In Vivo Study Of Contact Lens Wettability Biology Essay

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In 1963 Otto Wichterle discovered polyhydroxyl ethyl methacrylate and created the worlds first soft contact lens(Bennett and Weissman 2005). Since then there has been an improvement in manufacturing techniques, as well as an increase in the types of polymer used in the contact lens. Traditional types of contact lenses were made using hydrogel and water where the water transfers the oxygen from the air to the cornea. These types of lenses needed a high water content which made the lens suffer from dehydration if worn for a long time. Studies have shown that contact lens wearers often discontinue lens wear due to symptoms of dryness and discomfort. In 1998 Bausch & Lomb introduced new types of contact lens material (silicon hydrogel, silicon is gas permeable Dk 100 or greater) which can be worn for long periods. Hydrogel lens groupings are based on the water content and electrostatic charge (ionicity) of the polymer. According to the FDA Contact Lenses are commonly grouped into four groups based on water content (less than 50% water are considered to be low water content lenses, and greater than 50% water are high-water content) and on whether the lens surface is considered to be ionic (reactive) or non-ionic (less reactive). Group 1: Low water, Nonionic, Group 2: High water, nonionic, Group 3: Low water, ionic, Group 4: High water, ionic.

Developments in soft contact lens materials start in 1998 by Bausch & Lomb. They created first silicon hydrogel lens (Pure Vision). In 2001 CIBA Vision introduced the Focus Night and Day silicon hydrogel for up to 30 days of continuous wear. (Bennett and Weissman 2005). A number of materials have been created recently to improve the properties of soft contact lenses and increase the duration of wearing. Senofilcon A is an example of new-generation of silicone hydrogel material (ACUVUE OASYS); it is made of approximately 38% water and 62% Senofilcon A.

Contact lens wettability is defined as "the ability of tear film to spread and maintain itself over a contact lens surface" (Maldonado-Codina and Morgan 2007). The ability to keep the contact lens clean and with a high wettable surface is the key to long-term wearing. The blinking rate will be increased in contact lens wearers; hence we need to maintain the wettability of the lens surface to help in the blinking process. A poorly wettable surface will result in dry areas on the lens that will increase the deposition of lipids (Jones 2007).

There is a direct relationship between tear film and contact lens, since any effect of the tear film stability will affect the contact lens wettability. In my study a sequence of experiments will be done to investigate the contact lens wet angle; in variable contact lens areas, in different environments, using different contact lens materials and under different tear film chemistry. It is expected that this series of experiments will reveal the mean contact lens wet angle and the appropriate materials and environment. About 45 subjects (age 18-35) will be randomly chosen for this study. An Interferometry instrument will be used in all experiments, and the MATLAB analysis program will be used to analyze the data.

The pre- and post lens tear film plays a role in the contact lens wearers vision, however the post-lens tear acts as a smooth layer for the back contact lens surface which provides clear vision; if this layer is disrupted it will lead to a light scattering. The pre-lens layer contains lipid film which helps in reducing evaporation (Nichols and King-Smith 2003).


File:Surface tension.svgFile:Contact angle.svg

Wetting of different fluids. A shows a fluid with very little wetting, while C shows a fluid with more wetting. A has a large contact angle, and C has a small contact angle. 2) Contact angle of a liquid droplet wetted to a solid surface.

Clinically, most practitioners test the wettability of contact lenses by using a slit lamp to observe the spread of the tear film over the lens surface. Another method of studying the wettability of contact lenses is by measuring the angle between the liquid spread over the surface and the solid surface itself (figure 2). This is called the contact angle (CA) and it is used in many other areas of biomaterial research and with contact lens materials. The smaller the contact angle, the more lens wettability. A number of techniques exist to measure the contact angle, including Wilhelmy Plate, captive bubble and sessile drop (Jones 2007 ).

There are some factors that will affect the wettability of contact lenses, for instance: tear film component, environment (humidity & temperature), and contact lens materials.

Maldonado et al. (Maldonado-Codina and Morgan 2007) have investigated the water wet angle of 5 commercial silicon hydrogel contact lenses (Acuvue Advance, Acuvue Oasys, Focus Night and Day, O2 Optix, and PureVision). Sessile drop and captive bubble techniques were used. The only lens that showed a significant difference is Acuvue Advance (47% water content). In a comparison between the direct measurement from the blister and after 48 hours soaking in saline solution, the angle changed from 66° to 96°, respectively. Guillon et al. (Guillon and Guillon 1989) have studied the wettability of hydrogel lens overnight wear; two types of soft contact lenses (Igel 67 Prima and Bausch & Lomb 70) were tested in vivo, pre-lens tear film was assessed by non-invasive break up - time (NIBUT). Good wettability was obtained during the study while no significant difference was found between the two types of materials, but there was a little difference between the tear thickness of open and closed eyes; pre-lens tear thickness was found to be thicker immediately after waking. A study concentrating on the wettability of silicon hydrogel in the presence of tear film component has been done by Cheng (Cheng, Muller et al. 2004). Cheng used the captive-bubble technique to measure the advancing angle (when a liquid spreads over a surface) and the receding angle (when a liquid is absorbed from a surface) of two commercial silicon hydrogel lenses; he said both the advancing and the receding angle are related to contact lens wettability. He used three different types of solution for soaking the lenses (ex vivo), before applying the bubble over the lens surface. The advancing angle was found to be between 13° and 81° while the receding angle varied between 13° and 33°; this variation in angles depend on lens materials and soaking solution.

Method & Materials:

Depending on the principle of Newton's rings shape (figure 3), which uses different phases index to get optical oscillation, there are five different phases in the eye when a contact lens is worn: air, pre-lens tear film layer, contact lens, post-lens tear layer, and anterior surface of cornea, reflection from any two or more of these surfaces can cause interference (Nichols and King-Smith 2003).

An interferometer is an instrument designed and constructed to simplify the measurement of in vivo tear film topology, thickness, and wetting, using the principle of thin film interferometry. The use of a tear film interferometer permits objective assessment of the break up characteristics of the tear film on contact lenses and offers understanding for examining the dynamic changes in thickness, thickness distribution, and wetting properties throughout the sequence inter blink retro (Doane 1989). A special version of the MATLAB (figure 4) program will be used to measure the angle between the tear film and contact lens surface depending on the principle of thin film interference. The dry area is supposed to be the start point of measuring where the tear film thickness is almost zero.

Figure 3: Rings shape of soft lens in eye. Figure 4: Analasys Proces

The aim of the proposed research is to study the wettability of contact lenses, which may influence changes in tear film dynamics. A series of studies will be conducted to determine the effect of the wet angle of contact lenses on specific factors such as temperature and humidity, tear film chemistry, contact lens material.

First experiment: will measure the regional variation in wettability of a single type of contact lenses; the effect of the blinking process in the wettability of the lens will be shown. A single type of contact lens will be fitted for 10 subjects (one eye), to be kept in his\her eye between 10 -15 minutes for eye adaptation. The subject will be encouraged to keep his\her eye open as much as possible; during this time of no blinking, the tear film will start to evaporate and dry areas appear. A CM1300\1301 digital camera will be used to take a number of photos over time; these photos must be sharply focused to see the fringes (Figure 3). Eight markers will be used in the MATLAB program to measure the wet angle in four different area of the lens at the same time.

Second experiment: will measure the difference in wettability of different contact lens materials. Different types of contact lens materials will be fitted for the same five subjects during five separated visits. At each visit the subject will be fitted with different type of lenses and keep those between 10 -15 minutes for eye adaptation with a lens. The subject will be asked to blink once then encouraged to keep his\her eye open as much as possible. A digital camera CM1300\1301 will take a number of continues photos for a period of between 50 - 60 seconds. The same area of all different lenses will be analysed by using the MATLAB program.

Third experiment: will measure the effect of individual's tear chemistry on the wettability of a single contact lens material. Twenty subjects will be fitted with a single type of lens in one eye; each subject will keep the lens between 10 - 15 minutes for eye adaptation, then the subject will be asked to blink once and keep his\her eye open as much as possible. Immediately after blinking a digital camera CM1300\1301 will start to take a number of photos continuously. When a subject keep his\her eye open between 50 - 60 seconds dry areas will appear. The first fringes next to the dry area will be analysed to find the wet angle of the used lens.

Fourth experiment: will measure the change in wettability of a single type of contact lens in high (60%) and low (30 %) humidity areas. This experiment will be done in a special room under controlled temperature and humidity. A subject will be fitted with a contact lens in one eye then stay in the room for between 10 - 15 minutes to become accustomed to the contact lens. Subject will be asked to sit in front of Doan's instrument which is supplied by the digital camera CM1300\1301 and be asked to blink once then encouraged to keep his\her eye open during the time of photographing which is no more than 60 seconds. The number of photos which are taken will be compressed to AVI video then submitted for analysis by MATLAB program.