Toric Lens Implant
Here are some helpful guidelines for measuring, marking, and placement of the toric intraocular lens based on my participation in the development of the mathematics for Alcon’s AcrySof toric calculator, involvement in the 2002 phase 3 FDA study, and frequent implantation of this intraocular lens.
The video below (no sound) demonstrates how to mark the cornea before implantation of the Alcon AcrySof Toric IOL. With the patient sitting upright and looking at a distant target, the cornea is marked at the 3:00 and 9:00 positions using a Blakewell BubbleLevel. In the operating room, a Gimbel/Mendez fixation ring is aligned with these reference marks. Note that with the patient recumbent, there is about 10° of excyclotorsion. The implantation axis (24°) and the location of the incision (155°) are marked with the Boris Meridian Marker. All of the instruments shown are from Mastel Precision (no financial interest).
The Pre-Op Assessment
The pre-op assessment for the toric IOL has two fundamental parts:
1. Keratometry
First, the amount of corneal astigmatism that needs to be corrected must be determined. At least in our hands, a Javal-Schiötz type keratometer, or any form of manual keratometer, appears to yield an overall correlation with the amount and direction of the refractive astigmatism that we aim to correct. Recent data 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 them.
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 measure accurately. This is why a well-reasoned methodology for accurately measuring corneal astigmatism must be employed.
Distribution of Corneal Astigmatism
Normal Adult Population
IOLMaster auto-Ks, slit scanning Ks, Scheimpflug camera Ks, and 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, more minor additive errors associated with corneal marking and proper alignment. Under-corrections become the rule rather than the exception.
2. Topography
Second, we need to confirm that the astigmatism is regular, and for this purpose, a topographer is essential. However, remember that a topographer is primarily a “big picture” instrument for normal eyes and should generally not be used as a substitute for a keratometer. In other words, a topographer should not be considered a primary instrument for determining the axis and magnitude of corneal astigmatism.
The Temptation to “Automate and Delegate”
Suppose a surgeon wants to use an autokeratometer or a topographer. In that case, 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 were taken. The bottom line is that for the toric lens, a reliance on automation will result in a higher-than-normal number of undercorrections due to angular errors.
To calculate the spherical power of the toric IOL, you should use whatever your standard procedure is. 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 are three measurements within 0.25 D in each of the principal meridians.
However, for the Ks that are entered into Alcon’s Toric IOL Calculator, it is best to use the numbers from a manual keratometer. The rationale here is that the IOLMaster Ks and a manual keratometer will very often give the same average central corneal power. Still, the amount of astigmatism (the power difference between the two principal meridians) measured may be different. This is because the IOLMaster Ks samples a 2.5 mm zone while manual keratometry samples 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 meridians.
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). We have found that determining the steep and flat axis is far more reliable with a manual instrument, as we can take as much time as needed to align each axis precisely. Our surgeons and staff are somewhat uncomfortable with entirely relying on a computer algorithm for axis determination and prefer to determine this number manually 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 consider as we strive to achieve the highest level of accuracy and consistency in our results.
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.
Bottom line…
In our practice, the physicians personally do the pre-op Ks
for toric IOL patients using a Javal keratometer.
To achieve the best outcome with the toric IOL, a plan must be 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). Still, among all the methods we’ve tried, this one seems to yield 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 preoperative measurement of astigmatism, this should be an area approached with utmost care, accuracy, and consistency.
