New Page 1
Exploring the Relationship Between Materials
and Ocular Health and Comfort
As the debate continues, this expert shares the latest thinking on the most
By Graeme Young, M Phil, PhD, F COptom, DCLP, FAAO
May any contact lens material be described as truly biocompatible? One
definition suggests that a biocompatible contact lens material should have
minimal impact on comfort and ocular physiology and, at the same time, be
minimally affected by the ocular environment, as evidenced by deposits on the
lens.1 Given that so many contact lens complications can be traced to
material properties, there is an obvious shortfall in material biocompatibility.
Conventional hydrogels commonly can induce hyperemia, corneal swelling and
symptoms of dryness. Although firstgeneration silicone hydrogel lenses somewhat
alleviated these problems, other aspects of performance suffered. As we see with
high-Dk gas permeable materials, subtle differences in silicone hydrogel
materials can have significant effects on clinical performance.
In this article, I will explore the most important contact lens material
properties affecting clinical performance and how these relate to comfort.
Elastic modulus or, as I prefer, Young�s modulus, is an index of a contact lens
material�s ability to drape and align to the ocular surface. There is relatively
little variation in the moduli of conventional hydrogels. However, early
silicone hydrogel lens materials, with moduli up to five times greater than
conventional hydrogels (Table 1), showed that stiffer lenses can affect lens
performance. On the positive side, handling is easier, and silicone hydrogel
lenses are sometimes prescribed for this reason alone. Regarding lens fit,
centration is generally better with a higher modulus lens, however, a single
design is less likely to fit a wide range of ocular shapes. Some clinicians have
noted cases of poor edge alignment in terms of edge stand-off and indentation,
which can cause discomfort. As a result, some silicone hydrogel lenses may
require longer adaptation periods. This makes assessing a patient�s success with
these lenses more difficult because the absence of lens awareness is a useful
clinical indicator of fitting success.
High modulus also has been implicated in a
growing list of contact lens complications. Our study of silicone hydrogel
lenses has shed some light on the previously perplexing problem of superior
epithelial arcuate lesions (SEALs). On some eye shapes, relatively stiff lenses
are unable to flex enough to align with the peripheral cornea and the limbal
area. As a result, when clamped by the upper lid, the lens is forced against the
cornea, exerting extreme pressure in the area of greatest misalignment (Fig. 1).
Slightly higher rates of papillary conjunctivitis also modulus,
second-generation silicone hydrogels. Corneal molding also has been described
with high modulus silicone hydrogel lenses, particularly when inadvertently worn
inside out.7 These lenses may prove useful in orthokeratology.
Contact lens dehydration is poorly understood, despite many opinions on this
subject. To suggest that lower water content lenses are inherently less prone to
dehydration is probably an oversimplification. Similarly, to suggest that
materials showing the least on-eye dehydration are superior also can be
misleading because hydration is affected by a range of variables, such as eye
temperature, osmolarity and tear film stability.8 Nevertheless, a
relationship does seem to exist between dehydration and corneal desiccation
staining and symptoms. Figure 3 summarizes some of the correlations noted in
clinical studies. Although no direct correlation has been noted between
dehydration and corneal staining, the link is implied by the fact that staining
can be induced by lowering humidity, increasing evaporation or reducing lens
direct correlation between dehydration and discomfort have proved difficult to
replicate. One reason for this difficulty is that dehydration is probably
localized in problem areas, such as areas showing �smile� staining. Also, there
are probably several mechanisms for symptoms of dryness; pre-lens drying can
result in discomfort due to friction during blinking, while post-lens
desiccation can cause corneal irritation through disruption of the corneal
Several reports have suggested symptoms of dryness and end-of-day discomfort are
reduced with silicone hydrogel lenses.9-11 Early work suggests that,
once equilibrated in the eye, silicone hydrogel lenses maintain lens hydration
through the wearing period. However, further work is needed to elucidate
differences between materials and to determine the relative importance of this
versus surface wettability.
The wide range of measurements used to characterize contact lens wetting is an
indication of the complexity of this topic. Wetting angle has limited relevance
because it relates to saline, not tears. Researchers in the Biomaterials Group
at Aston University in Birmingham, England, have measured dynamic contact angle
with a procedure that simulates tear film breakup and consequent dehydration.
Some materials, such as lotrafilcon B, show better maintenance of wetting angle
under these conditions than others.12 Surface lubricity (coefficient
of friction) has been used recently to compare contact lens materials (Table 1).12
It is thought to relate to the level of friction sustained by the eyelid
crossing the lens surface, with higher lubricity allowing the lens to cross the
ocular surface with less awareness or irritation. Senofilcon A has been measured
as having lubricity approaching that of the cornea. Given that during the
average wearing period, the eyelid can travel a distance longer than the length
of a soccer field, minimizing friction must be a factor in maintaining comfort.
Vistakon has suggested that practitioners try a �touch test� with patients to
demonstrate the lubricity of senofilcon A compared with other materials.
The oxygen transmissibility recommended for daily contact lens wear has been
variously estimated between 24 and 35.13 This debate has spilled out
of the laboratories and educational arenas and into the courts, and I, for one,
don�t intend to be caught in the crossfire. Suffice it to say, those of us who
saw ultrathin polymacon lenses as a big step forward for cornea-kind are bemused
at the concern shown for corneas exposed to Dk/t of a mere 80 or so. Wherever
the threshold lies, I believe it is time for low water-content hydrogel lenses
to be given a dignified retirement.
In general, raising the Dk of silicone hydrogel materials has involved
increasing silicone content and decreasing water content at the expense of other
properties, modulus in particular. The imminent launch of the comfilcon A lens (CooperVision)
appears to be an exception. Early clinical trial reports suggest reduced adverse
events; however, we await the results of ongoing clinical studies to give a
It seems likely that as we move to newer generations of silicone hydrogel
contact lenses, we will see improvements in the balance of material properties.
However, there may be more oxygen to be had from hydrogels other than
silicone-containing materials. With methacrylate-containing hydrogels, the
strict correlation between water content and Dk and its limitations are well
understood. Other families of hydrogels, such as ethylene glycols, show a
similar link but with higher permeability (k), enabling hydrogels of higher Dk.
One development group (Ocutec, United Kingdom) has overcome the problem of
producing stable, transparent polyethylene glycol materials and is working on
contact lens applications.
Silicone hydrogel materials have been shown to attract relatively low levels of
lysozyme but high levels of lipid compared with ionic hydrogel lens materials.
As a result, we have seen a reappearance of white-spot deposits (Figure 4),
having previously celebrated their demise in conventional hydrogels with the
introduction of frequent replacement. Even when these deposits are barely
visible to the naked eye, patients begin to notice a foreign body sensation. The
prevalence of deposition varies among the silicone hydrogel materials, depending
on the type of system used to retain wettability: inherent wettability or face
treatment. However, it has been shown with galyfilcon A that most lipid
deposition can be avoided by the use of a rub-and-rinse step in the care
Finally, an often overlooked material property that influences ocular health is
UV inhibition. This can reduce one of the risk factors for patients developing
chronic UVinduced pathology such as pinguecula, cataract and macular problems.
Kwok and colleagues15 have shown that even when people wear
sunglasses (non-wrap-around types), UV radiation entering the eye obliquely is
focused on the nasal sclera, risking the formation of pterygium. Two currently
available silicone hydrogel contact lenses contain a UV inhibitor: galyfilcon A
and senofilcon A. These lenses should be used with, not as a substitute for,
UV-blocking sunglasses. (For an in-depth discussion of the effects of UV
radiation and protection methods, see �When a Contact Lens Is the Healthier
Choice� on page 30.)
Goal: True Biocompatibility
The relative performance of contact lens materials differs, in some cases,
gaining one advantage at the expense of other aspects of clinical performance.
Often these are only evident by clinical experience, and it is clear that we
need to improve our ability to predict clinical performance from measurement of
material properties. This is particularly important given that further
improvements in contact lens materials on the horizon will bring us closer to
the goal of true biocompatibility.
1. Williams DF. Biomaterials and biocompatibility. In: Fundamental Aspects of
Biocompatibility. Ed. Williams DF. CRC Press; 1981.
2. Sankaridurg PR, Sweeney DF, Sharma S, et al. Adverse events with extended
wear of disposable hydrogels: results for the first 13 months of lens wear.
3. Dumbleton KA. Adverse events with silicone hydrogel continuous wear. Contact
Lens Ant Eye. 2002;25:137-146.
4. L�fstr�m T, Kruse A. A conjunctival response to silicone hydrogel lens wear.
CL Spectrum. 2005; 20:42-44.
5. Fonn D, Dumbleton K, Papas E. Conjunctival appearance related to silicone
hydrogel lens wear. CL Spectrum. 2005;18:46-52.
6. Lin MC. Conjunctival flaps: What are they and do we need to worry? Editorial
in: www.siliconehydrogels.com, June 2006.
7. Bergenske P. Some topographical changes associated with silicone hydrogel
contact lenses may be due to inverted lenses. Presented at the American Academy
of Optometry meeting in Tampa, Fla., Dec. 9, 2004.
8. McCarey BE, Wilson LA. pH, osmolarity and temperature effects of the water
content of hydrogel lenses. Cont Lens Intra Lens J. 1982;8:158-167.
9. Nichols K. Considering silicone hydrogels for dry eye patients. CL Spectrum.
10. Long B, McNally J. The clinical performance of a silicone hydrogel lens for
daily wear in an Asian population. Eye Contact Lens. 2006;32:65-71.
11. Riley C, Young G, Chalmers R. Prevalence of ocular surface symptoms, signs,
and uncomfortable hours of wear in contact lens wearers: the effect of refitting
with daily wear silicone hydrogel lenses (senofilcon A). Eye & Contact Lens.
12. Ross G, Nasso M, Franklin V, et al. Silicone Hydrogels: Trends in Products
and Properties. Poster presented at the British Contact Lens Association
meeting, Brighton, UK; June 3-5, 2005.
13. Fonn D, Bruce AS. A review of the Holden-Mertz criteria for critical oxygen
transmission. Eye Cont Lens. 2005;31:247-251.
14. Nichols JJ. Deposition rates and lens case influences on galyfilcon A
silicone hydrogel lenses. Optom Vis Sci. 2006; 83:751-757.
15. Kwok LS, Kuznetsov V, Ho A, Coroneo MT. Prevention of the adverse photic
effects of peripheral light focusing using UV-blocking contact lenses. Invest
Ophthal Vis Sci. 2003;44:1501-1507.
Contact Lens Spectrum, Issue: May 2007