Every incision that is made in the cornea can have an effect on the corneal shape, curvature, and astigmatism. The larger the incision, the greater the effect. The position of the incision also makes a difference with limbal placement having less effect than clear corneal positioning. And even making an identical incision at two different places such as temporal limbus versus superior limbus will make a significant difference.
Many factors go into determining how much of an astigmatic effect and incision has:
- width of the incision
- a larger width gives more corneal flattening
- tunnel length of the incision
- a shorter tunnel length give more corneal flattening
- distance of the incision from the corneal center
- the farther away from the visual axis, the less astigmatic effect
- diameter of the cornea with respect to the incision
- the same 2.75 mm incision in a cornea with a diameter of 10 mm vs 12.5 mm will produce different astigmatic effects:
- for a 10 mm diameter cornea, the circumference is ∏ x diameter = 31.4 mm and the 2.75 mm incision is about 31 degrees of arc
- for a 12.5 mm diameter cornea, the circumference is 39.3 mm and the same 2.75 mm incision is only about 25 degrees of arc
- the same 2.75 mm incision in a cornea with a diameter of 10 mm vs 12.5 mm will produce different astigmatic effects:
- other properties of the cornea:
- thickness / pachymetry
- elasticity, which can vary considerably with age
- for non-penetrating incisions, the depth of the incision
- less effect for shallower grooves, more effect for deeper grooves
For patients with 1 diopter or less of corneal astigatism, corneal based incisions such as limbal relaxing incisions or full-thickness phaco incisions placed on axis can be enough to address the astigmatism. For 1.5 diopters of more of corneal astigmatism, a better choice tends to be a toric IOL to neutralize the astigmatism.
In the case presented here, the patient has about 2 diopters of with-the-rule corneal astigmatism with a steep axis of about 105 degrees. This patient is also unable to afford the cost of a toric IOL so we will attempt to lessen her corneal astigmatism by placing paired, full-thickness corneal incisions on the steep axis. This means moving the microscope so that we can operate from the superior position and then placing an inferior full-thickness corneal incision at the end of the case.
The pre-op measurements were:
MRx -3.50 +2.00 x 105 and K 44.00 x 105 / 42.00 x 015 (average K 43.00)
The best time to place this second incision is after the IOL is inserted and the anterior segment is still full of viscoelastic. This allows the anterior chamber to stay formed during the incision creation. Making the incision earlier in the case is not advised since it could leak during the phacoemulsification procedure which could lead to anterior chamber instability and complications such as a ruptured posterior capsule.
Our paired incisions were successful in treating about 1.5 diopters of the pre-existing astigmatism which left the patient with 0.5 diopters of with-the-rule astigmatism which she found to give excellent vision while being quite comfortable.
The final measurements after healing are:
MRx -0.50 +0.50 x 105 and K 43.25 x 105 / 42.75 x 015 (average K still 43.00)
This technique of paired phaco incisions has been described by many surgeons and many years ago. Watch the video and learn the technique.
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