Winter 2017

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20 CLSA EyeWitness Winter 2017 The contact lens power (CLP) can be determined by subtracting the LL from the spectacle refraction. Remem- ber to vertex the spectacle refraction if it is greater than ±4.00D. LL = BC – K 42.75D 44.00D -1.25D LL = 42.75D - 43.00D = - 0 . 2 5 D LL = -0.25 - 1.00 @ 180 by busy practitioners who do not have time to place multiple trial lenses on a patient. This fitting technique is com- pleted without the use of trial lenses. It primarily utilizes keratometry and refraction; however, topography, hori- zontal visible iris diameter (HVID), lid aperture width, and pupil size in light and dark may also be helpful. Some topographers have software systems which can aid in empirical lens design. The patient tends to have a better ini- tial experience with empirical fitting because the first lens that is placed on the eye was designed specifically for them with their refractive correction in place. The benefit of the lens is quickly demonstrated. Empirical Example: Keratometry: 43.00D @ 180 / 44.00D @ 090 Spectacle refraction: -2.50 -1.00 x 180 BC = average K – 0.75D When fitting GP contacts, it is im- portant to remember that the lacrimal lens (LL) also contributes power to the system. The lacrimal lens is the tear film between the contact lens and the cor- nea. The LL can be calculated by sub- tracting the keratometry values from the calculated BC. - 2.50 – 1.00 x 180 CLP = - (- 0.25 – 1.00 x 180) - 2.25DS The final spherical GP lens order would be as follows: BC: 42.75D (7.89mm) CLP: -2.25D OAD: 9.2mm Empirical fitting calculations can be completed as shown above or can be done by most lab consultants if provided keratometry and spectacle refraction. Diagnostic fitting involves using trial contact lenses in office. It is com- monly used for irregular corneas and specialized GP designs such as trans- lating multifocals. By placing a trial lens on the eye, the influence of eyelid position can be determined and ad- justments are made accordingly at the start of the fitting process. With an irregular cornea, such as with kerato- conus, using trial lenses provides an ac- curate assessment of how the lens will fit and center once ordered. This helps minimize the number of reorders re- quired to achieve a successful fit. The trial also provides a smooth refracting surface so a more precise contact lens power and best corrected visual acuity can be determined. Occasionally, ophthalmologists will refer patients for a diagnostic contact lens fitting to determine if potential improve- ment in vision with contacts is possible before completing a complicated surgical High-Dk Material Menicon Z Fluoroperm 151 Boston XO2 Optimum Extreme Boston XO Optimum Extra Paragon HDS 100 TYRO-97 Fluoroperm 92 Boston Equalens II Dk 163 151 141 125 100 100 100 97 92 85 Mid-Dk Material Optimum Comfort Fluoroperm 60 Boston EO Paragon HDS 58 ONSI- 56 Boston Equalens Dk 65 60 58 58 56 47 Low-Dk Material Fluoroperm 30 Optimum Classic Boston ES PMMA Dk 30 26 18 0 ygen permeability through the lens. Below are some of the most common lens materials available on the market categorized by Dk (above): Each material comes in select colors with the most common tints being blue, green, and clear. In order to differentiate the right and left lens- es, some practitioners will order both lenses in blue, then request a black dot or drill dot on the right. It is important to note that the black dot can wear off over time due to regular cleaning. For this reason, other practitioners will order the Right lens in gReen and the Left lens in bLue. This works well as the color never fades or wears off; how- ever, patients with light blue or green eyes may comment that their eyes ap- pear different colors. The last component to a GP lens and arguably one of the most import- ant is the power. For normal corneas, the contact lens power usually direct- ly correlates to the vertexed spectacle Rx. This is why when lenses are or- dered empirically, the lab consultant asks for the keratometry values and an updated refraction. When using diag- nostic lenses, once the ideal base curve is established using a trial lens with a known power, an over-refraction is completed to get the final contact lens power. Remember to vertex the over-refraction if it is > ±4.00D. Empirical vs. Diagnostic Fitting There are pros and cons to fitting con- tact lenses empirically vs. diagnostical- ly. Empirical fitting is commonly used

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