Accurate Lens Power Selection

Imaging and biometry contribute to improved outcomes. Roibeárd O’hÉineacháin reports

Newer technologies and IOL power formulae increase the predictability of cataract surgery outcomes, according to Professor Filomena Ribeiro MD, PhD, FEBO.

Accurate IOL power calculation requires a thorough examination of the cornea, from the tear film to the posterior surface cornea, with multiple measurements and modern IOL calculation formulae, she said.

“Accurate lens power selection demands biometric data validation and a proper IOL power calculation method. One of the golden keys is the preoperative evaluation of all the pathologies that could impact [both] the quality of vision and the probability [of achieving] emmetropia.”

Prof Ribeiro stressed the importance of considering the possibility of ocular surface disease, pupillary abnormalities, and zonular pathology. Regarding the ocular surface, she said to always be sure to treat dry eye before making corneal assessments. Research has shown treating dry eye before making corneal measurements increases the proportion achieving a postoperative refraction within 0.5 D of target by 8% and increases the proportion of patients with 0.5 D or less residual astigmatism from 40% to 75%.

“It is important to recognise the relevance of the corneal dioptric power, which represents two-thirds of the total dioptric power of the human eye’s optical system, and only small changes can be very significant in IOL power calculation.”

She noted that topography is essential to evaluate the regularity of the cornea but also to make sure there is no great difference between the value of the axis between the centre and the more peripheral values. It is also important to always select high-quality images and obtain several measurements (with more than one instrument), if possible.

TOTAL CORNEAL POWER Prof Ribeiro said the total corneal powers used in classical IOL calculation formulae are based on keratometric values for the anterior cornea curvature and a keratometric index which assumes a constant ratio of 82.2% between the posterior and anterior surface. However, nowadays it is possible to directly measure the posterior surface with Scheimpflug devices like the Pentacam® (Oculus) and optical biometers like the IOLMaster® (Zeiss). Using these measurements, surgeons can calculate the true total corneal dioptric power with Gaussian optics, vector addition, or ray tracing using Snell’s law.

“But these powers cannot be used directly—we need to convert them into equivalent K values because formulae expect keratometric values and there is a systematic difference between keratometric values and directly measured total corneal power.”

Some of the new formulae now have the option to include the directly measured posterior corneal surface values, although it is still under investigation whether using those measurements can improve visual outcomes and refractive accuracy in toric IOLs compared with standard index values. However, research suggests the directly measured total corneal power measurements may improve results in post-LASIK cases and smooth the usual undercorrection of against-the-rule astigmatism.

In the 2021 ESCRS practice survey, optical biometry and Scheimpflug tomography were the primary preoperative measurements used by 74% and 57% of respondents, respectively. In the same survey, similar proportions of respondents reported using Scheimpflug and optical biometry when making astigmatism power and AXIS decisions when implanting a toric IOL.

NEWER FORMULAE IMPROVING OUTCOMES In the last decade, many new and better formulae making use of the new biometers, resulting in significant improvements in predicting postoperative refraction. For example, in 2011, 40% of eyes achieved refraction within 0.25 D of target, and 75% did so within 0.5 D of target. By comparison, 2019 respective values were 50% and 80%.

“We can now reach an absolute prediction error of around 0.25 D and with a very good probability of success. And if you use more than one formula, you can have an interval of residual error prediction between two steps of dioptric powers and assess the probability of success in achieving emmetropia,” Prof Ribeiro said.

In addition to the use of directly measured total keratometry values and better IOL calculation formulae, factors contributing to these improvements in predictability include better axial length error correction in long eyes with empirical optimised axial length measurements, sum-of-segment measurements, better effective lens position estimations with more predictive components, and increased computational power that allows for the employment of linear regression and machine learning.

“We need strong preoperative evaluation and validation of all the data. We need to assess the associated pathologies. We need to validate all the measurements, if possible, with more than one instrument, compare the measurements with the population average values, and always do the spherical and toric calculation in every patient. And finally, to optimise outcomes, we need to evaluate our results,” Prof Ribeiro concluded.

“It is important to recognise the relevance of the corneal dioptric power, which represents two-thirds of the total dioptric power of the human eye’s optical system, and only small changes can be very significant in IOL power calculation.”

This presentation was made at the ESCRS Presbyopia IOL Forum during the ESCRS Virtual Winter Meeting 2022.

Prof Filomena Ribeiro MD, PhD, FEBO is Head of the Ophthalmology Department at Hospital da Luz, Lisbon, Portugal. fjribeiro@hospitaldaluz.pt