The Case Against Back-Toric Lenses in 2014
BY EDWARDS. BENNETT, OD, MSED, FAAO, & KEITH PARKER
While toric GP lenses may seem complicated, with experience they are easy to fit—very similar to fitting spherical GP lenses. They provide better vision and are subjectively preferred over soft toric lenses (Michaud et al, 2009); in addition, they are just as successful when fit empirically as compared to diagnostic lens application (Pitts et al, 2001).
Why Back Torics Aren’t Needed
However, there still exists the issue of back-surface toric (i.e., toricity on the back surface only) versus bitoric lens design use. As back-toric designs induce residual astigmatism (which, when corrected on the front surface, results in a bitoric design)—typically between 40% to 50% of the backsurface cylinder of the lens and dependent upon the refractive index of the material—the only time that back-toric lenses should be used is when the lens power toricity is 1.4 to 1.5 times the toricity of the back surface (i.e., the induced cylinder corrects the patient’s residual cylinder).
As this is uncommon and adds a layer of complexity to teaching and understanding GP designs, a strong argument can be made that a bitoric is the only GP design that should be used in patients who have high (i.e., >2.00D) corneal astigmatism.
In addition, a practitioner or laboratory consultant who has this understanding could alter the fitting relationship of a backtoric design to accommodate the refractive cylinder needed. What often occurs is a less-thanoptimum fitting relationship accompanied by less-than-optimum visual acuity. In contrast, bitoric lens designs, when properly designed, fit optimally and allow for the true refractive needs to be met in both meridians.
Both Similar to Manufacture
Back torics at one time were considerably easier to make compared to bitorics. Manufacturers could cut a back toric with some sort of toricity using a crimping or bending device and then block up the blank as normal and cut a spherical front curve.
In the past, making a bitoric required the laboratory to squeeze, bend, or crimp a spherical base curve cut into a blank until the radiuscope indicated that it had a desired toricity to match the toric shape of the cornea. Then, before the lens was prepared/ blocked for the front-curve process, the crimp was released a determined amount to satisfy the cylinder needed for the final prescription. The entire process was more time-consuming and complex than for a back toric. Therefore, back-surface torics were appealing to both laboratories and practitioners—despite the likely compromise in vision due to the induced cylinder—because of their simplicity of manufacturing and reduced cost to practitioners.
If you now fast forward to the present, laboratories no longer crimp lenses. They generate toric back and front surfaces independently of each other. Of course, the two surfaces have to be lined up properly in blocking and cutting, but the high-quality lathes in common use today do this consistently. In fact, most laboratories now charge the same for backtoric and bitoric lens designs.
Considering the improved quality of manufacturing, optimum fitting relationship and resultant vision, and availability of easy-touse resources to empirically design bitoric GP lenses, we argue that back-toric lenses are no longer a viable option for correcting high corneal astigmatism. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #218.
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 firstname.lastname@example.org. Keith Parker is president of Advanced Vision Technologies. He is a past president of the Contact Lens Manufacturers Association and currently serves as chair of the GP Committee.