- Email: [email protected]
The biomechanics of rigid contact lens removal
Contact Lens & Anterior Eye 28 (2005) 121–125 www.elsevier.com/locate/clae
The biomechanics of rigid contact lens removal Michael J. Collins a,*, Stephanie C. Voetz a, Nora Bretschneider b a
Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of Technology, Victoria Park Road, Kelvin Grove,Brisbane, QLD 4059, Australia b Fachhochschule Aalen, Studiengang Augenoptik und Hoerakustik, University of Technology and Business, Gartenstrasse 135, 73430 Aalen, Germany
Abstract The removal of rigid contact lenses from the eye, using the eyelids, is a relatively simple procedure. However, there is a sequence of biomechanical events underlying this procedure which are not well understood. By using high-speed videokeratoscopy (50 Hz), we have shown that during the lid-pull procedure the cornea typically shows a significant increase in with-the-rule astigmatism by an average of 2.19 D (axial power). The average increase in steep K power was 1.22 D (S.D. 1.05 D) and the average decrease in flat K power was 0.97 D (S.D. 1.05 D). This change in corneal topography increases the edge lift of the lens at 12 and 6 O’clock locations of the lens, enhancing the removal process. When the subject blinks, as the lid margins reach the lens edge, the lens flexes and the eye retracts by an average of 0.66 mm (S.D. 0.27 mm). This retraction again enhances the act of lens removal. # 2005 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. Keywords: Rigid contact lens; Cornea; Contact lens removal; Corneal astigmatism; Retraction; Proptosis
1. Introduction Rigid contact lenses are widely used in the correction of refractive error. While soft contact lenses are more commonly prescribed than rigid lenses, there are many patients and practitioners who prefer rigid lenses for various reasons, including quality of vision, eye health and lens cost. Rigid contact lenses can be removed from the eye using a number of methods. The most common technique is known as the blink technique [1] or one-finger method [2]. To use this technique, the subject opens the eyes, as wide as possible, then places the index finger on the outer canthus, pulls the lids outwards to tighten them and then blinks the lens out [1–5]. Another common removal technique is the two-handed method, in which the contact lens is pinched off the eye by pushing the lid margins towards the eye and the contact lens edges along the vertical meridian [1,2]. Rigid contact lens wearers can also use a lens sucker to remove their contact lenses. * Corresponding author. Tel.: +61 7 3864 5739; fax: +61 7 3864 5665. E-mail address: [email protected] (M.J. Collins).
In this study, we investigated the biomechanical events that occur during rigid contact lens removal, using the blink technique. We performed high-speed videokeratoscopy to investigate corneal changes during the lid pull procedure and measured eye retraction during the blink technique. Our results illustrate the biomechanical mechanisms underlying the process of rigid lens removal.
2. Methods Five subjects, three females and two males, took part in this study. The subjects were between 24 and 37 years of age with a mean age of 28 years and none of the subjects were regular rigid lens wearers. All subjects had normal corneal characteristics and no eye disease. Only the right eye was used in the study. To measure the changes of the corneal topography associated with rigid lens removal, we used a specially modified videokeratsoscope. The Medmont E300 corneal topographer (Medmont Pty Ltd. Melbourne, Australia) was modified to continuously acquire corneal topography at 50
1367-0484/$ – see front matter # 2005 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.clae.2005.04.001
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M.J. Collins et al. / Contact Lens & Anterior Eye 28 (2005) 121–125
videokeratographs per second (50 Hz). The details of these modifications have been outlined by Iskander et al. [6]. The subjects were seated at the Medmont E300 with their head stabilised on a headrest. The subjects were instructed to look at the fixation target and not to blink during the measurement period. After the instrument was adjusted, the measurement started and 10 s were recorded (500 videokeratographs). Between 3 and 5 s after the measurement had started, the subjects were asked to pull their eyelids outward, as if removing a rigid contact lens. The subjects were asked to put the right index finger at the position of the outer canthus before the measurements began, so as to improve the accuracy and timing of the procedure. The videokeratographs were converted into axial power and corneal height maps for subsequent analysis. About 20 baseline videokeratographs (prior to lid-pull) and another 20 videokeratographs, when the lids were pulled outward, were averaged for each subject. The method employed for averaging topography maps and statistically analysing the changes has been presented by Buehren et al. [7]. Each topography map is resampled into a common polar grid format so that data can be averaged at common locations. Statistics, such as t-tests can then be applied to test the significance of changes in topography at all locations within the maps. We analysed topography within the central 5 mm of the cornea for both conditions (i.e. natural and lid-pull conditions), because eyelashes restricted the vidoekeratoscope vertical map diameter in the lid-pull condition. To investigate the forces on a rigid contact lens during lens removal, we also had one of the subjects wear a Menicon Z rigid lens (8.0/9.6/ 3.00, tc = 0.13 mm) and attempt lid-pull lens removal. We used the videokeratoscope to acquire axial curvature maps with the lids opened wide and with lid margins at the edge of the lens during the lid-pull removal. The retraction of the right eye during blinking was measured, using a digital camera (Nikon Coolpix 995). The subjects placed their head on a headrest and were instructed to look straight ahead. Retraction of the right eye was measured, while the upper and lower eyelid of the right eye were manually restrained against the orbital rim. The subject was instructed to blink with the left eye, while the retraction of the right eye (with restrained eyelids) was measured with the digital camera, aligned at approximately 908 to the line of sight at a distance of 20 cm. A ruler (with millimetre graduations) was placed ‘‘in frame’’ beside the outer canthus of the right eye and was used to calibrate the amount of eye retraction in subsequent analysis, using image processing software.
( p < 0.0001) after pulling the eyelids outward. The average increase in steep K power was 1.22 D (S.D. 1.05 D) with a range from 0.31 to 3.03 D. The smallest increase of 0.31 D was found for subject 5. The flat K power decreased significantly ( p < 0.0001) in four subjects. For subject 5, we did not find a significant decrease of the flat K power ( p = 0.8). The average decrease in flat K power was 0.97 D (S.D. 1.05 D) with a range from 0.01 (for subject 5) to 2.77 D. Changes in flat and steep K power before and during lid-pull of a representative subject (subject 4) are shown in Fig. 1. To analyse the topographical changes in axial curvature, the 20 baseline maps and 20 lid-pull maps were averaged and a difference map (‘‘lid-pull’’ minus ‘‘baseline’’) was created for axial radius and corneal height (Fig. 2). In four of the five subjects, significant regions of change in axial curvature in the vertical and horizontal meridian were found ( p < 0.001). The subject with the smallest changes of topography following lidpull (subject 5) did show significant steepening of the lower cornea, while the upper cornea was largely unchanged. This probably reflects a difference in this subject’s application of the lid-pull technique, or could reflect the underlying anatomical differences. The corneal shape changes are also shown in a meridian analysis of the difference maps (Fig. 3). The horizontal meridian (08 and 1808) flattened for four of the five subjects and the vertical meridian (908 and 2708) became steeper for all subjects. The axial radius maps of the front surface of the rigid lens on-eye with the lids open and at the point of the lid-pull removal procedure is shown in Fig. 4. In the open eye condition, the surface of the lens is approximately spherical with minor vertical steepening. However, in the lid-pull condition, the lens surface becomes significantly astigmatic (with-the-rule), with about 0.75 mm difference in axial radii between the horizontal and vertical meridians. We repeated this procedure at separate time with the same subject and found a similar outcome. But it should be noted that the amount of astigmatic change on the lens surface would be influenced by factors, such as the strength of the lid-pull, the underlying corneal shape, the lens thickness and the lens material modulus. The average eye retraction during the simulated blink condition for the five subjects was 0.66 mm (S.D.
3. Results The change in flat and steep simulated K readings, obtained by the videokeratoscope has been analysed, using the average data of 20 baseline and 20 lid-pull maps. For all subjects, the steep K became significantly steeper
Fig. 1. Changes of flat K and steep K power of subject 4, recorded continuously with a high-speed videokeratoscope. While pulling the eyelids outwards the flat K power became flatter and the steep K power became steeper.
M.J. Collins et al. / Contact Lens & Anterior Eye 28 (2005) 121–125
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Fig. 2. Difference in axial radius, corneal height and significance maps for five subjects. The difference represents the cornea during lid-pull minus the baseline cornea, simulating removal of a rigid contact lens. The significance maps show the statistical significance ( p-values) of the measured changes.
0.27 mm) with a range from 0.4 to 1.1 mm. To illustrate the changes in eye position associated with rigid lens removal, one subject’s eye position was measured, using the high-speed videokeratoscope. The Medmont videokeratoscope provides a continuous output of eye position (anterior–posterior movement) via a position sensor at the leading edge of the Placido disk cone of the instrument. This
output is not normally available to the user, but is available in our modified instrument. The subjects manually restrained their upper and lower eyelids of the measured eye and were asked to ‘‘widen their eyes’’ and then perform a ‘‘forced blink’’, as if removing a rigid lens. The fellow eye was free to perform these tasks, but the measured eye was restrained. The associated changes in eye position showed a
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M.J. Collins et al. / Contact Lens & Anterior Eye 28 (2005) 121–125
Fig. 3. Change of the axial curvature in each meridian for all five subjects. In the horizontal meridian (08 and 1808), the axial radius became flatter for four subjects. For all subjects, the axial radius in the vertical meridian (908 and 2708) became steeper. Fig. 5. The high-speed videokeratoscope provides a continuous output of eye position (anterior-posterior movement). The subject manually restrained their upper and lower eyelids of the measured eye and were asked to ‘‘widen their eyes’’ and then perform a ‘‘forced blink’’ as if removing a rigid lens.
proptosis of the eye with eye widening and a retraction of the eye during the attempted forced blink (Fig. 5). In summary, the initial act of eye widening causes a slight proptosis of the eye and positions the eyelids past the edges of the rigid lens (Table 1). As the lids are pulled towards the ear, the cornea typically becomes significantly steeper along the vertical meridian and flatter along the horizontal meridian, thereby increasing edge lift along the vertical meridian. When the lids are positioned at the edges of the lens, the lens also flexes to become steeper vertically and the act of blinking then causes eye retraction and forces the lids behind the lens, which is leveraged off the cornea.
4. Discussion
Fig. 4. Axial radius of rigid lens surface on-eye. In the top panel, the eye is open wide. In the centre panel the lids have been pulled to reach the lens edge. In the lower panel, the difference in axial radius between the open eye and lid-pull conditions is shown.
We found that most subjects show significant corneal steepening in the vertical meridian and a flattening in the horizontal meridian of the cornea during the lid-pull procedure of rigid contact lens removal. Gullstrand [8] stated that the pressure of the lids on the eye produce a change in corneal curvature in the direction of with-the-rule astigmatism. It has also been reported that lifting the lids from the surface of the eye causes a decrease in with-the-rule astigmatism in corneas with more than 1.00 D of with-therule astigmatism [9] and that the cylindrical component of the refractive error increased significantly when the palpebral aperture was narrowed [10]. Therefore, it is not surprising that significant with-the-rule astigmatism arises when extra pressure is exerted by pulling the eyelids outward. There is a direct relationship between anterior–posterior eye position and the lid aperture during blinking. Retraction of the human eye during blinking has been observed in several previous studies [11–15]. Collins et al. [12] found that retraction was greater when lids were closed forcefully
M.J. Collins et al. / Contact Lens & Anterior Eye 28 (2005) 121–125
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Table 1 Sequence of biomechanical events in rigid lens removal Procedure
Eye position
Corneal and lens shape
Eyes widened Lids pulled Lids reach lens edge Forced blink
Proptosis Proptosis Proptosis Retraction
Cornea unaltered Cornea shows induced with-the-rule astigmatism (steeper vertically and flatter horizontally) Lens flexes to show induced with-the-rule astigmatism Cornea returns to normal shape and lens is removed
(0.96 mm) compared to normal blinking (0.44 mm). In this study, we found an average retraction of 0.66 mm which matches reasonably with the data, reported in previous studies. The rigid lens appears to flex along the vertical meridian as the lids reach the edge of the lens in the lid-pull procedure. The forced blink, associated with lens removal, almost certainly flexes the lens further and as the eye retracts, it is not surprising that the lens can sometimes appear to spring from the eye. We could not investigate the two-handed rigid lens removal technique with the high-speed videokeraoscope because the instrument’s cone is too close to the eye to allow access by the fingers. However, we expect that the technique would cause similar changes in corneal topography and potentially greater changes due to the more direct application of force. Some patients find difficulty in removing rigid lenses from the eye using the blink technique. These difficulties may arise from a range of factors, including technique used, anatomical differences and confidence. We did ask each of our five subjects to remove a rigid lens from their eye, while we observed their technique. Only one subject (subject 5) was unable to remove the rigid contact lens using the blink technique. This same subject showed the least amount of corneal change associated with the lid-pull technique (see Fig. 2) and we suspect that this reflects a difference in lid tension applied during the procedure (possibly anatomical or related to the subject’s technique).
5. Conclusions The act of removing rigid contact lenses, using the blink technique, is a procedure that most patients master with practice. However, underlying this simple procedure is a series of complex biomechanical events in the anterior eye. During the outward pull of the eyelids, the corneal curvature steepens in the vertical meridian and flattens in the horizontal meridian. These corneal changes create an increased edge lift at the 12 and 6 o’clock locations of the lens edge to facilitate lens removal. The act of forced
blinking at this point of the process, causes the eye to retract, which also enhances the rigid lens removal.
Acknowledgement We thank Nicola Pritchard, Brett Davis and Robert Iskander for their help with the high-speed videokeratoscope.
References [1] Efron N. Contact lens practice. Oxford/Boston: Butterworth–Heinemann, 2001. [2] Hales RH. Contact lenses: a clinical approach to fitting. Baltimore: Williams & Wilkins, 1978. [3] Obrig TE, Salvatore PL. Contact lenses. New York: Obrig Laboratories, 1957. [4] Stein HA, Slatt BJ. Fitting guide for rigid and soft contact lenses: a practical approach. St. Louis: Mosby, 2002. [5] Lowther GE. Contact lenses: procedures and techniques. Boston: Butterworths, 1982. [6] Iskander DR, Collins MJ, Davis BA. Evaluating tear film stability in the human eye with high speed videokeratoscopy. IEEE Trans Biomed Eng, in press. [7] Buehren T, Collins MJ, Carney L. Corneal aberrations and reading. Optom Vis Sci 2003;80:159–66. [8] Gullstrand A. The cornea. In: Southall JPC, editor. Helmholtz’s treatise on physiological optics. New York: Dover; 1962. p. 320–1. [9] Wilson G, Bell CH, Chotai S. The effect of lifting the lids on corneal astigmatism. Am J Optom Physiol Opt 1982;59:670–4. [10] Grey C, Yap M. Influence of lid position on astigmatism. Am J Optom Physiol Opt 1986;63:966–9. [11] Doane MG. Interactions of eyelids and tears in corneal wetting and the dynamics of the normal human eyeblink. Am J Ophthalmol 1980;89:507–16. [12] Collins M, Smyth W, Seawright J, Kelly S. The synkinesis between antero-posterior eye position and lid fissure width. Clin Exp Optom 1992;75:38–41. [13] Riggs LA, Kelly JP, Manning KA, Moore RK. Blink-related eye movements. Invest Ophthalmol Vis Sci 1987;28:334–42. [14] Evinger C, Shaw MD, Peck CK, Manning KA, Baker R. Blinking and associated eye movements in humans, guinea pigs, and rabbits. J Neurophysiol 1984;52:323–39. [15] Lydon D, Tait A. Lid-pressure: its measurement and probable effects on the shape and form of the cornea-rigid contact lens system. J Br Contact Lens Assoc 1988;11:11–22.
The biomechanics of rigid contact lens removal Michael J. Collins a,*, Stephanie C. Voetz a, Nora Bretschneider b a
Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of Technology, Victoria Park Road, Kelvin Grove,Brisbane, QLD 4059, Australia b Fachhochschule Aalen, Studiengang Augenoptik und Hoerakustik, University of Technology and Business, Gartenstrasse 135, 73430 Aalen, Germany
Abstract The removal of rigid contact lenses from the eye, using the eyelids, is a relatively simple procedure. However, there is a sequence of biomechanical events underlying this procedure which are not well understood. By using high-speed videokeratoscopy (50 Hz), we have shown that during the lid-pull procedure the cornea typically shows a significant increase in with-the-rule astigmatism by an average of 2.19 D (axial power). The average increase in steep K power was 1.22 D (S.D. 1.05 D) and the average decrease in flat K power was 0.97 D (S.D. 1.05 D). This change in corneal topography increases the edge lift of the lens at 12 and 6 O’clock locations of the lens, enhancing the removal process. When the subject blinks, as the lid margins reach the lens edge, the lens flexes and the eye retracts by an average of 0.66 mm (S.D. 0.27 mm). This retraction again enhances the act of lens removal. # 2005 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. Keywords: Rigid contact lens; Cornea; Contact lens removal; Corneal astigmatism; Retraction; Proptosis
1. Introduction Rigid contact lenses are widely used in the correction of refractive error. While soft contact lenses are more commonly prescribed than rigid lenses, there are many patients and practitioners who prefer rigid lenses for various reasons, including quality of vision, eye health and lens cost. Rigid contact lenses can be removed from the eye using a number of methods. The most common technique is known as the blink technique [1] or one-finger method [2]. To use this technique, the subject opens the eyes, as wide as possible, then places the index finger on the outer canthus, pulls the lids outwards to tighten them and then blinks the lens out [1–5]. Another common removal technique is the two-handed method, in which the contact lens is pinched off the eye by pushing the lid margins towards the eye and the contact lens edges along the vertical meridian [1,2]. Rigid contact lens wearers can also use a lens sucker to remove their contact lenses. * Corresponding author. Tel.: +61 7 3864 5739; fax: +61 7 3864 5665. E-mail address: [email protected] (M.J. Collins).
In this study, we investigated the biomechanical events that occur during rigid contact lens removal, using the blink technique. We performed high-speed videokeratoscopy to investigate corneal changes during the lid pull procedure and measured eye retraction during the blink technique. Our results illustrate the biomechanical mechanisms underlying the process of rigid lens removal.
2. Methods Five subjects, three females and two males, took part in this study. The subjects were between 24 and 37 years of age with a mean age of 28 years and none of the subjects were regular rigid lens wearers. All subjects had normal corneal characteristics and no eye disease. Only the right eye was used in the study. To measure the changes of the corneal topography associated with rigid lens removal, we used a specially modified videokeratsoscope. The Medmont E300 corneal topographer (Medmont Pty Ltd. Melbourne, Australia) was modified to continuously acquire corneal topography at 50
1367-0484/$ – see front matter # 2005 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.clae.2005.04.001
122
M.J. Collins et al. / Contact Lens & Anterior Eye 28 (2005) 121–125
videokeratographs per second (50 Hz). The details of these modifications have been outlined by Iskander et al. [6]. The subjects were seated at the Medmont E300 with their head stabilised on a headrest. The subjects were instructed to look at the fixation target and not to blink during the measurement period. After the instrument was adjusted, the measurement started and 10 s were recorded (500 videokeratographs). Between 3 and 5 s after the measurement had started, the subjects were asked to pull their eyelids outward, as if removing a rigid contact lens. The subjects were asked to put the right index finger at the position of the outer canthus before the measurements began, so as to improve the accuracy and timing of the procedure. The videokeratographs were converted into axial power and corneal height maps for subsequent analysis. About 20 baseline videokeratographs (prior to lid-pull) and another 20 videokeratographs, when the lids were pulled outward, were averaged for each subject. The method employed for averaging topography maps and statistically analysing the changes has been presented by Buehren et al. [7]. Each topography map is resampled into a common polar grid format so that data can be averaged at common locations. Statistics, such as t-tests can then be applied to test the significance of changes in topography at all locations within the maps. We analysed topography within the central 5 mm of the cornea for both conditions (i.e. natural and lid-pull conditions), because eyelashes restricted the vidoekeratoscope vertical map diameter in the lid-pull condition. To investigate the forces on a rigid contact lens during lens removal, we also had one of the subjects wear a Menicon Z rigid lens (8.0/9.6/ 3.00, tc = 0.13 mm) and attempt lid-pull lens removal. We used the videokeratoscope to acquire axial curvature maps with the lids opened wide and with lid margins at the edge of the lens during the lid-pull removal. The retraction of the right eye during blinking was measured, using a digital camera (Nikon Coolpix 995). The subjects placed their head on a headrest and were instructed to look straight ahead. Retraction of the right eye was measured, while the upper and lower eyelid of the right eye were manually restrained against the orbital rim. The subject was instructed to blink with the left eye, while the retraction of the right eye (with restrained eyelids) was measured with the digital camera, aligned at approximately 908 to the line of sight at a distance of 20 cm. A ruler (with millimetre graduations) was placed ‘‘in frame’’ beside the outer canthus of the right eye and was used to calibrate the amount of eye retraction in subsequent analysis, using image processing software.
( p < 0.0001) after pulling the eyelids outward. The average increase in steep K power was 1.22 D (S.D. 1.05 D) with a range from 0.31 to 3.03 D. The smallest increase of 0.31 D was found for subject 5. The flat K power decreased significantly ( p < 0.0001) in four subjects. For subject 5, we did not find a significant decrease of the flat K power ( p = 0.8). The average decrease in flat K power was 0.97 D (S.D. 1.05 D) with a range from 0.01 (for subject 5) to 2.77 D. Changes in flat and steep K power before and during lid-pull of a representative subject (subject 4) are shown in Fig. 1. To analyse the topographical changes in axial curvature, the 20 baseline maps and 20 lid-pull maps were averaged and a difference map (‘‘lid-pull’’ minus ‘‘baseline’’) was created for axial radius and corneal height (Fig. 2). In four of the five subjects, significant regions of change in axial curvature in the vertical and horizontal meridian were found ( p < 0.001). The subject with the smallest changes of topography following lidpull (subject 5) did show significant steepening of the lower cornea, while the upper cornea was largely unchanged. This probably reflects a difference in this subject’s application of the lid-pull technique, or could reflect the underlying anatomical differences. The corneal shape changes are also shown in a meridian analysis of the difference maps (Fig. 3). The horizontal meridian (08 and 1808) flattened for four of the five subjects and the vertical meridian (908 and 2708) became steeper for all subjects. The axial radius maps of the front surface of the rigid lens on-eye with the lids open and at the point of the lid-pull removal procedure is shown in Fig. 4. In the open eye condition, the surface of the lens is approximately spherical with minor vertical steepening. However, in the lid-pull condition, the lens surface becomes significantly astigmatic (with-the-rule), with about 0.75 mm difference in axial radii between the horizontal and vertical meridians. We repeated this procedure at separate time with the same subject and found a similar outcome. But it should be noted that the amount of astigmatic change on the lens surface would be influenced by factors, such as the strength of the lid-pull, the underlying corneal shape, the lens thickness and the lens material modulus. The average eye retraction during the simulated blink condition for the five subjects was 0.66 mm (S.D.
3. Results The change in flat and steep simulated K readings, obtained by the videokeratoscope has been analysed, using the average data of 20 baseline and 20 lid-pull maps. For all subjects, the steep K became significantly steeper
Fig. 1. Changes of flat K and steep K power of subject 4, recorded continuously with a high-speed videokeratoscope. While pulling the eyelids outwards the flat K power became flatter and the steep K power became steeper.
M.J. Collins et al. / Contact Lens & Anterior Eye 28 (2005) 121–125
123
Fig. 2. Difference in axial radius, corneal height and significance maps for five subjects. The difference represents the cornea during lid-pull minus the baseline cornea, simulating removal of a rigid contact lens. The significance maps show the statistical significance ( p-values) of the measured changes.
0.27 mm) with a range from 0.4 to 1.1 mm. To illustrate the changes in eye position associated with rigid lens removal, one subject’s eye position was measured, using the high-speed videokeratoscope. The Medmont videokeratoscope provides a continuous output of eye position (anterior–posterior movement) via a position sensor at the leading edge of the Placido disk cone of the instrument. This
output is not normally available to the user, but is available in our modified instrument. The subjects manually restrained their upper and lower eyelids of the measured eye and were asked to ‘‘widen their eyes’’ and then perform a ‘‘forced blink’’, as if removing a rigid lens. The fellow eye was free to perform these tasks, but the measured eye was restrained. The associated changes in eye position showed a
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M.J. Collins et al. / Contact Lens & Anterior Eye 28 (2005) 121–125
Fig. 3. Change of the axial curvature in each meridian for all five subjects. In the horizontal meridian (08 and 1808), the axial radius became flatter for four subjects. For all subjects, the axial radius in the vertical meridian (908 and 2708) became steeper. Fig. 5. The high-speed videokeratoscope provides a continuous output of eye position (anterior-posterior movement). The subject manually restrained their upper and lower eyelids of the measured eye and were asked to ‘‘widen their eyes’’ and then perform a ‘‘forced blink’’ as if removing a rigid lens.
proptosis of the eye with eye widening and a retraction of the eye during the attempted forced blink (Fig. 5). In summary, the initial act of eye widening causes a slight proptosis of the eye and positions the eyelids past the edges of the rigid lens (Table 1). As the lids are pulled towards the ear, the cornea typically becomes significantly steeper along the vertical meridian and flatter along the horizontal meridian, thereby increasing edge lift along the vertical meridian. When the lids are positioned at the edges of the lens, the lens also flexes to become steeper vertically and the act of blinking then causes eye retraction and forces the lids behind the lens, which is leveraged off the cornea.
4. Discussion
Fig. 4. Axial radius of rigid lens surface on-eye. In the top panel, the eye is open wide. In the centre panel the lids have been pulled to reach the lens edge. In the lower panel, the difference in axial radius between the open eye and lid-pull conditions is shown.
We found that most subjects show significant corneal steepening in the vertical meridian and a flattening in the horizontal meridian of the cornea during the lid-pull procedure of rigid contact lens removal. Gullstrand [8] stated that the pressure of the lids on the eye produce a change in corneal curvature in the direction of with-the-rule astigmatism. It has also been reported that lifting the lids from the surface of the eye causes a decrease in with-the-rule astigmatism in corneas with more than 1.00 D of with-therule astigmatism [9] and that the cylindrical component of the refractive error increased significantly when the palpebral aperture was narrowed [10]. Therefore, it is not surprising that significant with-the-rule astigmatism arises when extra pressure is exerted by pulling the eyelids outward. There is a direct relationship between anterior–posterior eye position and the lid aperture during blinking. Retraction of the human eye during blinking has been observed in several previous studies [11–15]. Collins et al. [12] found that retraction was greater when lids were closed forcefully
M.J. Collins et al. / Contact Lens & Anterior Eye 28 (2005) 121–125
125
Table 1 Sequence of biomechanical events in rigid lens removal Procedure
Eye position
Corneal and lens shape
Eyes widened Lids pulled Lids reach lens edge Forced blink
Proptosis Proptosis Proptosis Retraction
Cornea unaltered Cornea shows induced with-the-rule astigmatism (steeper vertically and flatter horizontally) Lens flexes to show induced with-the-rule astigmatism Cornea returns to normal shape and lens is removed
(0.96 mm) compared to normal blinking (0.44 mm). In this study, we found an average retraction of 0.66 mm which matches reasonably with the data, reported in previous studies. The rigid lens appears to flex along the vertical meridian as the lids reach the edge of the lens in the lid-pull procedure. The forced blink, associated with lens removal, almost certainly flexes the lens further and as the eye retracts, it is not surprising that the lens can sometimes appear to spring from the eye. We could not investigate the two-handed rigid lens removal technique with the high-speed videokeraoscope because the instrument’s cone is too close to the eye to allow access by the fingers. However, we expect that the technique would cause similar changes in corneal topography and potentially greater changes due to the more direct application of force. Some patients find difficulty in removing rigid lenses from the eye using the blink technique. These difficulties may arise from a range of factors, including technique used, anatomical differences and confidence. We did ask each of our five subjects to remove a rigid lens from their eye, while we observed their technique. Only one subject (subject 5) was unable to remove the rigid contact lens using the blink technique. This same subject showed the least amount of corneal change associated with the lid-pull technique (see Fig. 2) and we suspect that this reflects a difference in lid tension applied during the procedure (possibly anatomical or related to the subject’s technique).
5. Conclusions The act of removing rigid contact lenses, using the blink technique, is a procedure that most patients master with practice. However, underlying this simple procedure is a series of complex biomechanical events in the anterior eye. During the outward pull of the eyelids, the corneal curvature steepens in the vertical meridian and flattens in the horizontal meridian. These corneal changes create an increased edge lift at the 12 and 6 o’clock locations of the lens edge to facilitate lens removal. The act of forced
blinking at this point of the process, causes the eye to retract, which also enhances the rigid lens removal.
Acknowledgement We thank Nicola Pritchard, Brett Davis and Robert Iskander for their help with the high-speed videokeratoscope.
References [1] Efron N. Contact lens practice. Oxford/Boston: Butterworth–Heinemann, 2001. [2] Hales RH. Contact lenses: a clinical approach to fitting. Baltimore: Williams & Wilkins, 1978. [3] Obrig TE, Salvatore PL. Contact lenses. New York: Obrig Laboratories, 1957. [4] Stein HA, Slatt BJ. Fitting guide for rigid and soft contact lenses: a practical approach. St. Louis: Mosby, 2002. [5] Lowther GE. Contact lenses: procedures and techniques. Boston: Butterworths, 1982. [6] Iskander DR, Collins MJ, Davis BA. Evaluating tear film stability in the human eye with high speed videokeratoscopy. IEEE Trans Biomed Eng, in press. [7] Buehren T, Collins MJ, Carney L. Corneal aberrations and reading. Optom Vis Sci 2003;80:159–66. [8] Gullstrand A. The cornea. In: Southall JPC, editor. Helmholtz’s treatise on physiological optics. New York: Dover; 1962. p. 320–1. [9] Wilson G, Bell CH, Chotai S. The effect of lifting the lids on corneal astigmatism. Am J Optom Physiol Opt 1982;59:670–4. [10] Grey C, Yap M. Influence of lid position on astigmatism. Am J Optom Physiol Opt 1986;63:966–9. [11] Doane MG. Interactions of eyelids and tears in corneal wetting and the dynamics of the normal human eyeblink. Am J Ophthalmol 1980;89:507–16. [12] Collins M, Smyth W, Seawright J, Kelly S. The synkinesis between antero-posterior eye position and lid fissure width. Clin Exp Optom 1992;75:38–41. [13] Riggs LA, Kelly JP, Manning KA, Moore RK. Blink-related eye movements. Invest Ophthalmol Vis Sci 1987;28:334–42. [14] Evinger C, Shaw MD, Peck CK, Manning KA, Baker R. Blinking and associated eye movements in humans, guinea pigs, and rabbits. J Neurophysiol 1984;52:323–39. [15] Lydon D, Tait A. Lid-pressure: its measurement and probable effects on the shape and form of the cornea-rigid contact lens system. J Br Contact Lens Assoc 1988;11:11–22.