Axis Of Astigmatism Research Articles

Interaction of axial and oblique astigmatism in theoretical and physical eye models.

The interaction between oblique and axial astigmatism was investigated analytically (generalized Coddington's equations) and numerically (ray tracing) for a theoretical eye model with a single refracting surface. A linear vector-summation rule for power vector descriptions of axial and oblique astigmatism was found to account for their interaction over the central 90° diameter of the visual field. This linear summation rule was further validated experimentally using a physical eye model measured with a laboratory scanning aberrometer. We then used the linear summation rule to evaluate the relative contributions of axial and oblique astigmatism to the total astigmatism measured across the central visual field. In the central visual field, axial astigmatism dominates because the oblique astigmatism is negligible near the optical axis. At intermediate eccentricities, axial and oblique astigmatism may have equal magnitude but orthogonal axes, which nullifies total astigmatism at two locations in the visual field. At more peripheral locations, oblique astigmatism dominates axial astigmatism, and the axes of total astigmatism become radially oriented, which is a trait of oblique astigmatism. When eccentricity is specified relative to a foveal line-of-sight that is displaced from the eye's optical axis, asymmetries in the visual field map of total astigmatism can be used to locate the optical axis empirically and to estimate the relative contributions of axial and oblique astigmatism at any retinal location, including the fovea. We anticipate the linear summation rule will benefit many topics in vision science (e.g., peripheral correction, emmetropization, meridional amblyopia) by providing improved understanding of how axial and oblique astigmatism interact to produce net astigmatism.

The effect of astigmatism axis on visual acuity.

To evaluate the effect of astigmatism axis on uncorrected distance visual acuity (UDVA) in emmetropic eyes that underwent laser refractive surgery. This retrospective study included patients who underwent laser in situ keratomileusis or photorefractive keratectomy between January 2000 and December 2015 at the Care-Vision Laser Centers, Tel Aviv, Israel. Eyes with a 3-month postoperative spherical equivalent between -0.5 D and 0.5 D were included in this study. Eyes with ocular comorbidities and planned ametropia were excluded. Study eyes were divided into 3 groups according to the steep astigmatic axis: with the rule (WTR) (60-120), oblique (31-59 or 121-149), and against the rule (ATR) (0-30 or 150-180). The UDVA of these 3 groups was compared. The oblique group was divided into oblique ATR and oblique WTR, which were compared with each other. A total of 17,416 consecutive eyes of 8,708 patients were studied. The WTR eyes (n = 10,651) had significantly better UDVA (logMAR 0.01 ± 0.08) than the oblique (n = 3,141, logMAR 0.02 ± 0.09) and ATR eyes (n = 3,624, logMAR 0.02 ± 0.10) (p<0.001). The oblique WTR group had significantly better UDVA than the oblique ATR group (p<0.001). The UDVA of the oblique and ATR groups was similar. Stepwise multiple regression analysis showed that the group accounted for 15% of the UDVA variance (p = 0.04). The astigmatic axis has a small but significant effect on UDVA in emmetropic eyes; WTR was better than oblique and ATR astigmatism. Therefore, when correcting astigmatism, it may be preferable to err towards WTR astigmatism.

Topography of the corneal epithelium and Bowman layer in low to moderately myopic eyes

To compare the epithelium (air-tear film) and Bowman layer's specular topographies in patients having low to moderate myopia corrected by photorefractive keratectomy (PRK). Rothschild Foundation, Paris, France. Prospective interventional case series. Anterior corneal specular Placido topography using the OPD-Scan II Placido-based topographer was performed in eyes of patients having PRK for myopia before and after epithelial removal. The differences in axial keratometry (K), asphericity (Q value), astigmatism magnitude (toricity), and axis were computed in the 1st, 3rd, and 5th central corneal millimeter zones. The study comprised 51 patients (90 eyes). The mean difference in axial mean K was 0.56 diopter (D)±0.26 (SD), 0.56±0.27 D, and 0.56±0.24 D in the 1st, 3rd, and 5th central millimeter rings, respectively. The mean difference in the magnitude of epithelium-induced astigmatism was 0.42±0.43 D×90, 0.41±1.60 D×78, and 0.02±1.82 D×83 (positive cylinder) in the 1st, 3rd, and 5th central millimeter rings, respectively. Corneal astigmatism shifted toward an increased with-the-rule orientation after epithelial removal. The mean difference in asphericity (over the central 6.0mm zone) was -0.07±0.21. These differences were significant (P<.0001) and independent of age, sex, central pachymetry, and spherical equivalent. In low to moderately myopic eyes, the topography of Bowman layer was significantly steeper than that of the epithelium. The epithelial layer tended to reduce the magnitude of Bowman layer's astigmatism and prolateness. None of the authors has a financial or proprietary interest in any material or method mentioned.

Agreement of corneal measurements between dual rotating Scheimpflug–Placido system and Placido-based topography device in normal and keratoconus eyes

To compare the anterior corneal measurements between Placido-based topography and dual-Scheimpflug topography in healthy and keratoconus eyes. Optometry Research Group, Instituto Universitario de Oftalmobiología Aplicada, University of Valladolid, Valladolid, Spain. Comparative case series. The mean simulated keratometry (K), flat K, steep K, astigmatism power, corneal astigmatism axis, J0, J45, maximum corneal power point, and white-to-white (WTW) distance were collected and compared between healthy eyes and keratoconus eyes. The study evaluated in 56 healthy eyes and 56 keratoconus eyes. Placido-based topography underestimated all topographic values except J45 and WTW in healthy eyes and J0, maximum corneal power point, and WTW in keratoconus eyes, with statistically significant differences in astigmatism (healthy), flat K (keratoconus), axis (keratoconus), J0, J45, and WTW (P<.05). Healthy eyes showed better agreement (95% limits of agreement: simulated K -0.13 to 0.40; steep K -0.30 to 0.59; flat K -0.29 to 0.51; astigmatism -0.60 to 0.64; J0 -1.15 to 1.13; J45 -1.10 to 1.20; maximum corneal power point -0.70 to 1.17; WTW -0.96 to 0.76mm) than keratoconic eyes (simulated K -2.84 to 4.55; steep K -2.80 to 5.21; flat K -3.68 to 4.70; astigmatism -1.90 to 2.95; J0 -2.85 to 3.20; J45 -3.21 to 3.05; maximum corneal power point -7.00 to 4.51 D; WTW -1.00 to 0.88). Healthy eyes showed better agreement than keratoconus eyes between Placido-based and dual-Scheimpflug topography. Both instruments could be used interchangeably with caution in healthy eyes, but not in keratoconus management. None of the authors has a financial or proprietary interest in any material or method mentioned.

A novel color-LED corneal topographer to assess astigmatism in pseudophakic eyes.

PurposeTo assess the accuracy of corneal astigmatism evaluation measured by four techniques, Orbscan IIz®, Lenstar LS900®, Cassini®, and Total Cassini (anterior + posterior surface), in pseudophakic eyes.Patients and methodsA total of 30 patients (46 eyes) who had undergone cataract surgery with the implantation of a monofocal intraocular lens (AcrySof IQ) were assessed after surgery. For each eye, subjective assessment of astigmatism and its axis was performed. Minimum, maximum, and mean keratometry and astigmatism and its axis were evaluated using the four measurement techniques. All measurements were compared with the subjective measurements. Agreement between each measurement technique and subjective assessment was evaluated using Bland–Altman plots. Linear regressions were performed and compared.ResultsLinear regression analysis of astigmatism axis showed very high R2 for all models, with Total Cassini showing the least difference to the unit slope (0.052) and the least difference to a null constant (3.790), although not statistically different from the other models. Regarding astigmatism value, the Cassini and Total Cassini models were similar and statistically better than the Lenstar model. Cassini and Total Cassini showed better J0 compared with Orbscan.ConclusionOn linear regression models, Cassini and Total Cassini showed the best performance regarding astigmatism value. Cassini and Total Cassini also showed the least J0 deviation from the Cartesian origin compared with Orbscan, which had the lowest performance. Total corneal measurement with the color LED topographer seems to be a better technique for astigmatism assessment.

An easy and practical method for toric intraocular lens implantation: marking corneal astigmatic axis at slit-lamp.

The objective of this study was to present a practical method of marking the corneal astigmatic axis for the patient sitting at the slit-lamp before toric intraocular lens (IOL) implantation. Eighteen eyes of 18 patients, who underwent uncomplicated phacoemulsification, with an implantation of Acrysof toric IOL were included. We marked the astigmatic axis while the patient sitting at the slit-lamp before surgery. The patient was asked to look at a distant target at head height with the fellow eye. Using the rotator switch, the slit light of the slit-lamp was just turned on to the steep astigmatic meridian in the orthograde position. Then, two tips of the astigmatic meridian were marked with a marking pen, where the slit light crossed at the limbus 180° away. Preoperative corneal and postoperative refractive astigmatism values were compared. Uncorrected and corrected postoperative visual acuities (UDVA and BCVA) and IOL rotations at early and late periods were noted. The mean age and mean follow-up were 63.6±14.6years and 9.4±5.3months (range 3-16months), respectively. Mean postoperative UDVA and BCVA at Snellen chart were 0.62±0.21 and 0.82±0.13, respectively. Mean preoperative keratometric and mean postoperative refractive astigmatism values were 2.48±0.87 D and 0.66±0.48 D, respectively. Reduction of astigmatism was significant (p<0.01). The mean rotation at 1week and that at last follow-up were 2.1°±3.1° and 2.3°±3.0°, respectively. Marking corneal astigmatic axis at slit-lamp is a simple and effective method in toric intraocular lens implantation. Surgeon does not need additional instrument except a slit-lamp and a marking pen, and can complete the marking task in just one setting.

Changes in astigmatism and corneal higher-order aberrations after phacoemulsification with toric intraocular lens implantation for mild keratoconus with cataract.

To assess the changes in astigmatism and higher-order aberrations (HOAs) after toric intraocular lens (IOL) implantation for mild non-progressive keratoconus with cataract. We prospectively examined 19 eyes of 19 consecutive keratoconic patients (mean age±standard deviation, 63.1±9.1years) who underwent phacoemulsification with toric IOL implantation. We determined uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), refractive astigmatism, corneal astigmatism, corneal HOAs and astigmatic axis rotation both preoperatively and 3months postoperatively. Logarithm of the minimal angle of resolution (logMAR) UDVA was significantly improved from 1.14±0.50 preoperatively to 0.46±0.33 postoperatively (Wilcoxon signed-rank test, p<0.001). LogMAR CDVA was also significantly improved from 0.27±0.45 preoperatively to -0.01±0.09 postoperatively (p<0.001). In 5 of 6 eyes in which the target refraction was emmetropia, postoperative UDVA was better than 20/32. The achieved spherical equivalent correction was within ±0.5diopters (D) of the targeted correction in 13 (68%) eyes and 1.0 in 18 (95%) eyes. The refractive astigmatism was significantly decreased from -1.92±1.73D preoperatively to -0.70±0.60D postoperatively (p=0.006). The corneal astigmatism changed from 2.89±1.30D preoperatively to 2.98±1.09D postoperatively (p=0.492), which was not statistically significant. The corneal HOAs for a 4-mm pupil was changed from 0.47±0.23µm preoperatively to 0.52±0.26µm postoperatively (p=0.211), which was not statistically significant. According to our experience, toric IOL implantation for mild keratoconic patients having rigid gas-permeable lens intolerance appears to be effective for reducing refractive astigmatism without a significant induction of corneal HOAs.

Changes in the axis of astigmatism and in fundus torsion following inferior oblique muscle weakening

To study the changes in fundus torsion and in the axis of astigmatism following inferior oblique (IO) myectomy in patients with inferior oblique overaction (IOOA). The degree of fundus torsion and corneal astigmatism were prospectively evaluated before and 3months after IO myectomy in patients with IOOA grade +2 or more in one or both eyes and an astigmatic error of ≥1 D. Fundus torsion was evaluated by measuring the disk foveal angle (DFA) using fundus photography. The axis of astigmatism was identified from the anterior sagittal map using Pentacam corneal imaging. A total of 54 eyes of 27 patients were included. Patients were divided into two groups: those with esotropia and those with exotropia. All patients had a preoperative DFA >8°, which decreased postoperatively in both groups (P<0.01). Postoperatively, there was incyclorotation of the axis of astigmatism by >5° in 80% of the esotropic group and 75% of the exotropic groups (P<0.01). Incyclorotation of the axis of astigmatism occurs after IO myectomy. Measurement of the change in the axis of astigmatism can be used to assess the torsional changes after IO myectomy in patients with IOOA.

Comparison of keratometry and astigmatism measured by Pentacam and IOL Master

Objective To compare the difference of keratometry and astigmatism measured by Pentacam and IOL Master in age-related cataract operations. Methods The inspection of involved patients was taken by the same operator.The data of keratometry and corneal astigmatism were the average of three sets. Paired t tests ,A simple linear correlation analysis and the Bland-Altman method were used to evaluate the difference, correlation and agreement of keratometry respectively. Wilcoxon signed rank test and simple statistical analysis were performed to compare the difference of corneal astigmatism. Results The mean keratometry value were (44.14±1.49) D (male), (44.73±1.55) D (female) for Pentacam, and (44.27±1.50) D (male)、(44.86±1.56) D (female) for IOL Master. There was significant statistical difference in keratometry value both in the terms of instrument and sex.The correlation of keratometry was high(r=0.986). The absolute value of the maximum difference was 1.19 D in the 95% limits of agreement of Bland-Altman histogram. The astigmatism degree and axis were (0.77±0.52) D and (85.38±53.36)° for Pentacam, (0.90±0.61) D and (85.38±53.36)° for IOL Master.There was obivous statistical difference in astigmatism degree between the two instruments, while, no great difference in axis.The percent of astigmatism axis difference above 10° was 44.4%. Conclusions Both Pentacam and IOL Master can measure corneal curvature and astigmatism of cataract patients accurately.There are good correlation and poor consistency. Key words: Pentacam; IOL Master; Keratometry; Corneal astigmatism

난시안에서 주경선 간 시력차이와 교정시력의 손실

목적: 난시교정의 중요성을 강조하기 위해 주경선 간 시력차이 유무와 교정시력의 손실정도를 알아보고자 하였다. 방법: 평균 나이 <TEX>$22.75{\pm}2.36$</TEX>세의 난시성 굴절이상자 64명(122안)을 대상으로 하였다. 난시안을 완전교정 한 후 열공판 중앙의 가는 틈을 난시교정 축방향과 <TEX>$90^{\circ}$</TEX> 반대방향으로 일치시킨 상태에서 시력을 각각 측정하였다. 결과: 경선 간 시력차이가 존재하지 않는 경우는 52안이었고, 경선 간 시력차이가 존재하는 경우는 70안이었다. 단안시력이 1.0 미만인 경우는 52안 중 14안, 70안 중 24안으로 나타났다. 난시도는 경선 간 시력차이가 존재하는 경우가 경선간 시력차이가 존재하지 않는 경우보다 높게 나타났으며, 단안시력이 1.0 미만의 경우가 더 높은 것으로 나타났다. 단안시력은 주경선 중 시력이 높게 나타나는 경선과 높은 상관성을 보였다. 좋은 시력을 유지하기 적합한 안경 교체주기는 6~12개월로 분석되었다. 결론: 난시의 지속적인 저교정은 경선 간 시력차이를 일으키는 원인이 되므로 임상에서 난시안의 최종처방은 완전교정을 목표로 하여야 한다. Purpose: This study was performed to investigate the difference of meridional visual acuity and the loss of corrected visual acuity (VA) in order to emphasis the importance of astigmatic correction. Methods: 64 subjects (122 eyes) aged <TEX>$22.75{\pm}2.36years$</TEX> participated in this study. After full correction of astigmatic refractive error, VA was measured in which the direction of the slit filter was matched with astigmatic axis and <TEX>$90^{\circ}$</TEX> to the astigmatic axis. Results: 52 eyes showed no difference in VA between the two direction. However 70 eyes had difference VA between them. 14 out of 52 eyes and 24 out of 70 eyes had under 1.0 in monocular VA. The astigmatic degree was higher in the existence of VA difference between the two direction than non-existence. The difference is higher with under 1.0 monocular VA. Monocular VA is closely related to the focal line having better VA in the principal focal line. Glasses replacement period was analyzed as 6~12 months for the preservation of better VA. Conclusions: The final glasses prescription has to be given with full correction because continued under-correction for astigmatism causes meridional VA difference.

Prevalence of corneal astigmatism before cataract surgery.

The purpose of this study was to describe and analyze the prevalence and pattern of corneal astigmatism in cataract surgery candidates. In a prospective cross-sectional study, preoperative demographics, and keratometric and refractive values of cataract surgery candidates were collected from January 2013 to December 2014. Axial length (AL) and flat and steep keratometry measurements were optically measured by a partial coherence interferometry device (IOLMaster). This study consisted of 2156 eyes of 1317 patients with a mean age of 64.92±11.48 (SD) (30-88years). The mean of AL was 23.33±1.37mm, and the mean of corneal astigmatism was 1.12±1.10 diopter (D) (range 0.0-7.00), in all patients. Furthermore, the mean of flat and steep keratometry were 43.70±1.70 and 44.83±1.79 D, respectively. Corneal astigmatism was 1.50 D or less in 1590 eyes (73.7%), more than 1.50 D in 566 eyes (26.2%), 3.00 D or more in 161 eyes (7.4%), WTR in 796 eyes (36.9%), ATR in 1010 eyes (46.8%), and oblique in 350 eyes (16.2%). ATR astigmatism axis significantly increased with the increase in age. Corneal astigmatism of most cataract surgery candidates fell between 0.50 and 1.50 D. The results of our study however is confined to our demographics might provide useful data for cataract patients, surgeons, and intraocular lens manufacturers for different purposes.

Effect of posterior corneal astigmatism on power calculation and alignment of toric intraocular lenses: Comparison of methodologies

To compare the accuracy of different methods that consider posterior corneal curvature in toric intraocular lens (IOL) calculations. Ein-Tal Eye Center, Tel-Aviv, Israel. Retrospective comparative case series. Consecutive cases of toric IOL implantation and preoperative measurements by optical biometry with optical low-coherence reflectometry (OLCR) (Lenstar LS 900) and a Scheimpflug camera (Pentacam) were retrospectively reviewed. Five methods of toric IOL calculation were compared as follows: (1) anterior corneal astigmatism using OLCR, (2) application of the Baylor nomogram, (3) posterior tomography combined with anterior corneal measurements using vector summation, (4) the Scheimpflug camera's true net power, and (5) total corneal refractive power. Toric IOL astigmatic power and axis, aiming for the lowest residual astigmatism, were selected according to these methods. Simulated residual refraction was calculated for each method based on manifest refraction and measured IOL alignment more than 3 weeks after surgery. The study included 115 eyes of 92 patients. The median simulated residual astigmatism was lower when based on vector summation of anterior and posterior astigmatisms than with calculations based on anterior corneal measurements only, application of the Baylor nomogram, true net power, and total corneal refractive power readings (0.49 diopters [D] versus 0.70 D, 0.60 D, 0.64 D, and 0.76 D, respectively) (P < .001). Residual astigmatism after toric IOL implantation can be reduced by appropriate consideration of the posterior corneal astigmatism. Using methods that take into account the effect of the posterior cornea in toric IOL calculations is suggested. Drs. Assia and Kleinmann are consultants to Hanita Lenses, Israel. Dr. Abulafia received a speaker's fee from Haag-Streit AG. No other author has a financial or proprietary interest in any material or method mentioned.

Characterization of anterior corneal high-order wavefront aberrations and correlations with astigmatism in Chinese elderly subjects

This study aimed to investigate the distribution of total high-order aberrations (HOAs) Z3±1, Z40, and Z3±3 of anterior cornea and their correlations to astigmatism. In addition, the relationship between different axes of astigmatism and Z3−1 or Z31 was analyzed. The HOAs in the central 6-mm zone of the cornea were measured with an iTrace wavefront analyzer in 556 cataractous eyes at the Tianjin Eye Hospital in China. Simulated keratoscope readings (Sim k) were also measured with the analyzer. The corneal total HOAs Z3±1, Z40 and Z3±3 exhibited a non-normal distribution. Significant correlations were found between the topographic-simulated keratometric power (ΔSim k) with the axis at 180°±5° and absolute value of Z3−1 (P=0.008), the ΔSim k with the axis at 90°±5° and absolute value of Z31 (P<0.001), and the ΔSim k and total HOAs (P<0.001), Z3±1 (P<0.001), and Z3±3 (P<0.001). No significant correlation was found between ΔSim k and Z40 (P=0.404). In conclusion, corneal astigmatism with the axis near the horizontal position was mainly associated with the value of Z3−1. Corneal astigmatism with the axis near the vertical position was mainly related to the value of Z31. HOAs except Z40 are correlated with astigmatism.

Influence factors of estimation errors for total corneal astigmatism using keratometric astigmatism in patients before cataract surgery

To evaluate the influence factors of the estimation errors for total corneal astigmatism using keratometric astigmatism in patients preparing for cataract surgery. EYE and ENT Hospital of Fudan University, Shanghai, China. Prospective observational study. Eyes of patients preparing for cataract surgery were measured with Pentacam Scheimpflug imaging device. Keratometric astigmatism was obtained using the anterior corneal surface measurement and the keratometric index (1.3375) while neglecting the posterior corneal surface measurement. The Scheimpflug-measured total corneal astigmatism was derived by vector analysis of the astigmatism on both corneal surfaces. The study comprised 374 eyes of 374 patients 45 to 84 years old. The mean absolute error in magnitude and mean absolute error in angle comparing keratometric astigmatism with Scheimpflug-derived astigmatism was 0.18 ± 0.14 diopter (D) and 7.7 ± 11.0 degrees, respectively. The mean magnitude of the error vector was 0.24 ± 0.14 D. The error in magnitude was significantly larger in eyes with against-the-rule anterior astigmatism, while error in angle was larger in eyes with with-the-rule and oblique anterior astigmatism. Multiple regressions showed that 4 predictors (difference in anterior-posterior astigmatism axis, magnitude of posterior astigmatism, magnitude of keratometric astigmatism, and axial length [AL]) were significantly associated with the absolute error in magnitude. Predictors including the difference in the anterior-posterior astigmatism axis, magnitude of posterior astigmatism, magnitude of keratometric astigmatism, and age were significantly associated with the absolute error in angle and magnitude of the error vector. Neglecting posterior corneal astigmatism yielded significant estimation errors in total corneal astigmatism in patients preparing for cataract surgery. Estimation errors were significantly influenced by the difference in the anterior -posterior astigmatism axis, magnitude of posterior astigmatism, keratometric astigmatism, AL, and age. None of the authors has any conflicts of interest to disclose.

Corneal Diameter as a Factor Influencing Corneal Astigmatism After Cataract Surgery.

To evaluate the corneal horizontal diameter [white-to-white (WTW) distance] as a factor influencing surgically induced astigmatism (SIA) and postoperative astigmatism. A total of 330 eyes with corneal astigmatism ≤1.5 D underwent cataract surgery with phacoemulsification. A 3-step, superotemporal for the right eye and superonasal for the left eye, clear corneal incision of 3.0 mm was made. Four groups were created according to WTW distance: group A ≤11.6 mm, group B 11.7 to 11.9 mm, group C 12.0 to 12.2 mm, and group D ≥12.3 mm. SIA was calculated by vector analysis using the Alpins method. We noted all cases, in which a change greater than 0.5 diopters (D) in astigmatism took place and a change greater than 20 degrees in axis torque, despite axis direction, on the first and sixth postoperative months. SIA was found in group A 0.98 D ± 0.6 (SD), B 0.79 D ± 0.43 (SD), C 0.68 D ± 0.45 (SD), and D 0.53 D ± 0.32 (SD) at the first postoperative month. At the sixth postoperative month, SIA was 0.77 D ± 0.43 (SD), 0.69 D ± 0.34 (SD), 0.62 ± 0.36 (SD), and 0.49 D ± 0.27 (SD), respectively. A change greater than 0.5 D in corneal astigmatic power at the first and sixth months postoperatively was significantly lower in eyes with WTW distance 12.0 to 12.2 mm and ≥12.3 mm in comparison with eyes with WTW distance ≤11.6 mm and 11.7 to 11.9 mm (P < 0.05). Changes greater than 20 degrees in astigmatic axis at the first and sixth postoperative months were not significantly different according to the horizontal corneal diameter. WTW distance should always be measured preoperatively when planning cataract surgery and should be accounted for in cases of large and small corneas.

Mathematical Method for Analysis of the Refractive Outcome after Toric Intraocular Lens Implantation

Purpose: We aimed to develop a self-designed software programmed with a mathematical method that analyzes the refractive outcome after toric intraocular lens (IOL) implantation, taking the axis misalignment of the toric IOL into consideration. Methods: A mathematical method that can analyze the refractive outcome after toric IOL implantation was devised. Compared with the conventional method, which performs the analysis based on precise alignment of the IOL, optional meridian orientations of the toric IOL were taken into account in this method. Self-designed computer software was developed using the mathematical method. Results: Relatively high accordance was achieved between the mathematical analysis and the actual postoperative outcome. The mean predicted spherical power was 0.37 ± 0.74 dpt, and the mean measured spherical power was 0.37 ± 0.71 dpt (paired t test, p = 0.98). The mean predicted cylindrical power was -1.35 ± 0.86 dpt, and the mean measured cylindrical power was -1.42 ± 0.85 dpt (p = 0.27). The mean predicted change in the astigmatic axis was 33.7 ± 11.8°, and the mean measured parameter was 32.5 ± 15.4° (p = 0.49). Conclusions: The advantage of the present mathematical method is that the postoperative refractive outcome can be analyzed under IOL misalignment.

Analysis of Changes in Anterior, Posterior and Total Corneal Astigmatism after On-Axis Cataract Surgery

Purpose: To assess the changes in anterior, posterior, and total corneal astigmatism after cataract surgery with on-axis clear corneal incision cataract surgery. Methods: This study included 48 eyes (24 eyes with ‘with-the-rule [WTR]’ and 24 eyes with ‘against-the-rule [ATR]’) that underwent phacoemulsification and intraocular lens insertion through on-axis clear corneal incision. The ATR group with vertically steep axis of posterior corneal astigmatism was divided into subgroups 1 and 2 for the opposite axis. Autorefraction, uncorrected and best-corrected visual acuities were measured. Corneal astigmatism (anterior, posterior and total) was measured using Pentacam preoperatively and 1 week, 1 month, and 2 months postoperatively. Results: Multivariate linear regression analysis of preoperative data showed positive correlations among anterior, posterior and total astigmatism. Anterior corneal astigmatism showed a significant decrease in both WTR and ATR groups in all measured points (all p 0.05) and significant decrease in the ATR subgroup 1 (p 0.05). According to correlation analysis based on trend line equations, 1.7 diopters of anterior astigmatism could expect 0.3 diopters of posterior astigmatism and 0.5 diopters of total astigmatism for the WTR group and 0.4 diopters of anterior astigmatism could expect 0.2 diopters of posterior astigmatism and 0.4 diopters of total astigmatism for the ATR group. Conclusions: Considering the majority of cataract patients have vertically steep posterior corneal astigmatism, temporal incision for ATR patients is generally effective. Moreover, for patients with WTR astigmatism of more than 1.7 diopters or ATR astigmatism greater than 0.8 diopters, additional preoperative correction based on posterior astigmatism is needed for more precise prediction of postoperative total corneal astigmatism. J Korean Ophthalmol Soc 2016;57(1):25-35

The location of incision in cataract surgery and its impact on induced astigmatism.

The purpose of the present study is a systematic review of previous studies on choosing the best incision site for the correction of astigmatism in cataract surgery and assessing the amount of surgically induced astigmatism (SIA) with each approach. Regardless of astigmatism axis, studies show that using an on-axis incision is associated with favorable results for 0.5-1.0 diopter (D) of astigmatism. In cases with more than 1.0 D astigmatism, paired on-axis incisions can be appreciably efficient in astigmatism correction and cause at least 1.5 D SIA. Considering the amount of SIA, a temporal incision is the best approach when the patient has minimal amounts of corneal astigmatism preoperatively. At higher levels of astigmatism, if no other astigmatism correction method is used simultaneously, the temporal incision is used less frequently; however, since it is associated with the least SIA, it is still the choice site when another correction method is used. The temporal incisions in cataract surgery are associated with little SIA and are appropriate choices for mild preoperative astigmatism. At higher levels of preoperative astigmatism, superior incisions are associated with better results when combined methods are not applied.

The application of vector analysis for evaluation of astigmatism correction in the corneal refractive surgery

Since sixty percent of ametropes obtain astigmatism, which has influence on the visual quality, correcting the astigmatism is always the focus of concerns during visual correction procedures especially for the corneal refractive surgery. The postoperative spherical equivalent or residual cylindrical dioptors was used as quantitative index to evaluate the correction of astigmatism previously; however, such results neglect the effect of astigmatic axis shift on the treatment. Taking astigmatism as a vector parameter could describe the magnitude and direction of astigmatism accurately, thus it was increasingly applied in the evaluation of astigmatism correction. This paper reviews the present vector analysis methods, evaluation indexes and its application for the correction of astigmatism in the corneal refractive surgery.

근시성 난시안에서 난시축 변화가 시력에 미치는 영향

The purpose of this study is to compare the status of eyesight according to the change of astigmatism axis in myopic astigmatism and to minimize errors in making astigmatic glasses based on accurate optometry and prescription. The subjects were 93 males and females(186 eyes) who have myopic astigmatism without any ocular disease or systemic disease. We performed comparative analysis on the status of visual acuity according to the change of astigmatism axis to 5, 10 and 15 degree in corrected eyesight 1.0. The direct astigmatism was the most common astigmatism type among the 186 eyes. After all subjects were perfectly corrected into 1.0, the change of astigmatism axis affected eyesight; The results suggested that the more change was made in astigmatism axis, the worse their eyesight would become. The main astigmatism type was changed from direct astigmatism to inverse astigmatism as age increased. The change of the astigmatism axis resulted in failing of corrected eyesight. Therefore, the convergence of examination and correction for astigmatism strength and axis is necessary when conducting refraction inspection for astigmatism.