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Optics and Vision Science
Advances in Toric
Contact Lens Design
New methods for stabilizing toric lenses have greatly improved their performance and
patients’ comfort, but some design features may cause prism in the optical zone.
By Giovanna E. Olivares, OD
K
eeping toric soft contact lenses rotationally stable
on the eye has always been a challenge for lens
designers. Traditionally achieved by differential
weighting or prism ballasting, stabilization now
relies on better ballasting and stabilization zones that leverage the power of the blink, thereby improving the lenses’
performance and patients’ comfort.
Owing to these changes in design, my colleagues and I
can analyze the quality of the lenses’ optics instead of merely counting successful versus unsuccessful fits. We recently
found, for example, that the degree of vertical prism present
in toric soft contact lenses varies considerably, up to 1.00Δ
of vertical prism or more, depending on the design.1
In this article, I discuss why, whether it matters, and how
to elicit more information about quality of vision from
patients who wear toric contact lenses.
DESIGN APPROACHES
An estimated 35% of prospective contact lens wearers
have some degree of clinically significant astigmatism,2 but
historically, for a variety of reasons (chiefly discomfort and
unstable vision), many of them have avoided wearing toric
contact lenses. Current-generation toric lenses have caused
this paradigm to shift, and the correction of even small
amounts of astigmatism (0.75 D) with toric contact lenses
is becoming commonplace. The change is due primarily to
innovations in lens design, manufacturing, and materials.
The first soft toric contact lens designs were prismballasted designs. Originally, these lenses were truncated,
leading to decreased comfort and increased lid interaction.
Later designs eliminated the truncation and added thickness throughout the lens, specifically in the bottom to
allow gravity to orient the lens.
There were a number of problems with this approach.
The thicker portion of the lens, particularly with the older
hydrogel materials, was less oxygen permeable, increasing
the likelihood of neovascularization and localized corneal
edema. Unpredictable interaction between the lower
lid and the prism-ballasted area was also problematic.
Toric contact lens wearers could feel the thicker portion
42 Advanced ocular care May/June 2014
A
B
C
Figure. Example of thickness profiles for three different toric
contact lenses. Mean vertical prism amount is indicated at
the top of each image in red. The central 6-mm region is
represented by a dotted red circle.
of the lens, contributing to a foreign body sensation and
discomfort. The increased lower lid interaction may also
have caused the lens to rotate unpredictably, reducing the
stability the ballasting was supposed to create. It may also
have sometimes led to transient blur and visual fluctuations that made toric soft contact lens wear initially unacceptable to many patients and eye care providers.
Dual-thin-zone designs were one attempt to resolve the
problems of prism ballasting. Lenses of this type had thinner
areas on both their superior and inferior portions. The pressure applied by the lids on the thin zones rotated the lens
into the correct position. The thicker areas in between the
lids, however, resulted in lid interaction and an unpredictable
fit. Fitting this type of design often increased chair time. Not
surprisingly, many practitioners simply recommended against
contact lenses for patients with astigmatism, and/or continued masking lower levels of cylinder with a spherical lens.
Two newer approaches have been more successful. The
periballast approach attempts to minimize the prism ballast to increase comfort and reduce lower lid interaction.
Periballast designs are more compatible physiologically,
offer more predictable rotation, and have better visual performance than the traditional prism ballast lenses. The gravity-dependent mechanism utilized by some of these lenses
is not optimal with some head movements (such as tilting
the head sideways) or extreme positions of gaze, making it
difficult for the lens to remain correctly positioned.
Since 2005, the approach we at Johnson & Johnson Vision
Optics and Vision Science
Table. Vertical Prism in Soft Toric
Contact Lenses
Care have taken with Acuvue for astigmatism is called
“blink stabilized design” (BSD). Based on years of research
on the mechanism of the blink—including studying lid
movement and mechanics as well as lens-lid interaction—
BSD uses four “active” zones placed in specific locations on
the lens’ surface. If the lens becomes misaligned, the natural
power of the blink will rotate the lens back into position.
When correctly aligned, the lens interacts very little with the
eyelids, regardless of eye movement or head position, so it
remains essentially stable during the blink. Even when the
lens is misaligned (eg, during insertion), interaction between
the upper and lower lids on its active zones quickly orients
the lens and helps keep it stable and in place.
VERTICAL PRISM IN CONTACT LENSES
We visually observed that designs using a prism or periballast design had residual prism (typically base down) in the
optical zone. This is an optical artifact from the prism. We
therefore evaluated vertical prism in currently commercially available soft toric lenses using an objective in vitro
measurement technique.
We tested several different toric soft contact lenses with
eight parameter combinations per product covering -6.00 D
sphere to +3.00 D sphere with -1.25 D cylinder at both the
90º and 180º axes.1 Thickness maps were obtained using
scanning transmission microscopy over the full surface
of the lenses (Figure). We used more than 650,000 thickness points in the central 6-mm optical zone of the lens to
determine the average change in thickness and to compute
prism via least-squares-fit of a linear equation.
As expected, lenses that do not use a base-down prism
design for stabilization were found to have no vertical prism
in the optic zone (Table). Most of the other lenses measured
had a mean vertical prism ranging from 0.50Δ to 1.20Δ.
CLINICAL RELEVANCE
Prism in the optic zone produces a slight shift of the
image being viewed. As long as the displacement due to
vertical prism is the same in both eyes, there is unlikely to
be any visual consequence. Nearly half of people with visually significant (≥ 0.75 D) astigmatism, however, have astigmatism in only one eye.2 In a patient with a toric contact
lens in one eye and a spherical or multifocal contact lens
in the other, prism-ballasted lenses will therefore create
vertical disparity between the two eyes, which may cause
symptoms of vertigo, double vision, tension in the forehead
muscles, or eye fatigue/strain.3,4 This problem is of particular concern in individuals with a fragile binocular system.
It can be challenging to predict who might struggle with
binocularity or have weaker fusional ability. Many individuals likely have anatomical imperfections that cause minor
exophoria, esophoria, or vertical phoria for which they sim-
Soft Toric Contact Lens
Stabilization
Design
Mean
Vertical
Prism
Acuvue Oasys for Astigmatism
(Johnson & Johnson)
Blink
stabilized
design (BSD)
0.01
1-Day Acuvue Moist for
Astigmatism (Johnson &
Johnson)
BSD
0.00
Air Optix for Astigmatism
(Alcon)
Modified
peri ballast
0.52
Biofinity toric (CooperVision)
Peri ballast
0.77
Avaira toric (CooperVision)
Peri ballast
0.75
Clariti toric (Sauflon)
Prism ballast
0.85
PureVision2 For Astigmatism
(Bausch + Lomb)
Modified
peri ballast
0.96
PureVision Toric (Bausch + Lomb)
Prism ballast
1.15
SofLens Toric (Bausch + Lomb)
Prism ballast
0.81
The amount of vertical prism varies considerably in
commercially available toric soft contact lenses, ranging
from 0.00 in BSD and dual-thin-zone lenses to more than
1.15 BSD in prism-ballasted lenses.
ply compensate in order to achieve fusion. Clinical studies
have not been conducted to fully characterize the clinical
effects of differences in base-down prism among different
contact lenses. The effects are likely to be subtle and may
be most noticeable with prolonged visual effort, such as at
the end of a long day at the computer.
The greater the disparity in the amount of vertical
prism and the lower an individual’s fusional reserves, the
more likely he or she is to experience the optical effects
of unilateral vertical prism. An artificially induced vertical
phoria, possibly caused by prism, is one factor to consider
when fitting toric contact lenses for monocular astigmats
or those requiring a mix of toric soft contact lens designs.
Future clinical work could further explain the impact of
vertical prism and other optical qualities of toric contact
lens designs. n
Giovanna E. Olivares, OD, is director, global
platform R&D, for Johnson & Johnson Vision Care.
Dr. Olivares may be reached at
(904) 443-3488; [email protected].
1. Hawke R, Osborn Lorenz K, Olivares G, et al. Vertical prism in toric soft contact lenses. Poster presented at: American
Academy of Optometry; October 25, 2013; Seattle, WA.
2. Young G, Sully A, Hunt C. Prevalence of astigmatism in relation to soft contact lens fitting. Eye Contact Lens. 2011;37:20-25.
3. Jackson DN, Bedell HE. Vertical heterophoria and susceptibility to visually induced motion sickness. Strabismus.
2012;20(1):17-23.
4. Bennett ES, Weissman BA. Clinical Contact Lens Practice. 2005, 496-7.
May/JUne 2014 Advanced ocular care 43