The constant incorporation of variables into the intraocular lens calculation formulae has been improving the calculation of the IOL to be implanted for years. The introduction of LA and keratometry were fundamental, but not sufficient. Little by little, variables have been added which, although not essential for the calculations, help to improve the efficiency of the formulas they generate.

Improving the accuracy of IOL calculations is becoming increasingly important as more and more trifocal and extended depth of focus (Edof) type lenses are implanted, which require much greater precision of the refractive outcome to maximise visual efficiency and not be penalised by unexpected refractive outcomes.

In the study by Ribeiro et al. (12) they evaluated the changes of each optical parameter on the refractive state of the eye and found that a 0.25 mm change in ACD measurement corresponds to an error of 0.10 D in an eye with an NA of 30.0 mm and 0.50 D in an eye with an AL of 20.0 mm. Following these results, studies were carried out assessing various ranges of ACD values to evaluate which formula was more effective. (13, 14).

However, formulas such as SRK/T (5) or Hoffer Q (3) which are still very frequently used today, did not include ACD in their variables.

Thickening of the lens occurs throughout life predominantly towards the anterior chamber (15). In an older population, as is often the case in the cataract population, the lens is thickened and therefore the depth of the ACD is shallower. Ribeiro et al. (12) found that for every 1% increase in lens thickness, the refractive error changes by 0.097, a 0.104 mm change in LT means a 0.25 D change in refractive error. Studies have already been published relating the different ocular parameters, including LT and the prediction error of the different formulas (16, 17).

However, there are no studies that support the incorporation of macular thickness as a variable to be taken into account in the calculation of the power of the lens to be implanted. One of the problems with ultrasonic biometry was that it measured the LA up to the inner limiting membrane, but the photoreceptor layer is in a more posterior area, close to Bruch's membrane (18, 19). Because of this, IOL power calculation formulae dating back to the 1970s added an average of 200 µm to the measured axial length to compensate for the difference.

The disparity was great, as the Binkhorst formula used a correction value of 0.25 mm, the Hoffer formula used a value of 0 and the Holladay formula used a value of 0.20 mm. However, the authors of the SRK/T formula decided that the retinal thickness factor was an interdependent variable of AL, with greater accuracy and prediction being observed using low values in long eyes and high values in short eyes, the relationship being determined by the following formula: retinal thickness = 0.65696-0.02029xAL (5). Nowadays, macular thickness is estimated at 250 microns as normal, but its great variability with respect to sex, age and different populations in healthy eyes has been studied (20). The introduction of optical biometers using the principle of partial coherence laser interferometry has improved the measurement of AL (21) but since its introduction into daily clinical practice, the influence of retinal thickness, in particular macular thickness, in improving the efficacy of biometric formulae has not been re-evaluated.

In our study, the significant predictor variables were axial length, mean preoperative K1 and K2 and preoperative macular thickness.

Unlike other authors, the anterior chamber depth did not improve the model and showed a high collinearity factor (VIF Variance Inflation Factor) with the LA. This is logical as the anterior chamber depth is part of the total axial length. The model achieves an adjusted R-squared of 0.96 indicating that the power of the implanted lens is 96% dependent on the value of the axial length, the mean of the preoperative Ks (the mean has been performed because of the excessive collinearity between the two Ks) and the preoperative macular thickness. The introduction of the preoperative macular thickness improves the R-squared of the model and is significant in the calculation of the coefficients. Table 5 shows the correlations between the most commonly used biometric formulae.