Download EXTENDED RANGE OF SILICONE HYDROGEL LENSES TO

COVER FOCUS
EXTENDED RANGE
OF SILICONE HYDROGEL
LENSES TO ADDRESS
HIGH AMETROPIAS
The prevalence of high refractive errors in the population has meant significant unmet
needs among contact lens options.
BY DESMOND FONN, MOptom, FAAO, AND PAUL CHAMBERLAIN
Visual distortions induced
by aberrations present in
high-prescription spectacle
lenses are well known and
can often result in unsatisfactory vision. These distortions
can be reduced substantially
by shifting the plane of the
lens to the ocular surface and reducing the thickness of the
lens, as with a contact lens. High-prescription spectacles also
create physical discomfort for the wearer. However, perhaps
the most appealing aspect of contact lenses as an alternative
to spectacles for high prescriptions is the elimination of poor
spectacle lens cosmetics.
PREVALENCE OF HIGH REFRACTIVE ERRORS
The literature does not contain a definitive classification
of “high refractive error,” and there is a considerable lack of
agreement regarding what constitutes “high” for what type
of refractive error.
Myopia
Classification of where “high myopia” begins ranges from
-5.00 to -8.00 D. This may be one reason that the reported
prevalence of high myopia varies considerably across countries and populations.1-3 For example, in studies conducted
in the past 15 years, the prevalence in the United States
was 1.6% for -8.00 D and greater,1 and in Israel it was 2.4%
for -6.00 D and greater.4 By contrast, the rates appear to be
much lower for older adults (age = > 40 years) in Australia,
the United States, and Western Europe (2%-4% for >
-5.00 D). In Taiwan, two studies have recorded rates of 21%
and 38.4% for -6.00 D
The younger age of onset
and greater,5,6 which
will
likely result in greater
clearly shows the extent
progression,
leading to higher
of the condition in that
levels
of
myopia
and increases
geographic region.
in
associated
eye
diseases
Aside from the
such
as
retinal
detachment,
alarming increase in
cataract, and glaucoma.
the prevalence of myopia in East Asian and
Western countries,
where estimates are
70% to 80% and 25% to
40%, respectively, onset of myopia is occurring at an earlier
age. The younger age of onset will likely result in greater progression, leading to higher levels of myopia and increases in
associated eye diseases such as retinal detachment, cataract,
and glaucoma.7 High myopia (> -8.00 D) in the United States
appears to have increased by a factor of eight since the early
1970s.7
“
Hyperopia
In most reports on hyperopia that may be classified as
high (ie, > +3.00 D), the prevalence ranges from 1% to 12%.
These publications do not, however, provide further subdivision beyond the classification of greater than +3.00 D.1,8-10
In the group of patients aged 20 to 39 years, the prevalence
of high myopia (> -5.00 D) was reported to be 7% to 8%, and
approximately the same in the group of 40- to 59-year-olds. The
prevalence of high myopia then decreased to 3% in the olderthan-60 group. Comparatively, the prevalence of high hyperopia (≥ +3.00 D) was 1% in the patients aged 20 to 39 years, 2.4%
in the 40- to 59-year-olds, and 10% in the over-60 group.1
JANUARY/FEBRUARY 2015 | ADVANCED OCULAR CARE 63
COVER FOCUS
Figure 1. Toric optic (5.75 D cylinder) with narrow steep
meridian (< 4.5 mm).
Astigmatism
Young and Hashemi have written the two papers that provide prevalence data on high astigmatism.11,12 The values range
from 2% to 3.4% for astigmatism greater than 3.00 D, but, as
with the hyperopia reports, they do not provide information
beyond the 3.00 D. Astigmatism is rarely an isolated refractive error. Young et al found that there were twice as many
patients with myopia and astigmatism as those with hyperopia
and astigmatism in the high astigmatic range.11
PRESCRIBING FOR HIGH REFRACTIVE ERRORS
Prescribing contact lenses for high refractive prescriptions has traditionally been rewarding for practitoners and
patients. Few practitioners would disagree that the professional satisfaction from a successful contact lens fitting in this
group is considerable, but the process can be challenging.
Historically, eye care practitioners have tried to fit high
ametropes with rigid gas permeable (RGP) lenses. However,
RGPs presented centration challenges and resultant visual
fluctuations, which can be even worse with highly astigmatic
corneas. These difficulties steered practitioners toward
hydrogel lenses, which invariably improved centration and
stability of the lens, thus eliminating visual complications.
Low-Dk hydrogel materials in high powers, and therefore with thicker geometries, are likely to cause hypoxia.
However, these clinical effects can easily be observed and
managed by practitioners.13-17
Silicone hydrogel lenses have significantly reduced and in
some cases eliminated these hypoxic effects without compromising high prescription lens designs. Companies that
manufacture molded silicone hydrogel or hydrogel lenses
provide a range of spherical lens powers extending from
64 ADVANCED OCULAR CARE | JANUARY/FEBRUARY 2015
Figure 2. Toric optic (5.75 D cylinder) with uncontrolled flat
meridian diameter.
approximately +8.00 D to -12.00 D. The spherical power
range for stock toric lenses is not quite as extensive (+6.00 D
to -10.00 D), and the cylinder powers of those lenses range
from -0.75 D to -2.25 D typically in 0.50 D steps. For most
of these lenses, cylinder axes are available from 0° to 180° in
10° steps. Prescriptions outside this spherical and astigmatic
range are available as custom-designed lathed lenses.
SUCCESSFUL FITTING IN A CUSTOM
POPULATION
The ideal contact lens—regardless of refractive power—
should fulfill the following criteria: provide excellent vision,
as good as or better than spectacle correction; be physiologically undemanding and biocompatible with ocular surfaces;
be centered over the pupil with an optical zone larger than
the pupil; and possess a posterior surface shape that closely
matches the contour of the cornea and limbus without
causing localized mechanical pressure. The lens should be
free to move during blinking or eye movement (lag) but not
excessively so as to cause discomfort or fluctuating vision.
Patients should be able to dispose of the lens and replace it
with an identical one so as to maintain the described criteria.
Contact lens manufacturers of stock molded silicone hydrogel lenses by and large provide products that satisfy those criteria, but only in the limited range of powers described earlier.
Reaching each of these performance standards is a greater
challenge in the high-prescription group of wearers, from
standpoints of both design and reproducibility.
DESIGN AND REPRODUCIBILITY CHALLENGES
Contact lens companies have traditionally not manufactured stock molded high-prescription lenses because of
of geometry involved in both sphere and toric contact lens
corrections.
To meet the objectives of satisfactory lens performance
criteria in high powers, a balance has to be struck between
minimizing the optical zone width—by reducing the thickness profile and blending of the junctional curvatures—
while maintaining a largely uniform center and edge thickness throughout the power range. The inability to achieve
this balance will likely result in a lens with unstable fitting,
poor comfort, and/or undesirable visual side effects.
These challenges are further exaggerated in toric lenses
because of the difference of meridional curvatures on the
posterior surface of the lenses. The critical performance
characteristics of toric lenses are cylinder axis predictability,
on-eye orientation, and, most important, stability during
wear.21 On the one hand, having greater differences in radius of curvature between the two principal meridians on the
COVER FOCUS
the potential for maintaining a large inventory with limited
patient demand. Besides the obvious lens design challenges
encountered with custom-lathing hydrogel lenses thin
enough to maintain oxygen transmissibility and comfort,
the lenses are custom-made to order, and therefore reproducibility may be a concern.
Manufacturing processes that may result in poor reproducibility will likely give rise to undesirable responses such
as inconsistent lens fitting, poor vision, and variable comfort. These custom lenses are often nondisposable. Even if
they are replaced every 3 months, the chances of surface
deposition, notably denatured protein and other contaminants, is greater, increasing the likelihood of ocular
complications.18-20
Reproducibility challenges notwithstanding, the likelihood of achieving a successful contact lens fit in the high
refractive error range is also encumbered by the extremes
TABLE 1. AVAILABLE POWERS BIOFINITY XR SPHERE
Power -20.00
Biofinity XR
sphere
-15.00 -12.50 -12.00 -10.00 -8.00 -6.50
-12.50D to -20.00D
in 0.50D steps
Base Curve =
Diameter =
8.60mm
14.0mm
-6.00 -5.00 +5.00 +6.00 +6.50
-6.50D to -12.00D
in 0.50D steps
-6.00D to +6.00D
in 0.25D steps
n Biofinity XR sphere
+8.00 +8.50
+6.50D to
+8.00D in 0.50D
steps
+10.00 +15.00
+8.50D to +15.00D in
0.50D steps
n Biofinity sphere
TABLE 2. AVAILABLE POWERS BIOFINITY XR TORIC
Biofinity XR
toric
Power
-20.00
-15.00
-12.50
-12.00
-10.00
-8.00
Base Curve =
8.70mm
-6.50
-6.00
-5.00
plano +5.00
Diameter =
14.50mm
+6.00
+6.50
+8.00
+8.50
+10.00
+15.00
Cylinder
-0.75
-1.25
-1.75
-6.50 to -10.00 in
0.50D steps in 10˚
Axis
-6.00 to +6.00 in 0.25D steps
in 10˚ Axis
+6.50 to _8.00D
in 0.50D steps in
10˚ Axis
-2.25
-2.75
-3.25
-3.75
-6.50D to -20.00D in
0.50D steps in 5˚ Axis
-6.00 to +6.00D in
0.25D steps in 5˚ Axis
+6.50D to +20.00D in
0.50D steps in 5˚ Axis
-4.25
-4.75
-5.25
-5.75
n Biofinity XR toric
n Biofinity toric
JANUARY/FEBRUARY 2015 | ADVANCED OCULAR CARE 65
COVER FOCUS
“
Cast-molded silicone hydrogel lenses
with an extended power range will
provide a useful addition to the
arsenal of practitioners.
posterior lens surface (because of high cylinder power) may
help with stability of axis orientation. On the other hand,
thicker lenses (because of higher powers) may be more
prone to movement and rotation because of lid torque. To
counteract that effect, all toric lenses have some form of
thickness ballasting or differential to stabilize the meridional orientation. Additionally, the optical zone from these
two principal meridians must be appropriately centered
and larger than the pupil.
This fundamental requirement for toric lens performance
is more challenging in eyes with high astigmatism. Back
optical zones are traditionally oval or elliptical in shape.
This is acceptable with low to moderate amounts of astigmatism, as the variations in curvature and thus thickness
are not great. However, as astigmatism increases, so does
the delta between the radii of curvature. This creates a
dual challenge to (1) provide adequate optical zone width
(Figure 1) and (2) not infringe on the peripheral stabilization zone of the lens as displayed in Figure 2.
A NEW APPROACH
CooperVision has enhanced the available power range of
its cast-molded Biofinity sphere and toric contact lenses, as
depicted in Tables 1 and 2.
The base curves and diameters of the extended power
range for spheres and torics are the same as for the lower
power Biofinity stock lenses: 8.60 and 14.0 mm for sphere,
and 8.70 and 14.5 mm for torics. All lenses use the same
comfilcon A material and have the same lens design principles as the core range versions and are manufactured
to the same thickness standards for edge profile, which is
critical for lens stability and comfort.
The toric optical zone has been designed to offer both
a wide optical zone in each meridian, which is consistent
regardless of power and axis, and to not infringe on the
peripheral stabilization zone.
SUMMARY
Cast-molded silicone hydrogel lenses with an extended
power range will provide a useful addition to the arsenal of practitioners. In addition to the many patients
who may benefit from an extended range of spheri-
66 ADVANCED OCULAR CARE | JANUARY/FEBRUARY 2015
cal lenses in high powers, the unmet needs of patients
who fall into the category of high myopia or hyperopia
with astigmatism (of both high and low powers) and
low myopia or hyperopia with high cylinder should be
fulfilled. Also, patients with keratoconus who have struggled with other forms of contact lenses may find these
lenses a useful alternative. Advances in lens design and
metrology innovations now also allow the manufacture
of extended-range multifocal and multifocal toric lenses
in silicone hydrogel material. n
1. Vitale S, Ellwein L, Cotch MF, et al. Prevalence of refractive error in the United States, 1999–2004. Arch Ophthalmol. 2008;126:1111-1119.
2. Sun J, Zhou J, Zhao P, et al. High prevalence of myopia and high myopia in 5060 Chinese university students in
Shanghai. Invest Ophthalmol Vis Sci. 2012;53:7504-7509.
3. Jung SK, Lee JH, Kakizaki H, Jee D. Prevalence of myopia and its association with body stature and educational
level in 19-year-old male conscripts in Seoul, South Korea. Invest Ophthalmol Vis Sci. 2012;53:5579-5583.
4. Bar Dayan Y, Levin A, Morad Y, et al. The changing prevalence of myopia in young adults: a 13-year series of
population-based prevalence surveys. Invest Ophthalmol Vis Sci. 2005;46:2760-2765.
5. Lin LL, Shih YF, Hsiao CK, Chen CJ. Prevalence of myopia in Taiwanese schoolchildren: 1983 to 2000. Ann Acad
Med Singapore. 2004;33:27-33.
6. Wang TJ, Chiang TH, Wang TH, et al. Changes of the ocular refraction among freshmen in National Taiwan University between 1988 and 2005. Eye (Lond). 2009;23:1168-1169.
7. Holden BA, Sankaridurg P, Smith E. Myopia, an underrated global challenge to vision: where the current data
takes us on myopia control. Eye (Lond). 2014;28:142-146.
8. Kempen JH, Mitchell P, Lee KE, et al; Eye Diseases Prevalence Research Group. The prevalence of refractive errors
among adults in the United States, Western Europe, and Australia. Arch Ophthalmol. 2004;122:495-505.
9. Ezelum C, Razavi H, Sivasubramaniam S, et al; Nigeria National Blindness and Visual Impairment Study
Group. Refractive error in Nigerian adults: prevalence, type, and spectacle coverage. Invest Ophthalmol Vis Sci.
2011;52:5449-5456.
10. Kim EC, Morgan IG, Kakizaki H, et al. Prevalence and risk factors for refractive errors: Korean National Health and
Nutrition Examination Survey 2008-2011. PLoS One. 2013;8(11):e80361.
11. Young G, Sully A, Hunt C. Prevalence of astigmatism in relation to soft contact lens fitting. Eye Contact Lens.
2011;37:20-25.
12. Hashemi H, Rezvan F, Yekta AA, et al. The prevalence of astigmatism and its determinants in a rural population
of Iran: the “Nooravaran Salamat” mobile eye clinic experience. Middle East Afr J Ophthalmol. 2014;21:175-181.
13. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Vascular response to extended wear of hydrogel lenses with
high and low oxygen permeability. Optom Vis Sci. 2001;78:147-151.
14. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Changes in myopic refractive error with nine months’ extended
wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci. 1999;76:845-849.
15. Papas EB, Vajdic CM, Austen R, Holden BA. High-oxygen-transmissibility soft contact lenses do not induce limbal
hyperaemia. Curr Eye Res. 1997;16:942-948.
16. Keay L, Sweeney DF, Jalbert I, Set al. Microcyst response to high Dk/t silicone hydrogel contact lenses. Optom Vis
Sci. 2000;77:582-585.
17. Covey M, Sweeney DF, Terry R, et al. Hypoxic effects on the anterior eye of high-Dk soft contact lens wearers are
negligible. Optom Vis Sci. 2001;78:95-99.
18. Pritchard N, Fonn D, Weed K. Ocular and subjective responses to frequent replacement of daily wear soft contact
lenses. CLAO J. 1996;22:53-59.
19. Kotow M, Holden BA, Grant T. The value of regular replacement of low water content contact lenses for extended
wear. J Am Optom Assoc. 1987;58:461-464.
20. Guillon M, Allary JC. Guillon JP, Orsborn G. Clinical management of regular replacement: Part 1. Selection of
replacement frequency. International Contact Lens Clinic. 1992;19:104-120.
21. Snyder C. Evaluation of “high cylinder” toric soft contact lenses. International Contact Lens Clinic. 1997;24:160-165.
Desmond Fonn, MOptom, FAAO
Distinguished Professor Emeritus at the University of Waterloo
School of Optometry in Waterloo, Ontario, Canada
n [email protected]
n Consultant to CooperVision
n
Paul Chamberlain
Senior manager of clinical research within CooperVision R&D
in Pleasanton, California
n [email protected]
n