Axial vs. Radial Edge Lift and Edge Clearance: Defining the Terms

KEITH PARKER, N.C.L.C.

You say radial, your lab says axial, your patients say "ouch!" Edge lift can make or break a lens design, so be sure you and your lab are speaking the same language.

The contact lens industry boasts a dynamic history through which we've witnessed changes in designs, materials, fitting philosophies and manufacturing techniques. Today, software programs in corneal topographers and computerized lathes can define the shape of the cornea and its relationship to the important paracentral and peripheral area of a contact lens. These tools help take the guesswork out of designing a well-fitting contact lens, but only if you and your lab have a clear, uniform definition of some important terminology.

DEFINING EDGE LIFT

Edge lift, one of the most important specifications determining the successful mechanics of rigid lenses, is also the most overlooked. There are two ways to specify edge lift -- axial and radial. Axial edge lift (AEL) is the distance between the apex of the lens edge and the continuation of the base curve, measured parallel to the lens axis. Radial edge lift (REL) is the distance between the apex of the lens edge and the continuation of the base curve, measured along the radius of (or normal to) the base curve (Fig. 1). Most computerized numerical controlled (CNC) lathing calculations are performed in terms of axial edge lift. So, if you provide a radial edge lift measurement to a lab that interprets it as an axial measurement, the contact lens you receive will differ from what you had intended. Software that can convert measurements from axial to radial and vice versa is now available, but it is useful only when your lab knows which type of data you have provided.

There will never be just one edge lift that will be optimum for everyone. The fitting philosophy (interpalpebral vs. lid attachment), environmental conditions (dry vs. humid) and corneal eccentricity will dictate the edge lift required. An appropriate edge lift leads to an optimum edge clearance, which allows each "custom" rigid gas permeable lens to perform as efficiently and comfortably as possible (TABLE 1)

DEFINING EDGE CLEARANCE

Edge clearance, the distance between the peripheral curve's highest point (the lens edge apex) and the peripheral cornea, can also be measured axially or radially. Axial edge clearance is the most commonly used by the lab and the practitioner.

You can calculate edge clearance only if you know the corneal rate of flattening (eccentricity). This peripheral cornea data is not easily obtained using the conventional keratometer. Corneal topography systems can provide average eccentricity values in given areas of the cornea, but edge clearance calculations based on these averages will never be entirely accurate. Therefore, the best way to determine the edge clearance of a lens is through fluorescein evaluation.

THE PROOF IS IN THE CONVERSIONS

Edge lift and edge clearance are measured in hundredths of a millimeter. For example, a normal peripheral system would have a radial edge lift of 0.08mm to 0.10mm. This would equal an axial edge lift of approximately 0.11mm to 0.14mm, depending upon the diameter. An axial edge clearance of 0.08mm equals a radial edge clearance of about 0.065mm at a diameter of 9.0mm on an average cornea. Table 1 illustrates the change in edge lift resulting from the change in the peripheral curves, their widths and the overall diameters of the contact lenses.

Keep in mind this rule of thumb: axial edge lift or clearance will always be more than radial edge lift or clearance, and the larger the diameter of the lens, the greater the difference. CLS

Keith Parker is general manager for Duffens Optical, a multibranch, wholesale, full-service ophthalmic laboratory in the midwest. He has specialized in the manufacture of custom rigid contact lenses for 22 years.

References are available upon request from the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #29. (Be sure to have a fax number ready.)