Electrosurgical keratoplasty. Clinicopathologic correlation

Radiofrequency heating of the cornea: an engineering review of electrodes and applicators

This paper reviews the different applicators and electrodes employed to create localized heating in the cornea by means of the application of radiofrequency (RF) currents. Thermokeratoplasty (TKP) is probably the best known of these techniques and is based on the principle that heating corneal tissue (particularly the central part of the corneal tissue, i.e. the central stroma) causes collagen to shrink, and hence changes the corneal curvature. Firstly, we point out that TKP techniques are a complex challenge from the engineering point of view, due to the fact that it is necessary to create very localized heating in a precise location (central stroma), within a narrow temperature range (from 58 to 76ºC). Secondly, we describe the different applicator designs (i.e. RF electrodes) proposed and tested to date. This review is planned from a technical point of view, i.e. the technical developments are classified and described taking into consideration technical criteria, such as energy delivery mode (monopolar versus bipolar), thermal conditions (dry versus cooled electrodes), lesion pattern (focal versus circular lesions), and application placement (surface versus intrastromal).

Effects of monopolar radiofrequency energy on ovine joint capsular mechanical properties

Radiofrequency energy may provide a relatively noninvasive method to stabilize joints with excessive laxity by thermally shrinking redundant joint capsular tissue. The authors determined the percentage of shrinkage associated with five radiofrequency treatment temperatures and evaluated the effect of this energy on the structural properties of joint capsular tissue in vitro. First, 36 adult sheep femoropatellar joint capsular specimens were treated with one of five treatment temperatures (n = 6 per group) or served as a control to determine tissue shrinkage. An additional 24 specimens were treated with three temperatures that resulted in different shrinkage: 45 degrees C, 65 degrees C, and 85 degrees C. Tissue stiffness, relaxation, and failure strength were determined for each specimen (n = 6 per group). Tissue shrinkage was correlated significantly with treatment temperature. There was a significant decrease in tensile stiffness in the 65 degrees C and 85 degrees C treatment groups. There were no significant differences between stress relaxation before treatment and after treatment. Relaxation properties after treatment were not different from each other or from control values either normalized to pretreatment values or expressed as raw data. Failure strength was not affected significantly at any temperature.

Continuous-wave diode laser thermokeratoplasty: first clinical experience in blind human eyes

PURPOSE

To evaluate the safety and stability of laser thermokeratoplasty (LTK) with a continuous-wave diode laser in blind human eyes and to optimize parameters for a study in sighted eyes.

SETTING

Department of Ophthalmology, Medical University Lübeck, Germany.

METHODS

A continuous-wave diode laser was set to emit radiation with a wavelength of 1.854 microns (Group 1, n = 4) or 1.870 microns (Group 2, n = 4) and 100 to 150 mW power for 10 seconds. A focusing handpiece was coupled with an application mask and fixed by partial vacuum to the conjunctiva or cornea. The radiation was focused into the corneal stroma between 400 and 600 microns in Group 1 and set to 1000 microns in Group 2. Eight (Group 1, single ring) or 16 (Group 2, double ring) coagulations were applied.

RESULTS

The refractive change increased with higher laser power and smaller ring diameters. Two rings of coagulations provided higher and more stable refractive changes of up to 5.66 diopters (D) than a single ring. The refractive effect stabilized between 3 and 6 months postoperatively. At 1 year, mean refractive change was +0.99 D +/- 0.39 (SD) in Group 1 and +2.32 +/- 2.24 D in Group 2. Extensive endothelial damage occurred in Group 1 but was minimal in Group 2.

CONCLUSIONS

Diode LTK was used to treat hyperopia safely and effectively. Regression occurred mainly in the first 3 postoperative months. With a wavelength of 1.870 microns, corneal endothelial damage was limited.

Correction of Hyperopia with Non-contact Ho:YAG Laser Thermal Keratoplasty

PURPOSE

To assess the potential of non-contact holmium:YAG laser thermal keratoplasty to safely and effectively correct hyperopia up to +5.00 diopters (D).

METHODS

Non-contact laser thermal keratoplasty with a holmium:YAG laser using energies ranging from 215 to 255 mJ with 6, 7, and 8-min ablation zone diameters, was applied to 57 sighted eyes of 42 hyperopic patients in a controlled study. All patients were followed for 15 months after surgery.

RESULTS

Preoperative spectacle-corrected visual acuity returned to baseline levels from 2 to 6 weeks after treatment. Regression of effect was evident in all eyes, and after 15 months was total in 18 eyes (31.5%). The mean cycloplegic refraction before surgery was +3.80 +/- 0.22 D (range, +1.50 to +5.00 D); after surgery, it was +1.40 +/- 0.12 at 3 months, +1.69 +/- 0.19 D at 6 to 8 months, and +1.73 +/- 0.16 D at 15 months. At the end of the study, 41 eyes (71.9%) had an uncorrected visual acuity of 20/40 or better. Thirty-two eyes (58.8%) were within +/- 1.00 D of the intended refraction. Patient age and central pachymetric values were significantly correlated with regression (ANOVA, (p less than .005).

CONCLUSIONS

Regression of initial effect can be large with non-contact holmium:YAG laser thermal keratoplasty. We think the technique works best up to +3.00 D in older individuals with central corneal thickness less than 525 microns. Algorithms to improve the final results should include an initial calculated overcorrection adjusted on variables that influence regression such as patient age and corneal thickness.

Correction of Hyperopia Induced by Photorefractive Keratectomy Using Non-contact Ho:YAG Laser Thermal Keratoplasty

PURPOSE

To evaluate the safety and effectiveness of non-contact holmium:YAG laser thermal keratoplasty in correcting hyperopia induced by photorefractive keratectomy (PRK).

METHODS

Non-contact holmium:YAG laser thermal keratoplasty was applied to 14 eyes with significant hyperopia induced by PRK. The mean spherical equivalent refraction before holmium:YAG laser thermal keratoplasty was +4.20 +/- 1.80 diopters (D) (range, +1.75 to +6.25 D). The results were evaluated 12 months after holmium:YAG laser thermal keratoplasty.

RESULTS

No sight-threatening complications occurred. Recovery of spectacle-corrected visual acuity took from 2 to 6 weeks. The immediate significant myopic shift that developed in all eyes gradually receded over 6 to 8 weeks. All eyes were relatively stable after 9 months. At 12 months, there was no statistically significant difference (p < .005) between the mean preoperative spectacle-corrected visual acuity (0.71 +/- 0.12) and the mean postoperative uncorrected visual acuity (0.65 +/- 0.28). At 12 months there was a mean increase of 4.60 +/- 1.20 D in central keratometric power. Total regression did not occur in any eye.

CONCLUSIONS

Non-contact holmium:YAG laser thermal keratoplasty offers a safe and effective alternative to correct PRK-induced hyperopia.

Wound healing in response to keratorefractive surgery

Over one million Americans have undergone refractive keratoplasty since the introduction of radial keratotomy into the United States in 1978. There are now a number of alternative techniques available for reshaping the corneal surface to alter ocular refractive errors. Numerous technologic advances in the past decade now enable us to perform these procedures in a safer and more reliable fashion. The ability to control precisely the refractive outcome, however, continues to elude us and appears to be limited, in part, by interindividual variability in the wound healing response. Presently, we review the corneal wound healing response to various keratorefractive approaches and suggest some interventional strategies which might enable us to modulate more precisely our refractive results.

Holmium laser thermokeratoplasty

PURPOSE

Corneal curvature can be altered by shrinking stromal collagen with a pulsed solid-state holmium: YAG laser in a procedure termed laser thermokeratoplasty.

METHODS

The authors performed laser thermokeratoplasty in 40 human cadaver eyes using a ring pattern of 32 spots, each spot having a diameter of 300 microns.

RESULTS

The amount of induced corneal steepening decreased as ring diameter was increased in 1 mm increments, with 22.2 +/- 3.3 and 3.7 +/- 2.0 diopters (D) of central steepening with diameters of 3 and 7 mm, respectively. Results of histologic examination showed a cone-shaped zone of increased stromal hematoxylin uptake extending posteriorly for 90% of stromal thickness. Energy levels greater than those needed to induce topographic changes produced limited endothelial injury in rabbit corneas and, in some cases, intraocular inflammation. A computerized, finite element model of the globe demonstrated central corneal steepening as a result of heat-induced stromal contraction to a depth of 75% corneal thickness.

CONCLUSIONS

These data support previous studies indicating that central corneal topography can be modified by heating corneal stroma in a controlled fashion with the mid-infrared holmium:YAG laser.