The performance of IOL materials is determined by several factors, including their ability to inhibit posterior capsular opacification (PCO), their biocompatibility, their haptic memory, the occurrence of glistenings or calcifications in the material, and the visual quality they provide to implanted patients. In this overview, we discuss the characteristics of IOL materials, how lens material affects outcome variables, and the suitability of IOL materials for microincision cataract surgery (MICS).
TYPES OF MATERIALS
PMMA. PMMA IOLs with sharp optic edges result in relatively low PCO rates.1,2 Heparin–surface-modified PMMA IOLs have been used in uveitis patients with good results. Due to their low price, PMMA IOLs are a viable option for cataract surgery in developing countries where manual expression techniques (ie, extracapsular cataract extraction) with large incision sizes are used. Currently, PMMA IOLs are used for sulcus-placement and sulcus-suture techniques due to their high overall rigidity, resulting in good centration and resistance to tilt. Anterior chamber and iris-fixated IOLs are also made of PMMA and are not associated with uveal inflammatory reaction.
Foldable hydrophobic acrylate is currently the most commonly used material group.3 These polymers of acrylate are foldable at room temperature and have very low water content, high refractive indices, and usually a strong plastic memory. These characteristics also make the material suitable for the haptics of one-piece, open-loop IOLs. Hydrophobic acrylate unfolds in a controlled fashion and has been shown to have good uveal and excellent capsular biocompatibility.
One drawback of this material group has been intralenticular changes. Small water inclusions in the optic material, called glistenings, can occur in hydrophobic materials; this has predominantly been seen with the AcrySof (Alcon Laboratories, Inc., Fort Worth, Texas) material. Over time, the glistenings can increase, but evidence does not indicate any relevant effects on visual function. Another drawback to this material group has been dysphotopsias. The most common positive dysphotopsia reported was edge glare, which is due to internal reflections at the rectangular edge of the AcrySof IOL under mesopic conditions with a large pupil. Edge glare is typically induced by a light source from the side and reported by patients as a peripheral arc of light.4 As a result of changes in optic geometry, these dysphotopsias have been reduced significantly with newer hydrophobic acrylic models. A small proportion of patients reported negative dysphotopsias; these are perceived as a scotoma in the temporal peripheral visual field and are reported more frequently with materials of high refractive index.
Hydrophilic acrylate. Sometimes referred to as hydrogel, hydrophilic acrylate is a heterogeneous material group of materials with high water content. These lenses are cut in a dehydrated state and then hydrated and stored in solution. The water content varies widely between IOLs and can be as high as 38%. A recent metaanalysis of PCO with these materials showed that hydrophilic acrylic lenses are slightly more prone to PCO than hydrophobic acrylic or silicone lenses.5 This may be due to the high water content being more conducive to lens epithelial cell (LEC) ingrowth. Another reason may be that the optic edges of IOLs in this group are never as sharp as with hydrophobic materials.6 As a result, the bend of the capsule at the optic edge is less sharp and a less effective barrier to regenerating LECs.
One major problem with previous hydrophilic acrylic lens designs, primarily the Hydroview IOL (Bausch + Lomb, Rochester, New York) and the Aquasense IOL (Ophthalmic Innovations International Inc.; now Aaren Scientific, Ontario, Canada), was opacification of the optic material due to calcification.7,8 Patients with these opacified lenses required subsequent explantation due to poor optical quality. It must be said, however, that the majority of hydrophilic lenses have never shown such problems.
Silicone. Silicone, a hydrophobic material, has been used since the first foldable IOL models were introduced. In the past decade, there has been a continual decline in the use of silicone IOLs. Silicone is a good IOL material, especially with regard to its PCO-blocking effect,9 but it cannot be used for a one-piece, open-loop lens. This lens design is the preferred choice for use with preloaded injectors that allow implantation through incisions smaller than 2.2 mm. When an injector is used to implant multipiece silicone-optic IOLs through a small incision, the surgeon faces the risk of tearing the optic at the optic-haptic junction or kinking the haptics during injection. Silicone IOLs with plate-haptic designs typically show higher rates of PCO due to insufficient capsular bending in the haptic region.
PCO. One of the prime outcome variables used to determine the long-term success of IOLs is PCO. In a Cochrane review of randomized controlled trials,5 a significantly higher PCO rate and a higher rate of Nd:YAG capsulotomy were found with hydrophilic acrylic IOLs compared with hydrophobic optic materials, including hydrophobic acrylic and silicone. There is no clear evidence for a difference between the hydrophobic materials, although in several studies silicone IOLs had slightly lower PCO rates than hydrophobic acrylics. There is an ongoing debate among cataract surgeons about which of the hydrophobic materials—hydrophobic acrylic or silicone— should be preferred with respect to PCO inhibition. The role of IOL optic material in PCO development remains unclear with the exception of hydrogel lenses, which have been shown to produce worse visual acuity results. This may partly be caused by their less sharp optic edge designs due to the manufacturing processes used.
Biocompatibility. The cellular reaction seen on an IOL is an important indicator of its biocompatibility, which can be divided into two factors, uveal and capsular reaction. Hydrophilic acrylic materials show good uveal biocompatibility with less flare and fewer cells on the IOL optic surface, but poorer capsular biocompatibility than hydrophobic materials. The poorer capsular biocompatibility manifests clinically as stronger tendency for LEC growth onto the IOL optic surface and especially as higher rates of PCO. Hydrophobic acrylic lenses show a higher incidence of giant cell reaction on the surface of the IOL. Despite good capsular biocompatibility, the uveal biocompatibility of hydrophobic acrylic lenses seems worse than that of silicone IOLs. Modern silicone lenses with sharp-edged optics have demonstrated excellent uveal and capsular biocompatibility.
Light filtering. All IOL materials today include ultraviolet (UV) light-blocking chromophores to filter UV light. Animal and in vitro experiments demonstrated blue light to be harmful to the retina due to short-wavelength, high-energy light causing damage by inducing oxidative stress at the retinal level. Although this has not been proved in humans, some manufacturers have introduced yellow-tinted IOLs to filter short-wavelength light. A yellow lens has two potential drawbacks: reduction in color contrast sensitivity, especially under mesopic conditions, and alteration of the melatonin production in the brain, causing a change in the circadian rhythms that are guided by blue light levels in the eye. To date, no study has demonstrated that yellow lenses cause significant loss in color contrast sensitivity; however, this may be due to a lack of sensitivity in the psychophysical tests used. Two patients at our center with a yellow lens in one eye and a standard, fully transparent nonyellow lens in the other could clearly differentiate the two. Both patients described the vision of the eye implanted with the yellow lens as a little dirtier than the other.
A recent clinical trial10 showed that blue-light–filtering IOLs negatively affect contrast sensitivity and blueyellow foveal threshold when compared with UVonly— filtering IOLs. Although the differences were small, the results suggested that in patients with high demands in color vision surgeons should bilaterally implant the same IOL type and avoid mixed implantation of a blue-light-filtering IOL in one eye and a UVonly— filtering IOL in the other.
The trend toward MICS has generated a need for IOLs that can be implanted through an injector with a cartridge tip of 2.2 mm or smaller. Most current IOLs that can pass through such small cartridges are made of hydrophilic acrylic materials. Because three-piece IOL designs with thin haptics made of filaments run the risk of undergoing deformation during implantation through such small openings, essentially all microincision IOLs are one-piece. However, due to the lack of rigidity of open-loop haptic designs made with hydrophilic IOL materials, most current microincision designs are plate-haptic style. These appear to have significantly higher PCO rates compared with open-loop designs due to large areas of insufficiently sharp capsule bending at the plate haptics. It will be interesting to see whether hydrophobic materials can also be passed through such small openings without risking optic damage.
Oliver Findl, MD, MBA, is Director of Ophthalmology at the Hanusch Hospital, Vienna, Austria, and a Consultant Ophthalmic Surgeon at Moorfields Eye Hospital, London. He is also the Head of the Vienna Institute of Research in Ocular Surgery (VIROS), Vienna, Austria. Dr. Findl states that he has no financial interests in the products or companies mentioned. He may be reached at e-mail: firstname.lastname@example.org.
Sabine Wanderer, MD, is a resident in ophthalmology at the Hanusch Hospital, Vienna, Austria. Dr. Wanderer states that she has no financial interests in the products or companies mentioned. She may be reached at e-mail: email@example.com.