Current accuracy of a simple Gaussian optics-based thin lens intraocular lens power calculation formula

Abstract

 

Purpose

 

To investigate the accuracy of a Gaussian optics-based thin lens intraocular lens (IOL) power calculation formula, implementing partial coherence interferometry (PCI) biometry and recent insights in the effective corneal refractive index (ECRI).

 

Methods

 

PCI biometry (Haag-Streit Lenstar LS 900) was performed in 71 eyes of 71 patients scheduled for cataract surgery with implantation of a monofocal, non-toric IOL (Alcon SA60AT). Three months postoperatively, biometry was repeated, and the refractive result was assessed using the Nidek ARK-530A autorefractor. Multiple linear regression analysis was used to find variables predictive of pseudophakic anterior chamber depth (ACD). Pseudophakic ACD was then predicted for each eye based on these variables. IOL power (P_i) was calculated for each eye using a thin lens formula based on Gaussian optics (Olsen, JCRS 1987), using the preoperative biometry, predicted pseudophakic ACD, and an ECRI of 1.328 (Ho, JCRS 2008). Based on the actual refractive result, we calculated the IOL power (P₀) that would have been required on hindsight to attain the target postoperative refraction. We then calculated the mean absolute error (MAE) of our formula as well as that of the Haigis, Hoffer Q, Holladay 1, and SRK/T formulas as the average of the absolute differences between P_i and P₀. In addition, we assessed the effect of a change in ECRI on the MAE of our formula.

 

Results

 

Mean axial length was 24.29 ± 1.59 mm (range 21.20 - 28.46). Variables predictive of pseudophakic ACD were phakic ACD, lens thickness, axial length, and average corneal radius. MAE of our formula was 0.41 ± 0.31 dioptres. MAEs of the other formulas are shown in table 1. The differences were not statistically significant (Kruskal-Wallis test, p=0.77). Either increasing or decreasing ECRI caused the MAE of our formula to increase, as shown in figure 1.

 

Conclusions

 

With PCI biometry and recent insights in the ECRI, it is possible to accurately calculate IOL power with a simple Gaussian optics-based thin lens IOL calculation formula. In addition, our study confirms recent work of others suggesting that the true ECRI is lower than values that have traditionally been used.

 

 

Table 1. MAEs (dioptres ± standard deviation) of our formula and 4 other formulas

 

 

Figure 1. Effect of a change in ECRI on MAE of our formula. Red, blue and green lines represent ECRI of 1.328, 1.3315 and 1.3375, respectively

 

Keywords: 567 intraocular lens • 743 treatment outcomes of cataract surgery