Contact Lens Case Reports
Aspheric Lenses for Keratoconus
By Patrick J. Caroline, FAAO, & Mark P. André, FAAO
For the past 50 years, the most common contact lens fitting approach for keratoconus has been the three-point-touch technique. This technique uses a small-diameter (7.8mm to 8.8mm), steep lens to create a “landing zone” on the cornea at 9 o'clock, at the apex, and then again at 3 o'clock (Figure 1).
A recent study by Cristina Kenney, MD, PhD (2003), has suggested that accumulative traumas to the epithelium in patients who have keratoconus can alter the corneal proteins, resulting in disruptions in Bowman's layer (with subsequent corneal scarring) and further thinning of the stroma. Therefore, in recent years there has been a paradigm shift toward minimizing epithelial trauma through the use of more apical clearance fitting philosophies.
Figure 1. Two keratoconus fitting philosophies: three-point-touch and apical clearance.
Focus on Apical Clearance
Today, this is most commonly accomplished through the use of aspheric lens designs that incorporate eccentricities between 0.80 and 1.60. As with all aspheric lenses, the higher the eccentricity number, the greater the amount of lens flattening.
Our patient is a 28-year-old male who has moderate keratoconus in his right eye only. Keratometric readings were OD 48.37 @ 030/51.37 @ 130 with an apical radius of curvature of 64.00D (5.25mm) (Figure 2). The base curve and eccentricity of the diagnostic lens was determined using the Medmont (Precision Technology) lens design software. A base curve was selected that provided between 20 microns and 50 microns of apical clearance and an eccentricity that provided “landing” of the lens approximately 3mm to 4mm from center (Figure 3). On-eye the fluorescein pattern very much emulated that seen with the fluorescein simulation (Figure 4).
Figure 2. Our patient's corneal topography OD.
Figure 3. Diagnostic lens calculation with the Medmont software.
Figure 4. Fluorescein pattern of the patient's posterior aspheric lens on-eye.
Modern posterior aspheric lenses for keratoconus can accomplish a number of desirable outcomes: 1. they can clear the apex of the cornea, thereby minimizing trauma to the corneal epithelium; 2. they can provide a wide area of midperipheral lens landing to stabilize the lens on the highly asymmetric and ectatic cornea; and 3. the larger overall lens diameters (9.2mm to 10.8mm) often provide greater comfort by minimizing excessive lens rocking and movement. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #186.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.