EyeWitness

Winter 2017

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INTRODUCTION During the 1970's, the United States Food and Drug Administration (FDA) began regulating medical devices as well as pharmaceutical agents. 1 From that point forward, all types of con- tact lens modalities, materials, and solutions have been closely regulated by the FDA Center for Devices and Radiological Health. It is important to communicate to patients that con- tact lenses are not a fashion accesso- ry. They are a medical device which requires monitoring and regular fol- low-up to ensure that health and vision are maintained. To help reinforce the point, it is important that a patient have a recent comprehensive eye exam with dilation to rule out any other possible reasons for decreased best corrected visual acuities. When fitting contact lenses, it is key to look at the ocular surface closely. An unhealthy eye will not be able to support a contact lens, which may lead to microbial keratitis, limbal stem cell deficiency, or other issues. When examining the ocular surface, look at all the structures that a contact lens may interact with – this includes not only the cornea, but also the con- junctiva, tear film, pupils, and eyelids. The cornea is the clear, refracting surface of the eye and is responsible for roughly 66% of the overall eye power. The cornea must remain clear and in- tact in order for a patient to see clearly. 18 CLSA EyeWitness Winter 2017 Contact Lens Fitting 101 CE Refractive error, corneal shape, and patient motivation provide guidance for choosing the best initial design Brianna Ryff, OD, FAAO & Florencia Yeh, OD, FAAO The limbus is the transition zone be- tween the cornea and the white part of the eye, and it houses stem cells. Gas permeable (GP) and soft contact lens- es interact with the cornea and limbus throughout the wearing time of the lens so it is important to look for any signs of corneal staining or other irreg- ularities. The conjunctiva is a clear but vas- cularized mucous membrane that lines the inside of the eyelids (palpebral con- junctiva) and lies over the white part of the eye (bulbar conjunctiva). Both the palpebral and bulbar conjunctival physiology may be impacted by soft contact lenses and large GP lenses. Signs of conjunctival irritation may present as staining when sodium fluo- rescein is used; as increasing conjuncti- val injection; or as papillae. While only several microns thick, the tear film plays a large component to the health of the eye. The tears help flush out debris from the eyes and also helps keep corneal cells in good health. Corneal and conjunctival dryness will only be exacerbated by contact lens wear, so maintaining a healthy tear film is essential. Looking at a patient's eyelid po- sition and laxity are also something to note, especially in the case of fitting GP contact lenses. If the lids are too loose, it may be difficult for a patient to remove a GP contact lens or it may be difficult for a translating GP con- tact lens to sit on the lower lid without slipping underneath the lid margin. If the lids are too tight, it is possible for a GP contact lens to be squeezed out of the eye. If the palpebral aperture, the height between the upper and lower lid, is narrow, it may lead to difficulty in removing a soft contact lens as they tend to be a larger diameter than GP lenses. In that case, a GP lens may be preferred for that patient. GP LENSES All GP lenses are made using a lathe to cut various curves onto the front and back surfaces of a gas permeable poly- mer button. This allows a more custom- ized fit and prescription for each pa- tient. The largest and central curve of the lens is the base curve (BC). The base curve is selected by diagnostically using a fitting set with trial lenses to determine ANATOMY OF A GP LENS BC = Base Curve SC = Secondary Curve IC = Intermediate Curve PC = Peripheral Curve OZD = Optical Zone Diameter OAD = Overall Diameter PCW = Peripheral Curve Width OZD = diameter of the BC OAD = BC + SC + PC PCW = SC + PC

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