The Optical Function of Keratoprostheses

Abstract

The optical function of acrylic cylinders used in keratoprostheses is demonstrated by using a water-filled minicamera as a model of the aphakic human eye. Lengths and diameters of the optical cylinders are important factors influencing the visual fields. The dimensions to be chosen, depends on the thickness of the cornea and the supporting tissues. The most suitable combination of visual field, magnification and diameter of the retinal image field is obtained with optical cylinders with a concave posterior surface. Using a fixed radius of curvature of the posterior surface, the radius of the anterior surfaces are calculated to make the eye emmetropic. With such a cylinder of length 4.5 mm and diameter 2.1 mm a circular visual field of 48" is obtained. With cylinder length 6.0 mm the visual field is at least 35'. The retinal image is about 190/0 larger than that of the normal phakic eye. Cylinder length exceeding 6.0 mm require a greater diameter to provide an adequate visual field. With cylinder lengths up to (3.0 mm and diameters not less than 2.1 mm, the diameter of the retinal image field is at least 12.0 mm. Accidental obliquity of the optical cylinder or off-center implantation give the possibility of undesired projection of the image field outside the macula. The obliquity or degree of off-center implantation tolerance is calculated. The retinal image may be improved by darkening the side of the cylinder.

Optical Functional Properties of the Boston Keratoprosthesis

“…In eyes with Type I KPros we found about 50° in the temporal direction, compared with the 44° computed, and in Type II, about 40° temporally. These results are also consistent with the 70° diameter measured by Sokol4 for a similarly-dimensioned prosthetic of 4.5 mm length and 3.1 mm diameter, and with the about 100° computed by Rol et al5 for a 3 mm diameter and 3 mm length. While these are very wide fields, it is obvious that in a situation where a patient has one normal eye and the other suffers from severe corneal damage, expanding the visual field is not a good reason for implanting KPro in the damaged eye.…”

“…The reflected light would result in diffusion circles or circular stripes which can cause a decrease in visual acuity and contraction of the visual field 18. Sokol et al4 painted the side of the cylinders black, resulting in improved image sharpness and contrast, particularly in the periphery. We were able to illustrate a similar phenomenon in the KPro using the computer simulation, assuming that the inner surface of the optical cylinder behaves as a perfect polished mirror.…”

“…However, little has been published about the visual performance and optical properties,4, 5 with discussions usually limited to visual acuity changes over time. Indeed, excellent visual acuity results have been demonstrated for the Boston KPro,6 as in the current study where, with selected patients, average acuity with the KPro was 20/30.…”

Keratoprosthesis II. Results obtained after implantation of 27 dismountable two‐piece prostheses A retrospective, follow‐up study

“…The visual fields of our patients were not recorded. However, studies by Sokol et al (1977) have shown a field of about 35 degrees through a 6 mm long optical cylinder, a finding which seemed to correspond well with the general visual performance of our patients. Three eyes with a certain preoperative light perception became completely amaurotic during follow-up.…”

Optical Functional Performance of the Osteo-Odonto-Keratoprosthesis

“…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.…”