OM t of c.iii - Vision Research Coordinating Center - Washington ...

1/1/2005 Chapter 8 Corneal Curvature and Topography page 8-1
8.1 Introduction
Chapter 8
Corneal Curvature and Topography
Corneal curvature and topography are measured primarily by central
keratometry according to the protocol that follows. CLEK Participating Clinics who
have Tomey Technologies’ TMS videokeratographic devices perform
videokeratography according to the protocol outlined below. Clinics without TMS
devices that have other videokeratographic devices should store at least one video
image from each eye on site at the Participating Clinic. Disks for corneal topography
data storage are supplied to these designated Clinics after they have been certified for
this procedure. Disks are returned to the Chairman’s Office for storage and future data
analysis only from Participating Clinics with TMS devices.
8.2 Keratometry
Keratometry is a long accepted method of measuring central corneal curvature.
Although its reliability is well described, its usefulness in measuring keratoconic
corneas is unclear. It has been chosen as the primary measure of corneal curvature for
the CLEK Study because of the availability of keratometers across Participating Clinics,
the ease with which keratometry can be performed, and the inadequacy of more
modern devices for assessment of irregular corneas.
8.2.1 Eyepiece Focusing
The keratometer eyepiece is focused prior to each use as follows:
(1) Hold a white card at the front of the instrument housing of the keratometer or
use the white occluder if present on the instrument. The purpose of using the white
background is to allow greater ease in viewing the black eyepiece reticule (black cross).
(2) The eyepiece is turned all the way counterclockwise into most plus power.
(3) Both eyes are kept open as the Clinician looks into the eyepiece to decrease
accommodation. The eyepiece is then slowly rotated clockwise until the black cross first
clears.
8.2.2 Calibration
The following protocol is used for calibrating the keratometer on a weekly basis
by the Clinician or the Back-up Clinician. Calibration results should be logged in the
Clinic.