**4. IOL power calculation**

An accurate biometry is a precondition not only for toric IOLs but also for regular IOL power calculation. The axial length may be measured by either ultrasonic biometry or optical systems, and SRK/T, Holladay 2, Hoffer Q, and Barrett formula are recommended to be used to calculate sphere power. Nguyen et al. adjusted the power of an existing hydrophobic acrylic IOL by a femtosecond laser [40], which is definitely a promising idea.

There are several toric calculators available for surgical planning that have been developed to predict postoperative cylinder power, such as Barrett toric calculator [41], Holladay toric calculator, and Alcon toric calculator(the revised Alcon toric calculator is a derivation of the Barrett calculator). In general, an ideal IOL power calculation formula should take into account the posterior corneal curvature, the effective lens position (ELP), as well as the SIA. And there are a few formulas available such as Abulafia-Koch linear regression formula [42], Baylor nomogram (a method from Koch) [43], Barrett formula, Abulafia-Koch formula, etc.

#### **4.1 IOL power calculation considering posterior cornea**

A few online toric IOL calculators have been revised to take into account the contribution of the posterior cornea in IOL power calculation, but it proved itself valuable. The Baylor nomogram which incorporates the posterior corneal curvature has been observed to be more precise than traditional Alcon and Holladay toric calculator without posterior corneal astigmatism compensation [44]. However, the revised AcrySof toric calculator incorporates the Barrett toric algorithm, which takes into account both the ELP and the posterior corneal astigmatism, and had better predictability than the Baylor nomogram as well as Holladay and traditional Alcon toric calculator [44]. Other toric IOL calculators such as TECNIS calculator also incorporate posterior corneal astigmatism compensation.

#### **4.2 IOL power calculation considering ELP**

Failing to consider the anterior chamber depth and cornea thickness may result in inaccurate calculations, especially in eyes with extremes of axial lengths [45]. As mentioned above, the revised AcrySof online toric calculator and iTrace toric planner takes into account the ELP [14, 46]. The TECNIS calculator incorporates the anterior chamber depth based on the axial length and keratometry values [46], and the Holladay formula incorporates the ELP in its calculations.
