Intraocular lenses (lOLs) are used after cataract extraction to replace the opaque crystalline lens of the eye (Apple et al., 1984). I01.S consist of an optical portion and haptics that support the optical portion in its proper place in the eye (Fig. 5). lOLs may be placed in the anterior chamber, in the pupil, and in the posterior chamber. The last type are most commonly used at this rime; they are usually placed within the posterior capsule of the crystalline lens, which remains in the eye after the lens contents have been removed surgically (Fig. 5). A large variety of 101. designs and shapes are available; the choice does not necessarily depend on need but rather on the preferences of surgeons and manufacturers.
I he requirements of IOL materials are good optical properties and biocompatibilfty with the surrounding tissues. Although oxygen or metabolite permeability is irrelevant for IOIjs, one may have to be concerned with the potential absorption in the IOL material of aqueous humor proteins or a topical or systemic drug given to a patient wearing an IOL, particularly if the IOL material is a hydrogel or silicone rubber.
Most lOLs are made ot poly(methyl methacrylate), and the hapucs are often made of the same material or polypropylene fiber (Apple et ai., 1984;. Filtration of UV light by UV-absorb-ing moieties polymerized into the IOL is desirable to protect the retina (Miller, Ï987).
Corneal astigmatism may result from tissue distortions occurring as a consequence of the uneven healing of the wound made when the 101, was implanted. There ss currently a strong interest in developing soft 101-s, which can be inserted in the eye through smaller surgical incisions that are required for implanting rigid tenses. A smaller incision may result in a lower incidence of astigmatism. Soft lOLs have been made of HEMA or other hydrogcls, which can be inserted into the eye fully hydrated or m the dehydrated state; in the latter case they will swell msttu to their equilibrium hydration (Barrett et al., 1986). Flexible lOLs are made also of silicone rubber and of alkyl a cry la te copolymers
Biopolymers in the form of a viscoelastic solution are also used in IOL, implantation. The corneal endothelium is an extremely delicate cell layer and can be irreversibly damaged upon contact with an IOL, during or after insertion. The surgeon must be extremely careful not to touch the corneal endothelium with the IOL or with any instrument used during surgery. Highly viscous, and preferably viscoelastic, solutions of biopolymers such as sodium hyaluronate, chondroitin sulfate, or hydroxypropyl methylcelluose are useful adjuncts in IOL. implant surgery for maintaining anterior chamber depth
HO. 5. (Top) Schematic representation oi a typical intraocular tens implant with a central optical portion and the hapnes or side-arms thai hold the lens in tile eye, (Bottom! A schematic representation of the anterior segment of the eye with an intraocular lens placed into the empty crystalline lens hag.
during introduction of the implant and for preserving the corneal endothelium (Fernandez-Vigo et al., 1989). Other important developments may be the surface modification of lOLs with permanent hydrophilic or hydrophobic coatings.
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