Standardizing sum-of-segments axial length using refractive index models

Measurement of in vivo lens shapes using IOLMaster 700 B-scan images: Comparison with phakometry

PURPOSE

This study compared in vivo crystalline lens shape measurements using B-scan images from the IOLMaster 700 with phakometry.

METHODS

Twenty-four young adult participants underwent IOLMaster 700 and phakometry measurements under cycloplegia (1% cyclopentolate). The IOLMaster 700 generated B-scan images along six meridians in 30° increments, which were analysed using custom MATLAB software to determine lens surface radii of curvature. Phakometry measurements were obtained using Purkinje images reflected from the lens surfaces.

RESULTS

The IOLMaster 700 image analysis method yielded a lower mean anterior lens surface spherical equivalent power (+6.20 D) than phakometry (+7.55 D); however, the two measurements were strongly correlated (R(21) = 0.97, p < 0.0001). The astigmatic power vectors (J0 and J45) for the anterior lens surface were significantly higher for the IOLMaster 700 measurements, with only J0 showing a significant moderate positive correlation (R(21) = 0.57, p = 0.005). For the posterior lens surface, the IOLMaster 700 measurements had a higher mean spherical power (+14.28 D) compared to phakometry (+13.70 D); however, a strong positive correlation (R(21) = 0.90, p < 0.0001) was observed. No significant correlations were noted for posterior lens surface astigmatic vectors (J0 and J45). The IOLMaster 700 estimates for the equivalent lens mean spherical power were slightly lower than those for phakometry, with a mean difference of -0.72 D, and both methods were positively correlated (R(21) = 0.94, p < 0.0001).

CONCLUSIONS

The findings demonstrate that IOLMaster 700 B-scan image analysis technique provides similar estimates of lens surface powers to phakometry. These results highlight the potential of the IOLMaster 700 to provide measurements of lens shape, informing future research and clinical use.

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.

Evaluating the Predictability of Postoperative Target Refraction Using the Prototype of a New Intraoperative Aberrometer

Despite all the progress in cataract and refractive lens surgery, refractive surprise is common in clinical practice. A significant postoperative refractive error is particularly annoying - and contributes to the patient's dissatisfaction with the procedure and the surgeon - when a multifocal IOL, an EDOF-IOL or a toric IOL has been implanted. The relatively new technology of intraoperative aberrometry offers the surgeon the option to intraoperatively measure the eye and its refraction, either directly after lens extraction and/or following IOL implantation. Currently, three different systems are available. In a number of studies, the technology has shown a better refractive predictability than preoperative biometry. Besides giving an evaluation of the prototype of a new intraoperative aberrometer, the I-O-W-A system, we also present our results on the influence of the kind of anaesthesia chosen and of two different IOL designs on the predictability of intraoperative aberrometry.

Dual study on the sum-of-segments method for axial length measurement: is it better?

PURPOSE

To evaluate the sum-of-segments (SOS) method for optical axial length (AL) measurements.

SETTING

Department of Ophthalmology, University of Aarhus, Denmark, and Private practice, Copenhagen, Denmark.

DESIGN

Retrospective observational study.

METHODS

2 retrospective datasets were included. The first dataset comprised 1491 university cataract cases measured with the LENSTAR LS900 preoperatively and 1 to 4 months postoperatively. The second dataset comprised 904 lens surgery cases with refractive follow-up to study the accuracy of intraocular lens power calculation. The prediction accuracy was evaluated as the difference between the observed and the expected refraction.

RESULTS

The mean difference between the preoperative and the postoperative AL readings was -0.06 mm and -0.020 mm for the standard and the SOS AL method, respectively, however with a larger variation for the SOS AL method ( P < .01). For the second dataset, the SOS method was found to increase the accuracy of the SRK/T and the Holladay formulas. With the Olsen formula, the SOS method was found to be worse ( P < .01). The highest accuracy was found using standard AL with the Olsen formula, with a mean absolute error of 0.24 diopter (D) and 89.8% of the cases within ±0.5 D.

CONCLUSIONS

The SOS method improved the accuracy of the classical formulas probably because the optical path is a better representative of the true AL. The Olsen formula already incorporates an optical path correction, and this may be the reason for the lack of improvement with the SOS method.

Update on Biometry and Lens Calculation - A Review of the Basic Principles and New Developments

These days, accurate calculation of artificial lenses is an important aspect of patient management. In addition to the classic theoretical optical formulae there are a number of new approaches, most of which are available as online calculators. This review aims to explain the background of artificial lens calculation and provide an update on study results based on the latest calculation approaches. Today, optical biometry provides the computational basis for theoretical optical formulae, ray tracing, and also empirical approaches using artificial intelligence. Manufacturer information on IOL design and IOL power recorded as part of quality control could improve calculations, especially for higher IOL powers. With modern measurement data, there is further potential for improvement in the determination of the axial length to the retinal pigment epithelium and by adopting a sum-of-segment approach. With the available data, the cornea can be assumed to be a thick lens. The Kane formula, the EVO 2.0 formula, the Castrop formula, the PEARL-DGS, formula and the OKULIX calculation software provide consistently good results for artificial lens calculations. Excellent refractive results can be achieved using these tools, with approximately 80% having an absolute prediction error within 0.50 dpt, at least in highly selected study populations. The Barrett Universal II formula also produces excellent results in the normal and long axial length range. For eyes with short axial lengths, the use of Barrett Universal II should be reconsidered; in this case, one of the methods mentioned above is preferable. Second Eye Refinement can also be considered in this patient population, in conjunction with established classic third generation formulae.