Download Submission to the American Optometric Association Contact Lens and

Submission to the American Optometric Association Contact Lens and
Cornea Section, Student Research Awards Committee
for
“Unique Applications of Silicone Hydrogel Lenses”
By: Brett Sobieralski
[email protected]
15 Lillian Drive
Belleville, IL 62226
(618) 210-0493
Third year Doctor of Optometry Student, Class of 2009
University of Missouri-St. Louis, College of Optometry
Silicone Hydrogel Contact Lenses: A New Approach
Brett A. Sobieralski
University of Missouri - St. Louis College of Optometry
1
Table of Contents
Page
Introduction
3
The Basics
3
Toric Silicone Hydrogels
4
Custom Silicone Hydrogels
5
Dry Eye: Post-menopause, Autoimmune, and Environmental
6
Recreational Uses
9
Occupational Uses
9
Cosmetic Uses
10
Multifocals or Monovision: The Presbyope
11
Keratoconus: the Piggyback
13
Pediatric Uses
14
Therapeutic Contact Lenses
17
Conclusion
19
References
20
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No material in the contact lens industry is more popular right now than silicone hydrogel.
According to Milton Hom, O.D. (1), “There is little doubt that silicone hydrogel materials will
displace conventional contact lens materials over time for both daily and continuous wear.” In
fact, sales of silicone hydrogel contact lenses increased 109 percent in 2006 alone (2). In 2007,
sales of silicone hydrogel contact lenses represented 46 percent of the total United States contact
lens market compared to just 15 percent in 2004 (3). Not only are sales continuing to increase,
but research comparing different silicone hydrogel materials as well as studies testing new
applications for silicone hydrogel materials continue to occur at increased rates. Contact lens
manufacturers are recognizing the demand researchers are putting on contact lens materials with
increased water content and oxygen permeability. In fact, the popularity and recent increase in
sales is forcing every major contact lens company to come out with its own patented silicone
hydrogel material to compete on the global market. Therefore, not only is the contact lens
industry challenging contact lens researchers to test more unique applications for improved
silicone hydrogel materials, but we are starting to see more innovative uses for these popular
contact lenses.
THE BASICS
Silicone hydrogel contact lenses are composed of silicone, which offers higher oxygen
permeability, and conventional hydrogel material, which offers increased wettability. Combining
these two materials creates a lens that combats hypoxia, neovascularization, corneal edema, and
dry eye. However, one downfall of combining these two very different materials is that they
leave the surface of the silicone hydrogel contact lens hydrophobic, which becomes a problem
when trying to put a hydrophobic contact lens on an already very hydrophobic epithelial surface
of the cornea. Therefore, either the surfaces of silicone hydrogel lenses are treated with a plasma
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surface treatment, treated through a plasma oxidation process, or the lens itself will have an
internal wetting agent like polyvinyl pyrrolidone (PVP) to make itself more hydrophilic. These
silicone hydrogel treatment options make the lenses more wettable and thus less likely to adhere
to the cornea.
Moreover, every silicone hydrogel material is given a Dk number which represents its
ability to transmit oxygen (oxygen permeability); the higher the number, the more oxygen the
lens can transmit. Dividing the Dk number by the thickness of the contact lens gives a Dk/t
number, which represents the true ability of a particular contact lens to transmit oxygen. Holden
and Mertz (4) set the criterion for the critical level of oxygen required to avoid overnight corneal
edema (4.1 percent), and established a minimum oxygen transmissibility (Dk/t) of 87 x 10-9 (cm
X mL O2) (s X mL X mmHg)-1 to prevent overnight edema (5). La Hood et al. (6) later revised
the critical level of oxygen required to avoid overnight corneal edema to 3.2 percent, using data
from a greater number of subjects (n = 41). While, Harvitt and Bonnano (7) estimated a
minimum lens Dk/t of 125 x 10-9 (cm X mL O2) (s X mL X mmHg)-1 to allow for normal oxygen
distribution to the stroma (8). These numbers are important for corneal physiology and health,
but they also serve as the standards used by contact lens companies and researchers to create new
silicone hydrogel materials and innovative applications for silicone hydrogel lenses.
TORIC SILICONE HYDROGELS
Most toric soft contact lenses utilize a prism ballasted, slab off, or periballasting design,
which makes the lenses thicker in one region (usually inferior). Inferior thickness aligns the axis
of the toric lens via gravity and the pressure of the lids pushing the lens against the eye during a
blink (9). Moreover, increasing the thickness of the toric lens material will reduce the Dk/t value
of the lens, leading to a compromised lens-to-cornea fitting relationship and increased risk of
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hypoxia and neovascularization (9). Silicone hydrogel contact lenses, however, have been proven
to reduce the signs of corneal hypoxia and neovascularization, especially in extended and
continuous wear patients (10). Therefore, silicone hydrogel materials along with innovative toric
designs allow for proper oxygen delivery, higher Dk/t values, and thus ideal conditions for toric
soft lens wear with minimized corneal swelling.
Moreover, corneal swelling or edema is the cause of increased myopia (myopic creep) in
contact lens patients, and it is associated with contact lens-induced, chronic, corneal hypoxia.
Myopic creep is the direct result of contact lens over-wear, especially low-Dk lenses or lenses of
increased thickness; i.e. toric soft lenses. In fact, the U.S. Food and Drug Administration (FDA)
recognizes corneal swelling to be a direct result of wearing conventional soft contact lenses on
an extended wear schedule (11). In addition, the differential thickness of toric soft lenses may
induce uneven effects, and cause a cylinder correction shift and other optical aberrations
resulting from edema caused by corneal swelling (12). However, a study by Moezzi (11) et al.
demonstrated that the results of corneal swelling induced by overnight lens wear and a return to
baseline (no swelling) were the same in two test groups wearing two popular silicone hydrogel
lenses. Both lenses, Comfilcon A and Lotrafilcon A, exhibited minimal corneal swelling with a
return to baseline and are suitable for overnight wear based on the low amount of induced
corneal swelling. Therefore, the use of high Dk/t lenses or silicone hydrogel toric lenses not only
eliminates corneal swelling and the effects of myopic creep, but minimizes optical aberrations
and corneal cylindrical changes resulting from corneal edema.
CUSTOM SILICONE HYDROGELS
The added silicone in a silicone hydrogel contact lens causes an increased rigidity
(modulus) leading to possible mechanical trauma or corneal distortion (13). Modulus, in
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optometric terms, refers to how well a material resists bending and deforming. The higher the
modulus of a contact lens, the more likely it is for that particular lens to resist deformation. Most
silicone hydrogel contact lenses have a relatively high modulus compared with HEMA-based
hydrogels (14). Having a high modulus becomes a problem for silicone hydrogel contact lenses
as parameters like corneal diameter, base curve, peripheral asphericity and overall saggittal
height of the cornea are affected by these stiffer silicone lenses. Therefore, contact lens
manufacturers are developing new silicone hydrogel contact lenses that benefit patients who
cannot fit into standard contact lens parameters. These contact lenses are known as silicone
hydrogel made-to-order lenses (SiHy MTO) and they are manufactured on an individual basis
using high precision lathing (15). SiHy MTO’s are available in three diameters, fourteen base
curves, and powers from +20.00D to -20.00D (15). While, the lenses are not yet FDA approved
for extended or continuous wear they have a quarterly replacement schedule on daily wear terms.
Therefore, patients who previously could not wear silicone hydrogel contact lenses resulting
from extreme parameter problems can have custom made-to-order lenses and an alternative to
refractive or corneal surgery.
DRY EYE: POST-MENOPAUSAL, AUTOIMMUNE, AND ENVIRONEMNTAL
A survey released by Pritchard et al. (16) shows that nearly 34 percent of patients will
discontinue contact lens wear once within five to six years of initially wearing contact lenses
specifically resulting from discomfort, dryness, and red eyes. Of those 34 percent that
discontinue wear the first time, 77 percent will attempt to wear contact lenses again, only to have
48 percent discontinue wear a second time (16). In fact, the group that initially discontinued lens
wear stated in the survey that improved comfort and relief of dryness were the two primary
factors that could improve their contact lens wear (16). In addition, several other studies have
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illustrated that discomfort and dryness are the major reasons patients discontinue conventional
soft lens wear (17 - 20). Schafer et al. (21) has illustrated that dryness symptoms are not
completely eliminated with the use of silicone hydrogel lenses. However, dryness symptoms
were significantly reduced and stabilized with silicone hydrogel contact lens wear over a threeyear period, even in continuous wear. Chalmers et al. (22) has illustrated that symptoms of
dryness were less frequent in silicone hydrogel contact lens wearers over one year compared
with wearers of conventional soft lenses. In addition, subjects wearing silicone hydrogel lenses
in the study reported that they were less likely to discontinue contact lens wear resulting from
dryness, or remove their contact lenses as a result of discomfort (22).
Moreover, dryness complaints are skewed towards specific age groups and gender. In the
United States alone, 15 percent of individuals aged 65 to 84 years have keratoconjunctivitis
sicca, defined by Schein et al. (23) as having at minimum one dry eye symptom often or all of
the time. In addition, women are one and a half to three times more likely than men to have dry
eye, especially in postmenopausal women using hormone replacement therapy (HRT) and
women with premature ovarian failure (POF) (24 - 27). In fact, postmenopausal women with dry
eye symptoms often present in clinical practice with reports of increasing difficulty with contact
lens wear and a decreased quality of life resulting from visual complaints (28). However,
research is presently being conducted to study the effects of silicone hydrogel materials and
postmenopausal women. Kelly Nichols O.D. has been researching the effects of dry eye in
postmenopausal women with a grant from the National Institutes of Health (NIH). According to
Doctor Nichols, “As a clinician and scientist, I am looking forward to the development of bifocal
contact lenses in silicone hydrogel materials. If these materials can help prevent dryness
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symptoms, then it is reasonable to expect a silicone hydrogel bifocal contact lens could help keep
postmenopausal women from discontinuing lens wear.” (29)
In addition, dry eye is the most common complaint among people diagnosed with
Sjögren’s Syndrome, rheumatoid arthritis, and other autoimmune diseases (23, 30). Moreover,
cancer survivors may experience dry eye as a complication of bone marrow transplantation
resulting from systemic chemotherapy, radiation therapy, or ocular graft-versus-host disease
(GVHD) (31). According to Jack et al. (32), dry eye symptoms are a relatively common
complication of chronic GVHD, likely a result of conjunctival and lacrimal gland dysfunction
resulting in tear deficiency and an often severely damaged ocular surface. A different study by
Russo et al. (33) has demonstrated that extended-wear silicone hydrogel contact lenses can be
used to manage dry eye symptoms secondary to other causes of severe dry eye disease, including
infiltrative, infectious, and autoimmune causes. In this study, not only did the visual acuity
results from the subjects increase but ocular surface disease index (OSDI) scores decreased
significantly after only one month of silicone hydrogel contact lens wear. The OSDI is scored
between 0 and 100 with higher scores representing more severe dry eye symptoms (33).
Therefore, while some practitioners are reluctant to prescribe contact lenses to autoimmune
patients, studies have demonstrated that even ocular findings secondary to systemic disease can
be managed with silicone hydrogel contact lenses.
In fact, dry eye is difficult to diagnose with so many causes: age, diet, medications,
hormonal and environmental changes. A complete and comprehensive review of systems (ROS)
is critical in narrowing a specific diagnosis for dry eye, especially those pertaining to
environmental changes. A study by Young et al. (34) has demonstrated that in challenging
environments predisposed to dry eye, refitting subjects who had previously worn conventional
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soft contact lenses into silicone hydrogel contact lenses improved comfort significantly in
simulated environments. The environments simulated during the study were as follows: sitting in
an air-conditioned or heated car, using a computer, driving at night, riding in an airplane,
napping or sleeping, being in high altitudes, reading, and being in a dusty, polluted, or smoky
environment (34). Therefore, instead of exacerbating dry eye symptoms in certain environments
contact lenses can now offer relief to dry eye patients.
RECREATIONAL USES
Patients enjoying outdoor activities can benefit from continuous wear silicone hydrogel
contact lenses. Recreational activities such as camping and hiking place patients at the mercy of
the wilderness, and most do not have the ability to disinfect or handle their contact lenses in a
hygienic manner (1). Therefore, continuous wear decreases the handling of the patient’s contact
lenses and consequently reduces the risk of ocular infection. Moreover, high altitude recreational
activities such as mountain climbing, skiing, or snowboarding offers silicone hydrogel contact
lenses a perfect opportunity to showcase the benefit of having increased oxygen permeability, as
the level of oxygen in the atmosphere is already reduced with such activities (35). In addition,
the benefit of continuous wear will also allow the patient the ability of not having to insert and
remove contact lenses during high altitude recreational activities.
OCCUPATIONAL USES
Certain occupations call for stringent and irregular working hours with little time to
correct visually related problems. Firefighters, police officers, armed forces, EMT’s, on-call
doctors and residents, even parents of a new-born baby labor in round-the-clock hours while
maintaining a definitive need for clear vision. Extended and continuous wear contact lens
schedules fit perfectly into the lifestyles of such occupations. However, there are certain
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industries that demand clear, comfortable vision but do not necessarily need continuous wear.
One industry that is gaining optometric attention is the airline industry, with a large number of
pilots and stewards complaining of dry eye symptoms (36). A study by Uchiyama et al. (36) has
demonstrated that low relative humidity environments, like those found in pressurized airline
cabins during commercial air travel, cause increases in evaporation of a patient’s tear film with
and without dry eye. The study found that the average evaporation rate of each subjects tear film
increased 99.72 percent when exposed to low relative humidity (36). The study concluded that
the preventative use of moisturizing eye drops (artificial tears) should be considered for all
people encountering low relative humidity environments; including air passengers, pilots and
crew; especially those with dry eye (36). In addition, the concurrent use of silicone hydrogel
contact lenses would also help alleviate the symptoms of dryness and discomfort resulting from
the increasing amounts of oxygen reaching the cornea, while increasing the wear time in low
humidity environments.
COSMETIC USES
Sales of cosmetic contact lenses are on the rise, especially in the silicone hydrogel
market. According to Jeffrey D Johnson O.D., “Over the next three years we can expect that
companies will extend their silicone hydrogel technology to encompass the multifocal and
colored segment markets.” (2) Currently, cosmetic silicone hydrogel contact lenses make up
seven percent of the total U.S. contact lens market, which is one percent more than the multifocal
market at six percent (3). Manufacturers are realizing the need for both markets, especially for
extended wear cosmetic lenses. The reasons patients need extended wear cosmetic contact lenses
include ametropia correction, a need to change iris shape and color, and to mask blind or
disfigured eyes (37). Moreover, people who wear cosmetic lenses utilize extended wear time
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averages to mask ocular disfigurements (37). Therefore, silicone hydrogel contact lenses can
benefit patients needing to cover up an ocular disfigurement or a congenital anomaly with
increased oxygen permeability throughout the day.
MULTIFOCALS OR MONOVISION: THE PRESBYOPE
In previous statistics, dry eye is found to be prevalent in high accordance in patients over
40 years of age. In addition, inadequate tear quality found in the elderly population can be
exacerbated by medical conditions and the accompanied use of systemic medications (38).
Therefore, just as silicone hydrogels can help alleviate discomfort and dryness in everyday
contact lens patients, so to can they help alleviate dry eye experienced in presbyopic contact lens
patients.
One problem, however, in contact lens wearing presbyopes is that they require some type
of multifocal lens, and unfortunately the multifocal market for silicone hydrogel contact lenses in
the U.S is currently at six percent; the lowest of any market (3). Moreover, other factors like
decreased interest in sleeping in contact lenses, and the expensive cost of multifocal contact
lenses are keeping presbyopic patients from buying silicone multifocal lenses. It is not that the
demand is not there, in fact according to Rex Ghormley O.D. in a recent interview with Kelly
Nichols O.D. (29), “the next company to develop a good silicone hydrogel bifocal lens will hit a
home run.” Therefore, the task for most developers is creating a contact lens that is thin,
wettable, and breathable, but also available in a variety of add powers.
Currently, there are only two silicone hydrogel contact lens options for presbyopic
patients: monovision or a single multifocal (balafilcon A) brand. Monovision offers clear vision
at various distances and is less time consuming to fit and less expensive for patients. However,
monovision decreases both depth perception and stereopsis since the patient is losing
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binocularity, and it does not offer intermediate distance correction for those computer users.
However, monovision is still effective; it has a success rate of 59 to 67 percent in adapted contact
lens wearers (39). In addition, with the advent of silicone hydrogel toric contact lenses, the
correction for low amounts of astigmatism will improve monovision success rates. Also, many
monovision patients exhibiting latent hyperopia and a need for higher plus powered (thicker)
contact lenses will benefit from the increased oxygen permeability in silicone hydrogel contact
lenses (38). One other factor to consider in all pre-presbyopic patients is that most are long-term
hydrogel lens wearers and can be suffering from corneal oxygen deficiency (38). Therefore,
silicone hydrogel contact lenses are the perfect complement for monovision, as they decrease the
signs of corneal oxygen deficiency (neovascularization and limbal redness) through improved
corneal oxygenation (37).
Compared to monovision, multifocal contact lenses have the benefit of having distance,
near, and intermediate correction. They also do not disrupt stereopsis and depth perception,
allowing for an overall improved quality of vision. Studies have illustrated that multifocal
contact lenses are preferred over their monovision counterparts. A 2-month crossover study by
Richdale et al. (40) comparing soft lens multifocals and soft lens monovision satisfaction has
demonstrated that among 38 subjects, 76 percent reported that they preferred soft lens multifocal
contact lenses to only 24 percent preferring soft lens monovision. Another crossover study by
Situ et al. (41) had successful soft lens monovision wearers fit into bifocal soft contact lenses for
6-months to assess subjective visual ratings. Results demonstrated that among 40 subjects, 68
percent reported that they preferred multifocal soft contact lenses, to only 25 percent preferring
their original monovision soft lenses. Reasons for the results included increases in the subject’s
field of view and contrast sensitivity (41). Therefore, whether it is monovision or multifocals,
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there is no substitute for silicone hydrogels improved oxygen exchange and good wettability
characteristics for presbyopic patients.
KERATOCONUS: THE PIGGYBACK
Keratoconus is a progressive thinning of the central cornea, leading to increased corneal
steepness upon topographic readings. Keratoconus is marked by frequent changes in spectacle
prescription caused by increased nearsightedness and irregularities in astigmatism (41). Thus, the
only non-surgical method to provide effective vision is by wearing contact lenses (42).
A number of different lens options exist for the treatment of keratoconus. One such
method is the piggyback lens system, which utilizes a rigid gas permeable (RGP) contact lens fit
on top of a soft contact lens. The combination of lenses, however, in the presence of a
compromised cornea can greatly increase the chances for corneal hypoxia and other
complications (43). In addition to the reduced oxygen transmission associated with wearing two
contact lenses, a thickened tear film can appear between the RGP/ soft lens interface, further
impeding oxygen transmission (42). However, with improved RGP and silicone hydrogel lens
materials that offer higher oxygen transmission (Dk/t), piggyback lens systems are becoming
practical options for keratoconic patients.
Research is showing that the combination of a silicone hydrogel contact lens with a
higher-Dk RGP contact lens can meet the requirements necessary to reduce corneal edema and
prevent corneal hypoxia (44). One study using 20 keratoconic subjects by Rodio-Vivadelli and
Gundel (44) has illustrated that out of 20 piggyback fits from previous RGP wearers, only one
subject felt more discomfort after wearing a silicone hydrogel/ RGP piggyback combination for
one month than wearing their RGP lenses alone. In addition, results from a one-month follow-up
after completion of the study demonstrated that 17 of the original 20 piggyback fits were
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continuing to wear the piggyback lens system equipped with a silicone hydrogel soft lens fulltime, while two of the remaining three were wearing the piggyback lens system only part-time
(44). Also, prior to the piggyback lens fitting in the study, 18 of the 20 keratoconic eyes
demonstrated three and nine o’clock staining with an average grading of one (44). One month
after the successful piggyback fittings, only two of 20 keratoconic eyes demonstrated three and
nine o’clock staining with an average grading of one (44). Thus, not only did the silicone
hydrogel piggyback system improve comfort but it also improved corneal health in patients with
compromised corneas.
Moreover, the use of silicone hydrogel contact lenses in combination with the piggyback
lens system can help symptomatic keratoconic patients avoid surgery. One study by Tsubota et
al. (45) demonstrated that 10 of 11 keratoconic subjects who could not wear RGP lenses
resulting from constant pain or inadequate lens fitting were able to avoid penetrating keratoplasty
and benefit from the increased comfort and oxygen-permeable from the piggyback lens system.
The study also proved that while RGP lenses may be uncomfortable for some patients initially,
the concurrent use of a soft contact lens improves comfort in some patients. In addition, RodioVivadelli and Gundel (43) demonstrated that it is often possible to fit a piggyback lens system
using a patient’s own RGP lenses. Therefore, the relative ease of fitting the piggyback lens
system in addition to the noted benefits of silicone hydrogel lenses offers a viable option for any
optometrist searching for an alternative to surgery.
PEDIATRIC USES
When it comes to treating amblyopia, several treatment options exist. Currently, the
Pediatric Eye Disease Investigator Group (PEDIG), formed in 1997, is conducting research on
eye disorders in children, specifically amblyopia (46). At this time, PEDIG has published 10
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studies known as the Amblyopia Treatment Studies (ATS), and each study investigates different
treatment options for amblyopia. Most of the ATS’s involve some form of patching to treat
amblyopia, whether pharmaceutical (penalization), slip-on, adhesive, or opaque (occlusion) like
a tinted contact lens. Tinted silicone hydrogel contact lenses offer unique contact lens patching
options for pediatric amblyopia. Instead of using pharmaceutical treatments that could cause
ocular side effects or wearing a cosmetically unappealing pirate-patch, tinted silicone hydrogel
contact lenses can provide a practical alternate for patching regimens. One study by Anderson et
al. (47) demonstrated that daily wear opaque contact lens treatment is a successful occlusion
method for amblyopia; especially in older children (mean age of 7.5 years). In addition, the
cosmetic appearance of contact lenses will be more socially acceptable to a child rather than a
patch or a dilated eye.
In fact, contact lens wear dramatically improves how children and teens feel about their
appearance and increases participation in recreational activities. One study by Walline et al. (48)
demonstrated that Pediatric Refractive Error Profile (PREP) scores, a pediatric quality-of-life
survey, improved in 169 children (8-12 years of age) and teen subjects (13-17 years of age) fit
into silicone hydrogel contact lenses, after previously wearing spectacles to correct refractive
error. The study was known as the Contact Lens in Pediatrics (CLIP) study, and it was the first
clinical investigation to compare children and teens using silicone hydrogel lenses (49). The
section of the CLIP study comparing the function and quality of life benefits in wearing silicone
hydrogel contact lenses demonstrated that 97.2 percent of children and 98.8 percent of teens
reported being very satisfied or satisfied with their contact lenses at three months. While 71.2
percent of children and 78.5 percent of teens reported preferring contact lenses slightly more
than spectacles at three months (48).
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However, even with advancements in silicone hydrogel materials many practitioners are
reluctant to prescribe contact lenses in children and teens for a number of different reasons. Most
notably are perceived poor compliance, increased chair time, and potential self-induced trauma
(50). Several studies have demonstrated, however, that contact lenses can be beneficial in
pediatric cases, especially problematic pediatric cases. One of these cases is in pediatric aphakic
patients. A study by Ozbek et al. (50) demonstrated that extended wear contact lenses provided a
safe treatment option for childhood aphakia, providing low rates of complications and dropouts
in 86.7 percent of subjects. In addition, visual acuity was either maintained or improved in 69.8
percent of subjects. Therefore, silicone hydrogel lenses are an excellent choice for any aphake, as
wearing a high plus power for long periods of time throughout the day will require increased
oxygen permeability to maintain good corneal health. Children will also benefit from improved
cosmesis with silicone hydrogel contact lenses, and a minimal difference in retinal image size
will be experienced by unilateral aphakes wearing contact lenses rather than spectacles (50).
Another benefit of silicone hydrogel contact lenses in children is found in pediatric
nystagmus cases. Nystagmus patients have reduced visual acuities resulting from the smearing of
the retinal images caused by involuntary eye movements. Wearing spectacles only distorts vision
in nystagmus patients as they cannot consistently look through the optical center of the lens.
Significant distortion can occur in patients with high myopia, i.e. looking through the periphery
of highly corrected myopic spectacle lenses (51). Therefore, silicone hydrogel lenses allow
nystagmus patients to look through the optical center of the lens at all times, even with eye and
head movements. In fact, by turning the head and looking in extreme gaze positions the patient
may experience a null point where their nystagmus is dampened. Contact lenses offer the
opportunity for the patient to experience clear vision while looking through their null point,
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whereas myopic spectacles would blur the null point while looking through the periphery of the
lenses. In fact, many practitioners have found that using contact lenses in pediatric nystagmus
patients improves visual acuity, and often reduces the amplitude and frequency of the nystagmus.
A study by Abrams (52) et al. has reported increases in visual acuity, and a decrease in the
frequency of the nystagmus in five pediatric nystagmus patients fitted with contact lenses.
THERPEUTIC CONTACT LENSES
Therapeutic contact lenses have several uses, some of which include treating acute and
chronic ocular pathologies, drug delivery, protecting the cornea (especially after corneal
refractive surgery), and improving comfort while relieving painful symptoms. Currently there are
three silicone hydrogel contact lenses that are FDA approved for therapeutic use as a bandage
contact lens: Lotrafilcon A, Balafilcon A, and Senofilcon A. Several factors make silicone
hydrogel contact lenses ideal therapeutic lenses. The higher modulus of silicone hydrogel contact
lenses resists the chances of adhering to irregularly shaped corneas and possibly pulling out
stitches in corneal grafts (53). The water content of silicone hydrogels continues to improve,
which will increase the reservoir available for drug uptake and delivery to the ocular surface;
Comfilcon A has a water content of 48 percent, the highest of any silicone hydrogel (53). More
importantly, the increased oxygen permeability is necessary to facilitate cellular division by
limbal stem cells, synthesize corneal proteins and lipids, and to maintain proper corneal pH and
osmotic balance (54). A study by Gürdal et al. (54) demonstrated that Lotrafilcon A contact
lenses used in therapeutic use for experimentally induced bullous keratopathy did not damage
limbal stem cells, which could be susceptible to constant mechanical trauma and hypoxia from
other therapeutic contact lenses. Therefore, the protection of stem cells in the limbal region by a
silicone hydrogel therapeutic contact lens is vital for the proliferation of new corneal cells and
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the recovery of corneal lesions. Ideally, therapeutic contact lenses should minimize hypoxia,
mechanical trauma, and tear film disruption, while promoting the recovery of the condition
affecting the cornea (55). Many studies have confirmed the benefit of using silicone hydrogel
contact lenses in therapeutic situations (56 - 59).
Other studies have illustrated that the increased modulus, wettability, and oxygen
permeability of silicone hydrogels make them ideal therapeutic lenses, especially for situations
that call for continuous wear. One study by Kanpolat and Omur in 2003 (60) demonstrated that
Lotrafilcon A contact lenses used in a therapeutic manner for a variety of ocular pathologies
were successfully used in a continuous wear manner without sight-threatening complications
related to contact lens wear. The mean duration of wear for the 50 subjects was 64 +/- 90 days on
a monthly wear schedule, with only one subject developing a culture-negative sterile corneal
infiltrate that was successfully treated (60). A continuous wear schedule for therapeutic treatment
is advantageous for patients and conditions that necessitate a potential increase in infection, or
for whom frequent insertion and removal can be associated with epithelial trauma or pain (60).
In addition, therapeutic silicone hydrogel contact lenses can be used in mechanical
protection for conditions such as trichiasis induced by age-related entropion. The overall
diameter of the contact lens should offer complete limbal coverage but avoid excessive
movement that could otherwise damage compromised tissue (54). Along with adult entropion,
pediatric epiblepharon can lead to trichiasis in children. A study by Bendoriene et al. (61)
illustrated that complete and partial corneal healing occurred in 27 of 29 eyes with traumatic
corneal conditions fit with Lotrafilcon A. The mean duration of continuous wear was 17.8 days
(range of 1 to 131 days of continuous wear) with the average age for the subjects being nine
years (range 2 months to 17 years) (61). Therefore, silicone hydrogel contact lenses are safe to
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use in children even in extended and continuous wear schedules, and can improve pediatric
ocular pathologies.
One of the most important uses of therapeutic contact lenses is the use of silicone
hydrogels as bandage contact lenses after corneal refractive surgery. Photorefractive keratectomy
(PRK) and Laser-assisted sub-epithelial keratomileusis (LASEK) utilizes silicone hydrogel
bandage contact lenses in post-refractive treatment for proper corneal healing resulting from its
increased oxygen permeability. In fact, the general procedure of PRK and LASEK surgery
requires the patient to wear a bandage contact lens for three to five days after surgery on an
extended wear schedule to promote epithelial healing. The only contact lenses FDA approved for
extended and continuous wear are silicone hydrogels. Furthermore, a study by Szaflik et al. (62)
demonstrated that in 30 subjects, 60 eyes receiving LASEK, 86 percent of eyes had a good or
very good corneal epithelium rating after 4-days of wearing post-operative bandage contact
lenses. An average of 99 percent of subjects rated symptoms of pain as either absent or mild
throughout the study (62). Therefore, even after corneal refractive surgery both ophthalmologists
and optometrists alike trust silicone hydrogel contact lenses to reduce the risk of corneal edema
and infection in their patients.
CONCLUSION
Silicone hydrogel materials are providing more unique contact lens options that did not
exist a few years ago. Whether it is treating dryness symptoms in postmenopausal women,
providing custom made-to-order options for parameter-troubled patients, or teaming together
with rigid gas permeable contact lenses to improve comfort in patients with keratoconus, these
lenses are expanding the entire scope of practice for the optometric profession. In fact, with Dk/t
values and water content of silicone hydrogel materials continuing to improve contact lens
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manufacturers are expanding silicone hydrogel lines to encompass new directions; such as
therapeutic specialty lenses, and possibly in the near future toric multifocal lenses and a hybrid
lens for treatment of keratoconus made from silicone hydrogel materials. We are even starting to
see silicone hydrogel contact lenses delay or even eliminate the need for corneal and refractive
surgery. Therefore, the pressure is on contact lens researchers to continue to discover unique
applications for improved silicone hydrogel materials, and it is our job as practitioners to utilize
the many advantages silicone hydrogel contact lenses continue to present.
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