When to Fit GPs Empirically
BY EDWARD S. BENNETT, OD, MSED, FAAO
Empirical fitting of GP lenses—originally promoted more than 20 years ago (Ames, 1993)—has become an optimum method to help ensure success with this modality. With the current popularity of soft lenses, it’s important to be able to fit out of inventory and to provide acceptable vision with the initial lens experience. A positive visual experience may also help patients be less cognizant of the initial awareness.
Which Lens Types Work Best?
Empirical design is a good option in the following GP lens types.
Spherical Lenses Modern CNC lathes and other advances in lens design and manufacturing have resulted in empirically designed spherical lenses that rival the results of those fit diagnostically. The lens designs often have aspheric or pseudo-aspheric peripheries, an ultrathin center thickness, and a consistent edge design. In addition, topography software programs and laboratories’ nomograms and calculators can help achieve first-fit success. Also, the GP Lens Institute (www.gpli.info) and eyedock.com offer online calculators (Bennett, 2014).
Multifocal Lenses Presbyopes, notably current spectacle or soft lens wearers, benefit from the immediate gratification of good vision at all distances often experienced with GP multifocal lenses (Benoit and Ames, 2010). Aspheric designs, in particular, can offer first-fit success with empirical fitting.
Segmented, translating lens designs are best fit with a diagnostic fitting set to accurately assess factors such as seg position, lens relationship to the lower lid, and translation with downgaze. There is one exception for which practitioners provide anatomical measurements (i.e., lower pupil margin to lower lid, horizontal visible iris diameter, vertical fissure design, pupil diameter, etc.) to help optimize success via empirical ordering.
Toric Lens Designs While GP toric lens design is usually perceived as complex, empirical fitting of GP torics can be more successful when compared to soft lenses (Michaud et al, 2009) and has resulted in a comparable success rate when compared to diagnostic fitting (Pitts et al, 2001). The first empirical program to be introduced was the Mandell-Moore Guide (Mandell and Moore, 1988). It is a very simple method to determine bitoric powers and base curve radii, and it is available as both a form and as a calculator (www.gpli.info/mandell-moore-bitoric-calculator). Clarke Newman, OD, FAAO, developed the Newman GP Toric Guide (www.gpli.info/newman-gp-toric-guide), a downloadable form that can help practitioners determine the base curve radii, powers, and other lens parameters. Finally, the GPLI Toric and Spherical Lens Calculator shows the tear film powers graphically, how the powers and base curve radii are derived, and considerations pertaining to lens rotation (www.gpli.info/lens-calculator).
Hybrid Lens Designs SynergEyes recommends empirical fitting of its Duette Progressive, requiring only the manifest refraction, keratometry values, the add power, and the dominant eye.
The Bottom Line
Obviously, there are some designs for which empirical fitting would not be desirable; most segmented, translating designs and lens designs for managing keratoconus, post-surgical, and other irregular cornea patients should be fit diagnostically to ensure a satisfactory lens-to-cornea fitting relationship. Likewise, scleral lenses benefit from diagnostic fitting to ensure proper central clearance and peripheral scleral alignment.
That said, empirical fitting, when indicated, not only eliminates the initial fitting step, it also can compete effectively with inventory fitting of soft contact lenses while providing the long-time benefit of high-quality vision. CLS
For references, please visit www.clspectrum.com/references and click on document #234.
Dr. Bennett is assistant dean for Student Services and Alumni Relations at the University of Missouri-St. Louis College of Optometry and is executive director of the GP Lens Institute. You can reach him at email@example.com.