Abstract
Purpose:
Recent reports have focused on methods to improve centration of the Boston keratoprosthesis (KPro) during surgery. However, the optical effects of decentration and consequently tilting of the device are unknown. We investigate these effects on image quality using a computer model.
Methods:
A model of the PMMA KPro was created in Zemax (Focus Software Inc, San Diego, Calif). Computerized ray-tracing technique was used to simulate the image projected on the retina in an eye with a perfectly centered KPro, and eyes with various degrees of KPro decentration and corresponding tilt. The degree of tilt was calculated based on the radius of curvature of KPro backplate of 8.0 mm. The spot diagrams for a wavelength of 587.6 nm and a pupil diameter of 3.0 mm were derived and the simulated images on the macula are presented.
Results:
The perfectly centered KPro (0 mm decentration) had a tight point spread function which resulted in the formation of a high quality image. Decentration of the KPro by 0.8, 1.5, and 2.3 mm (backplate abutting the angle) with a corresponding respective tilt of 5.6, 11.4, and 17.0 degrees was simulated. Spot diagrams demonstrating the change in image quality of a point object across the retina showed increased astigmatism and peripheral distortions of the retinal image with increasing decentration and tilt. This became significant with a decentration of 1.5 mm and beyond. Simulated images illustrate the resultant distortions. The addition of asphericity to the anterior lens surface of the KPro resulted in improved off-axis image quality while maintaining good potential visual acuity.
Conclusions:
Low amounts of decentration and tilt of the KPro are of little consequence on image quality projected on the retina, however, distortion of the projected image occurs with higher amounts of decentration. Our results illustrate the importance of attempting good centration of the device during surgery. Alternatively, aspherically optimizing the lens surface during manufacturing would result in dampening of the distortion effect. The model also confirms our previous observation that tilting of the KPro result in astigmatism on refraction.