Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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c l s p e c t r u m . c o m 27 C O N T A C T L E N S S P E C T R U M S P E C I A L E D I T I O N 2 0 1 7 alternative transport mechanism for oxygen as opposed to the aqueous phase in conventional soft materials, and this was originally exploited in GP materials with TRIS. The principle of combining silicone-containing mono- mers to increase oxygen permeability with hydrophilic monomers to increase comfort on the eye is simple, but combining these two components has proved extremely challenging. The simplest approach is to try to combine hydrophilic HEMA with TRIS, the molecule widely used in the production of GP materials. Unfortunately, these two compounds are immiscible, and the hydrated polymer is often opaque, indicating that phase separa- tion has taken place in this system. Various approaches have been used to solve this fundamental problem, re- sulting in the successful evolution of these materials. One approach has been the modification of the TRIS molecule to make it more hydrophilic. Many patients have described the insertion of hydrophilic groups in the linking region of this molecule between the meth- acrylate polymerizable group and the branched silicone section analogous to silicone rubber. Another approach was the use of macromers (preassembled, large mono- mers that contain specific functional units), which often cannot be combined in the form of simple monomers. In this way, the elements of silicone rubber and fluorine can be combined with hydrophilic polyethylene glycol units. These macromers have also been developed over many years, illustrating that despite another relatively simple concept, its execution proved difficult. More recent materials use a combination of these ap- proaches. They contain a modified TRIS derivative, and they also make use of macromer molecules. In addition, many of these products contain hydrophilic polymers that act as wetting agents such as polyvinylpyrrolidone (PVP). This component is a linear homopolymer com- posed entirely of vinylpyrrolidone, which is added to the monomer formulation prior to the lens molding process. The monomer mixture is then polymerized around the titioners. The characteristics of low-, mid-, and high-Dk materials follow. • Low Dk. The oxygen permeability of these materi- als is about 30 barrers, and they have good stability in maintaining the base curve dimensions to which they are cut. Visual acuity is usually excellent, and the lenses have good wettability and resist scratching. • Mid Dk. The oxygen permeability of these materi- als is about 75 barrers, and they also have good stability in maintaining the base curve dimensions to which they are cut. Visual acuity is very good, and the lenses have reasonable wettability and scratch resistance. • High Dk. The oxygen permeability of these mate- rials is greater than 100 barrers, and they usually have good stability in maintaining the base curve dimensions to which they were cut. Visual acuity is good, and the lenses have adequate wettability but can be more prone to scratching. The inclusion of silicone-containing monomers to increase permeability typically has a negative impact on surface wettability, given the hydrophobic nature of silicone. The silicone component is usually softer than other components used in GP formulations, and this may lead to increased movement in the base curve after the lens has been cut. It can result in a surface that has a lower hardness factor and is prone to surface scratches. It has also been suggested that visual acuity seems to de- crease as the permeability of GP materials increases. SILICONE HYDROGEL MATERIALS Silicone- and fluorine-containing lenses provide an LENS MA TERIALS LOW Dk MID Dk HIGH Dk Permeability * *** ***** Stability ***** ***** **** Wettability ***** **** *** Visual Acuity ***** ***** **** Durability ***** **** *** BALANCING LENS PROPERTIES T A B L E 1 S o f t l en s e s h a v e u n d e r g o n e a s i g n i f i c a n t tr a n s f o r m a t i o n .

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