The Excimer Laser
Current technology can achieve a high level of success in moderate and high astigmatic correction.
By Maria Clara Arbelaez, MD; and Thomas Magnago, Dipl-Ing
Due to recent advances in refractive surgery, the complexities of correcting irregular astigmatism can now receive closer attention than ever before. Today, subjective evaluation of the patient's refractive deficit may not produce an accurate depiction of his preexisting astigmatism. To minimize poor postoperative results, the golden rule of refracting a patient is to select the measurement with the lowest cylindrical value. In cases in which multiple measurements of the same cylindrical value can be used, we should choose the one with the axis closest to the 180— of with-the-rule astigmatism. In cases where these rules do not help, we should select the one that shows the lowest higher-order aberrations.
Although we have strategies to select the most accurate preoperative measurements, we must devise proper treatment strategies that translate these into the best astigmatism correction. We believe that excimer lasers are the best technology to reach this goal; they can compensate actively for all kinds of cyclotorsional errors and customize the treatment zone.
Newer ablation algorithms for correction of spherocylindrical values are typically aspheric, compensating for biomechanical influences and thereby preserving existing higher-order aberrations, retaining the patient's habitual visual impressions and avoiding the need for neural adaptation. These sophisticated algorithms also take into account the geometric shape of the anterior cornea using keratometry (K) readings of the two individual axes. These readings allow calculation of energy loss due to reflection (Fresnel loss) and efficiency loss due to the geometric change of the spot shape.1,2 The higher the cylindrical refraction, the more significant the benefit of these calculations. If we do not consider the individual loss in both axes, a 5.00 D correction of astigmatism will result in approximately 0.30 D of residual irregularity, which will not create a perfect cylindrical shape. Not considering the loss of energy would result in approximately 0.60 D of irregularity.
COMPENSATING FOR CYCLOTORSION
With the patient in position under the excimer laser, the laser should compensate for static cyclotorsion, adjusting the ablation profile to match the upright cyclotorsional position of the eye. This subsequently creates the foundation for dynamic cyclotorsional compensation, in which eye movements are tracked intraoperatively. We have found that the static cyclotorsional change between the upright and the supine position is within ±2.5° of rotation in 39% of patients and within ±5° in 72% (Figure 1A). Regarding intraoperative dynamic cyclotorsion, 42% of patients are within 1° of rotation (Figure 1B).
The six-dimensional eye tracker of the Amaris (Schwind eye-tech-solutions, Kleinostheim, Germany)3 reacts to lateral eye movements, eye rolling, and variations in the z-axis that describe the height position. Arba-Mosquera et al4 showed that, at a cylindrical value of 3.00 D, a small rotation of 2° changes the refraction by 0.25 D; a 5° rotation immediately shifts the effect by 0.50 D.
Another key to successful astigmatism correction is the diameter of the optical zone. We typically choose an optical zone of 6.5 to 7 mm; however, in some cases it is safe to go even larger. Wide optical zones are ideal for long-term stability, whereas smaller optical zones can cause optical disturbances such as halos and glare.
Centration of the correction can be based on the pupil center or the corneal vertex; as long as the difference between the two is less than 250 µm, this choice will not significantly affect the treatment. We have found that a 5.00 D astigmatic correction with an offset of 200 µm will result in approximately 0.10 D of residual astigmatism, whereas an offset of 400 µm will result in almost 0.25 D of residual astigmatism. Mrochen et al5,6 and de Ortueta7 have shown similar results. Considering the mean values for photopic pupillary offset7 and cyclotorsion,8,9 we can conclude that centration is more important than cyclotorsion.
THE CLINICAL OUTCOME
The patient's refraction, the cyclotorsional alignment, and the geometrical shape, optical zone, and centration of the treatment all influence the clinical outcome of cylindrical treatments. If any of these values is improperly used, the result should be thought of as residual astigmatism and not an undercorrection, except in the case of an imperfect refraction.
The surgeon must learn to deal with the combined effects and compounding influences that can result in unexpected outcomes in astigmatic treatments. Let us assume a simple combination: The treatment is carried out on the pupil center, located 300 µm from the corneal vertex; this is combined with a 5° torsional misalignment and use of an averaged K-reading rather than the appropriate K values for both axes. For a 5.00 D astigmatic treatment, this combination of errors will lead to almost 0.70 D of residual astigmatism.
In a retrospective study of 50 eyes with preoperative astigmatism greater than 2.00 D, we analyzed astigmatic LASIK outcomes at 6 months.10 All aspheric treatments using a nonwavefront–guided ablation profile were planned with the Amaris' ORK-CAM software module and performed as aberration-free treatments. LASIK flaps were created using the LDV femtosecond laser (Ziemer Group, Port, Switzerland).
Preoperatively, the mean manifest defocus refraction was -3.08 ±2.32 D (standard deviation [SD]; range -7.13 to -1.00 D); the mean manifest astigmatism magnitude was 3.54 ±0.85 D (range, 2.00 to 4.75 D); the mean manifest cardinal astigmatism was 1.26 ± 3.29 D (range, -3.49 to 4.60 D); the mean manifest oblique astigmatism was -0.04 ±1.46 D (range, -2.15 to 2.05 D; Figure 2); and the vectorial mean of the manifest astigmatism was -1.26 D at 179°.
At 6 months postoperative, the mean residual defocus was -0.12 ±0.25 D (range, -0.75 to 0.75 D); the mean residual astigmatism was 0.50 ±0.26 D (range, 0.00 to 1.25 D); the mean residual cardinal astigmatism was 0.10 ±0.43 D (range, -0.86 to 1.12 D); the mean residual oblique astigmatism was -0.02 ±0.35 D (range, -0.76 to 0.66 D); and the vectorial mean of the residual astigmatism was -0.10 D at 173°. Thirty-eight eyes (76%) were within ±0.50 D of the attempted astigmatic correction (Figure 3), and 46 eyes (94%) were within ±1.00 D.
Postoperatively, 36% of patients gained 1 or more lines of distance BCVA, and 4% lost 1 line. No eye lost 2 or more lines of distance BCVA (Figure 4). Additionally, the magnitude of the potential coupling factor was less than 5%, far below other reported values.11,12
We have had the opportunity to work with different excimer laser brands and generations; the characteristics of this technology enable exceptional correction of moderate to high astigmatism. It is an appropriate mode for treating patients with irregular astigmatism.
Maria Clara Arbelaez, MD, practices at the Muscat Eye Laser Center, Muscat, Sultanate of Oman. Dr. Arbelaez states that she has no financial interest in the products or companies mentioned. She may be reached at tel: +96824691414; fax: +96824601212; e-mail: email@example.com.
Thomas Magnago, Dipl-Ing, is Head of Division Customer Support, Schwind eye-tech-solutions GmbH & Co. KG, Kleinostheim, Germany. Mr. Magnago may be reached at tel: +49 6027 508 296; fax: +49 6027 508 208; e-mail: firstname.lastname@example.org.