Consultant's Corner
Loaner Cone Sets Help Simplify Keratoconus Fitting
by Michael A. Johnson, FCLSA
Director of Consultation Services
Fitting a keratoconic patient with GP lenses was once considered a difficult and laborious task; however, the availability of corneal topography and simplified cone diagnostic sets have made a significant improvement in fitting success. Art Optical’s AKS (Art Keratoconic System) loaner cone series was developed with the simplified philosophy of controlling the optical zone diameter according to the base curve relationship. As the base curve increases in steepness, the optical zone diameter decreases to prevent mid-peripheral seal off. This allows a steeper base curve to be placed over the apex of the cone with a minimal central touch while maintaining adequate peripheral and mid-peripheral clearance and assuring proper tear circulation.
The AKS loaner system utilizes standard and extended range parameters which will cover the majority of moderate to advanced cones. The standard set has base curves that range from 7.20mm to 6.00mm in .10mm steps. All lenses have a diameter of 8.5mm while the optical zone diameter reduces as the base curve steepens. For example, a base curve of 7.10 will have an optical zone of 7.10, a base curve of 6.70 will have an optical zone of 6.70 and a base curve of 6.50 will have an optical zone of 6.50. By controlling the optical zone to relatively match the base curve, you will avoid creating seal off or 360 degree bearing in the mid-periphery of the lens fit. The powers in the loaner set are also controlled to increase in minus as the base curve steepens. This will help to minimize the amount of over refractive power needed to obtain optimum acuity for the patient. The extended range loaner set follows the same principles as the standard cone set with the exception of the base curve range which is from 6.20mm to 5.00mm in .10mm steps. The extended set will cover advanced cones that exceed 60.00 diopters in apex curvature.
For keratoconus patients who present corneal curvature that is flatter than the range intended for the standard loaner set, Art Optical recommends using the Boston Envision lens design. We carry loaner sets in Envision with base curves from 8.30mm to 7.00mm in .10mm steps with a standard power of -3.00. Envision is an excellent design for beginning cones due to its bi-aspheric back surface which provides a better fit relationship on the majority of irregular corneas.
Corneal topography has made a major impact on keratoconus. It is recommended to have a topography done for all cone patients whenever possible to aid in the fitting process. In many cases, lenses can be designed off the axial maps which show the shape, steepness and position of the cone. When trial fitting is necessary, the topography will provide a better starting point than central keratometer readings due to the inability of the keratometer to pick up the curvature at the apex of the cone.
GP lenses are a necessity for patients with moderate to advanced degrees of keratoconus. Art Optical’s loaner cone series (AKS) can help simplify the fitting process. If possible, having a corneal topography done will help to a greater extent. You can call our consultation direct line at 1-800-566-8001 to discuss keratoconus and how our AKS loaner system can help you.
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