The Eyecryl ASHFY600 IOL showed high resistance to glistening formation using an established laboratory accelerated aging model. Furthermore, compared to the well-established AcrySof SN60WF, the ASHFY600 had a lower mean glistening grade. In general, glistening numbers were higher in the central part of the lens compared to the periphery in the AcrySof IOLs, corresponding to the lens thickness, which is highest in centre of the IOL optic. Due to the overall low number of glistenings in the ASHFY600 IOLs, mean values for the central section and the periphery did not differ significantly (0.7 and 0.5, respectively).
In general, hydrophobic acrylate has some advantages over other IOL materials. Lenses made of hydrophobic acrylate show a lower tendency to develop posterior capsule opacification in comparison to those made of polymethyl methacrylate (PMMA) or hydrophilic acrylate. Complications associated with hydrophilic acrylate lenses like IOL calcification have not been described in hydrophobic IOL material. Hydrophobic acrylate IOLs can be cost-effectively produced and offer good handling during small incision cataract surgery.
Despite these benefits, hydrophobic acrylic IOLs are prone to develop glistenings. This long-term change in the material can worsen the lens’ optical performance.[8, 9] In recent research, our group has examined the nature of this deterioration in vision that is attributable to glistenings. Our colleagues, Weindler et al. demonstrated that a large number of glistenings is needed to affect the central image quality. They induced varying amounts of glistening in monofocal AcrySof IOLs and evaluated glistenings’ impact on the image quality by measuring the lenses' modulation transfer function (MTF) and Strehl ratios. The MTF value was reduced from 0.580 in clear control lenses to 0.533 in lenses with over 500 MV/mm2 at a special frequency of 100 lp/mm and a 3-mm-aperture. Thus, glistenings have a rather small effect on the central image quality but their main effect is in changing another optical performance parameter, as a recent study by our group has shown. Labuz et al. found that straylight increases proportionally to the number of microvacuoles per square millimetre. Glistenings were induced in six different hydrophobic IOL models. IOLs with a mean central number of 3532 MV/mm2 showed elevated straylight levels of 19.3 deg2/sr, which would result in difficulties for patients while driving.[9, 13] Fortunately, in the presented study, mean glistening numbers were lower in both of the IOL models under test, suggesting improvements in these hydrophobic materials.
In 2013, Thomes and Callaghan reported on the continuous improvements (for which they unfortunately do not provide details) in manufacturing process of the Acrysof copolymer intended to reduce the incidence of glistening formation. They compared AcrySof lenses manufactured in 2003 with those made in 2012. The 2012 manufactured AcrySof demonstrated a significant reduction in glistening number (39.9 ±35.0 MV/mm2) compared to lenses produced in 2003 (315.7 ±149.4 MV/mm2). Our results showed similar values for Acrysof produced in 2017, with a mean number of central glistenings of 41.84 (±27.67) MVs/mm² suggesting a maintenance of the improved process that leads to the reduced glistening formation.
The Eyecryl ASHFY600 IOL is made from a hydrophobic acrylate polymer (Table 1).
The Eyecryl lens is manufactured by lathe-cutting the polymer which is different to the way Acrysof IOL is made, which is cast-moulding manufactured. Possibly the Eyecryl lens retains a more homogenous copolymer distribution within the final IOL whereas the cast-moulding procedure of the Acrysof lens might be rearranging the polymer distribution. In cast-moulding, care must be taken to avoid the development of inhomogeneities that can re-distribute co-polymers, chances which would make the lens susceptible to further material changes such as microvacuole formation. In a comparative clinical study, Nishihara et al found that lathe-cut lenses show better long-term stability (regarding surface light-scattering) compared to cast-moulded lenses.
After shaping the lens by lathe-cutting or cast-moulding, a subsequent step in manufacturing usually includes a polishing process. This stage has been shown to be the potential cause of postoperative material changes in hydrophilic acrylic lenses from a series of lenses affected by opacification, the residual polishing materials, like Aluminium Oxide, might have remained on the lens surface and provoking the postoperative clouding of the lenses.
Thus, the IOL production process as well as the polymer are crucial elements in providing a lens with a resistance to material changes. Our results suggest that lathe-cutting a lens is superior to cast-moulding and we consider the new technologies, such as laser-cutting the lens, might further improve IOL manufacturing.
Another approach to reduce the tendency for glistening formation is to improve the polymer by introducing hydrophobic IOL polymer compositions that have increased hygroscopy. Hygroscopy describes a material’s ability to absorb and hold water inside the material. Water entering the material connects with the hydrophilic groups, thus avoiding water accumulation in vacuoles or pockets and forming glistenings. The more hygroscopic a material is, the higher its equilibrium water content (EWC) under certain environmental conditions. Apart from the composition of the material, the EWC depends on the concentration of salts in its surrounding solution and the environmental temperature. Early hydrophobic materials for IOLs had low hygroscopy: the AcrySof material introduced in the 1990s has an EWC as low as 0.1–0.5%. Some of the new generation hydrophobic materials incorporate a certain amount of acrylate with hydrophilic groups, thus leading to equilibrium water contents around 4% to 5 %. Only a few companies disclose the exact copolymer composition used for their IOLs. One known composition is that of the enVista IOL made by Bausch & Lomb (New York, USA). Its copolymer consists of 3 different monomers: poly(ethylene glycol) phenyl ether acrylate (40%), 2-hydroxyethyl methacrylate (HEMA, 30%) and styrene (26%), cross-linked by ethylene glycol dimethacrylate (4%) - collectively called PHS copolymer. Due to the hydrophilic groups of the HEMA the material has a higher EWC of about 4% and shows a low tendency towards formation of glistenings. Another new generation hydrophobic polymer formulation by PhysIOL (Liège - Belgium) also contains an (undisclosed) amount of a hydrophilic monomer to provide an equilibrium water content of 4.9%, again this offers a low tendency for glistening formation.
As described above, even though improvements in the Acrysof material between 2003 and 2012 led to an increasing resistance to glistening formation, one can still induce glistenings in these lenses. Glistenings - even a low number of them is considered undesirable, and the Alcon company recently introduced a new material, named Clareon, that is considered to show minimal tendency towards glistening formation. The company does not disclose its exact material composition; Clareon’s EWC is around 1.5%. Several other IOL manufactures that have now addressed the problem of glistenings in their hydrophobic acrylic intraocular IOL materials: Vivinex (Hoya, Singapore), Tecnis (Johnson&Johnson, New Jersey, USA) and RayOne (Rayner, Hove, UK). In our laboratory, in-vitro accelerated aging studies have confirmed that lenses made of these materials and the Eyecryl ASHFY600 IOL are “glistening-free”. As this study was conducted in an in-vitro environment, results cannot be transitioned to the clinic without restriction. Therefore, long-term clinical studies have to confirm the lower amount of glistenings in IOLs made of advanced hydrophobic materials.