Michael Baertschi, MSc Mmed Education FAAO
Michael Wyss, MSc FAAO
Simon Bolli, Eidg dipl Augenoptiker
Marc Fankhauser, Eidg dipl Augenoptiker
Reverse geometry designs offer a fascinating approach for fitting (R)GPs after penetrating keratoplasty (PKP). Due to their unique geometry, the contact lens mimics the topography of a post-PKP cornea, which allows us to minimize the physiological impact while providing a superb optical result. Central keratometry readings in such cases do not relate to the peripheral curvature of the host cornea. Therefore, starting the fitting procedure in the peripheral host cornea is important. It is equally important to evaluate the sagittal depth needed to vault the graft-host interface.
A parallel fitting of the central cornea will not achieve a deep enough sagittal depth (’sag’) to vault the graft host interface, and the (R)GP lens will lift off in the periphery. Changing the base curve to create a deeper sag will result in excessive vaulting of the central cornea, which will lead to decreased visual performance and to contact lens adherence. Working with reverse geometry lenses offers the unique possibility to influence sagittal depth by changing the reverse zone only.
A 55-year-old female underwent a PKP and was fitted with a reverse geometry (R)GP lens. The first lens exhibited a perfect alignment fit in the periphery, but excessive central vaulting. (Figure 1)
Figure 1: Reverse geometry lens (V9) with excessive central vaulting.
There are three options to reduce the sag. Two are to fit the base curve flatter or to fit a flatter periphery, but with these options we have to compensate the whole geometry. For example, if you flatten only the base curve, the periphery will lift off of the cornea as a consequence. Reverse geometry designs offer the option to reduce only the sag, which is accomplished by making the reverse curve smaller. In our example, the first lens had a reverse curve with an additional sag depth of 90 microns (Falco Terminology V9). To achieve a better fit, the reverse curve depth was reduced to 40 microns (Falco Terminology V4), while all other parameters were similar to the previous lens. (Figure 2)
Figure 2: Schematic drawing of sag change from V9 to V4.
The fluorescein pattern showed a much better central alignment, while the peripheral fit was not influenced in any way. (Figure 3)
Figure 3: Reverse geometry lens (V4) with reduced sagittal depth.
Reverse geometry lenses offer a fascinating approach for fitting (R)GPs after PKP. Manipulating sagittal depth by changing the reverse curve is a huge advantage that simplifies the fitting procedure.
Our next I-site Newsletter contribution will concentrate on peripheral fitting possibilities such as peripheral toric or quadrant specific designs
Michael Baertschi was the senior optometrist at the University Eyehospital Basel from 2000 to 2007. He is the owner of Kontaktlinsenstudio Baertschi in Bern, Switzerland and the CEO of Eyeness AG in Bern. Michael graduated from Pennsylvania College of Optometry as M.Sc. Optom. and from the University of Bern as M.med. Educ. Michael Baertschi is a fellow of the American Academy of Optometry and president of the Swiss Interlens group.
Michael graduated from Olten SHFA in Switzerland and did his MSc at the Hochschule Aalen Germany (in cooperation with New England College of Optometry and Pacific University, USA). Since 1999 he has worked in a private practice (kontaktlinsenstudio Baertschi in Bern, Switzerland) as Optometrist for specialty contact lens fitting. Additionally, he is an adjunct Faculty Member at the New England College of Optometry USA, Hochschule Aalen Germany, TVCI in Prague (Czech Republic) and FHNW Optometry in Olten Switzerland. Michael is a clinical investigator for several Industry Partners and has published or lectured on several topics in the contact lens field throughout the world. Michael is a Fellow of the American Academy of Optometry and serves as a the vice chairman of the Admittance Committee for new Fellows outside the USA.