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Daniel R. Neal, Thomas D. Raymond, Stephen Farrer, Wei Xiong, David Baer; Corneal Topography Tilt and Coma measured with a combined Aberrometer and Full Gradient Corneal Topographer. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5903.
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© ARVO (1962-2015); The Authors (2016-present)
The importance of fixation, pupil center and reference axis in ocular measurements has been pointed out recently by Applegate, et al (J Cat. Ref. Surg 2009 36:139-152). We investigate the relationship of CT tilt to both CT coma, and WF coma measured with the a combined WF aberrometer and full gradient corneal topographer (FGCT).
Fifty subjects were measured multiple times with the combined instrument. A common fixation target ensures the measurements are captured along the line of sight. The FGCT and WF measurement axis are co-aligned to ±2 mRadians, with coordinate systems that are spatially registered to ±50 microns; the FGCT and WF data are taken with 100 msec of each other. A total of 154 WF and FGCT such measurements are reconstructed over the natural WF pupil diameter by fitting with 6th order Zernike polynomials. The FGCT are divided by -0.3375 to convert them to optical phase delay. The FGCT tilt terms are analyzed with respect to correlation and linear regression to FGCT coma and WF coma terms.
Figure 1(a) shows the relationship between horizontal FGCT tilt (CTZ11) and FGCT coma (CTZ13). The CT coma was found to be highly correlated to both FGCT tilt components with correlation coefficients of 0.91 (p<0.001) and 0.90 (p<0.001) for vertical tilt/coma and horizontal tilt/coma, respectively. In contrast, as shown in Figure 1(b), the FGCT tilt is highly uncorrelated to WF coma with correlation coefficients of (0.052, p=0.51 and 0.079, p=0.32, respectively).
We find that FGCT tilt is highly correlated to corneal coma but highly uncorrelated to WF coma demonstrating a strong compensation for corneal coma associated with corneal tilt. These data support the compensation mechanism proposed by Tabernero and Artal ( J. Opt. Soc. Am. A 2007, 24:3274-3283). They also highlight the importance of measurements taken with a combined instrument, and that FGCT tilt plays an important role in ocular measurements.
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