The squared, sharp-edged optic intraocular lens design (2001)

In a recent survey of practice styles and preferences of members of the American Society of Cataract and Refractive Surgery,1 respondents indicate that at least 76% of all implanted intraocular lenses (IOLs) are made of foldable materials. Forty-nine percent are acrylic; in the United States, mainly the hydrophobic acrylic AcrySof (Alcon) models. One major factor in the increased popularity of this IOL design is the low rate of posterior capsule opacification (PCO).2,3 This and other factors associated with IOLs with a square, sharp-edged optic design are investigated in this issue. Posterior Capsule Opacification A sharp capsular bend obviously inhibits lens epithelial cell (LEC) proliferation onto the posterior capsule. Nishi and coauthors4 have demonstrated that if a discontinuous capsular bend is created by an IOL with sharp edges, the bend may induce contact inhibition of migrating LECs regardless of the material composition of the lens. Experimental studies have demonstrated this with the AcrySof IOL and also with poly(methyl methacrylate) (PMMA) and silicone IOLs with rectangular sharp edges.4–7 The low PCO rate of silicone IOLs with a sharp edge design has been reported,8,9 although a comparative study with other IOLs has not been published. An additional reason for the low PCO rate with the AcrySof IOL has been discussed: adhesion of the hydrophobic acrylic foldable material to the capsule.10,11 In an article in this issue, Nishi and coauthors compare the AcrySof IOL with different degrees of round edges with the AcrySof IOL with sharp edges. They found that the preventive effect on PCO was lost when the optic edges were rounded. The material component of the AcrySof IOL may also affect PCO prevention, but the main factor appears to be the formation of a discontinuous capsular bend to prevent LEC migration. Edge Glare The edge design of the IOL optic can affect its optical and mechanical performance after implantation.12 The reported optical effects are glare, halos, peripheral arcs of light, and other unwanted optical images.9,13–16 Rounded IOL edges distribute reflected glare image over a greater area than do sharp edges.17 Rounded edges or squared nonreflective edges reduce the potential for edge glare phenomena that appear to the patient as a thin crescent or partial ring.17,18 Reflections Reflections of an IOL can be divided into external and internal light phenomena. The externally reflected light from an IOL may be visible to an outside observer (eg, in patients with an AcrySof IOL); the internal reflections are experienced as glare by the pseudophakic patient. In this issue, Erie and coauthors examine the reflected light rays of IOLs made of different materials—silicone (refractive index [RI] = 1.43), PMMA (RI = 1.49), and acrylic (RI = 1.55)—and different optic designs (equi-biconvex and unequal biconvex). They found that IOLs with unequal biconvex optics (less curvature on the anterior surface of the IOL) and made of high refractive material increase the potential for postoperative internal and external light reflections. All these findings point to an obvious problem: Intraocular lenses with sharp edges made of hydrophobic acrylic decrease the rate of PCO significantly, but this IOL design (AcrySof with sharp edges and a flatter anterior optic surface, the optic design that has been used recently) and IOL material (high RI foldable acrylic) increase glare and external and internal reflections. Two additional factors should be considered when evaluating the optic edge glare problem: scotopic pupil size and anterior capsule opacification (ACO). A large scotopic pupil might increase unwanted optical images at night or at low illumination conditions. In an eye inwhich the capsulorhexis is smaller than the IOL optic diameter, ACO might decrease the glare phenomena because the IOL optic edge is covered with fibrosed anterior lens capsule. The strongest reaction of the anterior capsule, so-called anterior capsule whitening, is usually seen with silicone IOLs. Further investigations of silicone IOLs with sharp edges and an increased anterior optic curvature in an equi-biconvex IOL design will help us understand their effect on PCO, edge glare, and reflections. Cataract surgery with foldable IOL implantation has become one of the safest and most successful surgical procedures worldwide. Fast rehabilitation and long-term patient satisfaction are the advantages of this procedure. However, as shown in the articles in this issue, IOL designs with major advantages might also have disadvantages. Investigating these issues is necessary to further improve cataract surgery and IOL implantation.