How Can We Improve Toric Intraocular Lens Calculation Methods? Current Insights

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

PURPOSE

To 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.

METHODS

A 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.

RESULTS

There 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.

CONCLUSIONS

Although 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.

Prevalence data for total corneal astigmatism in cataract patients

PURPOSE

To report the prevalence data for total corneal astigmatism (TCA) in cataract patients.

METHODS

The authors retrospectively collected and analyzed the preoperative biometric data of the patients who underwent cataract surgery in the Department of Ophthalmology, Peking University Third Hospital, from January 2019 to May 2023.

RESULTS

The mean age of the 10817 patients was 71 ± 10 years; the male/female ratio was 4653/6164. The mean TCA obtained by the IOLMaster 700 (Carl Zeiss Meditec AG, Jena, Germany), the Abulafia-Koch (AK) formula, and the Barrett toric calculator was 1.11 ± 0.81 diopter (D), 1.13 ± 0.75 D, and 1.12 ± 0.74 D respectively, which was significantly greater than the mean standard keratometric (K) astigmatism (0.99 ± 0.75 D) obtained by IOLMaster 700. Against-the-rule (ATR) astigmatism was dominant in all the TCA measurements, and its proportion increased with age. TCA measurements by different methods exhibit high variability, with a total of 1574 (8.9%) data sets from 1016 (9.4%) patients showing a difference larger than 0.5 D in at least one pair of TCA measurements.

CONCLUSION

The use of TCA rather than K astigmatism significantly influenced the choice of intraocular lenses (IOLs) as more patients would be candidates for toric IOLs. It was essential to carefully compare and select TCA obtained with multiple methods for optimal postoperative visual quality.

Functional Approach to IOL Selection in Eyes With Combined Cataract and Keratoconus With an Option for Refractive Lens Exchange

PURPOSE

To evaluate spherical intraocular lens (IOL) implantation for cataracts in keratoconic eyes followed by optional refractive toric lens exchange to improve uncorrected visual acuity.

METHODS

This retrospective study evaluated cataract surgery outcomes in keratoconic eyes. Eyes treated with a spherical IOL targeted for -2.00 diopters (D) either achieved acceptable manifest refraction and desired exchange with a toric IOL (Group 1); achieved satisfactory manifest refraction and chose to use spectacles or contact lenses (Group 2); or did not achieve acceptable refraction and used contact lenses (Group 3). Group 4 had single-stage toric IOL implantation with plano target. Corrected and uncorrected distance visual acuity (CDVA and UDVA) and keratometry were analyzed.

RESULTS

Groups 1 to 4 had 18, 23, 18, and 26 eyes, respectively. A staged toric exchange resulted in significantly better (P = .02) UDVA (mean: 0.15 logMAR; 20/25 Snellen) than initial toric IOL implantation (0.24 logMAR; 20/30 Snellen). All toric IOL exchange eyes achieved 20/30 or better CDVA and 94% had 20/40 or better UDVA. Mean manifest cylinder significantly decreased from 3.39 D before lens exchange to 1.10 D postoperatively.

CONCLUSIONS

Initial implantation of a spherical IOL in keratoconic eyes allows basing toric calculations on the manifest refraction, which may be more reliable than keratometry measurements in keratoconic eyes. UDVA after staged toric IOL exchange was significantly better than after initial toric IOL implantation. Importantly, by staging use of toric lenses, the authors avoided cases where patients required a rigid contact lens after a toric IOL was implanted. [J Refract Surg. 2024;40(4):e207-e217.].

Short-term clinical results with a trifocal diffractive toric intraocular lens using an optimized preoperative and intraoperative protocol

PURPOSE

To evaluate the short-term clinical outcomes of a specific toric diffractive trifocal intraocular lens (IOL) implanted following an optimized clinical protocol in a large population.

METHODS

Retrospective analysis of 337 eyes of 231 patients (mean age, 62.2 years) undergoing cataract surgery with implantation of the trifocal diffractive IOL AT.LISA tri toric 939M/MP (Carl Zeiss Meditec). A strict and careful clinical protocol was followed, including an accurate measurement of corneal astigmatism, use of a latest generation IOL power calculator, photography-based method intraoperative control of IOL alignment and IOL reposition at 1 week postoperatively if needed. Clinical outcomes in terms of visual acuity, refraction, efficacy of astigmatic correction analysed by vector analysis and patient satisfaction were evaluated during a 3-month follow-up.

RESULTS

A total of 82% and 98% of eyes achieved a postoperative uncorrected distance visual acuity of 0.00 and 0.10 logMAR or better, respectively. Furthermore, 99.7%, and 100.0% of eyes showed a postoperative spherical equivalent within ± 0.50 D and ± 1.00 D, with 97.9% of eyes having a postoperative cylinder ≤ 0.50 D. Uncorrected near and intermediate visual acuities were 0.2 logMAR or better in 89.0% and 99.1% of eyes, respectively. Mean difference vector, magnitude of error and angle of error were 0.02  ±  0.14 D, 0.02  ±  0.13 D and 0.11  ±  1.18°. Patient satisfaction was referred as high or very high by 97.6% of patients.

CONCLUSIONS

The implantation of the trifocal toric IOL evaluated following a careful clinical protocol provides an efficacious visual rehabilitation and astigmatic correction, leading to high levels of patient satisfaction.

Online intraocular lens calculation

PURPOSE OF REVIEW

To showcase the majority of online intraocular lens (IOL) calculation tools and highlight some of their characteristics.

RECENT FINDINGS

Online tools are available for preoperative and postoperative IOL-related calculations, including IOL power and toricity selection for standard patients, patients who underwent prior refractive surgery, keratoconus, limbal relaxing incisions for astigmatism management, realignment of a misplaced or rotated toric IOL, surgical induced astigmatism (SIA), formulae comparison, and other tools.

SUMMARY

As there are new online developments and technology is advancing rapidly, we hope that this review will assist ophthalmologists in becoming acquainted with a large variety of online tools.

Ray tracing optimization: a new method for intraocular lens power calculation in regular and irregular corneas

To develop a novel algorithm based on ray tracing, simulated visual performance and through-focus optimization for an accurate intraocular lens (IOL) power calculation. Custom-developed algorithms for ray tracing optimization (RTO) were used to combine the natural corneal higher-order aberrations (HOAs) with multiple sphero-cylindrical corrections in 210 higher order statistical eye models for developing keratoconus. The magnitude of defocus and astigmatism producing the maximum Visual Strehl was considered as the optimal sphero-cylindrical target for IOL power calculation. Corneal astigmatism and the RMS HOAs ranged from - 0.64 ± 0.35D and 0.10 ± 0.04 μm (0-months) to - 3.15 ± 1.38D and 0.82 ± 0.47 μm (120-months). Defocus and astigmatism target was close to neutral for eyes with low amount of HOAs (0 and 12-months), where 91.66% of eyes agreed within ± 0.50D in IOL power calculation (RTO vs. SRK/T). However, corneas with higher amounts of HOAs presented greater visual improvement with an optimized target. In these eyes (24- to 120-months), only 18.05% of eyes agreed within ± 0.50D (RTO vs. SRK/T). The power difference exceeded 3D in 42.2% while the cylinder required adjustments larger than 3D in 18.4% of the cases. Certain amounts of lower and HOAs may interact favourably to improve visual performance, shifting therefore the refractive target for IOL power calculation.

Predicted vs measured posterior corneal astigmatism for toric intraocular lens calculations

PURPOSE

To evaluate the astigmatic correction obtained with a toric intraocular lens using the keratometric readings (Ks) from a swept-source optical coherence tomography (SS-OCT) biometer and the Barrett toric formula with its predicted posterior corneal astigmatism (PCA) value and to compare the results with those expected by using the OCT Ks and a measured PCA from a scheimpflug topographer and by using the SimKs and the measured PCA from the Scheimpflug topographer.

SETTING

Private practice, Lynwood, California.

DESIGN

Retrospective observational study.

METHODS

All measurements were performed by the SS-OCT biometer and the Scheimpflug topographer and using the Barrett toric formula.

RESULTS

We evaluated 122 eyes of 122 patients. The mean absolute errors in predicted residual astigmatism for the entire series were 0.41 ± 0.19 diopters (D) (0.00 to 0.85 D) using the OCT Ks and predicted PCA, 0.45 ± 0.25 D (0.00 to 1.01 D) using the OCT Ks and measured PCA, and 0.49 ± 0.25 D (0.00 to 1.30 D) using the SimKs and measured PCA. The statistically significant differences between the errors had a P value of .062 for the entire series (n = 122), .26 for the subgroup with against-the-rule astigmatism (n = 68), .47 for the subgroup with oblique astigmatism (n = 11), and .05 for the subgroup with with-the-rule astigmatism (n = 43). The percentage of eyes within ±0.50 D were 74% (n = 90), 71% (n = 87) and 64% (n = 78) (P = .13) and within ±0.75 D were 99% (n = 121), 95% (n = 116) and 84% (n = 102) (P < .001), respectively.

CONCLUSIONS

The Barrett toric formula and its predicted PCA performed better with the OCT K readings than with the topographer SimKs and a measured PCA.

Modification of intraocular lens insertion using 4-flanged fixation with a standard cartridge and a 2.4 mm corneal incision in eyes with no capsular support

A modification of intraocular lens (IOL) implantation, using the 4-flanged IOL fixation technique, through a 2.4 mm corneal incision using a standard cartridge and injector, is presented. An IOL with 4 eyelets is used, through which a 6-0 polypropylene suture is threaded on one side and then loaded into a provided cartridge and inserted to the anterior chamber through a 2.4 mm corneal incision. Using the handshake technique, the suture ends are eventually threaded through the eyelets and secured outside the eye with the creation of 4 flanges. This technique was used in 6 eyes of 5 consecutive patients with the absence of capsular support. During all follow-up visits, the IOLs were well centered and stable, and the flanges were buried in the sclera and covered with the conjunctiva. No complications were recorded. This modification simplifies the technique and reduces the well-known complications of large corneal incisions.

Cataract Surgical Planning Using Online Software vs Traditional Methods: A Time/Motion and Quality of Care Study

Purpose

The objective of this study is to compare two methods of preoperative planning for placement of intraocular implants: traditional paper documentation/electronic scans versus a novel online planning software. The aim was to compare the time required and number of data points manually transcribed with each method and to explore whether differences in surgical accuracy could be identified between the two methods in a small sample of 40 patients.

Methods

In this study, preoperative planning was performed twice for all enrolled patients: once through the traditional method and once using an online planning software system. The total time spent and number of data points manually transferred were recorded for each method.

Results

Of the 40 patients enrolled, the mean total surgical planning time was 239 ± 190 seconds for the traditional method vs 63 ± 31 seconds with the online planning software (P<0.00001). The mean number of data points transcribed was 7.2 ± 7.2 for the traditional method vs 0.9 ± 1.7 with the online planning software (P<0.0000001). No statistically significant differences were noted in terms of accuracy of prediction of the spherical equivalent surgical outcome.

Conclusion

In comparison to traditional methods, a significant reduction in time required for surgical planning and the manual transcription of data points was noted when a comprehensive online surgical planning software was used. This has important implications for the efficiency and likely the safety of surgical planning.

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The role of posterior corneal power in 21st century biometry: A review

PURPOSE

Intraocular lens (IOL) calculation and biometry have evolved significantly in recent decades. However, present outcomes are still suboptimal. Our objective is to summarize the results reported in the literature with regard to a new variable, the value of the relationship between anterior and posterior corneal curvature in the biometric calculation of IOL power.

METHODS

We have created a narrative revision of the existing evidence regarding the posterior to anterior corneal curvature ratio in IOL calculation.

RESULTS

The corneal posterior/anterior ratio (P/A ratio), also called Gullstrand ratio, has a standard deviation of 2.4% in normal people, hence causing a possible IOL power miscalculation error of up to 0.75 diopters (D). This error is magnified in pathological corneas or in those with previous refractive surgery. Including the P/A ratio in the IOL formula reduces errors in the calculation of IOL power.

CONCLUSIONS

Measurement of the posterior corneal surface should be recommended prior to IOL calculation, given the demonstrated results regarding the P/A ratio for IOL power calculation. Regarding toric IOL calculation, we suggest incorporation of all internal astigmatic vectors, for instance, posterior corneal surface, IOL tilt induced toricity, and retinal astigmatism. All of these factors may improve surgical outcomes.