Accuracy Of Intraocular Lens Power Calculation Research Articles

Change in Axial Length Pre and Post Cataract Surgery Measured with IOL Master

Objective: Cataract surgery with intraocular lens (IOL) implantation tries to get a refractive result nearby to the optic emmetropia. So an accurate IOL power calculation is mandatory, and axial length (AL) measurement is one of the most influential parameter in this calculation. IOL Master is one of the most popular devices to measure AL, which uses the same group refractive index of the lens for all patients, independently of the cataract grade. But lens refractive index is higher as cataract grade increases. Nowadays cataract tends to be operated earlier, so moderate cataracts are the most extracted in clinical practice. Therefore, it is important to study the influence of moderate lens opacity in AL measurements. The objective of this study was to compare the AL value measures with IOL Master, before and after uncomplicated cataract surgery in patients with moderate cataracts. Material and methods: This study included 153 eyes of 105 patients (67.51 ± 13.56 years old) undergoing uncomplicated cataract surgery. Lens opacity was determined with the LOCSIII scale and AL was measured using optical biometry (IOL Master; Carl Zeiss Meditec) before and one month after intervention. Intraoperative measurements (ultrasound time and fluid volume) were also registered. Results: Mean preoperative AL of 25.10 ± 3.19 mm (range 20.54 to 36.06; IC95% 24.59 to 25.60 mm) and postoperative of 24.88 ± 3.16 mm (IC 95% 24.37 to 25.39; range 20.43 to 35.79 mm). The mean AL difference before and after surgery was 0.19 ± 0.05 mm (p=0.549 ANOVA) with agreement limits from 0.09 to 0.29 mm. It was observed more difference in eyes with larger AL (r2=0.14 p<0.01). The average cataract grade in each category was: nuclear opacity 2.25 ± 1.00 (range 1 to 5) (p=0.564 ANCOVA), cortical opacity 2.04 ± 0.73 (range 0 to 4) (p=0.543 ANCOVA), posterior subcapsular opacity 0.44 ± 0.90 (range 0 to 4) (p=0.563 ANCOVA), and nuclear color 2.40 ± 1.05 (range 0 to 5) (p=0.558 ANCOVA), without statistical significant effect in the AL difference pre and after cataract surgery. The ultrasound time used during the surgery was 43 ± 29 seconds (p=0.525 ANCOVA) and the fluid volume was 4.73 ± 1.31 (p=0.560 ANCOVA) cubic centimeters without statistically significant relationship between these parameters and the difference in the AL measurement. Conclusions: The difference in AL measurement with IOL Master before and after cataract surgery in eyes with low grade of cataract should be related with IOL Master reproducibility especially in extreme-long eyes. Moderate cataract showed non-statistically effect on AL measurement with IOL Master biometry.

Accuracy of Corneal Power Measurements for Intraocular Lens Power Calculation after Myopic Laser In situ Keratomileusis.

Purpose:To evaluate the accuracy of corneal power measurements for intraocular lens (IOL) power calculation after myopic laser in situ keratomileusis (LASIK).Methods:The study evaluated 45 eyes with a history of myopic LASIK. Corneal power was measured using manual keratometry, automated keratometry, optical biometry, and Scheimflug tomography. Different hypothetical IOL power calculation formulas were performed for each case.Results:The steepest mean K value was measured with manual keratometry (37.48 ± 2.86 D) followed by automated keratometry (37.31 ± 2.83 D) then optical biometry (37.06 ± 2.98 D) followed by Scheimflug tomography (36.55 ± 3.08). None of the K values generated by Scheimflug tomography were steeper than the measurements from the other 3 instruments. Using equivalent K reading (EKR) 4 mm with the Double-K SRK/T formula, the refractive outcome generated 97.8% of cases within ± 2 D, 80.0% of cases within ± 1 D, and 42.2% of cases within ± 0.5 D. The best combination of formulas was “Shammas-PL + Double-K SRK/T formula using EKR 4 mm.”Conclusion:Scheimflug tomography imaging using the Holladay EKR 4 mm improved the accuracy of IOL power calculation in post-LASIK eyes. The best option is a combination of formulas. We recommended the use the combined “Shammas-PL ± Double-K SRK/T formula using EKR 4 mm”h for optical outcomes.

Paying attention to optimization and combination of the preoperative biomeasurement with intraocular lens power calculation in cataractous eyes

As the development of surgery related technology and clinical application of premium intraocular lens (IOL), cataract surgery has been in the era of refractive surgery. To restore the refractive system by surgery has been focused on and challenges have been brought up in preoperative measurement and accurate IOL power calculation methods. It is necessary to know developments of the related measurement technologies and the possible error resources to decrease refractive errors. Different means of measuring axial length, cornea curvature and anterior chamber depth were related here, co-operated with varied IOL power calculation methods, such as standard power, traditional formulas and real ray tracing method. Researchers should pay close attention to the preoperative measurement and IOL power calculation methods in cataract patients, the combination of accurate measurement with different calculation methods can help to acquire an increasingly satisfactory visual result. Key words: Cataract/surgery; Biometrical measurement; Axial length; Corneal curvature; Anterior chamber depth; Intraocular lens

Accuracy of corneal power measurements for intraocular lens power calculation after myopic laser in-situ keratomileusis

Purpose The aim of the study was to evaluate the accuracy of corneal power measurements for intraocular lens (IOL) power calculation after myopic laser in-situ keratomileusis (LASIK). Setting The study was conducted at the Faculty of Medicine, Alexandria University, Egypt. Patients and methods The study was conducted on 45 eyes that had undergone myopic LASIK previously. Corneal power was measured using manual keratometry, automated keratometry, IOLMaster, and Pentacam. Different hypothetical IOL power calculation formulas were used in each case. Results The steepest mean K value with manual keratometry (37.48 ± 2.86 D) followed by automated keratometry (37.31 ± 2.83 D) and the IOLMaster mean K value (37.06 ± 2.98 D) were steeper than any other mean K value measured by Pentacam. Using 4 mm Holladay equivalent K reading (EKR) with the double-K SRK/T formula, the refractive outcome was good, with 97.8% of cases within ± 2 D, 80.0% of cases within ± 1 D, and 42.2% of cases within ± 0.5 D. The best combination of formulas was 'Shammas-PL+double-K SRK/T using EKR 4 mm'. Conclusion Pentacam Scheimpflug imaging using EKR at 4 mm improved the accuracy of IOL power calculation in post-LASIK eyes. The best option is a combination of formulas. It is recommended to use the mean of the combination of 'Shammas-PL+double-K SRK/T with EKR 4 mm'.

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

Evaluation of Orbscan II plus non-history double-K method in intraocular lens power calculation for patients after refractive surgery

To explore the accuracy of intraocular lens (IOL) power calculation with non-history method using the keratometric value derived from one zone of Orbscan II topography after laser in situ keratomileusis (LASIK). For this retrospective study, 20 eyes from 20 patients undergoing myopic LASIK were included from August 2008 to November 2011. Posterior corneal curvature in peripheral 7 to 10 mm fitting zone of postoperative cornea was measured with Orbscan II topography. The ratio between anterior cornea and posterior cornea was used to calculate the preoperative total corneal power of each eye. The predicted postoperative spherical equivalent was calculated by the calculated preoperative total corneal power, power of implanted IOL, refractive error before cataract surgery and eye length from IOL-Master into the double-K method for SRK-T formula. The predicted postoperative spherical equivalent was also calculated by double-K method for SRK-T formula with clinical history. The mean difference between actual and predicted postoperative spherical equivalent, i.e. mean absolute error (MAE), were compared. (1) Non-history method: MAE of 11 eyes was within ± 0.5 D (55%), MAE of 15 eyes within ± 1.0 D (75%) and MAE of all eyes within ± 2.0 D. (2) Clinical-history method: MAE of 10 eyes was within ± 0.5 D (50%), MAE of 14 eyes within ± 1.0 D (70%) and MAE of all eyes within ± 2.0 D. (3) Comparison of non-history and clinical-history methods: The MAE of non-history method was (0.61 ± 0.59) D and the MAE of clinical-history method (0.68 ± 0.55) D. Paired t-test showed no statistically significant difference between two methods (t = -1.248, P = 0.227). The non-history method using the keratometric value derived from one zone of Orbscan II topography after LASIK provides an accurate calculation of IOL power and it can be used as a substitute for incomplete clinical data.

Refractive Error after Cataract Surgery Combined with Descemet Stripping Automated Endothelial Keratoplasty

To assess refractive error after cataract surgery combined with Descemet stripping automated endothelial keratoplasty using adjusted keratometric (K) readings to calculate intraocular lens (IOL) power. Prospective, interventional case series. One eye of 39 consecutive patients with Fuchs endothelial dystrophy and cataract were included. To calculate IOL power before surgery, the Javal steep and flat K readings (Haag-Streeit) were adjusted by reducing their value by -1.19 diopters (D); the axial length was measured by immersion biometry, and the standard A-constant was used. Surgery included phacoemulsification, IOL (Acrysof SN60AT; Alcon) implantation within the capsular bag, and Descemet stripping automated endothelial keratoplasty using posterior lamella prepared with a 300-μm head microkeratome (Moria). The absolute prediction error (absolute difference between predicted and achieved refraction) was assessed 6 months after surgery. The mean power of the implanted IOL was 23.22 ± 2.90 D. The mean predicted and achieved refractions were -0.27 ± 0.26 D and -0.23 ± 0.73 D, respectively. The mean absolute prediction error was 0.59 ± 0.42 D (range, 0.05 to -1.52 D). The postoperative spherical equivalent fell within ±0.50 D, ±1.00 D, and ±2.00 D of the predicted refraction in 55.5%, 83.3%, and 100% of cases, respectively. Had the IOL power been calculated without adjusting the K readings, the mean absolute prediction error would have been significantly higher (0.86 ± 0.62 D; P = .04). In this study, in which posterior lamellae were prepared using a 300-μm head microkeratome, adjusting preoperative K readings by -1.19 D led to accurate IOL power calculation and highly predictable refractive error after cataract surgery combined with Descemet stripping automated endothelial keratoplasty.

Accuracy of a dual Scheimpflug analyzer and a corneal topography system for intraocular lens power calculation in unoperated eyes

To evaluate the accuracy of the corneal power measurements by a dual Scheimpflug analyzer (Galilei) and a corneal topography system (Keratron) for intraocular lens (IOL) power calculation in unoperated eyes. Private practice. Case series. The IOL power was calculated for eyes having phacoemulsification and in-the-bag IOL implantation by entering into the Hoffer Q, Holladay 1, and SRK/T formulas 3 corneal power measurements: (1) simulated keratometry (K) by corneal topography, (2) simulated K derived by anterior corneal curvature only using the dual Scheimpflug analyzer, (3) total corneal power derived by anterior and posterior corneal curvatures through ray tracing using the dual Scheimpflug analyzer. The prediction error was calculated as the difference between the predicted and the measured refraction 1 month postoperatively. Forty-three consecutive patients were enrolled. The mean arithmetic error was zero for all combinations of measurements due to constant optimization. Using the Hoffer Q formula, the mean absolute prediction errors were 0.23 diopters (D) ± 0.22 (SD) for the corneal topographer simulated K, 0.21 ± 0.18 D for the Scheimpflug simulated K, and 0.27 ±0.20 D for the Scheimpflug total corneal power (P>.05). No statistically significant different results (compared with the Hoffer Q formula) were obtained with the Holladay 1 and SRK/T formulas. Both devices provided corneal power measurements that led to accurate IOL power calculation. The total corneal power by the dual Scheimpflug system is the first corneal power measurement shown to lead to accurate IOL power calculation without using the standard keratometric index. Drs. Savini, Barboni, and Carbonelli have no financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.

Effects of Pterygium on Ocular Aberrations

To compare high-order aberration between the eyes with pterygium and normal fellow eyes. Ophthalmic Research Center and Department of Ophthalmology Labbafinejad Medical Center Shahid Beheshti University of Medical Sciences Tehran, Iran. This comparative cross-sectional study comprised 31 patients with unilateral pterygium. NIDEK OPD-Scan wavefront aberrometry was performed in all patients in both eyes. Then, aberometric parameters were compared between the eyes. Sixty-two eyes of 31 patients (23 men and 8 women) with a mean age of 42.5 ± 5.9 years were included in the study. Root mean square (RMS) of total higher-order aberration increased statistically significantly in the involved eyes compared with normal fellow eyes (1.85 ± 2.22 and 0.36 ± 0.44 μm; P = 0.001). All RMS of Zernike orders and all RMS of different aberrations were higher in the involved eyes group and were statistically significant except for spherical aberration. The most significant differences belonged to RMS of total trefoil (1.37 vs. 0.25 μm) and RMS of total coma (0.37 vs. 0.14 μm) (P = 0.0001). With decreasing the uninvolved optical zone (increasing the size of pterygium), RMS of all aberrations increased and the differences were statistically significant except for spherical aberration. Pterygium and its size have significant influence on high-order aberrations of the eye especially on measured total coma and total trefoil in compared with normal fellow eye.

Pseudoaccommodating bifocal intraocular lens implantation in eyes with previous corneal refractive surgery - refractive outcomes: case series of 8 eyes

OBJECTIVE: Accurate intraocular lens power calculation may be a challenging issue in eyes with previous corneal refractive surgery. METHODS: The authors present a series of 8 eyes (4 previously submitted to hyperopic laser in situ keratomileusis and 4 to radial keratectomy) submitted to phacoemulsification and implantation of pseudoaccommodating bifocal intraocular lens. Double-K SRK/T formula has been used. RESULTS: Acceptable refractive visual outcomes have been achieved. CONCLUSION: Pseudoaccommodating bifocal intraocular lenses can be safely implanted in patients with previous corneal refractive surgery.

Cataract surgery after refractive surgery

To review recent contributions addressing the challenge of intraocular lens (IOL) calculation in patients undergoing cataract extraction following corneal refractive surgery. Although several articles have provided excellent summaries of IOL selection in patients wherein prerefractive surgery data are available, numerous authors have recently described approaches to attempt more accurate IOL power calculations for patients who present with no reliable clinical information regarding their refractive history. Additionally, results have been reported using the Scheimpflug camera system to measure corneal power in an attempt to resolve the most important potential source of error for IOL determination in these patients. IOL selection in patients undergoing cataract surgery after corneal refractive surgery continues to be a challenging and complex issue despite numerous strategies and formulas described in the literature. Current focus seems to be directed toward approaches that do not require preoperative refractive surgery information. Due to the relative dearth of comparative clinical outcomes data, the optimal solution to this ongoing clinical problem has yet to be determined. Until such data are available, many cataract surgeons compare the results of multiple formulas to assist them in IOL selection for these patients.

Accuracy of modern intraocular lens power calculation formulas in refractive lens exchange for high myopia and high hyperopia

To determine the accuracy of the Holladay 2, Hoffer Q, SRK/T, and Haigis intraocular lens (IOL) power calculation formulas in high myopic and high hyperopic refractive lens exchange (RLE) with hydrophobic acrylic foldable IOLs. Department of Ophthalmology, Johann Wolfgang Goethe University, Frankfurt am Main, Germany. Preoperatively, axial length (AL) was measured by partial coherence interferometry with the IOLMaster. The RLE was performed by phacoemulsification with posterior chamber IOL implantation in the capsular bag. Optimization of the lens constants for each IOL type was done separately for eyes with a positive lens and eyes with a negative lens. Prediction error (predicted refraction - postoperative refraction) and mean absolute error were back-calculated for the Holladay 2, Hoffer Q, SRK/T, and Haigis formulas. Sixty-three eyes (44 myopic, AL >or=26.0 mm; 19 hyperopic, AL <or=22.0 mm) were included. In myopic and hyperopic RLE, all 4 formulas had an overall good performance. In myopic RLE, the Haigis formula performed better. In myopic and hyperopic RLE, optimization of lens constants improved the accuracy of IOL power calculation.

Accuracy of Biometry and Intraocular Lens Power Calculation With Partial Coherence Interferometry in High Myopia

Purpose: To investigate the accuracy of biometry and intraocular lens (IOL) power calculation using partial coherence interferometry (IOL Master®) in highly myopic patients with axial lengths of 26 mm or greater. Methods: Patients with axial lengths equal to or greater than 26 mm who had undergone cataract surgery were enrolled. IOL power was calculated using IOL Master and/or applanation ultrasonography with the SRK/T formula. Twenty-seven eyes using both IOL Master and applanation ultrasonography were included in a paired group, and forty-eight eyes using the IOL Master only and twenty-five eyes using applanation ultrasonography only were included in unpaired groups. The differences between the predicted refraction and the actual refraction were compared and analyzed. Results: In the paired study, the axial lengths in patients using IOL Master (29.14±2.32 mm) were significantly longer than those of patients using applanation ultrasonography (28.57±2.23 mm) (p0.05). In the unpaired study, the MAEs of the IOL Master and applanation ultrasonography groups were 0.61±0.61D and 0.65±0.63D, respectively. Conclusions: In eyes with axial lengths of 26.0 mm or greater, the accuracy of IOL power calculation with IOL Master using the SRK/T formula was comparable to that with applanation ultrasonography.

Improved accuracy of intraocular lens power calculation with the Zeiss IOLMaster

This study aimed to demonstrate how the level of accuracy in intraocular lens (IOL) power calculation can be improved with optical biometry using partial optical coherence interferometry (PCI) (Zeiss IOLMaster) and current anterior chamber depth (ACD) prediction algorithms. Intraocular lens power in 461 consecutive cataract operations was calculated using both PCI and ultrasound and the accuracy of the results of each technique were compared. To illustrate the importance of ACD prediction per se, predictions were calculated using both a recently published 5-variable method and the Haigis 2-variable method and the results compared. All calculations were optimized in retrospect to account for systematic errors, including IOL constants and other off-set errors. The average absolute IOL prediction error (observed minus expected refraction) was 0.65 dioptres with ultrasound and 0.43 D with PCI using the 5-variable ACD prediction method (p < 0.00001). The number of predictions within +/- 0.5 D, +/- 1.0 D and +/- 2.0 D of the expected outcome was 62.5%, 92.4% and 99.9% with PCI, compared with 45.5%, 77.3% and 98.4% with ultrasound, respectively (p < 0.00001). The 2-variable ACD method resulted in an average error in PCI predictions of 0.46 D, which was significantly higher than the error in the 5-variable method (p < 0.001). The accuracy of IOL power calculation can be significantly improved using calibrated axial length readings obtained with PCI and modern IOL power calculation formulas incorporating the latest generation ACD prediction algorithms.

The AS biometry technique—A novel technique to aid accurate intraocular lens power calculation after corneal laser refractive surgery

Intraocular lens power (IOL) calculation for cataract surgery has been shown to be inaccurate after photorefractive keratectomy (PRK), laser-assisted subepithelial keratectomy (LASEK) and laser in situ keratomileusis (LASIK). Many techniques exist to calculate corneal power with varying results and require the clinician to be aware of the pitfalls of IOL power calculation in post-refractive eyes. The AS biometry method proposed here is a simple method which does not rely on the calculation of corneal power. This new method is compared to the current gold standard the clinical history method (CHM). Twenty-nine eyes of 15 patients had routine biometry prior to LASIK, LASEK or PRK. The range of pre-operative spherical equivalent refractive error was −5.37 to +4.00 diopters. The post-operative refraction was measured at 3–6 months. The IOL power calculation was calculated using the AS biometry method and the CHM. The two methods were compared using the Student's paired t-test and the Bland Altman technique. There was no statistical difference between the AS biometry method and the CHM. The paired Student's t-test comparing the AS biometry method and the CHM showed no statistical difference, t = 0.33 with a p-value of 0.75, at a 95% confidence interval. The authors conclude that the AS biometry technique is as accurate as the CHM. The former is a simpler method which avoids many of the pitfalls and confounding factors involved in IOL power calculation following corneal excimer laser surgery. However, like the CHM it requires measurements prior to laser surgery.

Accurate intraocular lens power calculation after myopic laser in situ keratomileusis, bypassing corneal power

To describe a novel method for calculating intraocular lens (IOL) power after myopic laser in situ keratomileusis (LASIK) without using the inaccuracies of the post-LASIK corneal power. Department of Ophthalmology, Wake Forest University Eye Center, Wake Forest University School of Medicine, Winston Salem, North Carolina, USA. This retrospective chart review comprised 9 eyes of 9 patients who had phacoemulsification after LASIK using our method for IOL calculation. This new method assumes the patient never had myopic LASIK to calculate IOL power and then targets the IOL at the pre-LASIK amount of myopia. The pre-LASIK keratometry values, pre-LASIK manifest refraction, and the current axial length are placed in the Holladay formula, bypassing the post-LASIK corneal power. In theory, assuming that the patient had satisfactory LASIK results, the correct IOL can then be determined. The mean spherical equivalent postoperative refraction was +0.03 diopter (D) +/- 0.42 (SD) (range -0.625 to +0.75 D). In all 9 eyes, our method consistently chose the most accurate and precise IOL compared with other methods. The new method of calculating IOL power after LASIK provided excellent results and the most accurate and precise results to date.

Intraocular lens power calculation using ray tracing following excimer laser surgery

To evaluate intraocular lens (IOL) power calculation using ray tracing in patients presenting with cataract after excimer laser surgery. Ten eyes of seven consecutive patients who presented for cataract surgery following excimer laser treatment without any pre-refractive biometry data were enrolled in this prospective clinical study. Preoperatively, IOL power calculation was performed using a ray tracing software called OKULIX. Keratometry data (C-Scan) were imported and axial length (IOLMaster) was entered manually. Accuracy of IOL power calculation was investigated by subtracting attempted and achieved spherical equivalent. Mean spherical equivalent was -3.51+/-2.77 D (range -10.38 to -0.5 D) preoperatively and -1.01+/-1.08 D (range -2.5 to +0.75 D) postoperatively. Mean error was 0.31+/-0.84 D, mean absolute error was 0.74+/-0.46 D, and IOL calculation errors ranged from -1.39 to +1.47 D. A total of 40% of eyes were within +/-0.5 D, 70% within +/-1.0 D, and 100% within +/-1.5 D. Three eyes with corneal radii over 10 mm showed calculation errors exceeding +/-1.0 D. Mean best-corrected visual acuity increased from 20/60 to 20/30 postoperatively. IOL power calculation after excimer laser surgery can be difficult, especially when pre-refractive keratometry values are not available. In these cases, ray tracing combined with corneal topography measurements provides reliable and satisfactory postoperative results. However, it is advisable to be careful when calculating IOL power for eyes with corneal radii exceeding 10 mm because of slightly higher prediction errors.

Stereopsis in bilaterally pseudophakic patients

Purpose: To investigate the stereoacuity in patients who have had implantation of a monofocal intraocular lens (IOL) in each eye and to identify the principal risk factors for impaired stereoacuity. Setting: Hayashi Eye Hospital, Fukuoka, Japan. Methods: One hundred patients scheduled for bilateral phacoemulsification and monofocal IOL implantation were recruited. Near stereoacuity was measured approximately 2 weeks after surgery using the Titmus test. Various parameters that may be associated with stereoacuity in pseudophakic patients were measured using the New Aniseikonia Test (Handaya Corp.). Parameters included age, visual acuity, spherical equivalent, astigmatism, axial length, pupil diameters, IOL decentration and tilt, and aniseikonia. Stepwise multiple regression analysis was performed to identify independent predictors of impaired stereoacuity. Results: The mean stereoacuity was 57.1 seconds of arc ± 36.9 (SD); 90 patients (90%) had a good stereoacuity (ie, 100 seconds of arc or better). Simple regression analysis showed that a larger difference in spherical equivalent between fellow eyes and older age was significantly associated with poor stereoacuity. The best final model ( R 2 = 0.375) in the multiple regression analysis identified a greater difference in spherical equivalent to be the most significant predictor of impaired stereoacuity, followed by older age and a larger pupil diameter. Conclusions: Ninety percent of patients with bilateral pseudophakia had useful stereoacuity. The main risk factor for poor stereoacuity was a greater difference in the spherical equivalent between the eyes, followed by older age and a larger pupil diameter. These results indicate the need for an accurate preoperative IOL power calculation.

Intraocular lens power calculation after corneal refractive surgery: Double-K method

Purpose: To determine the accuracy of a method of calculating intraocular lens (IOL) power after corneal refractive surgery. Setting: Department of Ophthalmology, Hospital de Gipuzkoa, San Sebastián, Spain. Methods: The SRK/T formula was modified to use the pre refractive surgery K-value (Kpre) for the effective lens position (ELP) calculation and the post refractive surgery K-value (Kpost) for IOL power calculation by the vergence formula. The Kpre value was obtained by keratometry or topography and the Kpost, by the clinical history method. The formula was assessed in 9 cases of cataract surgery after laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) in which all relevant data were available. Refractive results of the standard SRK/T and the double-K SRK/T were compared statistically. Results: The mean IOL power for emmetropia and the achieved refraction (mean spherical equivalent [SE]), respectively, were +17.85 diopters (D) ± 3.43 (SD) and +1.82 ± 0.73 with the standard SRK/T and +20.25 ± 3.55 D and +0.13 ± 0.62 D with the double-K SRK/T. No case in the standard SRK/T group and 6 cases (66.66%) in the double-K group achieved a ±0.5 D SE. Conclusion: Double-K modification of the SRK/T formula improved the accuracy of IOL power calculation after LASIK and PRK.

Ultrasound axial length measurement in biphakic eyes

Purpose To determine the calculations to correct the error in ultrasound measurement of a phakic eye with a phakic intraocular lens (IOL) (biphakic eye) to increase the accuracy of IOL power calculation should cataract surgery become necessary. Methods Mathematical derivations were performed to calculate the error caused by a phakic IOL while measuring an eye using an average sound velocity. A simple model was developed to allow correction of the axial length (AL). Results If the eye is measured at an average velocity of 1555 meters/second, the following calculation can be used depending on the material of the phakic IOL: AL corrected = AL 1555 + X × T, where T is the central thickness of the phakic IOL and X = +0.42 (0.41 – 0.42) for poly(methyl methacrylate), –0.59 (0.56 – 0.59) for silicone, +0.11 (0.10 – 0.12) for Collamer, and +0.23 (0.23 – 0.24) for acrylic. Conclusion By obtaining the dioptric power of the phakic IOL and central thickness from the manufacturer and knowing the material of the lens, a mathematical correction can be made to arrive at the true ultrasound AL in a biphakic eye.