Laser Thermal Keratoplasty Research Articles

Fractional Beer-Lambert law in laser heating of biological tissue

<abstract><p>In this article we propose an alternative formulation to model a thermal-optical coupled problem involving laser heating. We show that by using the Fractional Beer-Lambert Law (FBLL) instead of the Beer-Lambert Law (BLL) as the governing equation of the optical problem, the formulation of the laser heat source changes, along with consequently, the distribution of temperatures. Our theoretical findings apply to laser thermal keratoplasty (LTK), used to reduce diopters of hyperopia. We show that the FBLL offers a new approach for heat conduction modeling of laser heating, which is more flexible and could better fit the data in cases where the BLL approach does not fit the data well. Our results can be extended to laser heating of other biological tissues and in other general applications. Our findings imply a new insight to improve the accuracy of thermal models, since they involve a new formulation of the external heat source rather than the heat equation itself.</p></abstract>

Corneal Ectasia After Laser In Situ Keratomileusis After Laser Thermal Keratoplasty

To report a case of corneal ectasia following hyperopic laser in situ keratomileusis in a patient who had previous laser thermal keratoplasty. Case report. We report a case of a 66-year-old emmetropic man initially presented with complaint of difficulty reading without correction. Laser thermal keratoplasty (LTK) was performed on the non-dominant right eye, resulting in successful monovision. Three years later, the patient presented with decreased unaided near vision caused by hyperopic regression of the LTK. Conventional hyperopic laser in situ keratomileusis (LASIK) was performed, again resulting in successful monovision. More than three years later, the patient returned with worsening near vision. A focal corneal ectasia was noted in the same location as the 6 o'clock LTK leukoma. To our knowledge, this is the first report of ectasia occuring after LASIK following LTK. Consideration should be given to performing photorefractive keratectomy (PRK) instead of LASIK following thermal keratoplasty.

Comparative Analysis of Postoperative Changes in Higher Order Aberrations Following LASIK and Laser Thermal Keratoplasty

Comparative Analysis of Postoperative Changes in Higher Order Aberrations Following LASIK and Laser Thermal Keratoplasty

Laser in situ keratomileusis for astigmatism following laser thermal keratoplasty

We describe 2 cases in which a staged laser in situ keratomileusis procedure was used to treat residual hyperopia and astigmatism after laser thermal keratoplasty. The procedure successfully reduced the spherical equivalent refractive error and residual astigmatism. All eyes maintained their best spectacle-corrected visual acuity and had an uncorrected visual acuity of at least 20/25 at the end of follow-up.

Theoretical investigations of the processes of laser interaction with ocular tissues for laser applications in ophthalmology

Theoretical investigations and the results of computer modeling of the optical, thermophysical, and thermochemical processes during laser interaction with ocular tissues are reviewed in this paper. Physical-mathematical models and results of numerical simulation of the processes are presented. The computer modeling was applied for investigations of laser heating and coagulation of ocular tissues for treatment of retina diseases and intraocular tumors, cyclophotocoagulation of the ciliary body for treatment of glaucoma, and laser thermal keratoplasty of the cornea. The influence of radiation parameters on the selectivity of laser coagulation of laminated ocular tissues is considered. The results obtained are of essential interest for laser applications in ophthalmology and can be used for investigation of heating and coagulation of tissues in different fields of laser medicine.

Very High-frequency Ultrasound Analysis of Non-contact Holmium Laser Thermal Keratoplasty Treatment Spots

To objectively measure the corneal treatment spots in vivo using very high-frequency ultrasound (VHFU) after non-contact laser thermal keratoplasty (LTK) to better understand the variability and regression of refractive outcomes. In an institutional setting, VHFU was performed on 128 spots (8 eyes of 4 patients) using an immersion scanning technique 1 to 2 years after LTK with a single element focused transducer (50 MHz arc scanning ultrasound). Biometric techniques were used to evaluate the treatment spot depth, corresponding corneal thickness, and spot profile between patients, eyes (left/right), and by location on the cornea. The identical technique was used in a rabbit immediately after LTK to compare ultrasound versus histologic findings. The mean treatment spot penetration depth ranged between 0.373 and 0.533 mm, representing 64% to 78% of the corneal thickness compared to previous reports of 80% to 90%. Treatment spot depth, the corresponding corneal thickness, and percentage of overall cornea penetrated differed significantly across patients. Treatment spot depth was not significantly related to the level of applied laser energy (230 to 258 mJ) (0.082 Pearson sign). Spot profiles were not uniformly cone-shaped; W- and wedge-shaped were also identified. Ultrasound findings in the rabbit were similar to histology results and confirmed evidence of epithelial remodeling. Very high-frequency ultrasound of 128 treatment spots after non-contact LTK demonstrates epithelial remodeling and inconsistencies in penetration depth and profile.

A New Analysis for Laser Thermal Keratoplasty Using a Focused Laser/Reply

A New Analysis for Laser Thermal Keratoplasty Using a Focused Laser/Reply

Non-contact Holmium:YAG Laser Thermal Keratoplasty for Hyperopia: Two-year Follow-up

We evaluated the safety and efficacy of the non-contact holmium:YAG laser thermal keratoplasty (LTK) for the treatment of mild to moderate hyperopia without astigmatism. A prospective, non-comparative case series included 50 eyes of 28 patients (aged > or =40 years) who had stable refraction and an astigmatic component < +0.50 diopters (D). We applied the non-contact pulsed holmium:YAG laser to treat the hyperopic spherical component using the Hyperion LTK System. All patients had minimum 12-month follow-up and 64% (18 patients) had 24-month follow-up. The mean age of patients was 48.4 +/- 8.23 years (range: 40 to 62 years). The preoperative hyperopic mean spherical equivalent refraction was +2.32 +/- 0.975 D (range: +1.00 to +4.75 D). Postoperatively, the subjective manifest refraction decreased from the preoperative mean value of +2.32 D to a mean -0.09 D at 1 month after surgery and regressed to +0.315 D at the last follow-up examination, resulting in a mean correction of +2.005 +/- 0.81 D at 24 months after surgery. Preoperatively, mean uncorrected visual acuity in LogMAR units was 0.798 +/- 0.353 and at 12 months after surgery, it was a mean 0.108 +/- 0.136. Keratometric power increased from 42.595 +/- 1.949 D before surgery to 44.605 +/- 1.626 D at 24 months after surgery. Holmium:YAG LTK was an acceptable alternative for the correction of mild to moderate hyperopia in this middle-aged population.

Intraocular lens power calculation after incisional and thermal keratorefractive surgery

Purpose: To evaluate the efficacy of corneal topography in determining the central corneal refractive power in intraocular lens (IOL) power calculations after incisional and thermal keratorefractive surgery. Setting: Oregon Eye Institute, Eugene, Oregon, USA. Methods: This retrospective review comprised 20 eyes (14 patients) that had cataract extraction with IOL implantation or refractive lens exchange after radial keratotomy, hexagonal keratotomy, or laser thermal keratoplasty. The effective refractive power (EffRP) of the Holladay Diagnostic Summary on the EyeSys Corneal Analysis System was used to determine the central corneal refractive power, which was input into the Holladay 2 IOL calculation formula. Results: Eighty percent of eyes achieved a postoperative spherical equivalent refraction within ±0.50 diopter of emmetropia. Conclusion: The use of the EffRP increases the likelihood of an acceptable refractive outcome after cataract or refractive lens exchange surgery in eyes with a history of keratorefractive surgery.

Anterior chamber angle biometry with ultrasound biomicroscopy after diode laser thermal keratoplasty

Purpose: To evaluate the effect of diode laser thermal keratoplasty (DTK) on the geometry of the anterior chamber angle with ultrasound biomicroscopy (UBM). Setting: Department of Ophthalmology and Optometry, Paracelsus Private Medical University, Salzburg, Austria. Methods: In 9 eyes, a Rodenstock DTK® (ProLaser Medical Systems) was used to correct hyperopia (n = 6) or hyperopic astigmatism (n = 3). Hyperopia was treated with a single- or double-ring pattern and hyperopic astigmatism, with additional spots in each of the flat half meridians. Ultrasound biomicroscopy measurements of the chamber angle were obtained preoperatively and 3 months postoperatively using a Humphrey UBM 840 system with a 50 MHz transducer. Quantitative measurements were performed at 3-, 6-, 9-, and 12-o'clock positions using a standardized method. Results: No statistically significant differences between the preoperative and postoperative chamber angle measurements were detected. Conclusions: The results of the UMB measurements were surprising given the flattening of the corneal periphery found by computerized topography, a finding that suggests narrowing of the chamber angle. Shortening of the corneal tissue may result in centripetal traction that affects the trabecular meshwork and results in a relative opening of the chamber angle structures.

Changes in keratometric corneal power and refractive error after laser thermal keratoplasty

Purpose: To evaluate the effect of laser thermal keratoplasty (LTK) on corneal power and refractive error to develop a logical approach to calculating accurate intraocular lens (IOL) power for cataract surgery. Setting: Department of Ophthalmology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea. Methods: Laser thermal keratoplasty was performed in 27 eyes of 23 patients. Preoperatively and postoperatively, the refractive error was measured and the corneal power obtained by manual keratometry and topography. The changes in keratometric corneal power and refractive error after LTK were compared. Results: The mean age of the 15 women and 8 men was 45.0 years ± 4.6 (SD) (range 43 to 61 years). The mean preoperative refractive error was +1.43 ± 0.97 diopters (D) (range 0 to +3.63 D) at the spectacle plane and +1.46 ± 1.01 D (range 0 to +3.79 D) at the corneal plane. The mean postoperative refractive error was −0.44 ± 1.07 D (range −2.24 to +2.18 D) at the spectacle plane and −0.44 ± 1.08 D (range −2.18 to +2.23 D) at the corneal plane. After surgery, corneal powers measured by manual keratometry were significantly smaller than those measured by topography ( P<.001) and refractive error changes were significantly smaller than keratometric changes ( P<.001). Conclusions: After LTK, corneal power measured by manual keratometry was smaller than that measured by corneal topography and changes in corneal power measured by conventional keratometric instruments were greater than changes in refractive error. This difference should be considered in calculating IOL power in post-LTK eyes to prevent undesirable hyperopia after cataract surgery.

Diode laser thermal keratoplasty for hyperopia and hyperopic astigmatism in patients younger than 40 years.

We performed a prospective, nonrandomized investigation of contact continuous wave diode laser thermal keratoplasty (DTK) for correction of hyperopia and hyperopic astigmatism. The goal of surgery was not to achieve emmetropia but to investigate the refractive effect in patients younger than 40 years. Twenty eyes with spherical hyperopia (Group A) and 15 eyes with hyperopic astigmatism (Group B) were treated with two rings; treatment zone diameter of the inner ring was 6 or 7 mm. Each ring consisted of eight spots for hyperopia correction; for astigmatism correction four additional pairs of spots were applied around the flat corneal meridian. Mean change in manifest spherical equivalent refraction 18 months postoperatively in Group A was 2.00 +/- 0.90 D (Group B, 15 mo, 1.80 +/- 0.60 D). Mean increase in keratometric power was 1.20 +/- 0.60 D (1.30 +/- 0.60 D). Mean refractive astigmatism reduction was 0.10 D (1.70 D). Mean paired differences per month for regression between spherical equivalent manifest refraction/keratometric power was 0.12/0.04 D (0.06/0.05 D) from 9 to 12 months, 0.01/0.04 D (0.06/0.06 D) from 12 to 15 mo, and -0.03/ +/- 0 D from 15 to 18 months. Mean uncorrected visual acuity improved from 20/100 to 20/32 (20/63 to 20/32). No eye lost more than 1 line of BSCVA. The amount of correction showed an age dependency. DTK is a minimally invasive, low-risk procedure, and was effective for correction of low hyperopia and low to moderate hyperopic astigmatism in patients less than 40 years.

Surgery for 4 refractive errors in 1 patient

We report a case of cataract extraction with implantation of a multifocal intraocular lens (IOL) after photorefractive keratectomy for myopia and astigmatism and subsequent laser thermal keratoplasty for surgically induced hyperopia. Good refractive results were obtained using standard biometry techniques for calculation of the IOL power.

Diode laser thermal keratoplasty to correct hyperopia.

We investigated long-term efficacy, predictability, stability, and safety of diode laser thermal keratoplasty (DTK) to correct hyperopia. DTK was performed on 24 eyes (18 patients). Eight eyes with high hyperopia (mean +4.75 +/- 0.63 D; range +3.50 to +5.50 D) received 12 pairs of coagulation spots at 6-mm and 7-mm treatment zone diameters; eight eyes with low hyperopia (mean +2.25 +/- 0.40 D; range +1.50 to +2.75 D) received eight coagulation spots at 8 mm, and eight eyes with low hyperopia (mean +1.50 +/- 0.46 D; range +1.25 to +2.25 D) were treated to induce mild myopia (-1.50 D) in the non-dominant eye for monovision using eight pairs of spots at 7 and 8-mm diameters. Minimum follow-up was 18 months. Mean decrease in cycloplegic refraction at 18 months was 5.00 +/- 0.38 D in the high hyperopia group, 1.75 +/- 0.19 D in the low hyperopia group, and 3.25 +/- 0.27 D in the presbyopia group. Mean increase in uncorrected visual acuity (UCVA) at 18 months was 8.125 +/- 2.1 Snellen lines in the high hyperopia group, 6.625 +/- 0.744 lines for low hyperopia; decrease of 1.00 +/- 1.85 line occurred in the presbyopia group. Near UCVA in the presbyopia group improved by 3.875 +/- 0.83 Jaeger lines. Best spectacle-corrected visual acuity (BSCVA) was restored by 3 months in all eyes. DTK was an effective and fairly safe procedure, with reasonable predictability and stability. Nomograms for laser energy level, treatment zone diameter, and number of spots need improvement.

Intraoperative wavefront monitoring during laser thermal keratoplasty.

Low energy laser thermal keratoplasty (LTK) is being proposed as an alternative to conventional holmium LTK in an effort to minimize corneal tissue necrosis, regression, and induced astigmatism. Real-time wavefront monitoring during surgery is also being proposed as a method for regulating induced aberrations and the treatment end point. We evaluated the feasibility of intraoperative wavefront monitoring during low energy LTK. An investigational prototype system combining a Sunrise Hyperion laser with a Shack-Hartmann wavefront sensor (Wavefront Sciences COAS) was used to treat 20 hyperopic eyes with a low energy application of eight spots of 18mJ/shot (144mJ/pulse) at 5 Hz in two rings of 6 mm and 7-mm diameter, while monitoring the treatment with real-time dynamic wavefront sensing up to 10 captures/second of 4th order Zernike data. The integrated wavefront-monitored LTK unit showed good stability in measuring dynamic intraoperative wavefront refractions before, during, and after the low energy LTK treatment. The readings did not interfere with the application of holmium laser pulses, nor did the LTK treatment distort the captured wavefront readings. A steady improvement of the wavefront-derived spherical equivalent refraction followed the laser treatment pattern, with a mean attempted spherical equivalent refractive change of 1.66 D and mean achieved change of 1.71 D. Intraoperative wavefront monitoring was feasible during low energy LTK. With future adaptation, it might be possible to dynamically relay the captured wavefront information to the laser during ongoing treatment, to intraoperatively control the surgical endpoint for multiple aberration terms.

Two-Year follow-up of noncontact holmium laser thermokeratoplasty for the correction of low hyperopia

Purpose: To summarize the 2-year results of laser thermal keratoplasty with a holmium:yttrium-aluminumgarnet (Ho:YAG) laser and the Sunrise Corneal Shaping System and assess the procedure's safety, efficacy and predictability in correcting hyperopia in a phase III clinical intervention case series.Methods: The Ho:YAG laser was used to correct low hyperopia (manifest spherical equivalent of +0.75 to +2.50 dioptres [D], with manifest cylinder of 1.00 D or less) in 38 eyes of 28 patients 40 years of age or older. Laser pulses were delivered to the cornea in 2 radially placed, concentric, 8-spot rings 6.0 and 7.0 mm in diameter. The last follow-up was at 2 years.Results: The preoperative uncorrected visual acuity (UCVA) at distance was less than 20/40 in 82% of the eyes and at near was less than 20/32 in 42%. At 2 years the distance UCVA was 20/40 or better in 100% of the eyes and 20/20 or better in 84%, and the near UCVA was 20/32 or better in 97% of the eyes and 20/20 or better in 8%. The difference between the preoperative and postoperative UCVA was statistically significant (p < 0.01). The correction at 2 years was within 0.50 D of that intended in 92% of the eyes and within 1.00 D in 100%. The reduction in mean spherical equivalent at 2 years was statistically significant (p < 0.0001). No loss of 2 lines or more in best-corrected visual acuity occurred after the I st month of follow-up.Interpretation: The Ho:YAG Sunrise Corneal Shaping System is safe and effective for the treatment of low hyperopia and astigmatism. It provides a predictable refractive outcome at 2 years of follow-up. However, our results show a tendency towards regression by 2 years.

Treatment of simple hyperopia: comparison of laser in situ keratomileusis and laser thermal keratoplasty

Purpose To evaluate and compare the efficacy, stability, and safety of laser in situ keratomileusis (LASIK) and laser thermal keratoplasty (LTK) for the treatment of simple hyperopia. Setting John Hill Eye and Laser Centre, Cape Town, South Africa. Methods This retrospective study comprised consecutive patients having primary treatment of simple hyperopia of up to 3.0 diopters (D) with astigmatism of 0.5 D or less. Treatment methods were as follows: Group 1 (81 eyes), LASIK with the Nidek EC-5000 excimer laser; Group 2 (69 eyes), LASIK with the LaserSight LSX® excimer laser; and Group 3 (84 eyes), LTK with the Sunrise holmium:YAG laser. Results The hyperopia decreased in all 3 groups. Both LASIK groups were stable by 1 month; there was continued regression in the LTK group for up to 18 months. The percentage of eyes achieving uncorrected visual acuities of 20/20 and 20/40 at 3 months were Nidek, 41% and 92%, respectively; LaserSight, 50% and 90%, respectively; and LTK, 21% and 89%, respectively. No eye lost more than 2 lines of best spectacle-corrected visual acuity. Surgically induced astigmatism (SIA) was evident in all 3 groups; it was highest in the LTK group. The mean posttreatment astigmatism was –0.47 D ± 0.40 (SD) (range 0 to −1.50 D) in the Nidek group, –0.45 ± 0.40 D (range 0 to −1.25 D) in the LaserSight group, and –0.81 ± 0.51 D (range 0 to −2.25 D) in the LTK group. The enhancement rates were 16.75%, 22.57%, and 38.30%, respectively. Because of the SIA, 61% of the LTK enhancements were corrected with LASIK. Conclusions All 3 treatment methods corrected hyperopia, but stability was achieved early in both LASIK groups, allowing early enhancement when necessary. Because LTK cannot currently correct astigmatism, many of the LTK repeat treatments required LASIK procedures. For these reasons, LASIK remains my preferred method to treat simple hyperopia up to +3.0 D.

US trends in refractive surgery: 2002 ISRS survey.

To determine the latest trends in refractive surgery in the United States. The full U.S. membership of the International Society of Refractive Surgery (ISRS) (approximately 900 members) was mailed the 2002 refractive surgery survey dealing with volumes, types, preferences of refractive surgery performed, and use of emerging technology. Questions regarding RK, AK, PRK, LASIK, LASEK, intracorneal ring segments (ICRS), laser thermal keratoplasty (LTK), conductive keratoplasty (CK), phakic intraocular lenses (PIOL), and clear lens extractions (CLE) were examined in the survey. Procedure preference for low, moderate, and high myopia, and hyperopia, were compared with the results from the surveys of the previous 5 years. Preference for unilateral versus bilateral same-day surgery, laser type, and microkeratome choice were also compared with the survey data from previous years. Incidence and frequency of co-management of refractive surgery patients were compared with 1999-2001 data. New questions regarding pupil measurement/documentation, wavefront aberrometry, and custom ablations were incorporated into the 2002 survey. As refractive surgery grows in the mainstream of ophthalmology, trends and changes in the United States continue to be elucidated by this professional organization survey. LASIK continues to dominate for refractive errors between -10.00 to +3.00 D. LASIK, LASEK, CLE, PIOL, and CK appear to have bright futures, whereas, RK, ICR, and LTK are on the decline. VISX continues to be utilized 2:1 over all other lasers combined, and instrumentation pupillometry is preferred 2:1 over pupil gauge cards. Currently, wavefront aberrometry and custom ablations are minimally employed but appear poised to be the wave of the future.

Cataract surgery after holmium:YAG laser thermal keratoplasty

A 64-year old man had noncontact holmium:YAG (Ho:YAG) laser thermal keratoplasty (LTK) performed in the left eye on March 10, 1998, and in the right eye on January 11, 1999. The patient achieved 1.3 diopters (D) and 1.4 D of corneal steepening in the right and left eye, respectively, which was the desired amount as his refractive error before Ho:YAG LTK was low. At the 3-month postoperative examination of the left eye, cortical cataracts were observed in both eyes. Approximately 1 year later, bilateral cataract extraction was recommended because of patient-reported decreased vision at distance and near and difficulty with vision in the presence of glare. Cataract surgery and intraocular lens (IOL) implantation was performed in both eyes in August 1999 using the keratotomy readings taken after noncontact Ho:YAG LTK to calculate IOL power. Although slight flattening of the cornea occurred after cataract extraction, the refractive outcomes achieved by noncontact Ho:YAG LTK were generally preserved.

Conductive keratoplasty for the correction of low to moderate hyperopia: 1-year results on the first 54 eyes

ObjectiveTo document the 1-year postoperative clinical results of the safety, efficacy, and stability of conductive keratoplasty (CK) to correct low to moderate hyperopia. DesignProspective, nonrandomized, self-controlled trial. ParticipantsFive surgeons at four centers performed CK on the first 54 eyes of a multicenter, 2-year clinical trial. Treated eyes had +0.875 to +4.00 diopters (D) of hyperopia and ≤0.75 D of cylinder. The nature of this procedure was explained to all participating patients who signed informed consent forms prior to undergoing the procedure. InterventionLow energy, high-frequency current was applied directly into the peripheral corneal stroma through a delivery tip inserted at 16 or more treatment spots. An early nomogram was used for the first 54 eyes with an intended refraction of plano. Main outcome measuresData from all 54 eyes were analyzed for safety and stability. A subset of 30 of the 54 eyes was found that had been treated with the appropriate number of spots with the early nomogram. These eyes were categorized as current nomogram eyes; the data for these eyes were analyzed for efficacy and predictability, as well as stability and safety. All patients reported on satisfaction and quality of vision after surgery. ResultsPreoperatively, the manifest refractive spherical equivalent refraction (MRSE) of the 30 current nomogram eyes was +1.57 D. At 1 year postoperatively, uncorrected visual acuity was 20/20 or better in 57% of the eyes and 20/40 or better in 93%. The MRSE was within 0.50 D in 46%, within ±1.00 D in 93%, and within ±2.00 D in 100%. No eye lost ≥2 lines of best spectacle-corrected visual acuity at 1 year postoperatively nor had an induced cylinder of ≥2.00 D. The MRSE changed a mean of 0.25 D ± 0.43 between 3 and 6 postoperative months, 0.16 D ± 0.38 between 6 and 9 postoperative months, and 0.07 D ± 0.38 between 9 and 12 postoperative months. Refractive stability appeared to be attained by 6 months postoperatively. ConclusionsConductive keratoplasty appears to be safe, effective, and stable for correcting low to moderate spherical hyperopia. Stability appeared by the 6-month follow-up visit. For the eyes treated with the current CK nomogram, uncorrected visual acuity, predictability, and stability are as good as or better than those obtained with hyperopic laser in situ keratomileusis or noncontact laser thermal keratoplasty.