Size Of Ablation Zone Research Articles

Differences in Ablation Size in Porcine Kidney, Liver, and Lung After Cryoablation Using the Same Ablation Protocol

The purpose of our study was to assess the variation in size of acute necrosis and the variation in thermal map measured during cryoablation in multiple organs using the same ablation protocol for each organ. Eight female pigs underwent one cryoablation per organ of kidney, lung, and liver performed with open surgery with a 2.4-mm cryoprobe. A 12- and 8-minute double-freeze cycle was used. Intratissue temperatures were monitored using 16-gauge thermometers spaced at 5.0-mm increments from the cryoprobe. The comparison of results among tissues was performed using the multiple analysis of variance. The -20 degrees C thermal diameter was correlated with tissue damage. The kidneys, lungs, and liver were removed and examined histologically for a pathologic complete coagulative necrosis zone. A single 2.4-mm cryoprobe had a mean ice ball diameter in kidney, lung, and liver of 38.5 +/- 4.7, 35.5 +/- 3.6, and 32.5 +/- 2.7 mm, respectively. A mean -20 degrees C thermal diameter was achieved at 24.07 +/- 1.38 mm in kidney, 12.76 +/- 3.0 mm in lung, and 8.8 +/- 3.7 mm in liver by means of regression analysis. The acute pathologic complete coagulative necrosis zone size was 21.0 +/- 1.56 mm (kidney), 11.6 +/- 1.48 mm (lung), and 8.0 +/- 1.20 mm (liver). The inherent characteristics of different organs manifest different ablation zone sizes during cryoablation despite the same ablation protocol being used. This information should be factored into planning for ablation procedures.

Laparoscopic Cryoablation for Small Renal Masses: Three-Year Follow-up

Objectives To report our experience with laparoscopic renal cryoablation for patients who have completed a minimum of 3 years of follow-up. Methods From July 2000 to March 2005, 81 patients underwent laparoscopic renal cryoablation for renal masses. Of these 81 patients, 31 (38%) underwent laparoscopic renal cryoablation for 36 tumors and have completed a minimal follow-up of 3 years (mean 45.7 months). The postoperative follow-up protocol consisted of serial contrast-enhanced computed tomography or magnetic resonance imaging at 1 day, 1, 3, 6, and 12 months, and yearly thereafter. Results Twenty-seven tumors were partially exophytic, five were totally endophytic, and four were hilar tumors. The mean operative time was 2.9 hours, with a mean estimated blood loss of 97 mL. The mean renal tumor size was 2.1 cm. In early follow-up, the ablation zone was larger than the tumor but subsequently diminished to the original tumor size 6 months postoperatively. Thereafter, the ablation zone size decreased. The biopsy results revealed that 22 tumors (61%) were malignant and 14 (39%) were benign. The renal tumor 3-year cancer-specific survival rate was 100%, and no patient developed metastatic disease. One patient demonstrated return of abnormal enhancement within the cryolesion during follow-up, suggesting tumor recurrence. One patient had a hemorrhage and urinary leak after cryoablation of an endophytic tumor and was treated conservatively. Conclusions Renal cryoablation is safe and offers a minimally invasive nephron-sparing alternative. The oncologic adequacy of renal cryoablation requires long-term follow-up data, but the intermediate-term data seem equivalent to that achieved with extirpative therapy.

Radiofrequency ablation with a high-power generator: Device efficacy in an in vivo porcine liver model

Purpose: The purpose of this study was to test the feasibility and efficacy of using a high-power generator with nondeployable electrodes to create large zones of coagulation in an in vivo porcine liver model.Methods: With approval from our institution's research animal care and use committee, 12 female swine (mean weight = 55 kg) were anesthetized and received RF ablation at laparotomy. Twenty-nine ablations were performed in four groups using: (i) a conventional 200-W generator and cluster electrode (n = 4), or an experimental prototype 250-W generator and (ii) a single, 17-gauge electrode (n = 9), (iii) a cluster electrode (n = 8) or (iv) three electrodes spaced 2.0 cm apart in a triangular configuration (n = 8). In the three-electrode group, power was applied by switching between electrodes using a prototype switching device. All electrodes were internally cooled. Ablation zone size, shape and generator data from each group were compared using a mixed-linear model with animals modeled as random effects.Results: The high-power generator was able to increase significantly the zone of coagulation. Mean (±SD) ablation diameter was largest in the switched group (4.31 ± 0.7 cm) followed by the cluster (3.98 ± 0.5 cm) and single-electrode (3.26 ± 0.5 cm) groups. Mean diameter in the high-power single-electrode group was no different than the low-power cluster group (3.25 ± 0.4 cm, p = 0.98). Circularity measures were high (>0.75) in all groups.Conclusions: Coupling a high-power generator and switching device is feasible. At higher powers, the switching device creates larger zones of ablation than cluster or single electrodes. Single-electrode ablations created with the prototype high-power generator were equivalent to those produced with the cluster electrode at conventional lower powers.

Real‐time monitoring of radiofrequency ablation of rabbit liver by respiratory‐gated quantitative temperature MRI

To evaluate the feasibility and precision of magnetic resonance imaging (MRI) thermometry for monitoring radiofrequency (RF) liver ablation in vivo and predicting the size of the ablation zone. At 1.5T, respiratory-triggered real-time MR temperature mapping (the proton resonance frequency (PRF) method) was used to monitor RF ablation in rabbit liver (N = 6) under free breathing. The size of the ablation zones, as assessed by histological analyses, was compared with that predicted from MR thermal dose (TD) maps or derived from conventional T1-weighted (T1w), T2-weighted (T2w), and T1w gadolinium (Gd)-enhanced (T1w-Gd) images acquired immediately after the ablation, and on days 4 and 8 postprocedure. MR temperature uncertainty remained under 1-2 degrees C even during RF deposition. The TD maps were shown to be more predictive and precise than the other MR images, with an average predictive precision for the final ablation zone size of about 1 mm as compared to the histologically proven lesion on day 8. Quantitative temperature MRI during RF ablation is feasible and offered a precise indication of the ablation zone size in this preclinical study based on the lethal dose threshold.

Quantitative magnetic resonance temperature mapping for real-time monitoring of radiofrequency ablation of the liver: an ex vivo study

We evaluated the feasibility and accuracy of real-time magnetic resonance (MR) thermometry for monitoring radiofrequency (RF) ablation in the liver. Continuous MR temperature mapping was used to monitor bipolar RF ablations performed in ex vivo livers with and without flow using two parallel electrodes. Macroscopic inspection of ablation zones was compared with thermal dose maps (TDm) and T1-weighted inversion recovery turbo spin echo (IR-TSE) images for their size and shape and the influence of flow. Pearson's correlation (r), Bland and Altman tests and kappa (chiK) tests were performed. The mean differences in ablation zone size between macroscopic and TDm and IR-TSE measurements were +4 mm and -2 mm, respectively. TDm was well correlated with macroscopy (r=0.77 versus r=0.44 for IR-TSE). TDm was found to be more precise for shape recognition (chiK=0.73 versus chiK=0.55 for IR-TSE) and for detection of an intact ring of liver due to the cooling effect of flow which was impossible with IR-TSE. Simultaneous monitoring of RF ablation by MR thermometry is feasible and reliable for predicting the shape of ablation zones and the impact of the heat-sink effect of flow. Further studies are needed to confirm these results in vivo.

Radiofrequency Ablation of the Liver in a Rabbit Model: Creation of Artificial Ascites to Minimize Collateral Thermal Injury to the Diaphragm and Stomach

To assess whether the creation of artificial ascites during radiofrequency (RF) ablation of the subcapsular portion of the liver can minimize collateral thermal injury to the diaphragm and stomach. A total of 20 percutaneous RF ablation procedures were performed in the livers of 10 rabbits (control, n = 5; experimental, n = 5) with use of an internally cooled electrode (1-cm active tip). In the experimental group, artificial ascites was established before RF ablation by dripping 320 mL of normal saline solution via a 20-gauge sheathed needle to separate the liver from the diaphragm and stomach. In each rabbit, two subcapsular ablation zones were made in the inferior tip of the left lobe of the liver adjacent to the stomach and in the far dome of the right lobe next to the diaphragm consecutively. After the animals were killed 3 days after the procedure, the frequency, size, and degree of thermal injury were compared between the experimental and control groups. The degree of thermal injury was graded by visual inspection according to a four-point scoring system. Representative cases underwent gross and histologic analysis. Artificial ascites was achieved successfully with a single puncture in all rabbits in the experimental group. Mean procedure time for the formation of artificial ascites was 9 minutes. There was no difference in the size of the RF ablation zones in the liver between the two groups (P > .05). Thermal injury in the adjacent organs was observed significantly more frequently in the control group compared with the experimental group (diaphragm, 100% vs 0; stomach, 80% vs 20%; P < .05). Creation of artificial ascites may be a simple and useful technique that can be used to reduce the frequency and severity of thermal injury during RF ablation of subcapsular hepatic tumors.

Multipolar Radiofrequency Ablation of Hepatic Tumors: Initial Experience

Institutional review board approval and patient informed consent were obtained. Use of a multipolar radiofrequency (RF) ablation device in patients with hepatic malignancy was prospectively evaluated with regard to feasibility, achieved ablation zone size and shape, technical effectiveness, and complications. Nineteen malignant liver tumors were treated with the multipolar resistance-controlled RF ablation system, with which up to three internally cooled bipolar coagulation electrodes can be operated simultaneously. Postinterventional imaging was performed with dynamic contrast material-enhanced magnetic resonance (MR) imaging and MR imaging-based three-dimensional planimetry. Complete tumor destruction was achieved in 18 of 19 tumors. Mean ablation zone volume was 52 mL +/- 45 (standard deviation). Thirteen patients were treated with a percutaneous approach; six, with an intraoperative approach. Maximum ablation size was 91 x 62 x 79 mm with the percutaneous and 73 x 98 x 74 mm with the intraoperative approach. Of the 18 completely evaluable ablation zones, 13 were concentric, two were moderately eccentric, two were eccentric, and one was wedge-shaped. The multipolar RF ablation device achieves large ablation zones and has high technical effectiveness in treating hepatic tumors.

MR Imaging Follow-up after Percutaneous Radiofrequency Ablation of Renal Cell Carcinoma: Findings in 18 Patients during First 6 Months

To prospectively evaluate the magnetic resonance (MR) imaging findings seen within the first 6 months after radiofrequency (RF) thermal ablation of renal cell carcinoma (RCC). After providing written informed consent, 18 patients (17 men, one woman; mean age, 71.2 years) with RCC underwent MR imaging-guided percutaneous RF thermal ablation, which was performed by using protocols approved by a comprehensive cancer center protocol committee and the institutional review board for human investigation. The study was Health Insurance Portability and Accountability Act compliant. Follow-up unenhanced T2-weighted MR images and unenhanced and gadolinium-enhanced T1-weighted MR images were acquired immediately, 2 weeks, 3 months, and 6 months after ablation. Thermal ablation zone size was analyzed, and contrast-to-noise ratios (CNRs) were calculated from the signal amplitudes of the thermal ablation zone, perirenal fat, and normal renal cortex on the MR images. Statistical analyses were performed by using the paired Student t test. P < .05 was considered to indicate statistical significance. The mean follow-up time was 16.1 months (range, 6.0-41.2 months). The mean sizes of the thermal ablation zones were 6.8, 7.0, 6.1, and 4.7 cm2, respectively, at immediate, 2-week, 3-month, and 6-month follow-up MR imaging examinations. Thermal ablation zones were uniformly hypointense and had a surrounding bright rim on T2-weighted images and were predominantly hyperintense on T1-weighted images. Thin rim enhancement with central hypointensity was noted on the gadolinium-enhanced images. Gadolinium-enhanced T1-weighted and unenhanced T2-weighted MR images showed significantly higher CNRs than unenhanced T1-weighted MR images. Residual tumor was detected after RF thermal ablation in two cases and was best seen on unenhanced T2-weighted and gadolinium-enhanced T1-weighted MR images. After initially increasing in size within the first 2 weeks, renal RF thermal ablation zones involuted during the remainder of the MR imaging follow-up period.

A long-term study of photorefractive keratectomy*112-year follow-up

Objective To evaluate long-term refractive stability of excimer laser myopic photorefractive keratectomy (PRK). Design A long-term (12 years) prospective follow-up study. Participants Sixty-eight patients (56.6%) of the original cohort of 120 who participated in the first United Kingdom excimer laser clinical trial underwent detailed clinical assessment at 12 years after myopic PRK. Intervention Myopic PRK was performed using the Summit Technology UV 200 excimer laser with a 4-mm ablation zone. Patients were allocated to 1 of 6 treatment groups based on their preoperative refraction. Each group received one of the following spherical corrections: −2, −3, −4, −5, −6, or −7 diopters (D). Patients in each group received an identical treatment, and therefore, emmetropia was not the primary aim. Main outcome measures Refractive stability, refractive predictability, best spectacle-corrected visual acuity (BSCVA), and corneal haze. Results The postoperative refraction remained stable at 12 years, with no significant change in mean spherical equivalent refraction between 1, 6, and 12 years. Seventy-five percent of patients who underwent a −2-D correction and 65% of patients who received a −3-D correction were within 1 D of intended correction at 12 years. Fifty-seven percent of the −4-D group and 50% of the −5-D group were within 1 D, and this was further reduced to 25% and 22% in the −6-D and −7-D groups, respectively. Four percent had residual corneal haze, and 12% had persistent nighttime halos at 12 years. Dry eyes were encountered in 3% of patients, and none of the eyes developed corneal ectasia in the long term. Conclusions In myopic PRK, refractive stability achieved at 1 year was maintained up to 12 years with no evidence of hyperopic shift, diurnal fluctuation, or late regression in the long term. Corneal haze decreased with time, with complete recovery of BSCVA. Night halos remained a significant problem in a subset of patients due to the small ablation zone size.

A long-term study of photorefractive keratectomy: 12-year follow-up

Objective To evaluate long-term refractive stability of excimer laser myopic photorefractive keratectomy (PRK). Design A long-term (12 years) prospective follow-up study. Participants Sixty-eight patients (56.6%) of the original cohort of 120 who participated in the first United Kingdom excimer laser clinical trial underwent detailed clinical assessment at 12 years after myopic PRK. Intervention Myopic PRK was performed using the Summit Technology UV 200 excimer laser with a 4-mm ablation zone. Patients were allocated to 1 of 6 treatment groups based on their preoperative refraction. Each group received one of the following spherical corrections: −2, −3, −4, −5, −6, or −7 diopters (D). Patients in each group received an identical treatment, and therefore, emmetropia was not the primary aim. Main outcome measures Refractive stability, refractive predictability, best spectacle-corrected visual acuity (BSCVA), and corneal haze. Results The postoperative refraction remained stable at 12 years, with no significant change in mean spherical equivalent refraction between 1, 6, and 12 years. Seventy-five percent of patients who underwent a −2-D correction and 65% of patients who received a −3-D correction were within 1 D of intended correction at 12 years. Fifty-seven percent of the −4-D group and 50% of the −5-D group were within 1 D, and this was further reduced to 25% and 22% in the −6-D and −7-D groups, respectively. Four percent had residual corneal haze, and 12% had persistent nighttime halos at 12 years. Dry eyes were encountered in 3% of patients, and none of the eyes developed corneal ectasia in the long term. Conclusions In myopic PRK, refractive stability achieved at 1 year was maintained up to 12 years with no evidence of hyperopic shift, diurnal fluctuation, or late regression in the long term. Corneal haze decreased with time, with complete recovery of BSCVA. Night halos remained a significant problem in a subset of patients due to the small ablation zone size.

MRI-guided radiofrequency thermal ablation of normal lung tissue: in vivo study in a rabbit model.

The purpose of this study was to assess the feasibility of MRI to guide and monitor radiofrequency ablation of normal pulmonary tissue in a rabbit model. Percutaneous puncture and lung radiofrequency ablation were performed in six New Zealand white rabbits under MRI control using a 0.2-T open MRI scanner. Technical feasibility and complication detection were evaluated. The ablation zone appearance and size were assessed using MRI, CT, and gross pathology. Interclass correlation coefficients (ICCs) of the maximum short-axis diameters of the lesions on gross pathology and the corresponding diameters as measured on each MRI pulse sequence and on CT scans were calculated. MRI guidance of percutaneous puncture and radiofrequency ablation of pulmonary tissue is feasible. A pneumothorax was detected and treated using MRI. In the specimen, the mean coagulation necrosis diameter was 9.8 mm. The T1-weighted spoiled gradient-echo fast low-angle shot images showed the highest ICC (0.81) for the thermal lesion diameter. Our results indicate that MRI guidance is feasible and useful for radiofrequency ablation of normal pulmonary tissue.

Quality of vision and patient satisfaction after LASIK.

The purpose of this article is to review the literature and find characteristics that lead to improved patient satisfaction and better quality of vision. Flatter preoperative corneal curvature is a risk factor for starbursts after laser-assisted in situ keratomileusis (LASIK). Pupil size has not been found to be correlated with night vision symptoms. Wavefront-guided ablations reduce higher-order aberrations in comparison with traditional LASIK. Night vision symptoms are correlated with younger age, greater correction/increased ablation depth, enhancement, and decreased ablation diameter. Contrast sensitivity has been found to initially decrease after LASIK, returning to baseline 6 to 12 months postoperatively. LASIK has quickly become the refractive procedure of choice around the world. Quality of vision and patient satisfaction after LASIK can be difficult to assess because of the many variables that must be considered to accurately measure these endpoints. Preoperative characteristics such as: increased patient age, decreased corneal toricity, or increased pupil size reduce patient satisfaction. Intraoperative factors like decentration, ablation-zone size, active eye tracking, and wavefront guided ablations affect quality of vision. Finally, postoperative factors such as night vision symptoms, reduced contrast sensitivity, and re-treatment can lead to a decline in patient satisfaction. Eliminating or limiting these variables may lead to increased patient satisfaction and higher quality of vision after LASIK.

Pupil size and the ablation zone in laser refractive surgery: considerations based on geometric optics

Purpose To determine whether the currently accepted method of selecting a minimum ablation zone size for refractive surgery based on dark-adapted pupil diameter is substantiated by geometric optical analysis. Setting Department of Ophthalmology and Visual Sciences, Texas Tech University Health Sciences Center, Lubbock, Texas, USA. Methods An optical model of the anterior segment was developed to calculate the effective corneal refractive diameter (ECRD), which is the diameter of the area of cornea that refracts all incident light rays arising from objects along the line of sight though the physical pupil (PP). The concept of the entrance pupil (EP) was reexamined and developed, and the ECRD was calculated over a range of corneal curvatures (K), anterior chamber depths (ACDs), and EP sizes. The model was generalized to include oblique light rays. Calculations were performed using MatLab Optimization Toolbox® software (The MathWorks). Results For a given EP size, the ECRD was significantly influenced by K and slightly influenced by ACD. For objects on the line of sight, the ECRD was smaller than the EP in all cases. Regarding rays from objects in the periphery, the ECRD expanded rapidly as the angle of oblique incidence increased. Conclusions For objects on the line of sight, the ECRD is always smaller than the clinically measured pupil (EP) because the EP is substantially magnified relative to the PP. Ablation zones larger than the EP should, in theory, prevent scattered or defocused light rays from contributing to the foveal image. When considering objects in the periphery, the increase in ECRD is sufficiently rapid that current refractive procedures cannot stop scattered light from these objects from contributing to the retinal image.

Theoretical analysis of peripheral imaging after excimer laser corneal refractive surgery for myopia

Purpose To explore theoretically the retinal point images in the peripheral fields of eyes that have had excimer laser refractive surgery. Setting University research laboratory. Methods Model eyes were based on Navarro’s finite schematic eye, the eyes being made myopic by an increase in axial length. To simulate photorefractive keratectomy (PRK), the anterior shape and thickness of the cornea were modified. Variables included pupil size, ablation zone size, preexisting refractive error, and the addition of a blending zone. Image-quality criteria for each retinal point image were its size and the angular separation of the centroids of those parts of the image produced by rays passing through ablated and unablated corneal zones. Results In the peripheral visual field, the boundary between the ablated and unablated cornea caused a separation of the retinal image of a single point into 2 parts. The separation increased with the preexisting refractive error. Image quality was correspondingly reduced by ablation. As pupil size increased, the field angle at which the retinal image doubling first occurred decreased. Increasing the diameter of the ablation zone or using a blending zone increased the angle at which the doubling first occurred, and the blending zone improved image quality considerably. Chromatic effects appeared to be relatively unimportant. Conclusions This analysis provides further evidence of the disadvantages of small central ablation zones in excimer laser refractive surgery and of the advantages of well-designed blending zones in improving postsurgical peripheral image quality. Image quality in the peripheral field of the pseudoemmetropic post-PRK eye is generally worse than in a naturally emmetropic eye, even though the axial image quality may be similar.

Mathematics of laser in situ keratomileusis for high myopia

Purpose: To determine the maximal ablation that can be safely performed with laser in situ keratomileusis (LASIK) to maintain long-term corneal integrity.Setting: TLC The Windsor Laser Center, Windsor, Canada.Methods: The pretreatment protocols for the VISX Star, Summit Omnimed, and Chiron Technolas 116 excimer lasers generally apply 1 to 2 pm per diopter (D) at an optical zone of 3.0 mm or less to avoid the postoperative central islands that can occur with broad-beam excimer lasers. The ablation depth per diopter for the VISX Star, Summit Omnimed, Chiron Technolas 116, and Chiron Technolas 217 excimer lasers ranges from 10 to 24 pm per diopter depending on the size and number of ablation zones and the excimer laser used.Results: Previous experience with lamellar surgery suggests that at least 250 pm of central posterior stromal tissue should be preserved to maintain long-term corneal integrity and avoid postoperative corneal ectasia. If a 160 μm flap is created for LASIK, the average 550 pm cornea will have 140 pm of corneal stroma available for ablation. Depending on the excimer laser and ablation nomogram used, the maximal LASIK correction for the average cornea ranges from 9.8 to 15.0 D.Conclusion: The preoperative corneal thickness and the depth of the excimer laser ablation must be evaluated before LASIK to ensure that adequate posterior corneal stroma is preserved.

Five year results of photorefractive keratectomy for myopia

Purpose: To evaluate the long-term stability, complications, and causative factors in eyes that had photorefractive keratectomy (PRK) for myopia.Setting: Kangnam St. Mary's Hospital, Seoul, Korea.Methods: This study evaluated the results of PRK in 201 myopic eyes with a consecutive 5 year follow-up. Preoperative myopia ranged from 2.25 to 12.50 diopters (D), with astigmatism of less than 1.50 D. The excimer laser was set to a maximum correction of −6.50 D at a 5.0 mm diameter ablation zone. Patients with more than 7.00 D of myopia had double-pass PRK with two different ablation zone sizes (5.0 and 4.5 mm). The data were statistically analyzed using polynomial regression for evaluating long-term stability and myopic regression and Cox's proportional hazard model for evaluating causative factors.Results: An uncorrected visual acuity better than 20/25 was achieved in 62.4% of eyes. The main complication after PRK was myopic regression. Mean refractive error 5 years after PRK was −2.43 ± 1.90 D. It was −1.49 ± 0.60 D in moderately myopic eyes (less than 6.50 D) and −3.55 ± 2.31 D in highly myopic eyes (over 7.00 D). According to our evaluation, the possible causative factors for myopic regression were pre-PRK refraction (P < .0001) and post-PRK corneal haze (P = .01); their relative risks were 3.33 and 1.93, respectively. Multivariate analysis eliminated the corneal haze factor.Conclusion: Myopic regression occurred as long as 5 years after PRK, with the most important factor for myopic regression being pre-PRK refraction.

Excimer Laser Technology: Key Concepts for the Ophthalmologist

Since its initial development, excimer laser technology has advanced at a rapid rate over the last decade. The underlying principles of excimer technology have not changed significantly during this time. However, the modality of laser delivery has advanced to incorporate scanning slit and flying spot techniques as well as the more established diaphragm method. Laser calibration has become important as a way to increase laser accuracy, as has the development of eye tracking technology to allow the laser to follow eye position in real time during laser treatment. Ablation-zone size and edge contour have emerged as critical points in our understanding of patient response to surgery in terms of optical side effects and wound healing. Astigmatism, hyperopia, and laser retreatments are undergoing intense investigation and these aspects of the field should be clarified in the near future. Solid state lasers have recently undergone clinical trials and offer ablation at lower energy levels per laser pulse with a r...

Excimer Laser Photorefractive Keratectomy for Myopia: Comparison of 4.00- and 5.00-Millimeter Ablation Zones

To date, there has been no systematic study of the effects of ablation zone diameter on the outcome of photorefractive keratectomy. To address these issues, we examined a series of eyes with bilateral corrections using different-sized ablation zones. Thirty-three patients underwent bilateral photorefractive keratectomy (Summit Excimed UV200, Waltham, Mass) with identical dioptric corrections in both eyes, except first eyes had 4.00-millimeter and second eyes had 5.00-millimeter ablation zones. Identical postoperative eyedrop regimens were used in both eyes of each subject and the interval between treatments was 12 months. The mean depth of the programmed central ablation was 24 microns in eyes treated with 4.00-millimeter and 39 microns with 5.00-millimeter zones. There was no statistically significant difference in the preoperative refraction between first and second eyes. Mean changes in refraction at 1, 3, 6, 9, and 12 months were significantly greater in eyes treated with 5.00-millimeter ablation diameters (p < .001). No eyes treated with 4.00-millimeter zones were overcorrected, but five eyes (15%) treated with 5.00-millimeter beams had a refraction greater than +1.00 diopter (D) at 12 months postoperatively. There was no significant difference in the amount of anterior stromal haze between the two eyes at any stage. In 14 patients, less night halo was noticed in the eye treated with a 5.00-millimeter zone. Using a computer program, halo measurements were made in both eyes of 12 patients whose pre- and postoperative refractions were within 0.50 D. The magnitude of halo was significantly less in eyes treated with 5.00-millimeter zones (p < .01). Despite greater depths of stromal ablation with 5.00-millimeter diameters, there was no increased anterior stromal haze or postoperative regression of refraction. The biological and physical constraints governing the optimum size of the photorefractive keratectomy ablation zone are discussed.

Night vision after excimer laser photorefractive keratectomy: haze and halos.

A series of 85 patients with myopia, up to -6.00D, was treated by photorefractive keratectomy (PRK), using a 5 mm diameter ablation zone. At six months, 38 patients (45%) reported slight disturbances of night vision, nine (11%) of whom had significant problems. Perturbations of night vision after PRK are seen as starbursts and halos around lights. Corneal haze produces the starbursts, which are usually transient. In contrast, halos are myopic blur circles and may be persistent. Using a computer program, halos after PRK were found to be significantly larger than those in emmetropes and myopes corrected with spectacles (p < 0.01). The halos were diminished by using either artificial pupils or negative lens over-correction. In patients with identical bilateral PRK corrections, except for the ablation zone size, the magnitude of the halo was less with 5 mm than 4 mm zones (p < 0.01). Patients treated with 5 mm reported fewer problems attributable to halo than with the 4 mm ablation diameters (p < 0.01). Halos and pupil diameters were measured in nine patients with significant impairment of night vision haze. Those with starbursts had small hyperopic shifts, minimal halos and high haze and light scatter measurements, whilst patients with halos had large hyperopic shifts, little haze and large pupil diameters. Patients with persistent halo problems benefited from either negative lens over-correction or miotics at night.

Design and Development of a New 193-Nanometer Excimer Laser Surgical System

The 193-nanometer excimer laser is used for photoablative reprofiling of the cornea to correct refraction errors and to treat corneal pathologies. To use the unique properties of the excimer laser, some technical features should be provided by the excimer laser system. A new excimer laser system is presented. The instrument has a fluence sensor for precise measurement of the actual fluence just before surgery, a new delivery system including an integrator system to homogenize the laser beam, a steelband with apertures and a zoom optic to get the desired form of central reprofiling, and an alignment system to reduce the possibility to decenter the optical zone. The fluence before surgery can be measured very precisely with the fluence sensor. The ablation zone size is at least 8 mm in diameter with a homogenous laser beam profile resulting in a smooth surface of the treated material. Different forms of ablations can be created using the steelband with different forms of apertures. The center of the ablation zone can be found using the alignment system. The new excimer laser system fulfills some important technical features for optimal use of the excimer laser in corneal surgery.