A-Constant Optimisation For Aspira-Axa Iol For Combined Phacoemulsification And Pars Plana Vitrectomy

The aim of this study was to optimize the A-constant for the 7mm implant with 4 point haptics support (ASPIRA-aXA, Human Optics, Germany)  when used for combined phakovitrectomy.

Department of Ophthalmology, Philipps University of Marburg, Germany

We obtained data from 104 eyes of 101 patients. Mean age was 64.4 ± 8.9 years, median age 66 (4 3to 83) years. The indications for surgery were as follows:  39 retinal detachments, 34 macular pucker or vitreomacular tractions, 27 (partial) macular holes  and 4 vitreous opacities.  All eyes had phakovitrectomy with ASPIRA-aXA IOL from 01/2018 till 07/2021. For preoperative calculation we used optical biometry (IOL-Master 700; Carl Zeiss Meditec AG, Germany) and SRK/T formula with the A constant of 119,568 as suggested by the manufacturer for pure phacoemulsification surgery. The postoperative refraction had to be 4 weeks or longer. For the optimisation we used iteration as well as the non-linear optimization algorithm  (Levenberg-Marquardt).

The mean axial length was 24.69 mm ± 1.72 with a median of 24.32 mm (min 21,83mm, max. 30,79 mm). The new A constant was calculated as 119,154 using regression analysis and 119,171 using Levenberg-Marquardt method. The predictability distribution was: ≤ 0.5 D in 56.7 %,  ≤ 1.0 D in 92.3 % and ≤ 2.0 D in 100%. A subgroup of  92 eyes was created after the removal of 12 outliers delivering an A constant of 119,167 and predictability of ≤ 0.5 D in 58.7%, ≤ 1.0 D in 94.6 % and ≤ 2 D in all eyes. The median of prediction error was 0.5 D of myopisation.

We recommend the A constant of 119.16 (instead of 119.57) for the  SRK-T formula for phakovitrectomy. The limitation of this study was a higher number of myopic patients and more challenging situation for the optical biometry measurements