Astigmatism is determined by the shape of the eye and determined predominantly (80%) by the shape of the cornea, the clear dome over the iris and pupil. Instead of being spherical like a basketball, an eye with astigmatism is oblong like a football. Because the shape of the eye is different at the long and short axes (think of the shape of a football), light from these two axes cannot come into focus at the retina simultaneously. Hence vision will be blurry unless astigmatism is corrected. Correction of astigmatism is usually done with glasses by grinding “cylinder” into the lens to increase the power of the lens only in one axis. Special contact lenses called “toric” contact lenses can also compensate for astigmatism in a similar way. The first number of a prescription for glasses or contact lens is the spherical error which tells you the degree to which you are nearsighted or farsighted. The degree of astigmatism is designated by the second number (in units called diopters) on the prescription followed by the axis in degrees. If there is no second number, then there is no astigmatism correction.
When we perform cataract surgery, the cloudy lens in the eye is removed and replaced by an intraocular lens implant. Insurance will cover a standard lens that will correct near or farsightedness to decreased dependency on glasses for distance vision, but this will only work if you do not have astigmatism. In order to correct mild astigmatism up to one diopter, laser cataract surgery can be performed to flatten the steep axis of the cornea by creating relaxing incisions in the peripheral cornea (in addition to increasing safety and precision, correcting astigmatism is another side benefit of laser cataract surgery). 1.5-4.0 diopters of astigmatism is best corrected using a toric intraocular lens implant. In order to implant these lenses, the surgeon will need to make sure the magnitude and axis of the astigmatism is correct. 4 or more instruments are used to make sure all measurements are consistent before permanently implanting the lens. One of these instruments is a corneal topographer which maps the shape of the cornea showing a topographic map of the cornea so we could see where the steep and shallow axes are. Using this map, we can also determine whether the astigmatism is “regular” or “irregular”. Patients who have irregular corneas from conditions such as keratoconus, corneal dystrophies affecting the surface of the cornea, or scars may not be good candidates for toric lens implants.
Cataract surgery to correct astigmatism is performed in the same way. The only difference with laser correction of astigmatism is the relaxing incision on the cornea which takes only a few seconds with a special high speed laser called a femtolaser. With a toric implant, the lens has marks that need to be positioned at the correct axis. After anesthetic drops are given, the axis is marked on the surface of the eye. Once the lens is implanted, the lens is rotated to align it to the correct axis. This gives a ballpark position of alignment. To increase the accuracy of the alignment even further, we use a special instrument called “intraoperative wavefront aberrometry” to actually measure the eye’s refractive error both before and after we put in the lens implant. This offers the opportunity to confirm that the power and axis of the lens we are using is correct and takes into account the shape of the whole eye and not just the cornea. If you have cataracts and astigmatism, make an appointment with one of our cataract specialists to discuss your options. We use the latest technology to give you the best opportunity to decrease your dependency on glasses after cataract surgery.