Modified double-K method for intraocular lens power calculation after excimer laser corneal refractive surgery

Intraocular Lens Power Calculation in Eyes After Myopic Laser Refractive Surgery and Radial Keratotomy: Bayesian Network Meta-analysis

PURPOSE

To compare the accuracy of formulas for calculating intraocular lens power in eyes after myopic laser refractive surgery or radial keratotomy.

DESIGN

Bayesian network meta-analysis.

METHODS

PubMed, Embase, the Cochrane Data Base of Systematic Reviews, and the Cochrane Central Register of Controlled Trials databases were searched for retrospective and prospective clinical studies published from January 1, 2012, to August 24, 2022. The outcome measurement was the percentage of eyes with a predicted error within the target refractive range (±0.50 diopter [D] or ±1.00 D).

RESULTS

Our meta-analysis includes 24 studies of 1172 eyes after myopic refractive surgery that use 12 formulas for intraocular lens power calculation. (1) A network meta-analysis showed that Barrett true-K no history, the optical coherence tomography (OCT) formula, and the Masket formula had a significantly higher percent of eyes within ±0.50 D of the goal than the Haigis-L formula, whereas the Wang-Koch-Maloney formula showed the poor predictability. Using an error criterion of within ±1.00 D, the same 3 formulas performed slightly better than the Haigis-L formula. Based on performance using both prediction error criteria, the Barrett true-K no history formula, OCT formula, and Masket formula showed the highest probability of ranking as the top 3 among the 12 methods. (2) A direct meta-analysis with a subset of 4 studies and 5 formulas indicated that formulas did not differ in percent success for either the ±0.5 D or ±1.0 D error range in eyes that had undergone radial keratotomy.

CONCLUSIONS

The OCT, Masket, and Barrett true-K no history formulas are more accurate for eyes with previous myopic laser refractive surgery, whereas no significant difference was found among the formulas for eyes that had undergone radial keratotomy.

Accuracy of the PEARL-DGS Formula for Intraocular Lens Power Calculation in Post-Myopic Laser Refractive Corneal Surgery Eyes

PURPOSE

To investigate the accuracy of the PEARL-DGS formula for intraocular lens (IOL) power calculation in post-myopic laser refractive corneal surgery eyes.

DESIGN

Retrospective case series.

METHODS

A total of 139 eyes of 139 patients (mean axial length: 27.4 ± 2.1 mm) who had prior myopic laser refractive corneal surgery and subsequent cataract surgery using Tecnis ZCB00 from March 2018 to February 2023 were included. Refractive outcomes of 5 formulas (Barrett True K, Haigis-L, Hoffer-QST, PEARL-DGS, and Shammas-PL) were evaluated. Prediction error was defined as the difference between the measured and predicted postoperative refractive spherical equivalent using the IOL power actually implanted. Mean prediction error (MPE), median absolute prediction error (MedAE), and mean absolute prediction error were calculated.

RESULTS

Without constant optimization, the PEARL-DGS resulted in a MPE of +0.05 ± 0.65 diopters (D), whereas the other formulas resulted in myopic shifts. The MedAEs of the formulas were 0.39, 0.53, 0.65, 0.85, and 1.11 D for the PEARL-DGS, Hoffer-QST, Barrett True K, Shammas-PL, and Haigis-L, respectively, in order of magnitude (P < .05). With constant optimization, there were no statistically significant differences in the MedAEs among the 5 formulas (P = .388).

CONCLUSIONS

In comparison to other IOL formulas, the PEARL-DGS resulted in better refractive outcomes after cataract surgery in post-myopic laser refractive corneal surgery eyes without constant optimization. We suggest that PEARL-DGS be considered as the first choice for IOL power calculation in these eyes when the clinicians do not have their optimized constants.

Reply to Cione et al. Comment on "Iida et al. 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. J. Clin. Med. 2022, 11, 522"

We appreciate the insightful comments [...].

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.

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.

Corneal Topography for Intraocular Lens Selection in Refractive Cataract Surgery

The purpose of this review is to evaluate the usefulness of corneal topography to select premium intraocular lenses (IOLs), including aspherical IOLs, toric IOLs, and multifocal IOLs, in refractive cataract surgery. Corneal topography can detect corneal regular astigmatism, corneal irregular astigmatism (higher-order aberrations [HOAs]) including spherical aberration, and corneal shape abnormalities after corneal refractive surgery. Surgeons can explain to the patients with significant corneal HOAs about its effect on postoperative visual function before surgery. Multifocal IOLs should not be selected for such eyes. For eyes with abnormal corneal shape, appropriate IOL power calculation formulae can be applied. In the case of toric IOLs, regular astigmatism and corneal HOAs should be checked. Before implanting an aspheric IOL, it is ideal to confirm spherical aberration of the cornea is not below the normal range. Because corneal HOAs, abnormal corneal shape after corneal refractive surgery, corneal regular astigmatism, and corneal spherical aberration increase postoperative refractive errors and poor vision quality with premium IOLs, corneal topography before cataract surgery is helpful in screening patients who are not appropriate candidates for premium IOLs.

Accuracy of intraocular lens formulas using total keratometry in eyes with previous myopic laser refractive surgery

Objectives

This comparative study aimed to determine if total keratometry (TK) from IOLMaster 700 could be applied to conventional formulas to perform IOL power calculation in eyes with previous myopic laser refractive surgery, and to evaluate their accuracy with known post-laser refractive surgery formulas.

Methods

Sixty-four eyes of 49 patients with previous myopic laser refractive surgery were evaluated 1 month after cataract surgery. A comparison of the prediction error was made between no clinical history post-laser refractive surgery formulas (Barrett True-K, Haigis-L, Shammas-PL) and conventional formulas (EVO, Haigis, Hoffer Q, Holladay I, and SRK/T) using TK values obtained with the optical biometer IOLMaster 700 (Carl Zeiss Meditec), as well as Barrett True-K with TK.

Results

The mean prediction error was statistically different from zero for Barrett True-K, Barrett True-K with TK, Haigis-L, Shammas-PL, and Holladay I with TK. The mean absolute error (MAE) was 0.424, 0.671, 0.638, 0.439, 0.408, 0.424, 0.479, 0.647, and 0.524, and median absolute error (MedAE) was 0.388, 0.586, 0.605, 0.298, 0.294, 0.324, 0.333, 0.438, and 0.377 for Barrett True-K, Haigis-L, Shammas-PL, Barrett True-K TK, EVO with TK, Haigis with TK, Hoffer Q with TK, Holladay I with TK, and SRK/T with TK, respectively. EVO TK followed by Barrett True-K TK and Haigis TK achieved the highest percentages of patients with absolute prediction error within 0.50 and 1.00 D (68.75%, 92.19%, and 64.06%, 92.19%, respectively)

Conclusions

Formulas combined with TK achieve similar or better results compared to existing no-history post-myopic laser refractive surgery formulas.

Network Meta-analysis of No-History Methods to Calculate Intraocular Lens Power in Eyes With Previous Myopic Laser Refractive Surgery

PURPOSE

To systematically compare and rank the predictability of no-history intraocular lens (IOL) power calculation methods after myopic laser refractive surgery.

METHODS

PubMed, Embase, the Cochrane Library, and the U.S. trial registry (www.ClinicalTrial.gov) were used to systematically search trials published up to August 2019. Included were case series studies reporting the following outcomes in patients with cataract undergoing phacoemulsification after laser refractive surgery: percentage of eyes with a refractive prediction error (PE) within ±0.50 and ±1.00 diopters (D), mean absolute error (MAE), and median absolute error (MedAE). A network meta-analysis was conducted using the STATA software version 13.1 (STATACorp LLC).

RESULTS

Nineteen studies involving 1,098 eyes and 19 formulas were identified. A network meta-analysis for the percentage of eyes with a PE within ±0.50 D found that ray-tracing (Okulix), intraoperative aberrometry (Optiwave Refractive Analysis [ORA]), BESSt, and Seitz/Speicher/Savini (Triple-S) (D-K SRK/T), and Fourier-Domain OCT-Based formulas were more predictive than the Wang/Koch/Maloney, Shammas-PL, modified Rosa, Ferrara, and Equivalent K reading at 4.5 mm using the Double-K Holladay 1 formulas. With regard to ranking, the top four formulas as per the surface under the cumulative ranking curve (SUCRA) values for the percentage of eyes with a PE within ±0.50 D were the Okulix, ORA, BESSt, and Triple-S (D-K SRK/T). With regard to MAE, the ORA showed lower errors when compared to the Shammas-PL formula. In this regard, the top four formulas based on the SUCRA values were the Triple-S, BESSt, ORA, and Fourier-Domain OCT-Based formulas. The SToP (SRK/T), ORA, Fourier-Domain OCT-Based, and BESSt formulas had the lowest MedAE.

CONCLUSIONS

Considering all three outcome measures of highest percentages of eyes with a PE within ±0.50 and ±1.00 D, lowest MAE, and lowest MedAE, the top three no-history formulas for IOL power calculation in eyes with previous myopic corneal laser refractive surgery were: ORA, BESSt, and Triple-S (D-K SRK/T). [J Refract Surg. 2020;36(7):481-490.].

Accuracy of intraocular lens power calculation formulae after laser refractive surgery in myopic eyes: a meta-analysis

Background

To compare the accuracy of intraocular lens power calculation formulae after laser refractive surgery in myopic eyes.

Methods

We searched the databases on PubMed, EMBASE, Web of Science and the Cochrane library to select relevant studies published between Jan 1st, 2009 and Aug 11th, 2019. Primary outcomes were the percentages of refractive prediction error within ±0.5 D and ±1.0 D.

Results

The final meta-analysis included 16 studies using seven common methods (ASCRS average, Barrett True-K no history, Double-K SRK/T, Haigis-L, OCT formula, Shammas-PL, and Wang-Koch-Maloney). ASCRS average yielded significantly higher percentage of refractive prediction error within ±0.5 D than Haigis-L, Shammas-PL and Wang-Koch-Maloney (P = 0.009, 0.01, 0.008, respectively). Barrett True-K no history also yielded significantly higher percentage of refractive prediction error within ±0.5 D than Shammas-PL and Wang-Koch-Maloney (P = 0.01, P < 0.0001, respectively), and a similar result was found when comparing OCT formula with Haigis-L and Shammas-PL (P = 0.03, P = 0.01, respectively).

Conclusion

The ASCRS average or Barrett True-K no history should be used to calculate the intraocular lens power in eyes after myopic laser refractive surgery. The OCT formula if available, can also be a good alternative choice.

Comparison of the Accuracy of Newer Intraocular Lens Power Calculation Methods in Eyes That Underwent Previous Phototherapeutic Keratectomy

PURPOSE

To evaluate the accuracy of intraocular lens (IOL) power calculations using ray tracing software in patients who had undergone phototherapeutic keratectomy (PTK).

METHODS

In this retrospective case series, 37 eyes of 22 patients (mean age: 69.4 years; range: 56 to 85 years) who underwent cataract surgery after PTK were reviewed. The prediction error, defined as the difference between the estimated postoperative spherical equivalent and the postoperative manifest refraction at the spectacle plane, was calculated using the following formulas: OKULIX (Tedics, Dortmund, Germany), PhacoOptics (IOL Innovations ApS, Aarhus, Denmark), Barrett True K No History (NH), and Camellin-Calossi. The PhacoOptics formula was used in three different ways: historical method (H), no history method (NH), and C-constant method (C). The median values of the arithmetic and absolute prediction errors among these six IOL calculation methods were compared.

RESULTS

The median arithmetic errors (in diopters [D]) and percentages of eyes within ±0.50 D of the absolute errors were as follows: OKULIX (0.33, range: -2.20 to 2.50, 30.6%), PhacoOptics (H) (-0.12, range: -3.28 to 4.85, 22.2%), PhacoOptics (NH) (-0.25, range: -2.08 to 1.70, 48.4%), PhacoOptics (C) (0.04, range: -1.40 to 2.18, 48.5%), Barrett True K (NH) (-0.35, range: -1.90 to 1.89, 48.6%), and Camellin-Calossi (-0.19, range: -1.78 to 1.47, 59.5%).

CONCLUSIONS

The PhacoOptics, especially the C-constant method (C), and Camellin-Calossi formulas were good options for calculating IOL powers in eyes that underwent PTK. [J Refract Surg. 2019;35(5):310-316.].

Calculation of Unknown Preoperative K Readings in Postrefractive Surgery Patients

Purpose

To determine the unknown preoperative K readings (Kpre) to be used in history-based methods, for intraocular lens (IOL) power calculation in patients who have undergone myopic photorefractive keratectomy (PRK).

Methods

A regression formula generated from the left eyes of 174 patients who had undergone PRK for myopia or for myopic astigmatism was compared with other methods in 168 right eyes. The Pearson index and paired t-test were utilized for statistical analysis.

Results

The differences between Kpre and those obtained with the other methods were as follows: 0.61 ± 0.94 D (range: −3.94 to 2.05 D, p < 0.01) subtracting the effective treatment, 0.01 ± 0.86 D (range: −2.61 to 2.34 D, p = 0.82) with Rosa's formula, −0.02 ± 1.31 D (range: −3.43 to 3.68 D, p = 0.82) with the current study formula, and −0.43 ± 1.40 D (range: −3.98 to 3.12 D, p < 0.01) utilizing a mean K (Km) of 43.5 D.

Conclusions

These formulas may permit the utilization of history-based methods, that is, the double-K method in calculating the IOL power following PRK when Kpre are unknown.

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).

Comparison of the accuracy of intraocular lens power calculations for cataract surgery in eyes after phototherapeutic keratectomy

PURPOSE

To compare the accuracy of several methods of intraocular lens (IOL) power calculations used for cataract surgery in eyes treated with phototherapeutic keratectomy (PTK) that results in changes in the anterior corneal surface and axial length; these results make power calculations less predictable.

METHODS

We evaluated the medical records of 23 eyes of 13 patients (mean age, 68.8 years; range 62-80 years) who underwent cataract surgery after PTK at Keio University Hospital, Tokyo, Japan. The prediction error, defined as the difference between the estimated postoperative spherical equivalent and the postoperative manifest refraction at the spectacle plane, was calculated using five formulas: SRK/T, Haigis-L, Shammas-PL, Camellin-Calossi, and OKULIX ray tracing software. We compared the median values of the arithmetic and absolute prediction errors among the five formulas.

RESULTS

The median arithmetic errors after cataract surgery for the five formulas were 0.70 D (diopter) (range -0.41 to 2.78), -0.96 D (range -2.14 to 0.81), -0.81 D (range -1.89 to 1.15), -0.04 D (range -1.35 to 1.47), and 0.68 D (range -0.61 to 2.50), respectively.

CONCLUSION

The Camellin-Calossi formula is a good option for calculating IOL powers in eyes that underwent PTK.

Evaluation of Equivalent Keratometry Readings Obtained by Pentacam HR (High Resolution)

Purpose

To assess the repeatability of Equivalent Keratometry Readings (EKRs) obtained by the Pentacam HR (high resolution) in untreated and post-LASIK eyes, and to compare them with the keratometry (K) values obtained by other algorithms.

Methods

In this prospective study, 100 untreated eyes and 71 post-LASIK eyes were included. In the untreated group, each eye received 3 consecutive scans using the Pentacam HR, and EKR values in all central corneal zone, the true net power (K_net) and the simulated K (SimK) were obtained for each scan. In the post-LASIK group, each eye received subjective refraction and 3 consecutive scans with the Pentacam HR preoperatively. During the 3-month post-surgery exam, the same examinations and the use of an IOLMaster were conducted for each eye. The EKRs in all zone, the K_net, the mean K (K_m) by IOLMaster and the K values by clinical history method (K_CHM) were obtained. The repeatability of the EKRs was assessed by the within-subject standard deviation (S_w), 2.77S_w, coefficient of variation (CV_w) and intraclass correlation coefficient (ICC). The bonferroni corrected multiple comparisons were performed to analyze the differences among the EKRs and K values calculated by other algorithms within the 2 groups. The 95% limits of agreement (LoA) were calculated.

Results

The EKR values in all central corneal zone were repeatable in both the untreated group (S_w≦0.19 D, 2.77S_w≦0.52 D, CV_w≦1%, ICC≧0.978) and the post-LASIK group (S_w≦0.22 D, 2.77S_w≦0.62 D, CV_w≦1%, ICC≧0.980). In the untreated group, the EKR in 4mm zone was close to SimK (P = 1.000), and the 95% LoA was (-0.13 to 0.15 D). The difference between K_net and SimK was -1.30±0.13 D (95% LoA -1.55 to -1.55 D, P<0.001). In the post-LASIK group, all the EKRs were significantly higher than K_CHM (all P<0.001). The differences between the EKR in 4mm zone and K_CHM, the EKR in 7mm zone and K_CHM, K_net and K_CHM, K_m and K_CHM, SimK and K_net were 0.64±0.50 D (95% LoA, -0.33 to 1.62 D), 1.77±0.88 D (95% LoA, 0.04 to 3.51 D), -0.98±0.48 D (95% LoA, -1.92 to -0.04 D), 0.64±0.53 D (95% LoA, -0.40 to 1.68 D), and 1.73±0.20 D (95% LoA, 1.33 to 2.13 D), respectively.

Conclusions

The EKRs obtained by the Pentacam HR were repeatable in both untreated eyes and post-LASIK eyes. Compared to the total corneal power obtained by the clinical history method, the EKR values generally overestimated the total corneal power in post-LASIK eyes. So, further calibrations for the EKR values should be conducted, before they were used for the total corneal power assessment in post-LASIK eyes.

Demographics of patients having cataract surgery after laser in situ keratomileusis

PURPOSE

To retrospectively assess the demographics of patients having cataract surgery in eyes with previous laser in situ keratomileusis (LASIK).

SETTING

Department of Ophthalmology, Kitasato University, Kanagawa, and Sanno Hospital, Tokyo, Japan.

DESIGN

Retrospective case series.

METHODS

This study evaluated eyes of consecutive patients scheduled for cataract surgery after previous LASIK (Group 1). The control groups comprised eyes with axial lengths (ALs) matched with ALS in Group 1 (Group 2) and all eyes scheduled for cataract surgery (Group 3). Assessed were age, sex, corrected distance visual acuity, manifest refraction, keratometry (K) readings, corneal astigmatism, and corneal higher-order aberrations (HOAs).

RESULTS

Group 1 comprised 40 eyes of 40 patients; Group 2, 606 eyes of 606 patients; and Group 3, 3642 eyes of 3642 patients. The mean age at cataract surgery of patients in Group 1 was 54.6 years ± 8.1 (SD), which was significantly younger than in Group 2 (by approximately 10 years) and Group 3 (by approximately 15 years) (P < .001, Student t test). In Group 1, 70.0% of patients were men, a significantly higher percentage than in Groups 2 and 3 (P < .05, Fisher exact test). The rate of corneal HOAs was significantly higher in Group 1 than in Groups 2 and 3 (P < .05, Student t test). There were no significant differences in other demographics except in K readings.

CONCLUSION

A long AL and an increase in corneal HOAs might contribute to a tendency for cataract surgery to be performed earlier in eyes in which LASIK has been performed.

FINANCIAL DISCLOSURE

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

Intraocular lens power calculation following laser refractive surgery

Refractive outcomes following cataract surgery in patients that have previously undergone laser refractive surgery have traditionally been underwhelming. This is related to several key issues including the preoperative assessment (keratometry) and intraocular lens power calculations. Peer-reviewed literature is overwhelmed by the influx of methodology to manipulate the corneal or intraocular lens (IOL) powers following refractive surgery. This would suggest that the optimal derivative formula has yet been introduced. This review discusses the problems facing surgeons approaching IOL calculations in these post-refractive laser patients, the existing formulae and programs to address these concerns. Prior published outcomes will be reviewed.

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.

IOL power calculation after corneal refractive surgery

PURPOSE

To describe the different formulas that try to overcome the problem of calculating the intraocular lens (IOL) power in patients that underwent corneal refractive surgery (CRS).

METHODS

A Pubmed literature search review of all published articles, on keyword associated with IOL power calculation and corneal refractive surgery, as well as the reference lists of retrieved articles, was performed.

RESULTS

A total of 33 peer reviewed articles dealing with methods that try to overcome the problem of calculating the IOL power in patients that underwent CRS were found. According to the information needed to try to overcome this problem, the methods were divided in two main categories: 18 methods were based on the knowledge of the patient clinical history and 15 methods that do not require such knowledge. The first group was further divided into five subgroups based on the parameters needed to make such calculation.

CONCLUSION

In the light of our findings, to avoid postoperative nasty surprises, we suggest using only those methods that have shown good results in a large number of patients, possibly by averaging the results obtained with these methods.

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.