Fitting Bitoric GP Lenses
By Edward S. Bennett, OD, MSEd, FAAO
It is not uncommon practice today to initially fit soft toric contact lenses on patients exhibiting ≥2.00D of corneal astigmatism. Some of the rationale for this decision is based upon factors such as initial comfort and ease of fitting. However, when highly astigmatic (and predominantly soft toric lens-wearing) patients were given the opportunity to be fit into both soft toric and toric back surface GP lenses, approximately 74 percent preferred the vision of GPs and nearly 58 percent preferred to remain in GPs (Michaud et al, 2009).
In addition, the initial comfort of GPs is being addressed via advances in manufacturing technology, resulting in thinner lenses with consistently high edge quality and good central and peripheral corneal alignment.
Improving Lens Design
The introduction of simple empirical methods of designing back-surface and bitoric lenses also has greatly simplified the fitting process while also often resulting in successful fits. The standard for more than 20 years has been the Mandell-Moore Guide (Mandell and Moore, 1988). This guide and the accompanying calculator are available at www.gpli.info. Enter the keratometry values and refraction, and the toric base curve radii and accompanying powers are provided. This is the most popular empirical method for designing toric back surface and bitoric GP lenses (Blackmore et al, 2006) and has also resulted in an equivalent success rate to bitoric diagnostic lens fitting (Pitts et al, 2001).
Another new and dynamic empirical method has recently been introduced: the “GPLI Spherical and Toric Lens Calculator,” also available at www.gpli.info. The toric calculator, developed by Tom Quinn, OD, MS, FAAO, shows optical crosses of the refractive and keratometric values, the tear lens powers, and the resultant base curve radii and powers of the toric GP contact lens. It also provides useful comments as to whether rotation will have any effect on vision. Figure 1 shows an example.
Figure 1. An example of fitting using the GPLI Spherical and Toric Lens Calculator.
What about the other lens parameters? Actually, determining the base curve radii and powers has always been perceived as the largest hurdle to jump over in GP toric back surface design. With advancements in manufacturing of these designs, you can rely on your laboratory to provide its recommended lens material as well as the other parameters (overall/optical zone diameter, center thickness, peripheral curve radii).
If you desire to design the lens, keep in mind that the lens material, overall diameter, and center thickness (in most plus power meridian) will essentially be the same as for a spherical lens. I recommend a tricurve toric periphery in which 1mm is added to the base curve radii to establish the secondary curve radii, and 3mm is added to the base curve radii to establish the peripheral curve radii. In the example in Figure 1, the toric peripheral curve radii would be (rounded off ) 8.9mm/8.5mm and 10.9mm/10.5mm. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #189.
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.