For most patients, we take for granted some contact lens material properties because they usually aren’t that critical. But in some cases, thinking about these properties can be the difference between a lens that is tolerable and one that is great. Paying closer attention to these material properties can provide significant improvements in contact lens performance.

Water Content

Hydrogel materials range in water content from about 25% to 80% when equilibrated in 0.9% saline solution. For traditional hydrogels, oxygen permeability (Dk) is directly related to water content and maxes out at about 40. Silicone hydrogels tend to have low- to mid-water content, and Dk is only partially dependent on it. Either way, there are pros and cons to lower and higher water contents. Lower-water materials tend to dehydrate slower and less by weight, are more deposit resistant, and tend to be more durable, allowing thinner designs. Higher-water materials may be more wettable and hydrating, especially with newer technologies that reduce dehydration.

Environmental factors—such as humidity, temperature, tear osmolarity, and pH—also affect hydration. Because of newer material technologies that decrease surface dehydration, water content may not be the primary factor in determining hydration-related comfort, but it is still worth considering when troubleshooting patient symptoms.

Index of Refraction

Soft lens materials range in index of refraction from about 1.37 to 1.44, while GP materials range from about 1.41 to 1.54. As with spectacle lenses, contact lens designs can be made somewhat thinner with higher-index materials.

For GP lenses, index is worth considering for designs such as back-surface torics and aspheric multifocals, especially in higher powers. Greater refractive correction for astigmatism and presbyopia can be attained with the same curvatures when higher-index materials are used. This may be even more apparent with thick, high-powered scleral lenses, though a more important consideration is material Dk.

For back-surface-toric GP lenses in particular, material choice helps determine the amount of astigmatism corrected; too low an index will undercorrect, while too high an index will overcorrect. The difference can be as much as 25%. If opting for a high-index GP material, the only major concern would be surface wettability.

Ultraviolet Blocking

Some, but not all, soft and GP materials can block ultraviolet (UV) radiation. Contact lenses are classified as either Class 1 (blocks 90% of UVA and 99% of UVB radiation) or Class 2 (blocks 70% of UVA and 95% of UVB). UV-blocking lenses, therefore, do not completely block UV light and are not as effective as sunglasses, which should block at least 99% of UVA and UVB rays. However, they provide added protection from UV light that passes through and around sunglass lenses. They are especially helpful for patients who are more at risk for UV-related problems, such as those who spend a lot of time outdoors, are lightly pigmented, or have ocular conditions such as keratoconus.

Surface Wettability

The lens surface is of utmost importance in determining comfort. Wettability is often discussed as a key factor in maintaining good comfort and vision, though it is not easily defined.

A material’s wetting angle can be measured, but this has not had great predictability for on-eye surface wetting (Gleason and Doane, 1987). Lubricity, or the reduction in friction by a lubricant, is a measure that appears to be correlated to comfort (Vidal-Rohret al, 2018) and can be improved in many ways. For example, wetting agents can be added to the lens matrix or to the packaging solution, and manufacturing techniques can bind a hydrophilic layer to a hydrophobic core. CLS

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