[Extent of damage to different intraocular lenses by neodymium:YAG laser treatment--an experimental study]

Neodymium:YAG laser capsulotomy frequently damages the intraocular lens (IOL). This damage, mainly caused by acoustic shock wave and thermal conduction, results in opacities in the IOL, which may cause glare or image degradation. Because of the introduction of new IOL materials in cataract surgery, investigation of YAG laser resistance of these IOL materials seems to be necessary.

A total of 17 standard IOLs of different types and classes of materials were tested as follows: Class I: Acrylate a) PMMA, compression molding, b) PMMA, compression molding, heparin-surface modified, c) acrylate/methacrylate copolymer; Class II: Silicone: a) Polydimethylsiloxane, b) Polydimethyldiphenylsiloxane; Class III: Hydrogel: a) poly-hydroxyethylmethacrylate (poly-HEMA), b) polyacrylate-hydrogel; Class IV: Thermoset polymer: methylmethacrylate, hydroxyethylmethacrylate, ethylene glycol dimethacrylate. Each IOL was placed in a rectangular transparent acrylic test chamber filled with balanced salt solution and subjected to irradiation from a Q-switched Nd:YAG laser. The laser beam was focused on the posterior surface and inside of the IOLs. The optic of the IOL was divided into four target zones and each zone was subjected to 40 bursts inside the lens and 40 bursts on the posterior surface of the lens. Laser power settings were: 1.1 mJ, 1.1 mJ with 0.4 mm defocus, 2.1 mJ and 4 mJ and one burst was applied (wave length 1064 nm, fundamental mode, duration 7 +/- 2 nanoseconds, spot size 15 microns in air). Following exposure, each lens was examined by light microscopy for the interior damage and by scanning electron microscopy for the posterior surface damage. For quantitative analysis, the extent of each superficial damage was evaluated by an image analysis system using at least original magnification x 1400.

Each IOL material demonstrated specific morphologic damage patterns. Intralenticular damage: Class I: cracks with radiating fractures with smaller extent in group Ic; Class II: blistered snowball-like inclusions; Class III: localized small holes, exception: IIIb: with very short radiating fractures; Class IV: stellar pits with short radiating fractures. For silicone superficial posterior damage sites a splash crater pattern with irregular melted edges was observed, while acrylate damage sites demonstrated a melted or stellate crater pattern with slightly raised edges. The silicone, poly-HEMA and the acrylic IOLs containing HEMA presented highest YAG laser resistance with the smallest amount of posterior damage in comparison to PMMA-IOLs. There was no marked increase in damage size in these IOL materials with higher energy exposure in this set-up.

For each material consistent and characteristic specific morphologic damage patterns were observed. Foldable optic materials were more resistant against Nd:YAG-laser photodisruption than rigid optic materials. Individual laser strategies for each IOL-material and design should be deducted.