• Your corneas
    Eye Health

    Cornea at the Corners – Function & Facts

    December 5, 2018

    The cornea is the transparent part of the eye through which you can see the iris and pupil. The outermost layer of the eye is composed of the sclera (white part of the eye) and the cornea. The outermost layer of the eye extends as the white sclera from behind the eye until the front of the eye where it becomes transparent and formed by the cornea. The cornea form one-sixth of the eye’s outer layer.

    What’s the main function of the cornea?

    The function of the cornea is to refract “bend” entering light rays to the eye to make them focused and forming images. Its power of focusing light rays (with the power of eye’s lens) determines where the light rays will be focused in the eye. The more the power, the shorter the distance in which light rays are focused.

    The cornea contributes about two-thirds of the whole refractive power (ability to focus light rays) of the eye. The power of the cornea is not the same in all people, like those who are prescribed with monthly Biofinity lenses. When the power of the cornea increases, the distance in which it focuses light rays decreases. Normal corneal power focuses light rays on the retina. If the corneal power is more than normal, light rays are focused in front of the retina. If the cornea has less power than normal, light rays focused behind the retina.

    The main factor that determines the refractive power amount of the cornea is its curvature.

    – The cornea is curved in two perpendicular meridians. When the two meridians have different curvatures, the cornea takes the shape of the basketball. When the two meridians have the same curvature, the cornea takes round shape like the football and contact lenses that have marked down prices should be able to attach to it without issues.

    – The more the curvature of the cornea, the more the power it has. Steeply-curved corneas refract light rays more and focus them in shorter distances than less curved corneas.

    – When the two meridians of the cornea have the same curvature, they have the same refractive power. They focus light rays on the same plane in the eye. This happens in the normal eye, myopia, and hypermetropia like in the below image.
    Diagram of refractive issues in the eye
    – When the curvature is different in the two corneal meridians, they have different refractive power from each other. So, they focus light rays into two different distances (planes) in the eye. This happens in astigmatism like in the image below.

    Normal vs Astimatic EyeThe cornea and the eye’s lens constitute all the refractive power of the eye. So, the location of the images in the eye is determined by the sum of the two powers.

    Facts about the cornea

    – The cornea is the most sentient part of the body. It is innervated by dense nerve fibers. It is very sensitive to any approaching movement or object. Its nerves send signals to close the eyelids immediately if anything approaches.

    – The cornea has no blood vessels. It gets nutrition and oxygen from the tears of the eye and the aqueous humor.

    – The cornea is transparent because it has no blood vessels. Also, the nerve fibers in the cornea are demyelinated keeping it transparent.

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  • contact lenses can cause corneal abrasion
    Eye Health

    Contact Lenses and Reshaping the Cornea

    November 1, 2018

    Contact lenses sit on the surface of the eye over the cornea. The lenses can be made of a soft silicone material or a rigid material called gas permeable or hard, PMMA lenses. Both types of lens material, if worn improperly or for too long, can cause the cornea to change shape.

    Understanding Why the Shape of The Cornea is Important for Contact Lens Wearers

    The cornea is the thin, clear outer layer of the eye. Light passes through the cornea and through the crystalline lens of the eye and to the retina. Wearing contact lenses can sometimes change the shape of the eye. When this happens, many problems can occur within the eye, including oxygen deprivation, vascular problems, and even altered vision.

    When a person first gets their contact lenses, the eye doctor measures the cornea with a tool called a keratometer. This machine measures the surface of the eye, and the doctor uses the measurements to prescribe a lens designed to fit the front surface of the eye. Proper fit is important for both good vision and good eye health.

    How Contact Lens Wearers Can Avoid Reshaping the Cornea

    Maintaining hygiene and resting your eyesTo help reshape a warped cornea or avoid a cornea becoming damaged:

    • Make sure the lenses are sitting on the eye properly. The best way to tell if the lens is correctly on the eye is to have the eye doctor or contact lens technician recheck the fit. This can be done quickly in a short appointment called a recheck. Lenses can tighten and loosen after wearing them for several weeks or months.
    • Give the eyes a break. Wearing time, the length of time per day lenses are permitted to be worn, varies depending on the type of lens and individual, general eye health. An eye care professional determines the proper wearing time during the initial contact lens fitting. Wearing lenses too long can affect the cornea through lack of oxygen to the eye. Following the wearing schedule and wearing glasses occasionally gives the eyes a time to rest and breathe.
    • Inspect contact lenses regularly for tears, cracks or rips. A lens can be damaged but not feel any different. Wearing damaged lenses can affect fit and cause corneal problems.
    • Practice proper lens hygiene. Cleaning all types of contact lenses regularly helps avoid deposits that can accumulate on the surface of the contacts. If lenses are continued to be worn, there is a risk of potentially serious side effects including corneal damage.
    • Follow a regular exam schedule. Contact lenses are a medical device and are regulated by law for patient protection with expiration dates. Wearing expired lenses can potentially cause many problems, including corneal fit.

    Corneal Reshaping with Contacts Can Be Intentional

    Recently, eye doctors have been using a technique to intentionally reshape the cornea through Orthokeratology (Ortho-K) or Corneal Refractive Therapy (CRT). This is done by wearing a special contact lens at night that gently reshapes the surface of the eye, but the results are temporary and must be repeated each night. This procedure is usually used on people who are nearsighted.

    Contact lenses are one of the most common medical devices used today. They are convenient and fairly easy to use. Properly wearing the lenses and taking good care of them is key to avoiding problems such as corneal swelling or reshaping.

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