EyeWitness

Winter 2017

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19 CLSA EyeWitness Winter 2017 around 9.2mm. It can be difficult, how- ever, to measure the width of each indi- vidual curve beyond the base curve. Of- ten for ease of analysis, the peripheral curve width (PCW) is measured which is the sum of all outer curve widths that are present in a given lens design (Ex: PCW = SC + PC). The transition zone between the OZ and peripheral curves may be a dis- tinct, sharp junction or it may be a grad- ual transition zone. The junction be- tween curves can trap debris and inhibit tear exchange. Smoothing out these junctions is referred to as blend. Blend- ing the transition zones between curves allows for optimal patient comfort and lens performance. Blend is usually con- sidered light, medium, or heavy. With a light blend, each individual junction is still easily seen when viewing with a magnifier. In a medium blend, shadows provide clues for where each junctional transition occurs. These transitions are extremely difficult to judge in a heavy blend or aspheric lens design. In a heavy blend, the peripheral curve width is estimated only by measur- ing the shadows. The GP lens edge also plays an important role in patient comfort. A rounded edge with a thin taper is ideal. This is often referred to as a "plus" edge pro- file. If the edge is too thick, it can cause more lid awareness and discomfort. The edge thickness is also dependent upon the pow- er of the contact. For high minus and high plus powers, consider asking a GP lens consultant about lenticular designs to maximize comfort and fit. A plus lenticular can reduce the edge thickness of high minus lenses (> -5.00D). A minus lenticular on plus or low minus lenses can reduce inferior decentration of a lens by providing an edge which the superior lid can grasp. If an edge is too pointed or sharp, it can dig into surrounding tissue causing mild lacerations on the palpebral or bulbar conjunctiva and cornea. Fortunately, if a lens modification unit is available, the lens edges can quickly be modified or polished in office to promote maximal patient comfort. Another GP lens component which is dependent upon contact lens power is the center thickness (CT). A plano lens has a center thickness of 0.18mm to 0.20mm. The final center thickness is determined by the lab based on contact lens power, the amount of astigmatism present, and Dk (oxygen permeability) of the material used. A minimum thick- ness of 0.10mm to 0.12mm is required for most GP designs to prevent the lens from being too thin and decreasing the structural integrity which can lead to lens flexure. With larger diameter GP designs such as scleral lenses, keeping the center thickness to a minimum is important to promote maximal oxy- gen permeability. Center thickness in sclerals usually ranges from 0.30mm to 0.40mm. In addition to center thick- ness, the lens material can also impact oxygen permeability. There are many GP polymers available with various levels of oxygen permeability or Dk values. In general, as the Dk increases, the wettability of the lens decreases. For this rea- son, many practitioners choose a mid-Dk material (30-65) for corneal GP lenses. Lower Dk materials tend to be more dura- ble whereas high Dk materials are softer, thus more prone to scratches and warping. Presby- opic patients tend to have bet- ter comfort in mid-Dk lenses as opposed to high Dk lenses. All large diameter GP lenses are typically made in high-Dk ma- terials to promote maximal ox- which curvature optimally fits the pa- tient's corneal shape. The base curve can also be calculated empirically using keratometry (K) or topography sim-K values. Frequently, labs use empirical calculations based on the keratometry values, spectacle prescription, and their specific lens design to determine the best starting curvature. Both methods will be discussed in more detail. Beyond the base curve, a series of flatter curves are used to help center the lens, achieve a desired lens-lid interaction (such as lid attachment), and improve patient comfort. Most spherical GP lenses are manufactured as bicurve (BC + PC), tricurve (BC + SC + PC) or tetracurve (BC + SC + IC + PC) designs. The most common GP designs are tricurve (Fig- ure 1 & 2) or aspheric. The optic zone diameter (OZD) or diameter of the base curve can be mea- sured using a 7x reticle magnifier as can the overall diameter (OAD) of a lens. The average OAD of a GP is usually Figure 1 Figure 2

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