Protein and Lipid Deposition of Silicone-Hydrogel Contact Lens Materials

Lyndon Jones, PhD FCOptom DipCLP DipOrth FAAO (Dip CL) FIACLE Associate Professor School of Optometry Associate Professor Departments of Physics, Chemistry & Chemical Engineering (cross-appointed) Associate Director Centre for Contact Lens Research School of Optometry, University of Waterloo, Waterloo OntarioCanada N2L 3G1

Michelle Senchyna, BSc, PhD Assistant Professor School Of Optometry and Faculty of Biology University of Waterloo

Silicone-hydrogel contact lens materials represent a new family of biomaterials, whose properties are unlike any other previously developed for contact lens use. The incorporation of siloxane groups into the base hydrogel material has produced materials that have substantially improved oxygen transmission characteristics compared with conventional soft contact lens materials 1-3 and the results from clinical studies conducted to-date indicate that the number of physiological complications induced by the overnight use of such materials is significantly less than that seen with conventional materials. ^4-11

Detailed explanations of the development of silicone hydrogel materials for contact lenses and their polymer chemistry have been described in detail elsewhere. ^{2, 12-16} The use of silicon-containing flexible contact lenses is not new, as silicone-elastomeric lenses have been used for therapeutic and paediatric applications for many years. ¹⁷ These lenses offer exceptional oxygen transmission, but the migration of siloxane moieties to the material surface results in the production of extremely hydrophobic surfaces, resulting in marked lipid deposition. ¹⁸ To overcome this, the surfaces of the two commercially available silicone-hydrogel lenses are surface treated, ^{2, 13, 19, 20} in an attempt to improve the wettability of the materials and to reduce the degree of deposition that would occur on non-treated materials. The surfaces of Focus Night & Day (lotrafilcon) lenses are permanently modified in a gas plasma reactive chamber to create a permanent, ultrathin (25nm), high refractive index, continuous hydrophilic surface. ^{13, 21} PureVision (balafilcon) lenses are surface treated in a gas plasma reactive chamber which transforms the silicone components on the surface of the lenses into hydrophilic silicate compounds. ^{2, 19, 20} Glassy, discontinuous silicate “islands” result, ^{19, 20} and the hydrophilicity of these areas "bridges" over the underlying hydrophobic balafilcon A material.

The deposition of contact lenses with substances derived from the tear fluid is a well-known clinical complication, resulting in reductions in comfort, ^{22 vision 23} and increased inflammatory responses. ²⁴ Hydrogel materials rapidly spoil with constituents from the tear film, particularly proteins, ^{25, 26} lipids ²⁷ and mucins. ²⁸ The adsorption of proteins and lipids at the contact lens interface is dependent upon a number of factors. Notable amongst these are material water content, ^{29, 30} surface charge, ^{29, 31, 32} wearing period ³³ and age of the lens material. ³⁴ Increasing water content and/or ionicity of the lens material greatly enhances protein deposition, ^{25, 29-32, 35-37} with lysozyme being detectable on FDA group IV lenses after wearing times for as little as one minute. ³⁸ Whilst group IV lenses tend to predominantly deposit lysozyme, neutral group II lens materials (particularly those containing vinyl pyrrolidone) have a tendency to deposit lipid. ^{33, 39, 40}

Information concerning the degree of protein and lipid deposition that occurs on silicone-hydrogel lens materials is of significant clinical importance, as these lenses are intended for in-eye use for up to 30 days without removal. It is imperative that materials worn in this way deposit as little material from the tear film as possible, in order to minimise the potential visual and inflammatory complications detailed above.

Results from our group ^{41, 42} and others ^43-45 indicate that the amount of lysozymedeposited on silicone-hydrogel lens materials is significantly less than that seen on group IV traditional contact lens materials. Figure 1 indicates that silicone-hydrogels typically deposit less than 10 µg of lysozyme after being worn in-eye for 30 days, as compared with an etafilcon-lens (Acuvue), which deposits in the region of 1000 µg of lysozyme after one week of extended-wear.

Figure 1 Degree of lysozyme deposition measured on Focus Night & Day (lotrafilcon), PureVision (balafilcon) and Acuvue (etafilcon). The lotrafilcon and balafilcon lenses were worn for 30 nights continuously and the etafilcon lens was worn for 6 nights without removal. The degree of lysozyme deposition was significantly different between all three lens types (p<0.001).

Despite surface treatment of the lens materials, in vitro wetting angle assessment of the two silicone-hydrogel materials indicates that they remain relatively hydrophobic compared with conventional hydrogels. ^{44, 46, 47} Lipids preferentially deposit onto hydrophobic surfaces and data thus far indicates that lipid deposition on silicone-hydrogels can be a problem for certain patients. ⁴¹ Figure 2 reveals data from our laboratory, which indicates that certain classes of lipids preferentially deposit onto silicone-hydrogel lens materials and that silicone-hydrogels deposit greater quantities of lipid than conventional ionic lens materials.

Figure 2 Degree of lipid deposition measured on Focus Night & Day (lotrafilcon), PureVision (balafilcon) and Acuvue (etafilcon). The lotrafilcon and balafilcon lenses were worn for 30 nights continuously and the etafilcon lens was worn for 6 nights without removal. The degree of lipid deposition was significantly different between all three lens types (p<0.001).

Lipid deposition onto conventional lens materials is highly patient dependent ^{33, 34, 48-50} and this fact is similarly observed with silicone-hydrogel lens materials. Figures 3 and 4 reveal various lipid deposition patterns seen in certain subjects using silicone-hydrogel lenses.

Figure 3 Lipid deposition in the form of lens calculi (jelly-bumps) on a silicone-hydrogel lens. Picture courtesy of Brian Tompkins.	Figure 4 Lipid deposition in the form of a heavy film on the lens surface of a silicone-hydrogel lens. Picture courtesy of Brian Tompkins.
Click to enlarge	Click to enlarge

If subjects are seen to be depositing their silicone-hydrogel lenses with lipid then moving to non vinyl pyrrolidone-containing materials (such as Proclear or Acuvue) should reduce this phenomenon. Further options include adding surfactant cleaners containing alcohol (such as Miraflow) or moving to more frequent periods of replacement. ³³

In summary, currently available, first-generation silicone-hydrogel lens materials have provided clinicians with materials that allow safe, oedema-free overnight wear for up to 30 continuous nights. They deposit only small amounts of protein from the tear film, but certain patients do have problems related to the deposition of lipids on these materials, due to the relatively hydrophobic nature of the lens surfaces. Subsequent iterations of siloxane-hydrogels should be designed with surfaces that are ideally more hydrophilic, in an attempt to optimise their biocompatibility with the tear film.