How to best measure the thickness of the PCTF, PLTF, and PoLTF is of considerable interest to clinicians and researchers; however, no agreement has been achieved regarding the optimal method. Doane
9 developed a tear film interferometer that applied a contour plot to the measurement of the PLTF thickness. Fogt et al.
10 used interferometry of spectral oscillations to determine the PLTF thickness. Bruce and Brennan
11 and Bruce and Mainstone
19 used biomicroscopy to observe the PoLTF in specular reflection. Prydal et al.
5 found the PCTF thickness to be 34 to 45 μm by noninvasive interferometry and 41 to 46 μm by confocal microscopy. Wong et al.
20 and Creech et al.
21 obtained theoretical predictions of PCTF thickness as 8.0 and 10.4 μm, respectively. King-Smith et al.
6 reported that the average thickness of the PCTF was 2.7 μm. Nichols and King-Smith
22 found both PLTF and PoLTF to be 2.3 μm, when measured by interferometry. In contrast, Lin et al.
23 reported a value of 11.5 μm for the PoLTF when determined by optical pachometry. Although the technologies and methods described above can be used to estimate tear film thickness with higher resolution than OCT, they cannot rapidly acquire large numbers of ultrahigh-resolution images that are necessary to precisely evaluate tear dynamics.
Commercial and custom-made OCT devices have been used to study many aspects of the ocular anterior segment such as the whole cornea, the epithelium, anterior chamber width, depth and angle, the flap after laser and others.
24–28 The high resolution of the time domain OCT instrument, approximately 10 μm, made it useful to investigate the tear menisci and tear film.
8,29 Wang et al.
7,8 presented distinct images of the cornea, contact lenses, and tear film and reported PCTF, PLTF, and the PoLTF thicknesses of approximately 3.3, 3.8, and 4.6 μm, as determined by indirect calculation. Recent studies demonstrated that a high-resolution SD-OCT or an ultrahigh-resolution SD-OCT could obtain higher quality images and detect more details of the ocular anterior segment.
30,31 Kaluzny et al.
30 indicated that the high-resolution OCT instrument, with a 4- to 6-μm longitudinal resolution, was a promising device in contact lens research and practice. However, the tears on the contact lens surface were not included in their study, and tear films were not evident in their images. In a previous study, we used a custom-built, ultrahigh-resolution (∼3 μm) OCT instrument to precisely estimate tear menisci and tear films on and underneath contact lenses and to record the interaction between the lens edge and the ocular surface.
3 In addition, we compared the fitting characteristics of two lenses with different materials, back curves, and edge designs and suggested that rational lens design might improve the fitting characteristics by improving tear exchanges around the lens edge. Therefore, the SD-OCT instrument was considered to be a promising tool for studying tear dynamics on the lens. However, because of the small subject sample in that study, conclusions regarding the true PLTF and PoLTF thickness were not drawn.
In the present study, we used ultrahigh-resolution SD-OCT and confirmed its theoretical depth resolution of 3 μm by visualizing a PLTF of 3.2 μm in one subject. As long as the tear film was thicker than 3.2 μm, it was detectable and calculated directly. In addition, the PoLTF was visualized and calculated directly immediately after lens insertion in some subjects, as suggested by Wang et al.
3 To the best of our knowledge, this is the first time the PoLTF has been quantified directly from images without the aid of artificial tears. The increased tear film is mainly the result of reflex tearing,
32 as we avoided applying soaking solution to the eye when the lens was inserted. In some subjects, there was no detectable increase in the PoLTF. This means only that the PoLTF did not exceed the 3-μm limit of resolution. It is possible that these subjects had poor sensitivity or good adaptability to soft contact lens wear and did not have any significant reflex tearing. Immediately after lens insertion or eye drop instillation, the PLTF and PoLTF were readily visualized. However, after 3 minutes of lens wear, they were not visible in most subjects. Thus, normally the PLTF and PoLTF during lens wear are too thin to be visible by ultrahigh-resolution OCT. The central thicknesses of the PCTF, PLTF, and the PoLTF were close to but a little thinner than those reported by King-Smith et al.,
6 Nichols and King-Smith,
22 and Wang et al.
7 This slight difference may be due to the differences in sample size, the race of the subjects recruited, and the evident improvement of the longitudinal resolution of the instrument.
To investigate the effect of 1 drop on the PLTF and the PoLTF, we monitored these variables for 10 minutes after instillation on the lens. The results indicated that the lubricating drops used to improve ocular comfort during lens wear do not flow around the lens edge into the space between lens and cornea. Thus, the drops may relieve only the friction between eyelid and contact lens by increasing the PLTF. When the artificial tears were placed on the concave surface of lens before insertion, the PoLTF was readily apparent. However, it was quickly extruded by blinking. Thus, the PoLTF cannot be maintained during lens wear, even when extra tears are added.
The SD-OCT instrument used in the present study has opened a new era in the evaluation of the different fitting characteristics of various lens designs and materials.
3 Our results suggest that soft contact lens design and materials must to be improved so that they hold the PoLTF and enhance the rate of tear exchange beneath the lens. Only in this way can the dry eye symptoms induced by contact lens wear be alleviated and the health of the ocular surface be sustained.
3
In the present study, the measurements of the PCTF, PLTF, and PoLTF thicknesses were limited to the central location. Not much is known about the topographic thickness of the tear films away from the optical center; this subject will be explored in another study. The image processing of the tear films by manual manipulation and not by automated software may be another concern. We assumed that the thickness of the contact lens and the cornea used in the indirect calculations of tear films did not change during the approximately 10 minutes of lens wear. Any thickness changes in the lens and cornea that did occur could introduce measurement errors for the indirect calculations. In addition, lens decentration during imaging may be another concern. After fitting the lens on the eye, we used a slit lamp biomicroscope to evaluate the fitting, especially the centration. Good centration of less than 1 mm was achieved on each eye. Although we were aware of variation of the lens fitting during blinking, the images were taken during the first gaze. Slight decentration appeared not to have an impact on the direct visualization of the tear film; however, some variation may have been introduced into the indirect calculation. Using test lenses, we measured the variation of lens thickness. The variation of the central 1-mm zone was approximately 1.3 μm between the thinnest and thickest points. In an attempt to offset the systematic error due to decentration of the lens, we processed multiple A-scans along both sides of the apex to arrive at the results. Last, in this one-visit study, we defined normal healthy subjects as individuals without any previously diagnosed dry eye. We did not perform any tear function tests, like tear break-up time or the Schirmer test. The absence of testing may have allowed some undetected variation, if some of our subjects had subclinical dry eye. Further studies are needed to correlate the measurements with these tests.
In summary, the ultrahigh-resolution SD-OCT instrument is a novel and promising tool for evaluating the tear dynamics during contact lens wear. With this instrument, we have shown that the tears cannot be held underneath the soft contact lens, and extra lubricating drops used clinically for improving ocular comfort did not alter the PoLTF.
Supported by Research Grant R03 EY016420 and Center Grant P30 EY014801 from The National Institutes of Health/National Eye Institute, and by Bausch & Lomb, Allergan, and Research to Prevent Blindness.
Disclosure:
Q. Chen, None;
J. Wang, Bausch & Lomb (F), Allergan (F);
A. Tao, None;
M. Shen, None;
S. Jiao, None;
F. Lu, None
The authors thank Britt Bromberg, PhD, for providing editing services for this manuscript.