The Enigmatic Cornea and Intraocular Lens Calculations: The LXXIII Edward Jackson Memorial Lecture

BCLA CLEAR Presbyopia: Evaluation and diagnosis

It is important to be able to measure the range of clear focus in clinical practice to advise on presbyopia correction techniques and to optimise the correction power. Both subjective and objective techniques are necessary: subjective techniques (such as patient reported outcome questionnaires and defocus curves) assess the impact of presbyopia on a patient and how the combination of residual objective accommodation and their natural DoF work for them; objective techniques (such as autorefraction, corneal topography and lens imaging) allow the clinician to understand how well a technique is working optically and whether it is the right choice or how adjustments can be made to optimise performance. Techniques to assess visual performance and adverse effects must be carefully conducted to gain a reliable end-point, considering the target size, contrast and illumination. Objective techniques are generally more reliable, can help to explain unexpected subjective results and imaging can be a powerful communication tool with patients. A clear diagnosis, excluding factors such as binocular vision issues or digital eye strain that can also cause similar symptoms, is critical for the patient to understand and adapt to presbyopia. Some corrective options are more permanent, such as implanted inlays / intraocular lenses or laser refractive surgery, so the optics can be trialled with contact lenses in advance (including differences between the eyes) to better communicate with the patient how the optics will work for them so they can make an informed choice.

Intraocular lens power calculation in patients with irregular astigmatism

PURPOSE

Accurate intraocular lens (IOL) calculation in subjects with irregular astigmatism is challenging. This study evaluated the accuracy of using Scheimpflug-derived central 2-mm equivalent keratometry reading (EKR) values for IOL calculation in irregular astigmatism.

METHODS

This retrospective study included subjects (31 eyes of 30 patients) who underwent cataract surgery and IOL calculation using the 2-mm central EKR methods. We compared prediction error (PE) and absolute PE (APE) outcomes using SRK/T and Barrett Universal II formulas for keratometry data obtained from the IOLMaster 500 and Pentacam (anterior corneal sim k) devices.

RESULTS

Cataract surgery and IOL calculation using the 2-mm central EKR methods resulted in improved visual acuity (uncorrected: from 1.13 ± 0.38 to 0.65 ± 0.46 logMar, p < 0.01; best-corrected: from 0.45 ± 0.24 to 0.26 ± 0.20 logMar, p < 0.01) after surgery. The percentage of subjects with best-corrected visual acuity of 6/6 was 22%, < 6/9 was 58%, and < 6/12 was 71%. For both the SRK/T and the Barrett formulas, the PE was similar to those obtained by IOLMaster (> 0.14) but lower than those obtained by the anterior corneal sim k (p < 0.02). IOLMaster provided keratometry reading in only 23/31 (74.1%) of cases.

CONCLUSIONS

The use of Scheimpflug central 2-mm EKR for IOL calculation in irregular astigmatism was beneficial in terms of visual acuity improvement. It had comparable refractive prediction performance to the IOLMaster 500 and better than the anterior corneal sim K. The 2-mm EKR method can be used when IOLMaster cannot provide a reliable reading in abnormal corneas.

Measured Corneal Astigmatism Versus Pseudophakic Predicted Refractive Astigmatism in Cataract Surgery Candidates

PURPOSE

To compare standard and total corneal astigmatism measurements to the predicted pseudophakic (nontoric) refractive astigmatism in candidates for cataract surgery.

DESIGN

A retrospective, cross-sectional study.

METHODS

A single-center analysis of consecutive eyes measured with a swept-source optical coherence tomography biometer at a large tertiary medical center between February 2018 and June 2020. Corneal astigmatism was calculated based on standard keratometry astigmatism (KA), total corneal astigmatism (TCA), and predicted refractive astigmatism (PRA) for a monofocal nontoric intraocular lens (IOL) implantation calculated by the Barrett toric calculator using the predicted posterior corneal astigmatism (PRA_{(Predicted-PCA)}) and the measured posterior corneal astigmatism (PRA_{(Measured-PCA)}) options. Separate analyses were performed for each eye.

SETTING

Ophthalmology Department, Shaare Zedek Medical Center, Jerusalem, Israel.

RESULTS

In total, 8152 eyes of 5320 patients (4221 right eyes [OD] and 3931 left eyes [OS], mean age 70.6±12.2 years, 54.2% females) were included in the study. The mean vector values (centroid) for KA, TCA, PRA_{(Predicted-PCA)}, and PRA_{(Measured-PCA)} were 0.07 diopters [D] at 19.5°, 0.27 D at 7.5°, 0.44 D at 2.9°, and 0.43 D at 179.3°, respectively (P < .01), for OD and 0.02 D at 150.3°, 0.23 D at 169.7°, 0.40 D at 179.4°, and 0.42 D at 169.5°, respectively (P < .01), for OS. More than 73% of eyes had a PRA >0.5 D.

CONCLUSIONS

Standard and total corneal astigmatism measurements differ significantly from the PRA by the Barrett toric calculator. The PRA, rather than the KA or TCA, should be used as the reference guide for astigmatism correction with toric IOL implantation.

Ocular Biometric Characteristics Measured by Swept-Source Optical Coherence Tomography in Individuals Undergoing Cataract Surgery

PURPOSE

To study the distribution of ocular biometric parameters using a swept-source optical coherence tomography (SS-OCT) biometer in adult candidates for cataract surgery.

DESIGN

A retrospective cross-sectional study.

METHODS

This is a single-center analysis of consecutive eyes measured with the IOLMaster 700 SS-OCT biometer at a large tertiary medical center between February 2018 and June 2020.

RESULTS

Three thousand eight hundred thirty-six eyes of 3836 patients were included in the study. The mean ± SD age was 72.3 ± 12.8 years and 53% were female. The mean biometric values were as follows: total corneal power (44.17 ± 1.70 diopters [D]), total corneal astigmatism (TCA; 1.11 ± 0.87 D), mean posterior keratometry (-5.87 ± 0.26 D), posterior corneal astigmatism (-0.26 ± 0.15 D), axial length (AL; 23.95 ± 1.66 mm), anterior chamber depth (ACD; 3.18 ± 0.42 mm), lens thickness (LT; 4.49 ± 0.47 mm), white-to-white distance (WTW; 11.92 ± 0.44 mm), central corneal thickness (CCT; 0.54 ± 0.04 mm), angle alpha (0.49 ± 0.17 mm), and angle kappa (0.34 ± 0.17 mm). There were sex-related differences in all biometric parameters save for LT (P = .440), angle kappa (P = .216), and corneal astigmatism (P = .103). Biometric parameters demonstrated correlations between AL, WTW distance, ACD, and LT (P < .001). Age correlated with all parameters (P < .001) except CCT and posterior keratometry. Angle alpha and angle kappa magnitudes also correlated (P < .001). The prevalence of patients with TCA ≥0.75 D, 1.0 D, and 1.5 D were 59.1%, 43.4%, and 22.6%, respectively.

CONCLUSIONS

Age significantly correlated with most of the biometric parameters and significant differences between sexes were noted. In addition, the high prevalence of TCA and relatively large angle alpha and angle kappa magnitudes were noted among subjects. These data can be relevant in planning local and national health economics.

Evaluating the prediction accuracy of the Hill-RBF 3.0 formula using a heteroscedastic statistical method

PURPOSE

To evaluate the accuracy of the Hill-RBF 3 formula, with and without direct measurements of total corneal power, using a heteroscedastic statistical method for analysis.

SETTING

Department of Ophthalmology, Shaare Zedek Medical Center, Jerusalem, Israel.

DESIGN

Retrospective, consecutive case series.

METHODS

Records of consecutive patients who underwent routine cataract surgery between February 2018 and June 2020 were retrospectively reviewed. The prediction accuracy of the Hill-RBF 3.0 formula was compared with that of the Barrett Universal II, Emmetropia Verifying Optical 2.0, Haigis, Hill-RBF 2.0, Hoffer Q, Holladay 1, Holladay 2, Kane, Olsen, and SRK/T formulas, based on biometry measurements by swept-source optical coherence tomography (SS-OCT) with standard keratometry (K), SS-OCT with total keratometry (TK), and an optical low-coherence reflectometer (OLCR). Statistical analysis was applied according to a heteroscedastic statistical method with SD of prediction errors as the main parameter for formula performance.

RESULTS

The study included 153 eyes of 153 patients. The SD values that were obtained by Hill-RBF 3.0 (0.266 to 0.285 diopters [D]) were significantly lower compared with those by Hill-RBF 2.0 (0.290 to 0.309 D), Hoffer Q (0.387 to 0.407 D), Holladay 1 (0.367 to 0.385 D), Holladay 2 (0.386 to 0.401 D), and SRK/T (0.377 to 0.399 D) formulas (P < .036). The prediction accuracy of the Hill-RBF 3.0 was similar across the SS-OCT (K), SS-OCT (TK), and OLCR methods of measurement (P > .51).

CONCLUSIONS

The Hill-RBF 3.0 was more accurate than the Hill-RBF 2.0 and older generation formulas and had similar prediction accuracy compared with new generation formulas. The use of TK did not provide significant improvement to its prediction accuracy.

Accuracy of intraocular lens power calculation for cataract surgery after deep anterior lamellar keratoplasty

BACKGROUND

The purpose of this study was to assess the accuracy of intraocular lens power (IOL) formulas for cataract surgery after deep anterior lamellar keratoplasty (DALK).

METHODS

This retrospective study included eyes which had previously undergone DALK and underwent standard phacoemulsification with monofocal IOL implantation between January 2012 and January 2021 at Ospedali Privati Forlì "Villa Igea" (Forlì, Italy). The predicted spherical equivalent (SE) was calculated using the Barrett Universal II, Emmetropia Verifying Optical (EVO), Haigis, Hoffer Q, Hoffer QST, Holladay 1, Holladay II, Kane and SRK/T formulas. Prediction error (PE) was calculated as the actual postoperative SE refraction minus the SE predicted refraction.

RESULTS

Eighty-two eyes of 82 patients were included. The mean PE was negative using all formulas. Friedman test revealed a statistically significant difference of the median absolute PE (MedAE) among the different IOL formulas (P = 0.005). On the basis of the MedAE, the formulas were ranked as follows: SRK/T (0.805 D), Kane (0.810 D), EVO (0.845 D), Hoffer QST (0.847 D), Barrett (0.895 D), Holladay 1 (0.915 D), Haigis (1.010 D) and Hoffer Q (1.070 D) formulas.

CONCLUSIONS

All formulas had a tendency towards a myopic refractive shift in post-DALK eyes. Although the SRK/T, Kane, EVO and Hoffer QST formulas were more accurate, predictability of refractive outcomes was lower than in virgin eyes.

[Monte Carlo simulation of biometric effect sizes and their influence on the translational ratio of corneal astigmatism in the cylinders of toric intraocular lenses]

Hintergrund und Zielsetzung

Torische Kapselsacklinsen bieten heutzutage eine zuverlässige Option der permanenten Korrektur eines Hornhautastigmatismus. Zur Ermittlung der für den gewünschten Ausgleich erforderlichen Linsenstärke kann der Operateur entweder auf die in seinem Biometriegerät implementierten Berechnungsmodi oder auf den vom Linsenhersteller angebotenen Kalkulationsservice zurückgreifen. In vielen Fällen wird dabei allerdings keine klassische Linsenberechnung aus biometrischen Daten durchgeführt, sondern nur mit einer vereinfachten Abschätzung gearbeitet, die den Hornhautastigmatismus in den Torus der tIOL übersetzt. Dieses dann zumeist als durchschnittlicher Standardwert genutzte Übersetzungsverhältnis kann jedoch eine erhebliche Schwankungsbreite aufweisen, sodass im ungünstigsten Fall eine Unterkorrektur des refraktiven Zylinders um bis zu 12,5 % oder eine Überkorrektur um bis zu 17 % resultieren kann. Ziel dieser Studie war es aufzuzeigen, welche biometrischen Einflussgrößen das Verhältnis zwischen dem zu korrigierenden Hornhautastigmatismus und dem für dessen Vollkorrektur notwendigen Torus einer Kapselsacklinse bestimmen.

Methoden

Aus der WEB-Plattform IOLCon wurden 16.744 Datensätze extrahiert, und anhand der präoperativen biometrischen Größen und dem postoperativen sphärischen Äquivalent wurde zunächst die axiale Position der Kapselsacklinse formelunabhängig abgeleitet. Anschließend wurde, basierend auf der Propagation sphärozylindrischer Vergenzen, der entsprechende Brechwert einer emmetropisierenden Kapselsacklinse ermittelt. Das Übersetzungsverhältnis als Quotient aus dem Torus der Linse und dem Hornhautastigmatismus wurde mit einer Monte-Carlo-Simulation auf seine potenziellen Einflussgrößen hin untersucht.

Ergebnisse

Die Monte-Carlo-Simulation zeigt, dass nicht von einem konstanten Übersetzungsverhältnis ausgegangen werden kann. Für die hier zugrunde gelegten klinischen Fälle ergibt sich ein mittleres Übersetzungsverhältnis von 1,3938 ± 0,0595 (Median 1,3921) mit einer Spannweite von 1,2131 bis 1,5974. Den größten Einfluss hat hierbei die axiale Position der Kapselsacklinse – je weiter posterior sich diese befindet, desto höher ist das Übersetzungsverhältnis. Aufgrund der Korrelation der axialen Linsenposition mit der Augenlänge kann die Augenlänge als indirekte Einflussgröße gewertet werden. Der Äquivalentbrechwert sowie der Astigmatismus der Hornhaut besitzen keinen nennenswerten Effekt auf das Übersetzungsverhältnis.

Diskussion

In einer ganzen Reihe von Berechnungsmodulen wird die Kalkulation des Torus der Kapselsacklinse dahingehend vereinfacht, dass dieser mittels eines einfachen konstanten Umrechnungsfaktors aus dem gemessenen Hornhautastigmatismus abgeleitet wird. Die vorliegende Studie zeigt jedoch, dass diese Vereinfachung zu deutlich fehlerhaften Ergebnissen führen kann. Dementsprechend wird eine individuelle Berechnung des Torus der IOL aus gemessenen biometrischen Größen (z. B. mittels Vergenzpropagation, Matrizen oder mittels Full-aperture-Raytracing) empfohlen.

Accuracy of Artificial Intelligence Formulas and Axial Length Adjustments for Highly Myopic Eyes

PURPOSE

To compare the accuracy of artificial intelligence formulas (Kane formula and Radial Basis Function [RBF] 2.0) and other formulas, including the original and modified Wang-Koch (MWK) adjustment formulas for Holladay 1 (H1-MWK) and SRK/T (SRK/T-WK and SRK/T-MWK), the Barrett Universal II (BUII), the emmetropia-verifying optical (EVO), and the Haigis equation in highly myopic eyes.

DESIGN

Retrospective consecutive case-series study.

METHODS

A total of 370 eyes with an axial length (AL) ≥26.0 mm of 370 patients were enrolled, and subgroup analyses was performed based on ALs. The median absolute error (MedAE), the percentages of eyes with hyperopic outcome and within ±0.25 diopters (D), ±0.50 D, and ±1.00 D of prediction error were determined.

RESULTS

Overall, the Kane equation had the lowest MedAE (0.26 D), followed by H1-WK (0.27 D) and H1-MWK (0.28 D). There were no significant differences in MedAE among the Kane equation, the RBF 2.0, the BUII, the H1-MWK, and the H1-WK, whereas the Kane equation had a significantly lower MedAE than EVO (P < .001), SRK/T-MWK (P = .001), SRK/T-WK (P = .006), and Haigis (P < .001). In extremely myopic eyes with an AL ≥30.0 mm (n = 115), the Kane equation had a significantly lower MedAE than the RBF 2.0 (P = .001), the EVO (P = .019), the BUII (P = .013), and the Haigis method (P = .005), whereas no significant differences were found among the Kane, H1-MWK, and H1-WK equations.

CONCLUSIONS

The Kane equation was comparable to RBF 2.0, BUII, H1-MWK, and H1-WK in highly myopic eyes and was better than RBF 2.0 and BUII in extremely myopic eyes. The Kane, H1-MWK, and H1-WK methods were equally accurate in eyes with high to extreme myopia.

Recent developments in intraocular lens power calculation methods-update 2020

For many decades only a few formulas have been available to calculate the intraocular lens (IOL) power for patients undergoing cataract surgery: the Haigis, Hoffer Q, Holladay 1 and 2 and SRK/T. In recent years, several new formulas for IOL power calculation have been introduced with the aim of improving the accuracy of refraction prediction in eyes undergoing cataract surgery. These include the Barrett Universal II, the Emmetropia Verifying Optical (EVO), the Kane, the Næser 2, the Olsen, the Panacea, the Pearl DGS, the Radial Basis Function (RBF), the T2 and the VRF formulas. Although most of them are unpublished so that their structure is unknown, we give an overview of each formula and report the results of the studies that have compared them. Their performance in short and long eyes is provided and a special focus is given on the issue of segmented axial length, which is a promising method to obtain more accurate outcomes in short and long eyes. Here, the group refractive index originally developed for the IOLMaster may not represent the best method to convert the optical path length into a physical distance. The issue of posterior and total corneal astigmatism (TCA) is discussed in relation to toric IOLs; the latest formulas for toric IOLs and their results are also reported.

Post-small incision lenticule extraction phacoemulsification with multifocal IOL implantation: A case report

A 54-year-old patient presented with cataract, 5 years after undergoing SMILE for high myopia in both eyes. He was motivated in achieving spectacle free vision and his post SMILE-induced aberrations were minimal, due to which he was found suitable for a trifocal IOL implant. Of the various methods considered, the IOL power predicted by a novel combined telecentric keratometry and swept source OCT-based method was finally selected. One month post-operatively, the patient achieved a binocular UDVA of 20/20p and near vision of N.6, suggesting that newer IOL formulae could be superior in providing satisfactory outcomes in post refractive patients.

Image-guided lens extraction surgery: a systematic review

A systematic review of the recent literature regarding the current image-guided systems used for cataract surgery or refractive lens exchange was performed based on the PubMed and Google Scholar databases in March 2018. Literature review returned 21 eligible studies. These studies compared image-guided systems with other keratometric devices regarding their accuracy, repeatability and reproducibility in measurement of keratometric values, astigmatism magnitude and axis, as well as in IOL power calculation. Additionally, the image-guided systems were compared with conventional manual ink-marking techniques for the alignment of toric IOLs. In conclusion, image-guided systems seem to be an accurate and reliable technology with measurements of high repeatability and reproducibility regarding the keratometry and IOL power calculation, but not yet interchangeable with the current established and validated keratometric devices. However, they are superior over the conventional manual ink-marking techniques for toric IOL alignment.

Cataract Surgery following Sequential Myopic and Hyperopic LASIK

We report a case of patient dissatisfaction after sequential myopic and hyperopic LASIK in the same eye. We discuss the course of management for this patient involving eventual cataract extraction and intraocular lens (IOL) implantation with attention to the IOL power calculation method used.

Reviewing the visual benefits of femtosecond laser-assisted cataract surgery: Can we improve our outcomes?

Femtosecond laser-assisted cataract surgery (FLACS) was introduced in 2009 and has increasingly been incorporated into surgical practice. The automation of three key aspects of cataract surgery was expected to deliver a significant improvement in both refractive and safety outcomes. The published literature has not yet shown consistent refractive improvement above conventional techniques. The purpose of this paper is to review current FLACS refractive outcomes and explore factors that may have contributed to the current findings and whether future improvements are possible.

Accuracy of Different Topographic Instruments in Calculating Corneal Power after Myopic Photorefractive Keratectomy

PURPOSE

To compare the corneal power measurements obtained using different topographic instruments after myopic photorefractive keratectomy (PRK).

METHODS

Patients with myopia who were candidates for corneal refractive surgery were sequentially included. Pre-PRK and six months post-PRK corneal powers were measured using Javal manual keratometer, Orbscan II, Galilei, Tomey TMS4, and EyeSys 2000 topographers. Measured values were compared with those obtained using the clinical history method (CHM).

RESULTS

This study included 66 eyes of 33 patients. The lowest keratometric measurements were obtained using the Galilei topographer (42.98 ± 1.69 diopters, D) and the highest measurements were obtained using the Javal manual keratometer (43.96 ± 1.54 D) preoperatively. The same order was observed postoperatively. Effective refractive power (EffRP) measured using EyeSys was most similar to the values obtained using CHM (ICC, intraclass correlation coefficient = 0.951), followed by the total corneal power measured using the Galilei system (ICC = 0.943). The values obtained using the adjusted EffRP formula (EffRP - 0.015*Δ Refraction - 0.05) were more consistent with the values obtained using CHM (ICC = 0.954) compared to those obtained with the adjusted average central corneal power formula measured using the Tomey system (ICC = 0.919).

CONCLUSION

Post-PRK corneal powers measured using the adjusted EffRP formula were the most similar to values obtained using CHM.

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.