Ocular-Biometry Driven Changes Of Add-Power Demand In Normal Eyes: Ray-Tracing Simulations

Clinical investigations have shown substantial variability in reported defocus curves of patients implanted with a multifocal intraocular lens (IOL) of the same model. Although each multifocal-lens type has its unique and fixed addition value, the position of the secondary visual-acuity peak detected in defocus-curve measurements may show high intersubject variability. In this study, the add-power demand dependency on ocular biometry was assessed by means of ray-tracing simulations.    

David J. Apple Center for Vision Research, Department of Ophthalmology, University of Heidelberg, Germany.

IOL-Master biometry reports of patients scheduled for routine cataract surgery were collected. In total, 34 eyes were included with the following parameters used to build a personalized eye model: corneal keratometry (KER), anterior chamber depth (ACD), and axial length (AXL). Ray-tracing software was applied for pseudophakic eye simulations. Each model featured a biconvex lens with radii of curvature adjusted for best distance vision. Then, trial glasses with -4D, -3D, -2D, and -1.5D were placed with a vertex distance of 12 mm. The required IOL-power change to induce a desirable near/intermediate effect at the spectacle plane was calculated with respect to the far-point condition.                  

The mean (±standard deviation) KER was 7.79 ±0.24 mm, the ACD was 3.22 ±0.60 mm, and the AXL was 23.77 ±1.34 mm. In this population, the lowest and highest add power (IOL plane) varied by 0.39D, 0.52D, 0.76D, 0.98D for the -1.5D, -2D, -3D and -4D trial glass, respectively. A high correlation was found between the observed add-power change and the ACD, indicating a linear increase of the add-power demand for larger ACD values. By contrast, the AXL and the KER demonstrated poor or no correlation with the required IOL-power adjustment. 

The ACD, which determines the effective lens position, affects the add-power demand of a pseudophakic patient despite a full far-distance correction. Given a fix addition value of multifocal IOLs, patients with shallower ACDs (e.g., hyperopes) may have their near focus closer than preoperatively planned. By contrast, a higher add-power lens may be required for eyes with a deeper anterior chamber (e.g., myopes). The reported change may contribute to the variability in defocus curves of multifocal patients observed clinically.