Topography and Custom Design
contact lens case reports
Topography and Custom Design
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Recently strides have been made in the topography-based contact lens fitting modules incorporated into many modern corneal mapping systems. One system is the new MasterFit II software that accompanies the Zeiss Atlas 9000 corneal topographer (Carl Zeiss Meditec). It is also compatible with Atlas models 993 and 995.
MasterFit II software is based on the principle that an optimum-fitting GP contact lens should accomplish three objectives: the base curve of the lens should be steep enough to clear the central cornea by 10 to 20 microns; the lens should land on the midperipheral cornea at 3 o'clock and 9 o'clock with approximately 5 microns of clearance; and the GP lens should maintain unobstructed movement along the vertical meridian.
A More Accurate Fit
Our patient is a 32-year-old female with a five-year history of decreasing lens comfort and intermittent blurred vision with a range of toric soft lenses. Her spectacle prescription was OD −4.50 −1.50 x 005 and OS −4.50 −1.50 x 170. Simulated Ks were OD 41.87 @ 10/43.62 @ 100 and OS 42.00 @ 170/43.87 @ 080.
Figure 1 shows the fitting relationship of a GP lens designed with the MasterFit II software on the patient's right eye, with a base curve radius 0.37D steeper than flat K, or 42.25D. The lens provides the optimum fitting relationship with 18 microns of apical clearance, landing at 3 o'clock and 9 o'clock, and 40 to 50 microns of clearance along the vertical meridian. Today, the patient enjoys excellent comfort and VA with her custom GP lenses.
Figure 1. The optimum fitting relationship for the patient's cornea made possible with a lens fitted 0.37D steeper than flat K.
Figure 2 shows the fitting relationship of a 41.75D contact lens fitted 0.50D flatter than the optimum lens. Note that the apical clearance is now less than 5 microns and the contact lens “hits” the corneal apex and decenters on the eye.
Figure 2. The fitting relationship of a lens fitted 0.50D flatter than the optimum fit.
Figure 3 shows the fitting relationship of a 42.75D lens fitted 0.50D steeper than the optimum lens. The software demonstrated the resulting apical clearance that is excessive at 35 microns.
Figure 3. The fitting relationship of a lens fitted 0.50D steeper than the optimum fit.
The MasterFit II software recommends the most appropriate diagnostic GP lens design for each patient's individual topography and refractive error. Additionally, the software allows practitioners to set their own preferences to customize individual fitting philosophies. CLS
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.
Contact Lens Spectrum, Issue: January 2010