Intraocular lens power calculation for combined cataract surgery, vitrectomy and peeling of epiretinal membranes for macular oedema

Purpose: This study aimed to compare preoperatively calculated and postoperatively achieved refraction following phacoemulsification and intraocular lens (IOL) implantation combined with pars plana vitrectomy with internal limiting membrane or epiretinal membrane removal. Methods: Twelve eyes of 12 patients who underwent a combined procedure were enrolled in this study. Ultrasound axial length and macular thickness (optical coherence tomography, OCT) measurements were performed pre- and postoperatively. Intraocular lens power was calculated with the SRK/T formula and the planned refraction was compared with results of postoperative automatic refractometry. Comparisons between IOL calculations using the measured axial length and calculations using an adjusted axial length (by adding the macular thickness measured on OCT to the ultrasound axial length) were also performed. Results: Axial length increased (p = 0.02), whereas macular thickness decreased (p = 0.002). The planned refraction was 0.63 +/- 0.38 dioptres, whereas the postoperative refraction error was - 0.16 +/- 0.48 D (p = 0.02), with a mean difference of - 0.79 +/- 0.24 D. During case-to-case comparisons, differences were observed between IOL calculations using the measured axial length and calculations using an adjusted axial length (0.63 +/- 0.38 D versus 0.13 +/- 0.48 D, p = 0.005). Correlation analysis showed a significant relationship between the 'adjusted planned' and the measured postoperative ametropia (r = 0.93, p < 0.001). Conclusions: The observed myopic shift due to erroneous IOL calculation results from underestimation of the axial length due to a thicker macula and calls for preoperative adjustment of ultrasound axial length when biometry is performed by this method.