Asymmetric Keratoconus Designs
contact lens case reports
Asymmetric Keratoconus Designs
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
In clinical practice we use the term keratoconus to describe a spectrum of diversely shaped conditions in which the only common denominator may be central or paracentral corneal steepening. Yet, it's the topography beyond the central and paracentral cornea that can significantly influence the optics of the cornea and the success of a contact lens design.
To better understand modern keratoconus contact lens fitting techniques, it's imperative to appreciate the common central and midperipheral topography changes associated with the condition.
Figure 1. Corneal mapping of the patient's right eye, note the corneal asymmetry at 12 o'clock and 6 o'clock.
Figure 2. Corneal mapping. Note the symmetry along the horizontal meridian.
Modern corneal mapping techniques demonstrate that in keratoconus there's often a topographical asymmetry along the vertical meridian of the cornea with paracentral steepening in the inferior, nasal quadrant. The inferior ectasia results in an island of flatter-than-normal cornea approximately 180 degrees away, usually in the more superior, temporal quadrant (Figure 1).
Figure 3. Fluorescein patterns of a traditional KC design and the QuadraKone design.
Additionally, the flatter superior corneal topography becomes one of the more important considerations in fitting rigid lenses for keratoconus. It's at the flatter superior portion of the cornea that the rigid lens will be tightest. Along the horizontal meridian, there's a surprising degree of corneal symmetry (Figure 2).
In recent years we've seen new designs called quadrant-specific lens designs. These incorporate different radii in the lens periphery to better align the flatter superior cornea and the steeper inferior cornea. One such design is the QuadraKone Design from Tru-Form Optics.
When we fitted the right eye of a 24-year-old moderate keratoconus patient with a traditional (symmetrical) keratoconus lens design, the lens resulted in excessive superior impingement and excessive inferior clearance. We ultimately refit him in the asymmetric Tru-Form QuadraKone design (Figure 3), which resulted in improved lens alignment along the vertical meridian at 12 o'clock and 6 o'clock.
Not Just One Answer
To better manage keratoconus, be aware that no one design provides adequate results in all cases. It's also important not to design lenses based on information provided by central keratometry readings. Rely on the central and peripheral information you obtain from corneal mapping and/or through detailed fluorescein evaluation of diagnostic lenses.
Advanced quadrant-specific designs offer practitioners and patients another alternative in managing these often highly asymmetric corneas. CLS
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences and SynergEyes, Inc. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision and SynergEyes, Inc.
Contact Lens Spectrum, Issue: February 2008