A Technique to Measure Eyelid Pressure Using Piezoresistive Sensors

Abstract-In this paper, novel procedures were developed using a thin (0.17 mm) tactile piezoresistive pressure sensor mounted on a rigid contact lens to measure upper eyelid pressure. A hydrostatic calibration system was constructed, and the influence of conditioning (prestressing), drift (continued increasing response with a static load), and temperature variations on the response of the sensor were examined. To optimally position the sensor-contact lens combination under the upper eyelid margin, an in vivo measurement apparatus was constructed. Calibration gave a linear relationship between raw sensor output and actual pressure units for loads between 1 and 10 mmHg (R 2 = 0.96). Conditioning the sensor prior to use regulated the measurement response, and sensor output stabilized about 10 s after loading. While sensor output drifts slightly over several hours, it was not significant beyond the measurement time of 1 min used for eyelid pressure. The error associated with calibrating at room temperature but measuring at ocular surface temperature led to a very small overestimation of pressure. Eyelid pressure readings were observed when the upper eyelid was placed on the sensor, and removed during a recording. When the eyelid pressure was increased by pulling the lids tighter against the eye, the readings from the sensor significantly increased

Eyelid pressure on the cornea

The eyelids play an important role in lubricating and protecting the surface of the eye. Each blink serves to spread fresh tears, remove debris and replenish the smooth optical surface of the eye. Yet little is known about how the eyelids contact the ocular surface and what pressure distribution exists between the eyelids and cornea. As the principal refractive component of the eye, the cornea is a major element of the eye’s optics. The optical properties of the cornea are known to be susceptible to the pressure exerted by the eyelids. Abnormal eyelids, due to disease, have altered pressure on the ocular surface due to changes in the shape, thickness or position of the eyelids. Normal eyelids also cause corneal distortions that are most often noticed when they are resting closer to the corneal centre (for example during reading). There were many reports of monocular diplopia after reading due to corneal distortion, but prior to videokeratoscopes these localised changes could not be measured. This thesis has measured the influence of eyelid pressure on the cornea after short-term near tasks and techniques were developed to quantify eyelid pressure and its distribution. The profile of the wave-like eyelid-induced corneal changes and the refractive effects of these distortions were investigated. Corneal topography changes due to both the upper and lower eyelids were measured for four tasks involving two angles of vertical downward gaze (20° and 40°) and two near work tasks (reading and steady fixation). After examining the depth and shape of the corneal changes, conclusions were reached regarding the magnitude and distribution of upper and lower eyelid pressure for these task conditions. The degree of downward gaze appears to alter the upper eyelid pressure on the cornea, with deeper changes occurring after greater angles of downward gaze. Although the lower eyelid was further from the corneal centre in large angles of downward gaze, its effect on the cornea was greater than that of the upper eyelid. Eyelid tilt, curvature, and position were found to be influential in the magnitude of eyelid-induced corneal changes. Refractively these corneal changes are clinically and optically significant with mean spherical and astigmatic changes of about 0.25 D after only 15 minutes of downward gaze (40° reading and steady fixation conditions). Due to the magnitude of these changes, eyelid pressure in downward gaze offers a possible explanation for some of the day-to-day variation observed in refraction. Considering the magnitude of these changes and previous work on their regression, it is recommended that sustained tasks performed in downward gaze should be avoided for at least 30 minutes before corneal and refractive assessment requiring high accuracy. Novel procedures were developed to use a thin (0.17 mm) tactile piezoresistive pressure sensor mounted on a rigid contact lens to measure eyelid pressure. A hydrostatic calibration system was constructed to convert raw digital output of the sensors to actual pressure units. Conditioning the sensor prior to use regulated the measurement response and sensor output was found to stabilise about 10 seconds after loading. The influences of various external factors on sensor output were studied. While the sensor output drifted slightly over several hours, it was not significant over the measurement time of 30 seconds used for eyelid pressure, as long as the length of the calibration and measurement recordings were matched. The error associated with calibrating at room temperature but measuring at ocular surface temperature led to a very small overestimation of pressure. To optimally position the sensor-contact lens combination under the eyelid margin, an in vivo measurement apparatus was constructed. Using this system, eyelid pressure increases were observed when the upper eyelid was placed on the sensor and a significant increase was apparent when the eyelid pressure was increased by pulling the upper eyelid tighter against the eye. For a group of young adult subjects, upper eyelid pressure was measured using this piezoresistive sensor system. Three models of contact between the eyelid and ocular surface were used to calibrate the pressure readings. The first model assumed contact between the eyelid and pressure sensor over more than the pressure cell width of 1.14 mm. Using thin pressure sensitive carbon paper placed under the eyelid, a contact imprint was measured and this width used for the second model of contact. Lastly as Marx’s line has been implicated as the region of contact with the ocular surface, its width was measured and used as the region of contact for the third model. The mean eyelid pressures calculated using these three models for the group of young subjects were 3.8 ± 0.7 mmHg (whole cell), 8.0 ± 3.4 mmHg (imprint width) and 55 ± 26 mmHg (Marx’s line). The carbon imprints using Pressurex-micro confirmed previous suggestions that a band of the eyelid margin has primary contact with the ocular surface and provided the best estimate of the contact region and hence eyelid pressure. Although it is difficult to directly compare the results with previous eyelid pressure measurement attempts, the eyelid pressure calculated using this model was slightly higher than previous manometer measurements but showed good agreement with the eyelid force estimated using an eyelid tensiometer. The work described in this thesis has shown that the eyelids have a significant influence on corneal shape, even after short-term tasks (15 minutes). Instrumentation was developed using piezoresistive sensors to measure eyelid pressure. Measurements for the upper eyelid combined with estimates of the contact region between the cornea and the eyelid enabled quantification of the upper eyelid pressure for a group of young adult subjects. These techniques will allow further investigation of the interaction between the eyelids and the surface of the eye

Eyelid pressure : inferences from corneal topographic changes

Purpose:The cornea is the principal optical element of the eye so the regularity of its surface topography is critical for visual optics. However the cornea is known to be susceptible to forces exerted by the eyelids. These corneal changes are bands of 'wave-like' change that are parallel to the position of the eyelid margin. There is little known about eyelid pressure on the cornea which is dependent on eyelid force and the contact area. By analysing the depth and width of corneal topography changes after various downward gaze tasks, inferences could be drawn about upper and lower eyelid pressure. Methods:Corneal topography changes due to eyelid pressure were measured using the Medmont E300 Corneal Topographer (Medmont Pty. Ltd. Victoria, Australia), for eighteen subjects aged between 18 and 29 years. Four conditions were considered, consisting of two downward gaze angles (20° and 40°) and two visual tasks (reading and staring). The amplitude and width of the ‘wave-like’ changes were analysed for each of the four conditions and for both upper and lower eyelids. Anterior eye digital photography was used to determine the position of the eyelids in downward gaze and the width of Marx's line. Results:For each condition the average peak-to-valley amplitudes of corneal change were between 1.4 and 2.4 µm. For the upper eyelid, the downward gaze angle magnitude had a significant impact on the peak-to-valley amplitude (p<0.001), with corneal changes after the 40° tasks being 25% greater than after the 20° tasks. The topographical changes showed a characteristic 'wave-like' pattern, with an outer peak, a valley and an inner peak (closer to corneal centre). The upper eyelid produced a larger outer peak compared to the inner peak (p<0.001). The corneal changes after the 40° downward gaze tasks were greater for the lower eyelid than for the upper eyelid (p<0.01). The amplitude of corneal change produced by the upper eyelid was associated with the width of Marx's line (R2=0.32, p<0.05). Conclusions:Analysis of the eyelid-induced corneal topography changes gives insight into the eyelid pressure in different situations. The upper eyelid seems to exert greater pressure on the cornea in larger downward gaze angles. The asymmetrical surface shape (outer versus inner peaks) suggests that the upper eyelid is angled when in contact with the cornea. In 40° downward gaze, it can be inferred that the lower eyelid exerts greater pressure on the cornea than the upper eyelid. There was some evidence that Marx's line is the site of frictional contact between the eyelids and the cornea

Corneal refractive changes due to short-term eyelid pressure in downward gaze

Purpose: To assess corneal refractive changes after visual tasks of 15 minutes duration and their association with eyelid morphology. \ud Setting: Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Queensland, Australia. \ud Methods: Eighteen young subjects with normal ocular health were recruited. Corneal topography was measured with a videokeratoscope prior to and after four conditions consisting of two downward gaze angles (20 degrees and 40 degrees) and two types of visual tasks (reading and steady fixation). Anterior eye photography in downward gaze was used to determine the eyelid angle, tilt and position with respect to the cornea. \ud Results: There were significant changes in corneal refractive power after the 15 minute downward gaze tasks. The largest group mean corneal sphero-cylindrical change was +0.33/-0.30x84 after reading in 40 degrees downward gaze (4 mm corneal diameter). The refractive changes after the 40 degrees tasks were significantly larger than the changes after the 20 degrees tasks (p<0.001). The changes in refractive RMSE were significant for all conditions, except the 20 degrees steady fixation task, for 4 and 6 mm analysis diameters (p<0.05). Significant correlations were found between some aspects of eyelid morphometry and corneal refractive change. \ud Conclusion: The pressure of the eyelids on the cornea in short-term downward gaze resulted in optically and clinically relevant corneal changes. Correlation between the refractive corneal changes and eyelid parameters suggests that the angle, shape and position of the eyelids influence the nature of the corneal changes. When high accuracy is required, refraction should be qualified in terms of the visual tasks undertaken prior to assessment

Hardware modifications to enhance the eye surface profiler

Purpose The Eye Surface Profiler (ESP) is an instrument that estimates the shape of the cornea, limbus and a portion of the sclera. Sodium fluorescein is instilled into the ocular tear film, the ESP projects blue fringe patterns from two directions and the resulting green fluorescent emission patterns are analysed. The ESP is a useful clinical and research tool and we found that its operation could be enhanced. The focusing system is relatively insensitive to changes in position and, although crucial to the operation of the instrument, tear film fluorescence cannot be viewed before data is collected. We addressed these issues to enhance its operation. Methods A video camera was attached to the ESP to view the instrument’s focusing spots from a second direction. The operator can then position the ESP at a more repeatable distance (Z) from the eye. X/Y alignment is handled via the original ESP focusing system. A blue LED ring light, operated via a footswitch, was also attached to the camera to give a live view of tear fluorescence. When consistent fluorescein coverage was observed, the operator switches off the ring light and collects data immediately with the original flash system. To investigate repeatability, five maps were collected for the right eyes of 3 subjects using A) the original ESP focusing technique (ESPf) and B) our new camera and focusing technique (NEWf). Maps for each subject for each technique were filtered to remove artefacts then averaged to derive a standard deviation map (SD at each map grid point). To compare ESPf with NEWf, the SD maps for all subjects were averaged and split into ‘corneal’ (central 10 mm diameter) and ‘limbal/scleral’ (outside 10 mm diameter) regions. Results The ‘corneal’ average SD was 8.6 um for ESPf and 4.8 um for NEWf (44% reduction). The ‘limbal/scleral’ average SD was 19.6 um for ESPf and 11.1 um for NEWf (43% reduction). Conclusions The hardware modifications to the ESP have enhanced focusing precision and reduced variability between maps for the eyes tested. By viewing tear film fluorescence prior to collecting data, we have also ensured that tear film artefacts have less impact on data quality

Tear Film Surface Quality with Soft Contact Lenses Using Dynamic Videokeratoscopy

Purpose: To investigate changes in tear film surface quality after commencing soft contact lens wear. Methods: Tear film surface quality (TSQ) was assessed during the interblink period using dynamic videokeratoscopy at 25 Hz. A quantitative value of TSQ is derived for each raw Placido ring image. Eleven young subjects with normal tear characteristics participated in the study. Dynamic videokeratoscopy was taken three times per day; in the morning, at lunchtime, and in the afternoon. This was done on two baseline days (bare eye) and on the first and seventh days of lens wear for a conventional hydrogel lens and following a week of no lens wear, for a further week of silicone hydrogel lens wear. Additionally clinical tests to assess TSQ were conducted and subjects were also asked to rate the subjective dryness of their eyes. Results: All lens wear measurements showed a significant worsening of TSQ compared to bare eye measurements (repeated measures ANOVA, P0.05). The subjective rating of dryness correlated with TSQ values (Pearson's r=0.62, P<0.05) for the bare eye condition, but not during contact lens wear. TSQ derived from the right and left bare eyes of the same individuals showed a significant correlation (Pearson's r=0.61, P<0.05). Conclusions: The measurement of TSQ using dynamic videokeratoscopy differentiates between bare eye and lens wearing conditions. It also shows a small systematic improvement in tear surface quality during the first day of silicone hydrogel lens wear and a significant association with subjective dryness for the bare eye condition

Using optical coherence tomography to assess corneoscleral morphology after soft contact lens wear

Purpose. To evaluate the use of optical coherence tomography (OCT) to assess the effect of different soft contact lenses on corneoscleral morphology. Methods. Ten subjects had anterior segment OCT B-scans taken in the morning and again after six hours of soft contact lens wear. For each subject, three different contact lenses were used in the right eye on non-consecutive days, including a hydrogel sphere, a silicone hydrogel sphere and a silicone hydrogel toric. After image registration and layer segmentation, analyses were performed of the first hyper-reflective layer (HRL), the epithelial basement membrane (EBL) and the epithelial thickness (HRL to EBL). A root mean square difference (RMSD) of the layer profiles and the thickness change between the morning and afternoon measurements, was used to assess the effect of the contact lens on the corneoscleral morphology. Results. The soft contact lenses had a statistically significant effect on the morphology of the anterior segment layers (p <0.001). The average amounts of change for the three lenses (average RMSD values) for the corneal region were lower (3.93±1.95 µm for the HRL and 4.02±2.14 µm for the EBL) than those measured in the limbal/scleral region (11.24±6.21 µm for the HRL and 12.61±6.42 µm for the EBL). Similarly, averaged across the three lenses, the RMSD in epithelial thickness was lower in the cornea (2.84±0.84 µm) than the limbal/scleral (5.47±1.71 µm) region. Post-hoc analysis showed that ocular surface changes were significantly smaller with the silicone hydrogel sphere lens than both the silicone hydrogel toric (p<0.005) and hydrogel sphere (p<0.02) for the combined HRL and EBL data. Conclusions. In this preliminary study, we have shown that soft contact lenses can produce small but significant changes in the morphology of the limbal/scleral region and that OCT technology is useful in assessing these changes. The clinical significance of these changes is yet to be determined