The Intraocular Lens

Although the existence of the intraocular crystalline lens, usually referred to as the lens, in the eye was recognized by the scientists of the Hellenistic period (about 2000 years ago), it was 400 years ago that the real role of the lens in vision was properly understood, and truly scientific approaches to the lens measurements and properties began to be applied only in the nineteenth century [1]. For instance, the first to weigh the human lens was Smith in 1883 [2]. 

The lens is positioned between the aqueous humor and vitreous body of the eye. The lens refracts the light which enters the eye through the pupil and focuses it on the retina. The lens (i) provides refractive power to the optical system of the eye (ii) provides the accommodation necessary for normal vision (iii) maintains its own transparency and (iv) absorbs UV radiation and blue light, both deleterious to the subsequent ocular segments. 

The lens is a biconvex body similar to a flattened globe. For descriptive piuposes, it has two poles (anterior and posterior), an equator, and therefore two diameters (polar, or lens thickness, and equatorial). 

The lens is composed of epithelial cells which become anuclear and elongated as they are displaced further toward the center. Because of the enormous length finally attained by these cells, they are referred to as lens fibers. The lens is surrounded by a transparent acellular capsule of variable thickness. A proper epitheUum underlies the capsule along the anterior side and equator, but not under the posterior capsule. The superficial layers of cells and fibers constitute the lens cortex, and the lens 

Handbook of Biomaterial Properties. Edited by J. Black and G. Hastings. Published in 1998 by Chapman Hall, London. ISBN 0 412 60330 6. 

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Another example of a biomaterial is the intraocular lens, which have been commonly used to treat cataracts. They were traditionally made of inflexible materials, but more recently consist of poly(methyl methacrylate) and soft flexible materials such as silicone and acrylic. The first person to successfully implant an intraocular lens was Sir Harold Riley at the St Thomas Hospital in London in 1949. The first lenses were made of glass, were heavy, and carried several risks including infection, inflammation, loosening of the lens, lens rotation, and night time halos (Thompson, 2007). These problems, now less frequent, still occur today in a small fraction of more than one million intraocular lenses that are implanted annually in the USA. 

One example for type 3 in Table 9 is the intraocular lens (IOLs), and the validation may be performed as follows according to Figure 2. 

A thorough examination should be performed to determine the cause of bullous keratopathy. The specific treatment plan depends on both the cause and severity. Examination of the endothelium, internal structures, and fundus can be enhanced by the use of topical hyperos-motics to decrease epithelial edema. Internal examination is essential to determine if there is corneal touch by the intraocular lens or vitreous face and to rule out cystoid macular edema or intraocular inflammation. 

Kruger A, Amon M, Abela-Formanek C, Schild G, Kolodjaschna J, Schauersberger J. Effect of heparin in the irrigation solution on postoperative inflammation and cellular reaction on the intraocular lens surface. J Cataract Refract Surg 2002 28 87-92. 

D.J. Apple, J. Sims, Harold Ridley and the invention of the intraocular lens, Surv. Ophthalmol. 40 (1996) 279—292. 

FDA approved intraocular injections include mio-tics, viscoelastics, and viscoadherents and an antiviral agent for intravitreal injection. The approved intraocular miotics, carbachol (Miostat ) and acetylcholine (Miochol ), are injected into the anterior chamber at the end of cataract surgery to constrict the pupil and allow the iris to cover the implanted intraocular lens. Carbachol is formulated in a BSS vehicle in sterile water for injection at a physiological pH... 

Tetracaine, an ester of para-aminobenzoic acid (PABA), has been widely used for topical anesthesia of the eye. It is currently available in a 0.5% solution. Its onset, intensity, and duration of anesthesia are comparable with those of proparacaine and benoxinate (Figure 6-2). Onset of anesthesia sufficient to permit tonometry or other minor procedures involving the superficial cornea and conjimc-tiva is 10 to 20 seconds, and duration of anesthesia is 10 to 20 minutes. It has been reported, however, that the 1% solution produces anesthesia lasting nearly an hour. Tetracaine 1% has also been used successfully to provide anesthesia during phacoemulsification cataract surgery and intraocular lens implantation. 

Ponte F, C.illino S, Faranda F, et al. Intraocular dapiprazole for the reversal of mydriasis after cataract extraction with intraocular lens implantation. J Cataract Refract Surg 1991 17 785-789. 

Patients who have had cataract extraction with implantation of an intraocular lens (lOL) often have pupils that dilate less well than they did preoperatively.The poorer pupillary response probably relates to the amount of iris trauma occurring at surgery. The difference in mydriatic response may affect evaluating and treating peripheral retinal abnormalities in aphakic and pseudophakic eyes. However, even with maximally dilated pupils often the capsulotomy is the limiting fector. 

Figure 20-11 Posterior chamber intraocular lens entrapped within the pupillary aperture after dilation. (Courtesy Hernan Benavides, O.D.)...

Three basic types of cataract extraction are available intracapsular cataract extraction, exttacapsular cataract extraction with nuclear expression, and extracapsular cataract extraction with phacoemulsification-aspiration. Intraocular lens (lOL) implantation typically is performed at the time of cataract extraction to help correct postoperative refractive error. Extracapsular cataract extraction with phacoemulsification-aspiration is the predominant surgery today, whereas the relative numbers of planned extracapsular cataract extraction, and particularly intracapsular cataract extraction, procedures have diminished. 

Assia El, Legler UFC, Merrill C, et al. CUnicopathologic smdy of the effect of radial tears and loop fixation on intraocular lens decentration. Ophthalmology 1993 100 153-158. 

The substitution of the crystalline lens for a transparent intraocular lens (lOL) has become an integral part of almost all cataract operations. Because the implantation of the lOL is done through a small incision, foldable lenses have been developed, some of which are PHEMA hydrogels (refractive index, 1.44 water content, 38%o) or HEM A copolymers with... 

Kohnen, T. The variety of foldable intraocular lens materials. J. Cataract Refract. Surg. 1996, 22, 1255-1257. 

Hyaluronic acid is considered to have low inflammatory and antigenic potential, and has been used in various intraocular procedures. In addition to filtration bleb formation, it has been used to protect the corneal endothelium during intraocular lens implantation and keratoplasty, to reform the anterior chamber, to push back a bulging vitreous face, and in retinal detachment surgery as a vitreous replacement. 

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