Peripheral defocus of polymethylmethacrylate spherical optic monofocal intraocular lenses

Abstract

Purpose : To evaluate the peripheral defocus of monofocal intraocular lenses (IOLs), thereby quantifying the accuracy of laser ray tracing (LRT) measurements of peripheral defocus.

Methods : Data were acquired on 14 polymethylmethacrylate spherical optic monofocal IOLs (overall length 12.5 mm, optic diameter 6.0 mm). The power (D) of the IOLs used were 17.5, 18, 18.5, 19, 19.5, 20, 20.5, 21, 22, 22.5, 23, 23.5, 24 and 24.5. IOLs were placed in a custom-built combined LRT and optical coherence tomography (OCT) system (Ruggeri et al, BOE 2018). Ray trace experiments were performed for incidence angles ranging from -30° to +30° in 5° increments using a raster scan (6 mm x 6 mm) with 0.25 mm spacing. An image sensor mounted on a positioning stage below the IOL acquired ray-intercept spots at different axial positions and a custom program calculated the ray slopes from the spot positions. At each angle, lens power was calculated by finding the axial position that minimizes the root-mean-square diameter of the spot pattern formed by the central 169 rays corresponding to the central 3 mm zone. For each IOL, the measured relative peripheral defocus (Figure 1) was compared to the value obtained using optical theory. The theoretical peripheral defocus was the position of the circle of least confusion calculated assuming that the IOL is a thin lens with the aperture stop located at the plane of the lens.

Results : The measured peripheral defocus (D) increased significantly (all p≤0.001) with increasing incidence angle (IOL power adjusted mean of +0.27, +0.78, +1.85, +3.25, +5.54 and +8.60 at ±5°, ±10°, ±15°, ±20°, ±25° and ±30° respectively). The IOL power mean difference (D) between the measured and predicted peripheral defocus (Figure 2) was +0.03, -0.21, -0.24, -0.74, -0.76 and -0.59 at ±5°, ±10°, ±15°, ±20°, ±25° and ±30° respectively.

Conclusions : The IOL power increases with increasing field angle corresponding to a shift towards myopic peripheral defocus. The LRT-OCT system accurately measures the peripheral power.

This is a 2021 ARVO Annual Meeting abstract.

 

Figure 1: Scatterplot showing the relative peripheral defocus (difference between off-axis and on-axis power) for each intraocular lens at various delivery angles.

 

Figure 2: Scatterplot showing the difference between the measured and predicted peripheral defocus for each intraocular lens at various delivery angles.