Double peak axial length measurement signal in cataract patients with epiretinal membrane.

PURPOSE: To evaluate the accuracy of axial length (AL) measurement for intraocular lens (IOL) calculation in patients with cataract and epiretinal membrane (ERM). METHODS: This prospective, cross-sectional study was performed in cataract patients with ERM. All subjects were sent for standard optical biometry, prepared for cataract surgery. Signals of AL measurement were detected as double peaks and recorded as AL1 (first peak), and AL2 (second peak). The IOL power was calculated from AL1 and AL2, and reported as IOL1 and IOL2. The IOL2 was chosen for cataract surgery in all cases. Postoperative predictive errors were compared between IOL1 and IOL2. RESULTS: Thirty-seven eyes from 37 patients were included. Mean AL1 was significantly shorter than AL2 (23.13 ± 1.28 vs. 23.60 ± 1.34 mm, p < 0.001), resulting in higher power of IOL1 than IOL2 (mean difference was 1.53 ± 0.96 diopters, p < 0.001). At 3-months post-operation, twenty-nine eyes (78.4%) (95% CI 62.8%-88.6%) showed refractive error within ± 0.5 diopter and all eyes were within ± 1.0 diopter. Postoperative predictive errors including mean arithmetic error (ME) and mean absolute error (MAE) of IOL2 were significantly lower than those of IOL1 (ME: IOL1 vs. IOL2, -0.94 ± 0.91 vs. 0.08 ± 0.51; MAE: 0.97 ± 0.88 vs. 0.39 ± 0.33 diopter, all p < 0.001). CONCLUSIONS: AL measurement in ERM can be detected as a double peak signal during biometric measurement. The IOL power calculated from the first and second peak signals is significantly different. However, the IOL power derived from the second peak signal provides better refractive outcomes. The results suggest that the second peak signal represents an accurate AL measurement.

Efficacy of Swept-source Optical Coherence Tomography in Axial Length Measurement for Advanced Cataract Patients.

SIGNIFICANCE: A major limitation of standard time-domain optical coherence tomography-based biometers (TD-OCT) is an inability to measure the axial length (AL) in advanced cataract. A new device that uses swept-source optical coherence tomography (SS-OCT) allows better light penetration. Hence, a considerable number of cataract patients who failed AL measurement by TD-OCT can be recovered by SS-OCT. PURPOSE: The purposes of this study were to evaluate the efficacy of an SS-OCT for AL measurement in advanced cataract patients and to identify characteristics of lens opacity that impede the AL measurement. METHODS: Advanced cataract patients who were unable to obtain AL measurement using a standard TD-OCT-based optical biometer (IOLMaster500; Carl Zeiss Meditec, Jena, Germany) were recruited in this study. The AL was remeasured using SS-OCT (IOLMaster700), followed by measurement with immersion ultrasonography (IU). The percentage of patients who achieved AL measurement by SS-OCT was recorded. The AL obtained from SS-OCT was then verified by comparing with the AL derived from IU. The cataract type of each patient was classified according to standard Lens Opacity Classification III score. The association between characteristics of cataract and successful AL measurement by SS-OCT was analyzed. RESULTS: Sixty-four eyes that failed AL measurement from TD-OCT were included. Fifty-six eyes (87.5%) were able to be measured by SS-OCT (95% confidence interval, 77.23 to 93.53%). The AL obtained by SS-OCT showed very high agreement with those derived from IU (intraclass correlation coefficient, 0.99). There was no statistically significant correlation between characteristics of lens opacity and the capability of SS-OCT for AL measurement (P > .05). However, there was a trend toward an inability to measure the AL in cataracts with a high grade of lens opacity. CONCLUSIONS: The efficacy of SS-OCT-based optical biometer was excellent. Of the patients with advanced cataract who failed the AL measurement by TD-OCT, 87.5% could be recovered by SS-OCT. However, there was no specific type of lens opacity associated with a failure of AL measurement using SS-OCT.

Comparison of refractive outcomes using conventional keratometry or total keratometry for IOL power calculation in cataract surgery.

PURPOSE: To compare the refractive outcomes following cataract surgery using conventional keratometry (K) and total keratometry (TK) for intraocular lens (IOL) calculation in the SRK/T, HofferQ, Haigis, and Holladay 1 and 2, as well as Barrett and Barrett TK Universal II formulas. METHODS: Sixty eyes of 60 patients from Siriraj Hospital, Thailand, were prospectively enrolled in this comparative study. Eyes were assessed using a swept-source optical biometer (IOLMaster 700; Carl Zeiss Meditec, Jena, Germany). Posterior keratometry, K, TK, central corneal thickness, anterior chamber depth, lens thickness, axial length, and white-to-white corneal diameter were recorded. Emmetropic IOL power was calculated using K and TK in all formulas. Selected IOL power and predicted refractive outcomes were recorded. Postoperative manifest refraction was measured 3 months postoperatively. Mean absolute errors (MAEs), median absolute errors (MedAEs), and percentage of eyes within ± 0.25, ± 0.50, and ± 1.00 D of predicted refraction were calculated for all formulas in both groups. RESULTS: Mean difference between K and TK was 0.03 D (44.56 ± 1.18 vs. 44.59 ± 1.22 D), showing excellent agreement (ICC = 0.99, all p < 0.001). Emmetropic IOL powers in all formulas for both groups were very similar, with a trend toward lower MAEs and MedAEs for TK when compared with K. The Barrett TK Universal II formula demonstrated the lowest MAEs. Proportion of eyes within ± 0.25, ± 0.50, and ± 1.00 D of predicted refraction were slightly higher in the TK group. CONCLUSIONS: Conventional K and TK for IOL calculation showed strong agreement with a trend toward better refractive outcomes using TK. The same IOL constant can be used for both K and TK.

Ocular Injury due to Potassium Permanganate Granules.

PURPOSE: We report a rare case of ocular injury due to potassium permanganate (KMnO4) granules in a child. METHODS: This is a retrospective case report. RESULTS: A 2-year-old boy was transferred to our emergency room with severe pain in his right eye, inflamed eyelids, and brownish stains on his fingers. Chemical injury was suspected. Copious eye irrigation was immediately performed. Diffuse brownish splotches were then observed at the inferior bulbar conjunctiva. Otherwise, systemic organs were intact. Complete eye exam under general anesthesia revealed a 5-mm epithelial defect at the central cornea, along with generalized conjunctival injection and limbal ischemia, inferiorly. Multiple semi-dissolved granules of KMnO4 trapped in the inferior fornix were identified. The chemical particles were gradually washed out and removed; however, the brownish stains remained. The patient received preservative-free steroid, antibiotic eye drops, and lubricants as regular management for mild to moderate degree of ocular burn. Pseudomembrane developed early and transformed into symblepharon within a few days after the injury. Membrane adhesion was lysed, and more aggressive medications were then substituted. Commercial amniotic membrane (PROKERA®) was also applied to promote wound healing and to prevent recurrence of symblepharon. The ocular surface was eventually restored, and corneal transparency was preserved. CONCLUSION: Ocular injury with the granular form of KMnO4 is rare. Its toxicity is comparable to concentrated KMnO4 solution. However, the dissolved particles that had been absorbed in the stained conjunctiva were continuously released and damaged the ocular surface more than we primarily anticipated. Awareness of this condition and prompt management yield a good treatment outcome.

Clinical comparison of a new swept-source optical coherence tomography-based optical biometer and a time-domain optical coherence tomography-based optical biometer.

PURPOSE: To evaluate the repeatability and reproducibility of a newer swept-source optical biometer and to compare it with a standard partial coherence interferometry (PCI) biometer. SETTING: Siriraj Hospital, Mahidol University, Bangkok, Thailand. DESIGN: Prospective comparative study. METHODS: One hundred eyes from 100 cataract patients were enrolled in this study. Each patient was measured with 2 optical biometers, a newer swept-source optical biometer (IOLMaster 700) and a standard partial coherence interferometry biometer (IOLMaster 500) by 2 independent operators. The keratometry, axial length (AL), anterior chamber depth, white-to-white corneal diameter, and intraocular lens (IOL) power, calculated by the SRK/T and the Haigis formulas for each device, were recorded. Intraoperator repeatability and interoperator reproducibility of both devices were analyzed using intraclass correlation coefficients (ICCs). Agreement of ocular biometry and IOL power between the 2 devices was evaluated using the Bland-Altman method. RESULTS: The repeatability and reproducibility of the swept-source and standard biometers were high for all ocular biometry parameters (ICC, 0.93-1.00). The agreement between the 2 biometers was also high (ICC, 0.92-1.00). The IOL powers obtained from both devices were not distinct. Because of the density of the cataracts, the AL in 5 eyes could be measured only by the swept-source biometer. CONCLUSIONS: Repeatability and reproducibility of a swept-source optical biometer was excellent and agreement with a standard biometer was very high. Better lens penetration ability and AL measurements were obtained with the swept-source biometer than with the standard biometer. FINANCIAL DISCLOSURE: No author has a financial or proprietary interest in any material or method mentioned.

Comparison of corneal astigmatism measurements of 2 optical biometer models for toric intraocular lens selection.

PURPOSE: To compare corneal astigmatism measurements obtained using a new optical biometer (AL-Scan) and a standard biometer based on partial coherence interferometry (IOLMaster 500) for toric intraocular lens (IOL) selection. SETTING: Siriraj Hospital, Mahidol University, Bangkok, Thailand. DESIGN: Prospective comparative study. METHODS: Keratometry (K) readings were taken with the 2 biometers in eyes of cataract patients. The corneal astigmatism readings from the 2 devices were compared. The cylinder measurements were transformed into J0 and J45 vector components for analysis, and the devices' values were compared using the paired t test. Correlation coefficients were calculated. The Bland-Altman method was used to evaluate the agreement between the sets of measurements. RESULTS: One hundred thirty-seven eyes (81 patients) were enrolled. The magnitude of astigmatism between the new biometer 2.4 mm zone and the standard biometer was similar (P = .19). There was a statistically significant difference between values from the new biometer 3.3 mm zone and the standard biometer (P < .05). There was good correlation in the overall magnitude of astigmatism between devices. There was a trend of low correlation toward the flat cornea (K <43.0 diopters [D]), low corneal cylinder (<1.0 D), and with-the-rule astigmatism. The IOL toricity calculated from both devices was similar. CONCLUSIONS: Corneal astigmatism measurements with the 2 biometers were similar in magnitude, especially in 2.4 mm zone of the new biometer. There was no difference in the toric IOL selections between the devices. FINANCIAL DISCLOSURE: No author has a financial or proprietary interest in any material or method mentioned.

Comparison of ocular biometry and intraocular lens power using a new biometer and a standard biometer.

PURPOSE: To compare the repeatability and reproducibility of ocular biometry and intraocular lens (IOL) power obtained with a new optical biometer (AL-Scan) and a standard optical biometer (IOLMaster 500). SETTING: Siriraj Hospital, Mahidol University, Bangkok, Thailand. DESIGN: Prospective comparative study. METHODS: Two independent operators measured eyes with cataract using both biometers. The keratometry values, axial length, anterior chamber depth, white-to-white (WTW) corneal diameter, and IOL power calculated using the Holladay 1 formula obtained with each device were recorded. Intraoperator repeatability and interoperator reproducibility of both devices were analyzed using the intraclass correlation coefficient (ICC). The agreement in ocular biometry and IOL power between the 2 devices was evaluated by the Bland-Altman method. RESULTS: The study recruited 137 eyes of 81 patients. The repeatability and reproducibility of both devices were high for all ocular biometry measurements (ICC, 0.87-1.00). Except for the WTW corneal diameter (ICC, 0.44), the agreement between the biometers was also high (ICC, 0.98-0.99). The IOL powers calculated by the Holladay 1 formula were similar between the 2 biometers. CONCLUSION: The new optical biometer provided excellent repeatability and reproducibility for all ocular biometry. Agreement with the standard optical biometer was good except for the WTW corneal diameter.

Accuracy of Holladay 2 formula using IOLMaster parameters in the absence of lens thickness value.

BACKGROUND: The accuracy of the Holladay 2 (H2) formula is well-documented. This formula requires seven variables to estimate effective lens position (ELP) for the IOL power calculation. The lens thickness (LT) value is one of the required variables. Interestingly, the IOLMaster, which is one of the most commonly used optical biometers, can provide all the required ocular variables except LT value. It has become a pertinent issue to evaluate the accuracy of theH2 formula when it is used without the LT value. The purpose of this study was to evaluate the results when using the H2 formula, without the LT value, and compare such results to those obtained using the Haigis formula and the Hoffer Q formula. METHODS: The Institutional review board (IRB) gave their approval for the conduct of this prospective comparative study. One hundred and sixty-three eyes of 143 cataract patients from the Ophthalmology Department, Siriraj Hospital, Thailand were recruited. All eyes were measured using the IOLMaster (Carl Zeiss Meditec, Jena, Germany) for keratometry (K), axial length (AL), anterior chamber depth (ACD), and horizontal white-to-white (WTW) corneal diameter. Then, the LT measurement was obtained by A-scan ultrasonography (Quantel Axis-II, Quantel Medical, USA). Every patient underwent uncomplicated phacoemulsification by a single surgeon (NC) with a single technique using a single IOL model. Post-operative refraction was obtained at 3 months. The mean absolute errors (MAEs), median absolute errors (MedAEs) and percentage of the eyes within ±0.25, ±0.50, and ±1.00 D of predicted refraction was calculated for H2 formula both with and without LT input, Haigis, and Hoffer Q formula. The results were also classified into a group of short AL (<22.0 mm), average AL (22.0 to 24.5 mm) and long AL (>24.5 mm). RESULTS: There was no statistically significant difference in either MAEs or MedAEs of all formulas in all AL groups including the H2 with and without LT. There was a trend toward lower MAEs and MedAEs for H2 in the long AL group. Percentage of the eyes within ±0.25, ±0.50, and ±1.00 D of predicted refraction were similar in all AL groups. CONCLUSION: The preliminary results of this study showed that the H2 formula performed well even without the LT value. It was comparable to the Haigis and Hoffer Q formulas.

Agreement of IOL power and axial length obtained by IOLMaster 500 vs IOLMaster 500 with Sonolink connection.

BACKGROUND: The accurrate and expedient ocular biometry is essential for modern cataract surgery. IOLMaster 500, one of the most popular partial coherence interferometry (PCI) device, has been widely used. However, with the PCI device, it is difficult to obtain the axial length through densely opaque media. With the current version of IOLMaster 500, a unique feature is added to link with the Synergy immersion A-scan ultrasound (sonolink connection). In case of failure to measure axial length by IOLMaster 500, the axial length can be obtained by ultrasound, and then transferred to IOLMaster 500 for the IOL power calculation. This study aims to compare the results and evaluate the agreement between IOL power and axial length obtained by IOLMaster 500 and IOLMaster 500 with sonolink connection. METHODS: A prospective study of 60 eyes in 60 mild-to-moderate cataract patients was conducted under Institutional Ethics Committee approval. Keratometry (K) and axial length (AL) of all eyes were measured using IOLMaster 500 (Carl Zeiss, Germany), then IOL power was generated using Holladay 1 formula (group 1). After 5 min, the K measurements were repeated with IOLMaster 500 and the AL were measured again using the Synergy A-scan ultrasound (Accutome, USA). Then, the AL data were transferred to IOLMaster 500 via the sonolink connection to generate the IOL power using the same setting (group 2). The IOL power and AL were compared between the two groups, and the agreement was evaluated using intraclass correlation coefficient (ICC) and the Bland-Altman method. RESULTS: The mean IOL power in group 1 was 21.04 + 2.36 D and group 2 was 21.03 + 2.36 D. The mean AL in group 1 was 23.35 + 0.86 mm and in group 2 was 23.36 + 0.86 mm. There was no statistically significant difference in IOL power and AL between the two groups. The agreements in IOL power and AL between both groups were high (ICCs = 0.997 for IOL power and 0.993 for AL) CONCLUSIONS: The IOL power and AL derived from both groups were similar. The agreements between them were high.

Comparison of corneal astigmatism and axis location in cataract patients measured by total corneal power, automated keratometry, and simulated keratometry.

PURPOSE: To compare the corneal astigmatism (magnitude and axis location) derived by total corneal power (TCP), automated keratometry, and simulated keratometry. SETTING: Siriraj Hospital, Mahidol University, Bangkok, Thailand. DESIGN: Prospective comparative study. METHODS: Eyes with previous ocular surgery or abnormalities were excluded. All patients were examined with the ARK 730A autokeratometer and the Galilei analyzer. The steepest and flattest corneal power along with the steepest axis of the TCP, automated keratometry, and simulated keratometry were recorded. Vector analysis (J0 and J45) was calculated. Analysis of variance with Bonferroni correction was performed for multiple comparisons. Outcome measures were the magnitude and axis location of astigmatism. RESULTS: One hundred eyes of 100 cataract patients were randomly selected. There was no statistically significant difference in the mean steepest axis between TCP (93.31 ± 68.75 [SD]), automated keratometry (94.24 ± 64.78), and simulated keratometry (92.42 ± 64.30). However, the mean magnitude of astigmatism measured by TCP (1.23 ± 0.75) was significantly higher than that measured by automated keratometry (0.93 ± 0.68) (P=.01) but not than that measured by simulated keratometry (1.08 ± 0.68) (P=.43); there was no statistically significant difference in J0 or J45. Twenty two (40%) of 54 eyes with more than 1.00 diopter of TCP astigmatism had more than 10 degrees of axis difference from automated keratometry. CONCLUSIONS: The magnitude of TCP astigmatism was higher than that of automated keratometry. The axis location was similar. However, there was more than 10 degrees of axis difference between automated keratometry and TCP in patients with high astigmatism. FINANCIAL DISCLOSURE: No author has a financial or proprietary interest in any material or method mentioned.

Spherical aberration from myopic excimer laser ablation for aspheric and non-aspheric profiles.

PURPOSE: To evaluate the difference in the spherical aberration (SA) induced by an aspheric and a non-aspheric myopic ablation profile with the MEL80 excimer laser (Carl Zeiss Meditec). METHODS: This was a prospective randomized paired-eye controlled study of 15 patients (30 eyes) who underwent LASIK using the Hansatome microkeratome (160 µm) (Bausch & Lomb) and the MEL80 using a 6 mm optical zone. For each patient, one eye was treated with an aspheric (Aberration Smart Ablation [ASA]) profile and the other eye was treated with a non-aspheric (Tissue Saving Ablation [TSA]) profile, assigned at random. Patients were measured before and 3 months after LASIK with the wavefront supported custom ablation aberrometer (Carl Zeiss Meditec), and SA was measured with an analysis zone of 3 to 7 mm in 0.5 mm intervals. The area under the curve of SA plotted against analysis zone (RAWS) was calculated. RESULTS: The mean spherical equivalent refraction was -2.43 ± 1.53 D for the ASA group and -2.54 ± 1.47 D for the TSA group (p = 0.87). There was a statistically significant increase in SA for an analysis zone of 6.0 to 7.0 mm for the ASA group and for an analysis zone of 4.5 to 7.0 mm for the TSA group. At the 6 mm zone, the increase in SA was 0.032 in the ASA group and 0.069 in the TSA group. The induction of SA per diopter was 0.024 µm/D in the ASA group and 0.035 µm/D in the TSA group. The RAWS parameter increased by 34.8% in the ASA group and 74.4% in the TSA group. CONCLUSIONS: There was less induction of SA in myopic LASIK with an aspheric ablation profile than with a non-aspheric ablation profile.

Refractive outcomes of femtosecond LASIK for myopic correction at Siriraj Hospital, Thailand.

OBJECTIVE: To evaluate refractive outcomes of myopic femtosecond LASIK at Siriraj Hospital, Thailand. MATERIAL AND METHOD: A retrospective study of patients who underwent femtosecond LASIK at SiLASIK center Siriraj hospital, Mahidol University, Thailand,from April 2009 to April 2010 was conducted. All patients had completely normal preoperative eye examination. All LASIK procedures were performed using Intralase femtosecond laser (Abbott Medical Optics Inc (AMO)) and VISX star S4 excimer laser (AMO). Postoperative follow-up included visual acuity (ETDRS chart) and manifest refraction at 1 day, 1 week, 1 month and 3 months. The flap thickness was evaluated at 3 months using Visante optical coherence tomography (OCT Carl Zeiss Meditec). RESULTS: One hundred and twenty eight eyes of 64 patients were included in the study. Mean preoperative manifest refraction spherical equivalent (MRSE) was -5.47 +/- 2.28 diopters (range, -1.88 to -12.63 diopters). At 3 months post-operation, the MRSE was -0.26 +/- 0.39 diopters (range, + 0.25 to -2.00 diopters) and 95.31% were within + 1.00 diopter of intended correction. There was 98% of eyes achieved uncorrected distance visual acuity (UDVA) 20/40 or better and 73% achieved UD VA 20/20 or better There was 31% of eyes gained one line and 15% lost 1 line of corrected distance visual acuity (CDVA). The mean error of flap thickness (difference between actual and intended central flap thickness) was + 0.23 +/- 1.77 microm for 120 microm flap setting and + 0.22 +/- 0.97 microm for 110 microm flap setting. The flap morphology was planar configuration. CONCLUSION: Myopic femtosecond LASIK at Siriraj Hospital provided satisfactory refractive outcomes and high accuracy of the flap thickness.

Precision analysis of posterior corneal topography measured by Visante Omni: repeatability, reproducibility, and agreement with Orbscan II.

PURPOSE: To evaluate the repeatability and reproducibility of posterior corneal curvature and posterior corneal elevation best-fit sphere (BFS) obtained with the Visante Omni (Carl Zeiss Meditec) and to compare the results with the Orbscan II (Bausch & Lomb). METHODS: Thirty eyes from 30 healthy volunteers were included in this study. All patients were examined 5 times with the Visante Omni and Orbscan II by 2 independent operators. The posterior corneal curvature (3- and 6-mm zone) and posterior corneal elevation BFS (5- and 8-mm zone) were generated for each system. Intraoperator repeatability and interoperator reproducibility and agreement between the systems were evaluated using the intraclass correlation coefficient (ICC) and the Bland-Altman method. RESULTS: The repeatability of posterior corneal curvature and posterior corneal elevation BFS measured by the Visante Omni was high for all analysis zones (ICC, 0.99 to 1.00). The reproducibility also showed similar results (ICC, 0.99 to 1.00). Agreement between the Visante Omni and Orbscan II was high for posterior corneal curvature (ICC, 0.94 to 0.97) and posterior corneal elevation BFS (ICC, 0.96 to 0.98) with 95% limits of agreement at -0.26 to 0.22 diopters for posterior corneal curvature and 0.11 to 0.69 mm for posterior corneal elevation BFS. CONCLUSIONS: The Visante Omni provides good repeatability and reproducibility of posterior corneal topography. Overall agreement with the Orbscan II system was high.

Development of a refractive error quality of life scale for Thai adults (the REQ-Thai).

OBJECTIVE: To develop a scale for measuring refractive error quality of life (QOL) for Thai adults. MATERIAL AND METHOD: The full survey comprised 424 respondents from 5 medical centers in Bangkok and from 3 medical centers in Chiangmai, Songkla and KhonKaen provinces. Participants were emmetropes and persons with refractive correction with visual acuity of 20/30 or better An item reduction process was employed by combining 3 methods-expert opinion, impact method and item-total correlation methods. The classical reliability testing and the validity testing including convergent, discriminative and construct validity was performed. RESULTS: The developed questionnaire comprised 87 items in 6 dimensions: 1) quality of vision, 2) visual function, 3) social function, 4) psychological function, 5) symptoms and 6) refractive correction problems. It is the 5-level Likert scale type. The Cronbach's Alpha coefficients of its dimensions ranged from 0.756 to 0. 979. All validity testing were shown to be valid. The construct validity was validated by the confirmatory factor analysis. A short version questionnaire comprised 48 items with good reliability and validity was also developed. CONCLUSION: This is the first validated instrument for measuring refractive error quality of life for Thai adults that was developed with strong research methodology and large sample size.

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.

Estimation of posterior corneal power for IOL calculation after myopic LASIK.

PURPOSE: To estimate the posterior corneal power for intraocular lens (IOL) calculation after myopic LASIK. METHODS: A retrospective study of 50 patients (92 eyes) who underwent uneventful myopic LASIK was conducted. Pre- and postoperative data (12 months) were collected including refraction and Orbscan II (Bausch & Lomb) for simulated keratometry (Sim K), central corneal thickness, and posterior corneal power. Statistical analysis was performed to determine the correlation between the postoperative posterior corneal power and other variables. An empirical formula was generated to predict the postoperative posterior corneal power. The net corneal power was calculated based on the new formula and Gaussian optics formula in an additional 31 eyes that underwent myopic LASIK and compared to the Orbscan total optical power map. The net corneal power was retrospectively applied to calculate the IOL power in an additional 10 patients who underwent cataract extraction after myopic LASIK. The back-calculation for emmetropic IOL power was performed and compared to the IOL power derived from the net corneal power. RESULTS: By using multiple linear regression, a formula to predict postoperative posterior corneal power can be computed from postoperative Sim K, central corneal thickness, and the amount of myopia treated with R2 of 0.63 (standard error =0.25) (P<.05). The net corneal power was highly correlated to the total optical power (R2=0.96); no statistically significant difference (paired t test) was noted (P>.05). The error of the IOL power derived from the net corneal power was -0.30+/-0.20 diopters. CONCLUSIONS: Corneal power after myopic LASIK can be estimated using postoperative Sim K, central corneal thickness, and the amount of myopia treated, which can be used for the assessment of IOL power after myopic LASIK.

Diurnal variation of higher order aberrations in human eyes.

PURPOSE: To evaluate diurnal variation of higher order aberrations in human eyes. METHODS: A prospective study of 20 myopic eyes was performed. Cycloplegic aberrometry (Zywave) and refraction were measured at 2-hour intervals for five time points beginning at 10:00 AM. Changes in measured magnitude for third and fourth order Zernike coefficients were compared over time. Analysis of variance (ANOVA) was performed to test for statistically significant differences across time points for each Zernike coefficient. A separate prospective study was performed of 11 myopic eyes to determine and compare diurnal variation with the repeatability coefficient of individual Zernike coefficients measured using the Zywave aberrometer. RESULTS: Mean refraction was -0.75 +/- 1.25 diopters (D) sphere and -0.57 +/- 0.43 D cylinder. No statistically significant change (ANOVA) was noted over the period of observation for any individual Zernike coefficients. Only Z3(-3) (vertical trefoil) showed an average increase of 0.029 microm (P = .98) towards the end of the day; however, this trend was not statistically significant. The standard deviation of individual Zernike coefficients ranged from 0.035 microm (Z4(4)) to 0.165 microm (Z3(-1)). CONCLUSIONS: No statistically significant diurnal variation of higher order aberrations was measured by Zywave aberrometer. The trend of Z3(-3) increasing later in the day was most likely a Zywave measurement error artifact.

Performance of conventional biometry vs. integrated laser interferometry with keratometry device in intraocular lens measurement.

OBJECTIVE: To compare the performance of the Conventional Biometry (CB) (Applanation Ultrasound and Keratometry) and the Integrated Laser Interferometry with Keratometry Device (LI) in the measurement of Intraocular Lens (IOL). MATERIAL AND METHOD: A prospective study of 100 eyes in 50 cataract patients was conducted. The IOL measurement using the LIfollowed by the CB was done on all eyes. The keratometry (K), axial length (AL), IOL power, and time required for both methods were compared in the same subject by ANOVA with repeated measurement. RESULTS: The LI could not obtain the AL in 14%. There were 21% that the signal to noise ratio was below 1.6. The AL obtained by the LI was significant higher compared to the CB by mean of 0.28 mm (p < 0.05). The K showed no statistically significant difference (p = 0.05). There was statistically significantly higher IOL power calculated by the LI compared to the CB by mean of 0.63 D (p < 0.05). The time required for the LI was 2.77 +/- 1.44 min and the CB was 9.63 +/- 3.82 min (p < 0. 05). CONCLUSION: The LI required less time than the CB in measurement of the IOL but could not measure the AL in the group of patients with dense cataracts. The LI give a higher IOL power compared to the CB.

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.