Total Corneal Power Research Articles

Repeatability and agreement of two swept-source optical coherence tomographers and scheimpflug imaging for measurements of corneal curvature

PurposeTo evaluate the repeatability and agreement of two swept-source optical coherence tomographers and Scheimpflug imaging for corneal curvature in healthy subjects to obtain data on the clinical application of a new device.MethodsThis prospective study was conducted in January and February 2021 with 100 healthy subjects at the Eye Hospital of Wenzhou Medical University. Simulated keratometry (Sim-K), posterior keratometry (Kp), total corneal power (TCP), and total corneal astigmatism (TCA) were measured with CASIA2, Anterion, and Pentacam. Within-subject standard deviation (Sw), repeatability coefficient (RC), coefficient of variation (CoV), and intraclass correlation coefficient (ICC) were used to assess inter-device repeatability. Bland–Altman analysis was performed to determine inter-device agreement.ResultsThe three devices showed good repeatability for Sim-K, Kp, and TCP with all ICC > 0.980. Pentacam showed the highest repeatability (ICC ≥ 0.993; ICC ≥ 0.993) while the CASIA2 demonstrated the lowest repeatability (ICC: ≥ 0.986; ICC: ≥ 0.985) for Sim-K and TCP. Anterion and CASIA2 revealed better repeatability (ICC ≥ 0.998; ICC ≥ 0.981) for Kp than Pentacam (ICC ≥ 0.980). Pentacam and Anterion showed good repeatability for TCA (ICC: 0.935 and 0.916), whereas the CASIA2 showed moderate repeatability (ICC: 0.836). Three instruments demonstrated good agreement with the maximum absolute 95% Limits of agreement (LoA) of 1.00 D for Sim-K, Kp, and TCP. Wide LoA were found for TCA with the maximum absolute 95% LoA ≥ 0.66 D between the three devices.ConclusionsIn healthy subjects, the three devices (Pentacam, Anterion and CASIA2) displayed comparable repeatability and accuracy for SimK, Kp, and TCP, and could be used interchangeably for these parameters. However, TCA measured by the three devices was not interchangeable.Trial registrationChinese Clinical Trial Registry Center (10/10/2020, ChiCTR2000038959).

The Homburg-Adelaide toric IOL nomogram: How to predict corneal power vectors from preoperative IOLMaster 700 keratometry and total corneal power in toric IOL implantation.

The purpose of this study is to compare the reconstructed corneal power (RCP) by working backwards from the post-implantation spectacle refraction and toric intraocular lens power and to develop the models for mapping preoperative keratometry and total corneal power to RCP. Retrospective single-centre study involving 442 eyes treated with a monofocal and trifocal toric IOL (Zeiss TORBI and LISA). Keratometry and total corneal power were measured preoperatively and postoperatively using IOLMaster 700. Feedforward neural network and multilinear regression models were derived to map keratometry and total corneal power vector components (equivalent power EQ and astigmatism components C0 and C45) to the respective RCP components. Mean preoperative/postoperative C0 for keratometry and total corneal power was -0.14/-0.08 dioptres and -0.30/-0.24 dioptres. All mean C45 components ranged between -0.11 and -0.20 dioptres. With crossvalidation, the neural network and regression models showed comparable results on the test data with a mean squared prediction error of 0.20/0.18 and 0.22/0.22 dioptres2 and on the training data the neural network models outperformed the regression models with 0.11/0.12 and 0.22/0.22 dioptres2 for predicting RCP from preoperative keratometry/total corneal power. Based on our dataset, both the feedforward neural network and multilinear regression models showed good precision in predicting the power vector components of RCP from preoperative keratometry or total corneal power. With a similar performance in crossvalidation and a simple implementation in consumer software, we recommend implementation of regression models in clinical practice.

Comparison of keratometry and total corneal power, as measured by an SS-OCT-based optical biometer, for intraocular lens power calculation in Asian eyes.

To investigate whether standard keratometry (K) or total corneal power (TCP) leads to more accurate refractive outcomes for intraocular lens (IOL) power calculation. Public hospital. Retrospective evaluation of a diagnostic test instrument. Preoperatively, all patients underwent optical biometry with Anterion, a swept-source optical coherence tomography device providing both K and TCP. The same IOL model was implanted in all cases. The whole sample was divided into a training dataset, used to optimize the formula constants, and a testing dataset, used to investigate the spherical equivalent prediction error (SEQ-PE) of 8 IOL power formulas. Trueness, precision, and accuracy were evaluated by means of the robust 2-sample t test. Cochran Q test was performed to assess whether the percentage of eyes with an SEQ-PE within each threshold was significantly different; in such an event, the McNemar test was then applied. Both the training and testing datasets included 317 eyes. No significant differences were detected for trueness because of constant optimization. Precision and accuracy were better when K was entered, although a statistically significant difference was observed only with the EVO (precision: P = .02 and accuracy: P = .03) and Haigis ( P < .01 for both precision and accuracy) formulas. No significant differences were observed for the percentage of eyes with an absolute SEQ-PE within any threshold. With most formulas, IOL power calculation is not more accurate when TCP is used instead of K.

Repeatability of Automatic Measurements by an Anterior Segment Swept-Source OCT Biometer in Patients With Keratoconus.

To evaluate the repeatability of automatic measurements of a new anterior segment optical coherence tomographer (ANTERION; Heidelberg Engineering) and their agreement with an anterior segment optical coherence tomography device combined with Placido disc corneal topography (MS-39; CSO) in patients affected by keratoconus. Fifty-four consecutive patients were included. Three measurements were performed with the ANTERION and one with the MS-39. Repeatability was assessed by means of within-subject standard deviation, coefficient of variation (CoV), and intraclass correlation coefficient (ICC). Agreement was investigated with the 95% limits of agreement. The paired t-test and Wilcoxon matched-pairs test were performed to compare the measurements of the different devices. Repeatability of ANTERION measurements was high, with an ICC greater than 0.98 for all parameters. Many parameters revealed a CoV of less than 1% and a CoV within 5% was obtained for astigmatism measurements. The ANTERION measured a significantly higher corneal power and the MS-39 more negative posterior keratometric values. These differences were mirrored by a moderate agreement for mean simulated keratometry and poor agreement for total corneal power and posterior keratometry. The ANTERION revealed high repeatability of automatic measurements and good agreement with the MS-39 for many parameters in patients affected by keratoconus, but for most parameters the two instruments cannot be considered interchangeable. [J Refract Surg. 2024;40(7):e445-e452.].

Total and simulated keratometry measurements using IOLMaster 700 and Pentacam AXL after small incision lenticule extraction.

Calculating the intraocular lens (IOL) in patients after corneal refractive surgery presents a challenge. Because an overestimation of corneal power in cases undergone this surgery leading to a subsequent under-correction of IOL power. However, recent advancements in technology have eliable measurement of total corneal power. The aim of this research was to assess the agreement in simulated keratometry (SimK) and total keratometry (TK) values between IOLMaster 700 and Pentacam AXL. The study involved 99 patients (99 eyes) undergone small incision lenticule extraction (SMILE) surgery. Each patient underwent scans using IOL Master 700 and Pentacam AXL. The following parameters were recorded: SimK1, SimK2, Total K1 (TK1), and Total K2 (TK2) for IOLMaster 700; and SimK1, SimK2, True Net Power (TNP) K1, TNPK2, Total Corneal Refractive Power (TCRP) K1, and TCRP K2 for Pentacam AXL. Agreement between the two devices was evaluated using Bland-Altman plot, while paired t-test was utilized to compare any differences in the same parameter by both instruments. The results revealed a strong correlation between the two devices.Noticeable comparability was identified for all SimK variables. However, there were noticeable differences in TK measurements as well as TK1-TNPK1, TK2-TNP K2, TK1-TCRP K1, and TK2-TCRP K2 parameters when comparing the two devices. The IOLMaster 700 consistently measured steeper values than the Pentacam AXL, with significant and clinically relevant differences of 1.34, 1.37, 0.87, and 0.95 diopters, respectively. While there was a noticeable correlation between the IOLMaster 700 and Pentacam AXL in SimK measurements, a marked difference was noted in TK values. The two devices cannot be used interchangeably when quantifying TK values.

Differences Between Simulated Keratometry and Total Corneal Power in Eyes With Keratoconus and a Formula to Improve IOL Power Calculation Results.

To compare simulated keratometry (SimK) and total corneal power (TCP) in keratoconic eyes, to determine whether the differences are systematic and predictable and to evaluate an adjusted TCP-based formula for intraocular lens (IOL) power calculation. In a consecutive series of keratoconic eyes, measurements of SimK, TCP, posterior keratometry, and anterior and posterior corneal asphericities (Q-values) were retrospectively collected. The difference between SimK and TCP was linearly correlated to the biometric parameters. In a separate sample of keratoconic eyes that had undergone cataract surgery, IOL power was calculated with the Barrett Universal II, Hoffer QST, Holladay 1, Kane, and SRK/T formulas using the SimK and an adjusted TCP power. The respective prediction errors were calculated. A total of 382 keratoconic eyes (271 patients) were enrolled. An increasing overestimation of SimK by TCP was detected from stage I to III, with a significant correlation between the SimK and TCP difference and SimK in the whole sample (P < .0001, r2 = 0.1322). Approximately 7% of cases presented an underestimation of SimK by TCP. IOL power calculation with the adjusted TCP improved outcomes, achieving a maximum of 80% of eyes with a prediction error within ±0.50 diopters with the Hoffer QST, Holladay 1, and Kane formulas. Overall, SimK overestimated TCP. Such a difference could not be predicted by any variable. The proposed TCP-adjustment formula (TCPadj = TCP + 0.56 diopters) in keratoconic eyes for IOL power calculation might be valuable for improving refractive outcomes. [J Refract Surg. 2024;40(4):e253-e259.].

Total keratometry for toric intraocular lens calculation: comparison from two swept-source optical coherence tomography biometers.

To compare the astigmatism prediction accuracy of total keratometry (TK) from the IOLMaster 700 and total corneal power (TCP) from Anterion based on swept-source optical coherence tomography (SS-OCT) technology in toric intraocular lens (toric IOL) calculation. A retrospective observational study. Total corneal astigmatism (TCA) were obtained using IOLMaster 700 and Anterion. Z CALC 2.0 was used to calculate the expected postoperative refractive astigmatism in conjunction with TCA. Prediction errors (PE) in refractive outcomes was analyzed 1 month postoperatively using the vector analysis by the Holladay method, including the mean vector PE magnitude, percentage of cases with vector PE in certain intervals, and the centroid PE. A total of 56 eyes from 56 patients were enrolled in the study with an insertion of an AT TORBI 709 toric IOL. The difference in mean vector PE of postoperative refractive astigmatism between TK and TCP was not statistically significant (0.48D versus 0.46D, P = 0.281). TK and TCP yielded 27.3 and 40.0% of eyes with vector PE ≤ 0.25D, and 58.2 and 63.6% with vector PE ≤ 0.5D (both P > 0.05), respectively. TK and TCP resulted in similar ATR centroid PE of 0.10D@35° ± 0.60D and 0.15D@22° ± 0.57D, respectively, and there were no significant differences between x-PE component and y-PE component. IOLMaster 700 and Anterion provided comparable astigmatic predictability in toric IOL implantation using total keratometry and Z CALC 2.0.

Intraocular lens power calculation after corneal refractive surgery

Challenges in IOL power calculation in eyes after corneal refractive surgery are caused by difficulties in accurately determining corneal refractive power and in predicting the effective lens position. Three factors contribute to inaccurate corneal refractive power estimation ‐ inaccurate measurement of anterior corneal curvature by standard keratometry or corneal topography, inaccurate calculation of total corneal power from the anterior corneal curvature can result from using the standardized value for the refractive index of cornea (1.3375) and finally changes in posterior corneal curvature. There are three categories of methods proposed based on the use of historical data acquired prior to the corneal refractive surgery (formulas using only historical data, no historical data or a combination of historical data and current data). The most frequently used methods of IOL power calculation in eyes after corneal refractive surgery are discussed and their accuracy are compared. The latest studies and meta‐analyses in this area are presented. Despite availability of new measurement technologies and development of new IOL calculation formulas, further advances are needed to improve outcomes of cataract surgery in eyes that have undergone corneal refractive surgery.

Performance of IOL calculation formulas that use measured posterior corneal power in eyes following myopic laser vision correction.

To compare the predictive accuracy of the biometer-embedded Barrett True-K TK and new total corneal power methods of intraocular lens (IOL) power calculation in eyes with prior laser vision correction (LVC) for myopia. Academic clinical practice. Retrospective case series. IOL power formulas were assessed using measurements from a swept-source optical coherence biometer. Refractive prediction errors were calculated for the Barrett True-K TK, EVO 2.0, Pearl-DGS, and HofferQST, which use both anterior and posterior corneal curvature measurements. These were compared with the Shammas, Haigis-L, Barrett True-K No History (NH), optical coherence tomography, and 4-formula average (AVG-4) on the ASCRS postrefractive calculator, and to the Holladay 1 and 2 with non linear axial length regressions (H1- and H2-NLR). The study comprised 85 eyes from 85 patients. Only the Barrett True-K TK and EVO 2.0 had mean numerical errors that were not significantly different from 0. The EVO 2.0, Barrett True-K TK, Pearl-DGS, AVG-4, H2-NLR, and Barrett True-K NH were selected for further pairwise analysis. The Barrett True-K TK and EVO 2.0 demonstrated smaller root-mean-square absolute error compared with the Pearl-DGS, and the Barrett True-K TK also had a smaller mean absolute error than the Pearl-DGS. The Barrett True-K TK and EVO 2.0 formulas had comparable performance to existing formulas in eyes with prior myopic LVC.

Comparison of two swept-source optical coherence tomography devices, a Scheimpflug camera system and a ray-tracing aberrometer in the measurement of corneal power in patients with cataract

PurposeTo assess the differences and similarities in the corneal curvature obtained by two swept-source optical coherence tomography (SS-OCT) devices, Scheimpflug imaging system and one ray tracing aberrometer in patients with cataracts. Moreover, this study aimed to compare the differences in posterior corneal (PK), total corneal (TK) and true net power (TNP) measurements among the IOLMaster 700, CASIA2, and Pentacam.MethodsA total of 200 eyes of 200 patients (116 female, 58%) were enrolled in this study, with a mean age of 65.9 ± 9.5 years. The flattest (Kf), steepest (Ks), and mean cornal powers (Km), J0, and J45 were obtained using two SS-OCT-based biometric devices, one rotating camera system and one ray-tracing aberrometer. The PK, TK and TNP values were also measured using these devices. To evaluate the differences and similarities between the devicves, the Friedman test, Pearson correlation coefficient (r), intraclass coefficient correlation (ICC) and Bland‒Altman plots with 95% limits of agreement (LoA) were used, and boxplots and stacked histograms were generated to describe the distributions of the data.ResultsThere were no significant differences between the IOLMaster 700 and Pentacam for any of the keratometry values. Additionally, there were no significant differences between the IOLMaster 700 and iTrace in evaluating J0 and J45. Bland‒Altman plots revealed relatively wide LoA widths, almost larger than 1 diopter for the keratometry values and almost larger than 0.5 diopter for J0 and J45 values among the four devices. In terms of PK and TK values, significant differences and low ICCs were found among the three devices.ConclusionsAlthough strong correlations and good agreement were found among the IOLMaster700, CASIA2, Pentacam and iTrace for Kf, Ks, Km and J0, J45, it seems that the measurements should not be used interchangeably because of the wide LoA widths and the presence of significant differences among the devices. Similarly, due to significant differences and low ICCs, the PK, TK and TNP values obtained by IOLMaster 700, CASIA2, and Pentacam should not be used interchangeably.

An update on intraocular lens power calculations in eyes with previous laser refractive surgery.

There is an ever-growing body of research regarding intraocular lens (IOL) power calculations following photorefractive keratectomy (PRK), laser-assisted in-situ keratomileusis (LASIK), and small-incision lenticule extraction (SMILE). This review intends to summarize recent data and offer updated recommendations. Postmyopic LASIK/PRK eyes have the best refractive outcomes when multiple methods are averaged, or when Barrett True-K is used. Posthyperopic LASIK/PRK eyes also seem to do best when Barrett True-K is used, but with more variable results. With both aforementioned methods, using measured total corneal power incrementally improves results. For post-SMILE eyes, the first nontheoretical data favors raytracing. Refractive outcomes after cataract surgery in eyes with prior laser refractive surgery are less accurate and more variable compared to virgin eyes. Surgeons may simplify their approach to IOL power calculations in postmyopic and posthyperopic LASIK/PRK by using Barrett True-K, and employing measured total corneal power when available. For post-SMILE eyes, ray tracing seems to work well, but lack of accessibility may hamper its adoption.

Repeatability and Interobserver Reproducibility of a Swept-Source Optical Coherence Tomography for Measurements of Anterior, Posterior, and Total Corneal Power.

The aim of this work is to evaluate the intraobserver repeatability and interobserver reproducibility of corneal power measurements obtained with a swept-source optical coherence tomographer (CASIA 2, Tomey, Japan) in healthy subjects. A total of 67 right eyes from 67 healthy subjects were enrolled. Two experienced observers measured each eye three times consecutively with the CASIA 2. Corneal power values were recorded as simulated keratometry, anterior, posterior, and total corneal power. Parameters were flattest keratometry (Kf), steepest keratometry (Ks), mean keratometry (Km), astigmatism magnitude, astigmatism power vectors J0 and J45. Intraobserver repeatability and interobserver reproducibility of the CASIA 2 were assessed by the within-subject standard deviation (Sw), test-retest repeatability (TRT), coefficients of variation (CoV), and intraclass correlation coefficients (ICCs). Double-angle plots were used for astigmatism vector analysis. The CASIA 2 had high repeatability for all corneal power values, with Sw values ≤ 0.17 diopters (D), TRT ≤ 0.46 D, and ICCs ranging from 0.866 to 0.998. Interobserver reproducibility was also high, showing all Sw values ≤ 0.10 D, TRT ≤ 0.27 D, and ICCs ≥ 0.944. The reproducibility of the average of three consecutive measurements (Sw 0.01-0.10 D, TRT 0.03-0.27 D, ICC 0.944-0.998) was higher than the reproducibility of single measurements (Sw 0.01-0.17 D, TRT 0.03-0.47 D, ICC 0.867-0.996). The CASIA 2 showed high intraobserver repeatability and interobserver reproducibility for anterior, posterior, and total corneal power measurementsin 6.0-mm diameter area. In addition, we suggest that using the average of three consecutive measurements can improve reproducibility between observers, compared to single measurements only.

Prediction of corneal power vectors after cataract surgery with toric lens implantation-A vector analysis.

Intraocular lenses are typically calculated based on a pseudophakic eye model, and for toric lenses (tIOL) a good estimate of corneal astigmatism after cataract surgery is required in addition to the equivalent corneal power. The purpose of this study was to investigate the differences between the preoperative IOLMaster (IOLM) and the preoperative and postoperative Casia2 (CASIA) tomographic measurements of corneal power in a cataractous population with tIOL implantation, and to predict total power (TP) from the IOLM and CASIA keratometric measurements. The analysis was based on a dataset of 88 eyes of 88 patients from 1 clinical centre before and after tIOL implantation. All IOLM and CASIA keratometric and total corneal power measurements were converted to power vector components, and the differences between preoperative IOLM or CASIA and postoperative CASIA measurements were assessed. Feedforward neural network and multivariate linear regression prediction algorithms were implemented to predict the postoperative total corneal power (as a reference for tIOL calculation) from the preoperative IOLM and CASIA keratometric measurements. On average, the preoperative IOLM keratometric / total corneal power under- / overestimates the postoperative CASIA keratometric / real corneal power by 0.12 dpt / 0.21 dpt. The prediction of postoperative CASIA real power from preoperative IOLM or CASIA keratometry shows that postoperative total corneal power is systematically (0.18 dpt / 0.27 dpt) shifted towards astigmatism against the rule, which is not reflected by keratometry. The correlation of postoperative CASIA real power to the corresponding preoperative CASIA values is better than those as compared to the preoperative IOLM keratometry. However, there is a large variation from preoperative IOLM or CASIA keratometry to the postoperative CASIA real power of up to 1.1 dpt (95% confidence interval). One of the challenges of tIOL calculation is the prediction of postoperative total corneal power from preoperative keratometry. Keratometric power restricted to a front surface measurement does not fully reflect the situation of corneal back surface astigmatism, which typically adds some extra against the rule astigmatism.

The accuracy of the trifocal IOL calculation using equivalent K-readings and total corneal power in different zones.

To identify the equivalent K-readings and total keratometry zones that is optimally suitable for calculating the IOL spheroequivalent according to 7 formulas. The study included 40 patients (40 eyes) who underwent uneventful femtosecond laser-assisted cataract surgery and refractive lens exchange (RLE) with implantation of a trifocal diffractive IOL (PanOptix, Alcon inc.). Targeted emmetropia was achieved in all patients, no distance and near correction was needed. Retrospective IOL calculations were performed utilizing 7 formulas (SRK/T, Holladay 1 and 2, Haigis, Hoffer Q, Barrett Universal 2, Olsen) and Pentacam keratometry data: Holladay equivalent K-readings, total optical power by ray tracing (TCRP) centered on the apex and pupil in 10 zones (from 0.5 to 5 mm in 0.5 mm increments). For each formula/zone/map combination: postoperative predicted refraction (PPRs), mean absolute errors (MAEs), and median absolute errors (MedAEs) were analyzed. According to EKR, the Haigis formula showed the lowest error in the central zones up to 3.5 mm, the TCRP zone for Holladay I and II formulas 4.0-4.5 mm, for HofferQ and SRK/T formulas 4.5-5.0 mm, and for Olsen and Barrett II Universal-5 mm. The use of keratometry data (EKR, TCRP) in the formulas adapted to SimK, with the correct choice of the evaluation zone of keratometric data, will increase the chance of hitting the refractive target.

Comparison of total corneal power measurements obtained with different devices after myopic keratorefractive surgery.

To analyze the differences, agreements, and correlation among total corneal power parameters generated by different instruments after myopic keratorefractive surgery. The prospective cross-sectional study included patients who underwent myopic keratorefractive surgery and received measurements of corneal power 3mo after surgery. Automated keratometer was used for the measurement of simulated keratometry (SimK), swept-source optical coherence tomography (SS-OCT) based biometer for total keratometry (TK), anterior segment-OCT for real keratometry (RK), and Scheimpflug keratometer for the true net power (TNP), the total corneal refractive power (TCRP) and equivalent K-readings (EKR). The differences among these parameters were analyzed, and the agreements and correlation between SimK and other total corneal power parameters were investigated. A total of 70 eyes of 70 patients after myopic keratorefractive surgery were included. The evaluated corneal power parameters were as follows: SimK 38.32±1.93 D, TK 37.54±2.12 D, RK 36.64±2.09 D, TNP 36.56±1.97 D, TCRP 36.70±2.01 D, and EKR 37.55±2.00 D. Pairwise comparison showed that there were significant differences (P<0.001) among all parameters except for between TK and EKR, RK and TNP, RK and TCRP (P=1.000, 1.000, 1.000, respectively). The limits of agreement between SimK and TK, RK, TNP, TCPR, and EKR were 1.08, 1.08, 1.43, 1.48, and 1.73 D, respectively. All parameters showed good correlation with SimK, and the correlation coefficients were 0.995, 0.994, 0.983, 0.982, and 0.975. Among the corneal power parameters after myopic keratorefractive surgery, the value of SimK is the largest, followed by TK and EKR, with TCRP, RK, and TNP being the smallest. The differences among the parameters may be attributable to the different calculation principles. Correct understanding and evaluation of corneal power parameters can provide a theoretical basis for taking advantage of the total corneal power to improve the accuracy of intraocular lens calculation after keratorefractive surgery.

Prediction of total corneal power in keratoconus using anterior surface data

ABSTRACT Clinical relevance Keratoconus results in an increase in anterior and posterior curvatures and a reduction in corneal thickness. Anterior corneal ectasia is partially compensated by remodelling the corneal epithelium. Therefore, there is an alteration in the relationship between corneal surfaces and variation in corneal power. The variation in corneal power is one of the sources that induces errors in IOL power calculation. Background This study aimed to assess a method for predicting total corneal power in keratoconus using several anterior surface parameters at 3 mm and 4 mm. Methods Tomographic data obtained using Pentacam (Oculus, Germany) were analysed from 280 eyes of 140 patients with keratoconus using anterior and posterior keratometry, anterior Q-value at 8 mm, central corneal thickness, Kmax location and value, and true net power at 4 mm (TNP). Calculated total corneal power (TCPc) at 3 mm was obtained using the Gauss formula. Predicted total corneal power at 3 mm (TCPp3) and 4 mm (TCPp4) was obtained from univariate (TCPp3u and TCPp4u) and multivariate linear regression formulae (TCPp3m and TCPp4m). SimK, anterior Q-value, vertical location, and Kmax value were used in the multivariate formulae. Mean absolute error (MAE) and median absolute error (MedAE) were also calculated. Absolute frequencies within dioptric ranges of all formulas divided for keratoconus grading were evaluated. Results TCPc and TNP exhibited a good correlation (R2 = 0.58, p < 0.05) with a higher dispersion above 50 D of corneal power. Highly significant correlations were observed between TCPp3u and TCPc (R2 = 0.978, p < 0.05) and TCPp3m and TCPc (R2 = 0.989, p < 0.05). Lower but significant correlations were observed between TCPp4u and TNP (R2 = 0.692, p < 0.05) and between TCPp4m and TNP (R2 = 0.887, p < 0.05). The best results for TCP prediction at 3 and 4 mm were obtained with TCPp3m and TCPp4m as follows: MAE of TCPp3m was 0.24 ± 0.20 (SD) D with MedAE of 0.20 D, while MAE of TCPp4m was 0.96 ± 0.77 D with MedAE of 0.80 D. The 3 mm multivariate regression formula results in higher absolute frequencies of prediction errors in the total eyes within 0.5 D (93%) than the univariate formula (81%). At 4mm, the multivariate regression formula has a lower percentage within 0.5 D (32%) than the univariate formula (41%), but the percentage of the multivariate formula is higher within 1 D (63%) than the univariate formula (56%). Conclusion All formulas show a decrease in accuracy with increasing grades of keratoconus. Multivariate linear regression formulae using only anterior surface data can predict TCP with good approximation in eyes with keratoconus in cases where posterior surface parameters are unavailable. The vertical location of Kmax and the anterior asphericity could play a relevant role in the prediction of total corneal power in keratoconus.

Theoretical Relationship Between the Anterior-Posterior Corneal Curvature Ratio, Keratometric Index, and Estimated Total Corneal Power.

To predict the relationships between the keratometric index value that would match the total Gaussian corneal power and its related variables: anterior and posterior radii of curvature of the cornea, anterior-posterior corneal radius ratio (APR), and central corneal thickness. The relationship between the APR and the keratometric index was approximated by calculating the analytical expression for the theoretical value of the keratometric index, which would make the keratometric power of the cornea equal to the total paraxial Gaussian power of the cornea. The study of the impact of variations in the radius of anterior and posterior curvature and central corneal thickness showed that the difference between exact and approximated best-matching theoretical keratometric index was less than 0.001 for all of the performed simulations. This translated to a variation in the total corneal power estimation of less than ±0.128 diopters. After refractive surgery, the estimated optimal keratometric index value is a function of the preoperative anterior keratometry, the preoperative APR, and the delivered correction. The larger the magnitude of myopic corrections, the greater the increase in postoperative APR value. It is possible to estimate the most compatible value of the keratometric index that allows simulated keratometric power to equal the total Gaussian corneal power. The obtained equations enable the evaluation of the impact of corneal variables such as the APR on the ideal keratometric index value. The use of 1.3375 for the keratometric index results in an overestimation of the total corneal power in most clinical situations. [J Refract Surg. 2023;39(4):266-272.].

Quantifying Corneal Topographic Astigmatism (CorT Total) in Keratoconic Eyes.

To determine optimal corneal regions from which to derive corneal topographic astigmatism (CorT) in kerato-conic eyes. In this retrospective study, potential measures of corneal astigmatism are calculated from raw total corneal power data (179 eyes from 124 patients) from a corneal tomographer. The measures are derived from annular corneal regions varying in both extent and center position, and evaluated according to the variability of the ocular residual astigmatism (ORA) in the cohort. This variability is quantified by the ORArms, which is the root-mean-squared distance of the ORAs from their summated vector mean in double angle space. The lower the ORArms, the better the corneal astigmatism measure corresponds to manifest refractive cylinder. Corneal astigmatism measures derived from regions centered on corneal vertex had ORArms values (mild: 1.07 diopters [D], moderate: 1.61 D, severe: 2.65 D) as low or lower than other measures derived from regions centered on thinnest point, corneal apex (front or back), or pupil center. Corneal astigmatism measures derived from a region centered 30% of the way toward thinnest point from corneal vertex appeared to have even lower ORArms values (mild: 1.05 D, moderate: 1.45 D, severe: 2.56 D). None of the corneal astigmatism measures corresponded closely with manifest refractive cylinder for severe keratoconus (ORArms > 2.50 D). For keratoconic eyes, the CorT should be derived from an annular region centered 30% of the way toward thinnest point from corneal vertex, although when the keratoconus is mild, a standard corneal-vertex-centered CorT performs just as well. [J Refract Surg. 2023;39(3):206-213.].

Modification of the Barrett Universal II formula by the combination of the actual total corneal power and virtual axial length.

This study aims to investigate whether a combination of the total corneal power (TCP) and virtual axial length (AL) based on Gaussian optics makes the refractive prediction accuracy of the Barrett Universal II (BUII) formula better than the conventional anterior keratometry (K) and axial length. The TCP and the virtual AL were calculated in two ways: the corneal index strategy and the TK index strategy. The former uses the corneal refractive index n1 as a variable, and the latter uses the TK index nx as a variable. In a dataset of 225 eyes, the calculated TCP and the virtual AL were input into the BUII formula along with the anterior chamber depth, lens thickness, and white-to-white measured with the IOLMaster 700, and the refractive prediction accuracy was evaluated by the mean numerical prediction error (MNE), standard deviation (SD), mean absolute prediction error (MAE), median absolute prediction error (MedAE), percentages of eyes with prediction error (PE) within ± 0.50 diopter, and IOL formula performance index (FPI). The refractive prediction outcomes also underwent subgroup analyses and were compared with those of the anterior keratometry-based BUII-K of the IOLMaster 700. In the corneal index strategy, the FPI had the highest value at approximately n1 = 1.346. In the TK index strategy, the FPI had the highest value at approximately nx = 1.3858. There was no tendency for the refractive prediction outcomes of the BUII-n1 = 1.346 and the BUII-nx = 1.3858 to be inferior to those of the BUII-K, particularly in the medium range of subgroups. The combination of the actual TCP and the virtual AL based on Gaussian optics may lead to a better refractive prediction accuracy of the BUII formula than that of BUII-K.

Agreement between a new swept-source ocular coherence tomography and a Placido disc-dual Scheimpflug ocular biometric devices.

To evaluate the agreement between two biometry devices, the Heidelberg Anterion and the Galilei G6 Lens Professional. Eyes were scanned with both biometry devices. Analysis of inter-device agreement was conducted for the following metrics: flat (K1), steep (K2) and mean K (Km) for anterior, posterior and total cornea, lens thickness (LT), central corneal thickness (CCT), anterior chamber depth (ACD), white to white (WTW) and axial length (AL). Generalised Estimating Equations were used to account for inter-eye correlation. Bland-Altman analysis was conducted to derive the mean difference (MD) and limits of agreement (LoA) between devices. Differences were deemed clinically significant if they would result in a change in post-operative refraction of 0.25D or more. 159 eyes of 91 patients were included. For the anterior cornea, no significant MD was found for K1 (-0.11D) and K2 (-0.10D), although a significant MD was found for Km (-0.10D). For posterior cornea, while there were no significant MDs between devices, the LoAs were wide for both posterior K1(-0.70, 0.68) and posterior K2 (-1.01, 1.29). For total corneal power, significant MDs were found in K1 (0.36D), and Km (0.26D) but not for K2 (0.17D). Significant MDs were found for LT (0.179mm), CCT (-0.005mm), ACD (-0.111mm) and WTW (-0.158mm), but not for AL (-0.021mm, p > 0.05).Conclusion: There are statistically but not clinically significant differences between Anterion and Galilei G6 Lens Professional in anterior Km, LT, CCT, ACD and WTW. Measurements of the posterior and total cornea are not interchangeable between devices.