Planning to have cataract surgery and have astigmatism? Make sure to choose a toric intraocular lens implant to correct astigmatism and see clearly without glasses. Astigmatism is when the shape of the eye is not spherical like a basketball, but oblong like a football. It distorts vision unless corrected by glasses or toric contact lenses. The steep axis can be horizontal, vertical or anywhere in between and lenses used to correct it must be oriented in the same axis. A toric intraocular lens is implanted once the cataract has been removed and will correct astigmatism in addition to any pre-existing near or farsightedness. With all of these refractive errors corrected, glasses will no longer be required for distance vision. The difference between a toric implant and a standard implant is that a standard implant will only correct near or farsightedness. If there is a significant amount (>1 diopter) of residual astigmatism after cataract surgery, glasses will still be required to get the best vision.
Astigmatism is measured by magnitude and axis (if there is a second number in your prescription called cylinder, then you have astigmatism). The magnitude is measured in diopters and the axis, which indicates the direction of the astigmatism, is measured in degrees. Astigmatism between 1-4 diopters can be corrected using a toric intraocular lens implant. Before surgery, the magnitude and axis of astigmatism is precisely measured using multiple methods. In addition to checking the glasses or contact lens prescription, the curvature of the anterior cornea (80% of astigmatism is attributable this) is measured using multiple instruments to check for consistency. Some instruments can even measure the small amount of astigmatism attributable to the back surface of the cornea (about 0.5 diopters). The power and axis of the toric implant required to correct astigmatism is selected with the help of a computer program that will also factor in a small amount of astigmatism that is induced by the incision for the cataract surgery.
Cataract surgery will proceed in the usual fashion with the following exceptions. The position of the eye is first marked with the head in the upright position. Sometimes the eye rotates when lying down. Since the axis of astigmatism is critical, marking the eye will allow us to take this rotation into account. The final axis as previously determined by the computer is then marked on the eye just before cataract surgery. Once the implant is placed, it is rotated to this axis and the surgery is completed.
Before the implant is inserted the power and axis of the astigmatism can be confirmed with a special instrument in the microscope and compared against the preoperative measurement. This allows for last minute adjustments to make sure the best implant is used. Once the implant is placed on axis, the instrument takes a second measurement to see if there is any residual astigmatism. The computer on this instrument will allow the surgeon to fine tune the axis of the lens to minimize residual astigmatism.
A new technology will soon become available to make marking the eye obsolete. Instead, an instrument in the clinic will remember the location of the blood vessels in the eye and orient itself with this natural landmark. Instead of marking the axis on the eye, the axis of the lens implant as measured in the clinic is projected on the eyepiece of the surgeon’s microsope. Through this heads-up-display, the surgeon sees where he needs to make his incisions and the correct axis for the toric lens in order to optimize astigmatism correction. The image even locks on and follows the blood vessels so the marks will follow the eye if it moves. These amazing technologies all help to improve the precision and accuracy of toric lens implants to correct astigmatism during cataract surgery. There are no downsides to these toric lenses as they do not increase glare, decrease the quality of vision, or increase the risk of cataract surgery. Unlike contact lenses, it cannot be felt and causes no irritation as these implants are internal.
Rest assured that at North Shore Eye, we guarantee our results for life. Most of our cataract surgeons are also seasoned LASIK and PRK surgeons. If there is any residual astigmatism affecting vision even after the technologies described, it is reassuring to know that enhancement LASIK or PRK is always available at no additional charge after cataract surgery. However, this is rarely done because our preoperative instruments are so good at predicting the correct lens power, magnitude and axis of astigmatism. Therefore, if you have cataracts and astigmatism, don’t settle for the standard lens and get a toric lens so that you can get rid of your glasses.