The squarest edge

Key Points

There is little evidence, and no conclusive proof, of how 'sharp' the optic edge must be to prevent LEC migration effectively. To investigate further, we designed a study to evaluate the edge profile of several commercially available IOLs using environmental scanning electron microscopy (ESEM). In contrast to ordinary scanning EM, this technique does not require the specimen to be dehydrated, which, of course, would distort a hydrophilic IOL material. Seventeen IOLs of different materials and designs - but all marketed as 'square edged' - were selected from prominent European manufacturers. To maintain consistency, each of the 17 IOLs used in the study had a refractive power of 20.0 D. The IOLs were scanned using a Quanta 200F field emission gun ESEM (FEI Co.). The technique is sophisticated and requires meticulous care, and each IOL was processed individually.

In brief, the IOLs were mounted on a platform with a central groove by an experienced electron microscopist using a simple microscope. A pill of black carbon wax was plastered in the centre of the groove on the platform; the IOL was then slotted in the groove, so that one end of the IOL optic stood vertically embedded in the wax. Due to their unique design, some IOLs (such as the Bausch & Lomb Akreos series, the HumanOptics 1CU, and the Lenstec Tetraflex) required the cutting of a haptic for stable mounting to obtain the best scans. Utmost care was taken to identify the anterior and posterior optic edges before the IOLs were scanned.²

Interesting findings

Clinical reports suggest that square edge hydrophilic IOLs are more likely to develop visually significant PCO over time, which we believe this study suggests may be a consequence of the manufacturing process. Hydrophilic IOLs are lathe-cut from dehydrated blocks, possibly with some tumble polishing to remove burr, and then rehydrated. As the IOL swells, the sharpness of the edge profile is lost: this demonstrates the inter-relationship and restrictions that IOL material places on engineering, and may account for the rounder edge profile of the Rayner C-flex, Rayner Superflex, Bausch & Lomb Akreos, Bausch & Lomb Akreos AO MI60, and Lenstec Tetraflex IOLs. By contrast, the two HumanOptics hydrophilic IOLs had a good square edge, indicating that it is possible to manufacture hydrophilic IOLs with high quality square edges.

The study also gives us an idea of how sharp the IOL edge must be to prevent PCO. The Hoya AF-1 (UY) IOL, with a radius of 19.9 μm, has comparatively poor PCO performance, whereas the lenses with a radius of curvature of less than 10.0 μm appear to have good PCO performance. The link between lower PCO incidence and smaller edge profile has been corroborated by other studies, indicating that the minimum edge profile (radius of curvature) of the posterior optic edge should be in the region of 10.0 μm. We predict that IOLs with a greater radius of curvature will also have a comparatively poorer PCO performance.

Above all, it is essential to remember that not all square edges are created equal; as always with cataract surgery, attention to detail pays off.

References

1. J. Hancox, et al. J. Cataract Refract. Surg. 2008;34(9):1489–1494.

2. M.A. Nanavaty, et al. J. Cataract Refract. Surg. 2008;34(4):677–686.