contact lens case reports
Reverse Geometry Lens Designs and Keratoconus
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
The use of apical clearance, reverse geometry lens designs for keratoconus has always been an intriguing topic. This is especially true in light of new research indicating that accumulative epithelial traumas in keratoconus can alter the biochemistry of the stroma and result in further tissue compromise. Therefore, minimizing epithelial trauma may, in the long term, be in keratoconus patients' best interest.
Given an already small posterior optic zone of 6.0mm, there are two ways to change the sagittal height of a GP lens: one is to steepen or flatten the base curve radius, the other is to raise or lower the lens through the use of a reverse curve radius.
Reverse Geometry Lens Fitting
Our patient is a 38-year-old female with a history of late onset keratoconus. Figure 1 shows the patient's corneal maps with apical radii of OD 53.75D (6.30mm) and OS 52.75D (6.40mm). Using the OrthoTool software (www.orthotool.com), we designed 11.0mm reverse geometry lenses with peripheral radii to align her peripheral corneas and reverse curve radii to clear the corneal apices. Figure 2 shows the specifications of the right contact lens (base curve 47.00D [7.20mm]) and the actual fluorescein pattern. It is clear that the 6.90mm reverse curve resulted in a lens with apical touch or inadequate sagittal depth. Figure 3 shows the left eye in which the same base curve radius and a reverse curve of 7.25mm resulted in an optimum amount of apical clearance.
Figure 1. The sagittal height of the lens can be raised and lowered through changes in the reverse curve radius.
Figure 2. Reverse geometry lens design and fluorescein pattern for the right eye. Note the apical touch.
Figure 3. Reverse geometry lens design and fluorescein pattern for the left eye. Note the apical clearance.
A new right lens was designed with a base curve radius of 47.00D (7.20mm), with a steeper reverse curve radius incorporated to increase the sagittal height an additional 100 microns. Figure 4 shows that the lens has cleared the corneal apex. The patient's final visual outcome was stable at 20/25 OD and 20/20 OS, with all-day wearing comfort. CLS
Figure 4. Reverse geometry lens design and fluorescein pattern for the right eye with the steeper reverse curve radius.
For references, please visit www.clspectrum.com/references.asp and click on document #175.
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.