Intraocular lens implantation - Optometry Today

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Sponsored by Module 3 Part 11 to the iris. The third generation of IOLs, independently produced by Epstein and Binkhorst, used the pupillary part of the iris diaphragm for anatomical fixation. However, this led to luxation of the lens if the pupil dilated unexpectedly. These pupillary fixated IOLs are now obsolete. A second development was the use of a Medallion lens fixed to the iris by a suture, however late degradation of the suture led to dislocation of the lenses. The next development was the Iris-claw lens with a fixating mechanism based on the capture of a fold of iris tissue at the two ends of the IOL. Modern anterior chamber IOLs (1970 to present day) For an anterior chamber lens to be safe and effective there should be minimal contact with the drainage angle, stability within the anterior chamber with no movement in the angle, no iris chafing and no endothelial touch. Modern anterior chamber IOLs were designed to achieve this by using flexible open loop lenses made of PMMA. They have the advantage of not requiring an intact posterior capsule for implantation and can be implanted into eyes even after posterior capsule rupture (occurring during complicated cataract surgery or after intracapsular surgery). Modern anterior chamber IOLs allow far better fixation Table 2 Advantages and disadvantages of the IOL types www.optometry.co.uk than the early anterior chamber IOLs and corneal complications are rare. However, they are associated with a higher incidence of CMO and retinal detachment than posterior chamber IOLs. Modern posterior chamber IOLs (1975 to present day) The first posterior chamber IOLs were made of PMMA with either PMMA, polypropylene or polyamide haptics. They require the presence of a posterior capsule and can either be placed in the sulcus or the capsular bag. Capsular bag placement has been shown to be superior to sulcus placement in terms of centration and the rate of posterior capsular opacification (PCO). The advantage of posterior chamber IOLs is that they are placed in the position of the original crystalline lens leading to a more physiological situation with optical benefits. An additional advantage is that posterior chamber IOLs are situated away from the delicate structures of the anterior chamber including the cornea, the aqueous outflow channels, the iris and the ciliary body. This leads to a lower incidence of corneal problems, UGH and pupil block. When the IOL is placed within the capsular bag, contact with uveal tissues is completely avoided. The intact posterior capsule is associated with a decreased incidence of CMO and retinal detachment 5 . Type of IOL Advantages Disadvantages Ridley PC IOL optical uveitis secondary glaucoma hyphaema decentration/dislocation Early AC IOLs do not require posterior capsule - corneal complications: (rigid, closed loop) capsule usually removed therefore (decompensation, oedema, no PCO pseudophakic bullous keratopathy, IOL corneal touch) CMO, uveitis, UGH subluxation, dislocation Iris-supported IOL do not require posterior capsule iris complications: (iris chafing and erosion, pupil changes, pupillary block, PAS - Peripheral Anterior Synechiae) Modern AC IOLs do not require posterior capsule CMO retinal detachment (flexible, open loop) better fixation corneal complications rare Modern PC IOLs less corneal problems require intact zonules & posterior capsule less CMO less retinal detachment less UGH less pupil block optical Foldable IOLs small incision expensive less astigmatism decentration quicker rehabilitation rupture bag when unfolding safer spontaneous dislocation IOL material PMMA is the gold standard material for use in IOL manufacturing. It was the first material to be used, and has withstood the test of time as the majority of IOLs implanted worldwide today are still made of it. PMMA benefits from inducing a minimal intraocular inflammatory reaction, is not adversely affected by ultraviolet light and is not biodegradable in the eye. Because of this, it maintains a smooth surface even when in contact with vascular and metabolically active tissue. PMMA is also relatively inexpensive. However, one disadvantage is that it is rigid and so requires a larger incision for insertion than the newer foldable materials. Surface modifications of PMMA with heparin have been found to reduce the inflammatory cell precipitates found on the anterior IOL surface post-operatively, therefore, this lens modification is useful in patients expected to have more precipitation, such as uveitics. With the development of small incision phacoemulsification surgery, a whole new range of foldable lens implant materials have been developed. Early foldable IOLs, however, were not successful, because they elbowed and folded as the capsular bag contracted. The use of capsulorhexis dramatically reduces this decentration of foldable materials. The advantages of foldable IOLs are due to the smaller incision size required for their insertion. They are usually self-sealing and do not require suturing. Smaller incisions produce less astigmatism, allow quicker visual rehabilitation with stable refraction after a couple of weeks compared to six to eight weeks for larger wounds, and are safer with less iris prolapse and dehiscence. The disadvantage of foldable IOLs is that they are more expensive than PMMA and have a higher incidence of decentration if a continuous curvilinear capsulorhexis is not used. When a new IOL material is developed, its biocompatibility needs to be studied. IOL biocompatibility within the human eye has three major aspects. These are the effect on the blood aqueous barrier, the cellular reaction on the anterior surface of the lens, and the effect on the lens capsule. Blood aqueous barrier changes can be assessed by the amount of inflammation (flare and cells) within the anterior chamber, which can be quantified using the laser flare and cell meter. Cells on the anterior surface of the implant can be examined post-operatively using specular microscopy and have been used extensively as a method of assessing the foreign body response to the IOL. The effect of the IOL on the capsule consists of lens epithelial cell proliferation and metaplasia leading to anterior and posterior capsular opacification, and IOL decentration. PCO occurs in 20-50% of patients two years after cataract surgery (Figure 3). It leads to a progressive deterioration in visual acuity and 29
ot Figure 3 This IOL shows a dramatic amount of opacification of the posterior capsule. The patient’s vision was significantly reduced and they required a Nd: YAG laser posterior capsulotomy contrast sensitivity, and in many cases patients do not seek medical attention, fearing that this is an untreatable age-related change. PCO can be relatively easily treated with Nd (neodymium):YAG laser posterior capsulotomy, where the Nd:YAG laser is used to make a hole Table 3 Posterior chamber intraocular lens design features Design feature Number of pieces Loop material Loop shape Optic design Surface modification IOL material Variations • One piece (disc or plate haptic) • Three piece (disc) Figure 4 A diamond shaped posterior capsulotomy can be seen in the central posterior capsule of this patient with significant PCO in the posterior capsule to clear the visual axis (Figure 4). This procedure, however, can be associated with a number of complications such as pitting of the IOL, intraocular pressure spikes, inflammation, CMO and retinal detachment. Many IOL manufacturers are concentrating their research on ways to prevent PCO, and there are now certain IOLs which are associated with a lower incidence of PCO. • Polypropylene (flexible, possible higher incidence of endophthalmitis) • PMMA (rigid) • Polyacrylic • Angulation (forward angulation increases pressure on posterior capsule and may decrease PCO, and increases distance of optic from iris. Less angulation may facilitate insertion) • J or C loops • Convex plano, plano convex, biconvex (Biconvex associated with the least image degradation with decentration and tilt and less PCO) • Length of the IOL (14mm for sulcus fixation, 12-12.5mm for in-the-bag fixation) • Size of optic (4.5-7mm. Larger optics may be better for patients with large pupils but need bigger incisions. Smaller optics for bag fixation) • Holes (to facilitate dialling into capsular bag) • Ridges (designed to facilitate laser capsulotomy, but associated with more PCO) • Sharp or round optic edge (less PCO when optic has a square edge than round, due to barrier to lens epithelial cell migration, e.g. Alcon AcrySof IOL; Pharmacia & Upjohn CeeOn 911; Rayner Centerflex IOLs) • Binding of hydrogels or heparin to the IOL surface (to make IOL more hydrophilic, found to reduce inflammatory cell precipitate) • Surface passivation (to make IOL more hydrophobic and oleophilic, aiming to repel lipids such as cell membranes and ocular tissues) • PMMA • Silicone • Hydrogel/hydrophilic acrylic • Hydrophobic acrylic Silicone elastomers (elastic polymers) are composed of highly cross-linked polysiloxane chains. They are compressible and can therefore be inserted through a small incision by folding. Silicone lenses are homogeneous, heat-resistant, autoclavable, moldable and compressible and highly transparent to visible light. The specific gravity of the material is low and therefore the lens is nearly weightless in aqueous. Silicone has excellent tensile and tear strength, and is extraordinarily flexible. Silicone IOLs can be either three pieces, with open loop prolene, polyamide or PMMA haptics, or plate haptic. Silicone has a lower refractive index (1.41-1.46) than PMMA (1.49) and consequently the lens is thicker, which may be an impediment to very high powers. Although the silicone lens is transparent to Nd:YAG laser radiation, the lens can be pitted when hit by the laser beam 6 . In fact, silicone IOLs have been shown to have the lowest threshold for laser-induced damage compared to PMMA and acrylic IOLs, and to suffer the greatest damage depth at each energy level tested 7 . Furthermore, the surface of a silicone IOL is hydrophobic and it has been shown that a higher percentage of cellular reactions occur on hydrophobic surfaces than hydrophilic ones 8 . A further disadvantage is that they become slippery when wet and, as a result, become difficult to handle. An additional clinical complication occurs if silicone oil is used to help re-attach the retina following a retinal detachment in a pseudophakic patient, as the oil adheres to silicone IOLs. In addition, silicone IOLs with flexible haptics or plate haptic design have a higher incidence of decentration than other IOL types. Plate haptic silicone IOLs are also at increased risk of anterior capsule opening contraction 9 . Incidences of capsular bag rupture have also been reported during IOL unfolding, and spontaneous dislocation of IOLs into the vitreous after YAG capsulotomy, particularly with the plate haptic design. More recent silicone IOLs are made of second-generation silicone and appear to be associated with less inflammatory cell precipitate than the earlier first-generation lenses. A recent silicone IOL design, CeeOn 911 (Pharmacia & Upjohn) has been manufactured with a square optic edge, which has been shown to be associated with a low incidence of PCO compared to many other IOL types. Hydrophilic acrylics include a broad class of polymer materials which swell extensively in water but are not water-soluble (previously referred to as hydrogels). These materials have variable size and physical properties according to the state of hydration. An example is polyhydroxyethylmethacrylate (PHEMA), which has a 38% water content. High-water content hydrogels are cross-linked polymers based on a hydrophilic monomer. They are soft and resemble living 30 November 2, 2001 OT www.optometry.co.uk
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Intraocular lens implantation - Optometry Today

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