Contact lenses suffer from the same problems of deposition that other biomaterials exhibit, being rapidly coated with a variety of proteins, lipids and mucins. The first event observed at the interface between a contact lens and tear fluid is protein adsorption. Protein deposits on contact lenses are associated with diminished visual acuity, dryness and discomfort and lid-related inflammatory changes.
The aim of this thesis was to determine the quantity and the conformational state of lysozyme deposited on contact lens materials over various time periods and also to determine the clinical relevance of protein deposits on contact lenses. The specific aims of each chapter of this thesis were as follows:
• Chapter 4: To determine the total lysozyme deposition on conventional and silicone hydrogel contact lens materials as a function of time by artificially doping lenses with 125I-labeled lysozyme.
• Chapter 5: To determine the conformational state of lysozyme deposited on conventional and silicone hydrogel contact lens materials as a function of time using an in vitro model.
• Chapter 6: To quantify the total protein, total lysozyme and the conformational state of lysozyme deposited on a novel, lathe-cut silicone hydrogel contact lens material after three-months of wear.
• Chapter 7: To determine the relationship between protein deposition and clinical signs & symptoms after one-day wear of etafilcon lenses in a group of symptomatic and asymptomatic lens wearers.
• Chapter 8: To determine the influence of individual tear proteins (lysozyme, lactoferrin and albumin) on the adhesion of Gram positive and Gram negative bacteria to conventional and silicone hydrogel contact lens materials.
• Chapter 4: Conventional hydrogel FDA group I (polymacon), group II (alphafilcon A and omafilcon A), group IV (etafilcon A and vifilcon A), polymethyl methacrylate and silicone hydrogel lens materials (lotrafilcon A, lotrafilcon B, balafilcon A, galyfilcon A and senofilcon A) were incubated in a lysozyme solution containing 125I-labeled lysozyme for time periods ranging from 1 hour to 28 days. After each time period, lysozyme deposited on contact lens materials was determined using a Gamma Counter.
• Chapter 5: Conventional hydrogel FDA groups I, II, IV and silicone hydrogel lens materials were incubated in lysozyme solution for time periods ranging from 1 hour to 28 days. After each time period, the lysozyme deposited on the lenses was extracted and the sample extracts were assessed for lysozyme activity and total lysozyme.
• Chapter 6: 24 subjects completed a prospective, bilateral, daily-wear, nine month clinical evaluation in which the subjects were fitted with a novel, custom-made, lathe-cut silicone hydrogel lens material (sifilcon A). After 3 months of wear, the lenses were collected and total protein, total lysozyme and active lysozyme deposition were assessed.
• Chapter 7: 30 adapted soft contact lens wearers (16 symptomatic and 14 asymptomatic) were fitted with etafilcon lenses. Objective measures and subjective symptoms were assessed at baseline and after hours 2, 4, 6 and 8. After 2, 4, 6 and 8 hour time points, lenses were collected and total protein, total lysozyme and active lysozyme deposition were assessed.
• Chapter 8: Three silicone hydrogel (balafilcon A, lotrafilcon B & senofilcon A) and one conventional hydrogel (etafilcon A) lens materials were coated with lysozyme, lactoferrin and albumin. Uncoated and protein-coated contact lens samples were incubated in a bacterial suspension of Staphylococcus aureus 31 and two strains of Pseudomonas aeruginosa (6294 & 6206). The total counts and the viable counts of the adhered bacteria were assayed.
• Chapter 4: Lysozyme accumulated rapidly on conventional hydrogel FDA group IV lenses, reached a maximum on day 7 and then plateaued with no further increase. PMMA showed a deposition pattern similar to that seen on lotrafilcon A and lotrafilcon B silicone hydrogel lenses. After 28 days, conventional hydrogel FDA group IV lenses deposited the most lysozyme.
• Chapter 5: After 28 days, lysozyme deposited on group IV lenses exhibited the greatest activity. Lysozyme deposited on polymacon, lotrafilcon A and lotrafilcon B exhibited the lowest activity. Lysozyme deposited on omafilcon, galyfilcon, senofilcon, and balafilcon exhibited intermediate activity.
• Chapter 6: The total protein recovered from the custom-made lenses was 5.3±2.3 µg/lens and the total lysozyme was 2.4±1.2 µg/lens. The denatured lysozyme found on the lenses was 1.9±1.0 µg/lens and the percentage of lysozyme denatured was 80±10%.
• Chapter 7: Correlations between subjective symptoms and protein deposition showed poor correlations for total protein/ lysozyme and any subjective factor, and only weak correlations between dryness and active lysozyme. However, stronger correlations were found between active lysozyme and subjective comfort.
• Chapter 8: Different tear proteins had varying effects on the adhesion of bacteria to contact lens materials. Lysozyme deposits on contact lenses increased the adhesion of Gram positive Staphyloccocus aureus 31 strain, while albumin deposits increased the adhesion of both the Gram positive Staphyloccocus aureus and Gram negative Pseudomonas aeruginosa 6206 & 6294 strains. Lactoferrin deposits increased the total counts of both the Gram positive and Gram negative strains, while they reduce the viable counts of the Gram negative strains.
• Chapter 4: Lysozyme deposition is driven by both the bulk chemistry and also the surface properties of conventional and silicone hydrogel contact lens materials. The surface modification processes or surface-active monomers on silicone hydrogel lens materials also play a significant role in lysozyme deposition.
• Chapter 5: The reduction in the activity of lysozyme deposited on contact lens materials is time dependent and the rate of reduction varies between lens materials. This variation in activity recovered from lenses could be due to the differences in surface/ bulk material properties or the location of lysozyme on these lenses.
• Chapter 6: Even after three-months of wear, the quantity of protein and the conformational state of lysozyme deposited on these novel lens materials was very similar to that found on similar surface-coated silicone hydrogel lenses after two to four weeks of wear. These results indicate that extended use of the sifilcon A material is not deleterious in terms of the quantity and quality of protein deposited on the lens.
• Chapter 7: In addition to investigating the total protein deposited on contact lenses, it is of significant clinical relevance to determine the conformational state of the deposited protein.
• Chapter 8: Uncoated silicone hydrogel lens materials bind more Gram positive and Gram negative bacteria than uncoated conventional hydrogel lens materials. Lysozyme deposited on contact lens materials does not possess antibacterial activity against all bacterial strains tested, while lactoferrin possess an antibacterial effect against certain Gram negative strains tested in this study.
This thesis has provided hitherto unavailable information on contact lens deposition and its influence on subjective symptoms and bacterial binding. These results suggest that protein deposition has a significant potential to cause problems. Therefore, it is important that practitioners advise their patients regarding the importance of lens disinfection and cleaning and appropriate lens replacement schedules. These results will also be useful for the contact lens industry and the general field of biomaterials research.||en