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N. L. Himebaugh, C. G. Begley, Z. Wu, R. L. Chalmers, K. Moody; Application of RMS Fit Error for Assessing Pre-lens Tear Film Break-up. Invest. Ophthalmol. Vis. Sci. 2009;50(13):535.
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Tear film break-up (TBU) causes local, often highly irregular changes in tear film thickness. The purpose of this study was (1) to determine if the wavefront metric root mean square fit error (RMS-FE) reflects TBU aberrations better than current wavefront metrics that smooth the wavefront and (2) whether RMS-FE correlates with visual changes measured by contrast sensitivity (CS) in contact lens (CL) wearers.
Soft CL wearers (n=28) kept one eye open for 18 sec while a 3-way concurrent measure of CS, retroillumination (RI), and wavefront aberrometry was taken every 1.5 seconds using a modified COASTM wavefront aberrometer (AMO Wavefront Sciences, LLC). A masked observer graded TBU from one randomly chosen eye on a scale of 0 to 4 in each RI frame. Higher order rms (RMS-HO), calculated from modal (Zernike) reconstruction of the wavefront (3rd - 6th order aberrations), and RMS-FE, equivalent to the difference between the zonal and model reconstruction of the wavefront, were calculated for a 6 mm pupil.
Five subjects had no TBU (grade 0) during the trial, and their ocular aberrations did not change significantly during the trial (mean change RMS-HO = 0.05±0.10 υm; mean change RMS-FE = 121.84±241.13 υrad; paired t-test, p>0.3). For the other 23 subjects, TBU (median=grade 2), RMS-HO and RMS-FE increased significantly at the end of the trial. The mean change in RMS-HO was 0.12±0.14 υm and in RMSFE was 1196.60±908.59 υrad (paired t-test, p<0.001). CS decreased significantly from 11.76 to 6.41±3.41 (paired t-test, p<0.001). RMS-HO and RMS-FE were both correlated with TBU grade and CS. The average correlations were RMS-HO and TBU (0.46±0.46); RMS-HO and CS (-0.50±0.54); RMS-FE and TBU (0.85±0.12); RMS-FE and CS (-0.74±0.23).
RMS-FE is highly correlated with the development of TBU and loss of CS over 18 sec of eye opening. RMS-FE removes the smoother, lower-order aberrations contained in modal reconstruction from the local, high-frequency aberrations in the zonal reconstruction. The RMS-FE yields an accurate metric for changes to the local tear film thickness and highly irregular wavefront that current metrics fail to represent.
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