Intraocular Lens Power Estimation Research Articles

Description of a new method to calculate the equator of the crystalline lens using AS-OCT images: Accuracy in non-dilated measurements.

To establish a methodology for objectively estimating the Lens Equatorial Plane (LEP) from clinical images, comparing LEP with dilated versus non-dilated pupils. A cohort of 91 eyes from 60 patients undergoing preoperative assessments for cataract surgery was evaluated. Anterior Segment Optical Coherence Tomography (AS-OCT) images were analysed under conditions of pharmacologically induced pupil dilation versus a non-dilated pupil. Geometrical parameters, including LEP, intersection diameter (ID), lens thickness (LT), anterior and posterior lens thickness were automatically calculated by applying standard image processing techniques to clinical AS-OCT images. Significant differences in lens parameters, including LEP, were observed between dilated and non-dilated conditions (all p < 0.001). A strong linear correlation was found across all geometrical variables under both conditions (r[LEP] = 0.64, r[ID] = 0.78, r[LT] = 0.99, all p < 0.001); enabling reliable correction of these differences. The study introduces an objective methodology for LEP calculation, emphasising the need to consider the eye's physiological state during preoperative measurements. Incorporating LEP into future intraocular lens (IOL) power calculation formulas and replacing the habitual effective lens position may potentially improve the accuracy of IOL power estimation and thus postoperative visual outcomes.

Accuracy of Intraocular Lens Power Estimation in Aphakic Eyes Filled with Silicone Oil

Purpose: To compare preoperative refractive power estimation and actual postoperative refraction after combined silicone oil removal and intraocular lens (IOL) sulcus implantation in silicone oil-filled aphakic eyes.Methods: Records of patients with silicone oil-filled aphakic eyes who received simultaneous silicone oil-removal and IOL sulcus implantation (Group1) and aphakic patients who received vitrectomy combined with IOL implantation (Group 2) were reviewed. Optical biometry including axial length measurements were obtained using IOL master&lt;sup&gt;®&lt;/sup&gt; 700 and predicted refractive errors, actual postoperative refractive errors were examined retrospectively.Results: Predicted refractive errors, calculated by subtracting 1 diopter (D) from myopic lens power closest to plano based on sulcus implantation, were 0.50 ± 0.12 D in Group 1, 0.45 ± 0.20 D in Group 2. Six months after operation, mean refractive errors measured -0.76 ± 0.49 D in Group 1, -0.23 ± 0.49 D in Group 2, showing significant myopic shift (&lt;i&gt;p&lt;/i&gt; &lt; 0.001, &lt;i&gt;p&lt;/i&gt; &lt;0.001). Group 1 showed larger significant myopic shift compared to Group 2 (&lt;i&gt;p&lt;/i&gt; = 0.001), especially in 3 eyes with insufficient filling of intravitreal silicone oil showing -1.5 D myopic shift.Conclusions: Ocular biometry of silicone oil-filled aphakic eyes undergoing simultaneous silicone oil removal and IOL sulcus implantation are difficult to measure, showing postoperative myopic shift compared to control. Incomplete intravitreal silicone oil filling showed larger refractive errors needing additional compensations.

Influence of Pilocarpine Eyedrops on the Ocular Biometric Parameters and Intraocular Lens Power Calculation.

To evaluate the influence of pilocarpine eyedrops on the ocular biometric parameters and whether these parameter changes affect the intraocular lens (IOL) power calculation in patients with primary angle-closure glaucoma (PACG). Twenty-two PACG patients and fifteen normal subjects were enrolled. Ocular biometric parameters including the axial length (AL), anterior chamber depth (ACD), lens thickness (LT), mean keratometry (Km), and white-to-white distance (WTW) were measured by using a Lenstar LS 900 device before and at least 30 minutes after instillation of 2% pilocarpine eyedrops. Lens position (LP) was calculated, and the IOL power prediction based on the ocular biometric parameters was performed using the Barrett Universal II, Haigis, Hoffer Q, Holladay I, or SRK/T formulas before and after pilocarpine application. In both PACG and normal groups, pilocarpine eyedrops induced a slight but statistically significant increase in the mean AL (0.01 mm for both groups) and mean LT (0.02 mm and 0.03 mm, respectively) but a significant decrease in the mean ACD (0.03 mm and 0.05 mm, respectively) and mean LP (0.02 mm and 0.04 mm, respectively). No significant changes in the mean Km and WTW were noticed in both groups. In addition, the IOL power calculation revealed insignificant changes before and after the pilocarpine instillation in both groups, regardless of the formula used. Pilocarpine eyedrops can induce slight changes in the ocular biometric parameters including the AL, ACD, LT, and LP. However, these parameter changes will not result in a significant difference in IOL power estimation.

Comparison of Intraocular Lens Power Estimation by Optical Biometry and Ultrasound Biometry in Cataract Surgery

Introduction: The evolution of modern technologies for cataract surgery has made it crucial for aiming emmetropia with highly defined vision. The key factor responsible for postoperative emmetropia is an accurate biometry, along with various other factors. Ultrasonic biometry is the gold standard method of Intraocular Lens (IOL) power calculation but the corneal indentation with the probe underestimate the axial length and result in a myopic shift which is overcome by the newer optical biometry devices, including swept source optical coherence biometry which uses infrared light to measure the ocular distances. Aim: To determine the precision and accuracy of IOL power calculation by ultrasound A-scan and optical IOL master and their refractive outcomes. Materials and Methods: This prospective, and observational study was conducted between September 2019 to February 2021 in 155 patients with cataract undergoing phacoemulsification in Kalinga Institute of Medical Sciences, KIIT University, Bhubaneswar, Odisha, India. All subjects underwent comprehensive ocular examination and biometry with two formulae {Sanders-RetzlaffKraff (SRK) and Holladay-I}. Biometry included corneal curvature (keratometry), axial length, anterior chamber depth, IOL power calculation, predicted refractive error. There were two broad groups. One group underwent biometry by ultrasound A-scan and the other group underwent optical biometry by IOL Master 700. The IOL power was calculated with the two formulae in both the groups. Comparisons between variables measured using the IOL master and A-scan were done using paired t-test. The p-values &lt;0.05 were considered statistically significant. Results: In a 18 month period, 155 eyes were consecutively enrolled in the study. The mean age of all enrolled patients was 62.1±8.65 years (range 34-80 years) with male:female ratio of approximately 1.25:1. The mean axial length measured by IOL master was higher (23.15 ± 0.85) than that by A-scan (22.96±0.81 diopters) with a mean difference of 0.197±0.35 mm (p-value &lt;0.001, paired t-test). The mean predicted IOL power was 20.81±1.84 diopters by IOL master and 21.13±1.62 by A-scan by SRK-II formula (p-value &lt;0.001). While mean predicted IOL power with Holladay-I by IOL Master 700 was 20.61±1.92 and 21.44±1.98 diopters by A-scan with a mean difference (-0.82±0.76 diopters) with a significant p-value &lt;0.001. Bland-Altman analysis plots showed almost perfect agreement between both methods regarding predicted IOL power. Conclusion: The swept source Optical Coherence Tomography (OCT) based IOL master 700 proved to be a faster non contact device to use with a shorter learning curve, higher accuracy in average axial length eye and less refractive surprises.

Evaluation of Barrett Universal II, SRK-T, and Haigis formulae for intraocular lens power calculation in myopes using optical biometry

Objective To assess the precision of Barrett Universal II, SRK-T, and Haigis formulas in high myopic eyes for the estimation of intraocular lens power, using optical biometry. Patients and methods This is an interventional prospective study that included 34 eyes with an axial length more than or equal to 26 mm, who underwent uneventful phacoemulsification. One month after the surgery, we recorded the postoperative refraction. Refractive prediction error (RPE), median absolute error (Med AE), and proportion of eyes with postoperative myopic and hyperopic outcomes were compared. Results The lowest mean RPE (−0.11±0.90 D), Med AE (0.15 D), and proportion of eyes with hyperopic outcomes (44.1%) were reported with the Barrett Universal II formula. They were all statistically lower than the results of SKR/T and Haigis formulas (mean RPE 0.16±0.99 and 0.31±0.98 D, respectively; Med AE 0.47 and 0.40 D, respectively; and percentage of hyperopic outcome 70.6 and 76.5%, respectively). Conclusion We concluded that the lowest mean RPE, median AE, and the lowest percentage of hyperopic outcome were reported with the Barrett Universal II formula.

Comparison of applanation ultrasound biometry with optical biometry for intraocular lens power estimation in cataract surgery and their impact on prediction error

Aim: The aim of this study is to determine the agreement between applanation ultrasound biometry (AUB) and optical biometry using Lenstar 900 for precataract surgery axial length measurement and intraocular lens (IOL) power estimation and their effect on postoperative refractive outcomes. Methods: A case record-based retrospective study of 229 eyes which underwent phacoemulsification with foldable IOL, in a private hospital setting was done. All the eyes were evaluated using AUB and optical biometry for IOL power prediction. The IOL power was chosen based on the optical biometry and the final refraction was used to calculate the prediction error (PE) for both optical and ultrasound biometry. The concordance coefficient and Bland Altman's limits of agreement were determined to examine the disagreement between the two technologies. Results: The mean axial length ± standard deviation (SD) in the study eyes by ultrasound was 23.46 ± 1.01 mm and 23.57 ± 0.99 mm by optical biometry (P = 0.19). The axial length of 4.37% of eyes could not be measured by optical biometry. The mean IOL power prediction ± SD in the study eyes by ultrasound was 20.98 ± 2.68 D and 20.89 ± 2.85 D by optical biometry (P = 0.72). The mean ± SD absolute value of the refractive PE was − 0.32 ± 0.44 D (median − 0.25, interquartile range: −0.75–0). All eyes achieved a postoperative visual acuity of 6/18 or better including 204 (89.87%) that had a visual acuity of 6/6. One hundred and sixty-four eyes (71.62%) eyes had a postoperative spherical equivalent of 0 to ± 0.5D at 30 days. Two hundred and twenty eyes (96.07%) had a postoperative spherical equivalent of 0 to ± 1.0 D. Conclusions: The findings of the study prove that carefully done AUB is comparable to optical biometry and should not be a deterrent in providing the best possible refractive outcomes in a majority of our cataract patients. To ensure that these results are replicable on a wider scale will necessitate adequate training of personnel in the art and science of biometry.

Comparison of Aphakic Refraction and Biometry-Based Formulae for Secondary In-The-Bag and Sulcus-Implanted Intraocular Lens Power Estimation in Children

Purpose: The aim of the study was to compare the accuracy of refractive outcomes in children undergoing secondary in-the-bag or cilliary sulcus intraocular lens (IOL) implantation, using aphakic refraction (AR)-based formulae (Hug and Khan) and biometry-based formulae (Holladay 1, Hoffer Q, SRK/T, and SRK II). Methods: In this retrospective study, a total of 65 eyes of 44 patients who underwent secondary in-the-bag or cilliary sulcus IOL implantation were included and divided into 2 groups: 39 eyes of the in-the-bag IOL group and the other 26 eyes of the sulcus-implanted IOL group. Holladay 1, Hoffer Q, SRK/T, and SRK II formulae were employed depending on the biometric data, while Hug and Khan formulae were used based on preoperative AR. The prediction error (PE) and the absolute value of predicted error (APE) were compared between the 2 groups and formulae. Results: In the in-the-bag IOL group, nonsignificant differences of APE were found among the 6 formulae, while the Holladay 1, Hoffer Q, SRK/T, and SRK II all demonstrated a significant hyperopic shift of median PE value compared to the Hug formula (p < 0.05, all), and Holladay 1 and SRK II also showed a significant hyperopic shift of PE compared to the Khan formula (p < 0.05, both). Higher percentages of eyes with PE <1 D were found using Hoffer Q and SRK/T. In the sulcus-implanted group, the Holladay 1, Hoffer Q, and SRK/T had a significantly smaller median value of APE than the Hug and Khan formulae (p < 0.05, all), and the SRK II had a significantly smaller median value of APE than the Hug formula (p < 0.05), while Holladay 1 had the lowest value of APE. Higher percentages of eyes within PE <1 D were found using Holladay 1, Hoffer Q, and SRK/T, while the highest one was SRK/T. Significantly larger hyperopic shifts of median PE value using all the 6 formulae were found in eyes with sulcus-implanted IOL than eyes with in-the-bag implanted IOL (p < 0.05, all). In the eyes of with in-the-bag implanted IOL, the Hug and Khan formulae had significantly smaller APE values when compared with the eyes with sulcus-implanted IOL (p < 0.05, both). Conclusions: Whether IOL was in the bag or implanted in the sulcus, almost all the formulae showed hyperopic shift, SRK/T showed the best accuracy. Biometry-based formulae were superior to AR-based formulae in accuracy of IOL power calculation, especially when IOL was implanted in the sulcus. In-the-bag IOL implantation should always be with higher priorities, especially when using AR-based formulae in IOL power calculation.

Comparison of refractive error in phacovitrectomy for epiretinal membrane using ultrasound and partial coherence interferometry.

To compare the postoperative refractive error (RE) using A-scan ultrasound (US) and partial coherence interferometry (PCI) after phacovitrectomy for idiopathic epiretinal membrane (iERM) and cataract. Eighty-eight participants (88 eyes) with iERM and cataracts underwent phacovitrectomies with internal limiting membrane removal. Postoperative RE was the main outcome measured, calculated by subtracting intended spherical equivalent (SE) from 6-month postoperative SE. Secondary outcomes included axial length (AL) measured by 2 methods, change in best-corrected visual acuity (BCVA), and change in central subfield thickness (CSFT). Mean postoperative RE using US showed greater myopic shift compared with that using PCI (-0.569 ± 0.571 D vs -0.169 ± 0.415 D, respectively, p&lt;0.001). The 6-month postoperative RE was within ±0.50 D in 43.2% (38/88) using US vs 84.1% (74/88) using PCI and within ±1.00 D in 84.1% (74/88) using US vs 96.6% (85/88) using PCI. Mean AL measured by US was shorter than that measured by PCI (23.50 ± 1.27 mm vs 23.58 ± 1.30 mm, respectively, p&lt;0.001). Postsurgery, BCVA improved from 0.374 ± 0.264 logMAR to 0.144 ± 0.124 logMAR (p&lt;0.001), and CSFT decreased from 449.2 ± 78.5 µm to 378.2 ± 47.0 µm (p&lt;0.001). The BCVA improvement significantly correlated with decreased CSFT (R = 0.268, p = 0.011). Estimation of intraocular lens power for phacovitrectomies for iERM and cataracts is more accurate when assessed by PCI than by US.

Accuracy of Intraocular Lens Power Estimation in Eyes Undergoing Phacovitrectomy for Proliferative Diabetic Retinopathy

2 , Goyang, Korea Purpose: To evaluate the accuracy of intraocular lens (IOL) power estimation and the factors associated with outcome in eyes undergoing combined phacovitrectomy for proliferative diabetic retinopathy. Methods: We performed a retrospective case review of 39 consecutive patients (44 eyes) that underwent phacovitrectomy for proliferative diabetic retinopathy. Axial lengths were measured using ultrasound (A-scan) and/or optical biometry (IOL Master). Achieved and predicted refractions were compared to calculate the mean postoperative refractive prediction error (ME) and the mean absolute prediction error (MAE). Systemic conditions of patients and several preoperative and postoperative factors re- lated to the postoperative refraction were analyzed. Results: The ME of 44 eyes were −0.23 ± 0.52 diopters (D) and -0.23 ± 0.47 D after 3 and 6 months, respectively (range, -1.40~+0.79 D). There was no statistically significant difference in the refractive outcomes between the refractive errors (p = 0.959). The MAEs were 0.45 ± 0.35 D and 0.40 ± 0.33 D after 3 and 6 months, respectively with no statistical significant differ- ence between the results (p = 0.196). When comparing ME in the 20 eyes that achieved both results, ultrasound was more accu- rate than optical biometry (p = 0.002, 0.002). The factors associated with more inaccurate ME and MAE after phacovitrectomy were diabetic nephropathy and neovascular glaucoma. Conclusions: Combined phacovitrectomy in proliferative diabetic retinopathy showed small biometric errors within the tolerable range in most cases. Patients with neovascular glaucoma and diabetic nephropathy had more inaccurate postoperative re- fractive power. Both optical biometry and ultrasound should be used to estimate axial lengths for improving the accuracy of IOL power calculation. J Korean Ophthalmol Soc 2015;56(5):737-744

ACCURACY OF INTRAOCULAR LENS POWER ESTIMATION IN EYES HAVING PHACOVITRECTOMY FOR RHEGMATOGENOUS RETINAL DETACHMENT

To evaluate the accuracy of intraocular lens power estimation in eyes having phacovitrectomy for rhegmatogenous retinal detachment. Retrospective case review of 100 consecutive eyes that underwent phacovitrectomy for rhegmatogenous retinal detachment. Axial lengths were measured using optical biometry and/or ultrasound A-scan. Achieved and predicted refraction were compared to calculate the mean postoperative refractive prediction error and the mean absolute prediction error. Factorial analysis of variance models were developed to assess outcome on the whole and that between the subgroups. Ninety-five eyes had postoperative refraction: 41 macula-on (43%) and 54 macula-off (57%). The mean postoperative prediction error was -0.34 ± 0.89 diopters. There was no statistical significant difference in the refractive outcomes between macula-on and macula-off groups (P > 0.05). Overall, using mean absolute prediction error as the outcome measure, optical biometry was more accurate than ultrasound (P = 0.040). However, significantly more ultrasound-measured axial lengths were selected for intraocular lens power estimation in macula-off group compared with the macula-on group (P = 0.016). Combined phacovitrectomy in rhegmatogenous retinal detachment included a small biometric error that was within the tolerable range in most cases. Both optical biometry and ultrasound should be used to estimate axial lengths, for macula-off rhegmatogenous retinal detachment cases, to improve the accuracy of intraocular lens power calculation.

Intraoperative Refractive Biometry for Predicting Intraocular Lens Power Calculation after Prior Myopic Refractive Surgery

To evaluate a new method of intraoperative refractive biometry (IRB) for intraocular lens (IOL) power calculation in eyes undergoing cataract surgery after prior myopic LASIK or photorefractive keratectomy. Retrospective consecutive cases series. We included 215 patients undergoing cataract surgery with a history of myopic LASIK or photorefractive keratectomy. Patients underwent IRB for IOL power estimation. The Optiwave Refractive Analysis (ORA) System wavefront aberrometer was used to obtain aphakic refractive measurements intraoperatively and then calculate the IOL power with a modified vergence formula obtained before refractive surgery. Comparative effectiveness analysis was done for IRB predictive accuracy of IOL power determination against 3 conventional clinical practice methods: surgeon best preoperative choice (determined by the surgeon using all available clinical data), the Haigis L, and the Shammas IOL formulas. Median absolute error of prediction and percentage of eyes within ±0.50 diopters (D) and ±1.00 D of refractive prediction error. In 246 eyes (215 first eyes and 31 second eyes) IRB using ORA achieved the greatest predictive accuracy (P < 0.0001), with a median absolute error of 0.35 D and mean absolute error of 0.42 D. Sixty-seven percent of eyes were within ±0.5 D and 94% were within ±1.0 D of the IRB's predicted outcome. This was significantly more accurate than the other preoperative methods: Median absolute error was 0.6, 0.53, and 0.51 D for surgeon best choice, Haigis L method, and Shammas method, respectively. The IOL power estimation in challenging eyes with prior LASIK/photorefractive keratectomy was most accurately predicted by IRB/ORA.

Comparison of aphakic refraction formulas for secondary in-the-bag intraocular lens power estimation in children

Intraocular lens (IOL) formulas based on aphakic refraction alone have been proposed as an alternative to biometry-based formulas. The purpose of this study was to determine the relative accuracy of these simplified formulas. Records of patients who received secondary in-the-bag (IOL) implants and for whom aphakic refraction was obtainable were retrospectively studied. The formulas for IOL power calculation by Hug (J Pediatr Ophthalmol Strabismus 2004;41;209-11) and Khan and by AlGaeed (Br J Ophthalmol 2006;90:1458-60; modified to account for greater K values in our population) were compared with the biometry-based Holladay 1 formula by calculating the absolute prediction error for each. Twenty eyes met the inclusion criteria. The average age at IOL implantation was 4.8 ± 3.4 years. The mean of the absolute value of the prediction error when we used the Hug's formula was 2.4 ± 2.1 D; with Khan's formula, 2.4 ± 2.0 D; and with biometry, 1.6 ± 1.4 D. None of these differences was statistically significant (paired t test: Hug vs Khan, P = 0.9; biometry vs Hug, P = 0.3; biometry vs Khan, P = 0.2). The absolute prediction error was more than 2 D in 8 of 20 eyes (40%) when we used Hug's formula, 11 of 20 eyes (55%) with Khan's formula, and 5 of 20 eyes (25%) with biometry. Although the 0.8 D reduction in accuracy of the refraction-based formulas was not statistically significant, we recommend continued use of biometry-based formulas for IOL power calculation. Aphakic refraction may be used to confirm IOL power prediction when biometry is difficult.

Intraoperative retinoscopy for intraocular lens power estimation in cases of combined phacoemulsification and silicone oil removal

To study the application of intraoperative retinoscopy for intraocular lens (IOL) power calculation in combined cataract extraction and silicone oil removal. Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India. This study comprised patients with silicone oil-filled eyes and visually significant cataract who had combined cataract extraction by phacoemulsification and silicone oil removal by a standard method through the pars plana route. Retinoscopy was performed with a streak retinoscope, standard vertex distance of 13.0 mm, and distance of 50.0 cm. All eyes had in-the-bag implantation of a foldable IOL with an A-constant of 118.4. The IOL power was calculated using the Ianchulev formula as follows: R x 2.01449. Postoperative refraction was performed at 3 months, and the spherical equivalent was calculated. Twelve eyes of 12 patients were evaluated. The mean emmetropic power calculated by intraoperative retinoscopy was 20.46 diopters (D) +/- 3.4 (SD) (range 13.09 to 25.18 D) and the mean refractive error, -0.45 +/- 0.63 D (range -1.00 to +1.00 D). The postoperative refractive error was within +/-0.50 D in 4 eyes (33.3%) and within +/-1.00 D in all eyes. Ten eyes (83.33%) had a postoperative refractive error in the range of 0.00 to -1.00 D using IOL power based on intraoperative retinoscopy. Intraoperative retinoscopy for IOL power calculation in combined cataract extraction and silicone oil removal gave satisfactory refractive outcomes, although further studies with more patients are required to confirm its usefulness and determine whether there are disadvantages.

Optical biometry in combined phacovitrectomy

To assess the accuracy of intraocular lens (IOL) power calculation using optical biometry in patients having combined phacovitrectomy for macular hole and macular pucker. Sunderland Eye Infirmary, North East England, United Kingdom. This single-surgeon retrospective case series review comprised eyes having combined narrow-gauge phacovitrectomy for macular hole or macular pucker. Intraocular lens power was calculated using the results of optical biometry and the Haigis formula. Achieved and planned refraction were compared to calculate the mean postoperative refractive prediction error (ME) and the mean absolute postoperative prediction error (MAE) and the percentage of eyes with an achieved refraction within +/-0.50 diopter (D) and +/-1.00 D of the planned refraction. The results were compared with those in a series of 42 eyes that had phacoemulsification after previous vitrectomy surgery and a series of 60 nonvitrectomized eyes that had uneventful phacoemulsification. Of the of 59 having combined phacovitrectomy, 39 had macular hole and 20 had macular pucker. There was no statistically significant difference in refractive outcomes between the phacovitrectomy group (ME -0.02, MAE 0.39) and the sequential phacoemulsification group (ME -0.10, MAE 0.38) (P = .82). There was a statistically significant difference between the phacovitrectomy group and the phacoemulsification-only group (ME 0.08, MAE 0.26) (P<.001). The use of optical noncontact biometry with the Haigis formula achieved a high degree of accuracy of IOL power estimation in patients having phacovitrectomy. There was no tendency toward a myopic shift, as has been reported using ultrasound axial length measurement.

Accuracy of intraocular lens power estimation in eyes having phacovitrectomy for macular holes

To report the accuracy of intraocular lens (IOL) power estimation in eyes having combined phacoemulsification and vitrectomy for macular holes and to compare the axial length (AL) in those eyes with that in the fellow eyes. Calderdale Royal Hospital, Halifax, West Yorkshire, United Kingdom. The mean and standard deviation of the refractive aim, achieved refraction, and postoperative prediction error (calculated as difference between achieved refraction and refractive aim) were determined in 40 patients who had phacovitrectomy with gas tamponade for the treatment of idiopathic macular holes. The percentage of patients with an achieved refraction within +/-0.50 diopter (D), +/-1.00 D, and more than 2.00 D of the refractive aim was recorded. The mean absolute error (MAE) of the postoperative prediction error was calculated. In addition, the AL in eyes with macular holes was compared with that in fellow eyes. Axial lengths were measured using applanation A-scan ultrasound. Of eyes having phacovitrectomy, 45.0%, 67.5%, and 90.0% achieved a postoperative refraction within +/-0.50 D, +/-1.00 D, and +/-2.00 D, respectively, of the refractive aim; 10.0% of eyes were more than -2.00 D from the refractive aim. The overall postoperative prediction error ranged from +1.64 D to -2.51 D. The mean refractive aim was +0.30 +/- 0.72 D and the mean achieved refraction, -0.09 +/-1.25 D. There was no clinically significant difference between the means. The mean postoperative prediction error was -0.39 +/- 1.01 D, suggesting a myopic overcorrection occurred postoperatively. The MAE of the postoperative prediction error was 0.83 D. The mean AL was 23.40 mm in operated eyes and 23.46 mm in fellow eyes. The achieved refraction after phacovitrectomy for macular holes was comparable to results after phacoemulsification alone. The myopic overcorrection after phacovitrectomy might be a result of the gas bubble causing forward displacement of the capsular bag and IOL or inaccuracies in AL and keratometry measurements. Aiming for residual hyperopia may counteract the overcorrection. There was no difference in AL between eyes with macular holes and fellow eyes.

Optimization of biometry for intraocular lens implantation using the Zeiss IOLMaster

To compare the accuracy of biometry using conventional A-scan ultrasonography and partial coherence interferometry, and to improve the accuracy of biometry by sequential audit of postoperative refractive error. The study was performed in three phases. In phase 1, 20 consecutive patients undergoing routine phacoemulsification underwent biometry using both A-scan ultrasonography and the Zeiss IOLMaster (ZIOLM). A single experienced optometrist refracted all patients 2 weeks after surgery. The errors between expected and achieved refraction were calculated and compared between the two methods. In phases 2 and 3, a further 22 and 20 patients, respectively, were recruited and only the ZIOLM was used for biometry. The manufacturer's suggested A-constant was refined and the error between expected and achieved refraction was calculated. In phase 1, the median unexpected error for the ZIOLM was +0.63 (interquartile range +0.368 to +1.015) and for A-scan biometry was - 0.24 (interquartile range - 1.335 to +0.802). In phase 1 65% of patients' postoperative refractions were found to be within 1.0 D of emmetropia using the ZIOLM (55% using A-scan biometry). Refinements to the A-constant improved this to 95% by phase 3. An error was identified in IOL power estimation with the ZIOLM, when using the manufacturer's recommended A-constant (recommended and previously optimized ultrasound A-constant 118.0; ZIOLM optimized A-constant 118.6). Serial modifications to the A-constant were successful in reducing the unexpected error to well within the tolerance limits of published international standards.

Understanding corneal topography.

With the increase in corneal laser refractive surgery, there is a greater need for precise evaluation of the corneal surface. Articles published in the past year have reported possible use of confocal scanning laser technology-based devices to image the cornea. Other studies have compared existing instruments and software in an effort to determine if data from different instruments are comparable. Topographic evaluation also has served to highlight long-term complications of procedures like radial keratotomy and the promise of newer surgical procedures like the intrastromal corneal ring segments. Studies of the corneal surface have helped refine surgical procedures like photorefractive keratectomy, laser in situ keratomileusis, penetrating keratoplasty, and cataract surgery. Topographic measures that serve as direct correlates of ocular visual performance, however, still remain elusive. Studies in the past year have confirmed that corneal topographic evaluation may be a powerful tool in the search for a genetic basis of keratoconus. Important areas for future research include precise determination of the power of the postrefractive surgery cornea to allow precise estimation of intraocular lens power in these eyes. Detecting the presence of past corneal refractive surgery in donor eyes also is likely to be a challenge. Despite the numerous advances in the field of corneal topography in the past year, there is still a need to present the data in a standardized format that is universal to all instruments and technologies.

The necessity of axial length A scan in intraocular lens power estimation.

The difficulties of accurate estimation of intraocular lens power (I.O.L.) are presented. Axial length A Scan measurement can overcome some of these and avoid the occasional large dioptre surprise. The results of 50 consecutive cases are shown and various methods of I.O.L. power estimation compared. These show that without A Scan, the occasional aniseikonic disaster will occur.