Optical Functional Properties of the Boston Keratoprosthesis

Abstract

Purpose To evaluate the optical characteristics of the Boston Keratoprosthesis (KPro), identify glare sources, evaluate possible glare control, and examine the benefit of implanting when the fellow eye has normal vision. Methods 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 due to 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 Mentor Brightness Acuity Test (BAT), with and without the dark-iris contact lens. The computed and measured visual fields were compared. Stereopsis was measured in patients with an intact fellow eye. Results The computed and measured MTFs 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 implanted with the Type II KPro. 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. Conclusion 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.

Optical Functional Performance of the Osteo-Odonto-Keratoprosthesis

“…11 Glare is a commonly reported symptom following OOKP surgery and has also been observed with other KPro surgery such as the Boston KPro. 6 The decline in visual acuity with increasing glare settings was similar to those reported with the Boston type 2 KPro.…”

“…Sayegh et al observed that the change in visual acuity in patients implanted with a Boston type 2 KPro was decreased in an AfroCaribbean patient presumed to be because the darker skin pigment served as better protection from glare from the surrounding cornea. 6 We did not observe this effect in our two patients of Asian and Afro-Caribbean descent.…”

“…Sayegh et al has demonstrated that glare causes a much greater decrease in visual acuity with the Boston Type 1 KPro compared with the Type 2 KPro where there is more corneal exposure to external light sources and that this is reduced if a black contact lens is used to cover the exposed surrounding cornea. 6 As the buccal mucous membrane overlies the OOKP and cornea, we would expect the effect of glare from transillumination of the surrounding cornea to be minimal.…”

“…The BAT was also used by Sayegh et al and hence we can directly compare results between the two studies on what are the two keratprosthesis devices with the best survival rates for end-stage corneal disease. 6 Light reflection from the wall of the cylinder could act as a source of glare resulting in a reduction of visual acuity and visual field. 14 KPro optical cylinder on computer simulations, assuming the inner surface of the cylinder acts like a mirror.…”

“…14 KPro optical cylinder on computer simulations, assuming the inner surface of the cylinder acts like a mirror. 6,15 However, they also suggested that the contribution to total glare from light reflected from the wall of the cylinder was minimal. As the OOKP optical cylinder has a longer tubular wall, the effect of glare in an OOKP eye may be greater than that with the Boston KPro.…”