Case Series: Advanced Biometric Calculation

Abstract

Relevance: Over the years, various mathematical models have been developed to optimize the calculation of intraocular lens power for cataract surgery. However, final refractive outcomes remain variable, especially in cases where certain ocular parameters deviate from the norm.

Purpose: Our goal is to describe different calculation methodologies that can be employed based on the patient's preoperative refractive error and surgical history, and to assess the predictive efficacy of these methods.

Case Reports: The first case involves a patient who had previously undergone myopic LASIK, for whom specific formulas such as Pearl-DGS, EVO, Barrett True K post-refractive, or the ASCRS calculator were used to determine the intraocular lens (IOL) power. All four formulas, taking pre-LASIK data into account, predicted an IOL power of +18.00D, aiming for the first negative in the spherical equivalent (SE). After implanting this lens, a residual refraction of -0.25D SE was achieved.

The second case concerns a patient with corneal astigmatism of -1.90D who was recommended femtosecond laser arcuate incisions based on the Castrop nomogram. According to this, given the patient’s age of 79 years and corneal astigmatism of -1.90D, two arcuate incisions with an arc length of 51º were made at the 86º meridian in the 8.5 mm optical zone. At the final follow-up, the patient had an SE of -0.75D.

In the third case, the patient had corneal astigmatism of -3.50D, and a toric IOL was chosen. The lens power was calculated using formulas such as Kane Toric, Barrett Toric, or the ZCT calculator (from the manufacturer). All formulas estimated a similar power of +22.00D with a cylinder of 6.00D. Post-surgery, a residual refraction of +0.25sph -0.75cyl 127º with an SE of -0.12D was obtained.

Conclusions: Thanks to the new mathematical models developed in recent years, which include critical parameters such as the power of the posterior corneal surface, pre-refractive surgery refraction and keratometry, lens thickness, and anterior chamber depth, as well as the integration of artificial intelligence into some methods, refractive surprises are becoming less frequent. However, unknown variables and others that are difficult to estimate continue to influence these results, such as the effective lens position (considered the main source of error), or alterations caused by surgery and potential complications, such as toric IOL rotation, which can significantly impact refractive outcomes.