Accuracy of intraocular lens formulas using total keratometry in eyes with previous myopic laser refractive surgery.

OBJECTIVES: This comparative study aimed to determine if total keratometry (TK) from IOLMaster 700 could be applied to conventional formulas to perform IOL power calculation in eyes with previous myopic laser refractive surgery, and to evaluate their accuracy with known post-laser refractive surgery formulas. METHODS: Sixty-four eyes of 49 patients with previous myopic laser refractive surgery were evaluated 1 month after cataract surgery. A comparison of the prediction error was made between no clinical history post-laser refractive surgery formulas (Barrett True-K, Haigis-L, Shammas-PL) and conventional formulas (EVO, Haigis, Hoffer Q, Holladay I, and SRK/T) using TK values obtained with the optical biometer IOLMaster 700 (Carl Zeiss Meditec), as well as Barrett True-K with TK. RESULTS: The mean prediction error was statistically different from zero for Barrett True-K, Barrett True-K with TK, Haigis-L, Shammas-PL, and Holladay I with TK. The mean absolute error (MAE) was 0.424, 0.671, 0.638, 0.439, 0.408, 0.424, 0.479, 0.647, and 0.524, and median absolute error (MedAE) was 0.388, 0.586, 0.605, 0.298, 0.294, 0.324, 0.333, 0.438, and 0.377 for Barrett True-K, Haigis-L, Shammas-PL, Barrett True-K TK, EVO with TK, Haigis with TK, Hoffer Q with TK, Holladay I with TK, and SRK/T with TK, respectively. EVO TK followed by Barrett True-K TK and Haigis TK achieved the highest percentages of patients with absolute prediction error within 0.50 and 1.00 D (68.75%, 92.19%, and 64.06%, 92.19%, respectively) CONCLUSIONS: Formulas combined with TK achieve similar or better results compared to existing no-history post-myopic laser refractive surgery formulas.

Comparison of corneal power obtained from VERION image-guided surgery system and four other devices.

PURPOSE: To assess the corneal keratometric values obtained using the VERION image-guided surgery system and other devices. METHODS: This study evaluated the right eyes of 115 cataract patients before intraocular lens (IOL) implantation through consecutive tests using 5 devices: VERION Reference Unit, Placido-based corneal topography (OPD-Scan III), monochromatic light-emitting diodes (LenStar LS900 and AL-Scan), and rotary prism technology (auto kerato-refractometer KR-8800). Analyzed parameters were corneal steep and flat keratometric values (Ks and Kf) and corneal astigmatism and axis. These parameters were evaluated using the one-sample two-tailed t-test and the 95% limits of agreement (95% LOAs) between the devices. RESULTS: The mean corneal cylinder value measurements were -0.97±0.63 D, -0.88±0.60 D, -0.90±0.69 D, -0.90±0.67 D, and -0.83±0.60 D with VERION, LenStar, AL-Scan (2.4 mm), OPD III, and KR-8800, respectively. Only KR-8800 showed a significant difference from VERION in the corneal cylinder value (P<0.05). The mean differences in the Kf and Ks of VERION compared to those of OPD III were 0.18±0.45 D and 0.17±0.38 D (P<0.05), respectively. The 95% LOAs of Bland-Altman analysis for the corneal astigmatism axis of the VERION with LenStar, AL-Scan (2.4 mm), OPD III, and KR-8800 were -26.25° to 58.71°, -20.61° to 47.44°, -25.03° to 58.98°, and -27.85° to 65.17°, respectively. CONCLUSION: None of the VERION parameters were significantly different from those of AL-Scan and LenStar. AL-Scan (2.4 mm zone) was especially similar to VERION. Wide LOAs are potential contributors to axis error in patients with toric IOL implants.

Analgesic effect of supplemental intracameral lidocaine during phacoemulsification under topical anaesthesia: a randomised controlled trial.

AIMS: To determine the analgesic effect of supplemental intracameral lidocaine 1% during phacoemulsification under topical anaesthesia, and to assess the risk factors associated with pain. METHODS: In a double-masked, randomised, clinical trial, 506 patients undergoing phacoemulsification under topical anaesthesia were randomised to receive a supplemental intracameral injection of either 0.5 cc of 1% lidocaine (277 patients, 54.7%) or balanced salt solution (BSS) (229 patients, 45.3%). Patients were interviewed by a trained interviewer using a standardised questionnaire. The main outcome measure was intraoperative pain, scored on a visual analogue scale of 0-10. Logistic regression was performed to assess ORs. RESULTS: 125 of 277 patients (45.1%) experienced pain in the lidocaine group, compared with 123 of 229 patients (53.7%) in the BSS group. The proportion of patients who experienced pain was significantly lower in the intracameral lidocaine group compared with the BSS group (multivariate OR 0.68, 95% CI 0.47 to 0.97; p=0.034). The median pain score (range) was 0.0 for intracameral lidocaine group compared with 1.0 for BSS group (p=0.039). Pain was more common in females (54.3% vs 43.6%; OR 1.56), non-Chinese (62.3% vs 46.9%; OR 2.13) and those who had previous cataract surgery to the fellow eye (55.3% vs 44.7%; OR 1.61). CONCLUSION: The use of 0.5 cc of 1% intracameral lidocaine during phacoemulsification under topical anaesthesia significantly reduces pain experienced by patients. Risk factors for pain include females, non-Chinese and previous cataract surgery.

Ethnic differences in higher-order aberrations: Spherical aberration in the South East Asian Chinese eye.

PURPOSE: To determine the distribution of higher-order corneal and ocular aberrations in a healthy refractive surgery population. SETTING: Island Hospital, Penang, Malaysia. METHODS: In this prospective observational study, 1 eye of ethnic Chinese refractive surgery patients was evaluated with an Orbscan II corneal topographer and a Zywave Hartmann-Shack aberrometer with a 6.0 mm pupil. Height data were analyzed to derive the higher-order aberrations (HOAs) from the 3rd to 5th Zernike order. RESULTS: The mean spherical equivalent in the 70 eyes evaluated was -6.46 diopters +/- 3.10 (SD). The mean total corneal HOA was 0.574 +/- 0.218 microm (range 0.269 to 1.249 microm) and the mean total ocular HOA, 0.525 +/- 0.354 microm (range 0.138 to 2.145 microm). There was no statistically significant correlation with age. The mean 3rd-order ocular aberration was 0.399 +/- 0.287 microm; the mean 4th-order, 0.297 +/- 0.223 microm; and the mean 5th-order, 0.108 +/- 0.101 microm. Corneal spherical aberration was greater than ocular spherical aberration (mean 0.312 +/- 0.114 microm versus 0.200 +/- 0.170 microm). Multilinear regression showed that the only dependent that predicted ocular spherical aberration was anterior corneal asphericity (r(2) = 0.227, F = 17.95, P<.001). CONCLUSION: Corneal and ocular aberrations in South East Asian Chinese eyes were significantly greater than that reported in other populations. Population differences in wavefront errors were significant, and this should be noted in patient management.

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.

A comparative analysis of intraocular lens power calculation methods after myopic excimer laser surgery.

PURPOSE: To compare the accuracy and predictability of different intraocular lens (IOL) power calculation methods in eyes after myopic excimer laser surgery. METHODS: Phacoemulsification and IOL implantation outcomes in 37 eyes of 37 patients with prior LASIK or photorefractive keratectomy were documented (amount of correction=-6.92+/-3.12 diopters (D), range: -2.00 to -13.00 D). The theoretical IOL power that would have resulted in emmetropia was calculated (IOLemme). Using the clinical history keratometry and biometry, the IOL power was calculated using the following methods: Sanders, Retzlaff, Kraff (SRK)-T, SRK-T Double-K (DK), Holladay 1 DK, Hoffer Q DK, Holladay 2 DK, Feiz-Mannis, and Ladas-Stark corneal bypass. The calculated IOL power was compared to IOLemme and used to determine the mean error and mean absolute error of refractive outcome for each eye. RESULTS: The calculated IOL power using the SRK-T, Feiz-Mannis, and Holladay 1 DK methods were significantly different from IOLemme. The lowest mean absolute error was achieved using the Hoffer Q DK method (0.75+/-0.52 D), Holladay 2 DK (0.75+/-0.62 D), SRK-T DK (0.76+/-0.60 D), and Ladas-Stark (0.83+/-63 D). With the SRK-T DK method, 51.4% of eyes were within +/-0.50 D of emmetropia and 67.6% of eyes were within +/-1.00 D. The Holladay 2 DK method had the highest percentage (81.1%) of eyes within +/-1.00 D and 45.9% within +/-0.50 D. CONCLUSIONS: The refractive results of IOL implantation using the same biometry data in eyes after LASIK can vary markedly. The SRK-T DK, Hoffer Q DK, and Holladay 2 DK methods resulted in the highest accuracy.

Meridional analysis for calculating the expected spherocylindrical refraction in eyes with toric intraocular lenses.

PURPOSE: To present a method of calculating the expected spherocylindrical refraction after toric intraocular lens (IOL) implantation and to compare it with the actual outcomes in a small group of patients. SETTING: Eye Institute, Tan Tock Seng Hospital, Singapore. METHODS: The expected refractions of the 2 principal meridians of the cornea (maximum keratometry and minimum keratometry) were calculated separately using the Holladay 1 formula with a modification to the data used for the effective lens position. The expected refraction was compared with the actual outcomes in 7 eyes that had phacoemulsification with toric IOL implantation. RESULTS: The mean preoperative keratometric cylinder was 2.07 diopters (D) +/- 0.97 (SD) (range 1.25 to 3.75 D), and the mean postoperative cylinder by subjective refraction was 0.68 +/- 0.40 D (range 0.25 to 1.50 D). The mean expected cylindrical refraction was 0.62 +/- 0.46 D (range 0.30 to 1.60 D). Five eyes (71.4%) were within +/-0.25 D of the expected refraction, and all eyes were within +/-0.50 D. There was a significant reduction in cylinder after toric IOL implantation (P<.001, paired t test). There was no statistically significant difference between the magnitude of the expected cylindrical refraction and the actual postoperative cylinder by subjective refraction (P = .604). CONCLUSION: Calculation of the expected spherocylindrical refraction with toric IOLs was achieved using meridional analysis.

Validity of the keratometric index: large population-based study.

PURPOSE: To determine the accuracy of the keratometric index of 1.3315 based on the Gullstrand model eye in predicting the power of the posterior cornea, Gullstrand's model was compared to a calculated keratometric index derived from actual measurements of the cornea. SETTING: Eye Institute, Tan Tock Seng Hospital, Singapore. METHODS: One eye of 2429 subjects with a mean spherical equivalent of -5.32 diopters (D) +/- 2.88 (SD) was measured with the Orbscan II (Bausch & Lomb). The following variables were analyzed: anterior radius of curvature (r(anterior)), posterior radius of curvature (r(posterior)), radius of keratometry (r(simK)), and central pachymetry. RESULTS: The r(anterior), r(posterior), and r(simK) were normally distributed, with a mean of 7.87 +/- 0.25 mm (95% confidence interval [CI], 7.38-8.36), 6.46 +/- 0.26 mm (95% CI, 5.95-6.97), and 7.71 +/- 0.27 mm (95% CI, 7.18-8.24), respectively. The mean ratio between the anterior corneal curvature and posterior corneal curvature was 1.22 +/- 0.03 (95% CI, 1.16-1.28). Based on the measurements of each eye, the mean calculated keratometric index, N(calc), was 1.3273 +/- 0.0013 (95% CI, 1.3248-1.3298). Using N(calc), the posterior corneal power was predicted to within +/-0.50 D of the actual posterior power in 98.3% of eyes. The mean absolute error between the actual and calculated posterior power was 0.157 +/- 0.123 D using N(calc) and 0.326 +/- 0.133 D using the Gullstrand model. CONCLUSION: Modifying the keratometric index increased the accuracy of predicting the posterior corneal power.

Relationship between the corneal surface and the anterior segment of the cornea: An Asian perspective.

PURPOSE: To determine the values for the anterior best-fit sphere (BFS) and posterior BFS in an Asian population using the Orbscan II (Bausch & Lomb) slit-scanning Placido disk corneal topographer. SETTING: Hospital-based refractive surgery center. METHODS: This prospective nonrandomized study comprised of 1 eye of 724 subjects. The eyes were measured with the Orbscan II. Default settings were used to generate relative elevation maps. Anterior BFS, posterior BFS, mean keratometry (K(mean)), spherical equivalent (SE), horizontal white-to-white size (WTW), and anterior chamber depth (ACD) were analyzed. RESULTS: The mean spherical equivalent (SE) in the 724 eyes was -5.32 diopters (D) +/- 2.88 (SD). The anterior BFS and posterior BFS were normally distributed, with a mean of 43.05 +/- 1.37 D and 52.43 +/- 2.03 D, respectively. The regression equation of anterior BFS and posterior BFS was posterior BFS = 0.443 + 1.208 anterior BFS (r(2) = 0.667, P<.01). The ratio of posterior BFS to anterior BFS was 1.22 +/- 0.03. The anterior BFS was significantly correlated to posterior BFS (P<.01), K(mean) (P<.01), and horizontal WTW (P<.01) but not to SE (P = .28). The posterior BFS was significantly correlated to K(mean) (P<.01) and the horizontal WTW (P<.01) but not to SE (P = .70). CONCLUSION: There was a close relationship between the various corneal parameters, which is significant in refractive surgery and intraocular lens calculation and design.

Efficacy of measuring visual performance of LASIK patients under photopic and mesopic conditions.

INTRODUCTION: The aim of the study was to compare the visual performance of LASIK eyes measured using high-contrast logMAR letter charts under bright (photopic) and dim (mesopic) conditions. MATERIALS AND METHODS: A total of 46 subjects (35 +/- 8 years of age) undergoing LASIK procedures were recruited for the study. The best spectacle-corrected visual acuity (BSCVA) of each subject was measured using the high-contrast ETDRS logMAR chart under photopic and mesopic conditions at 3 visits: preoperative (Pre), 1 month postoperative (Post1) and 3 months postoperative (Post3). The differences in logMAR scores for the right eyes only were analysed for the Pre-Post1 (n = 46), Pre-Post3 (n = 18) and Post1-Post3 (n = 16) comparisons. RESULTS: The logMAR scores of subjects were worse at the 1-month postoperative visit than preoperatively, and improvement in visual performance was seen at the 3-month postoperative visit. These changes in visual performance became more evident under mesopic conditions. The means and standard errors of the differences in logMAR scores for the Pre-Post3 (0.097 +/- 0.020) were slightly larger than those of the Pre-Post1 (-0.067 +/- 0.019) and Post1-Post3 (0.031 +/- 0.012) comparisons. Under mesopic conditions, the visual performance of the subjects was statistically significant for the 3 comparisons, but not under photopic conditions. CONCLUSION: High-contrast logMAR chart performed under mesopic conditions has the potential to replace visual acuity measured under photopic conditions in providing more reliable representation of postoperative visual outcomes of LASIK eyes. Eye doctors should consider performing this vision test routinely to gauge the success of LASIK surgery.

Corneal elevation indices in normal and keratoconic eyes.

PURPOSE: To determine an index that distinguishes keratoconus and keratoconus-suspect eyes from normal eyes with Orbscan (Bausch & Lomb) corneal elevation maps. SETTING: Department of Ophthalmology, Tan Tock Seng Hospital, Singapore, and iLaser Centre, Island Hospital, Penang, Malaysia. METHODS: In this initial phase of this multicenter nonrandomized retrospective study, 1 eye of 166 normal subjects, 15 keratoconus patients, and 11 keratoconus suspects was examined at 1 clinic. The anterior best-fit sphere (BFS), posterior BFS, anterior elevation, posterior elevation, and maximum and minimum keratometries were analyzed. Two indices with the highest sensitivity and specificity classifying these conditions were identified using receiver operating characteristic curves. These 2 indices were evaluated in a subsequent validation study using 1 eye of 221 normal subjects, 43 keratoconus patients, and 23 keratoconus suspects from another clinic. RESULTS: The anterior elevation and anterior elevation ratio (anterior elevation ratio = anterior elevation/anterior BFS) best classified the different groups. An anterior elevation ratio of 0.5122 mm or less had 99% sensitivity and 95.2% specificity while a ratio 16.5 mum or less had 80.1% sensitivity and 80.8% specificity in discriminating normal eyes from keratoconus and keratoconus suspects. The results were similar in the validation study. In addition, these anterior elevation and anterior elevation ratio cutoff values had high sensitivity and specificity in identifying keratoconus suspects from normal eyes in the validation study. CONCLUSION: Anterior corneal elevation parameters are clinically relevant measures for detecting keratoconus and suspected keratoconus eyes.

NeuroVision treatment for low myopia following LASIK regression.

PURPOSE: To evaluate a novel non-surgical method for improving vision in a refractive surgery patient. METHODS: A 45-year-old man who had undergone LASIK 5 years previously presented with blurred distance vision. Unaided vision in the right eye was 20/329-2) and 20/20 in the left eye. He enrolled for NeuroVision treatment (NeuroVision Pte Ltd, Singapore), a computer-based interface in which a repetitive set of visual excerises is performed for 10 to 12 weeks. RESULTS: After 35 sessions, unaided visual acuity in the right eye was 20/16(-3) and 20/20(-1) in the left eye, representing 2.8 lines of improvement in the right eye and 1.6 lines in the left eye. CONCLUSIONS: NeuroVision, a noninvasive treatment based on the concept of perceptual learning, is a benefit in cases in which surgical enhancement is not recommended.

Orbscan global pachymetry: analysis of repeated measures.

PURPOSE: The Orbscan II is a hybrid slit-scanning and Placido disc corneal topographer capable mapping global pachymetry over a 10-mm corneal diameter. In this study, the repeatability of the Orbscan global pachymetry was determined. METHODS: Five consecutive Orbscan examinations were performed on one eye of 20 healthy volunteers by one examiner in one session. Mean, standard deviation, range of readings, and coefficient of variance were calculated for the central, thinnest point and global pachymetry. RESULTS: Orbscan repeatability for the central pachymetry was 3.62 microm (95% confidence interval [CI], 0.57-6.66) with a coefficient of variance of 0.67% (95% CI, 0.09-1.25) and intraclass correlation coefficient (ICC) of 0.978 (95% CI, 0.959-0.990). Thinnest point pachymetry had a repeatability of 4.25 microm (95% CI, 1.19-7.31) with a coefficient of variance of 0.80% (95% CI, 0.19-1.42) and ICC of 0.973 (95% CI, 0.950-0.988). Global pachymetry had a coefficient of variance <1.5% in the central 4 mm and <<2.5% across the entire cornea. Global pachymetry showed an area of greatest repeatability (<< +/- 5 microm) within a central 3.0-mm horizontal and 4.0-mm vertical diameter. DISCUSSION: Orbscan is capable of giving repeatable pachymetry readings that are comparable to the ultrasound pachymeter within the central and thinnest point of the cornea. There is a gradual loss of repeatability toward the peripheral cornea possibly as a result of a lack of overlapping data points.

Corneal honeybee sting.

BACKGROUND: We report the complications and management of a retained bee sting injury to the cornea. The case highlights the acute and chronic management of an uncommon injury and its pathogenesis. METHODS: A 67-year-old man was attacked by a swarm of bees and was referred for severe chemosis on the right eye. A retained corneal bee stinger (ovipositor) was seen but removal was only partially successful. He subsequently developed a large corneal epithelial defect, anterior uveitis, intractable glaucoma, traumatic cataract, toxic optic neuropathy, and corneal scarring. We reviewed the literature on corneal bee sting injuries and their complications. RESULTS: Inflammation was controlled with topical steroids and the patient underwent a combined phacoemulsification and trabeculectomy with mitomycin-C for uncontrolled glaucoma. However, optic neuropathy did not resolve. INTERPRETATION: Corneal bee sting injuries are uncommon but can result in severe sight-threatening complications such as toxic optic neuropathy. Early recognition of the possible complications and appropriate treatment may help to prevent permanent loss of vision. Removal of a retained corneal bee stinger remains controversial.

Post-LASIK myopic shift after a trek in the North Pole.

Bilateral laser in situ keratomileusis was performed in a 37-year-old male ethnic Chinese adventurer. Eight weeks postoperatively, the patient made an unassisted trek to the geographic North Pole. After 2 weeks in this environment, a myopic shift of more than -1.50 diopters occurred in both eyes. The myopic shift resolved over 2 months when the patient returned to a tropical climate. Using Orbscan corneal topography, we propose that the myopic shift was attributed to biomechanical changes in the cornea induced by corneal dehydration in the Arctic environment.

Effect of higher-order wavefront aberrations on binocular summation.

PURPOSE: To determine the effect of individual Zernike wavefront aberrations on binocular summation and binocular visual acuity. METHODS: A 0.25-microm wavefront aberration of second, third and fourth order Zernike modes were introduced into a set of log minimum angle of resolution unit (logMAR) visual acuity charts convolved by CTView. Subjects were dilated and fitted with an artificial pupil of 3 mm. For each set of charts, right eye, left eye, and binocular acuity was measured. The gain in binocular visual acuity over monocular visual acuity was defined as binocular summation. The visual acuity lost binocularly reading aberrated charts was normalized for each subject and defined as the aberration induced loss in acuity. RESULTS: Binocular summation was 10.0% (95% limits of agreement 8.8 to 11.1%) in the unaberrated state and ranged from 17.3 to 3.4% in the Zernike modes studied. Binocular summation was greatest in defocus followed by coma and astigmatism. The aberration induced loss in monocular and binocular acuity was higher for Zernike modes with low angular frequency compared to those with high angular frequency. Linear regression showed a significant relationship between aberration-induced loss of visual acuity and binocular summation. CONCLUSIONS: A fixed amount of root mean square (RMS) aberration has a varied effect on binocular vision depending on the angular frequency and radial order of Zernike mode. Binocular vision has a positive effect in reducing the visual impact of aberrations as Zernike modes that suffer from the most loss of visual acuity also experience the greatest amounts of binocular summation.

Evaluation of biofilm formation on nylon sutures removed from clinically noninfected eyes after cataract surgery.

PURPOSE: To determine the presence of bacterial biofilm on nylon sutures removed from clinically noninfected eyes after cataract surgery. SETTING: The Eye Institute, Tan Tock Seng Hospital, Singapore, Singapore. METHODS: Sutures were removed from 10 eyes after cataract surgery at different time periods. Immediately after removal, the sutures were fixed and dehydrated. All sutures were viewed by scanning electron microscopy, and 6 were also viewed by transmission electron microscopy (TEM). RESULTS: There was no evidence of bacterial biofilm formation on the nylon sutures. Significant cellular debris was seen, mainly at the knots. Clusters of coccoid-shaped structures were visible; however, examination by TEM showed they were not bacteria. CONCLUSION: There was no evidence of biofilm formation on sutures removed after cataract surgery from clinically noninfected eyes.