Astigmatic Effect from Phaco Incision Placement
Many patients desire a refractive correction at the same time as cataract surgery. This makes sense because we can correct a large degree of spherical refractive error as well as astigmatism with proper planning. The phaco incision that we use during cataract surgery can have a significant effect on the astigmatism of the eye and it needs to be accounted for in our calculations.
Many ophthalmologists split astigmatism patients into two primary groups: eyes which have with-the-rule (WTR) astigmatism and eyes with against-the-rule (ATR) astigmatism (Figure 1). A smaller number of patients will have oblique astigmatism which typically has steep and flat meridians at about the 45 and 135 degree positions, the vast majority are either WTR or ATR. Our usual temporal phaco incision will affect these types of eyes differently.
With the Rule Astigmatism (WTR)
According to convention, patients with a steep corneal axis at about 90 degrees are the rule, thus this type of astigmatism is called WTR. The steep axis of the corneal astigmatism can be on either side of 90 degrees, within about a clock hour. This means that the typical range for WTR astigmatism is a steep axis of between 60 and 120 degrees. This type of WTR astigmatism is more commonly found in younger patients and myopic eyes, and less so among the seniors who compose our typical cataract population. With our alphanumeric characters, a small degree of WTR astigmatism my help to increase depth of focus for enhanced reading vision in eyes with little or no accommodation.
Against the Rule Astigmatism (ATR)
The cornea tends to slowly change over time and with age, patients typically develop a shift from either no astigmatism or WTR astigmatism, to a degree of ATR astigmatism. Among our cataract patients, who tend to be senior citizens, the most commonly seen astigmatism is ATR. Against-the-rule astigmatism has a steep corneal axis at the 180 degree meridian, with a span of a clock hour on either side, giving a range of 150 degrees to 30 degrees. This position means that our typical cataract surgery incisions which are placed temporally, tend to be near this steep axis.
Figure 1: Typical positions for With the Rule (WTR) astigmatism and Against the Rule (ATR) astigmatism.
Temporal Phaco Incisions
Using a temporal corneal incision for phacoemulsification gained in popularity about 20 years ago and it is now the most common entry site for cataract surgery. There are advantages to a temporal incision: it provides easier access to the anterior chamber even in patients with prominent brows or narrow palpebral fissures and it is farthest from the visual axis and therefore less prone to induce astigmatism. We have moved toward using smaller incisions, going from 3 to 3.5mm wide incisions to the smaller 2.2 to 2.8 incisions that are more commonly used now. While it is possible to make an even smaller incision for cataract surgery, the narrow lumens of the smaller instrumentation can impede the efficiency of the procedure. In addition, IOL choices are limited when it comes to designs which will fit through a sub-2 mm incision.
Corneal phaco incisions induce flattening and a decrease in the astigmatism where they are placed. Due to the corneal coupling effect, if one meridian of the cornea is flattened, then the corresponding meridian 90 degrees away will be steepened by approximately the same amount. This means that the net or average corneal power, as used in IOL power calculations, is minimally affected by the main phaco incisions, accessory incisions, or even limbal relaxing incisions.
Temporal Phaco Incision reduces ATR astigmatism
The total corneal astigmatism is the difference between the steep and flat axis powers. For example, if an eye has a keratometric power of 44 diopters at 180 degrees and 43 diopters at 90 degrees, the total astigmatism is the difference of 44 and 43, which equals 1 diopter of astigmatism (figure 2). With the phaco incision placed at the 180-degree position, and with the assumption that it will induce 0.5 diopters of flattening, the new keratometric values will be 43.75 diopters at 180 and 43.25 diopters at 90, giving a total astigmatism of 0.5 diopters (calculated by 43.75 minus 43.25). Note that the average corneal power before the incision (43.50 D) is the same as the average corneal power after the incision (43.50 D), thus illustrating the corneal coupling effect. We can see in this example that patients who have against-the-rule (ATR) astigmatism will benefit from a reduction in corneal astigmatism due to placement of the phaco incision at this axis.
Figure 2: The temporal phaco incision will actually help to reduce the pre-existing against-the-rule (ATR) astigmatism.
Temporal Phaco Incision will worsen WTR Astigmatism
For the smaller number of patients who have WTR astigmatism during the pre-operative consultation for cataract surgery, we must carefully consider the placement of the incisions. Since WTR astigmatism means the 90-degree meridian is steepest, placing the phaco incision at this superior position could be helpful but it would require the surgeon to change positions. In many patients, particularly those with deep set eyes, over-arching brows, or tight palpebral fissures, sitting superiorly and making the phaco incision at the 90-degree position may not be possible. We can still make our phaco incision temporally at the 180-degree position, but we must realize that this will worsen the corneal astigmatism (figure 3).
In this example, the eye has a total of 1 diopter of WTR astigmatism which is steep at 90 degrees with a corneal power of 44 diopters at 90 and 43 diopters at 180. Then the phaco incision is made temporally at the 180-degree position. This will induce 0.5 diopters of flattening at this position so that the new corneal powers become 42.75 diopters at 180 and 44.25 diopters at 90, thus giving a total of 1.5 diopters of corneal astigmatism. Note that the average corneal power both before and after the phaco incision is 43.50 diopters, thus having no effect on the IOL power calculation. In this example, the resultant 1.5 diopters could be addressed by either limbal relaxing incisions or use of a toric IOL.
Figure 3: The temporal phaco incision will worsen the pre-existing with-the-rule (WTR) astigmatism, which will need to be addressed separately.
We can even plan for the future shift from WTR to ATR astigmatism by choosing to leave our patients with just a little WTR astigmatism so that they have more years of less than one diopter of astigmatism. A patient who has 0.0 diopters of astigmatism after surgery can be expected to have 0.5 diopters ATR after 5 years and then 1.0 diopters ATR after 10 years. A better result may be to leave a patient with 0.5 diopters WTR after cataract surgery so that at 5 years the eye will have 0.0 diopters of astigmatism and then at 10 years, 0.5 diopters ATR. This gives more years with 0.5 diopters or less of astigmatism.
For patients who desire a refractive outcome at the time of cataract surgery, careful analysis and planning can reduce the pre-existing astigmatism and improve the final visual outcome.
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