MIOL implantation yielded good distant and near visual acuity in post-refractive surgery eyes, and the emmetropia target is also recommended for good near and distance visual acuity. In calculating mIOL power after refractive surgery, the Barrett True-K formula, which considers posterior corneal curvature as well as anterior corneal curvature, was more accurate than the Shammas-PL formula. However, eyes with history of larger amount of laser ablation in corneal refractive surgery (39.13 D and 26.59 mm were the mean values of K and AL in the present study, respectively) seems to show weaker predictive accuracy of IOL power calculation when using the Barrett True-K formula.
The predictive accuracy of the Barrett True-K formula was higher than that of the Shammas-PL formula, which is thought to be due to the fact that the Barrett True-K formula showed more hyperopic PE than Shammas-PL formula6. Although conventional formulas such as Shammas-PL and Haigis-L calculate IOL power based upon preoperative keratometry measuring only the anterior curvature, supplemental factors have been added to these formulas to reduce PE for eyes with prior corneal refractive surgery4,11. Specific methods for corneal power correction have been used with both Shammas-PL and Haigis-L to compensate for overestimated keratometry in eyes with prior corneal refractive surgery. However, the keratometry used in both formulas were not measured values, but expected values developed by using regression analysis (a no-history method). As a result, IOL power calculations with Shammas-PL and Haigis-L have shown more myopic PE in eyes with prior corneal refractive surgery than those preoperatively expected7.
The Barrett True-K formula more accurately calculates IOL power, representing the curvature of the entire cornea by including the posterior corneal curvature measurement6,11−13. In clinical studies reporting the results of cataract surgery after corneal refractive laser surgery, IOL power calculation using total keratometry which considers both anterior and posterior corneal curvature showed better results than conventional calculation using only the anterior corneal curvature14–19. Additionally, Shammas-PL based on total keratometry measured with an IOLMaster 700 biometer showed improved accuracy compared to that based on anterior keratometry in clinical studies by Lawless et al. and Yeo et. al.20,21 Recently, formulas using a ray-tracing method developed directly by the research institute were presented for more accurate IOL power calculation considering the anterior and posterior corneal curvature as well as the corneal thickness15,22−24. Keratometry calculated from directly measured corneal curvatures of both the anterior and posterior surfaces would improve refractive outcomes due to the correction of myopic refractive error along with conventional IOL power calculations in eyes with prior corneal refractive surgery.
In the present study, eyes with extreme biometry (mean K value of 39.13 D and mean AL of 26.59 mm) showed greater PE as calculated using Barrett True-K. IOL power calculation formulas for eyes with prior corneal refractive surgery are divided into history methods, which consider pre-refractive surgery data, and no-history methods, which are used when data from the refractive surgery are not available. Although there was no significant difference between the two methods4,11,25, there has been no single formula showing high accuracy in various biometry conditions including eyes after corneal refractive surgery26,27. Recently, Whang et al. reported in a retrospective study that the predictive accuracy of no-history IOL formulas depends on AL28. Barrett True-K was most accurate for AL less than 28 mm, Triple-S was most accurate for AL between 28 and 30 mm, and Shammas-PL was most accurate for AL 30 mm or more. In the present study, mean preoperative AL was 25.84 ± 1.81 mm and only two eyes had a preoperative AL of more than 30 mm; thus, further evaluation of prediction accuracy in the case of extremely long eyes requires further study.
Eyes with high PE as calculated by Barrett True-K (group B in Table 4) showed preoperative characteristics of lower K-values and longer axial length. These biometry results correspond to eyes that experienced larger amount of ablation during refractive laser surgery. Additionally, both IOL power selection targeting myopic refraction and postoperative myopic refractive outcome resulted in PE being myopic. Previous studies with earlier formulas recommended targeting myopic due to hyperopic refractive surprise when calculating IOL power in eyes with previous corneal refractive surgery27–31. Recent studies have reported that it is good to make the postoperative MR close to emmetropia in eyes having general corneal curvature, and the same results were reported for eyes which have undergone corneal refractive surgery32,33. However, targeting slightly myopic may improve near vision without compromising distant vision, especially in high myopic patients, which is the case in the real world practice. Based on our result, we recommend aiming emmetropia when using Barrett True-K formula for implanting mIOL in eyes that have undergone corneal refractive surgery. Moreover, this study shows that predictive accuracy is more important than mIOL type for postoperative visual outcomes of patients with prior corneal refractive surgery.
To measure postoperative refractive power of the eye, MR is recommended after mIOL implatation34. Pseudophakia with a diffractive multifocal IOL (Tecnis ZM900 or ReSTOR) showed similar refraction between autorefraction and MR measurements, including both spherical and astigmatic values34,35. However, for eyes with a refractive multifocal IOL (ReZoom and Lentis LS-312 MF30), spherical values were found to be underestimated by autorefraction compared to MR34,36,37. Interestingly, in the present study both EDOF IOL (Tecnis Symfony) and rotationally asymmetric multifocal IOL (Lentis LS-312 MF30) also led to underestimation of spherical power by means of autorefraction compared to MR. Postoperative refraction in patients with refractive or EDOF mIOL should be measured by MR.
However, there were some limitations in this study. IOL power calculation formulas with no-history methods were not compared to history methods in this retrospective study. Type of corneal refractive surgery was not identified due to a lack of information in some medical records. Although this was a relatively large-scale study comparing the results of different types of mIOLs, further prospective study based on a larger number with same type of mIOL is needed in order to improve the predictive accuracy of each IOL power calculation, especially in eyes that have undergone a large amount of corneal ablation.
In conclusion, MIOL implantation after corneal refractive laser surgery showed good refractive and visual outcomes. The Barrett True-K formula, which considers both anterior and posterior corneal curvature, was more accurate than the Shammas-PL formula, which considers only anterior corneal curvature. Targeting emmetropia is recommended when implanting mIOL in eyes that have undergone corneal refractive surgery. Especially in eyes that have undergone extensive corneal ablation, the previously large amount of laser ablation for cornea seems to be an important factor causing low predictive accuracy of IOL power calculation in cataract surgery.