Prototype base curve attachment for the topographer: what will replace the vanishing radiuscope?

With the disappearance of the radiuscope, alternative methods must be developed to accurately measure gas-permeable (GP) lenses. The purpose of this study was to design, manufacture, and test a novel attachment for the evaluation of spherical base curves of GP rigid contact lenses using a corneal topographer.

A topographer attachment was devised with the intent of measuring the surface radii of GP lenses. A front-surface mirror was fixed at a 45 degrees angle above a hemispherical GP lens mount taken from a radiuscope. The device was set in the chin rest of a Humphrey Atlas topographer (Carl Zeiss Meditec, Inc., Jena, Germany) such that the image from the back of a GP surface was focused. The base curve radii of 9 rigid polymer buttons nominally ranging from 5.00 mm to 9.00 mm in 0.5-mm steps were measured 3 times with a Neitz Auto CG (Neitz Instruments Co., Ltd., Tokyo, Japan), Reichert Radiuscope (Reichert Ophthalmic Instruments, Depew, New York), and Humphrey Atlas topographer using the base curve (BC) attachment.

An analysis of variance with replication and without interaction, using the main effects of measurement method (n = 5) and nominal button radius (n = 9), found that there was no statistically significant effect of the base curve measurement method (F[4, 122] = 1.23; P = 0.303), although there was a statistically significant effect of nominal button radius (F[8, 122] = 4.70; P = 0.003).

Although the optical system of the corneal topographer was designed for measurement of convex surfaces, measurement of base curve radii from concave surfaces was performed without a correction factor using the prototype attachment. The mean SimK and Axial radii were within the tolerance for GP base curve radius, 0.05 mm, cited in ANSI Z80.20-2004 and ISO 18369-2:2006. Thus, the clinical feasibility of this prototype BC attachment to support measurement of spherical base curve radii for GP lenses by a corneal topographer was demonstrated.