Testing Performed Before Surgery

Determining Intraocular Lens Power for your Eye

Today, there are several devices in use that help to determine intraocular lens power. The length of your eye and the power of your cornea are the primary measurements used to determine your IOL power. In addition, there are several other smaller factors that are included in this determination. Selecting an intraocular lens is a complex mathematical process that is not perfect, but it is much better than it used to be. Today, the formulas used to determine lens power are much more accurate than older formulas. However, there are some estimates used in these formulas, and they do not apply perfectly to every eye. Occasionally, the predicted lens power will differ from the lens power your eye actually requires. Unfortunately, this may not be apparent until the IOL has been placed in your eye, which can lead to under- or over-correction after your surgery. Your doctor may choose to perform these measurements after your initial examination, or possibly on another day if your cornea is dry or you wear contact lenses.

Measuring Corneal Topography

If you have an astigmatism or have chosen to have a premium IOL, your doctor will elect to check the corneal topography. This test tells the power (refracting strength, or ability to bend light) and shape of your cornea. This will help your doctor to determine if you are a candidate for astigmatism correction and what method of correction would be best for you. Some doctors will check this on every cataract patient. This test is not covered by most insurance carriers during a routine cataract surgery evaluation, so your doctor may discuss whether to perform this test for you.

There are three options to correct your astigmatism surgically:

Limbal relaxing incisions

Toric intraocular lenses

Laser vision correction (LVC)

Each of these procedures can treat astigmatism very effectively, but your doctor needs to perform specific measurements and tests to determine which procedure is best for you. Please see the page “Other Ways to Decrease Dependence on Glasses” for an in depth discussion of these procedures. Alternatively, you could wear glasses or contact lenses to correct your astigmatism.

For Contact Lens Wearers

It is important that you remove your contact lenses for a certain period of time prior to having your eyes measured for IOL power. This is because contact lenses reshape the surface of the cornea. The corneal power is one of the primary components used to predict the correct IOL power, so you will want it to be in its natural state when this measurement is taken. For soft contact lens wearers, one week is usually adequate to allow for the cornea to normalize. If you are a rigid gas permeable (RGP) wearer, normalization will take longer. The endpoint is a stable measurement in the cornea, and this will be after three to four weeks for someone wearing RGPs regularly. In unusual cases, it could be longer.

Additional Ancillary Tests

If you have coexisting eye disease in addition to your cataract, the doctor may need to perform one or more tests to evaluate these other diseases prior to your cataract surgery. This will help your doctor to determine the status of these diseases and the appropriate timing of your cataract surgery. These tests may include a formal visual field examination (peripheral vision testing), specialized optic nerve measurements, and specialized testing to evaluate the macula and retina, which may include fluorescein angiography and optical coherence tomography. Some of these tests may also be used to determine if you are a candidate for a premium IOL or laser vision correction.

© Vision Information Services, LLC, Mooresville, NC 2012