Winter 2017

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T he term nomenclature means to devise or choose a name for things, especially in a science. To apply terms to the different com- ponents that make up a contact lens is necessary to understand as a contact lens professional. The purpose of this article is to identify to the reader con- tact lens nomenclature. GAS PERMEABLE (GP) NOMENCLATURE Many nomenclature terms apply to both a GP corneal lens and scleral. However, there is additional nomencla- ture specific to scleral GP lens designs. Base curve (BC or central posterior curve CPC) • This identifies the primary fitting curve of the posterior surface • May be spherical, aspheric, or toric a. Spherical = one radius of curvature b. Aspheric = a group of curves with a specific degree of flattening or eccentricity value c. Toric (radii) two primary curves that are perpendicular or 90 degrees apart • May be expressed in millimeters (mm) or diopters (7.50mm / 45.00 D) • The radius of the optic zone Power (RX or prescription) • Equals the dioptric value measured by lensometry • May be expressed as back vertex power (BCV) or front vertex power (FVP) which is important to differen- tiate for documenting higher powers • May be spherical or toric • Results from the difference between the anterior surface and the poste- rior surface curvatures, the index of refraction of the material, and the center thickness Buddy Russell, FCLSA, COMT, FSLS The Anatomy of a Gas Permeable (GP) Contact Lens CLSA EyeWitness Winter 2017 16 Optic zone (OZ) • Linear width that is optically effective • Width of the BC (posterior OZ) • Width of the lenticular bowl (anterior OZ) • Higher powers may have a difference between the anterior OZ and the posterior OZ • Changes in size of optic zone affect the sagittal depth / fit of the lens • Expressed in mm Secondary curve (intermediate or transition curve) • In standard designs, the next flatter curve outside of the posterior OZ • In return curve designs (reverse geometry), the next steeper curve outside of the posterior OZ • Expressed in width and radius (. 35mm / 9.80mm ) • Can add more than one transition curve Peripheral Curve (PC) • The flattest curve on the posterior surface • Essential for tear exchange • Expressed in width and radius (.4mm / 12.50mm) Blend • The smoothing of the transition curves outside of the OZ • Heavy blend or ski blend describes no distinct junctions • A poor blend may cause poor com- fort and decreased tear exchange Chamber • The diameter of the posterior zone that vaults the ocular surface • Chamber size includes the OZ width and any other curves within the spe- cific design prior to the landing curves Haptic • The portion of the scleral lens that lands on the ocular surface • Interacts with bulbar and tarsal conjunctiva • Depending on the specific proprietary design, a variety of terms may be used to describe (examples - scleral zone, advanced peripheral system ) • May be toric • May be asymmetrical • May be spherical • May be a tangent (not a curve) Limbal zone or vault • The area of the posterior surface of a scleral lens that covers the limbus with some degree of vault Saggital depth • In geometry, the sag of a circular arc is the distance from the center of the arc to the center of its base • The depth is dependent on the base curve, the diameter and the curve system of the posterior surface EW Buddy Russell, FCLSA, COMT, FSLS, is currently the Director of Contact Lens Services at Thomas Eye Group in Atlanta, GA and the Chief Clinical Specialist for X-Cel Specialty Contacts. He serves as the National Contact Lens coordinator for the Infant Aphakia Treatment Study; peer reviewer for Cornea, Eye and Contact Lens Science and Research publications; and is a national/international lecturer and writer.

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