Tear film structure of the contact lens wearer
New techniques are proven in this thesis which allow the visual examination of the tear film on the cornea and on different types of contact lenses. The techniques are as follows: 1. A high magnification technique of photography using the biomicroscope and crossed polarised light, allows the measurement of tear film layer thicknesses and on the surface of rigid contact lenses and 'in vivo receding contact angle'. 2. A low magnification technique of photography allows the recording and the analysis of the superficial layers of the tear film over a wide area of the eye and contact lens surface. 3. A clinical instrument has been designed for the routine clinical observation of the tear film. It permits the visual measurement of the tear film break-up time (BUT) without the use of fluorescein. 4. A unique technique of in-vivo photography of the mucous coverage of the corneal epithelial surface and contact lens surfaces is presented. Such techniques improve the understanding of surface wettability problems. As a result of this work the appearance of the normal lipid layer of the pre-ocular tear film has been classified into seven grades within a thickness range of 0.02μm to 0.58μm. As we well as the normal, two examples of abnormal lipid layers are described. The manner by which Meibomian gland secretion occurs and performs its role in the tear film is illustrated and analysed. Forced gland secretion by pressure induced localised lipid thickness increase. Eyelid closure was seen to compress the lipid film and instillation of saline broke up the lipid film. It can be seen that the pre-soft lens tear film usually possesses a thin superficial lipid layer and an aqueous phase of limited dimensions. On the other hand the pre-PMMA rigid contact lens tear film rarely possesses a visible superficial lipid layer and its aqueous layer measured 1.5μm on average. The addition of the 'wetting' solution acts on the thickness of the aqueous phase which increases to 2.5μm and supports a minimal lipid layer. The use of contact lens materials of better wettability permits the formation of films of increased thickness (up to 5.5μm) with a visible superficial lipid layer which was seen to stabilise the film and retard its drying. The main differences in mucous coverage are described as follows. At the level of the basal layer of the tear film the mucous coverage takes a continuous undulated form on the corneal epithelium but a discontinuous sporadic distribution on soft lenses decreasing to sporadic occurrence on rigid lenses. Finally, because of the acquisition of the quantitative results, new infra-structures of the pre-lens tear film for soft and rigid lenses are proposed.