A doctor's perspective on what influences IOL lens design.
I would like consider the fundamental elements which are important to lens design. There are three main components that constitute an intraocular lens (IOL) - The Material, Optic and Haptic. I would like to discuss the factors which influence IOL design under these headings as well as introduce a new lens released by Croma at the ESCRS meeting in Barcelona.
Late onset internal crazing described as 'Snowflake degeneration' has been reported on several occasions with PMMA. Yellow discoloration has occurred with certain silicone lenses and vacuoles or glistenings occur frequently with Hydrophobic Acrylic lenses.
Although the glistenings do not appear to affect visual acuity in the majority of cases severe glistenings may affect contrast and there are reported cases of glistenings requiring IOL explantation.
Certain hydrophilic acrylic lenses have become opacified due to calcification and required explantation. The phenomenon has been reported to be a particular problem to certain manufacturers. In the one instance the problem was thought to be due to incompletely polymerized polymer blank or impurities, whilst the packaging was responsible for calcification with the Hydroview lens. David Apple has suggested that these specific instances can be considered as primary calcification due to polymer
problems as opposed to instances of secondary calcification due to an abnormal ocular environment. The latter can occur with hydrophilic acrylics or indeed any material. The earlier problems of primary calcification have been resolved and now millions of hydrophilic IOLs have now been implanted. Reported cases of calcification by manufacturers of hydrophilic acrylic IOLs are exceedingly rare and the material has several advantages as an IOL material.
Opacification of the posterior capsule varies with different lens designs and materials. The adhesive nature of different IOL materials is relevant although mechanical factors such as exerting pressure on the capsule and a square optic edge may be more important. A meta-analysis of randomized clinical trials indicated that acrysof and sharp-edged silicone IOLs are similarly effective in inhibiting PCO after cataract surgery.
Surgeons need to consider functional as well as structural factors when choosing the optic for their preferred IOL. Monofocal implants can be aspheric to improve contrast sensitivity or toric to reduce pre-existing contrast sensitivity. There is much discussion on the merits of neutral vs. negative asphericity but recent investigations suggest that simple contrast charts may lack sensitivity when comparing different clinical outcomes.
Multifocal implants are associated with contrast sensitivity, which may be unacceptable in some patients whilst accommodative implants tend to be unpredictable. The evidence suggests that there is minimal forward translation and psycho visual data confirming efficacy has been lacking.
Although most manufacturers have incorporated UV absorbers to protect the retina the need to reduce blue light radiation is more controversial. Epidemiological studies demonstrating progression of macula degeneration are inconsistent and there is speculation that depriving individuals of blue light may be detrimental.
Haptic design is a critical element in choosing an intraocular lens implant. Conventional J or C loop haptics distort the capsular bag and are not required with an intact capsulorhexis. A haptic that is sufficiently rigid to support the lens in the capsular bag is considered adequate. With single piece flexible IOLs, however, careful design is necessary to avoid distortion of the haptic and the ability to adapt to different dimensions of the capsular bag. Open loop haptics can be distorted by fibrosis of the capsule preventing fusion of the anterior and posterior capsule in the interval between the optic and haptic. Close looped soft haptics and plate haptics are rigid and resistant to fibrosis but are unable to adapt to the range of different capsular bag sizes encountered clinically. A strategy whereby the overall dimension of the IOL varies with axial length is used by several manufacturers to try and overcome this aspect of closed loop haptic behaviour.
An ideal haptic should be able to size automatically to different capsular bags without distortion whilst exerting posterior pressure on the posterior capsule with a broad arc of contact with the periphery of the capsular bag. A new haptic design for a single piece flexible lens, incorporating these features, has been developed by Croma as part of the new K1B IOL.