrical thermal footprint extending to 80% depth.
CORNEAL RESPONSE TO HEAT
Heating can induce collagen shrinkage up to one third of its native length. Thermal energy disrupts the hydrogen bonds of the tertiary collagen structure (without altering the primary structure), allowing the collagen triple helix to unwind partially and form new cross-links between amino acids. Heating human corneal collagen to temperatures of 55–58° C induces collagen shrinkage by approximately 7%. Heating past the shrinkage temperature, into the 65–78° C range, results in relaxation of the contracted collagen secondary to hydrolysis of the heat-labile cross-links. The aging process increases the number of thermally stable cross-links, raising the temperature threshold for collagen relaxation. Further elevation of the temperature (beyond 78° C) eventually leads to collagen fibers necrosis.
Appropriate elevation of corneal collagen fiber temperature results in contraction and subsequent flattening of the area of heating. Central heating of the cornea (to the 4?mm-diameter zone) results in central corneal flattening, decrease of the refractive power of the cornea, and a hyperopic shift. Peripheral heating of the cornea produces a beltlike effect of peripheral flattening with evident collagen stress lines emanating from each stromal burn, resulting in central steepening and increase of the refractive power of the eye ( Fig. 24-3 ). In general, the greater the number of peripheral burns or radials, and the smaller the optical zone beyond the 4.5?mm diameter, the greater the central steepening and the concomitant myopic shift ( Fig. 24-4 ). For astigmatic corrections, peripheral heating along a single meridian (the flatter meridian) causes central steepening along the meridian of treatment (similar to a wedge resection). A compound treatment for hyperopia with astigmatism can be designed by treating more of
Figure 24-3 Laser thermal keratoplasty treatment patterns used in some of the Sunrise clinical trials.
Figure 24-4 Corneal topography map after laser thermal keratoplasty, showing the optical zone of treatment.
the cornea or closer to the visual axis along the flattest meridian of the cornea. 
The effect of corneal collagen contraction tends to decrease with time in both human and animal studies ( Fig. 24-5 ), which might be explained by the production of new collagen by
Figure 24-5 Photographs of a cornea treated with laser thermal keratoplasty at different intervals after the procedure.
Figure 24-6 Probe applied to the corneal surface.
Figure 24-7 Noncontact holmium:yttrium-aluminum-garnet laser (Sunrise Technologies). Left, the slit-lamp delivery system; right, the laser unit, which has a fiber-optic cable to transmit the energy to the delivery system. (Reproduced with permission from Sunrise Technologies, Fremont, CA.)
corneal fibroblasts.  At least three key factors are believed to play a role in achieving adequate refractive results: the collagen shrinkage temperature, the collagen stability, and the keratocyte response. Because of the narrow temperature range for collagen shrinkage, thermal keratoplasty requires excellent control of corneal temperature. Normal corneal collagen appears to be very stable, with a probable half-life greater than 10 years, but the stability of thermally contracted collagen is not known. Minimal wound healing with a minimal inflammatory and keratocyte wound healing response would probably require minimization of temperature levels.
As the heating temperature increases, the likelihood of tissue destruction and an inflammatory response, with subsequent wound healing and remodeling, also increases. Normal energy levels from the noncontact Ho:YAG laser tested in rabbits produced the expected stromal scarring, but the endothelium appeared to be only minimally affected. With a maximal energy of 32 spots treated with at least 20?J/cm2 , the total endothelial loss was less than 1.2%.
HOLMIUM:YAG LASER THERMAL KERATOPLASTY AND CONDUCTIVE KERATOPLASTY
Ho:YAG laser devices provide adequate control to avoid overheating of the cornea past the shrinkage temperature, which could result in collagen relaxation and a wound healing response. The corneal penetration depth of this laser light, 480–530?µm, is ideal for stromal heating with minimal damage to adjacent tissue. The beam produces a cone-shaped temperature profile, which leads to more pronounced shrinkage of the collagen fibrils in the anterior than the posterior stroma, resulting in better refractive correction and long-term stability.
Two principal Ho:YAG laser delivery systems have been investigated:
The contact probe types previously manufactured by Summit Technology and Technomed ( Fig. 24-6 ), and
The noncontact device manufactured by Sunrise Technologies ( Fig. 24-7 ).
Figure 24-8 An eye immediately after undergoing conductive keratoplasty.
The CK approach is a contact method employing a disposable stainless steel tip that penetrates about 450?µm into the corneal stroma. The eyelid speculum is attached to the probe to allow for the electrical return path (see Figs. 24-2 and 24-8 ).
These systems produce different corneal temperature-time-space distributions. The laser contact mode procedure almost certainly heats stromal collagen to a higher average temperature, because of the delivery of approximately twice as much energy per spot (19?mJ × 25 pulses, versus 24–30?mJ × 10 pulses), at three times the pulse repetition frequency (15?Hz versus 5?Hz), and in a higher irradiance (strongly versus weakly focused) geometry. The CK approach may seem more invasive, but the temperature distribution is such that the treated zone is cylindrical (which seems to be more favorable than the conical distribution of the laser approaches).
Contact Laser Thermal Keratoplasty
Contact Ho:YAG lasers were manufactured by Summit Technology Inc. (Waltham, MA, USA) and Technomed (Baesweiler, Germany). They consisted of solid-state infrared lasers emitting electromagnetic radiation with a wavelength of 2.06?µm in 300?µsec pulses at 15?Hz repetition frequency and a pulse power of approximately 19?mJ. The lasers focally raise the stromal collagen temperature to approximately 60° C by delivering 25 pulses at each treatment location.
The energy is delivered to the corneal stroma through a quartz fiber-optic hand piece that is focused by a sapphire tip that provides a cone angle of 120°. The tip is applied to the corneal surface and is used to focus the laser energy to form a wedge-shaped collagen shrinkage zone that measures 700?µm in diameter at the corneal surface and reaches a depth of approximately 450?µm. 
In a typical treatment, each treatment location accommodates 25 pulses of 19?mJ per pulse. For low hyperopia, one ring of eight applications is placed in the peripheral cornea, at a larger optical zone. For higher hyperopia, a second ring of another eight applications is placed along the same radial meridians as the first eight spots but more peripherally. The probe is manually placed in contact with the cornea, after marking the locations for probe placement with an instrument similar to a radial keratotomy marker. In contact LTK, it is important to orient the hand piece perpendicular to the corneal surface during the treatment to obtain consistent coagulation profiles and to prevent decentration and irregular treatments. The epithelium generally sloughs at the treatment sites and is removed with a cotton tip. Patients generally have foreign body sensation for the first three postoperative days. For astigmatism treatment, two to four treatment spots are placed on either side of the flat meridian in a variable optical zone. 
Several reports on the clinical outcome of using contact LTK devices for the treatment of hyperopia revealed important regression, poor predictability, and significant
Figure 24-9 Change in induced astigmatism over time after laser thermal keratoplasty.
TABLE 24-1 — STABILITY OF CONDUCTIVE KERATOPLASTY (24-MONTH COHORT: ALL EYES TREATED)
Manifest reaction (n = 88)
0.04/month SD 0.13
0.04/month SD 0.13
0.03/month SD 0.04
Cycloplegic reaction (n = 79)
0.06/month SD 0.15
0.04/month SD 0.11
0.03/month SD 0.03
induced astigmatism             ( Fig. 24-9 ). Treatment of astigmatism, with two coagulation spots placed on each side of the flat meridian in the 8.5?mm zone, resulted in high regression. Thereafter, astigmatism treatment was tried in rabbits or human eye-banked eyes. The efficacy of LTK on corneas that have already undergone photorefractive keratectomy (PRK) might be different from the treatment of primary hyperopia or astigmatism, because Bowman’s layer is removed by PRK. Two reports showed that in eyes with hyperopia induced by PRK, LTK appeared to be considerably more successful than in eyes with primary hyperopia, even if the predictability of the method is low and astigmatism can be induced with the attempted spherical correction. 
CK uses a probe that heats the cornea through high-radiofrequency currents in eight peripheral locations (similar to LTK). Its major advantage is the greater stability of the refractive effect ( Table 24-1 ).
In CK, topical anesthetics are applied, followed by insertion of the eyelid speculum (which acts as a return pathway for the electrical current) without an eyelid drape. The CK tip is inserted in premarked peripheral corneal spots, and the treatment is applied according to published nomograms.
For lower hyperopic corrections, eight spots are applied at the 6?mm optical zone and eight spots are applied at the 7?mm optical zone. For greater hyperopic corrections (+2–+2.50D), 24 spots are applied (eight additional spots at the 8?mm optical zone). For even greater hyperopic corrections, 32 spots are applied ( Table 24-2 ).
The efficacy, stability, and safety of CK have been established. It is not clear why there are major differences in the stability of CK and LTK. Further studies are needed to determine the basis for these differences.
TABLE 24-2 — UNCORRECTED VISUAL ACUITY AFTER CONDUCTIVE KERATOPLASTY (EYES TREATED WITH CURRENT NOMOGRAM)*
1 Month (n = 354)
3 Months (n = 358)
6 Months (n = 352)
9 Months (n = 35)
12 Months (n = 354)
24 Months (n = 70)
UCVA 20/20 or better
UCVA 20/25 or better
UCVA 20/32 or better
UCVA 20/40 or better
FDA, Food and Drug Administration; UCVA, uncorrected visual acuity.
* No retreatment was performed during the study.
Noncontact Laser Thermal Keratoplasty
Noncontact Ho:YAG LTK treatments are performed with the Sun1000TM Corneal Shaping System and the newer Sunrise Hyperion LTK System, both manufactured by Sunrise Technologies (Fremont, CA, USA). They both consist of the Ho:YAG laser console, a slit lamp, and a polyprism beam-splitting optical tower delivery system. The Corneal Shaping System emits pulses of infrared light (2.13?m) with a pulse duration of 250?µsec full width at half maximum intensity, a pulse repetition frequency of 5?Hz, and adjustable pulse energy up to 300?mJ.  The device projects a ring pattern of up to eight spots on the cornea with an adjustable diameter in the 3–8?mm range. The system employs a fiber-optic noncontact delivery system mounted to a slit lamp to deliver up to eight simultaneous treatment spots. Each spot has a nominal spot diameter of 600?µm (containing 90% of the energy per shot) and a nonuniform energy distribution within the spot. The Sunrise Hyperion LTK System is an improved device that produces the same kind of laser light.
Topical anesthesia (tetracaine 1%) is applied, and a speculum is used to keep the eyelids open for 3 minutes to allow the tear film to dry. Epithelial drying can be achieved by blotting the cornea with a moist sponge. Centration is obtained by centering red helium neon (HeNe) laser tracer beams (wavelength 633?nm) around the entrance pupil while the patient is focusing on a flashing yellow HeNe light fixation source (Hyperion System). The laser is focused on the surface of the cornea using calibrated green HeNe laser-focusing beams (wavelength 543?nm).
In a typical treatment, each treatment location accepts 5–10 pulses to each spot simultaneously over a 1.4?sec exposure time using a pulse frequency of 5?Hz. The total pulse energy varies in a linear manner for each level of pretreatment refractive error and ranges from 228?mJ (for +0.75D correction) to 256?mJ (for +2.50D correction). For hyperopia, two concentric eight-spots rings of 8?mm and 7?mm centerline diameters are placed, with spots of 0.6?mm on radials (radial pattern, Fig. 24-10 ). The two rings are delivered in a centrifugal (inner to outer) pattern. If retreatment is necessary, the pattern can be rotated to a location between the eight original spots. Postoperatively, antibiotic and nonsteroidal anti-inflammatory drops are administered four times daily until the epithelium heals.
Hyperopia.        
Early in vitro studies with fresh swine eyes and human eye-bank eyes revealed that at smaller treatment zones, factors such as spot size, spot number, and energy density play a role in the refractive outcome. In the United States, the Corneal Shaping System has been tested in animal eyes, in poorly sighted patients, and in an early series of sighted patients.      In those trials, the histological changes after laser
Figure 24-10 Orientation of the spots delivered in a radial-pattern laser thermal keratoplasty treatment.
treatments were studied,  and it was also found that a greater change in curvature was produced using two-ring treatment instead of one-ring treatment.
Noncontact LTK treatments of hyperopic patients have been reported by several groups worldwide.          The phase I U.S. feasibility study began in November 1992, studying a total of 10 poorly sighted eyes. The phase IIa U.S. investigation started in September 1993, and 18 patients were enrolled. Thereafter, a change was made in the nomograms, and the Hyperion LTK System was used by the same company in both the explanted phase IIa U.S. study of 200 patients and the phase III U.S. study started in November 1997 that enrolled 200 additional patients.    The results in 612 eyes from 379 patients who participated in both phase IIa and phase III U.S. studies after 2 years follow-up have been presented by Aker and Brown. 
In this extended trial, the procedure appeared to be very safe, since no eyes lost more than two lines of best spectacle-corrected visual acuity. No laser-related adverse effects were reported during this clinical study, and the only adverse event reported was a transient increase in intraocular pressure. Corneal edema (0.2%) and pain (0.2%) were also reported. A mild foreign body sensation (an itchy, scratchy feeling requiring artificial tears) was observed in a small number of patients, mostly at the 1-month examination. The incidence of other symptoms (glare, photophobia, night vision difficulties, double vision) was extremely
Figure 24-11 Rate of change of refraction as a function of time elapsed after laser thermal keratoplasty. Parentheses enclose the rate of change per month at the reported interval. Horizontal lines indicate 95% confidence intervals.
low. Astigmatism less than 2.00D was induced in 4.2% of individuals in the second year (see Fig. 24-9 ). Regarding efficacy, at the 2-year examination, 69.4% of patients showed improvement of distance uncorrected visual acuity (UCVA) by two or more lines, with the mean improvement in UCVA being 2.8 lines. Near UCVA improved by two lines in 56.3% of patients at 2 years post-treatment. At 2 years post-treatment 76.4% of the individuals showed an induced reduction in their manifest hyperopia of 0.50D or more, with a mean reduction of hyperopic correction of 0.79D. No patients experienced a greater than 0.25D increase in hyperopia. Clinical outcomes were similar for both manifest and cycloplegic refraction.
In that study, the attempted correction was emmetropia in the early post-treatment period (third–sixth month). Two years after treatment, the proportion of eyes within 1.0D of emmetropia was 62.5%, up from 10.9% preoperatively. The remaining 37.5% of eyes not within 1.0D of emmetropia at the 2-year post-treatment examination were all undercorrected by more than 1.0D, with 4.2% of eyes being undercorrected by more than 2.0D. A review of the study revealed that the initial nomogram was approximately 30% underpowered, so on average, patients were undercorrected during treatment. The mean rate of change per month improved over the post-treatment period ( Fig. 24-11 ). Until the third postoperative month, the rate of refraction change was approximately 0.3D per month; thereafter, it declined to 0.1D per month and kept declining until the second postoperative year.
Retreatment was done in 10 patients participating in this expanded U.S. study by rotating the laser pattern to a location between the original spots, and this appeared to be safe and effective. International data on retreatment after undercorrection were collected by the authors by means of a questionnaire sent to eight physicians outside the United States. These physicians have not observed that the contact LTK procedure precludes the performance of subsequent ophthalmic procedures (PRK, laser in situ keratomileusis [LASIK]) or the use of contact lenses. The authors of this extended study concluded that noncontact LTK is a safe and effective method for the correction of +0.75–+2.50D of hyperopia, with a very low risk of potential complications or side effects.
Similar studies outside the United States also suggest that LTK is safe and effective for low hyperopia and has a low complication rate.  Good patient selection is the key to obtaining satisfactory results. Some authors think that the technique works best for up to +3.00D in older individuals with a central corneal thickness less than 525?µm.
Figure 24-12 Human cornea 6 weeks after laser thermal keratoplasty. Note the wedge-shaped area (apex toward the endothelium) of relatively homogeneous corneal stroma and acellularity.
Reports of noncontact LTK for treating overcorrection after myopic PRK,  LASIK, and corneal lamellar cutting found that LTK was more effective and stable than for primary hyperopia. This phenomenon may be attributed to the absence of Bowman’s layer or to low central corneal pachymetry. The wedge-shaped area of heating is evident histologically ( Fig. 24-12 ).
Laser Thermal Keratoplasty Overcorrection.
LTK overcorrections have been treated with hyperopic LASIK.  Hyperopic LASIK after LTK is safe and effective, without vision-threatening complications, and is a good alternative for hyperopic regression. Predictability and efficacy are less than with primary LASIK for hyperopia, but the procedure is equally safe.
All studies of pulsed LTK in congenital hyperopia with a minimum of 1-year follow-up have shown that the amount of correction is limited to a maximum of 2.50D with contact and 2.0D with noncontact devices. A change of more than 4.0D was achieved only if the central cornea was thinned by previous ablative surgeries.  The fact that the effect of LTK with a pulsed laser depends strongly on the pulse repetition rate led to the idea of using a continuously emitting laser source, such as a diode, to achieve a more steady temperature rise and to avoid temperature peaks. Diode LTK can induce a more lasting refractive change than pulsed holmium LTK.  Trials in porcine eyes  have shown that diode LTK can provide defined and uniform coagulation resulting in sufficient refractive changes. Although the potential exists for endothelial cell damage, diode LTK appears to be superior to pulsed holmium LTK ( Figs. 24-13 and 24-14 ). The first trial of the diode LTK technique in blind human eyes for treating hyperopia revealed that at a wavelength of 1.870?µm, corneal endothelial damage was limited, and the procedure appeared to be safe and effective. Regression occurred mainly in the first three postoperative months.
The most exciting direction of LTK is that of wave front-guided treatment. This has been shown to be possible via an adaptation to the currently available system, but it has not yet been approved by the Food and Drug Administration. The major advantage of this modification is that it is, theoretically, the only keratorefractive procedure for hyperopia wherein wave-front-guided treatment effects can be monitored and adjusted continuously during treatment. Energy adjustments during surgery may allow greater predictability, reduced astigmatism, and the ability to achieve a predetermined amount of overcorrection to ensure acceptable visual results for the longest possible duration.
This approach may also be used as an adjunct to other refractive procedures for hyperopia, including LASIK and CK. Real-time wave front-guided LTK will overcome several of the known limitations of LTK and may become an important adjunct procedure for the correction of hyperopia in the next decade.
Figure 24-13 Slit-lamp photographs 2 years after two-ring noncontact Ho:YAG laser thermal keratoplasty. A, The spots are almost imperceptible with direct slit-beam illumination. B, The spots are readily visible with sclerotic scatter.
Figure 24-14 Computerized videokeratographs following noncontact Ho:YAG laser thermal keratoplasty with two-ring application. A, Intervals for the topographical maps from upper left to lower right are preoperative and 1 week, 1 month, and 3 months postoperatively. B, Intervals for the topographical maps from upper left to lower right are 6, 12, 18, and 24 months postoperatively. This patient showed an increase in corneal steepening of 2.12D and a change in subjective manifest refraction (spherical equivalent) of -1.75D. The postoperative topographical maps demonstrate surgically induced peripheral corneal flattening and central corneal steepening with excellent stability between 12 and 24 months. Note the large (approximately 5?mm) central steepened zone.
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