Toric Power Research Articles

Prediction of refraction error after toric lens implantation with biometric input data uncertainties and power labelling tolerances.

The purpose of this study was to simulate the impact of biometric measure uncertainties, lens equivalent and toric power labelling tolerances and axis alignment errors on the refractive outcome after cataract surgery with toric lens implantation. In this retrospective non-randomised cross sectional Monte-Carlo simulation study we evaluated a dataset containing 7458 LenStar 900 preoperative biometric measurements. The biometric uncertainties from literature, lens power labelling according to ISO 11979, and axis alignment tolerances of a modern toric lens (Hoya Vivinex) were taken to be normally distributed and used in a Monte-Carlo simulation with 100 000 samples per eye. The target variable was the defocus equivalent (DEQ) derived using the Castrop (DEQC) and the Haigis (DEQH) formulae. Mean/median / 90% quantile DEQC was 0.22/0.21/0.36 D and DEQH was 0.20/0.19/0.32 D. Ignoring the variation in lens power labelling and toric axis alignment the respective DEQC was 0.20/0.19/0.32 D and DEQH was 0.18/0.17/0.29 D. DEQC and DEQH increased with shorter eyes, steeper corneas, equivalent lens power and highly with toric lens power. According to our simulation results, uncertainties in biometric measures, lens power labelling tolerances, and axis alignment errors are responsible for a significant part of the refraction prediction error after cataract surgery with toric lens implantation. Additional labelling of the exact equivalent and toric power on the lens package could be a step to improve postoperative results.

Meridional analysis for calculation of the toric power of phakic IOLs.

To present a reproducible method to calculate the toricity needed at the intraocular lens (IOL) plane with toric phakic IOLs (ICL, Staar Surgical) and compare its results with those obtained with the online calculator provided by the manufacturer. Retrospective case series. Private practice, Buenos Aires, Argentina. The formula originally described by Holladay to calculate the IOL power in phakic eyes was used to calculate the required spherical power along the less refractive meridian and along the more refractive meridian. Meridional analysis was applied to calculate the required toricity at the IOL plane and the surgically induced corneal astigmatism was incorporated into the calculations. The refractive cylinder predicted by this method and by the online calculator of the manufacturer were compared to the postoperative refractive cylinder by means of vector analysis. The possible changes in the ratio of toricity in patients with different amounts of astigmatism and anterior chamber depth are assessed in a theoretical section. In 35 eyes, the measured mean postoperative refractive cylinder was 0.09 D @ 99°, the mean predicted postoperative refractive astigmatism was 0.04 D @ 102° according to the manufacturer's online calculator and 0.09 D @100° according to our method. With both methods, 91.43% of eyes had an absolute cylinder prediction error within ±0.50 diopters. The method described in this article to calculate the toricity of phakic IOLs has a refractive accuracy similar to that of the original calculator developed by the manufacturer.

Calculation of Equivalent and Toric Power in AddOn Lenses Based on a Monte Carlo Simulation

Introduction: Additional lenses implanted in the ciliary sulcus (AddOn) are one option for permanent correction of refractive error or generate pseudoaccommodation in the pseudophakic eye. The purpose of this paper was to model the power and magnification behaviour of toric AddOn and to show the effect sizes with a Monte Carlo simulation. Methods: Anonymized data of a cataractous population uploaded for formula constant optimization were extracted from the IOLCon platform. After filtering out data with refractive spherical equivalent (RSEQ) between −0.75 and 0.25 dpt and refractive cylinder (RCYL) lower than 0.75, for each of the N = 6,588 records, a toric AddOn was calculated which transfers the refraction error from spectacle plane to AddOn plane using a matrix-based calculation strategy based on linear Gaussian optics. The equivalent (AddOnEQ) and toric (AddOnCYL) power of the AddOn and the overall lateral magnification change and meridional magnification were derived for the situations before and after AddOn implantation, and a linear modelling was fitted for all 4 parameters. Results: RSEQ is the dominant effect size in the prediction of AddOnEQ and overall change in magnification (ΔM), whereas the lens position (LP), corneal thickness (CCT), and mean corneal radius (CP_a) play a minor role. In a simplified model, AddOnEQ can be estimated by 0.0179 + 1.4104 RSEQ. RCYL and corneal radius difference (CP_ad) are the dominant effect sizes in the prediction of AddOnCYL and the change in meridional magnification (ΔM_mer), whereas LP, CCT, CP_a, and RSEQ play a minor role. In a simplified model, AddOnCYL can be predicted by −0.0005 + 0.0328 CP_ad + 1.4087 RCYL. Myopic eyes gain in overall magnification, whereas in hyperopic eyes, we observe a loss. Meridional distortion could be in general reduced to 35% on average with a toric AddOn. Conclusion: Our simulation shows that with a linear model, the equivalent and toric AddOn power, as well as overall change in magnification, meridional distortion before and after AddOn implantation, and the reduction in meridional distortion, can be easily predicted from the biometric data in pseudophakic eyes with moderate refractive error.

The Influence of Analysis Mode Selection on Prediction Accuracy of Corneal Astigmatism Using Pentacam.

Purpose: To evaluate the influence of analysis mode selection on prediction accuracy of corneal astigmatism using Pentacam.Methods: Fifty-nine eyes of 59 patients implanted with toric intraocular lenses (IOLs) were included in the retrospective study. Preoperative corneal astigmatism (total refractive power) measured with Pentacam was analyzed based on 2-, 3-, 4-, or 5-mm ring or zone mode either centered on corneal apex or pupil center. Actual corneal astigmatism was calculated based on residual astigmatism on the corneal plane, surgical-induced astigmatism, and effective toric power on the corneal plane. Prediction error, the difference between actual corneal astigmatism and measured astigmatism, was compared among different analysis modes. Influences of local topography on prediction error were also evaluated.Results: Based on the zone mode, prediction error was lower when centered on corneal apex than on pupil center at different diameters, whereas based on the ring mode, this difference was only seen at 2-mm cornea (all P < 0.05). When centered on the corneal apex, the zone mode showed lower prediction error than the ring mode at 4- and 5-mm corneas (both P < 0.001), regardless of asymmetric or symmetric astigmatism. In symmetric bowtie, the zone mode showed lower prediction error than the ring mode at 2-mm cornea of the small bowtie, and 4- and 5-mm corneas of the large bowtie (all P < 0.05).Conclusions: For toric IOL planning, the corneal apex may be a better reference center. At a cornea diameter ≥4 mm, the zone mode is more accurate than the ring mode. Local topography affects prediction accuracy in the symmetric bowtie.

Calculation of Toric Intraocular Lens Power with the Barrett Calculator and Data from Three Keratometers.

Aim To investigate the interdevice agreement for differences in toric power calculated using data on anterior corneal astigmatism obtained with corneal topography/ray-tracing aberrometry (iTrace), partial coherence interferometry (IOLMaster 500), and Scheimpflug imaging (Pentacam). Methods The analysis included 101 eyes (101 subjects) with regular astigmatism. The main outcome measures were corneal cylinder power, axis of astigmatism, and keratometry values. Toricity and toric IOL power were calculated using the online Barrett toric calculator. Interdevice agreement for measurement and calculation was assessed using a paired sample t-test and a nonparametric test. Results Significant interdevice differences were noted in the magnitude of astigmatism and flat, steep, and mean keratometry values between iTrace and IOLMaster (all P < 0.01); in flat, steep, and mean keratometry values (all P < 0.001) but not in the magnitude of astigmatism (P=0.325) between iTrace and Pentacam; and in the magnitude of astigmatism and steep and mean keratometry values (all P < 0.01) but not in flat keratometry values (P=0.310) between IOLMaster and Pentacam. The toric IOL power calculated using data from the three devices showed the following trend: iTrace > IOLMaster (0.49 ± 0.36, P < 0.001) and Pentacam (0.39 ± 0.42, P < 0.001) and Pentacam was <IOLMaster (−0.10 ± 0.39, P=0.009). There were differences in toricity calculated using data from the three devices (P=0.004). Conclusions Differences in toric IOL power and toricity calculated using anterior keratometry data from iTrace, IOLMaster 500, and Pentacam should be noted in clinical practice.

Comparison of astigmatism prediction error taken with the Pentacam measurements, Baylor nomogram, and Barrett formula for toric intraocular lens implantation

BackgroundTo evaluate and compare the astigmatism prediction errors taken with the Pentacam measurements, Baylor nomogram, and Barrett formula for toric intraocular lens (IOL) implantation.MethodsPhacoemulsification with toric Precizon IOL implantation was performed in 41 eyes with corneal astigmatism (range, 1 to 5 diopters (D)) determined by IOLMaster and SimK on Pentacam. Preoperative corneal astigmatism measurements were obtained from IOLMaster readings (IOLMaster, Baylor-IOLMaster, and Barrett-IOLMaster) and Pentacam readings (SimK, Baylor-SimK, Barrett-SimK, wavefront, true net power, total corneal refractive power, and vector derived by manual vector summation using corneal front and back astigmatism). Prediction error and intraclass correlation coefficient (ICC) between the measured (or calculated) astigmatism by IOLMaster and Pentacam and the estimated corneal astigmatism estimated by IOL toricity power and residual astigmatism were determined.ResultsThe centroid errors in prediction error with IOLMaster, SimK, Baylor-IOLMaster, Baylor-SimK, Barrett-IOLMaster, Barrett-SimK, wavefront, true net power, total corneal refractive power, and vector were 0.59@103, 0.61 @103, 0.37@161, 0.41@162, 0.24@171, 0.36@162, 0.42@106, 0.04@8, 0.07@82, and 0.03@82, respectively, in with-the-rule (WTR) astigmatism eyes at postoperative 3-month. They were 0.22@87, 0.20@74, 0.16@21, 0.54@10, 0.43@3, 0.33@19, 0.51@25, 0.31@58, 0.29@50, and 0.14@50 in against-the-rule (ATR) astigmatism eyes. Of the ten modalities, vector showed the lowest WTR astigmatism prediction error and the highest ICC between the predicted and the estimated corneal astigmatism for both WTR and ATR eyes.ConclusionVector summation using anterior and posterior corneal surface power taken with the Pentacam yields the least astigmatism prediction error and is a promising tool for determining toric IOL cylinder power.

Analysis of cataract surgery induced astigmatism: Two polar methods comparison

PurposeSurgically induced astigmatism (SIA) caused by the incision after cataract surgery may be calculated to improve IOL toric power calculation and achieve better visual outcome. SIA could be determined as the difference between preoperative and postoperative keratometry expressed in polar values using different equations. The objective of this study is to compare the SIA calculated with two different polar value analysis methods [Method #1: KP (90)/KP (135) developed to be used with incisions placed at 90° and Method #2: AKP/AKP (+45) developed to be used independently of the incision location]. MethodsPreoperative and one month postoperative data of 210 cataractous eyes (131 patients) undergoing uncomplicated cataract surgery were assessed. All incisions were performed at 11 o’clock (120°). No sutures were used in any patient. IOLMaster (Carl Zeiss Meditec, Dublin, Ireland) keratometry was used to polar calculation. ResultsThe average age was 66.25±12.33 years (range 22–89). SIA polar value data calculated with Method #1 were KP (90) −0.06±0.52D and KP (135) +0.05±0.91D and calculated with Method #2 were AKP −0.10±0.87D and AKP (+45) +0.02±0.02D. However, SIA value represented in traditional notation (diopters@axis in degrees) was the same value independently of the method used to calculate; +0.65@110.70°. ConclusionSIA value is independent of the polar method used to its calculation and slight variations in the incision position could be accepted without clinical relevant impact in SIA magnitude. Both methods [Method #1: KP (90)/KP (135) and Method #2: AKP/AKP (+45)] are useful to calculate SIA with superior incisions at 120°.

Refractive cylinder outcomes after calculating toric intraocular lens cylinder power using total corneal refractive power.

PurposeTo determine whether the total corneal refractive power (TCRP) value, which is based on measurement of both anterior and posterior corneal astigmatism, is effective for toric intraocular lens (IOL) calculation with AcrySof® Toric IOLs.Patients and methodsA consecutive series of cataract surgery cases with AcrySof toric IOL implantation was studied retrospectively. The IOLMaster® was used for calculation of IOL sphere, the Pentacam® TCRP 3.0 mm apex/ring value was used as the keratometry input to the AcrySof Toric IOL Calculator and the VERION™ Digital Marker for surgical orientation. The keratometry readings from the VERION reference unit were recorded but not used in the actual calculation. Vector differences between expected and actual residual refractive cylinder were calculated and compared to simulated vector errors using the collected VERION keratometry data.ResultsIn total, 83 eyes of 56 patients were analyzed. Residual refractive cylinder was 0.25 D or lower in 58% of eyes and 0.5 D or lower in 80% of eyes. The TCRP-based calculation resulted in a statistically significantly lower vector error (P<0.01) and significantly more eyes with a vector error ≤0.5 D relative to the VERION-based calculation (P=0.02). The TCRP and VERION keratometry readings suggested a different IOL toric power in 53/83 eyes. In these 53 eyes the TCRP vector error was lower in 28 cases, the VERION error was lower in five cases, and the error was equal in 20 cases. When the anterior cornea had with-the-rule astigmatism, the VERION was more likely to suggest a higher toric power and when the anterior cornea had against-the-rule astigmatism, the VERION was less likely to suggest a higher toric power.ConclusionUsing the TCRP keratometry measurement in the AcrySof toric calculator may improve overall postoperative refractive results. Consideration of measured posterior corneal astigmatism, rather than a population-averaged value, appears advantageous.

Evaluating the effect of splitting cylindrical power on improving patient tolerance to toric lens misalignment

Evaluating the impact of splitting toric power on patient tolerance to misorientation such as with intraocular lens rotation. University vision clinic. Healthy, non astigmats had +1.50D astigmatism induced with spectacle lenses at 90°, 135°, 180° and +3.00D at 90°. Two correcting cylindrical lenses of the opposite sign and half the power each were subsequently added to the trial frame misaligned by 0°, 5° or 10° in a random order and misorientated from the initial axis in a clockwise direction by up to 15° in 5° steps. A second group of adapted astigmats with between 1.00 and 3.00DC had their astigmatism corrected with two toric spectacle lenses of half the power separated by 0°, 5° or 10° and misorientated from the initial axis in both directions by up to 15° in 5° steps. Distance, high contrast visual acuity was measured using a computerised test chart at each lens misalignment and misorientation. Misorientation of the split toric lenses caused a statistically significant drop in visual acuity (F=70.341; p<0.001). Comparatively better acuities were observed around 180°, as anticipated (F=3.775; p=0.035). Misaligning the split toric power produced no benefit in visual acuity retention with axis misorientation when subjects had astigmatism induced with a low (F=2.190, p=0.129) or high cylinder (F=0.491, p=0.617) or in the adapted astigmats (F=0.120, p=0.887). Misalignment of toric lens power split across the front and back lens surfaces had no beneficial effect on distance visual acuity, but also no negative effect.

Correcting astigmatism with toric intraocular lenses: Effect of posterior corneal astigmatism

To evaluate the impact of posterior corneal astigmatism on outcomes with toric intraocular lenses (IOLs). Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. Case series. Corneal astigmatism was measured using 5 devices before and 3 weeks after cataract surgery. Toric IOL alignment was recorded at surgery and at the slitlamp 3 weeks postoperatively. The actual corneal astigmatism was calculated based on refractive astigmatism 3 weeks postoperatively and the effective toric power calculated with the Holladay 2 formula. The prediction error was calculated as the difference between the astigmatism measured by each device and the actual corneal astigmatism. Vector analysis was used in all calculations. With the IOLMaster, Lenstar, Atlas, manual keratometer, and Galilei (combined Placido-dual Scheimpflug analyzer), the mean prediction errors (D) were, respectively, 0.59 @ 89.7, 0.48 @ 91.2, 0.51 @ 78.7, 0.62 @ 97.2, and 0.57 @ 93.9 for with-the-rule (WTR) astigmatism (60 to 120 degrees), and 0.17 @ 86.2, 0.23 @ 77.7, 0.23 @ 91.4, 0.41 @ 58.4, and 0.12 @ 7.3 for against-the-rule (ATR) astigmatism (0 to 30 degrees and 150 to 180 degrees). In the WTR eyes, there were significant WTR prediction errors (0.5 to 0.6 diopters [D]) by all devices. In ATR eyes, WTR prediction errors were 0.2 to 0.3 D by all devices except the Placido-dual Scheimpflug analyzer (all P<.05 with Bonferroni correction). Corneal astigmatism was overestimated in WTR by all devices and underestimated in ATR by all except the Placido-dual Scheimpflug analyzer. A new toric IOL nomogram is proposed.

Anamorphic gradient index (GRIN) lens for beam shaping

In this short communication we are reporting a new kind of rod lens with toric power with moderately large power difference. These rods can be directly used in coupling power from a semiconductor laser to optical fiber or in free space communication to convert the beam shape. This rod may directly be butt-jointed to the laser, which may attract many application scientists. Moreover, anamorphic power in a GRIN lens can be generated by proper selection of geometry of the substrate for ion exchange. This may lead to a new kind of optical system that needs further exploration.

Clinical results after spherotoric intraocular lens implantation using the bag-in-the-lens technique

To evaluate the clinical results after implantation of a spherotoric intraocular lens (IOL) using the bag-in-the-lens (BIL) technique. Antwerp University Hospital, Department of Ophthalmology, Antwerp, Belgium. Evidence-based manuscript. Consecutive eyes with cataract and corneal astigmatism had implantation of a spherotoric BIL intraocular lens (IOL). The IOL was centered based on the patient's pupillary entrance using Purkinje reflexes of the surgical microscope light. The study enrolled 52 eyes of 35 patients (23 women) with corneal astigmatism ranging from 0.90 to 6.19 diopters (D). The toric power was between 1.00 D and 8.00 D. One-third of eyes had an additional ocular comorbidity (including amblyopia) that could influence the clinical outcomes; 5.2% had an irregular astigmatism up to 15 degrees. Twelve eyes had high myopia (axial length [AL] >26 mm) and 5 eyes, high hyperopia (AL <21 mm). The mean preoperative corrected distance visual acuity was 0.58 ± 0.25 (SD). Postoperatively, the uncorrected distance visual acuity (UDVA) was 0.5 or better in 92% of eyes, the mean UDVA was 0.85 ± 0.21 D, the mean magnitude of error was 0.05 ± 0.49 D, and the mean angle of error was 0.29 ± 0.89 degree. Astigmatism correction was successful in 82% of eyes. Spherotoric BIL IOL implantation yielded outcomes similar to those with other spherotoric IOLs, even in eyes with ocular comorbidity or irregular astigmatism up to 15 degrees.

Toric intraocular lens implantation: 100 consecutive cases

Purpose: To evaluate the first 100 consecutive cases of toric posterior chamber silicone intraocular lens (IOL) implantation by 2 community-based ophthalmologists. Setting: Two private practices in western Virginia, USA. Methods: Data on the first 100 consecutive toric IOL implantations in 81 patients were collected in a prospective manner. Cataract surgery was performed using topical anesthesia and phaco-chop, phaco-flip, or divide-and-conquer phacoemulsification. A Staar AA4203TF or AA4203TL IOL with a 2.00 diopter (D) or 3.50 D toric power was implanted using a lens injector. After the viscoelastic material was removed, the IOL was rotated to the desired orientation. The IOL orientation was assessed postoperatively at 1 day, 1 to 2 weeks, and the last visit. The mean follow-up was 23 weeks ± 17 (SD). Results: The IOLs performed in a predicable fashion. The mean astigmatism correction with IOLs within 15 degrees of the intended axis was 1.62 D with the 2.00 D IOL and 2.86 D with the 3.50 D IOL. Eleven patients had IOLs that were rotated more than 15 degrees away from the intended axes; in 3, the astigmatism was worse than preoperatively. The mean preoperative refractive and keratometric astigmatism was 2.48 D and 2.11 D, respectively. The mean postoperative astigmatism was 0.87 D and 2.05 D, respectively. At the last follow-up, half the patients had residual refractive astigmatism of 0.50 D or less and 45% had an uncorrected visual acuity of 20/30 or better. Of postoperative patients with a 20/30 visual acuity with or without correction, 52% could see this well no correction. Conclusions: The results show that toric IOL implantation can help an estimated 20% of patients with astigmatism achieve good vision and a reduced need for distance spectacles. If IOL rotation occurs, it is usually during the first week and can be remedied by repositioning the IOL at 1 week.