Accuracy and predictability of intraocular lens power calculation after laser in situ keratomileusis

Cataract surgery following refractive surgery: Principles to achieve optical success and patient satisfaction

A growing number of patients with prior refractive surgery are now presenting for cataract surgery. Surgeons face a number of unique challenges in this patient population that tends to be highly motivated to retain or regain functional uncorrected acuity postoperatively. Primary challenges include recognition of the specific type of prior surgery, use of appropriate intraocular lens (IOL) power calculation formulas, matching IOL style with spherical aberration profile, the recognition of corneal imaging patterns that are and are not compatible with toric and/or presbyopia-correcting lens implantation, and surgical technique modifications, which are particularly relevant in eyes with prior radial keratotomy or phakic IOL implantation. Despite advancements in IOL power formulae, corneal imaging, and IOL options that have improved our ability to achieve targeted postoperative refractive outcomes, accuracy and predictability remain inferior to eyes that undergo cataract surgery without a history of corneal refractive surgery. Thus, preoperative evaluation of patients who will and will not be candidates for postoperative refractive surgical enhancements is also paramount. We provide an overview of the specific challenges in this population and offer evidence-based strategies and considerations for optimizing surgical outcomes.

Intraoperative aberrometry compared to preoperative Barrett True-K formula for intraocular lens power selection in eyes with prior refractive surgery

To compare the predictive refractive accuracy of intraoperative aberrometry (ORA) to the preoperative Barrett True-K formula in the calculation of intraocular lens (IOL) power in eyes with prior refractive surgery undergoing cataract surgery at the Loma Linda University Eye Institute, Loma Linda, California, USA. We conducted a retrospective chart review of patients with a history of post-myopic or hyperopic LASIK/PRK who underwent uncomplicated cataract surgery between October 2016 and March 2020. Pre-operative measurements were performed utilizing the Barrett True-K formula. Intraoperative aberrometry (ORA) was used for aphakic refraction and IOL power calculation during surgery. Predictive refractive accuracy of the two methods was compared based on the difference between achieved and intended target spherical equivalent. A total of 97 eyes (69 patients) were included in the study. Of these, 81 eyes (83.5%) had previous myopic LASIK/PRK and 16 eyes (16.5%) had previous hyperopic LASIK/PRK. Median (MedAE)/mean (MAE) absolute prediction errors for preoperative as compared to intraoperative methods were 0.49 D/0.58 D compared to 0.42 D/0.51 D, respectively (P = 0.001/0.002). Over all, ORA led to a statistically significant lower median and mean absolute error compared to the Barrett True-K formula in post-refractive eyes. Percentage of eyes within ± 1.00 D of intended target refraction as predicted by the preoperative versus the intraoperative method was 82.3% and 89.6%, respectively (P = 0.04). Although ORA led to a statistically significant lower median absolute error compared to the Barrett True-K formula, the two methods are clinically comparable in predictive refractive accuracy in patients with prior refractive surgery.

Presbyopia correction after previous Intracor treatment: Combined implantation of a small-aperture and a non-diffractive extended-depth-of-focus lens

PURPOSE

We present the case of implantation of two different Extended depth of focus intraocular lenses (EDoF IOLs) in a patient with a history of unilateral intrastromal femtosecond laser treatment for presbyopia correction (Intracor).

OBSERVATIONS

The patient reported decreasing visual acuity at near distance and increasing spectacle dependence. Ten years earlier, he had Intracor treatment for presbyopia correction in his left eye. Corrected distance visual acuity (CDVA) was 0.08 logMAR for the right eye and 0.16 logMAR for the left eye. Apart from dysfunctional lens syndrome, the examination results were unremarkable. Phacoemulsification and subsequent IOL implantation was performed in both eyes. The left eye was implanted with an IC-8 (AcuFocus, Irvine, CA, USA), whereas the fellow eye was implanted with an AcrySof IQ Vivity IOL (Alcon, Fort Worth, TX, USA). Postoperatively, CDVA improved to 0.02 and 0.04 logMAR for the right and left eye. Uncorrected intermediate visual acuity (UIVA) was 0.24 logMAR for the right eye and -0.04 logMAR for the left eye, binocular UIVA was -0.04 logMAR. The patient reported a low level of photic phenomena and spectacle independence for far and intermediate distance.

CONCLUSIONS AND IMPORTANCE

Combined implantation of a non-diffractive and a small-aperture EDoF lens after previous unilateral Intracor treatment could successfully improve visual acuity at far and intermediate distance.

Development of a New Method for Calculating Intraocular Lens Power after Myopic Laser In Situ Keratomileusis by Combining the Anterior-Posterior Ratio of the Corneal Radius of the Curvature with the Double-K Method

Background: A new method, the Iida–Shimizu–Shoji (ISS) method, is proposed for calculating intraocular lens (IOL) power that combines the anterior–posterior ratio of the corneal radius of the curvature after laser in situ keratomileusis (LASIK) and to compare the predictability of the method with that of other IOL formulas after LASIK. Methods: The estimated corneal power before LASIK (Kpre) in the double-K method was 43.86 D according to the American Society of Cataract and Refractive Surgery calculator, and the K readings of the IOL master were used as the K values after LASIK (Kpost). The factor for correcting the target refractive value (correcting factor [C-factor]) was calculated from the correlation between the anterior–posterior ratio of the corneal radius of the curvature and the refractive error obtained using this method for 30 eyes of 30 patients. Results: Fifty-nine eyes of 59 patients were included. The mean values of the numerical and absolute prediction errors obtained using the ISS method were −0.02 ± 0.45 diopter (D) and 0.35 ± 0.27 D, respectively. The prediction errors using the ISS method were within ±0.25, ±0.50, and ±1.00 D in 49.2%, 76.3%, and 96.6% of the eyes, respectively. The predictability of the ISS method was comparable to or better than some of the other formulas. Conclusions: The ISS method is useful for calculating the IOL power in eyes treated with cataract surgery after LASIK.

Comparison of Corneal Power Calculation by Standard Keratometry and Total Keratometry in Eyes With Previous Myopic FS-LASIK

PURPOSE

To compare the accuracy of Total Keratometry (TK) and standard keratometry (K) with the IOLMaster 700 (Carl Zeiss Meditec) in evaluating the corneal refractive change in eyes with previous myopic femtosecond laser-assisted LASIK (FS-LASIK).

METHODS

A series of consecutive patients who had undergone myopic FS-LASIK was prospectively enrolled. The refractive change in spherical equivalent (ΔSE) was defined as the difference between the preoperative target correction entered into the laser software and the postoperative cycloplegic refraction. The difference between the postoperative and the preoperative K (ΔK) and the difference between the postoperative and the preoperative TK (ΔTK) were compared to the ΔSE. Only the right eye of each patient was selected for the statistical analysis.

RESULTS

Twenty-five eyes of 25 patients were enrolled. The mean ΔSE was -4.41 ± 1.68 diopters (D). The mean ΔK (-3.82 ± 1.60 D) revealed a statistically significant underestimation of the laser-induced refractive change (P < .0001), whereas the mean ΔTK (-4.36 ± 1.78 D) did not show any significant difference (P = .45). The difference between ΔK and ΔTK was statistically significant (P < .0001). Linear regression between the laser-induced refractive change and the individual difference between the postoperative K and TK disclosed a statistically significant relationship (r = -0.6930, r² = 0.4803, P < .0001), thus revealing that higher refractive corrections increase the difference between the postoperative values of K and TK.

CONCLUSIONS

TK does not underestimate the laser-induced corneal changes and can be considered a reliable option for intraocular lens power calculation after myopic excimer laser surgery. [J Refract Surg. 2021;37(12):848-852.].

Comparisons of intraocular lens power calculation methods for eyes with previous myopic laser refractive surgery: Bayesian network meta-analysis

PURPOSE

To compare the accuracy of the methods for calculation of intraocular lens (IOL) power in eyes with previous myopic laser refractive surgery.

SETTING

EENT Hospital of Fudan University, Shanghai, China.

DESIGN

Network meta-analysis.

METHODS

A literature search of MEDLINE and Cochrane Library from January 2000 to July 2019 was conducted for studies that evaluated methods of calculating IOL power in eyes with previous myopic laser refractive surgery. Outcomes measurements were the percentages of prediction error within ±0.50 diopters (D) and ±1.00 D of the target refraction (% ±0.50 D and % ±1.00 D). Traditional and network meta-analysis were conducted.

RESULTS

Nineteen prospective or retrospective clinical studies, including 1217 eyes and 13 calculation methods, were identified. A traditional meta-analysis showed that compared with the widely used Haigis-L method, the Barrett True-K formula, optical coherence tomography (OCT), and Masket methods showed significantly higher % ±0.50 D, whereas no difference was found in the % ±1.00 D. A network meta-analysis revealed that compared with the Haigis-L method, the OCT, Barrett True-K formula, and optiwave refractive analysis (ORA) methods performed better on the % ±0.50 D, whereas the Barrett True-K formula and ORA methods performed better on the % ±1.00 D. Based on the performances of both outcomes, the Barrett True-K formula, OCT, and ORA methods showed highest probability to rank the top 3 among the 13 methods.

CONCLUSIONS

The Barrett True-K formula, OCT, and ORA methods seemed to offer greater accuracy than others in calculating the IOL power for postrefractive surgery eyes.

Comparative clinical accuracy analysis of the newly developed ZZ IOL and four existing IOL formulas for post-corneal refractive surgery eyes

BACKGROUND

Intraocular lens (IOL) calculation using traditional formulas for post-corneal refractive surgery eyes can yield inaccurate results. This study aimed to compare the clinical accuracy of the newly developed Zhang & Zheng (ZZ) formula with previously reported IOL formulas.

STUDY DESIGN

Retrospective study.

METHODS

Post-corneal refractive surgery eyes were assessed for IOL power using the ZZ, Haigis-L, Shammas, Barrett True-K (no history), and ray tracing (C.S.O Sirius) IOL formulas, and their accuracy was compared. No pre-refractive surgery information was used in the calculations.

RESULTS

This study included 38 eyes in 26 patients. ZZ IOL yielded a lower arithmetic IOL prediction error (PE) compared with ray tracing (P = 0.04), whereas the other formulas had values like that of ZZ IOL (P > 0.05). The arithmetic IOL PE for the ZZ IOL formula was not significantly different from zero (P = 0.96). ZZ IOL yielded a lower absolute IOL PE compared with Shammas (P < 0.01), Haigis-L (P = 0.02), Barrett true K (P = 0.03), and ray tracing (P < 0.01). The variance of the mean arithmetic IOL PE for ZZ IOL was significantly smaller than those of Shammas (P < 0.01), Haigis-L (P = 0.03), Barrett True K (P = 0.02), and ray tracing (P < 0.01). The percentages of eyes within ± 0.5 D of the target refraction with the ZZ IOL, Shammas, Haigis-L, Barrett True-K, and ray-tracing formulas were 86.8 %, 45.5 %, 66.7 %, 73.7 %, and 50.0 %, respectively (P < 0.05 for Shammas and ray tracing vs. ZZ IOL).

CONCLUSIONS

The ZZ IOL formula might offer superior outcomes for IOL power calculation for post-corneal refractive surgery eyes without prior refractive data.

Contribution of Posterior Corneal Astigmatism to Total Corneal Astigmatism in a Sample of Egyptian Population

Purpose

To determine the percentage of contribution of the magnitude of posterior corneal astigmatism to total corneal astigmatism using Scheimpflug imaging.

Methods

This prospective cross-sectional study was conducted on 356 eyes of 356 patients, where the total corneal astigmatism was calculated by addition of anterior and posterior corneal astigmatism using vector analysis and then the percentage of posterior to total corneal astigmatism was calculated.

Results

The percentage of contribution of posterior to total corneal astigmatism was about 30% in patients with With The Rule astigmatism and about 8% in patients with Against The Rule astigmatism.

Conclusion

Posterior corneal astigmatism should not be neglected during calculation of total corneal astigmatism as neglecting posterior corneal astigmatism can result in errors during calculation and correction of astigmatism.

Comparison of intraocular lens calculation methods after myopic laser-assisted in situ keratomileusis and radial keratotomy without prior refractive data

AIM

To compare intraocular lens (IOL) calculation methods not requiring refraction data prior to myopic laser-assisted in situ keratomileusis (LASIK) and radial keratotomy (RK).

METHODS

In post-LASIK eyes, the methods not requiring prior refraction data were Hagis-L; Shammas; Barrett True-K no-history; Wang-Koch-Maloney; 'average', 'minimum' and 'maximum' IOL power on the American Society of Cataract and Refractive Surgeons (ASCRS) IOL calculator. Double-K method and Barrett True-K no-history, 'average', 'minimum' and 'maximum' IOL power on ASCRS IOL calculator were evaluated in post-RK eyes. The predicted IOL power was calculated with each method using the manifest postoperative refraction. Arithmetic and absolute IOL prediction errors (PE) (implanted-predicted IOL powers), variances in arithmetic IOL PE and percentage of eyes within ±0.50 and ±1.00 D of refractive PE were calculated.

RESULTS

Arithmetic or absolute IOL PE were not significantly different between the methods in post-LASIK and post-RK eyes. In post-LASIK eyes, 'average' showed the highest and 'minimum' showed the least variance, whereas 'average' and 'minimum' had highest percentage of eyes within ±0.5 D and 'minimum' had the highest percentage of eyes within ±1.0 D. In the post-RK eyes, 'minimum' had highest variance, and 'average' had the least variance and highest percentage of eyes within ±0.5 D and ±1.0 D.

CONCLUSION

In post-LASIK and post-RK eyes, there are no significant differences in IOL PE between the methods not requiring prior refraction data. 'Minimum' showed least variance in PEs and more chances of eyes to be within ±1.0 D postoperatively in post-LASIK eyes. 'Average' had least variance and more chance of eyes within ±1.0 D in post-RK eyes.

Intraocular lens power calculations in eyes with previous hyperopic laser in situ keratomileusis or photorefractive keratectomy

PURPOSE

To evaluate the accuracy of 7 intraocular lens (IOL) calculation formulas in patients with previous hyperopic laser in situ keratomileusis (LASIK) or excimer laser photorefractive keratectomy (PRK).

DESIGN

Retrospective case series.

SETTING

Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, and private practice, Mesa, Arizona, USA.

METHODS

The 7 formulas evaluated were the adjusted Atlas 0-3, Masket, Modified Masket, Haigis-L, Shammas-PL, Barrett True-K, and Barrett True-K No-History. The Masket and Modified Masket were calculated using the single-K version of Holladay 1 and Hoffer Q formulas; the adjusted Atlas 0-3 was calculated using the double-K version of Holladay 1 and Hoffer Q. The IOL power predicted by each formula was calculated by targeting the postoperative manifest refraction. The IOL prediction error was obtained by subtracting the predicted IOL power from the implanted IOL power. The mean IOL prediction error, median absolute refractive prediction error, and percentages of eyes within ±0.50 diopter (D) and ±1.00 D of the predicted refraction were calculated.

RESULTS

Twenty-one eyes of 21 patients were evaluated. There were no significant differences in the median absolute refractive prediction error or percentages of eyes within ±0.50 D or ±1.00 D of the predicted refraction between formulas or methods. The IOL mean prediction errors were comparable between the Holladay 1 and Hoffer Q calculations for all formulas except for a greater error for the double-K version of the Hoffer Q of the adjusted Atlas 0-3.

CONCLUSION

In eyes that had hyperopic LASIK or PRK, there were no significant differences in the accuracy between the 7 IOL calculation formulas.

Evaluation of total keratometry and its accuracy for intraocular lens power calculation in eyes after corneal refractive surgery

PURPOSE

To compare the accuracy of total keratometry (TK) and standard keratometry (K) from a swept-source optical coherence tomography biometer for intraocular lens (IOL) power calculation in eyes with previous corneal refractive surgery.

SETTING

Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA.

DESIGN

Retrospective case series.

METHODS

The differences between the TK and K and their association with K were assessed. For IOL power calculation, combinations of 1) K with Haigis, Haigis-L, and Barrett True-K, and 2) TK with Haigis (Haigis-TK) were used. The mean absolute error (MAE) and the percentages of eyes within prediction errors of ± 0.50 diopters (D), ± 1.00 D, and ± 2.00 D were calculated.

RESULTS

The study comprised 129 eyes. For Haigis, Haigis-L, Barrett True-K, and Haigis-TK, respectively, the MAEs were 0.72 D, 0.61 D, 0.54 D, and 0.50 D in the myopic laser in situ keratomileusis (LASIK)/photorefractive keratectomy (PRK) group, and 0.74 D, 0.68 D, 0.71 D, and 0.70 D in hyperopic LASIK/PRK group. For the radial keratotomy (RK) eyes, the MAEs were 0.66 D, 0.71 D, and 0.72 D for the Haigis, Barrett True-K, and Haigis-TK formulas, respectively. In the myopic LASIK/PRK group, the Barrett True-K and Haigis-TK produced significantly lower MAEs than did Haigis (P < .05). In the hyperopic LASIK/PRK and RK groups, there were no significant differences between the formulas in MAEs and percentages of eyes within the above prediction errors.

CONCLUSIONS

The performance of the combination of Haigis and TK in refractive prediction was comparable with Haigis-L and Barrett True-K in eyes with previous corneal refractive surgery.

A Survey of Intraocular Lens Explantation: A Retrospective Analysis of 23 IOLs Explanted during 2005

Purpose

To evaluate the indications, lens styles, perioperative findings, and results of intraocular lens (IOL) explantation or exchange performed in the authors department in 2005.

Methods

The retrospective analysis comprised 22 patients (23 eyes). Twenty-one eyes had previous phacoemulsification and IOL implantation, one eye secondary aphakic IOL, and one eye phakic IOL implantation. The indications for IOL explantation/exchange and perioperative complications were evaluated. The best-corrected visual acuity (BCVA) before and after surgery was compared.

Results

Time from initial surgery to explantation/exchange varied from 1 to 121 months, median value was 46 months. The IOLs were explanted using local anesthesia and in 21 eyes replaced with new lens. Indications for IOL removal were opacification of the IOL in 12 eyes, malposition of the IOL in 5 eyes, postoperative refractive error in 2 eyes, recurrent toxic anterior segment syndrome in 1 eye, pseudophakic dysphotopsia in 1 eye, endothelial cell loss in phakic anterior chamber IOL in 1 eye, and visual discomfort with intraocular telescopic lens in 1 eye. The mean BCVA (decimal scale) before and after IOL explantation/exchange was 0.562±0.279 and 0.627±0.276, respectively. There was no significant difference in visual acuity before and after IOL exchange (Wilcoxon test).

Conclusions

The most frequent indications for IOL explantation/exchange were opacification of the IOL and IOL malposition. Surgeries were uneventful in most cases. Final visual results have been largely good. Long-term follow-up of patients with various types of IOLs should be maintained.

Comparison of OKULIX ray-tracing software with SRK-T and Hoffer-Q formula in intraocular lens power calculation

Purpose

To compare the performance of OKULIX ray-tracing software with SRK-T and Hoffer Q formula in intraocular lens (IOL) power calculation in patients presenting with cataract.

Methods

In this prospective study, 104 eyes of 104 patients with cataract who underwent phacoemulsification and IOL implantation were recruited. Three IOL brands were used and for all eyes, IOL power calculation was performed using SRK-T, Hoffer Q formula and also OKULIX ray-tracing software. For all patients, axial length and keratometry data was obtained with IOLMaster 500 device and IOL power was determined using Hoffer Q and SRK-T formula. The IOL powers were also calculated using the OKULIX ray-tracing software combined with CASIA AS-OCT and IOLMaster 500 device. Optically measured axial length of eyes were inserted to OKULIX software from IOLMaster 500 device, and anterior and posterior tomographic and corneal pachymetry data was imported from CASIA AS-OCT into the OKULIX.

The performance of each calculation methods was measured by subtracting the predicted postoperative refraction from the postoperative manifest refraction spherical equivalent (MRSE). For each of the 3 methods, the mean absolute prediction error was determined, too.

Results

The mean value absolute prediction error by OKULIX, SRK-T and Hoffer Q formulas, respectively, were 0.42 (±0.03), 0.36 (±0.02) and 0.37 (±0.02). The mean absolute prediction error by OKULIX had no significant difference between three IOL groups (P = 0.96), and it was confirmed that there was no meaningful statistically difference in mean absolute prediction error between the OKULIX, SRK-T and Hoffer Q formula. (P = 0.25). Also in each group of implanted IOLs, all three formulas worked with the same accuracy. The prediction error using OKULIX were within ±0.50 diopter in 63.5% of eyes and within ±1.00 diopter in 94.2% of eyes.

Conclusion

OKULIX ray-tracing IOL power measurements provides reliable and satisfactory postoperative results, which are comparable to other 3rd generation formulas of SRK-T and Hoffer Q.

Effect of postoperative keratometry on quality of vision in the postoperative period after myopic wavefront-guided laser in situ keratomileusis

PURPOSE

To determine whether postoperative keratometry is a predictor of patient-reported satisfaction and night-vision phenomena after wavefront-guided myopic laser in situ keratomileusis (LASIK).

SETTING

Optical Express, Glasgow, United Kingdom.

DESIGN

Retrospective case series.

METHODS

Myopic eyes treated with wavefront-guided LASIK were analyzed in this study. All patients completed pre-operative and 1-month postoperative questionnaires, in which the satisfaction with visual outcomes and pre-operative and postoperative night-vision symptoms (glare, halos, starburst, ghosting/double-vision) were rated. Multivariate regression analysis was performed to determine factors associated with questionnaire outcomes.

RESULTS

This study evaluated 8672 myopic eyes of 4602 patients. The mean pre-operative manifest spherical equivalent was -3.72 diopters (D) ± 2.00 (SD) (range -0.50 to -11.00 D) and the mean pre-operative keratometry (K) value was 43.64 ± 1.43 D (38.38 to 49.00). At 1 month after surgery, 93.7% and 99.1% of eyes were within 0.50 D and 1.00 D of emmetropia, and 94.6% and 98.3% of eyes achieved monocular and binocular uncorrected-distance visual acuity(UDVA) of 20/20 or better, respectively. There were 48.7% of eyes that had the flat corneal meridian (minimum K) of 40.0 D or less. Although postoperative keratometry was a significant predictor of patient-reported satisfaction and the change in halo reports in the regression analysis, its relative contribution was very low and accounted for less than 0.50% of the variance explained by either model. Postoperative keratometry was not a significant predictor of a change in reports of glare, starburst, and ghosting or double vision.

CONCLUSION

In this large cohort of patients, postoperative keratometry played a minimal and clinically insignificant role in predicting post-LASIK halo visual phenomena and patient-reported satisfaction.

FINANCIAL DISCLOSURE

Dr. Schallhorn is a consultant to Abbott Medical Optics and Zeiss and a Global Medical Director for Optical Express. None of the other authors have a financial or proprietary interest in the products and materials presented in this paper.

Comparison of Newer Intraocular Lens Power Calculation Methods for Eyes after Corneal Refractive Surgery

PURPOSE

To compare the newer formulae, the optical coherence tomography (OCT)-based intraocular lens (IOL) power formula (OCT formula) and the Barrett True-K formula (True-K), with the methods on the American Society of Cataract and Refractive Surgery (ASCRS) calculator in eyes with previous myopic LASIK/photorefractive keratectomy (PRK).

DESIGN

Prospective case series.

PARTICIPANTS

A total of 104 eyes of 80 patients who had previous myopic LASIK/PRK and subsequent cataract surgery and IOL implantation.

METHODS

By using the actual refraction after cataract surgery as target refraction, predicted IOL power for each method was calculated. The IOL prediction error (PE) was obtained by subtracting the predicted IOL power from the power of the IOL implanted.

MAIN OUTCOME MEASURES

Arithmetic IOL PEs, variances of mean arithmetic IOL PE, median refractive PE, and percent of eyes within 0.5 diopters (D) and 1.0 D of refractive PE.

RESULTS

Optical coherence tomography produced smaller variance of IOL PE than did Wang-Koch-Maloney (WKM) and Shammas (P < 0.05). With the OCT, True-K No History, WKM, Shammas, Haigis-L, and Average of these 5 formulas, the median refractive PEs were 0.35 D, 0.42 D, 0.51 D, 0.48 D, 0.39 D, and 0.35 D, respectively, the percentage of eyes within 0.5 D of refractive PE were 68.3%, 58.7%, 50.0%, 52.9%, 55.8%, and 67.3%, respectively, and the percentage of eyes within 1.0 D of refractive PE were 92.3%, 90.4%, 86.9%, 88.5%, 90.4%, and 94.2%, respectively. The OCT formula had smaller refractive PE compared with the WKM and Shammas, and the Average approach produced significantly smaller refractive PE than all methods except OCT (all P < 0.05).

CONCLUSIONS

The OCT and True-K No History are promising formulas. The ASCRS IOL calculator has been updated to include the OCT and Barrett True K formulas.

TRIAL REGISTRATION

Intraocular Lens Power Calculation After Laser Refractive Surgery Based on Optical Coherence Tomography (OCT IOL); Identifier: NCT00532051; www.ClinicalTrials.gov.

Agreement between clinical history method, Orbscan IIz, and Pentacam in estimating corneal power after myopic excimer laser surgery

The purpose of this study was to investigate the agreement between the clinical history method (CHM), Orbscan IIz, and Pentacam in estimating corneal power after myopic excimer laser surgery. Fifty five patients who had myopic LASIK/PRK were recruited into this study. One eye of each patient was randomly selected by a computer-generated process. At 6 months after surgery, postoperative corneal power was calculated from the CHM, Orbscan IIz total optical power at the 3.0 and 4.0 mm zones, and Pentacam equivalent keratometric readings (EKRs) at 3.0, 4.0, and 4.5 mm. Statistical analyses included multilevel models, Pearson’s correlation test, and Bland-Altman plots. The Orbscan IIz 3.0-mm and 4.0 mm total optical power, and Pentacam 3.0-mm, 4.0-mm, and 4.5-mm EKR values had strong linear positive correlations with the CHM values (r = 0.90–0.94, P = <0.001, for all comparisons, Pearson’s correlation). However, only Pentacam 3.0-mm EKR was not statistically different from CHM (P = 0.17, multilevel models). The mean 3.0- and 4.0-mm total optical powers of the Orbscan IIz were significantly flatter than the values derived from CHM, while the average EKRs of the Pentacam at 4.0 and 4.5 mm were significantly steeper. The mean Orbscan IIz 3.0-mm total optical power was the lowest keratometric reading compared to the other 5 values. Large 95% LoA was observed between each of these values, particularly EKRs, and those obtained with the CHM. The width of the 95% LoA was narrowest for Orbscan IIz 3.0-mm total optical power. In conclusion, the keratometric values extracted from these 3 methods were disparate, either because of a statistically significant difference in the mean values or moderate agreement between them. Therefore, they are not considered equivalent and cannot be used interchangeably.

Intraocular lens explantation technique for one-piece acrylic lenses

PURPOSE

To present a simple technique to remove a one-piece, acrylic AcrySof (Alcon Laboratories Inc) intraocular lens (IOL) via the original incision.

METHODS

The AcrySof IOL is removed via the original (2.75-mm) incision, without cutting or folding the IOL or widening the incision. After the IOL is viscodissected from the capsular bag and brought into the anterior chamber, toothed forceps hold the optic through the incision while the manipulator enters the side-port incision and hooks onto the optic 180° away.

RESULTS

With the forceps pulling and the lens manipulator pushing the IOL toward the incision, the IOL will fold and be delivered.

CONCLUSIONS

A one-piece, acrylic (Acrysof) IOL can be removed without cutting or folding the lens and without widening the original 2.75-mm incision.

Comparison of retinal image quality with spherical and customized aspheric intraocular lenses

We hypothesize that an intraocular lens (IOL) with higher-order aspheric surfaces customized for an individual eye provides improved retinal image quality, despite the misalignments that accompany cataract surgery. To test this hypothesis, ray-tracing eye models were used to investigate 10 designs of mono-focal single lens IOLs with rotationally symmetric spherical, aspheric, and customized surfaces. Retinal image quality of pseudo-phakic eyes using these IOLs together with individual variations in ocular and IOL parameters, are evaluated using a Monte Carlo analysis. We conclude that customized lenses should give improved retinal image quality despite the random errors resulting from IOL insertion.

Orbscan II and double-K method for IOL calculation after refractive surgery

BACKGROUND

Precise IOL calculation in post-refractive surgery patients is still a challenge for the cataract surgeon. The purpose of this study is to test whether adding Orbscan II values into the double-K method improves IOL calculation in this group of patients.

METHODS

A prospective study with 43 eyes previously submitted to refractive surgery that underwent cataract extraction. IOL calculation was performed with double-K method. Post-K value was derived from Orbscan total-mean power map. The average corneal curvature of the general population (43.8D) was used as the pre-K value. Refraction results 30 days after surgery were compared with refraction that would be obtained if we used: (1) post-K values from keratometry, (2) post-K values from topography, and (3) pre-K values from Orbscan total-mean power. Anterior chamber depth measures obtained with the IOL Master and Orbscan II were compared.

RESULTS

Mean postoperative spherical equivalent (SE) was -0.25 ± 1.10 D in eyes submitted to radial keratotomy , -1.04 ± 1.42 D in eyes previously submitted to myopic Lasik, and +0.05 ± 1.76 D in those submitted to hyperopic surgeries. Had we inputted post-K values derived from keratometer and from topography, we would have obtained significantly higher postoperative refractive errors in eyes previously submitted to myopic refractive surgery (p < 0.05). Refractions using pre-K derived from the central 8 mm Orbscan instead of 43.8 D were similar in all studied groups (p > 0.05). Anterior chamber depth measured with IOL Master or Orbscan were similar.

CONCLUSIONS

Orbscan measurements used as the post-K values into the double-K method provide a precise IOL calculation, especially in post myopic refractive surgery patients.

Comparison of simulated keratometric changes induced by custom and conventional laser in situ keratomileusis after myopic ablation: retrospective chart review

PURPOSE

To determine the relationship between the achieved refractive change and the change in simulated keratometry (K) after myopic laser situ keratomileusis (LASIK) and compare this relationship between custom and conventional treatments.

SETTING

Department of Ophthalmology, University of California, Davis, Sacramento, California, and John A. Moran Eye Center, Salt Lake City, Utah, USA.

METHODS

The change in simulated K and the refractive change induced by custom myopic LASIK and conventional LASIK were determined. The relationship between the variables was analyzed by regression methods.

RESULTS

Custom treatment was performed in 106 eyes and conventional treatment in 224 eyes. Simple linear regression analysis did not fit the clinical observation when the refractive change was less than 2.00 diopters (D) of myopic correction with both treatments. Under the linear model and nonlinear model, each unit of refractive change yielded a greater change in corneal topographic power with custom treatment than with conventional treatment. With both treatments, the rate of change in simulated K was not constant and was much more variable with lower amounts of correction. The relationship was more constant and linear with larger amounts of refractive correction.

CONCLUSIONS

The relationship between the measured change in simulated K and the induced refractive change better fit a nonlinear relationship with smaller amounts of refractive correction in custom LASIK and conventional LASIK. Under all forms of analysis, custom treatments yielded a greater per-unit change in corneal curvature than conventional treatments, especially for refractive corrections of 4.00 D and higher.

Calculation of intraocular lens power using Orbscan II quantitative area topography after corneal refractive surgery

PURPOSE

To present the prospective application of the Orbscan II central 2-mm total-mean corneal power obtained by quantitative area topography in intraocular lens (IOL) calculation after refractive surgery.

METHODS

Calculated and achieved refraction and the difference between them were studied in 77 eyes of 61 patients with previous radial keratotomy (RK), RK and additional surgeries, myopic LASIK, myopic photorefractive keratectomy (PRK), or hyperopic LASIK who underwent phacoemulsification without complications in 3 eye centers. All IOL calculations used the average from the central 2-mm Orbscan II total-mean power of maps centered on the pupil without the use of previous refractive data. Six IOL styles implanted within the bag were used.

RESULTS

Using the SRK-T formula, the overall calculated refraction was -0.64+/-0.93 diopters (D). The overall achieved spherical equivalent refraction (-0.52+/-0.79 D; range: -3.12 to 1.25 D; 95% confidence interval [CI]: -0.70/-0.34 D) was +/-0.50 D in 53% of eyes, +/-1.00 D in 78% of eyes, and +/-2.00 D in 99% of eyes. The overall difference between the calculated and achieved refraction (0.12+/-0.93 D, P=.27; range: -2.18 to 2.62 D; 95% CI: 0.09/0.33 D) was +/-0.50 D in 39% of eyes, +/-1.00 D in 77% of eyes, and +/-2.00 D in 96% of eyes. This difference was +/-1.00 D in 77% of eyes with RK (P=.70), 82% of eyes with myopic LASIK (P=.34), and 90% of eyes with myopic PRK (P=.96). In eyes with RK followed by LASIK, a trend toward undercorrection was noted (P=.03). In eyes with hyperopic LASIK, a trend toward overcorrection was noted (P=.005).

CONCLUSIONS

In eyes with previous corneal refractive surgery, IOL power calculation can be performed with reasonable accuracy using the Orbscan II central 2-mm total-mean power. This method had better outcomes in eyes with previous RK, myopic LASIK, and myopic PRK than in eyes with hyperopic LASIK or RK with LASIK.

Use of the pentacam true net corneal power for intraocular lens calculation in eyes after refractive corneal surgery

PURPOSE

To assess the accuracy and validity of true net corneal power of the Pentacam system to provide a keratometry reading for calculating intraocular lens (IOL) power in postoperative refractive surgery eyes.

METHODS

Refraction, an automated keratometry reading, and true net corneal power were measured for 30 eyes that required cataract surgery and had previously undergone refractive surgery. Target refraction values calculated with the SRK/T formula using true net corneal power were compared with postoperative manifest refraction values.

RESULTS

Using true net corneal power, the mean deviation from the desired postoperative cataract refractive outcome was 0.47 +/- 0.56 diopters (D); the actual refraction was within +/- 0.50 D of the intended refraction for 70% of eyes (21/30) and within +/- 1.00 D for 93% of eyes (28/30).

CONCLUSIONS

The true net corneal power can be used as a keratometry reading for eyes with previous refractive surgery requiring cataract surgery.

Accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement

PURPOSE

To evaluate the accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement.

DESIGN

Prospective, observational study.

METHODS

The right eyes of 493 subjects were measured with a rotating Scheimpflug camera (Pentacam; Oculus, Wetzlar, Germany). The keratometric corneal astigmatism (KA) was obtained by using the anterior corneal surface measurement and the keratometric index (1.3375) while neglecting the posterior corneal surface measurement. The Pentacam-derived total corneal astigmatism (PA) was derived by doubled-angle vector analysis of the astigmatisms on both corneal surfaces.

RESULTS

The mean arithmetic and absolute estimation errors of the KA magnitude for the PA magnitude were -0.06 +/- 0.28 diopters (D) (range, -0.59 to 0.91 D) and 0.24 +/- 0.16 D (range, 0 to 0.91 D), respectively. The mean arithmetic and absolute estimation errors of the KA angle for the PA angle were -0.6 degrees +/- 12.7 degrees (range, -69.9 degrees to 83.4 degrees) and 7.4 degrees +/- 10.3 degrees (range, 0 degrees to 83.4 degrees), respectively. Among all eyes, 142 eyes (28.8%) had either a KA magnitude that differed by > 0.50 D from the PA magnitude or a KA angle that differed by > 10 degrees from the PA angle. For the 282 eyes with a KA magnitude exceeding 1.0 D (that are candidates for intraoperative correction of a preexisting astigmatism during cataract surgery), 29 eyes (10.3%) had either a KA magnitude that differed by > 0.50 D from the PA magnitude or a KA angle that differed by > 10 degrees from the PA angle.

CONCLUSIONS

Neglecting the posterior corneal surface measurement may lead to significant deviation in the corneal astigmatism estimation in a proportion of eyes.

The comparison of central and mean true-net power (Pentacam) in calculating IOL-power after refractive surgery

PURPOSE

To compare the accuracy of central true net corneal power (cTNP) and mean true net corneal power (mTNP) of the Pentacam system to give a keratometry (K) reading for calculating IOL (intraocular lens) power in eyes following refractive surgery.

METHODS

Refraction, an automated K-reading (Km), cTNP and mTNP were measured for 15 eyes that required cataract surgery and had previously undergone refractive surgery. The difference between postoperative manifest refraction values and target refraction values calculated with the SRK/T formula using cTNP were compared with the one using mTNP.

RESULTS

The mean deviation from the desired post-cataract refractive outcome was 0.60 diopter (D)+/-0.47 (standard deviation) using cTNP; 0.75+/-0.54 using mTNP (p=0.386). The actual refraction was within +/-0.50D of the intended refraction for 60% (cTNP) and 33.3% (mTNP) of eyes, and within +/-1.00D for 93% (cTNP) and 66.7% (mTNP) of eyes.

CONCLUSIONS

Although not statistically significant, the cTNP showed better accuracy than mTNP to give a keratometry (K) reading for post-refractive surgery eyes requiring cataract surgery.

Correction of residual hyperopia after cataract surgery using the light adjustable intraocular lens technology

PURPOSE

To determine whether residual hyperopia could be corrected postoperatively using the light adjustable lens technology in patients undergoing cataract surgery and light adjustable lens implantation.

DESIGN

Prospective, nonrandomized clinical trial.

METHODS

Fourteen eyes of 14 patients were studied. The manifest refraction, uncorrected visual acuity (UCVA), and best-corrected visual acuity (BCVA) were determined with follow-up time to determine the achieved refractive corrections and their stability.

RESULTS

Of 14 eyes, 13 eyes (92.9%) achieved +/- 0.25 diopters (D) of the target refraction at one day post lock-in, with 100% of the eyes achieving the targeted refractive adjustment within 0.5 D or better up to six months postoperative follow-up. All eyes treated show no change in manifest spherical refraction >0.25 D between one day post lock-in, and three and six months postoperative visits. The data demonstrate the stability of the achieved refractive change after the adjustment and lock-in procedures. The mean rate of change was 0.006 D per month, which is six times more stable than that of refractive procedures.

CONCLUSIONS

Residual hyperopia errors in the range of +0.25 to +2.0 D were successfully corrected with precision and significant improvement in UCVA and without compromising BCVA using the light adjustable intraocular lens technology.

Posterior corneal curvature measurements with peripheral fitting zones before and after myopic LASIK using Orbscan II

PURPOSE

To compare pre- and postoperative posterior corneal curvature measurements in peripheral fitting zones using the Orbscan II topographer in patients undergoing myopic LASIK.

METHODS

Retrospective analysis of preoperative and 3-month postoperative Orbscan II data of 194 eyes that underwent myopic LASIK at a university eye center. Posterior corneal power was estimated using the peripheral 7- to 10-mm fitting zones. The pre- and postoperative values were analyzed and compared.

RESULTS

The mean difference in estimated pre- and postoperative power of the posterior cornea was -0.04 +/- 0.16 diopters (P < .01).

CONCLUSIONS

The difference in posterior corneal curvature measurement following myopic LASIK using the peripheral fitting zone with the Orbscan II, as compared to the preoperative values, is clinically insignificant.

Intraocular lens power calculation after automated lamellar keratoplasty for high myopia

PURPOSE

To report intraocular lens (IOL) power calculation in 2 eyes that were highly undercorrected by previous myopic automated lamellar keratoplasty (ALK).

METHODS

A 35-year-old man underwent bilateral myopic ALK, which caused high residual myopia (-9.0 -4.0 x 171 and -9.5 -4.5 x 74). The patient then underwent cataract surgery with IOL implantation for cataract development. The double-K clinical history method was utilized, and satisfactory IOL power prediction results were obtained. Two no-history IOL power calculation methods (Rosa correcting factor method and Ferrara theoretical variable refractive index method), which involved axial length-dependent modification of the keratometer-measured corneal radius, and 1 no-history IOL power calculation method (Shammas' method), which involved axial length-independent modification of the keratometer-measured corneal power, were tested on these 2 eyes.

RESULTS

In both eyes, the double-K SRK-T clinical history method gave small IOL prediction errors (-0.66 and -0.81 D). The Shammas' no-history method gave a slightly higher IOL prediction error in the right eye (-1.67 D) and a small IOL prediction error in the left eye (-0.74 D). Unacceptable IOL power prediction errors would have resulted if Rosa's correcting factor method (-8.07 and -8.35 D) or Ferrara's theoretical variable refractive index method (-17.56 and -18.51 D) had been applied. When we utilized Rosa's method for the IOL power calculation by assuming that the previous ALK had fully corrected the refractive error, the predicted IOL powers were very close to the benchmark IOL powers (IOL power prediction errors: 1.16 and 0.37 D). When we utilized Ferrara's method with the same assumption, the IOL power prediction errors remained high (-6.32 and -7.16 D).

CONCLUSIONS

For patients who have had previous myopic ALK (and whose eyes are highly undercorrected) and who require cataract surgery and for whom the pre-ALK history is available, the double-K method appears to yield excellent predictive results. However, if the pre-ALK history is not available, the Shammas' no-history method appears to yield better results than the Rosa's or the Ferrara's method. High undercorrection by the previous ALK might have been one of the major reasons why Rosa's method resulted in a high IOL prediction error in these 2 eyes. The cause for the marked IOL prediction error by Ferrara's method in this case, however, remains to be determined.

Biometry and intraocular lens power calculation

PURPOSE OF REVIEW

Heightened patient expectations for precise postoperative refractive results have spurred the continued improvements in biometry and intraocular lens calculations. In order to meet these expectations, attention to proper patient selection, accurate keratometry and biometry, and appropriate intraocular lens power formula selection with optimized lens constants are required. The article reviews recent studies and advances in the field of biometry and intraocular lens power calculations.

RECENT FINDINGS

Several noncontact optical-based devices compare favorably, if not superiorly, to older ultrasonic biometric and keratometric techniques. With additional improvements in the internal acquisition algorithm, the new IOL Master software version 5 upgrade should lessen operator variability and further enhance signal acquisition. The modern Haigis-L and Holladay 2 formulas more accurately determine the position and the shape of the intraocular lens power prediction curve.

SUMMARY

Postoperative refractive results depend on the precision of multiple factors and measurements. The element with the highest variability and inaccuracy is, ultimately, going to determine the outcome. By understanding the advantages and limitations of the current technology, it is possible to consistently achieve highly accurate results.