Abstract
Abstract: :
Purpose: Recent studies have shown that the higher order aberrations due to the changes in the tear film can be readily measured using a Shack–Hartmann wavefront sensor (Montés–Micó et al., Ophthalmology 2004). The goal of this study is to demonstrate the effectiveness of measuring tear film dynamics quantitatively using the Shack–Hartmann wavefront sensor and its effect on retinal image quality. Methods: The wavefront aberrations of five subjects, 4 non–contact lens users (NCL) and 4 contact lens users (CL), were measured in real time (10 frames per second) with a Shack–Hartmann wavefront sensor. Subjects were asked to fixate, for as long as possible voluntarily, on a point source for the entire measurement process (NCL: 27.78±16.45 sec. CL: 17.07±10.17 sec.). CL subjects were measured immediately after contact lens removal. Several measurement sessions were performed for each subject (at least 30 min apart). Higher order rms and retinal image quality metrics such as Strehl ratio, MTF and convolved image were computed to quantitatively compare the two groups on how ocular aberrations and retinal image quality are affected by tear film dynamics. Results: The variability in higher order rms (HORMS) and retinal image quality was measured by means of standard deviation over time separately for the two groups. Our results show that variability was significantly higher for CL subjects (.0446±.0219µm HORMS) than NCL subjects (.0156±.0112µm HORMS). For HORMS, Strehl ratio and volume MTF, the variability for the CL group was approximately three times larger than that of the NCL group. The behavior of radially averaged MTF was nearly identical to that of the Strehl ratio. In CL subjects, all metrics used in this study suggest a greater rate of decrease on average in retinal image quality over time. Time–lapse image sequences of the wavefront, PSF and convolved retinal image confirm correlations between retinal image qualities and tear film break up as well. By observation, the behavior of the convolved retinal image correlates better with Strehl ratio and volume MTF than HORMS. Conclusions: The real time Shack–Hartmann wavefront sensing is capable of observing behavior differences in tear film dynamics. Strehl ratio and volume MTF are better metrics to assess tear film dynamics. This same technique can be used as an objective tool to diagnose dry eye symptoms.
Keywords: cornea: tears/tear film/dry eye • computational modeling • contact lens