Single versus three piece acrylic IOLs.

S ince their clinical introduction 10 years ago, hydrophobic acrylic IOLs have gone on to become the most popular foldable IOL category worldwide. According to the annual Leaming surveys of the American Society of Cataract and Refractive Surgery membership, hydrophobic acrylic has been the most preferred IOL material since 1998, and was favoured by 63% of respondents in the 2002 poll. This sustained popularity undoubtedly results from the excellent track record enjoyed by the first hydrophobic acrylic model—the three piece Alcon AcrySof— in terms of safety, biocompatibility, and capsular opacification. While some initially thought that the universally observed reduction in posterior capsule opacification (PCO) with this lens was mainly because of the acrylic material, 8 subsequent studies have shown that it is the truncated edge of the IOL that is the primary factor. The sharp posterior edge is able to indent the posterior capsule, forming a mechanical barrier to posterior lens epithelial cell (LEC) migration. Nishi et al demonstrated in rabbit experiments that if the edge of the AcrySof IOL was rounded the PCO advantage was lost. Furthermore, they demonstrated that any IOL with a squared edge, regardless of material, was able to inhibit LEC migration in rabbits by forming this ‘‘capsular bend.’’ Abela Formanek et al and Auffarth et al subsequently confirmed this finding in prospective clinical trials. 13 Finally, Buehl and co-authors conducted a prospective trial of 53 patients who each received AMO Sensar hydrophobic acrylic IOLs with or without a square edge in alternate eyes. The eyes receiving the truncated edge model developed less PCO at 1 year. Since the patient variables and the IOLs were otherwise identical, the barrier effect of the IOL edge was clearly more important than the acrylic material in preventing PCO. The potential optical tradeoffs of a squared IOL edge have also been well documented. Ray tracing studies have shown that the symptoms of arcuate light flashes at night are the result of edge reflections from peripheral sources of light. In addition, the high refractive index of the material led to the original AcrySof IOLs having a flattened anterior curvature. This could result in both external and internal reflections and dysphotopsias. In 2000, Alcon introduced a significantly different design—the single piece AcrySof. Having moulded and floppier haptics permits this IOL to be more easily injected through a small incision. Reduced wound size is particularly important for clear corneal cataract incisions. While the square edge design was maintained, modifications to reduce the aforementioned dysphotopsias were incorporated. The single piece models featured a frosted edge and a steeper anterior curvature—changes that have since been made to the three piece AcrySof models as well. In the brief ensuing period, the single piece AcrySof has become the most popular foldable IOL in the United States, where it accounts for 80% of total AcrySof sales. In Europe, 50% of Alcon’s AcrySof sales are the one piece models.