Optical functional properties of the Boston Keratoprosthesis

To evaluate the optical characteristics of the Boston Keratoprosthesis (KPro), identify glare sources, evaluate possible glare control, and examine the benefit of implantation when the fellow eye has normal vision.

Computed and optical-bench-measured point spread function (PSF) and glare sources were compared. A translucent plastic cornea was used to determine the impact of glare caused by scatter in the cornea and its control with a dark-iris tinted contact lens. The effect of glare in implanted eyes was measured with a brightness acuity test (BAT), with and without the dark-iris contact lens. Computed and measured visual fields were compared. Stereopsis was measured in patients with an intact fellow eye.

Computed and measured modulation transfer functions for the KPro were found to be very close to the diffraction limit. Both the model-eye measurements and patients' BAT glare responses identified that the hazy corneal graft surrounding the KPro is the main source of glare and can be controlled with a dark-iris contact lens. The lid effectively blocks the light that would be scattered in the hazy cornea of patients in whom the type II KPro was implanted. An intact fellow eye remains the dominant eye, with better acuity, and the KPro eye supports only minimal stereo ability and does not expand the binocular visual field.

Glare can be reduced significantly with the use of a contact lens with a dark iris. Implanting the KPro in a patient whose fellow eye has normal or near normal vision does not seem to improve visual function.