Hydrophilic vs hydrophobic: which is the better option? - Ophthalmology Times Europe






Hydrophilic vs hydrophobic: which is the better option?
Measuring IOL success in broader terms

Ophthalmology Times Europe
Volume 4, Issue 7

The design of intraocular lenses (IOLs) has evolved rapidly in recent years. Because of the significant advances that have been made, there are now numerous varieties of IOL on the market and in development; naturally, questions over which is the best model endure. In particular, there is discussion of whether hydrophilic or hydrophobic IOLs are the superior lenses.

"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.

Table 1: Hydrophobic IOL YAG rates.
"The incidence of post-implantation YAG at two years is very similar for hydrophilic and hydrophobic IOLs," claimed Dr Chassain (Table 1).1–7 "The high rate of short term success observed with hydrophilic IOL implantation might also yield encouraging results in later years," he continued.

"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."

Table 2: Relationship between light and macular degeneration incidence found
Although the harmful effect of blue light on the retina has not been proved definitively, Dr Chassain argues that it is indisputable. "Light photons with shorter wavelengths have more energy than those with longer wavelengths. This means that light from the blue end of the spectrum has more energy, and therefore more potential to cause damage to absorbent surfaces, such as the retina," he said.

Table 3: Relationship between cataract extraction and macular degeneration incidence found
Several large population studies, however, have not been able to reach agreement on the damaging effects of light and its link to macular degeneration (Table 28–14), and so, even if the cumulative information is aggregated, it remains difficult to establish this relationship definitively. If, however, the selection of populations is narrowed to those with eyes that have previously undergone cataract extraction, the relationship between phototoxicity and macular degeneration does seem apparent (Table 315–26).

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.

Figure 1: Retinal protection according to material (mean values for hydrophilic and hydrophobic groups). Courtesy of PhysIOL.
According to Figure 1, approximately 50% of 375 nm wavelength (blue) light is not blocked by a hydrophilic IOL. In addition, hydrophobic IOLs, even in the absence of a blue blocker, appear to have more efficient UV filters, thus offering better all-round protection from light, as discussed earlier.

"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.

Figure 2: YellowFlex IOL (PhysIOL).
The YellowFlex (Figure 2) is very similar to the SlimFlex IOL; it is hydrophilic and square-edged, and has a low incidence of PCO, but it has a blue-blocking yellow chromophore added to it. Thus this lens is able to block UV waves, in a similar manner to standard hydrophobic IOLs, and to block blue light, as standard hydrophobic blue-blockers do. "Additionally, as I mentioned earlier, these lenses are less likely to cause trauma to the eye during implantation than hydrophobic lenses," reminded 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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