Understanding and Working With Toric RGP Lenses
By Christopher Miller, ABOC, NCLEC
Release Date: April, 2013
Expiration Date: September 28, 2017
completion of this program the participant should be able to:
- Understand the features and differences of the three standard RGP toric lens designs.
- Know when to use each design.
- Learn how to use a radiuscope and lensometer to identify each design.
Christopher Miller started at SOLA Optical USA in 1977 as the Quality Control Department lead manufacturing CR-39 lenses. After several optical courses, Miller became an optician at a retail chain, passed both the ABO and NCLE exams, managed optical locations and then opened his own optical boutique. In the Caribbean and South America he helped several doctors open practices and ran the government of Guyana's optical business. In 2002, he returned to California and now works at a busy HMO as a contact lens fitter.
This course is approved for one (1) hour of CE credit by the National Contact lens Examiners (NCLE). Course CTWJM546-2.
RGP toric lenses are indicated
for high amounts of prescription astigmatism, high corneal
toricity or when a smaller diameter or other parameter is needed. They are
not readily available in a soft lens design
such as a steeper or flatter base curve.
However, identifying when to use each
of the different RGP toric designs can be
difficult, especially for the new contact
lens fitter. This course will clarify the different lens designs, suggest when a particular design is indicated, list the order of
frequency of usage and ways to identify
each type of lens.
DESIGNING AND ORDERING LENSES
Remember to always have the prescription in
minus cylinder form and to correct for vertex
distances for both sphere and cylinder powers for powers above ± 4.50 power. Things
can get tricky when designing for a monocular fit or when over-refracting so write calculations down a step at a time and always
double check your work before ordering.
In the following examples, for simplicity,
we'll use a 9.2 mm diameter for all lens
designs. However, as you know in the real
world we must carefully consider the size of
the cornea, previous lens size if any, fissure
opening, lid attachment, etc., and adjust
accordingly the diameter, sagittal depth or
base curve (BC) when needed.
- Review the prescription. Put in minus cylinder form and correct for vertex in both
- Review the keratometer (K) readings.
- Compare the cylinder power of the prescription and the corneal toricity from the
Is there less than 3.00D of corneal toricity? If less than 3.00D, can a spherical design
When over 3.00D corneal toricity, determine
the location; is it on the cornea or is it residual?
You will know it is on the cornea if the value of
the keratometer readings is close to being equal
to the prescription cylinder correction. Subtract K from the prescription cylinder. If the
cylinder correction on the prescription is equal
to the difference in the K readings, it then follows that the base curves, which are on the
concave or backside of the lens, will need two
curves to match the shape of the cornea. If the
cornea is spherical or only has a small amount
of toricity, then it is considered residual astigmatism, and the correction (two curves) will be
placed on the front of the lens.
We will begin by looking at spherical/single cut designs. This is the lens designed and
dispensed the most, followed by the three
different toric designs in the order of frequency of usage.
BASIC RGP DESIGNS IN THE ORDER
OF FREQUENCY OF USAGE
Sphere/single cut: The spherical or single
cut lens design is for spherical and sphero-cylinder prescriptions with up to 3.00D of
corneal toricity. This contact lens design
has only a spherical power and one base
curve. The lens is typically designed steeper
than the flattest corneal curve to create
enough of a tear lens layer to correct up to 3.00D of prescription cylinder power.
Request a chart from your lab that shows
the BC and diameter to order as the sagittal
depth changes with each diameter (see
sample chart). Fitters certainly become familiar with these over time, making
designing lenses an easy task.
- Determine the amount of corneal cylinder by finding the difference between
both K readings.
- Find the flattest K reading in the left
- Follow across the horizontal column to
the corneal cylinder listed at the top of
- Base curve to use is listed in the column
where the two intersect.
- For final lens power, add minus 0.25D
sphere power for every 0.25D the base
curve is steepened from the flattest K.
- Use the base curve listed.
- If the cornea cylinder exceeds 3.00D,
consider a bitorc design.
- Use this chart as a guide for selecting the
first base curve to try. Analyze lens
movement, centration and fluorescein
patterns to determine the need for a different fitting lens.
Prescription: OD -2.50 sph
K readings: OD 44.25/44.50
Lens design considerations
(from Fig. 1 below):
OD Fit 9.2 Dia. +0.25 flatter than flattest K
OS Fit 9.2 Dia. -0.25 steeper than flattest K
OD 44.00, 9.2, -2.25D sphere
OS 44.50, 9.2, -2.50D sphere
Bitoric: Use this lens design when there is
over 3.00D of prescription cylinder and
most if not all of it is from corneal toricity.
Bitorics aren't hard to design when you
think of them as two spherical designs on
one lens; +0.25 flatter for the flatter K and
+0.75 flatter for the steeper K.
EXAMPLE (RIGHT LENS ONLY)
Prescription: OD -3.50 -4.50 x180
K Readings: OD 43.00x180/47.50@090
4.50 D Corneal Toricity
Vertexed power @12 mm:
-3.50/-8.00 = -3.50/-7.25
OD -3.25/42.75 -6.50/46.75 9.2 dia. Blue
Front Toric: This lens design is used when
there is over 1.25D of prescription cylinder correction with a spherical or near
spherical cornea. This lens is ordered with
a cylinder power and axis. To prevent the
lens from rotating, the lens is ordered
with a prism base down to act as weighted
ballast or stabilizer. A prism of usually 1.5Δ base down will keep the lens from
rotating. The lens is also dotted at 6
o'clock for diagnostic purposes when dispensing and for evaluations at later visits.
The lab will do the dotting of the lens and
should be verified when received from the
lab. Do this before dispensing. Use the
lensometer positioning the reticle hairline
at 90 degrees and move the center of the
target to the point 1.5Δ base down. Confirm the prescription. Again, confirm that
the dot is at 6 o'clock when on the patient.
Wait approximately 15 minutes for the
lens to settle and the tear layer to stabilize.
Remember, left-add, right-subtract (LARS)
since this lens should be analyzed similar
to a soft lens. If the dot isn't at 6 o'clock it
needs to be re-ordered with corrected cylinder axis or with more prism ballast, or a
steeper/tighter fit if it rotates too much
Prescription: OD -2.50-2.50x180
K Readings: OD 44.00/44.00X090
Lens design considerations: Order the
prescription sphere, cylinder power and
axis from the doctor's prescription.
Include with 1.5Δ ballast and dot
@ 6 o'clock.
OD 44.00BC, 9.2 dia., -2.50 -2.50 x 180,
1.5Δ prism ballast, dot @6:00 Blue
Back Toric: This lens design is rarer than
the others and is used when there is a highly
toric cornea (3.00D or more) with a spherical
Prescription: OD +4.50
K Readings: OD 41.50/46.50 @ 090
Lens design considerations: Fit to the
corneal curves (on K) and order spherical
power from the prescription considering
any vertex change effects.
Power vertex corrected:
+4.50 at 12 mm = +4.75 at 0 mm vertex
OD 41.50/46.50 9.2 +4.75 Blue
AND FITTING CHANGES
Once the patient has their initial lenses
and has been wearing them for the initial
week, an evaluation of vision and fit is
required. Changing a power or the fit of a
toric lens isn't difficult, as long as you
keep things straight in your mind what it
is you are correcting and why. We'll use
the example for a bitoric from above to
The lens need is strong by 0.50D.
First Lens: Bitoric, -3.25/42.75 -6.50/46.75
9.2 dia. Blue
• Over-refraction, +0.50 sphere
• Add the power to both meridians
using the same previous curves.
Remake Order: -2.75/42.75 -6.00/46.75
9.2 dia. Blue
The bitoric lens looks too loose so it needs
to be steepened to "tighten" its fit. If
-0.50D steeper works to reduce excessive
movement and the power was excellent,
we'll want the same corrective power as
before. Therefore, we will need to compensate for the change in the tear lens to keep
the same power. Keep in mind the rule
Steeper Add Minus (SAM) and Flatter Add
Plus (FAP). Look closely and notice that
we have to change both curves and compensate for the tear lens for both curves.
First Lens: Bitoric -3.25/42.75 -6.50/46.75
9.2 dia. Blue
• Fit -0.50D steeper to tighten the fit.
Remake Order: -3.75/43.25 -7.00/47.50
Using the lens from the first example let's
say that we need to both steepen the fit by
-0.50 and change the power by +0.50
sphere. Do each step at a time carefully.
• -3.25/42.75 -6.50/46.75, 9.2, dia. Blue
• *Steepen fit -0.50D
• -3.75/43.25 -7.00/47.50 9.2 Blue
(Steepening the fit adds +0.50D to the
tear lens so change the lens power by
• Power check shows that an over-refraction of +0.50D is needed also
Remake Order: -3.25/43.25 -6.50/47.50,
Lens from first example needs a spherical-cylinder over-refraction.
First Lens: -3.25/42.75 -6.50/46.75
9.2 dia. Blue
• Over-refraction: -0.50-0.75x180.
• Add the first power to the first set and
the second to the second keeping the
same base curves for the same fit.
Remake Order: -3.75/42.75 -7.25/46.75
For other types of toric lenses the process
is similar, keeping in mind that these lenses
are rarely seen unless you work in a practice
that specializes in contact lenses. If the
over-refraction is a cylinder at an axis other
than the original, this creates a resultant
cylinder problem. There is a good tool at
opticampus.com for resolving two cylinders
at different axes.
IDENTIFYING TORIC LENSES
When a new patient visits and wears RGP
contacts, verify if they are toric. Using a
lensometer, check the power. Then with a
radiuscope check curvature.
Power: Is it spherical or cylindrical (two
powers) using a lensometer? Don't worry
about axis alignment unless the lens is dotted. This could indicate that the lens is a
right lens unless the lens is ballasted, and
the dot is at 6 o'clock when it's on the patient.
Curvature: Radiuscope readings would
show two distinctly different curves. Make
sure that the lines are sharp. If the lens is
toric the values for the two curves will be
distinct. If there are two values but blurred
and indistinct, the lens is most likely
warped and not toric. Confirm the location
of the toricity on the lens. Be aware that the
radiuscope only reads the back curvature of
the lens (base curve) and the lensometer
will tell you whether there are two powers.
Sphere/Single cut = Spherical Power/1 Curve
Bitoric = 2 Powers/2 Curves
Front Toric = 2 Powers/1 Curve.
Back Toric = 1 Power/2 Curves
For front toric lenses, the lens may or may
not have a dot on it as they come off when
polishing or after a year or two from daily
cleaning. You must know the cylinder axis
from the prescription to verify and re-dot
the lens. This is very tricky without a lens
holder of some sort. Fortunately, these lenses are rare but the author has had to verify
and do this several times over the years, and
the sense of accomplishment when you
have done your first one will be one of the
signs that you have reached an advanced
stage of fitting expertise.
A clear understanding of the different RGP
lens designs and a close evaluation of both
the prescription and keratomer readings
will help to quickly identify which lens
design to use for each situation. Careful
changes to the lenses from over-refractions
and curve changes will help confirm a