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B. M. Kim, M. P. Blair, J. de la Cruz; Macular Optical Coherence Tomography Signal Strength in Boston Type I Keratoprosthesis Patients at the Illinois Eye and Ear Infirmary. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1499.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate the causes of low signal strength in macular optical coherence tomography (OCT) scans of keratoprosthesis patients.
Boston Type 1 Keratoprosthesis patients with retinal OCT studies were included in this series. Post-operative macular OCT examinations were performed using the Stratus OCT. Data compiled from patient records included the type of keratoprosthesis (aphakic vs. pseudophakic), presence of any retro-prosthetic membrane, as well as OCT signal strength numbers.
A total of 5 patients and 7 studies were included in this series. The disease process leading to keratoprosthesis placement was variable (rheumatoid arthritis, Fuchs' corneal dystrophy, chemical burn, pseudophakic bullous keratopathy after anterior chamber lens placement, and Acanthamoeba keratitis). All patients had prior failed full thickness grafts (penetrating keratoplasty). Two of five patients were pseudophakic after the procedure while three of five were left aphakic. All patients had clear views to the posterior pole on clinical exam, and none of the OCT scans demonstrated macular edema. The overall average signal strength in the seven OCT studies was 3.50 (range of 0.83 to 6.00). There was no significant difference in OCT signal strength between eyes with pseudophakic keratoprostheses (3.93±1.46) and aphakic keratoprosthesis (3.83±2.05) (p=0.824). There was also no significant difference in the signal strength of patients with (3.79±1.83) or without (4.11±1.71) retro-prosthetic membranes (p=0.523); however, none of the membranes were dense enough to impair clinical views of the macula.
Macular OCT signal strength was diminished in all patients following Boston Type 1 Keratoprosthesis implantation, despite clear views to the posterior pole on clinical exam. There was no statistically detectable difference in signal strength between patients with pseudophakic vs. aphakic keratoprosthesis, or in patients with vs. without faint retro-prosthetic membranes. Multiple variables may play a role in poor signal strength. One possibility is that the calibration of the OCT machine is disrupted, due to the fact that light from the sample arm must pass through the thick poly(methyl methacrylate) (PMMA) material of the keratoprosthesis, which is not encountered by light in the reference arm. An additional factor may be reflection and/or refraction of light through the prosthesis, which may set up additional unanticipated interference patterns. An third factor may be that patients are often wearing contact lenses, which further degrade the quality of the scan.
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