contact lens case reports
New Keratoconus Design Fits Many Corneal Surfaces
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO

Lens designs range from small diameter apical clearance lenses to large diameter apical touch designs. Most seasoned practitioners agree that a number of different strategies must be employed to satisfy the diverse corneal topographies seen in keratoconus.

Figure 1. Corneal mapping OS.

The KBA Lens

Recently, a new GP lens design has found its place into our armamentarium of lenses for keratoconus. The design, the Keratoconic Bi-Aspheric Design or KBA, and is the brain child of Australian optometrists John Mountford and Donald Noack. The major features of the KBA design are its large, 10.2mm overall diameter and its adjustable posterior aspheric surface. Additionally, the anterior surface has a compensating aspheric curve that in effect renders the lens optically spherical. A software program accompanies the KBA diagnostic set to assist the practitioner throughout all phases of the lens calculations and the lens is manufactured through Precision Technologies in Vancouver.

Decreasing Lens Tolerance

Patient BN is a 26-year-old male with a long history of keratoconus that that has always been worse in his left eye. Keratometric readings on the right eye were 46.25/ 48.50 and 50.75/56.25 on the left eye. The patient reported good comfort and wearing time with his right lens, which was a traditional multicurve GP design. He reported decreasing OS lens tolerance resulting in a maximum wearing time of about six hours a day. Videokeratography OS revealed a highly asymmetric topography with the cornea 3.0mm above the midline 37.00 diopters and 3.0mm below the midline 57.00 diopters (Figure 1).

	Figure 2. KBA fluorescein pattern OS.

We diagnostically fitted the patient with the KBA design, selecting an initial base curve radius about 5.00D (0.60mm) steeper than the flat K reading of 50.75 diopters (6.65mm). The 55.50 diopters (6.10mm) lens, with a posterior eccentricity of 1.30, was evaluated with fluorescein. The diagnostic lens exhibited all of the characteristics of a well fitted KBA lens, good centration, slight apical clearance over the apex of the cone, 360 

degrees of peripheral clearance and 0.25mm to 0.50mm of vertical movement with the blink (Figure 2). If the diagnostic lens had shown an optimal apical pattern but a tight mid-periphery, the posterior asphericity could be increased to 1.40 (in steps of 0.1 up to 1.50) which flattens the lens peripherally. However, as the posterior surface eccentricity of the lens is flattened, the base curve radius needs to be steepened to maintain the desired sagittal depth across the apex. The KBA software guides the practitioner through the simple calculations necessary to maintain the desired sagittal depth.

Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.

Mark André is director of contact lens services at the Oregon Health Sciences University.