Stromal surface topography-guided custom ablation as a repair tool for corneal irregular astigmatism.

PURPOSE: To illustrate the concept of using a stromal surface topography-guided procedure for therapeutic repair after a complication following primary laser refractive surgery. METHODS: One case example of therapeutic retreatment for short nasal flap after primary LASIK performed in September 2000 is presented. The Artemis very high-frequency digital ultrasound arc-scanner (Arc-Scan, Inc., Morrison, CO) was used to obtain layered corneal thickness including epithelial thickness profile. Corneal front surface elevation was measured with the Orbscan II (Bausch & Lomb, Salt Lake City, UT). Stromal surface height was then calculated by subtracting epithelial thickness data from corneal front surface elevation data and used to calculate the ablation profile applied to the eye. The treatment was performed using the Ultralink system (ArcScan, Inc.), linking the ultrasound corneal thickness data with the Technolas 217c laser (Bausch & Lomb). Postoperative data were available at 30 days and 13 years. RESULTS: One month after treatment, the epithelial thickness map demonstrated that the difference in thickness between the thinnest and thickest points located 2.5-mm nasally was reduced by 26 µm (from 56 to 30 µm). The axial difference map demonstrated an increase in corneal curvature of approximately 4 diopters where the cornea was the flattest nasally, thereby reducing the corneal asymmetry. The anterior elevation map also showed a reduced depression nasally. The patient reported significant improvement of her night vision. CONCLUSIONS: This case example of stromal surface topography-guided treatment demonstrated a significant reduction in the irregularity of the stromal surface and an improvement in the topography, and the visual quality. Stromal surface topography-guided ablation might become the tool of the future for therapeutic repairs because it offers advantages over the current alternative of transepithelial phototherapeutic keratectomy.

Short term LASIK outcomes using the Technolas 217C excimer laser and Hansatome microkeratome in 46,708 eyes treated between 1998 and 2001.

PURPOSE: To report the outcomes of a high-volume, multi-surgeon, multicentre LASIK corporation between 1998 and 2001. METHODS: 46,708 eyes of 24,138 consecutive patients with myopic astigmatism had undergone LASIK using the Bausch & Lomb Technolas 217C excimer laser and Hansatome microkeratome. The study included 38 surgeons operating at 11 surgical centres. 31 surgeons underwent standardised training regardless of previous experience, which included didactic, observership and proctorship components. Mean attempted spherical equivalent refraction correction was -4.02 ± 1.93 D (range -0.50 to -12.00 D). Mean attempted cylinder correction was 0.78 ± 0.69 D (range 0.00 to 3.50 D). Median follow-up was 3 months. RESULTS: Postoperative data with at least 1 month follow-up was available in 35,360 eyes (76%) of 18,195 patients. Predictability: mean deviation from intended spherical equivalent refraction correction was -0.21 ± 0.47 D with 81% of eyes within ±0.50 D and 95% of eyes within ±1.00 D. EFFICACY: uncorrected distance visual acuity was 20/20 in 71% of eyes and 20/40 in 95% of eyes. SAFETY: two or more lines of corrected distance visual acuity were lost in 0.57% of eyes. Postoperative corrected distance visual acuity was worse than 20/40 in 0.029% of eyes. CONCLUSIONS: The short-term results of a high-volume, multi-surgeon LASIK Corporation were comparable with those reported in the Food and Drug Administration clinical trials during the same period.

Epithelial thickness profile changes induced by myopic LASIK as measured by Artemis very high-frequency digital ultrasound.

PURPOSE: To characterize changes in the corneal epithelial thickness profile induced by myopic LASIK. METHODS: This was a prospective study of 37 eyes of 19 myopic LASIK patients. Eyes were divided into three groups according to sphere in the maximum myopic meridian: low (-1.00 to -4.00 diopters [D]), moderate (-4.25 to -6.00 D), and high myopia (-6.25 to -13.50 D). The epithelial thickness profile was measured by prototype Artemis very high-frequency (VHF) digital ultrasound scanner (ArcScan Inc) across the central 10-mm corneal diameter preoperatively and between 3 and 6 months postoperatively. The epithelial thickness profile was determined by averaging the epithelial thickness within annular bands centered on the corneal vertex. The change in epithelial thickness profile was calculated as the difference between the preoperative and postoperative epithelial thickness profiles. RESULTS: The corneal epithelium thickened after myopic LASIK across the central 6 mm with maximum thickening centrally and progressively less thickening centrifugally in low myopia, and a more homogenous thickening in moderate and high myopia within the 5-mm diameter. The mean epithelial thickening at the corneal vertex was 7.41 +/- 1.09 microm, 9.29 +/- 1.22 microm, and 12.33 +/- 1.05 microm for low, moderate, and high myopia, respectively. The rate of epithelial thickening at the corneal vertex per diopter of myopia treated decreased with increasing myopia. CONCLUSIONS: Although the magnitude of epithelial thickening increased with increasing ablation depth, in accordance with Barraquer's Law of Thicknesses, the myopic refractive shift due to epithelial thickness profile changes was paradoxically more significant in low myopia than in high myopia.

Probability model of the inaccuracy of residual stromal thickness prediction to reduce the risk of ectasia after LASIK part I: quantifying individual risk.

PURPOSE: To measure the imprecision of microkeratome cuts, preoperative corneal pachymetry, and laser ablation depth and develop a statistical model to describe the probability of the residual stromal bed thickness (RST) after myopic LASIK being significantly thinner than predicted. METHODS: Preoperative corneal thickness, flap thickness, ablation depth, and RST were measured in 36 eyes by a prototype three-dimensional very high-frequency (VHF) 50 MHz digital ultrasound scanning device (<1.2 microm precision), precursor to the commercially available Artemis 2. All eyes had undergone LASIK with the Moria LSK-One microkeratome and the NIDEK EC-5000 excimer laser. Based on the statistically combined uncertainty (standard deviation) and bias (accuracy to intended value) of corneal thickness measurement, flap thickness, and ablation depth, a continuous probability function was devised describing the chance of obtaining an actual RST less than a specified "cut-off". The model was applied using the data collected from the cohort of eyes. The model was also applied using published flap thickness statistics on a series of microkeratomes. RESULTS: Precision (standard deviation) was 0.74 microm for VHF digital ultrasound measurement of pachymetry, 30.3 microm for Moria LSK-One flap thickness, and 11.2 microm for NIDEK EC-5000 ablation depth. Assuming negligible laser ablation depth bias, the model found the probability that the actual RST will be <200 pmicromgiven a target RST of 250 microm is 7.56% with the Moria LSK-One. The model applied to published flap statistics revealed a range of probabilities of leaving <200 microm given a target RST of 250 microm from <0.01% to 33.6%. CONCLUSIONS: The choice of microkeratome, laser, and pachymeter has a significant impact on the variation of the depth of keratectomy and thus on the risk of ectasia. This model together with high-precision microkeratomes, preoperative pachymetry, and knowledge of laser ablation precision would enable surgeons to determine the specific imprecision of RST prediction for individual LASIK cases and minimize the risk of ectasia.

Probability model of the inaccuracy of residual stromal thickness prediction to reduce the risk of ectasia after LASIK part II: quantifying population risk.

PURPOSE: To derive a statistical model to estimate the rate of excessive keratectomy depth below a selected cut-off residual stromal thickness (RST) given a minimum target RST and specific Clinical Protocol; apply the model to estimate the RST below which ectasia appears likely to occur and back-calculate the safe minimum target RST that should be used given a specific Clinical Protocol. METHODS: Myopia and corneal thickness distribution were modeled for a population of 5212 eyes that underwent LASIK. The probability distribution of predicted target RST error (Part I) was used to calculate the rate of excessive keratectomy depth for this series. All treatments were performed using the same Clinical Protocol; one surgeon, Moria LSK-One microkeratome, NIDEK EC-5000 excimer laser, Orbscan pachymetry, and a minimum target RST of 250 microm--the Vancouver Clinical Protocol. The model estimated the RST below which ectasia appears likely to occur and back-calculated the safe minimum target RST. These values were recalculated for a series of microkeratomes using published flap thickness statistics as well as for the Clinical Protocol of one of the authors-the London Clinical Protocol. RESULTS: In the series of 5212 eyes, 6 (0.12%) cases of ectasia occurred. The model predicted an RST of 191 microm for ectasia to occur and that a minimum target RST of 329 microm would have reduced the -rate of ectasia to 1: 1,000,000 for the Vancouver Clinical Protocol. The model predicted that the choice of microkeratome varied the rate of ectasia between 0.01 and 11,623 eyes per million and the safe minimum target RST between 220 and 361 microm. The model predicted the rate of ectasia would have been 0.000003: 1,000,000 had the London Clinical Protocol been used for the Vancouver case series. CONCLUSIONS: There appears to be no universally safe minimum target RST to assess suitability for LASIK largely due to the disparity in accuracy and reproducibility of microkeratome flap thickness. This model may be used as a tool to evaluate the risk of ectasia due to excessive keratectomy depth and help determine the minimum target RST given a particular Clinical Protocol.

Evaluating microkeratome efficacy by 3D corneal lamellar flap thickness accuracy and reproducibility using Artemis VHF digital ultrasound arc-scanning.

PURPOSE: To present a method that determines the efficacy of flap creation using a microkeratome. METHODS: Thirty-six eyes of 18 patients were included in the study, operated by two experienced surgeons using the Moria LSK-One microkeratome. Very high-frequency (VHF) digital ultrasound arc-scanning using Artemis technology was used to measure the Reinstein Flap Thickness Profile created by addition of the preoperative epithelial thickness profile to the postoperative stromal component of the flap. Descriptive statistics revealed central flap thickness accuracy and reproducibility. Univariate and multivariate regression analysis was used to determine correlations between central flap thickness and preoperative clinical parameters. Three-dimensional flap thickness maps demonstrated the mean, median, standard deviation, and range of thickness. RESULTS: Intended flap thickness was 160 microm. Mean central flap thickness was 161 microm in the right eye and 166 microm in the left eye. Central reproducibility (standard deviation of the mean) was 30.3 microm, with a range of 106 to 228 microm. Central flap thickness correlated to preoperative corneal thickness measured by VHF digital ultrasound but not by Orbscan. On qualitative analysis, no consistency among eyes was observed in the three-dimensional flap thickness map. Flaps were on average thinner in the center, but not in all cases. CONCLUSIONS: A systematic qualitative and quantitative statistical evaluation of microkeratome accuracy and reproducibility can be performed by measuring corneal flap thickness with arc-scanning VHF digital ultrasound.