Humectabilidad in-vivo e in-vitro en diferentes materiales de lentes de contacto y calidad de la película lágrimal

Resumen

Contact lenses have increased in popularity over the past five decades as an option for correcting refractive errors of the eye, with an estimated 140 million users worldwide (1) (2). Despite the introduction of new contact lens technologies and materials, disruption or "dropout" from contact lens wear continues to challenge the industry. Dryness and discomfort related to contact lenses continue to be reported by approximately 30% and 50% of lens wearers, primarily end-of-day discomfort (3) (4) (5) (6) (7). A recent survey indicated that 25% of respondents had discontinued wearing contact lenses permanently due to discomfort and dryness (8). Richdale and colleagues recently conducted a cross-sectional survey of 730 subjects and reported that 24.1% of subjects had permanently discontinued contact lens wear (9). The main reasons reported in the study for both dissatisfaction with contact lenses and discontinuation were ocular symptoms (dryness and discomfort) (9). A current review of the literature by Pucker and Tichenor found that contact lens abandonment was common in developed countries, with abandonment frequency ranging from 12.0% to 27.4% (pooled mean=21.7%). The most cited reason for wearers in wearers was discomfort while vision was the primary reason in neophyte contact lens wearers (10). These data are similar to a study conducted nearly 20 years ago that indicated that 26.5% of discontinuation was due to discomfort (11). The dropout rate remains high despite the advancement in lens materials and designs, and this may not come as a surprise; since when a contact lens is placed in the eye, the lens alters the normal structure of the pre-ocular tear film and affects its evaporation rate (12) (13). Therefore, it has been hypothesized that a normal tear film structure will probably never form for an extended period of time on the lens surface (14). It has been established that pre-lens tear film stability plays a critical role in successful contact lens wear as it can influence comfort and tolerance during lens wear (14). Earlier research in leporine models has shown that when the lipid layer of the tear film is removed evaporation rapidly increases approximately four-fold (38 x I0-7g/cm2/sec) (15) (16). A more recent paper studied the appearance and thickness of the tear lipid layer in a group of normal and dry eye subjects and reported that mean tear evaporation rate in the group with an intact lipid layer was found to be 1.42 g/m2/h, while in the group with no lipid layer or with abnormal color fringes was found to be 5.90 g/m2/h; this represents more than a four times greater tear evaporation rate (17). In recent years, a variety of non-invasive methods have become available to assess different factors affecting tear film stability, including pre-corneal tear lipid layer thickness (LLT) and tear film surface quality (TFSQ) over a contact lens surface (19) (20) (21) (22) (23) (24). Several studies have investigated pre-corneal LLT as an indirect measure of tear film stability (25). Isreb et al. found a correlation between LLT and Schirmer¿s test with anesthesia and fluorescein tear film break-up time and concluded that LLT is a reliable test for the diagnosis of dry eye (25). Athira et al. evaluated interference patterns of lacrimal lipid non-invasively using Tearscope Plus® and concluded that the appearance of the lipid layer did not differ between symptomatic and asymptomatic contact lens wearers, whereas non-invasive lens surface drying time was lower in symptomatic contact lens wearers than in asymptomatic contact lens wearers (26). Caroline et al. investigated the relationship between LLT and dry eye symptoms and found that patients reporting severe symptoms have relatively thin lipid layers of 60 nm or less, whereas approximately 3 of 4 patients without symptoms have relatively thick lipid layers of 75 nm or more (27). The development of daily disposable soft contact lenses and silicone hydrogel materials reveal two of the most notable improvements in the field of contact lenses over the past 30 years (28). Daily disposable contact lenses aimed to overcome many of the problems inherent in wearing frequently replacement soft contact lenses, such as the build-up of surface deposits that can reduce visual acuity, increase the risk of infection, the need for daily cleaning and disinfection, and reactions to lens care solutions that can lead to discomfort and even damage to the cornea (28). The objectives of this doctoral thesis work were the following: 1) To establish whether there is a difference in the stability of the pre-corneal tear film between symptomatic (Sx) and asymptomatic (ASx) contact lens wearers in terms of lipid layer thickness, non-invasive tear break-up time, and tear film surface quality. 2) Determine whether contact lenses manufactured with materials of different polymer chemistry of conventional hydrogel and silicone hydrogel impact differently the in-vivo wettability and stability of the pre-lens tear film. 3) To establish whether there is a correlation between the different indicators of the stability of the pre-corneal tear film, and its relationship with the subjective symptoms of comfort and dryness. The study consisted of two phases. In both phases, an assessment of the pre-corneal tear film was performed, without contact lens, measuring the thickness of the lipid layer, noninvasive tear break-up time and tear film surface quality. Subsequently, daily wear disposable contact lenses were fit contralaterally and two-week replacement lenses were fitted bilaterally. The study design was double-blind, randomized and required two clinic visits. Before lens fitting, the stability of the pre-corneal tear film was evaluated, then the contact lenses were fitted, and the same measures of tear film stability (pre-lens) were performed. In phase I, daily disposable wear lenses were used and worn for a period of 6 hours while in phase II the wearing time was 2 weeks. There was no statistical difference in the characteristics of pre-corneal tear film stability between the ASx and Sx groups. In Sx groups, a longer pre-corneal non-invasive tear break-up time is significantly associated with a thicker pre-corneal lipid layer thickness and poor pre-corneal tear film surface quality is associated with a shorter pre-corneal non-invasive tear break-up time. There is an association between in-vivo wettability and tear film surface quality regardless of the contact lens manufacturing material. The stability of the tear film is altered by contact lens wear, regardless of their manufacturing material, immediately after insertion, at 6 hours and 2 weeks of wear.