topography topics

Bitoric Lens Indications After Penetrating Keratoplasty

BY LORETTA B. SZCZOTKA, OD, MS, FAAO
September 2000

After penetrating keratoplasty (PK), trying to fit the first RGP lens can be frustrating. Keratometry readings are a start, but they provide only paracentral corneal curvature data and only a qualitative assessment of regularity. Corneal topography, on the other hand, assesses thousands of data points that can cover up to 80 percent of the corneal surface for the most common placido-based corneal topography units. This is beneficial when fitting the PK patient because relying on limited central keratometry data can be extremely misleading.

Choose the Best Design

Prior to selecting specific RGP parameters, the fitter must first decide which RGP design (spherical, aspheric, reverse geometry, front toric, back toric or bitoric) is indicated for the patient. A post-PK patient can have astigmatism in excess of 6.00D. Often the practitioner's or consulting RGP laboratory's first instinct is to select a back toric or bitoric design because of high corneal astigmatism. However, an RGP with back surface cylinder is indicated only when the astigmatism is fairly regular, symmetrical and extends throughout the graft and/or onto the host corneal tissue. Only topography can give you an accurate pre-fitting assessment prior to selecting or empirically ordering the first trial lens.

Don't Let Keratometry Fool You

Both patients highlighted have undergone penetrating keratoplasty procedures for Fuch's dystrophy (Patient A) or keratoconus (Patient B). Patient A has 8.50D of against-the-rule astigmatism, and Patient B has almost 6.00D of oblique astigmatism as measured by simulated (or manual) keratometry. Both eyes appear to be candidates for bitoric lenses due to the high corneal astigmatism indicated by keratometry.

However, the topography results significantly sway the interpretation of the measurements. Patient A has very regular and symmetrical corneal astigmatism which extends throughout the corneal map, as captured by the topography instrument (Figure 1). I designed a bitoric RGP lens by fitting 75 percent of the corneal toricity.

For Patient B, the corneal surface is not regular and symmetric. In fact, the steepest portion of the cornea is directly opposed by the flattest hemi-meridian (Figure 2). A bitoric lens is contraindicated because the steep and flat RGP meridians would not align properly along the corneal surface, and excessive lens impingement at the flattest corneal hemi-meridian (one o'clock quadrant) is expected by the steep RGP lens meridian. I successfully fit this patient into a standard tri-curve spherical lens design with a 7.15mm base curve and low to moderate Dk polymer to avoid lens flexure.

Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.