FITTING SOFT TORICS
Determining
Power for Disposable Soft Torics
Use
these tips to maximize your success with soft toric lens trial fitting.
By Richard G. Lindsay, BScOptom, MBA, FAAO
(DipCL), FVCO, FCLSA
The
majority of prescribed soft toric contact lenses are disposable, with a recent survey
revealing that less than 1 percent of new soft toric lens fits involved unplanned
lens replacement. The majority of disposable lenses have monthly, two-week or daily
replacement intervals, and disposable soft toric lenses are now available in these
three modalities. One advantage associated with disposable soft toric lenses is
that prospective patients can undergo a lens-wearing trial using a disposable soft
toric lens with the appropriate back vertex power.
Work with What You Have
Virtually all disposable soft toric designs come
in a stock range of lenses encompassing a certain number of cylindrical powers (such
as –0.75D, –1.25D and –1.75D), a set choice of spherical powers
(for example, from +6.00D to –9.00D) and cylinder axes in 5-degree or 10-degree
steps — usually the latter — most often covering the complete spectrum
from 0 degrees to 180 degrees. We're usually limited in our choice of back optic
zone radius (BOZR) and total lens diameter (TD) for these lenses also. Therefore,
assuming that you've chosen to use a particular type of disposable soft toric lens,
your main decision in fitting and prescribing these lenses generally relates to
the specification of back vertex power.
In specifying a back vertex power for the
disposable soft toric lens that you want to trial on a patient's eye, the limited
number of cylindrical powers and axes does place some restrictions on you. However,
you still need to decide between certain cylinder powers and axes (in addition to
determining the spherical component) when selecting this first (diagnostic) lens.
I'll explain what factors you need to consider to arrive at the best prescription.
|
|
|
Figure 1. The effect of lid action on lens
rotation for a toric soft lens with the prescription -2.00/-1.25 x 45 being
worn on the left eye. The upper lid will initially act on the thicker (135°)
meridian, thereby causing the lens to rotate temporally.
|
Allow for Vertex Distance
While we continually learned that we must convert
from spectacle refraction to ocular refraction (at the corneal plane) when determining
contact lens back vertex power, we often overlook this important principle in contact
lens practice.
The need to take the vertex distance into
account (for powers greater than ±4.00D) is especially important when correcting
astigmatism with soft toric lenses. For myopes, the degree of astigmatism in the
ocular refraction will always be less than the spectacle refraction. For hyperopes,
the ocular astigmatism will always be greater than the spectacle astigmatism.
Consider a patient who has a spectacle
correction of –5.00 –1.50 x10 worn at a vertex distance of 12mm. The
ocular refraction is therefore –4.75 –1.25 x10, so not taking the vertex
distance into account when performing a contact lens fitting on this patient would
result in an over-correction of the ocular cylinder. Conversely, for a hyperopic
patient who has a spectacle correction of +5.50 –1.00 x80 worn at a vertex
distance of 13mm, there is potential for under-correction of the ocular astigmatism
when prescribing a soft toric contact lens if you don't make appropriate allowance
for the vertex distance, given that the ocular refraction in this case is +6.00
–1.25 x80.
Err on Under-correcting the Cylinder Power
As I mentioned previously, virtually all
disposable soft toric lenses are available in a stock range of lenses encompassing
certain cylindrical powers, usually in 0.50D steps such as –0.75D, –1.25D
and –1.75D. Given that we generally work in 0.25D steps when determining
a patient's refractive error, ultimately this means that you'll often be faced with
the choice of under-correcting or over-correcting a patient's astigmatism when prescribing
disposable soft toric lenses.
When this situation arises — for example,
when you're fitting a patient who has a prescription of –2.00 –1.00
x90 with a disposable soft toric lens that comes only in cylindrical powers of –0.75D
and –1.25D — then it's best to under-correct the cylinder. The reason
for this is twofold. First, lens rotation becomes more significant as the cylinder
increases. Secondly, while Bernstein et al (1991) demonstrated that no statistically
significant masking of corneal cylinder occurs with standard thickness soft spherical
lenses, clinical experience has shown that soft toric lenses — with their increased
thickness — may mask up to 0.50D of corneal cylinder. This is especially the
case with disposable silicone hydrogel toric lenses, such as the PureVision Toric,
because of the higher modulus of the silicone hydrogel materials.
Allow for Lens Rotation Based on the Lens Thickness Profile
Most disposable soft toric lenses have cylinder
axes in 10-degree steps, usually covering the complete spectrum from 0 degrees to
180 degrees. So what should you do when the axis of the cylinder in the ocular refraction
is 45 degrees? Or 135 degrees? Or 15 degrees? Obviously, you'll need to make an
allowance for some lens rotation by virtue of the fact that you can order the cylinder
axes only in 10-degree steps. You'll therefore have to allow for either 5 degrees
of nasal rotation or 5 degrees of temporal rotation when ordering the diagnostic
disposable soft toric lens. Admittedly, most contact lens laboratories will permit
you to order two trial lenses for one eye so that you may experiment with a cylinder
axis that's 5 degrees to either side of the cylinder axis in the patient's ocular
refraction. Even so, your patients are more likely to be impressed and their chair
time reduced if your first diagnostic lens turns out to be the appropriate prescription.
Hanks
and Weisbarth (1989) showed that, on average, soft toric lenses will tend to rotate
nasally by about 5 degrees to 10 degrees, where nasal rotation is rotation of the
inferior aspect of the lens towards the nose. They also showed, however, that significant
variability exists among soft toric lens wearers in the actual amount and direction
of lens rotation. A major factor causing this variability is the thickness profile
of the soft toric lens.
The power of the lens determines
the lens thickness profile — in particular, the axis and magnitude of the astigmatic
correction. Gundel (1989) postulated that the principal factor affecting lens rotation
is the initial point of contact between the upper lid and the thicker meridian of
the lens. This effect is greatest for toric lenses that have oblique axes, and the
implication here is that notable rotational effects will occur as contact from the
upper lid will always affect one edge of the thicker meridian before the other.
As the upper lid comes down, it will force the lens down at this first point of
contact, causing it to rotate in a certain direction.
Consider a patient who has an ocular
refraction OS of –2.00 –1.25 x45. Assume that the patient is to wear
a soft toric lens with a similar prescription. As the upper lid comes down, it will
first act on the lens (and the 135-degree meridian) at around the 10 o'clock position
on the cornea. The downward motion on the lens at this point will cause it to rotate
temporally (Figure 1). Remember that to allow for temporal rotation on the left
eye, you should subtract the amount of rotation from the required cylinder axis.
Hence, the recommendation for this example is that the first disposable soft toric
lens to be placed on this patient's eye should have a back vertex power of –2.00
–1.25 x40.
Maximize the Spherical Component to Ensure Optimal Lens Stability
Prism ballasting is the most common method of
stabilization for disposable soft toric lenses. Most prism-ballasted lenses also
incorporate some form of dynamic stabilization in the form of lenticulation, comfort
chamfer or selective thinning of the inferior aspect of the lens. The main disadvantage
of dynamic stabilization is that the thickness differential at the edge of the lens
depends on the spherical power of the lens. In some designs, small spherical components
can significantly reduce the achievable thickness differentials, which may compromise
lens stability.
One way to overcome the decreased lens stability
of soft toric lenses that incorporate low or negligible spherical components is
to increase the spherical component of the lens. Contact lens practitioners are
generally quite happy to leave 0.50D of cylinder uncorrected when prescribing soft
spherical lenses. In doing so, they usually increase the spherical component of
the lens in lieu of correcting the cylinder. In other words, they prescribe the
spherical equivalent for the back vertex power. For example, a patient who has a
spectacle refraction of –1.25 –0.50 x90 will most likely be prescribed
a soft spherical lens with a back vertex power of –1.50D.
In fitting soft toric lenses that have
small spherical components, the same principle of increasing the spherical component
to compensate for an uncorrected cylinder can apply so as to improve lens stability.
Consider a patient who has a spectacle refraction of –0.25 –1.25 x180.
Rather than prescribing a soft toric lens with the same back vertex power as the
spectacle refraction, it may be better to reduce the cylindrical correction by 0.50D
and replace this with its spherical equivalent, thereby increasing the spherical
component by 0.25D. In this case, you would prescribe a soft toric lens with the
power –0.50 –0.75 x180. As I mentioned previously, it's generally
best to under-correct the cylinder (to reduce the effect of lens rotation and to
make allowance for possible masking of corneal cylinder by the toric lens). In addition,
increasing the spherical component in this situation will also result in greater
thickness differentials in the periphery of the lens, therefore improving the stability
of the soft toric lens on the eye.
Better First Fit Success
In summary, many clinicians initially treated
disposable soft toric lenses with skepticism because of concerns about on-eye performance
and reproducibility. But as it turns out, these lenses have greatly facilitated
the process of soft toric lens fitting by making it possible for contact lens practitioners
to perform lens-wearing trials on prospective patients.
When fitting disposable soft toric lenses,
the back vertex power will generally be the only parameter that you need to specify.
By careful consideration of the factors I've discussed, you can help to ensure that
the first lens you choose for a soft toric lens trial provides the optimal correction
for the patient.
For references, please visit
www.clspectrum.com/references.asp
and click on document #128.
Dr. Lindsay, a Diplomate and Fellow of
the American Academy of Optometry, is in private practice in Melbourne,
Australia and is also a Senior Fellow of the Department of Optometry and Vision
Sciences at the University of Melbourne.
Contact Lens Spectrum, Issue: July 2006