1. and the Corresponding Lens Water Content Complex Model Discussion
An Analytical Model of Tear Film pH, Soft (i.e., hydrogel) Contact Lens Anterior Surface pH,
and the Corresponding Lens Water Content
Morris R. Lattimore, O.D., Ph.D. Thomas H. Harding, Ph.D. Steven T. Williams, M.A.
MHSRS
Introduction
Figure 1
Figure 4
Extending prior to Operations Desert Shield / Desert Storm (ODS/S), from 1989 to 1991 the Army commissioned an expansive study on soft contact lens wear reference their operational usefulness by Apache Pilots. Operational environmental concerns regarding soft contact lens (CL) hydration was a major influence on practical contact lens wearability. The material physical parameters (as an ocular prosthesis) helped focus our study design. Our primary concern was in determining the relationship of the CLs to the internal bodily environment as influenced by the external geographic environment. While numerous investigators have shown that soft lens hydration is directly influenced by the pH of its in-vitro storage solution, none had demonstrated what transformations might occur to the hydrogel material, none had assessed CL immersion within the precorneal tearfilm.
Methods
Complex Model Discussion
Figure 2
Forty-one subjects were fitted with two types of extended wear soft contact lenses, differing in the ionic structure and their inherent water content. The volunteer subjects were followed quarterly for a year. The in-vivo hydrogel lens pH was obtained using a gel-surface pH electrode, which the investigator placed against the anterior surface of the worn contact lens as part of each examination. CL water content was assessed by refractometer measurement, as well as via gravimetric means, which correlated quite well with one another (R=0.98). Using a log-log conversion (Figure 3, bottom left) extended lens wear water content was plotted as a function of the extended lens wear anterior lens surface pH data.
The resulting graph in Figure 3 visually illustrates the differing linear slopes of each type of contact lens, which is attributed to their differing ionic status, as well as the secondarily related variability of their polar attraction for water molecules. These data were from 26 years ago, when simple hydrogel materials dominated the contact lens market. Current contact lens materials are dominated by complex silicone-hydrogel polymers, representing of 2/3 of the CL market share. Such compound polymers offer exceptional oxygen transmission. However, tear fluid exchange by the complex CL material is much less than what had been available with the simple hydrogels, resulting in frequent lens binding to the ocular surface. Figure 4 illustrates the actual CL immersion within the tearfilm. Additional complex silicone CL surface hydrophobic characteristics resulting in marked lipid deposition have been attributed to dry eye symptoms. frequently reported by up to 50% of silicone-hydrogel lens wearers.
Similar CL material linear behavior models of oxygen transmisivity, index of lipid deposition, and fluid exchange indices could lead to individually customized silicone-hydrogel contact lens selections for individual patients. As a secretory fluid, tear lipid content is very likely no different than that of internal bodily environs, which could impact other types of prosthetics. Prosthetic behavior within a body is directly related to the behavior of the materials of which they are made. If a prosthesis is dependent on maintaining joint flexibility, then a standard hydrogel material would best be utilized. Those requiring ease of oxygen transport through the prosthetic material, would best utilize the compound silicone-hydrogel material. In combination, these material characteristics can readily be determined via current standard chemical and biochemical means, but are rarely understood as to their applied adaptability based on their material interactions.
Results
Figure 3
The in-situ contact lens pH increased logarithmically across extended wearing time, reaching an asymptote at approximately 5 days’ wearing time (7.45 +/- 0.03, which matches many of the previously documented normal tear pH values). Figure 1 demonstrates the previous discussion. The in-situ hydrogel lens water content was theorized to be directly dependent upon the precorneal tearfilm pH. However, additional anticipated contributors to the process (tear availability, tear stability in terms of resistance to evaporation, as well as tear osmolarity) were hypothesized to provide only secondary and tertiary clinical contributions to the pH change over days’ extended wear. Lens water content plotted as a function of extended lens wear duration illustrated a logarithmic decrease. Again, the logarithmic functions levelled off by 4 to 5 days’ CL wear. A multifactorial analysis (examining both types of contact lenses, factored by water content, and number of days’ wear) revealed an overall statistical significance of p <