Contact Lenses-How They Work And How I Fit Them

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Image via Wikipedia

This is the first of another multi-part series, this time about Contact Lenses.

There are two major types of contact lenses, “Rigid” and “Soft” lenses. Each have their own distinct benefits and drawbacks, neither one is the best type of lens for every patient. Before I get too far into this discussion, I want to cover some very basic anatomy and optics.

Anatomy
The cornea is the clear dome of tissue that a contact lens rests on when worn. It is an amazing anatomical structure in that it has no blood vessels running through it. If it did, we would have to see through them, which would give us a disrupted or distorted view of the world around us. Wearing any contact lens for too long, a lens that doesn’t fit correctly or a lens that does not allow enough Oxygen to reach the cornea will damage the cornea, sometimes permanently and can prevent further contact lens wear. When I am evaluating the fit of any contact lens, the health of the cornea is priority number one. A lens that doesn’t allow the cornea to remain healthy can’t be worn successfully.

The eyelids are also a very important structure in the fitting and function of contact lenses. They not only protect the eye, they act as the windshield wipers for the cornea and contact lens. They remove debris from the surface of the lens and/or cornea as well as spread a fresh layer of tears over the lens and cornea. The lids will cause movement of the contact lens over the cornea, which is critical for successful lens wear. However, too much or too little lens movement can create both vision and health problems. Striking the right balance between lens movement yet lens stability is critical, especially with Rigid lenses. The eyelids also provide the oily portion of the tear film which is critical in maintaining the integrity of the tears. If the eyelid glands called “Meibomian Glands” produce too much or too little of the oily tear film component, the tears can become too thick or too thin, neither of which is a good thing.

Optics
All lenses (contact and eyeglass lenses) correct our vision by bending light to bring it into focus on the surface of the retina. By bending the light more or less, lenses bring our vision into focus. When an eye has more than two focus points, the patient is said to have “Astigmatism”. Notice the green bowtie shaped powers in the image below. This patient has a mild amount of astigmatism. Patients who do not have astigmatism will have much more circular power distributions on this test.

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This is quite simple to correct in eye glass lenses, but a bit more complex in contact lenses. In fact, until about 10 years ago, astigmatism correcting contact lenses were much more difficult to fit successfully, and it was not uncommon for a patient with astigmatism to fail to achieve consistently clear vision. New advances in contact lens designs and materials has now made clear vision for astigmatic patients the norm, although the lens selection and fitting process is a bit more complex than when just fitting nearsighted or farsighted correcting lenses.

When a patient has an irregular cornea, the eye may have many focal points, a condition referred to as “Irregular Astigmatism”. These patients do not see well with glasses or standard soft contact lenses. This condition can occur naturally due conditions such as “Keratoconus”, “Corneal Marginal Degeneration” or other ocular dystrophies and degenerations. But the most common cause of irregular astigmatism now is refractive surgery such as “RK”, “PRK” and “LASIK”. While the vast majority of patients who undergo these elective surgical procedures do very well, there are many who have minor or major complications and suffer from glare, seeing “halos” around lights, and distorted vision. I can correct these conditions the vast majority of the time by using custom designed rigid lenses, but these corrections are quite complex. The image below shows the degraded optical system of a patient who has significant irregular astigmatism from a cataract surgery complication, their implant is decentered. Notice how the different colors which represent different focal powers in the eye are quite jumbled and not symmetrical.

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I’m going to talk about how “Soft” and “Right contact lenses work and how I select or design them for each patient in the next installment of this blog series. The final segment will cover bifocal contact lenses as well as custom lenses to correct more irregular vision conditions. As always, don’t hesitate to contact me with questions!

Dr. Warren

www.warreneyecarecenter.com

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