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Optical Qualities of the Boston KeratoprosthesisRony R. Sayegh†1, Linda Avena Diaz1, Fernando Vargas-Martín2, Robert H. Webb2, Claes H.

Dohlman1, Eli Peli2

Optical Qualities of the Boston KeratoprosthesisRony R. Sayegh†1, Linda Avena Diaz1, Fernando Vargas-Martín2, Robert H. Webb2, Claes H.

Dohlman1, Eli Peli2

Better understanding of the visual function with the Keratoprosthesis (KPro) may lead to design and practice modifications that could further improved outcomes. Here we addressed:

(1) the effect, sources, and control of glare

(2) the binocular vision benefits of implanting a KPro in a patient with a fellow healthy eye

Optical Analysis

Zemax (Focus Software Inc, San Diego, CA) was used to derive theoretical optical parameters of the KPro: Visual fields, point spread function (PSF), spot diagrams, and sources of glare.

Artificial Eye

An optical bench setup was used to assess of the optical quality and the effect of glare on the retinal image. To test for the role of the hazy cornea, the KPro was inserted into a finely sand-blasted Boston scleral lens.

1 Massachusetts Eye & Ear Infirmary, Boston, MA.2 The Schepens Eye Research Institute, Boston, MA.† Correspondance: rsaye001@umaryland.edu

♦Our experiments indicate that the hazy cornea surrounding the implant is a major source of glare in KPro implanted eyes.

♦Glare was virtually eliminated when the KPro was implanted into an opaque iris in the artificial eye.

♦The tinted soft lens did reduce glare but not to the same extent. This is attributed to the larger diameter of the CL pupil.

♦In patients, the use of the same CL resulted in an improvement in visual acuity of about 38%. Type II KPro patients, with lid tissue around the nub, were even less sensitive to increasing glare.

♦The beneficial effect of a painted CL could potentially be mimicked by keeping as much of the natural iris intact during the surgery.

♦Visual fields in KPros are dependent on the diameter and length of the optical cylinder. In the case of the Boston KPro, these were found to be around 95° and 90° wide for Types I and II respectively, providing patients good peripheral vision. However the binocular field is not significantly improved for patients with intact fellow eye

♦Little benefit to stereo vision seemed to be derived from implanting a KPro in the presence of an undiseased contralateral eye with conserved vision.

♦The latter 2 points may advice against KPro for patient with intact fellow eye

In the artificial eye optical bench setup with Type 1 KPro installed in a metal iris an image of a distant point-source was consistent with the computed high quality optics.

A soft Kontur CL with an opaque (1% transmissive) iris placed on top of the scleral lens-KPro construct decreased the size of the surrounding scatter halo; however, it was not completely eliminated.

A halo of scattered light surrounding the central projection of the bright point source was observed in the artificial eye with the sand blasted simulated cornea, which could be eliminated by decreasing the aperture of the iris down to a 3-mm pupil.

Stereoscopic ExamStereo acuity and eye preference was evaluated for 3 patients with intact contralateral eye. In all, vision was better in the non-KPro-implanted eye (by 1 to 4 lines). The fellow eye was found do be dominant. Stereopsis at 16 inches ranged from 400-800 seconds of arc, reflecting minimal stereoscopic ability.

Additional testsGoldmann kinetic perimetry (V4e). Stereo acuity measured in patients with intact fellow eye (Wirt Stereo Fly Test, Stereo Optical Co., Chicago, IL). Suppression was evaluated using the suppression test provided in the Fly test. RESULTSComputed modulation transfer function (MTF) and point spread function (PSF) show that the system was close to diffraction-limited.

Light scattered from residual cornea

Light focused at retina

KPro

Iris (stop)

4 meters

Arial Image (retina)

Relay lens

CCD Camera

CCD chip (retina)

LED(1 mm)

Cornea

14.6 mm

Glare sensitivity

Glare sensitivity was measured with a Mentor BAT (Mentor O&O, Inc, Norwell, MA). Measurements were obtained with:

(1) a standard transparent contact lens (CL) in place

(2) a dark iris CL, 4-mm pupil (Kontur Kontact Lens, Richmond, CA)

INTRODUCTION:

METHODS :

RESULTS:

Evaluation of glare effectsZemax computation of glare from reflections off the many surfaces was minimal, even when assuming all surfaces to be 100% reflecting

limit field60

deg

The use of the opaque soft CL resulted in significant gain in measured visual acuity of approximately 3 lines under high glare setting (p = 0.01). Visual FieldsThe Zemax model shows a maximal visual field diameter of approximately 120 degrees for the type I KPro. Consistent results were obtained using Goldman perimetry. The fields for the Type I were about 90 degrees in diameter and for the type II about 80

□ Average change in acuity of 8 pts with type I KPro; ■ The same 8 pts while wearing the iris-painted CL; ∆ Change in acuity for2pts with type II KProNB: The pt with no loss of acuity due to glare (▲) is African American. The other was Asian

degrees.

Type I (Lt) & Type II (Rt) superimposed on normal fields

CONCLUSIONS:

AFFILIATIONS:

SUPPORT:

EP and FVM were supported in part by NIH grants EY05957 and EY12890