Download THE ART OF BIFOCAL CONTACT LENS FITTING

THE ART OF BIFOCAL CONTACT LENS FITTING
Eef van der Worp BOptom PhD FAAO
Introduction
Bifocal contact lenses have high potential in contact lens practice. The contact
lens wearer who turns presbyopic is looking for alternatives to glasses. Wearing lenses
are a step back from what they were used to and would make contact lenses more or less
redundant. It is a misconception that bifocal lenses do not work and it is also a
misconception that they are difficult to fit. However, it is challenging and sometimes
confusing to manage all the presbyopic correction options available today and the secret
to success is to find the right design for the right patient. Many articles and presentations
on this subject provide the practitioner with details about all presbyopic lens options that
are available, with the risk of being unable to see the trees for the forest. This
presentation aims on presenting an overview of basic principles in bifocal lens and
providing a simple and useful flow-chart to help the contact lens practitioner in making
this crucial decision and to make fitting of bifocals easier.
STEP 1: BIFOCALS OR NOT?
About half of the vision care population is presbyopic so there is hardly any
discussion on the need for a presbyopic solution. Many bifocal wearers in fact report how
satisfied they are to be the only ones in community that are able to read without glasses.
The constant need for reading glasses, frequent application of the device and the risk of
losing them is a serious consideration for many.
Monovision is an option that is often regarded as a first option. And in fact: for
beginning presbyopes this can be a good first step. Simply adding +0.50D to one
(usually, but not necessarily, the dominant) eye will aid the presbyope in doing near
work. However, practitioners have to make sure they are aware of this ‘system’, the
progressive nature of presbyopia and the need for other solutions in the (near) future. At
some point in time, monovision usually will not provide satisfactory visual results. There
seems little doubt amongst practitioners and in the literature about the superiority of
bifocal lens designs in both hydrogel and GP lens wear over monovision.
STEP 2: GP vs HYDROGEL
Even more so than in single vision contact lenses, there are many variables that
need to be considered when deciding on GP lenses over hydrogels. However, much more
than in the single vision fitting, the contact lens practitioner is in charge of the final
decision.
Variables to consider are:
HYDROGEL
GP
High demand
-
+
Contrast
-
+
Complex cornea
-
+
Tear film
+/-
+/-
Hypoxia
-
+
Extended wear
-
+
Dusty
+
-
(Near) Emmetrope
+
-
COST
-
+
VISION
PHYSIOLOGY
COMFORT
Taking all items into consideration, it can be concluded that hydrogel lenses do
not provide the same optimal visual quality that GP lenses will give, and also are less
ideal physiologically. In general: if superior vision is required, GP lenses are advised as a
first lens of choice.
STEP 1
STEP 2
HYDROGEL
GAS PERMEABLE
TRANSLATING
STEP 4
SIMULTANEOUS
ASPHERIC
CONCENTRIC
SEGMENT
TRIANGLE
TRIFOCAL
CONCENTRIC
STEP 5
STEP 3
BIFOCAL
Manufacturer
Manucaturer
Manufacturer
Manufacturer
Manufacturer
Manufactuerer
STEP 3: SIMULTANEOUS OR TRANSLATING?
Following step 2, the focus will first be on GP lenses, covering step 3, 4 and 5 of
the flow chart, after which hydrogel lenses will be discussed in the same matter and
following the same steps.
Typically simultaneous systems are truly bifocal, while translating systems are
considered bifocal. However, a number of translating designs offer bifocal vision as well.
When referring to the term bifocal, in fact bi- or bifocal is meant, unless specifically
stated.
RIGID GAS PERMEABLE LENSES
For GP lenses, the most critical question that can and should be answered is
whether a simultaneous or alternating design is desired by the patient. This can be
decided upon based on several variables, but visual requirement is by far the number one.
The basic decision is fairly simple: if good and stable vision is required for far and or for
near, a translating system is the first method of choice. If flexible and good intermediate
vision is required, simultaneous is the number one choice.
Computer
Work
Sport
Free time
Age
Contrast sensitivity
Pupils
Ac depth
Comfort
No previous GP wear
Movement
Topography
Astigmatism
Eccenicity cornea
High myopia
Hypoxia
Tear film quality
Movement
Lid position (rotation)
Eyelid shape
Eyelid location
Aperture size
Head position
Cost
Fitting
simultaneous
translating
35% >
< 35%
Mid
Far-near
Mid
Far-near
Mid
Far-near
Low addition
High addition
OK
Good
5mm<
5mm>
Small-normal
Deep
Normal
Reduced
Best
Less ideal
Limited movement
Must move
Centration essential
Centration desired
Limited
Possible
low e-value only (BS) Low and high e-value
Possible
Limited
Good
Decreased
Less important
More important
Not too much
Must move
No influence
Large influence
No influence
Large influence
No influence
Around limbus
No influence
Large influence
Any
Only on down gaze
Moderate
High
Less challenging
More challenging
Bifocal lens fitting often relies on flexible and creative lens fitting, including
combining different lenses or methods. Modified monovision is for instance an excellent
way of expanding your options. However, one should be reserved to combine
simultaneous and translating systems: essential differences in optics could result in
optical confusion for the patient.
STEP 4: DIFFERENT DESIGNS
When the most important decision has been made in step three, it is time to finetune this choice. Within the system that has been chosen there are a number of
possibilities.
Simultaneous
The term ‘simultaneous vision’ is a bit misleading. Two (or more) images are
projected on the retina simultaneously (not necessary all clear), but only one is clear to
the patient. Simultaneous lenses are typically aspheric, concentric or diffractive. The first
one is by far the most popular, but all systems will be covered in this presentation briefly.
Simultaneous lenses are very user friendly, easy to fit, relatively inexpensive and
popular: success rates up to 75% are not uncommon. In general, a simultaneous lens is
the ideal lens to start with: both for the beginning presbyope and for the beginning
practitioner. It should be fitted when critical vision is not required. For higher near
additions simultaneous designs may have their limitations and translating designs should
be considered.
ASPHERIC
Aspheric lenses can be subdivided in two main groups: front and back surface
geometry. These have in common that the addition power increases towards the periphery
and this automatically means that all aspheric GP bifocals are centre distance (CD)
designs. They are also true bifocals, not bifocals. Be aware that the maximum near
addition values provided by the manufacturer can not always be reached.
Back surface aspheric
The back surface of this lens has a prolate shape, meaning it flattens towards the
periphery and thus adding plus power. This peripheral flattening should be compensated
by fitting the lens significantly steep, otherwise the lens would be far too flat. But high
aspheric lenses on relatively low eccentricity corneas (fitted 2D, 3D or even 4D steeper
than k following the manufacturers’ guidelines) could lead to severe corneal distortion.
Fortunately, newer lens designs only need to be fitted 1D steeper than k.
Regular topography measurements are required with back surface aspheric bifocal
lenses and the practitioner should be aware of any signs of spectacle blur. In the authors’
opinion, back surface aspheric bifocals should be avoided when alternatives are available
that do not have the unwanted and uncontrolled ortho-K effect on the cornea.
Front aspheric
Front aspheric lenses do not to rely on the asphericity of the cornea. The shape of
the cornea is respected as in any other GP lens. The front surface progressively becomes
steeper towards the periphery (oblate shape), creating an increasingly progressive reading
addition.
CONCENTRIC
Also belonging to the simultaneous group but less common in everyday practice
is the concentric lens design, often called ‘annular’. It uses a far zone and a distance zone
and can be created both centre near (CN) and CD.
To confuse things a bit: there are also concentric translating designs. In fact the
concentric simultaneous designs might benefit from translating as well. But the aim with
these simultaneous lenses is to fit them well centered on the cornea, not relying on the
lower eyelid. Translating bifocals on the other hand aim to move excessively, rest on the
lower eyelid and should translate. Down gaze is essential with translating lenses, which is
less of a necessity in simultaneous concentric lenses.
DIFFRACTIVE
Diffractive lenses bend light that normally travels in a straight line and breaks up
rays of light into dark and light bands or into the colors of the spectrum. The closer the
spacing between the concentric circles, the higher the addition power. This construction
has been used on both soft and GP lenses in the past and on IOL’s, all with limited
success. Disadvantages are the significant loss of contrast and the limited optical
performance (especially with higher additions). Another disadvantage is the built up of
debris in the prism rings, which are located at the back surface of the lens. The major
positive effect of diffractive bifocals is that they are pupil independent unlike all other
designs.
Translating
SEGMENT DESIGN
Different segmented bifocal designs are available all with slightly different
characteristics. They have in common that their visual performance is excellent. The long
line/ executive segment bifocal is probably the most widely used lens, followed by
crescent segments and D-shaped segments.
The major advantage that GP lenses have over most hydrogel contact lenses, intra
ocular lenses and other refractive surgery procedures is that they are capable of
alternating (often used as a synonym for translating) and use different optics for different
tasks, very similar to spectacle glasses. The goal is to create a situation where upon down
gaze most of the reading section (not necessarily all of it) is in front of the pupil while in
primary gaze the pupil is minimally bothered by the near segment. They are particular
suitable for presbyopes with reading additions over +1.50D and/or those who require
excellent near vision performance in general. Reading additions up to +3.50 are promised
and with this lens design these are indeed feasible. They are fairly pupil independent, as
long as the overall diameter is increased accordingly.
All lenses except some of the concentric designs require stabilization, usually
with prism ballast. Bear in mind that prism ballast works according to the watermelon
seed principle, not by gravity. This means the upper eyelids plays a crucial role in
stabilizing these lenses. They are not as difficult to fit as often is thought, however: it is
fairly difficult to predict how a lens will function on an eye (a little bit easier in existing
GP lens wearers). Evaluation of the on-eye rotation is essential. Crescent shaped
segments designs can reduce rotation problems.
TRIANGLE
The Triangle shaped translating bifocal is a monocentric lens design with the
optical zones cut on the front of the lens that was developed in the Netherlands. Its design
incorporates a spherical distance zone, spherical near zone and a triangle shaped aspheric
intermediate zone.
In contrast with straight top translating lenses, less translating is necessary, so
with less movement the desired effect will be reached (28% less movement according to
the manufacturer). This suggests that the amount of prism can be reduced and relatively
large diameters can be used. It is less pupil diameter dependent than straight top bifocals.
TRIFOCAL
For patients who desire intermediate vision (such as computer workers) together
high near and far demands, this design or the triangle shaped lens are the options to
consider. True bifocals are just what they are: bi-focals. Trifocal designs offer the option
to deal with the intermediate distance. It should be noted that this works only with small
pupils, since the intermediate zone is usually only 1 mm large, and good centration is
more critical than with normal bifocals.
Some of the true executive/ long line bifocal designs also have an intermediate
zone at the transition from distance to near. However, this should be regarded as a
transition zone indeed, not providing a tri- or bifocal effect.
CONCENTRIC
As mentioned, concentric or annular design lenses are a bit confusing, since they
can work as either simultaneous or translating. The central zone of a translating
concentric lens is larger than in a simultaneous concentric lens and they should also move
easily over the cornea and meet the under eyelid to translate.
In translating designs the centre part is always for distance, the periphery for
reading (remember, concentric simultaneous lenses can be both CN and CD). Some
companies allow you to order the size of the central zone according to your desires;
others have this zone diameter related to the height of the addition.
STEP 5: MANUFACTURER
STEP 1
Once a lens concept and design have been assigned to a patient, a manufacturer
should be chosen. The amount of GP lens designs available worldwide is almost
everlasting, and it is impossible to cover all available lenses. However, all lens designs
available can be classified according to the presented schedule, making it easy for the
practitioner to categorize all bifocal lenses available worldwide. The GPLI in the USA
has a website (www.gpli.org) where all available lenses are listed and one can search on
manufacturer, lens design or on brand to find what you need.
HYDROGEL
TRANSLATING
STEP 4
SIMULTANEOUS
GAS PERMEABLE
ASPHERIC
CONCENTRIC
SEGMENT
STEP 5
STEP 3
STEP 2
BIFOCAL
Manufacturer
Manufacturer
Manufacturer
HYDROGEL
The same questions as with GP lenses can be applied to hydrogel lenses: is a
simultaneous or a translating design desired for this patient? The options with hydrogels
compared to GP lenses are however very limited. There are two translating hydrogel
lenses available worldwide. Although they can be helpful for some patients that do not
get enough reading addition with simultaneous hydrogel lenses, the visual outcome is not
comparable to GP lens designs.
For simultaneous lenses in general it is a rule that they should be stable on
an eye during lens wear in order to work well. From that perspective, hydrogel lenses are
very suitable for simultaneous lens designs. Different types of simultaneous hydrogel lens
designs will be discussed.
Within the group simultaneous hydrogel group there are a number of options.
Basically they can be subdivided into aspheric and concentric designs as with GP’s. But
even within these two groups essential differences exist. In this section, disposable lens
designs on the market will be mentioned as examples. Tear film deposits and hypoxic
conditions is even more important in presbyopic lens fitting than normally and frequent
replacement systems are desired and should be considered as a first choice.
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