Cross-cylinder technique enhances optical quality in astigmatic LASIK

A prolate, more physiological postoperative cornea is created, saving up to 60% of stromal tissue.

ZURICH — A new method of correcting astigmatism, named the “cross-cylinder” technique by its developers, Paolo Vinciguerra, MD, and Daniel Epstein, MD, treats both principal meridians, correcting half the magnitude of the astigmatism on the flattest meridian and the other half on the steepest.

Since the advent of excimer laser surgery, the correction of astigmatic errors has presented a special challenge. According to Prof. Epstein, of the University Hospital here, the commonly used treatment strategies do not create a sufficiently smooth corneal surface to achieve top-notch corneal quality.

“The reason for this,” Prof. Epstein said, “is that standard ablation patterns for astigmatic correction treat only the steeper meridian, leaving the flatter meridian unchanged. This, in turn, creates poor transition zones along the steeper meridian and unphysiologically abrupt dioptric curvature gradients, resulting in a midperipheral multifocality of the cornea. The poor transition zones achieved by treating only one meridian lead to an abnormal healing response, and cause an overcorrection of the sphere and undercorrection as well as regression of the cylinder.”

Consequently, patients experience regression, best-corrected visual acuity (BCVA) decrease and disturbing functional symptoms such as halos and glare.

Prolate optical zone, smooth transitions

According to the developers, “Our technique overcomes these problems by treating the cylinder with two ablations of the same power but with opposite signs (one plus, one minus). A prolate, almost physiological optical zone is created, with very smooth transition zones in the midperiphery of the cornea.”

After the cross-cylinder astigmatic correction, they explained, the resulting spherical central cornea is ablated by the dioptric magnitude obtained by calculating the spherical equivalent (SE) of the preoperative refraction, as in the following examples:

1. Preoperative refraction = –3.00 –4.00 × 180

The cross-cylinder ablation is performed in the following steps:

+ 2.00 × 90

–2.00 × 180

–5.00 (SE)

2. Preoperative refraction = +3.00 –5.00 × 135

+2.50 × 45

–2.50 × 135

+0.50 (SE)

Many advantages

---Ablation using the cross-cylinder technique in a patient with –4.00 –2.50 results in a prolate cornea.
FIGURE COURTESY OF PAOLO VINCIGUERRA, MD

When asked what the difference is between the cross-cylinder method and bitoric ablation patterns, Prof. Epstein answered that the results of the two techniques are quite different.

“The bitoric ablation does not create the symmetry necessary for obtaining a prolate (and more physiological) cornea. An asymmetric postoperative corneal shape produces optical aberrations,” he said. “Also, the bitoric ablation pattern is variable and, thus, different for different patients, depending on the magnitude of the sphere and cylinder. This makes it difficult to compare results. Moreover, it is not certain that a bitoric ablation nomogram developed for one excimer laser can achieve equal results when used with another laser.”

An important feature of cross-cylinder ablation is that of sparing stromal tissue. By spreading the ablation over a larger corneal area, less tissue is removed. Depending on the correction, up to approximately 60% of corneal tissue can be saved in comparison with standard astigmatic correction programs.

“Such a substantial sparing of tissue is of special interest in laser in situ keratomileusis (LASIK),” Prof. Epstein pointed out. “The range of LASIK correction is limited by the amount of tissue actually available for laser treatment once the flap thickness and the need for a residual stromal bed are accounted for. By consuming less tissue, the cross-cylinder technique increases LASIK’s potential for correction.”

Last but not least, the symmetric ablation achieved by the cross-cylinder strategy cuts short the extended healing process induced by the sharp transition edges of commonly used ablation patterns.

“Accordingly, there is less regression in cross-cylinder eyes,” Prof. Epstein said. “And, the prolate corneal shape produced (without midperipheral multifocality) explains why these eyes do not lose even one line of best-corrected visual acuity and are free from functional disturbances at night.”

Ablation without the cross-cylinder technique results in a red ring around the cornea. FIGURE COURTESY OF PAOLO VINCIGUERRA, MD	Ablation using the cross-cylinder technique again yields a prolate cornea. FIGURE COURTESY OF PAOLO VINCIGUERRA, MD

For Your Information:

• Paolo Vinciguerra, MD, can be reached at the Istituto Clinico Humanitas, Milan, Italy; (39) 02 55211388; e-mail: vincieye@tin.it. Dr. Vincignerra has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Prof. Daniel Epstein, MD, can be reached at Dept. of Ophthalmology, University Hospital, Zurich, Switzerland; (41) 629233830; e-mail: epsteye@bluewin.ch. Dr. Epstein has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.