Introduction
Conductive keratoplasty (CK) and laser thermokeratoplasty (LTK) were originally conceived for the treatment of hyperopia by applying energy to the midperipheral cornea, causing the central cornea to become steeper. LTK has been plagued by a history of regression of refractive effect. The regression of the refractive effect after CK is less than that after LTK, allowing CK to be used to treat presbyopia, the gradual decrease in the range of accommodative amplitude resulting from age-related changes in the crystalline lens. A better understanding of corneal and collagen response to heat, combined with improved heat delivery systems, may offer new promise for the future of thermal refractive surgery.
The cornea responds to increased temperature by flattening in the area of heating owing to corneal collagen contraction. If the cornea is heated centrally, then flattening occurs in the most optically active part of the cornea, and the refractive power of the cornea decreases, with the eye becoming relatively more hyperopic. This central flattening is the optical effect that has been sought in the treatment of centrally steep keratoconus corneas.
When the corneal temperature is increased peripherally, the contracting collagen causes peripheral flattening, with a concomitant beltlike effect and resultant central steepening. The peripheral heating may be carried out in an annular pattern or in multiple radials. In general, the greater the number of peripheral burns or radials and the smaller the optical zone, the greater the central steepening. When peripheral heating is brought in centrally to the range of a 4-mm-diameter optical zone, the effect begins to reverse and central flattening begins. The optical zone diameter at which central flattening occurs seems to be pattern and modality dependent. For example, the noncontact Ho:YAG laser used in a 32-spot ring at a 3-µm optical zone produces significant central steepening on human cadaver eyes, whereas the same laser used with four to eight spots at a 3-µm optical zone causes central flattening on fresh swine eyes. A pulsed CO 2 laser used in a continuous ring pattern at a 5.5-µm optical zone causes central flattening in human cadaver eyes.
In vivo human and animal models demonstrate that the effect of the corneal collagen contraction tends to decrease with time. This regression of effect may be due to the production of new collagen by corneal fibroblasts, although the actual reason for the apparent reversal of collagen contraction is not clear.
Historical Background
Heat has long been known to affect the curvature of the cornea. Cautery was used to treat keratoconus beginning with Gayet in 1879 and continuing until the first penetrating keratoplasty was performed by Castroviejo in 1936. In 1898, Lans first reported using cautery to decrease corneal astigmatism. In 1900, Terrien reported using cautery to correct severe astigmatism in an eye with Terrien’s marginal degeneration. In 1914, Wray also reported a case of astigmatism successfully treated with corneal cautery. In 1933, O’Connor reported the successful, but variable, 10-year follow-up results on a patient with high myopic astigmatism whom he had treated with corneal cautery.
Rowsey and coworkers reported in 1980 their initial work with a 1.6-MHz radiofrequency probe (the Los Alamos Probe). This probe used a circulating saline electrode to deliver energy 200 to 400 µm below the surface of the probe, allowing the stroma to be heated with relative sparing of the endothelial and epithelial areas. The recurring problem of regression of effect seen with other thermal keratoplasty procedures also plagued the Los Alamos Probe.
In 1981, Fyodorov (in Moscow) began using superficial peripheral corneal thermal treatments for hyperopia, called radial thermokeratoplasty . By 1984, he and the engineers at the Moscow Research Institute for Eye Microsurgery developed a fine-needle probe for deeper thermal keratoplasty.
Response of Corneal Collagen to High Temperature
Human corneal collagen shrinks when its temperature is increased to 55°C to 58°C. The covalent bonds of the primary collagen structure are not disturbed, but the high temperature provides the necessary energy to disrupt the hydrogen bonds of the tertiary collagen structure, allowing the collagen triple helix to partially unwind and form new cross-links between amino acid moieties with different collagen hydration levels. The actual amount of shrinkage depends on the mechanical tensions on the collagen; in the cornea, the shrinkage is approximately 7%. If the temperature of collagen is increased past its shrinkage temperature of 65°C to 78°C, then the contracted collagen relaxes as heat-labile cross-links are hydrolyzed. The aging process increases the number of thermally stable cross-links, raising the temperature required for relaxation of the collagen. Further elevation of temperatures can cause the collagen fibers to undergo necrosis. Temperature elevation in the cornea varies by proximity to the heating source.
Histopathologic evaluation of human corneas treated with a Gasset thermokeratoplasty probe offers some insight into the changes the cornea undergoes with heating from a 100°C to 130°C surface probe. The Bowman layer appears to be more affected by destruction and abnormalities than would be expected with keratoconus alone. There is also a marked loss of hemidesmosome complexes between the basement membrane and basal cells, which may be the cause of reepithelialization problems in some thermokeratoplasty cases. Other changes included corneal vascularization, epithelial thinning and irregularity, and stromal scarring.
Laser Thermokeratoplasty
Multiple types of lasers have been investigated for use in LTK, including hydrogen fluoride, cobalt:magnesium fluoride, erbium:glass, carbon dioxide, and the Ho: Yag diode. For each of these lasers, the light energy is absorbed by water in the corneal epithelium and stroma, and the heat is passively transferred to the stromal collagen. The heating allows collagen shrinkage with subsequent topographic and refractive changes in the cornea ( Fig. 25.1 ).
Holmium:YAG Lasers
There were two principal Ho:YAG laser delivery systems for LTK. The first is a contact probe type manufactured by Summit; the second is a noncontact type manufactured by Sunrise Technologies.
Contact Holmium:YAG
The contact Ho:YAG laser from Summit Technology emitted infrared electromagnetic energy at the wavelength of 2.06 µm and operates with 300-µs pulses at a repetition frequency of 15 Hz and a pulse power of approximately 19 mJ. The laser focally raises the stromal collagen temperature to approximately 60°C by delivering 25 pulses at each treatment location. The laser energy reaches the cornea through a fiberoptic handpiece with a sapphire tip that provides a cone angle of 120 degrees. When the tip is applied to the cornea and laser energy is delivered, it creates a cone-shaped zone of collagen contraction with a base diameter at the corneal surface of 700 µm and a depth of 450 µm.
Noncontact Holmium:YAG
The noncontact Ho:YAG laser from Sunrise Technologies functioned at a 2.13-µm wavelength with a 5-Hz pulse repetition frequency and a 250-µs pulse duration. The system employs a compact, solid-state laser with a fiberoptic noncontact delivery system mounted to a slit lamp to deliver one to eight simultaneous treatment spots, each approximately 600 µm in diameter.
Koch’s in vitro studies with fresh swine eyes and this laser looked at corneal topographic changes with varying treatment zones using four to eight spots, 10 pulses per spot, and an energy density of 8 to 11 J/cm. The topographic changes were measured with the EyeSys Corneal Analysis System (EyeSys Technologies). Treatment zones of 3.0 and 3.5 mm produced central corneal flattening of up to 9 diopters (D). Treatment zones of 4.0 to 4.5 mm produced no effect, and 5.0 mm or greater treatment zones caused central corneal steepening of over 4 D. The central steepening could be increased by using 16 treatment spots instead of 8 and by placing a second ring of 16 spots around the first. Increasing the energy density of the spots also increased the curvature changes. Astigmatic treatments were made with pairs of laser spots along the flat corneal meridian.
Moreira and colleagues treated human eye-bank eyes with the Sunrise noncontact Ho:YAG laser using a 300-µm spot size and a 9 J/cm 2 laser energy with a 32-spot treatment ring (four sets of eight spots; each set rotated by 11.25 degrees) at 3- to 7-mm treatment zones. The central cornea steepened at all of these treatment zones; less steepening occurred in a near-linear fashion with enlarging treatment zones. The treatment ring produced a beltlike contraction effect in the cornea. Moreira et al. also conducted rabbit histopathologic studies of the effects of noncontact Ho:YAG laser energy, as described earlier in the section on nonrefractive corneal responses to heat. The difference in refractive results between Koch’s and Moreira’s work indicates that at smaller treatment zones factors such as spot size, spot number, and energy density play a role in the refractive outcome.
Clinical Outcomes of LTK
Noncontact LTK treatments for hyperopia have been reported by several groups. The summary of the results of 612 eyes from 379 patients who participated in both the IIa and III US studies after a 2-year follow-up has been presented by Aker and Brown. In this extended trial, no eye lost more than two lines of best spectacle-corrected visual acuity (BSCVA). No laser-related adverse effects were reported; there was a transient increase in intraocular pressure. Corneal edema (0.2%) and pain (0.2%) were also reported, as well as a mild foreign body sensation, requiring artificial tears in a small number of eyes, mostly during the first postoperative month. Astigmatism of less than 2.00 D was induced in 4.2% of individuals in the second year. Regarding efficacy, at the 2-year examination, the mean improvement in distance uncorrected visual acuity (UCVA) was 2.8 lines, with 69% of patients having at least 2 lines of improvement.
The attempted correction in this study was emmetropia in the early posttreatment period (third to sixth month). Two years postoperatively, 62.5% of eyes were within 1.00 D of emmetropia, from 11% preoperatively. The remaining 37.5% of eyes not within 1.00 D of emmetropia were all undercorrected, mostly by up to 2.00 D (4% of eyes undercorrected by > 2.00 D). The rate of postoperative refraction change was much higher during the first 3 months, around 0.3 D per month, declining to 0.1 D per month thereafter during the 2-year follow-up period. Regression of the original refractive effect after LTK had led to the development of CK, in which the regression is less severe.
Contact Holmium:YAG
The contact Ho:YAG laser from Summit Technology emitted infrared electromagnetic energy at the wavelength of 2.06 µm and operates with 300-µs pulses at a repetition frequency of 15 Hz and a pulse power of approximately 19 mJ. The laser focally raises the stromal collagen temperature to approximately 60°C by delivering 25 pulses at each treatment location. The laser energy reaches the cornea through a fiberoptic handpiece with a sapphire tip that provides a cone angle of 120 degrees. When the tip is applied to the cornea and laser energy is delivered, it creates a cone-shaped zone of collagen contraction with a base diameter at the corneal surface of 700 µm and a depth of 450 µm.
Noncontact Holmium:YAG
The noncontact Ho:YAG laser from Sunrise Technologies functioned at a 2.13-µm wavelength with a 5-Hz pulse repetition frequency and a 250-µs pulse duration. The system employs a compact, solid-state laser with a fiberoptic noncontact delivery system mounted to a slit lamp to deliver one to eight simultaneous treatment spots, each approximately 600 µm in diameter.
Koch’s in vitro studies with fresh swine eyes and this laser looked at corneal topographic changes with varying treatment zones using four to eight spots, 10 pulses per spot, and an energy density of 8 to 11 J/cm. The topographic changes were measured with the EyeSys Corneal Analysis System (EyeSys Technologies). Treatment zones of 3.0 and 3.5 mm produced central corneal flattening of up to 9 diopters (D). Treatment zones of 4.0 to 4.5 mm produced no effect, and 5.0 mm or greater treatment zones caused central corneal steepening of over 4 D. The central steepening could be increased by using 16 treatment spots instead of 8 and by placing a second ring of 16 spots around the first. Increasing the energy density of the spots also increased the curvature changes. Astigmatic treatments were made with pairs of laser spots along the flat corneal meridian.
Moreira and colleagues treated human eye-bank eyes with the Sunrise noncontact Ho:YAG laser using a 300-µm spot size and a 9 J/cm 2 laser energy with a 32-spot treatment ring (four sets of eight spots; each set rotated by 11.25 degrees) at 3- to 7-mm treatment zones. The central cornea steepened at all of these treatment zones; less steepening occurred in a near-linear fashion with enlarging treatment zones. The treatment ring produced a beltlike contraction effect in the cornea. Moreira et al. also conducted rabbit histopathologic studies of the effects of noncontact Ho:YAG laser energy, as described earlier in the section on nonrefractive corneal responses to heat. The difference in refractive results between Koch’s and Moreira’s work indicates that at smaller treatment zones factors such as spot size, spot number, and energy density play a role in the refractive outcome.
Clinical Outcomes of LTK
Noncontact LTK treatments for hyperopia have been reported by several groups. The summary of the results of 612 eyes from 379 patients who participated in both the IIa and III US studies after a 2-year follow-up has been presented by Aker and Brown. In this extended trial, no eye lost more than two lines of best spectacle-corrected visual acuity (BSCVA). No laser-related adverse effects were reported; there was a transient increase in intraocular pressure. Corneal edema (0.2%) and pain (0.2%) were also reported, as well as a mild foreign body sensation, requiring artificial tears in a small number of eyes, mostly during the first postoperative month. Astigmatism of less than 2.00 D was induced in 4.2% of individuals in the second year. Regarding efficacy, at the 2-year examination, the mean improvement in distance uncorrected visual acuity (UCVA) was 2.8 lines, with 69% of patients having at least 2 lines of improvement.
The attempted correction in this study was emmetropia in the early posttreatment period (third to sixth month). Two years postoperatively, 62.5% of eyes were within 1.00 D of emmetropia, from 11% preoperatively. The remaining 37.5% of eyes not within 1.00 D of emmetropia were all undercorrected, mostly by up to 2.00 D (4% of eyes undercorrected by > 2.00 D). The rate of postoperative refraction change was much higher during the first 3 months, around 0.3 D per month, declining to 0.1 D per month thereafter during the 2-year follow-up period. Regression of the original refractive effect after LTK had led to the development of CK, in which the regression is less severe.
Conductive Keratoplasty
The ViewPoint CK System ( Fig. 25.2 ) used to perform the CK procedure consists of a radiofrequency energy-generating console, a handheld, reusable, pen-shaped handpiece attached by a removable cable and connector, a foot pedal that controls release of radiofrequency energy, and a speculum that provides a large surface for an electrical return path. Attached to the probe is a single-use, disposable, stainless-steel, Keratoplast tip, 90 µm in diameter and 450 µm long, that delivered the current directly to the corneal stroma ( Fig. 25.3 ). The tip has a proximal bend of 45 degrees and a distal bend of 90 degrees to allow access to the cornea over the patient’s brow and nasal regions. At the very distal portion of the tip is an insulated stainless-steel stop (cuff) that ensures correct depth of penetration.
