Hydrophilic vs hydrophobic: which is the better option?
"Hydrophilic IOLs tend to be easier for the surgeon to implant, and are therefore, particularly in the case of very small incisions, probably a more suitable surgical option than hydrophobic IOLs," said Christophe Chassain, MD, an anterior segment surgeon at the Clinique Beau-Soleil, Montpellier, France. "Postoperatively, though, hydrophilic lenses have a worse reputation for prevention of posterior capsule opacification (PCO), which traditionally has been a prime metric used to determine the success of IOL implantation."
PCO: investigating hydrophilic IOLs
To investigate the truth of this assumption, Dr Chassain conducted a two-year retrospective study to establish the rate of posterior capsule YAG procedures following implantation of the SlimFlex IOL (PhysIOL).
The SlimFlex is a 26% hydrophilic monobloc IOL with a 6 mm optic, and a 360° posterior square edge. It has an overall diameter of 10.5 mm, with four haptics at a 5° angle. The lens can be injected through a 2.4 mm incision. MicroSLIM — the microincision version of the lens, which can be injected through a 1.8 mm incision — is made from 25% hydrophilic material.
Dr Chassain implanted 344 SlimFlex lenses into 191 female and 153 male subjects with a mean age of 77 years (range: 46–94 years) and a mean dioptric power of +20.5 D (range: 9–28 D). Topical anaesthesia was used and, during the coaxial surgery, the posterior capsule was cleaned; FlexiRing intracapsular rings (IOLTECH Laboratories) were implanted into 34 eyes. A rhexis of 5.5 mm was achieved in 66% of cases; the rhexis IN position was 76% with an optic overlap.
Similar PCO rates with both lens types
"Of the 344 subjects implanted with the SlimFlex lens, 3.2% required further treatment with Nd:YAG posterior capsulotomy to combat PCO. Of these, four YAGs were performed in the 180 eyes with an IN rhexis; six in the 57 eyes with an IN/OUT rhexis; one in the four eyes with posterior fibrosis, and two in the 34 eyes implanted with a FlexiRing," reported Dr Chassain.
The mean follow-up period was 20.6 months (range: 11–28 months). At this stage, 68.6% of subjects had achieved a clear central posterior capsule.
"As square edges have been shown to reduce the rate of PCO, the low observed incidence of this complication with the SlimFlex IOL is potentially related to the true 360° posterior edge of the optic, which did not exist with earlier hydrophilic IOLs," Dr Chassain conjectured. "The 'surgeon factor', however, is also important; this is demonstrated by the vastly differing rates of PCO development between IN rhexis and IN/OUT rhexis eyes: 2.22% and 10.52%, respectively."
Determining success: looking beyond PCO
"Whilst PCO remains a significant factor, it should not be the sole criterion used to judge the success of an IOL," claimed Dr Chassain. "We should also consider less immediately obvious variables, such as long-term light damage to the retina."
A cadaver study conducted by James Dillon, PhD of Columbia University, New York, US reached a similar conclusion, determining that implantation of a standard IOL following cataract surgery increased the amount of light absorbed by the lipofuscin fluorophore A2-E by approximately a factor of 5.27 This indicates just how important it is for IOLs to offer effective protection from light.
Is a blue blocking IOL needed?
In vitro studies have concluded that, when a blue blocking IOL is implanted, less cellular death is observed in the retinal pigment epithelium and production of vascular endothelial growth factor (VEGF) is lowered.28, 29 It should be borne in mind, however, that these studies do not address the question of whether or not the phototrauma was induced by the implantation itself.
Is a blue light blocking filter, in addition to the ultraviolet (UV) filter that comes as standard in all IOLs, therefore really necessary?
"Bearing the collective results of these, separately performed, studies in mind, the conclusion that a standard UV filter is not sufficient to protect eyes from macular disease seems apparent," said Dr Chassain.
"Both hydrophilic and hydrophobic lenses incorporate UV filters, but hydrophilic IOLs are constructed from aliphatic materials: the UV filter is less efficient when compared with aromatic-based hydrophobic materials," he said. Although it should be remembered that some hydrophobic IOLs are made from a mixture of both materials. "The efficiency of the UV filter in lenses also, of course, depends on the lens manufacturer," Dr Chassain reminded.
"Admittedly only one clinical study so far has shown the long-term superiority of blue-blocking IOLs over standard UV-filtered IOLs five years after surgery," (Miyake showed that a Hoya blue-light filtering IOL causes less autofluorescence from the accumulation of lipofuscin30), "but this is because there is still a lack of epidemiologic works that seek specifically to compare macular disease incidence levels of populations using UV-filter IOLs with those using blue-blocking IOLs," said Dr Chassain.
Hydrophilic IOLs just as effective as hydrophobic
"In my opinion, the addition of a blue filter to a hydrophilic IOL will help to block UV wavelengths as well as blue light. Therefore, a blue filter incorporated into a hydrophilic IOL is an interesting proposition; I believe this combination has a greater likelihood of offering effective protection from phototoxicity than standard hydrophilic lenses," asserted Dr Chassain.
Dr Chassain stressed that short-term outcomes, such as PCO incidence, are important for determining IOL success, but there are other factors to be borne in mind in addition to this, such as how well the IOL protects the aphakic eye from light damage. He therefore reached the conclusion that, although hydrophobic IOLs offer better outcomes in these terms, there are newer models of hydrophilic IOL on the market that offer similar benefits.
"Whatever happens in the evolution of IOLs in the future, and whether or not blue blockers will one day be offered as standard, it is now clear that considering the prevention of PCO as the sole marker of IOL implantation success is short-sighted to say the least," Dr Chassain concluded.
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