Seeing through the Boston Keratoprosthesis: A detailed optical assessment with natural scene imaging and characterization of the resolving power.

Abstract

Purpose : To visualize the image formation through the Boston Keratoprosthesis (B-KPro) and assess its optical properties.

Methods : We simulated the realistic natural-scene imaging condition by building an imaging cage system which subjected the B-KPro's posterior surface to an aqueous medium. Specifically, the B-KPro (back focal length: 17mm) was centrally clamped with a diaphragm and was mounted on a water-filled cuvette. Images formed by the B-KPro were then relayed to a camera with a pair of achromatic doublets (magnification: 1x). Targets including a standard eye chart and the natural scenes were imaged to assess the image quality and any presence of optical aberrations. Imaging of the 1951 USAF Resolution Test Target and a pinhole was also performed in air with a 10x telescope to evaluate the B-KPro's resolution efficiency and modulation transfer function (MTF).

Results : The 20/20 line of the eye chart could be adequately resolved. It remained in focus within a field angle of 3.8 degrees, which is well within the foveal area that confers 50-100% of the highest visual acuity. Images of the outdoor scene were similarly in focus in the central field but became defocused peripherally. However, the peripheral images could be refocused by adjusting the imaging distance of the B-KPro. This suggests the presence of spherical aberration that may be corrected by introducing an aspheric shape in B-KPro's optical surfaces. Finally, the spatial frequency resolution achieved by the B-KPro device was 146 line pairs per millimeter (lp/mm) in air at an MTF value of 0.2, which is higher than 25 lp/mm for a human eye with a 3mm aperture. Compared to the theoretical resolution of 290.7 lp/mm, the resolution efficiency was 66% which is sufficiently high according to the ISO11979-2 recommendation for intraocular lenses.

Conclusions : The B-KPro provides sufficient resolution power to achieve 20/20 vision over an angular field of view of 3.8 degrees, as elucidated by the natural-scene imaging and benchtop resolution characterization. Further improvement in the optical design may reduce the spherical aberration and expand the surgical margins for implantation decentration which may otherwise lead to vision degradation.

This is a 2021 ARVO Annual Meeting abstract.