Posterior capsule opacification (PCO) is a multifactorial problem. Factors implicated in the pathogenesis of the condition include: patient status, such as age, diabetes, intraocular inflammation; surgical technique, such as capsulorrhexis size; and intraocular lens (IOL) material and design. Many clinical studies have shown that IOLs with a square-edged optic profile are associated with a lower incidence of PCO than those with a round-edged profile: it is now clear that the edge profile of an IOL is of paramount importance for PCO prevention.

Figure 1

Although the reason for this is much debated, there is good evidence, both clinical and experimental, to support the 'capsule compression theory'. This theory suggests that as the capsular bag fibroses and contracts after surgery, the IOL edge is pushed, firmly but gradually, against the posterior capsule, which exerts an increased pressure profile at the point on the posterior capsule where the posterior IOL edge and the posterior capsule are in contact, thereby creating a physical pressure barrier to lens epithelial cell (LEC) migration. By contrast, a round-edged IOL optic produces a continuous bend that is more curved and non-sharp, generating a lower pressure at this point.

Figure 2

Most manufacturers now produce square-edged IOLs, although the incidence of PCO varies between models. We carried out a prospective randomized trial of two IOLs marketed as having square edges. We implanted an Alcon AcrySof SN60AT IOL in one eye and a Hoya AF-1 (UY) IOL in the fellow eye, and monitored PCO levels across two years.¹ To our surprise, the Hoya IOL showed significantly more PCO (Figure 1), which prompted us to examine the edge profile. We found that there was a clear difference in edge sharpness between the two IOLs (Figure 2).

Seventeen IOLs go head-to-head

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

Figure 3

The posterior optic edge was focused sharply and the resultant image, which included a 200 mm scale marker at x500 magnification, was digitized. The edge profile was displayed on a computer screen and the outline traced using a computer mouse with purpose-designed software; this produced a line tracing of the edge profile in which each point on the line was represented digitally by a pair of pixel coordinates. The optic edge profile can be conceived of as a line of varying curvature, which can be represented mathematically by multiple points of varying curvature. In this way, the sharpness of the edge profile can be quantified by measuring the local radius of curvature at the point on the posterior edge with the smallest radius of curvature (Figure 3).

Interesting findings

Figure 4

The sharpest edge profiles were found on silicone and hydrophobic IOLs, but this masks some interesting results (Figure 4). All hydrophobic acrylics had an edge profile of less than 10 μm, with the exception of the Hoya AF-1 (UY) IOL, which had an edge profile of 19.9 μm; lenses made by HumanOptic were the only hydrophilic IOLs with edge profiles sharper than on those lenses made of hydrophobic or silicone materials.

Table 1: Radii-of-curvature of the posterior square-edges and edge thickness.

The mean radii of curvature in the silicone, hydrophobic acrylic and hydrophilic acrylic groups were 8.5±0.6 μm (range: 7.6–9.2 μm), 11.2±4.9 μm (range: 8.3–19.9 μm) and 16.0±3.4 μm (range: 10.6–19.6 μm), respectively (p<0.05). At 7.6 μm, the Bausch & Lomb SofPort AO IOL had the sharpest posterior optic edge profile; at 23.1 μm, the Lenstec Tetraflex IOL had the roundest posterior edge profile (Table 1).

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.