Reducing Spherical Aberration

Currently, performing excimer laser ablations frequently produces a large dioptric power change at the junction between the optical and transition zones. This dioptric gradient scatters light across the retina and can increase, rather than decrease, the point spread function for any given object. This junction also determines the size of the effective optical zone, through which the eye sees without visual disturbances after excimer laser surgery. I use the NIDEK EC-5000 excimer laser system (Nidek Inc., Fremont, CA) to reduce this dioptric gradient and improve the characteristics of the effective optical zone.

THE OBLATE SHAPE
Excimer lasers can alter the human eye's natural optics by changing its corneal shape from prolate to oblate. On topography, the postoperative oblate shape is identifiable by a red ring representing the high peak of tissue that surrounds the ablated tissue. This abrupt alteration in corneal shape has been linked to various undesirable effects such as nighttime glare and halos, decreased contrast sensitivity, and poor subjective vision despite excellent objective UCVA postoperatively.1 Smoothing out this ring of tissue could potentially decrease these undesirable effects by producing a more prolate corneal shape.

OPTIMIZING THE LASER PROFILE
The NIDEK EC-5000 has adjustable optical and transition zones, and researchers such as Paolo Vinciguerra, MD, of Milan, Italy, and Arturo Chayet, MD, of Tijuana, Mexico, have found that altering the laser profile can reduce the dioptric power gradient caused by laser ablation.2 Because the function between the optical and transition zones is nearly linear, changing the ablation depth alters the depth of the transition zone. Drs. Vinciguerra and Chayet found that decreasing the optical zone diameter to 5 mm and changing the transition zone to 9 mm (the maximum width possible with the MS-DOS software) pushes the ring of tissue farther out and thereby creates a more prolate corneal profile. In addition to excellent nighttime vision, patients whom they treated with aspheric ablation reported minimal-to-no change in nighttime glare, halos, and contrast sensitivity. Dr. Chayet also conducted a small study that demonstrated a slight increase in contrast sensitivity in some patients several months postoperatively.3 Although this finding may have been unexpected, it is more difficult to challenge when other physicians performing aspheric ablations anecdotally have reported similar results.

Further research by Nidek Inc.'s engineers and additional clinical trials showed that the best relationship between the optical and transition zones is essentially linear, represented by the midpoint (number four) of eight profiles tested. Profiles three through one were progressively more concave, and profiles five through eight were progressively more convex. Profile four is unavailable in the US, but users in this country may use number three, which is slightly more concave. The company has not yet determined when profile four will become available in the US.

NOMOGRAM CONFIGURATION
I began developing my own nomogram for aspheric ablation with the NIDEK EC-5000. After carefully selecting the first several patients, all of whom were low myopes, I set the optical zone at 5 mm and the transition zone at 9 mm. I intentionally undercorrected patients and then adjusted my nomogram accordingly. By the time I treated my sixth patient, my nomogram was fairly accurate (Figures 1 and 2). During the past 7 months, I have performed aspheric ablations on approximately 250 eyes with a nomogram of 0.76 X (Sph-((Cyl)(0.4))). If I wish, I can accommodate the corneas of patients over the age of 45 by decreasing the first modifier to 0.72. I can also account for changes in temperature and humidity between operating centers by raising or lowering the first modifier.

STUDY RESULTS
Corrections performed on the 125 study participants ranged between -1.00 and -12.00 D. I have accumulated data on nine patients, all of whom had 20/15 UCVAs postoperatively. At 1 week postoperatively, 63% of patients had UCVAs of 20/15, and this figure increased to 77% by 1 month. Although the patients whom I have treated with 6.5-mm optical zones and 7.5-mm transition zones did not lodge a high number of subjective complaints, I found that the frequency of these complaints decreased significantly with aspheric ablations. For instance, a helicopter pilot with 7-mm pupils on whom I performed a -4.00-D correction reported no unwanted nighttime visual phenomena 1 week postoperatively.

More importantly, I found a link between complaints of nighttime glare or halos made by patients who underwent aspheric ablations and the occurrence of postoperative dry eye, under- or overcorrection, or regression. After appropriate treatment, these patients reported good daytime and nighttime vision. Often, the patients I treated attained better visual acuities with aspheric ablations than they had with either contact lenses or spectacles, and they noticed no significant change in their nighttime vision from baseline as early as 1 week postoperatively.

According to my preoperative analyses, the majority of the patients whom I treated did not have significant amounts of higher-order aberrations. Postoperative evaluations using the NIDEK OPD-Scan (Nidek Inc.), which includes wavefront analysis, showed that most of them experienced little-to-no increase in higher-order aberrations.

I have not yet fully evaluated my enhancement rate. To date, I have only re-treated nine eyes, including those requiring enhancements due to nomogram adjustments.

PERSPECTIVE ON THE STUDY
The key to aspheric ablation appears to be combining a wide ablation zone, a moderately sized central optical zone, and a gradual transition zone to the 9-mm edge. This procedure creates a smoother, more prolate cornea that pushes the peaked ring of tissue surrounding the ablation zone out farther, particularly for high myopes. Aspheric ablations appear to decrease the amount of nighttime glare patients experience postoperatively without significantly altering their contrast sensitivity. Moreover, treatments using a large ablation zone appear to offer further improvements in contrast sensitivity compared with results with the 5.5-mm optical and 7.0-mm transition zones. Additionally, because the optical zone is 5 mm wide, the deepest point of an aspheric ablation appears to remove only approximately 12.5 to 13.0 mm of tissue per diopter.

TECHNICAL POINTERS
Aspheric ablations do present a few surgical obstacles. First, using a smaller central optical zone increases the need for carefully centering the laser treatment. Although I have found eye tracking devices unnecessary thus far, they may allow the creation of an even more accurate treatment profile.

Second, the LASIK flap must be larger than 9 mm to accommodate the aspheric ablation. I use the M2 microkeratome (Moria, Antony, France) to create a superior-hinged flap, and I find it beneficial to use the 8.5 setting, which creates a slightly smaller hinge and more circular flap.

Third, performing aspheric ablations takes slightly longer than conventional LASIK treatment, because the MS-DOS software must conduct the correction in two stages. Spherical and astigmatic corrections require four stages. Further, the slightly extended duration of treatment may affect corneal hydration during ablation.

CONCLUSION
The results that other surgeons and I have achieved with aspheric ablation have been gratifying, and the procedure appears to yield stable corrections. It also appears to reduce the need for LASIK enhancements, because, thus far, the overall enhancement rate for the entire study group has been less than 5%. The number of patients achieving UCVAs of 20/15 or better has been high and the subjective quality of vision excellent. Now, we must combine the use of this treatment profile with wavefront-guided ablation in order to reduce the induction of higher-order aberrations. This step may ultimately provide patients with 20/10 vision free of aberrations.

Manoj Motwani, MD, is Medical Director and Chief Refractive Surgeon at the Alpha Laser Center in San Diego. He does not hold any financial interest in the products and companies mentioned herein. Dr. Motwani may be reached at (858) 554-0008; lasikdoc@seealpha.com.
1. Holladay JT, Janes JA. Topographic changes in corneal asphericity and effective optical zone size following LASIK. J Cataract Refract Surg. 2002;28:942-947.
2. Vinciguerra P, Camesasca FI, Urso R. Reduction of spherical aberration with the Nidek NAVEX customized ablation system. Refract Surg. 2003;19:2(suppl):S195-201.
3. Chayet A. The latest clinical results with NAVEX. Paper presented at: NAVEX Workshop; September 6, 2002; Nice, France.