Characteristics of silicone hydrogel contact lenses
When it comes to silicone hydrogel contact lenses, the term "modulus" is often used. The Modulus refers to the way of applying stress to the degree of change in the shape of an elastic body. In other words, the Modulus expresses the ability of a material to resist deformation. The higher the modulus, the less the material will result deformed. The actual stiffness of the contact lens is determined not only by its form, but also by its geometry and by its thickness. Most part of the silicone contact lenses have a relatively high modulus when compared to the hydrogel materials based on HEMA. Since they do not cover the eyes as well as the conventional hydrogel contact lenses, the application needs to be more precise. The choice of a combination base curve/diameter that applies well in a hydrogel material may not behave the same way when a combination of a silicone material is chosen instead, because the corneal eccentricity, the outline of the limbus and the geometry of the sclera may have a better effect on the lens/cornea combination with a more rigid material. A thinner edge, which is rarely seen in a conventional hydrogel, is more characteristic of a silicone hydrogel material and the use of a more rigid base curve than the expected or of a more malleable material, with a lower modulus, is more often required to allow good applicability. The lenses CIBA Vision Night & Day and Bausch & Lomb's PureVision have a high modulus and are relatively rigid when compared to conventional hydrogel lenses. CIBA Vision O2OPTIX is softer and more flexible. ACUVUE OASYS and the forthcoming CooperVision Biofinity contact lenses are evensofter and the ACUVUE ADVANCE have the lowest modulus of all the current silicone contact lenses and are closer in its handling characteristics to the hydrogel HEMA.
The permeability to oxygen
A current hydrogel contact lens made of HEMA has a maximum of Dk value below 40; a current silicone hydrogel contact lens goes from a 69 Dk (Dk/t 86) of the Vistakon ACUVUE ADVANCE to a 140 Dk (Dk/t 175) of the CIBA Vision NIGHT & DAY.
Since the Dk/t of a lens depends on its central thickness, this fact varies among the different powers and it is always lower for the lenses with positive powers than those with negative powers.
The value of Dk/t given by each manufacturer is normally calculated on a lens with power to -3.00 diopters. The difference in thickness between the center of the lens and its peripheral zone does not reflect differences in the distribution of oxygen.
The passage of the oxygen through the material the lens is made of is also related to the "driving force", that is to say the partial difference in pressure through the lens. All the current silicone contact lenses, except ACUVUE ADVANCE, have a value of Dk/t of at least 100 and are approved by the US FDA for prolonged use whilst the lens ACUVUE ADVANCE, with a Dk/t value of 86, is approved for daily use.
The third characteristic of the silicone hydrogel material that must be considered is the innate hydrophobic nature of its surface, which can result as a significant attractive lipid, reducing the wettability and requiring the use of a surface treatment. Bausch & Lomb's PureVision contact lenses use a surface treatment with oxidized plasma: CIBA Vision O2OPTIX and NIGHT & DAY incorporate in the surface material a gaseous plasma that enhances both the wettability and the biocompatibility. Vistakon ACUVUE ADVANCE contains Hydraclear, an internal wetting agent, and the new ACUVUE OASYS material contains HYDRACLEAR PLUS, an increased amount of lubricating material to compensate for the higher silicone content.
The growth of the use of silicon material is extraordinary. The higher oxygen transmission typical of the silicone hydrogel material should significantly reduce the hypoxic complications, including chronic conjunctival infections, corneal neovascularization, microcysts and the changes of endothelial cells as the polimegetism and pleomorphism. Although the parameters are more limited to correct astigmatism and presbyopia, customers are encouraging the use of the silicone hydrogel lenses when it is possible to maintain or increase metabolism and corneal physiology, especially in younger patients or patients with signs and symptoms of hypoxic stress and diabetics, patients under immunosuppressive therapies, patients with corneal dystrophies, patients using the lens rarely or occasional patients, patients who wear the lenses for a long period or sleep for short periods with the lenses on and, in particular, patients who wish to wear their lenses for longer periods.
Silicone hydrogel material complications
The fact that the advantages of silicone hydrogel material are much more than its disadvantages, it does not mean that the material is free of complications. The high modulus of the material can cause upper epithelial arc lesions (SEAL) which appears as a split in the epithelium. These signs can be hidden by the eyelid because they are normally aproximately 1-3 mm below from the upper limb.
The silicone hydrogel materials may also cause mucinic deposits, which appear as small spherical translucent entities of various sizes that determine depressions on the ocular surface and disappear with the lens removal.
They are formed by the gradual movement of the lens along the tear film, which creates a cutting effect and cause small balls of tear film made of mucin material, proteins and lipids that are rolled by the movement of the eyelid and coated on the corneal surface.
Most wearers of silicone-based hydrogel contact lenses have never suffered from these complications and report great experience with an excellent visual acuity, excellent comfort and the benefits of high oxygenation of the eye. Patients who have negative experiences and adverse reactions with high modulus materials may continue to use the silicone hydrogel materials by choosing products with lower modulus.
The article "Silicone hydrogels: what, why and how" by Phyllis L. Rakow, Director of Contact Lens Services for Princeton Eye Group in New Jersey, was published in 2015 on Eyewitness Fourth Quarter magazine and can be downloaded from the English section of our site by clicking here -> english text
The excerpts were translated into Italian by non medical personnel, specialized in translation and may be subject to different interpretations based on medical knowledge. For a correct information it is recommended to refer exclusively to the English text.
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