Implanting the Alcon AcrySof Toric IOL.
Here are some helpful guidelines for measuring, marking and placement of
the Alcon AcrySof toric intraocular lens based my participation in the development
of the mathematics for original AcrySof toric calculator, participation in
the 2002 phase 3 FDA study and frequent implantation of this intraocular
The Pre-Op Assessment
The pre-op assessment for the AcrySof toric IOL has two fundamental
First, the amount of corneal astigmatism that needs to be corrected
must be determined. At least in our hands, a Javal,
type keratometer, or any form of manual keratometer seems to give a very good overall correlation with the amount and the direction of the refractive astigmatism that we're looking to correct. Recent data soon to be published in the Journal of Cataract and Refractive Surgery suggests that the Haag-Streit Lenstar is also an excellent choice for accurately determining the steep and the flat meridians and the power difference between.
It's also important to keep in mind that most patients will have lower values for corneal astigmatism. The smaller the amount of corneal astigmatism, the more difficult it is to accurately measure. This is why a well-reasoned methodology for the accurate measurement of corneal astigmatism must be employed. Click here for a chart that shows the distribution of corneal astigmatism for the normal adult population.
IOLMaster auto-Ks, slit scamming Ks, Scheimpflug camera Ks topographic sim-Ks can easily be off by 10°, which means a 33% reduction in the effect of the toric IOL. And if the steep axis measurement is off by 30°, you might as well have placed a spherical IOL. And stir into the mix a collection of almost universal, smaller additive errors associated with corneal marking and proper alignment, and under-corrections become the rule rather than the exception.
Second, we need to confirm that the astigmatism is regular and for
this purpose a topographer is essential. But, remember that a topographer
is mostly a "big picture" instrument for normal eyes and
should generally not to be used in place of a keratometer. Said another
way, a topographer should not be considered a primary instrument for
determining the axis and magnitude of corneal astigmatism.
The Temptation to "Automate and Delegate"
If a surgeon wants to use an autokeratometer, or a
topographer, that's perfectly OK, but they will have to be
willing to accept a greater number of under-corrections resulting from
an angular error than they would see if more care and time was taken.
The bottom line is that for the AcrySof toric, a reliance on automation
will result in a higher than normal number of under-corrections due
to angular errors.
Here, the temptation to “automate and delegate” should
To calculate the spherical power of the AcrySof toric IOL, you should
use whatever is your normal procedure. If you have an IOLMaster, we
would suggest that IOLMaster Ks be
used to calculate the spherical equivalent of the IOL power, keeping
in mind the fact that the validation criteria for any form of autokeratometry
is three measurements within 0.25 D in each of the principal meridians.
But, for the Ks that are entered into the AcrySof Toric IOL Calculator,
this is best done using the numbers from a manual keratometer. The
rational here is that the IOLMaster Ks and a manual keratometer will
very often give the same average central corneal power, but the amount
of astigmatism (the power difference between the two principal meridians)
measured may be different. This is because the IOLMaster Ks sample
a 2.5 mm zone while manual keratometry sample from a larger 3.0 mm
or 3.2 mm zone. Recall that the normal prolate cornea is more like
the tip of a rugby ball than the top of an orange and if we sample
a smaller area, we may see less of a difference between the two principal
Using a manual keratometer, the difference in power between the two
principal meridians is the amount of astigmatism to be corrected (what's
entered into the calculator). It has been our finding that the determination
of the steep and flat axis is far more reliable with a manual instrument
because we can take as much time as needed to align each axis exactly.
Our surgeons and staff are a little uncomfortable completely handing
ourselves over to a computer algorithm for axis determination and prefer
to determine this number ourselves using a manual instrument. Oftentimes,
the steep axis for the IOLMaster and a manual instrument will coincide,
but not always. This is certainly something to think about as we try
our best to achieve the highest level of accuracy and the most consistent
If you do not have a Javal keratometer, but only have a B&L manual
keratometer, a more accurate method for its use is as follows: Use
the horizontal drum to measure the axis and corneal power in the horizontal
meridian. Then rotate the same drum 90° and measure the axis and
power. The power difference between meridians is the astigmatism to
be corrected. The power in each meridian and the axis of each meridian
is what's entered into the AcrySof toric calculator.
Another requirement for accurate outcomes is that every surgeon know the amount of his or her surgically induced astigmatism. This easily accomplished using the Surgically
Induced Astigmatism (SIA) Calculator.
Not everything is best done by automation. In
our practice, the physicians personally do the pre-op Ks for toric
IOL patients using a Javal keratometer.
In order to achieve the best outcome with the AcrySof
toric IOL there has to be a plan that is followed. We know that manual
Ks are not the most high-tech instrument in the office, may have results
that vary from one operator to the next, and are not easily delegated
to any staff member (skill level often becomes an issue), but of all
the methods we've tried, this seems to give the most accurate and consistent
results. This is why manual keratometry was required for the original
phase 3 FDA study.
Since almost everything is based on the pre-operative measurement
of astigmatism, this should be one area that is approached with the
utmost care, accuracy and consistency.