Evaluating RGP Fit in Keratoconus

BY KARLA ZADNIK, O.D., PH.D.
DEC. 1996

The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a three-year national investigation of more than 1,200 patients diagnosed with keratoconus. CLEK Study patients undergo a rigorous annual examination that includes intensive visual assessment, corneal photography and measurement of quality of life. One of the aims of the study, which is sponsored by the National Eye Institute, is to assess how the eyecare community is currently prescribing rigid contact lenses for keratoconus patients.

ASSESSING HABITUAL LENSES

The CLEK Study employs several methods to determine patients' rigid contact lens fit. First, study investigators observe the fluorescein patterns of patients' habitual contact lenses, which are rigid and usually gas permeable.

Next, investigators photograph the fluorescein patterns of the habitual lenses (Fig. 1) and forward them to the CLEK Photography Reading Center at The Ohio State University College of Optometry, directed by Joseph T. Barr, O.D., M.S. There, masked photograph readers analyze the fluorescein patterns for image quality, and to gain a detailed description of the central and peripheral patterns.

FIG. 1: FLUORESCEIN PATTERN OF HABITUAL LENS

A STANDARDIZED FITTING SYSTEM

Timothy B. Edrington, O.D., of the Southern California College of Optometry, has developed an assessment method exclusively for the CLEK Study to determine how flat or steep contact lens practitioners are fitting keratoconus patients. Using this information, it may be possible to deliver a more standardized fitting system for keratoconus to the contact lens community.

First, the clinician applies a series of trial lenses to determine the flattest lens that shows a central or apical clearance fluorescein pattern. This is the First Definite Apical Clearance Lens, or FDACL (Fig. 2).

FIG. 2: FIRST DEFINITE APICAL CLEARANCE LENS (FDACL).

CLEK Study Trial lenses measure 8.6mm in diameter, and have a 6.5mm posterior optic zone, a secondary curve radius ranging from 8.25mm to 8.00mm, and base curves in 0.05mm increments ranging from 8.00mm to 5.00mm. All diagnostic lenses are made from PMMA with a third curve radius of 11.00mm and a third curve width of 0.2mm. The lenses are lightly blended and the center thickness is 0.13mm.

The starting point for the FDACL is the steep keratometric reading, and the lens with base curve closest to the steep K reading is applied. If the fluorescein pattern shows clearance, i.e., looks steep, then the next flattest lens is applied. If the fluorescein pattern shows apical touch, i.e., looks flat, then the next steeper lens is applied. Lenses are applied in sequence until the FDACL is found. [Editor's Note: You'll find the full story of Dr. Edrington's pioneering work for the CLEK Study in the June 1996 issue of Optometry and Vision Science.]

FDACL STILL IN TRIALS

This system may enable us to apply lenses in a similarly regimented way to determine the lens that will best fit the keratoconic cornea, especially mild to moderate cases. As the CLEK Study progresses, we will document, develop and report the degree to which the FDACL parameters conform to the habitual lens parameters and how successful patients are with lenses prescribed according to this system. Stay tuned! CLS

Dr. Zadnik is an assistant professor at The Ohio State University College of Optometry in Columbus.