Comparing Hydrogels and Silicone Hydrogels
See how a particular hydrogel lens fared going head-to-head with two silicone hydrogels.
By Brian Chou, OD, FAAO
hydrogel contact lenses are well recognized for their high oxygen transmissibility.
High-Dk/t lenses minimize hypoxic corneal changes that include microcysts, limbal
hyperemia and neovascularization. Meanwhile, the contact lens industry is correcting
some of the early surprises associated with silicone hydrogels such as solution
incompatibility. Some solution manufacturers have introduced lens care systems,
such as Opti-Free Replenish (Alcon), that are designed to be compatible with all
soft contact lenses including silicone hydrogels.
the market entry of lower-modulus silicone hydrogels should reduce modulus-related
surprises such as mucin-balls, superior epithelial arcuate lesions (SEALs), contact
lens papillary conjunctivitis (CLPC) and conjunctival epithelial flaps.
Finally, the contact lens market
is experiencing a notable shift in silicone hydrogel prescribing away from continuous
wear toward daily wear. This shift toward daily wear should further enhance clinical
safety for patients wearing silicone hydrogel lenses.
1. Conjunctival staining with Acuvue Advance.
Does More Oxygen = More Comfort?
While the evolution of silicone hydrogel lenses
continues, some unanswered questions still exist. Among them, does the high oxygen
transmissibility of silicone hydrogels convey optimal wearing comfort? Some of the
commercially available silicone hydrogel lenses are marketed in a way that implies
a direct connection between higher oxygen transmission and greater comfort. As a
result, many practitioners appear to believe that silicone hydrogels offer better
wearing comfort than do hydrogel materials.
This article reviews two recent studies that
compared the subjective comfort of omafilcon A (Proclear Compatibles, CooperVision),
a biomimetic hydrogel lens material, against two leading silicone hydrogel lenses.
Several papers in the peer-reviewed literature have demonstrated that contact lens
wearers of omafilcon A typically experience less symptoms and signs of lens-related
discomfort vs. wearers of other hydrogel materials, while others have not. The two
studies I review here were designed to compare the comfort and patient preference
of silicone hydrogels against the Proclear lens.
Proclear vs. Acuvue Advance
The first study (Iravani N et al, 2005) was presented
at the 2005 BCLA conference and won first place in the poster contest over more
than 60 entries. In this bilateral, two-part crossover study, 60 subjects wore omafilcon
A and galyfilcon A (Acuvue Advance, Vistakon) contact lenses in random succession
for four weeks, each with daily lens removal. Study participants were asked to use
Opti-Free Express (Alcon) to care for their test lenses. Although galyfilcon A is
labeled for two-week replacement, a four-week replacement schedule was selected
to allow for comparison against omafilcon A under similar time intervals. The researchers
performed clinical assessments and patients completed questionnaires at two weeks
and four weeks after beginning wear of each test lens.
Figure 2. Fluorescence from lipid and protein
deposition from a representative sample of lenses.
half (55 percent) of the subjects enrolled reported a maximum comfortable wearing
time (WT) of 10 hours or less with their habitual lenses. A significant number of
subjects reported that both omafilcon A and galyfilcon A lenses improved comfortable
WT compared with their habitual lenses. There were no significant differences between
the two test lenses with respect to comfort or comfortable WT.
The slit lamp findings were similar
between lens types except for conjunctival staining, which was graded significantly
higher for Acuvue Advance (Figure 1); at four weeks, conjunctival staining was noted
in 70 percent of eyes wearing Acuvue Advance compared with 33 percent of eyes wearing
Proclear Compatibles. The mean scores for limbal hyperemia were similar for both
lenses. Similar proportions of subjects stated a preference for each lens type;
for overall comfort, 46 percent of subjects expressed a preference for Proclear
Compatibles compared to 35 percent for Acuvue Advance, and 19 percent indicated
a post-study analysis of lens surface deposits, Acuvue Advance showed greater deposition
as graded by slit lamp than did Proclear Compatibles (Figure 2). During the study,
two of the Acuvue Advance lenses required replacement because of deposits (Figure
3) compared to none for the Proclear lenses.
With the exception of the conjunctival
staining and surface deposits with Acuvue Advance, the overall clinical performance
of Proclear lenses was similar to that of Acuvue Advance. Both lenses showed improved
comfort and WT compared to conventional hydrogel contact lenses. Other researchers
have since reported a higher-than-baseline level of conjunctival staining with Acuvue
Advance as well (Brennan, Coles, 2006).
Figure 3. Deposits on Acuvue Advance after
two weeks of daily wear for one subject.
Proclear vs. O2Optix
This second study (Morgan D et al, 2005) shared
similarities in design to the previous evaluation. It was a subject-masked, bilateral
crossover investigation. Forty subjects wore omafilcon A and lotrafilcon B (O2Optix,
CIBA Vision) lenses in random succession and were evaluated after two weeks and
four weeks of wear. The researchers asked study participants to use CIBA Vision
AOSept Plus (in the United Kingdom) or Clear Care (in the United States) with their
test lenses, wearing the lenses with daily removal and four-week replacement. As
in the earlier study, lotrafilcon B is labeled for two-week replacement, but a four-week
replacement schedule was selected to allow comparison against omafilcon A under
similar time intervals. The researchers performed clinical assessments and patients
completed questionnaires at two weeks and four weeks after beginning wear of each
Both O2Optix and Proclear offered
excellent overall clinical performance. The Dk/t of O2Optix (125) and
of Proclear (52) both exceed the minimum Dk/t of 35 reported by Harvitt and Bonnano
to avoid induced corneal swelling for daily wear. The current study showed similar
slit lamp findings except that the O2Optix lens was associated with less
limbal redness than was Proclear at the four-week point, which is consistent with
previous reports of reduced limbal hyperemia with high-Dk/t silicone hydrogels (Sweeney,
Mean comfort score on a 0 to 100 visual
analogue scale was 79.0 for Proclear, and 68.8 for O2Optix (two-tailed
P = 0.0046). Mean comfortable WT for the Proclear lenses was 11.83 hours and for
the O2Optix 10.75 hours (two-tailed P = 0.0563). There were no significant
differences between lens types for dynamic and static lens fit, visual acuity or
subjective ratings of visual quality. "Burning/stinging" and "dryness" symptoms
demonstrated a lower frequency with Proclear, but the difference wasn't statistically
significant. Sixty-eight percent of subjects preferred Proclear overall compared
to 32 percent for O2Optix, and 0 percent reported no preference. For
end-of-day comfort, 59 percent preferred Proclear as compared to 24 percent for
O2Optix, and 17 percent indicated no preference. Figure 4 summarizes
patient questionnaire results.
Factors that Influence Lens Comfort
Figure 4. Results of subject responses to a
forced-choice questionnaire at the conclusion of the study. P-values are from Fisher's
The clinical evaluations of
Proclear against Acuvue Advance and O2Optix suggest that for most
patients, Proclear offers equivalent if not better comfort. The logical
conclusion is that high Dk/t alone doesn't guarantee optimal lens-wearing
comfort. Further support for this conclusion comes from a study (Fonn, Dumbleton,
2003) in which investigators found that symptoms of dryness and discomfort were
no better with the silicone hydrogel material lotrafilcon A (Night & Day, CIBA),
than they were with three hydrogel materials including omafilcon A.
Taken in sum, it's likely
that multiple factors besides Dk/t � including lens modulus, wetting and
dehydration characteristics, edge profile and deposit resistance � collectively
influence lens-wearing comfort. The empirical "proof" is that many happy and
comfortable PMMA lens wearers exist. As we all know, PMMA is completely impermeable to oxygen. If high Dk/t was the sole
requisite for comfortable lens wear, no patient would have tolerated PMMA lens wear.
Looked at from a different angle, if
high Dk/t really is critical to wearing comfort, why aren't all wearers of hyper-Dk
GPs successful? Alas, some patients cannot tolerate the GP lens edge awareness
a factor that's completely independent of Dk/t. So does hypoxia cause dryness-related
discomfort? Perhaps that question is best addressed by Noel Brennan, MScOptom, PhD,
FAAO, and Philip Morgan, MCOptom, FAAO, who in their December 2005 Readers' Forum
"Another View of Oxygen Transmission" wrote, "There's absolutely no evidence in
the Medline-abstracted literature to back (such a) claim."
Neither the studies nor any anecdotal
clinical experience indicates that comfort and silicone hydrogels are mutually exclusive.
Riley and Chalmers at the 2005 American Academy of Optometry meeting demonstrated
that one new-generation silicone hydrogel for daily wear can alleviate some common
lens-related symptoms. With other daily wear silicone hydrogels in the industry
pipeline, we can expect to see additional improvements in wearing comfort. Research
and development efforts are shifting away from Dk/t toward other desirable lens
the key to contact lens comfort is not as simple as high Dk/t alone. Comfort is
a more complex equation which deserves further scientific clarification. Meanwhile,
practitioners should realize that despite the buzz surrounding silicone hydrogel
materials, omafilcon A remains a proven benchmark for daily wear lens comfort, possibly
for many years to come.
Dr. Chou is in private group practice in San
Diego. He is also a consultant for the California Board of Optometry and a clinical
investigator and consultant to CooperVision, SynergEyes and Ophthonix.
For references, please visit
and click on document #124.
Contact Lens Spectrum, Issue: March 2006