Intraocular lens power calculation after previous laser refractive surgery

Intraocular Lens Power Calculation in Eyes After Laser In Situ Keratomileusis or Photorefractive Keratectomy for Myopia

Intraocular power calculation is challenging for patients who have previously undergone corneal refractive surgery. The sources of prediction errors for these eyes are well known; however, the numerous formulas and methods available for calculating intraocular lens power in these cases are eloquent testimony to the absence of a definitive solution. This review discusses some of the available methods for improving the accuracy for predicting the refractive outcome for these patients. It focuses mainly on the methods available on the American Society of Cataract and Refractive Surgery (ASCRS) online calculator and provides some practical guidelines for cataract surgeons who encounter these challenging cases.

Myopic Laser Corneal Refractive Surgery Reduces Interdevice Agreement in the Measurement of Anterior Corneal Curvature

OBJECTIVES

To investigate interdevice differences and agreement in the measurement of anterior corneal curvature obtained by different technologies after laser corneal refractive surgery.

METHODS

The prospective study comprised 109 eyes of 109 consecutive patients who had undergone laser-assisted in situ keratomileusis (LASIK). Preoperative and postoperative corneal parameters were measured by Scheimpflug imaging (Pentacam), Placido-slit-scanning (Orbscan) and auto-keratometry (IOLMaster). Preoperative and postoperative anterior corneal curvatures (K readings) were compared between devices. Interdevice agreement was evaluated by Bland-Altman analysis.

RESULTS

Preoperatively, the difference of K reading for Pentacam-IOLMaster (0.04±0.20 D) was not statistically significant (P=0.059). The differences between Pentacam-Orbscan and Orbscan-IOLMaster were 0.20±0.34 D (P<0.001) and -0.17±0.29 D (P<0.001), respectively. After surgery, no difference was found for Pentacam-Orbscan (-0.05±0.38, P=0.136). The differences between Pentacam-IOLMaster and Orbscan-IOLMaster were 0.13±0.29 D (P<0.001) and 0.19±0.34 D (P<0.001). Preoperative interdevice agreement (95% limit of agreement [LOA]) between Pentacam and Orbscan, Pentacam and IOLMaster, and Orbscan and IOLMaster were 1.31 D, 0.79 D and 1.14 D, respectively. The 95% LOAs decreased to 1.47 D, 1.14 D, and 1.34 D after refractive surgery.

CONCLUSION

Corneal refractive surgery changed the preoperative and postoperative interdevice differences in corneal curvature measurements and reduced interdevice agreement, indicating that the devices are not interchangeable.

Corneal power evaluation after myopic corneal refractive surgery using artificial neural networks

Background

Efficacy and high availability of surgery techniques for refractive defect correction increase the number of patients who undergo to this type of surgery. Regardless of that, with increasing age, more and more patients must undergo cataract surgery. Accurate evaluation of corneal power is an extremely important element affecting the precision of intraocular lens (IOL) power calculation and errors in this procedure could affect quality of life of patients and satisfaction with the service provided. The available device able to measure corneal power have been tested to be not reliable after myopic refractive surgery.

Methods

Artificial neural networks with error backpropagation and one hidden layer were proposed for corneal power prediction. The article analysed the features acquired from the Pentacam HR tomograph, which was necessary to measure the corneal power. Additionally, several billion iterations of artificial neural networks were conducted for several hundred simulations of different network configurations and different features derived from the Pentacam HR. The analysis was performed on a PC with Intel^® Xeon^® X5680 3.33 GHz CPU in Matlab^® Version 7.11.0.584 (R2010b) with Signal Processing Toolbox Version 7.1 (R2010b), Neural Network Toolbox 7.0 (R2010b) and Statistics Toolbox (R2010b).

Results and conclusions

A total corneal power prediction error was obtained for 172 patients (113 patients forming the training set and 59 patients in the test set) with an average age of 32 ± 9.4 years, including 67% of men. The error was at an average level of 0.16 ± 0.14 diopters and its maximum value did not exceed 0.75 dioptres. The Pentacam parameters (measurement results) providing the above result are tangential anterial/posterior. The corneal net power and equivalent k-reading power. The analysis time for a single patient (a single eye) did not exceed 0.1 s, whereas the time of network training was about 3 s for 1000 iterations (the number of neurons in the hidden layer was 400).

Cataract surgery on the previous corneal refractive surgery patient

Cataract surgery in cases with previous corneal refractive surgery may be a major challenge for the ophthalmologist. The refractive outcome of the case deserves special attention in the preoperative planning process, which should be tailored for the type of prior refractive procedure: incisional, ablative under a flap, or on the corneal surface. Avoiding refractive surprise after cataract surgery in these cases is principally dependent on the accuracy of the intraocular lens calculation, together with the selection of the appropriate biometric formula for each case. Modern techniques for cataract surgery help surgeons to move toward the goal of cataract surgery as a refractive procedure free from refractive error. We give practical guidelines for the cataract surgeon in the management of these challenging cases.

New Scheimpflug camera device in measuring corneal power changes after myopic laser refractive surgery

PURPOSE

To assess the accuracy of a combined Scheimpflug camera-Placido disk device (Sirius, CSO, Italy) in evaluating corneal power changes after myopic photorefractive keratectomy (PRK).

METHODS

Two hundred and thirty-seven eyes of 237 patients that underwent myopic PRK with a refractive error, measured as spherical equivalent, ranging from -10.75 D to -0.5D (mean -4.63 ± 2.21D), were enrolled in this study. Corneal power evaluation using Sirius were performed before, 1, 3 and 6 months after myopic PRK. Mean simulated keratometry (SimK) and mean pupil power (MPP) were measured. Correlations between changes in corneal power, measured with SimK and MPP, and variations in subjective refraction, calculated at corneal plane, were evaluated using Pearson test at every follow up; differences between preoperative and postoperative data were evaluated with the Student paired t-test.

RESULTS

A good correlation has been detected between the variations in subjective refraction measured at corneal plane 1, 3 and 6 months after myopic PRK and both SimK (R(2) = 0.8463; R(2) = 0.8643; R(2) = 0.7102, respectively) and MPP (R(2) = 0.6622; R(2) = 0.5561; R(2) = 0.5522, respectively) but corneal power changes are statistically undervalued for both parameters (p < 0.001).

CONCLUSIONS

Even if our data should be confirmed in further studies, SimK and MPP provided by this new device do not seem to accurately reflect the changes in corneal power after myopic PRK.

Corneal Anterior Power Calculation for an IOL in Post-PRK Patients

PURPOSE

After corneal refractive surgery, there is an overestimation of the corneal power with the devices routinely used to measure it. Therefore, the objective of this study was to determine whether, in patients who underwent photorefractive keratectomy (PRK), it is possible to predict the earlier preoperative anterior corneal power from the postoperative (PO) posterior corneal power. A comparison is made using a formula published by Saiki for laser in situ keratomileusis patients and a new one calculated specifically from PRK patients.

METHODS

The Saiki formula was tested in 98 eyes of 98 patients (47 women) who underwent PRK for myopia or myopic astigmatism. Moreover, anterior and posterior mean keratometry (Km) values from a Scheimpflug camera were measured to obtain a specific regression formula.

RESULTS

The mean (±SD) preoperative Km was 43.50 (±1.39) diopters (D) (range, 39.25 to 47.05 D). The mean (±SD) Km value calculated with the Saiki formula using the 6 months PO posterior Km was 42.94 (±1.19) D (range, 40.34 to 45.98 D) with a statistically significant difference (p < 0.001). Six months after PRK in our patients, the posterior Km was correlated with the anterior preoperative one by the following regression formula: y = -4.9707x + 12.457 (R² = 0.7656), where x is PO posterior Km and y is preoperative anterior Km, similar to the one calculated by Saiki.

CONCLUSIONS

Care should be taken in using the Saiki formula to calculate the preoperative Km in patients who underwent PRK.

Accuracy of Scheimpflug Holladay equivalent keratometry readings after corneal refractive surgery in the absence of clinical history

BACKGROUND AND OBJECTIVE

To identify the most accurate combination of Pentacam's equivalent keratometry readings (EKR) and intraocular lens power formula when the clinical history is unavailable.

PATIENTS AND METHODS

A total of 18 patients underwent cataract surgery after refractive surgery. The Pentacam 4.5- and 3.0-mm EKR were combined with the SRK II, SRK/T, Hoffer-Q, and Holladay I and II formulas.

RESULTS

The smallest deviation from the predicted value was achieved by combining the 4.5 EKR with the Holladay II formula (mean arithmetic deviation, -0.2 ± 0.4 dpt).

CONCLUSION

The 4.5-mm EKR + Holladay II formula can accurately calculate intraocular lens power in patients with previous refractive surgery.

Corneal ray tracing versus simulated keratometry for estimating corneal power changes after excimer laser surgery

PURPOSE

To evaluate whether the refractive changes induced by excimer laser surgery can be accurately measured by corneal ray tracing performed by a combined rotating Scheimpflug camera-Placido-disk corneal topographer (Sirius).

SETTING

Private practices.

DESIGN

Evaluation of diagnostic test.

METHODS

This multicenter retrospective study comprised patients who had myopic or hyperopic excimer laser refractive surgery. Preoperatively and postoperatively, 2 corneal power measurements--simulated keratometry (K) and mean pupil power--were obtained. The mean pupil power was the corneal power calculated over the entrance pupil by ray tracing through the anterior and posterior corneal surfaces using Snell's law. Agreement between the refractive and corneal power change was analyzed according to Bland and Altman. Regression analysis and Bland-Altman plots were used to evaluate agreement between measurements.

RESULTS

The study evaluated 72 eyes (54 patients). The difference between the postoperative and preoperative simulated K values underestimated the refractive change after myopic correction and overestimated it after hyperopic correction. Agreement between simulated K changes and refractive changes was poor, especially for higher amounts of correction. A proportional bias was detected (r = -0.77; P<.0001), and the 95% limits of agreement (LoA) were -0.15 -0.14 × ±0.62 diopters (D). The difference between the postoperative and preoperative mean pupil power showed an excellent correlation with the refractive change (r(2) = 0.98). The mean pupil power did not overestimate or underestimate the refractive change. The 95% LoA ranged between -0.97 D and +0.56 D.

CONCLUSION

Corneal ray tracing accurately measured corneal power changes after excimer laser refractive surgery.

FINANCIAL DISCLOSURES

Dr. Calossi is consultant to Costruzione Strumenti Oftalmici. Dr. Carones is consultant to Wavelight Laser Technologie AG. No other author has a financial or proprietary interest in any material or method mentioned.

Intraocular lens power calculation by ray-tracing after myopic excimer laser surgery

PURPOSE

To investigate the refractive outcomes of intraocular lens (IOL) power calculation by ray-tracing after myopic excimer laser surgery.

DESIGN

Prospective, interventional case series.

METHODS

setting: Multicenter study. participants: Twenty-one eyes of 21 patients undergoing phacoemulsification and IOL implantation after myopic laser in situ keratomileusis or photorefractive keratectomy were enrolled. intervention: IOL power calculation was performed using internal software of a Scheimpflug camera combined with a Placido disc corneal topographer (Sirius; CSO). Exact ray-tracing was carried out after the axial length (measured either by immersion ultrasound biometry or partial coherence interferometry), target refraction, and pupil size had been entered. main outcome measures: Median absolute error, mean absolute error, and mean arithmetic error in refraction prediction, that is, the difference between the expected refraction (as calculated by the software) and the actual refraction 1 month after surgery.

RESULTS

The mean postoperative refraction was -0.43 ± 1.08 diopters (D), with a range between -1.28 and 0.85 D. The mean arithmetic error was -0.13 ± 0.49 D. The median and mean absolute errors were +0.25 D and 0.36 D, respectively. Also, 71.4% of the eyes were within ± 0.50 D of the predicted refraction, 85.7% were within ± 1.00 D, and 100% within ± 1.50 D.

CONCLUSIONS

Ray-tracing can calculate IOL power accurately in eyes with prior myopic laser in situ keratomileusis and photorefractive keratectomy, with no need for preoperative data.

Scheimpflug analysis of corneal power changes after myopic excimer laser surgery

PURPOSE

To assess the ability of corneal power measurements by a rotating Scheimpflug camera to measure the refractive change induced by myopic excimer laser surgery.

SETTING

G.B. Bietti Foundation-IRCCS, Rome, Italy.

DESIGN

Evaluation of diagnostic test.

METHODS

The following corneal power measurements by the Pentacam Scheimpflug camera were analyzed: average keratometry (K), true net power (calculated by Gaussian optics formula), and total corneal refractive power (TCRP) at 2.0 mm, 3.0 mm, and 4.0 mm, calculated by ray tracing on a ring and as the average of the zone inside the ring. The difference between the preoperative and postoperative values was compared with the subjective surgically induced refractive change (SIRC) and with the difference between the preoperative and the postoperative anterior corneal power measured by Placido corneal topography (Keratron).

RESULTS

In 36 consecutive eyes, the average K significantly underestimated the SIRC as determined by subjective refraction (-4.47 diopters [D] ± 1.81 [SD]) and corneal topography (-4.38 ± 1.81 D). The 3.0 mm and 4.0 mm ring total corneal refractive power significantly overestimated the SIRC. The remaining values did not show statistically significant differences with respect to the SIRC. The 3.0 mm zone TCRP and the 2.0 mm ring TCRP provided the lowest median difference compared with the SIRC (-0.07 D and -0.17 D, respectively) and the closest agreement.

CONCLUSIONS

The corneal power values provided by the Scheimpflug camera accurately reflected the SIRC after myopic excimer laser surgery. The best options seem to be the 3.0 mm zone TCRP and the 2.0 mm ring TCRP.

A new central-peripheral corneal curvature method for intraocular lens power calculation after excimer laser refractive surgery

PURPOSE

To propose the central-peripheral (C-P) method, which requires no data history to calculate intraocular lens (IOL) powers for eyes that underwent laser in situ keratomileusis (LASIK), and compare the accuracy of the C-P method with other IOL formulas for eyes after LASIK.

METHODS

Sixteen patients with cataract (25 eyes) who underwent myopic LASIK were analysed retrospectively. The C-P method is a modified double-K method using the SRK/T formula, in which the estimated pre-LASIK keratometric power calculated from the post-LASIK peripheral anterior sagittal power (also called the axial power) is used for the Kpre in the double-K method using the SRK/T formula, and the post-LASIK anterior sagittal power is used for the Kpost. We compared the accuracy of the C-P method with other popular IOL calculation formulas for use in eyes after LASIK.

RESULTS

The median values of the arithmetic and absolute prediction errors with the C-P method were 0.11 diopter (D) (range, -1.67 to 1.97 D) and 0.55 D (range, 0.02-1.97 D), respectively. The prediction error using the C-P method was within ±0.5 D in 48% of eyes, within -1.0 to +0.5 D in 60% of eyes, and within ±1.0 D in 68% of eyes. The C-P method resulted in a significantly higher percentage of eyes within ±0.5 D than the BESSt formula, Shammas-PL formula, true net power method, double-K method using 43.5 D for Kpre, and Feiz-Mannis method.

CONCLUSION

The C-P method may be a good option for calculating IOL powers in eyes undergoing cataract surgery after LASIK when the preoperative LASIK data are unavailable.

Comparison of methods to measure corneal power for intraocular lens power calculation using a rotating Scheimpflug camera

PURPOSE

To assess the accuracy of corneal power measurements provided by a Scheimpflug camera (Pentacam HR) for intraocular lens (IOL) power calculation in unoperated eyes and compare the results with those of simulated keratometry (SimK) performed with a Placido-disk corneal topographer (Keratron).

SETTING

Private practice.

DESIGN

Evaluation of diagnostic test.

METHODS

Eight Scheimpflug camera corneal power measurements were analyzed: (1) average K, (2) true net power calculated using the Gaussian optics formula, (3) total corneal refractive power at 2.0 mm calculated by ray tracing on a ring and (4) as the average of the zone inside the ring, (5) total corneal refractive power at 3.0 mm on a ring and (6) as the average of the zone inside the ring, (7) the equivalent K reading at 3.0 mm and (8) at 4.5 mm. The IOL power was calculated using the Hoffer Q, Holladay 1, and SRK/T formulas.

RESULTS

No statistically significant differences were observed between any corneal power measurements, including simulated K, in 41 consecutive patients. The latter showed slightly lower mean absolute errors with all 3 formulas (range 0.26 to 0.27 diopter [D]). The Scheimpflug camera gave the lowest median absolute errors with all formulas; that is, the 3.0 mm equivalent K reading with the Hoffer Q formula (0.18 D) and Holladay 1 formula (0.17 D) and the 2.0 mm total corneal refractive power ring with the SRK/T formula (0.18 D).

CONCLUSION

Corneal power measurements provided by the Scheimpflug camera and Placido disk corneal topographer displayed comparable accuracy in IOL power calculation.

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.

Intraocular lens power calculations after myopic laser refractive surgery: a comparison of methods in 173 eyes

PURPOSE

To evaluate and compare published methods of intraocular lens (IOL) power calculation after myopic laser refractive surgery in a large, multi-surgeon study.

DESIGN

Retrospective case series.

PARTICIPANTS

A total of 173 eyes of 117 patients who had uneventful LASIK (89) or photorefractive keratectomy (84) for myopia and subsequent cataract surgery.

METHODS

Data were collected from primary sources in patient charts. The Clinical History Method (vertex corrected to the corneal plane), the Aramberri Double-K, the Latkany Flat-K, the Feiz and Mannis, the R-Factor, the Corneal Bypass, the Masket (2006), the Haigis-L, and the Shammas.cd postrefractive adjustment methods were evaluated in conjunction with third- and fourth-generation optical vergence formulas, as appropriate. Intraocular lens power required for emmetropia was back-calculated using stable post-cataract surgery manifest refraction and implanted IOL power, and then formula accuracy was compared.

MAIN OUTCOME MEASURES

Prediction error arithmetic mean ± standard deviation (SD), range (minimum and maximum), and percent within 0 to -1.0 diopters (D), ±0.5 D, ±1.0 D, and ±2.0 D relative to target refraction.

RESULTS

The top 5 corneal power adjustment techniques and formula combinations in terms of mean prediction errors, standard deviations, and minimizing hyperopic "refractive surprises" were the Masket with the Hoffer Q formula, the Shammas.cd with the Shammas-PL formula, the Haigis-L, the Clinical History Method with the Hoffer Q, and the Latkany Flat-K with the SRK/T with mean arithmetic prediction errors and standard deviations of -0.18±0.87 D, -0.10±1.02 D, -0.26±1.13 D, -0.27±1.04 D, and -0.37±0.91 D, respectively.

CONCLUSIONS

By using these methods, 70% to 85% of eyes could achieve visual outcomes within 1.0 D of target refraction. The Shammas and the Haigis-L methods have the advantage of not requiring potentially inaccurate historical information.

Origins of the keratometer and its evolving role in ophthalmology

The keratometer, or ophthalmometer as it was originally known, had its origins in the attempt to discover the seat of accommodation in the eye. Since that early beginning, it has been re-invented a number of times, with improvements and modifications made in the original principles of its design for new applications that arose as ophthalmology advanced. The cornea is not only responsible for the majority of the refraction in the eye, but is also readily accessible for measurement and modification. The keratometer's ability to measure the cornea has allowed it to play a central role in critical advances in ophthalmic history. This review describes the origins and principles of this instrument, the novel applications that led to the keratometer's continued resurgences over its nearly 250-year history, and the modern devices that have borrowed its basic principles and are beginning to replace it in common clinical practice.

Intraocular lens power calculation after laser refractive surgery: corrective algorithm for corneal power estimation

PURPOSE

To evaluate an algorithm for corneal power estimation in intraocular lens (IOL) power calculation after myopic laser refractive surgery using direct corneal measurements.

SETTING

International Vision Correction Research Centre, University of Heidelberg, Heidelberg, Germany.

METHODS

Corneal parameters in normal eyes and eyes of refractive surgery cases were evaluated by rotating Scheimpflug imaging. Corneal optical power (K(optical)) calculated by a Gaussian optics formula was simplified as K(optical) = K(anterior) + K(2) (K(anterior) = anterior corneal power; K(posterior) = posterior corneal power; K(2) = K(posterior)--K(anterior) x K(posterior) x corneal thickness/1.376). The variation and change in K(2) induced by refractive surgery were analyzed. A corrective algorithm to calculate K(optical) using mean K(2) (-6.10 diopters [D]), K(corrective) = 1.114 x measured K - 6.10, was derived based on statistical analysis, which was in accordance with the modified Maloney method. The IOL power after refractive surgery was calculated using K(corrective).

RESULTS

The mean K(2) of normal and post-refractive corneas was -6.10 +/- 0.23 D and -6.16 +/- 0.17 D, respectively (P = .17). The mean refractive surgery-induced change in K(2) was -0.06 +/- 0.10 D. The variations in K(2) were small (95% confident interval, -6.55 to -5.65 [normal cornea]; -6.48 to -5.70 [pre-refractive]; - 6.49 to -5.83 [post-refractive)]. Using K(corrective) for IOL power calculation in post-refractive cases yielded mean absolute prediction errors of 0.58 +/- 0.52 D (Haigis), 0.59 +/- 0.49 D (double-K Hoffer Q), and 0.58 +/- 0.47 D (double-K SRK/T).

CONCLUSION

The algorithm that induced low error in corneal power estimation was relatively reliable in IOL calculation after myopic laser refractive surgery.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Cataract surgery after refractive surgery

PURPOSE OF REVIEW

To review recent contributions addressing the challenge of intraocular lens (IOL) calculation in patients undergoing cataract extraction following corneal refractive surgery.

RECENT FINDINGS

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.

SUMMARY

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.