An "Essential" Approach to Multifocal Fitting
Two practitioners offers guidelines for how
to succeed with fitting a particular family of multifocal lenses.
By Robert L. Davis, OD, FAAO, and Brad Cogswell,
affirmation that "one bifocal/multifocal design cannot fit every multifocal candidate"
has appeared in almost every article related to multifocal fitting over the last
30 years. However, I believe the Essential family of multifocal contact lens designs
from Blanchard Contact Lens, Inc. has come closer than any lens design to dispelling
The basic lens design mixes back aspheric
technology and annular configurations with a combination of many past multifocal
philosophies. The Essential family offers the choice of the Essential GP multifocal
lens and Essential Xtra, with the option of adding a front-surface annular component
called a concentric S-form add (CSA) enhancement to either design.
To describe and understand the
fitting approach is no easy task. First we'll describe the basic fitting design.
Then we'll look at the construction differences between the Essential and Essential
Xtra multifocal designs to appreciate the different patient opportunities for each
lens. Finally, a description of the Essential CSA and Essential Xtra CSA lens designs
will add to your knowledge and provide an additional method for satisfying patients'
visual expectations and meeting their optical requirements.
General Design and Fitting
The Essential GP Multifocal can be a translating
or simultaneous back-aspheric lens with low eccentricity to help prevent corneal
molding. "S-Form Technology," a proprietary manufacturing process, produces a smooth
flattening effect, creating a greater posterior power gradient and a greater near
point power effect. This differs from other aspheric lens designs, which incorporate
many curves cut into the lens to create the aspheric affect. Such lenses look like
a notched Fresnel prism under magnification and cause additional distortion and
contrast degradation as light refracts through the lens construction.
Determining Lens Diameter Fit
the Essential lens designs using an alignment fitting philosophy. The total lens
diameter selection follows the small/steep and large/flat paradigm. For lenses with
base curves of 7.3mm and steeper, select a 9.0mm diameter. The 9.5mm diameter lens
is preferred for patients who need a base curve of 7.4mm and flatter. Select a larger
lens diameter for better centration or to increase sagittal depth.
Base Curve Selection
Configure the initial base curve by the amount of corneal cylinder and the flat
corneal meridian. Fit the lens 0.50D steeper than flat K for patients who have less
than –0.75D of corneal cylinder. For patients who have –0.75D to –1.75D
of corneal cylinder, fit the lens 1.00D steeper than flat K. For patients who have
–1.87D or greater of corneal cylinder, make the initial lens 1.50D steeper
than the flat meridian.
problems with this lens design directly result from the lens not positioning appropriately.
A high-riding lens provides good near vision, but may be over-minused in its distance
prescription. To rectify this problem and position the lens correctly, use a steeper
base curve or larger diameter. A lens that rides low will create good distance vision
but poor near vision. To solve this lens-positioning problem, you must change the
fitting parameters. The lens will position more superior if you flatten the base
curve using a minus carrier or increase the overall size of the lens.
A lens that decenters nasally will
generate poor distance and near vision. Increasing the lens diameter and/or adding
a lens carrier to the design will change the lens configuration to facilitate proper
positioning. Let the lens position dictate the fitting parameters of the lens.
Fitting Approaches You
can fit the Essential and Essential Xtra lenses with either a lid-attachment fitting
approach, providing a translation style by fitting slightly flatter (manufacturer
recommended), or an intrapalpebral design philosophy, providing a simultaneous methodology
by fitting slightly steeper. The lid-attached fitting style will tuck underneath
the upper lid and move with the blink. The intrapalpebral fitting design will center
perfectly and translate very little with the blink.
The Essential and Essential Xtra lenses are available in three Series (I, II and
III) of aspheric lens designs, each having a greater amount of add correction contained
within a usable optic zone on the posterior surface. Each Essential/Essential Xtra
lens series has a lower sagittal depth or greater eccentricity, which in turn will
create a higher multifocal add effect. Use Series I for early to intermediate presbyopes
to incorporate up to a +1.25D aspheric power gradient for near and intermediate
vision correction. Series I features the largest portion of the optic zone intended
primarily for distance vision.
The Series II add zone begins closer
to the center of the lens, creating more add power as a result of increased eccentricity.
The Series II power gradient generates +1.50D to +2.00D for near and intermediate
The Series III add zone begins even
closer to the center of the lens and has the greatest amount of add correction,
providing +2.25D to +2.50D of add power. It has the smallest amount of distance
area within the optic zone intended for distance vision. The spherical central curve
provides the distance correction. As the add increases, the near portion of the
lens encroaches further into the central distance area. This enables a more precise
near acuity, but may adversely affect distance vision.
CSA Enhancement When
a patient needs additional add power because of pupil size, near power requirement
or lens positioning, you can include a CSA design onto the Essential or Essential
Xtra lens design. The Essential CSA design cuts a spherical annular near power curve
on the anterior surface of the lens (Figure 1) outside of the distance/intermediate
optics generated on the posterior surface. It provides additional add power in +0.50D
increments up to +2.00D, raising the combined posterior and anterior add power to
+4.00D. You can order the diameter of the annular near power curve in 0.3mm increments
depending on the patient's ocular and visual requirements. The standard distance/intermediate
zone diameter of the annular near power curve is 4.3mm, but you can reduce it to
4.0mm if the patient doesn't translate into the supplemental annular add zone or
increase it to 4.6mm if the standard annular add zone encroaches upon distance or
Add Power from Center of Lens
Lens Design Specifics
Essential Now that you know the
basics, we'll differentiate between the different lens designs. The original Essential
GP Aspheric Multifocal consists of three Series of individual lens designs that
differ in eccentricity and resulting distance optic zone areas. As the Series increases,
the multifocal addition increases and the useable distance area on the lens becomes
smaller (Table 1). The estimated distance zone size for Series I is 2.5mm, for Series
II 2.0mm and for Series III 1.7mm (according to the manufacturer), assuming that
approximately +0.25D doesn't affect the distance zone in this aspheric configuration.
You can modify the quality of distance acuity by selecting the Series that provides
optimal near vision. You need to find a balance between the distance and near zones
within the optic zone to achieve an optimal fit and symptom-free patients.
Essential Xtra The Essential
Xtra GP Aspheric Multifocal performs the same as the original Essential design by
changing the eccentricity of the contact lens in relationship to the cornea. The
increased rate of flattening of the contact lens relative to the corneal curvature
results in the "add" power. Therefore, by increasing the eccentricity of the lens,
you can increase the "add" power. This is how the three Series are differentiated
in both the Essential and Essential Xtra lens designs. The result is more "add"
power closer to the center of the lens in Series III compared to II and in Series
II compared to Series I. The eccentricity of the posterior aspheric power gradient
creates the configuration differences between the Essential and the Essential Xtra.
The Xtra's posterior power curve begins farther away from the center of the lens
and changes more rapidly compared with the Essential's design. This results in a
larger area in the center of the lens for distance vision that is unhindered by
add power. The more rapid change in the aspheric geometry allows the Xtra to develop
enough "add" power within a smaller radius of the lens (Table 2, estimated values
according to the manufacturer). The Essential Xtra lens has greater eccentricity
with a larger distance optical zone to provide better distance vision with less
distortion from the posterior aspheric curve as well as more power in the near zone
for Series III. It's constructed with less intermediate area because of the rapid
power delivered by the aspheric curve, although this doesn't subjectively compromise
Xtra Add Power from Center of Lens
The Essential lens design has many parameter choices,
so taking the first step may feel overwhelming. Understanding the lens design differences
is key in intuitively manipulating parameters according to patients' visual requirements.
Once you understand the differences among the Essential family of lenses and appreciate
their nuances, choosing lens parameters becomes a simple task.
Pupil size and visual demands are factors
that you must consider. Regarding pupil size, patients who have larger pupils will
generally do better with the Essential Xtra because the posterior aspheric power
curve begins further from the center of the lens. Patients who have large pupils
may also do better with a lower Series lens as the near power starts further away
from the center of the lens. For example, a Series I lens has the same near power
at 3mm from the center as a Series III has at 2mm from the center. The Series I
would ultimately limit the total near power available, but incorporating a CSA design
can increase the near power to the desired amount.
You need to explore visual demands
with each patient to determine which lens to select first. It's important to reveal
what the patient does most during the day (computer use, sewing, sports, etc.) so
you can give priority to these activities from a visual standpoint. A patient may
tell you that she sews, but her real concern is being able to see the flight of
a golf ball. This information may lead you to the optimal lens design and Series
The Essential Xtra might be the logical
choice for a patient whose priority is distance vision. Its larger central distance
area provides a less interrupted view from the near power. You could use a Series
I lens in an early presbyope or in someone who has few near demands. A patient who
has more near demands would benefit from a Series II lens design.
1. The CSA design provides up to +2.00D more add power when needed.
a patient's visual priority is with intermediate vision, then the Essential would
theoretically be the lens of choice because its more gradual eccentricity results
in a larger intermediate area. The most advantageous Series for patients who have
high intermediate demands depends on the stage of presbyopia and other visual tasks
that they regularly perform.
In patients who have high near
demands, the pupil size may direct your choice. Even though the near power begins
further away from the center of the lens in the Xtra design, its near vision can
be equal to or greater than the Essential because of the steeper eccentricity. But,
the near power in the Xtra may not be as effective in patients who have small pupils
or if the lens isn't translating properly. These patients may be unable to utilize
the near power effectively. Consider a Series III for most of these patients unless
they're early presbyopes or their distance vision is overly compromised; in such
cases use Series II Essential or Essential Xtra CSA.
Consider a CSA enhancement to either
design for patients who don't obtain adequate near vision from just the posterior
aspheric curvature. This typically occurs because of small pupils or poor lens translation.
By placing add power on the front surface of the lens, you can alleviate some of
the limitations in the aspheric lens design. Some patients cognitively prefer an
annular design modality. A Series I Essential or Essential Xtra CSA lens design
will solve these problematic lens adaptation issues.
The Essential Multifocal family of
lenses offers a complete distance and near range of powers to satisfy all visual
acuity requirements. Manipulating the Essential parameter options requires a learning
curve to satisfy the majority of presbyopic visual requirements. Understanding the
parameter alternatives available with the Essential Multifocal will enable fitting
changes during follow-up appointments to meet patients' expectations. This along
with listening to and understanding patients' needs will facilitate successful presbyopic
patient lens dispensing.
Dr. Davis has an eyecare specialty practice
outside Chicago. He is a diplomate of the Cornea and Contact Lens Section of the
AAO and a past chair of the Cornea and Contact Lens Section of the AOA. He is also
an inductee to the National Academy Practice in Optometry.
Dr. Cogswell is in practice in Oak Lawn, IL concentrating on problematic
contact lens cases. He has been involved with the research and development side
of vision care and has limited his interest to specialty contact lens
Contact Lens Spectrum, Issue: November 2005