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J Stoiber, W Hitzl, J Ruckhofer, G Grabner; Visual Acuity by Time Index (VATI) - A new method for assessment of "long-term" success in keratoprosthesis surgery . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4178.
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Purpose: To compare the "long-term" success of different methods of keratoprosthesis surgery is difficult, since the survival curves that are routinely used for postoperative evaluation generally do not take visual acuity into consideration. As this is THE crucial parameter from the patients point of view, a "Visual Acuity by Time Index" (VATI) has been developed as to make the surgical results over longer time periods more readily comparable. Methods: Best corrected visual acuity (BCVA) was measured pre- and postoperatively for n = 15 subjects at regular intervals with a maximal follow-up of 2700 days (equal to ≷ 7 years). In order to assess the average BCVA within a given time period [T1, T2] an index was designated as "visual acuity by time index" (VATI) that can be regarded as an extension of the usual mean. An estimation based on a nonparametric regression model was used to compute the mean BCVA at time t - denoted by f(t) - and next the area under the curve f(t) within the period [T1, T2] was divided by the length of passed time (T2 - T1). This consideration can be written as View OriginalDownload SlideView OriginalDownload Slide The index therefore expresses the average achieved BCVA within an arbitrary time interval [T1, T2]. In addition, 95% confidence intervals for the VAT-Index were computed. Results: In a series of 15 patients receiving an osteo-odonto-keratoprosthesis (OOKP) at the Salzburg Eye Clinic the following VAT indices in terms of 95% confidence intervals could be estimated: (0.32, 0.62) for the time period from surgery to month 6, (0.42, 0.75) from M6 to M12, (0.45, 0.87) from M18 to M24, (0.27, 0.80) from M30 to M36, (0.12, 0.69) from M42 to M48, (0.0, 0.7) from M54 to M60 and (0.15, 0.68) for the whole time period from surgery to month 60. Conclusion: The "visual acuity by time index" ("VATI") can be used to compare visual results of different techniques of keratoprosthesis over time at every requested time point and also within arbitrary time intervals. This index can also be used for the comparison of long-term study results following other types of treatment, such as photodynamic therapy, macular and glaucoma surgery and many different types of surgical refractive corrections.
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