Cataract & Refractive Surgery Today
http://www.crstoday.com/02_current/crst1102_12.html
The Bitoric and Cross-Cylinder Alternative
Arturo Chayet, MD, of Chula Vista, California, and Paolo Vinciguerra, MD, of Milan, Italy, pioneered the techniques of bitoric and cross-cylinder ablations.3-5 Both techniques divide the pre-existing astigmatism into hyperopic and myopic components. The key difference is that the cross-cylinder technique divides the preoperative cylinder into equal myopic and hyperopic components and treats the spherical equivalent of the preoperative refraction. Dr. Chayet and his colleagues introduced the bitoric LASIK ablation profile in order to correct mixed and simple myopic astigmatism by dividing the preoperative astigmatism into unequal ratios of myopic and hyperopic cylinder. Theoretically, this technique avoids causing a hyperopic shift in the spherical component of the refraction.
It is not surprising that both techniques were originally developed and patented6,7 by Nidek Co., Ltd., (Gamagori, Japan) for its EC-5000 excimer laser platform, because the circular ablation profile of the standard Nidek myopic cylinder ablation demonstrates greater hyperopic spherical coupling than other modern excimer laser platforms performing unimeridional (oval) ablation profiles. With the possible adoption of both of these ablation algorithms to the EC-5000, Nidek will be able to provide refractive surgeons with a detailed, cognitive, preoperative approach and one of the broadest astigmatic treatment ranges of any excimer laser platform currently available.
The primary goal of the bitoric and cross-cylinder techniques is to create a more stable, predictable optical result with fewer of the side effects commonly associated with unimeridional myopic cylinder treatments (Figure 1). One of the best times to use these alternate approaches to astigmatism treatment is when correcting mixed astigmatism induced by prior refractive procedures. During the preoperative surgical planning, it is important to note that these techniques differ in regard to the location and quantity of corneal tissue ablated. My personal clinical experience is that a cross-cylinder ablation provides the highest quality mesopic/scotopic vision in high astigmatic corrections. The cross-cylinder (50/50 astigmatic split) ablation treats the spherical equivalent of the manifest refraction with the cylinder of the same sign. I generally treat the myopic component first. For example:
Preoperative manifest refraction: -1.0 -5.0 X 180
Treatment Parameters
• First treatment: -3.50 -2.50 X 180
• Second treatment: plano +2.50 X 90
As a general rule, I will make my standard personalized-nomogram adjustments to each component of the treatment.
The cross-cylinder ablation picks up where the unimeridional, compound/myopic astigmatic ablation falters. The other key benefit of the cross-cylinder ablation is manifest in the form of less regression/greater stability of the treatment effect with higher-magnitude cylinder corrections. Cross-cylinder ablations are also tremendously useful when treating mixed astigmatism. For example:
Preoperative manifest refraction: +4.0 -6.0 X 180
Treatment Parameters
• First treatment: plano -3.0 X 180
• Second treatment: +1.0 + 3.0 X 90 (spherical equivalent added to cylinder of same sign)
The main caveat in using the cross-cylinder ablation approach is that it consumes more total tissue than the unimeridional treatment.
