Winter 2017

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Page 9 of 35

T his article will take you through the important parameters of a generic, gas permeable (GP) scleral lens design. Once we build a generic scleral lens, we will go through the sagittal depth ramifications of changing some of the parameters and the effect on the fit. Basic parameters & their importance to a successful fit 1. The Base Curve (BC) is the fundamental building block of a GP lens. The fit starts with the BC and all other parameters are added to provide optimal fit and comfort. 2. The Base Optical Zone (BOZ) is the diameter of the area that has all the optics and sets the overall fit. 3. The Over All Diameter (OAD) is the size of the lens and is a critical parameter to a successful fit. 4. The Peripheral System (PCs) is a series of curves that attempt to parallel the peripheral cornea and sclera. These curves are defined as: the first curve after the BOZ is the Secondary Curve (2nd), the next curve (s) are Intermediate Curves (IM) and the last curve is the Peripheral Curve (PC). Elements of a good scleral fit 1. The lens MUST vault the cornea limbus to limbus. The vault will create a tear layer that results in a sagittal depth (SAG). There are ongoing debates as to the optimal SAG when fitting scleral lenses. Designers have suggested sagittal values from 100 microns to 500 microns. The current consensus is 250 to 300 microns, but more clinical experience may settle this debate later. Be mindful that the nasal-superior quadrant will have the LEAST SAG. As long as there is some clearance in this quadrant, that is adequate. 2. The OAD needs to be large enough to have a proper landing zone on the sclera. Again designers/researchers disagree as to the proper starting OADs, but many scleral designs are in the range of 15.0 to 16.5 mm OADs. 3. The PC system must align to the sclera with no blood vessel impingement or seal off. Fitting issues and SAG ramifications Taking a hypothetical design and using it as our example, Consultant's Perspective Al Vaske, BA, NCLEC, is a former lab owner, lens designer, author and lecturer on irregular corneas. He is a Past President of the Contact Lens Manufacturers Association. He may be reached at Vaske & Company via alvaske@vaskecompany.com The Building Blocks of Gas Permeable Lenses By Al Vaske, BA, NCLEC we'll discuss the fitting issues: 1. A 7.00 BC/15.5 OAD/9.0 BOZ/3.658 SAG standard edge lens is trialed which yields a 100 micron tear layer. There are several choices to increase the tear layer to 300 microns: larger OAD, larger BOZ or steeper BC. We want 300 microns so our target SAG becomes 3.858 (300 target – 100 current = 200 more microns needed). Our options: Steeper BC: 6.50 BC/15.5 OAD/9.0 BOZ will yield 3.830 SAG. Larger BOZ: 6.85 BC/15.5 OAD/9.5 BOZ will yield 3.852 SAG. Larger OAD: 7.40 BC/16.0 OAD/9.5 BOZ will yield 3.868 SAG. 2. In the above example, we found from the trial fit that the 15.5 OAD/9.0 BOZ size was good for the eye but the standard edge was sealed off at the very edge. We need to flatten the PC. Our new fit lens is the 6.50 BC/15.5 OAD/9.0 BOZ with a SAG of 3.830 microns. A change from the 14.00 PC to a flatter 15.50 PC is needed to correct the edge seal off. Standard PC: 6.50 BC/15.5 OAD/9.0 BOZ PC: 9.00/1.0, 12.00/1.25, 14.00/1.0 will yield a SAG of 3.830. Flatter PC: 6.50 BC/15.5 OAD/9.0 BOZ PC: 9.00/1.0, 12.00/1.25, 15.50/1.0 will yield a SAG of 3.753 So to flatten the PC to 15.50 and keep the original target SAG of 3.830, we need a lens of: 6.30 BC/15.5 OAD/9.0 BOZ PC: 9.00/1.0, 12.00/1.25, 15.50/1.0. I trust you get the idea that any parameter change will affect the SAG and appropriate adjustments need to be made. Always talk with your CLMA lab consultant to get the needed numbers. EW 8 CLSA EyeWitness Winter 2017

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