After studying the effects of decentration and tilt on image quality in both aphakic and pseudophakic eyes with the type I Boston keratoprosthesis (KPro), researchers found decentration of up to 0.5mm had no significant impact on image quality.

However, they did find decentration of 0.9mm or more during surgical implantation can result in significant degradation in retinal image quality, including astigmatism, with the effect being greater in the pseudophakic eye.

The study, published in the June issue of Cornea, used an optical ray-tracing program to simulate the image projected onto the retina in an eye with a perfectly centered KPro and in eyes with varying degrees of KPro decentration and tilt. Decentration was modeled along a typical white-to-white distance of 12.0mm, and the corresponding tilt was calculated assuming a radius of curvature of 8.0mm—i.e, that of the KPro back plate. The researchers simulated both aphakic and pseudophakic eyes and analyzed the corresponding modulation transfer function curves, point spread functions and astigmatism.

Data showed the perfectly centered KPro produced a high-quality image with no induced astigmatism. Additionally, increasing decentration beyond approximately 0.5mm resulted in poorer image quality with a more pronounced effect in the presence of an intraocular lens. Using models of the normal eye as a threshold, image degradation due to decentration became clinically significant at approximately 1.4mm and 0.9mm for the aphakic and pseudophakic cases, respectively.

Helms, RW, Zhao X, Sayegh RR. Keratoprosthesis decentration and tilt results in degradation in image quality. Cornea. 2018;37(6):772-77.