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EW Supplement: Clinical ViewPoint Wavefront Guided vs. Optimized



   

Optical Zone Comparisons


By Charles R. Moore, M.D., F.I.C.S.
 
 

“Large, true optical zones can result in better quality-of-vision at night when the pupil enlarges and help to prevent roubling night vision symptoms.” Charles R. Moore, M.D., Medical Director of International EyeCare Laser Center in Houston. He is a consultant to WaveLight AG.

 

Refractive surgery these days is about creating enthusiastic patients with exceptional quality of vision. The fact is the quality of the patient's post-operative vision particularly at night, with the specter of glare and halos, is related in part to the true optical zone size. This is something which has been a key part of the Allegretto Wave system (WaveLight Laser Technologie AG, Erlangen, Germany) from the start, but which many other companies have been modifying in an attempt to keep up, for some time.

Precision Counts


In the past, we assumed that the optical zones predicted by lasers matched the zones ablated. However, too often the ablated optical zone was considerably smaller than the intended optical zone size. Unfortunately, this is true even with the newest laser technologies, such as the Visx Star S4. With this laser, the blend zone encroaches on the true optical zone.
When Visx first obtained approval of their wavefrontguided CustomVue platform, this did not include the measurement or treatment of higher-order aberrations. The Visx CustomVue platform was also only originally approved to treat up to 6 D of myopia and 3 D of cylinder, which has recently been expanded. The Allegretto Wave’s wavefront-optimized approach was initially approved to treat myopia up to -12D and 6D of astigmatism as well for hyperopia up to +6D and 5D of astimatism.
It wasn't until Visx developed the Star S4 WaveScan, that they changed the algorithm to increase the optical zone size to more favorably compare to those created by the standard wavefront-optimized Allegretto Wave platform.

Topographical Evidence


The trouble is that when the effective optical zone is too small it can result in poor outcomes for patients. Consider the following cases, involving patients who had undergone standard Visx Star S3 standard treatment (Figure 3) and Star 4 CustomVue/wavefront-guided (Figure 2). In these treatments, the ablated optical zone sizes were significantly smaller than the predicted ones - all resulting in poor outcomes. To correct the problems that resulted, wavefront- optimized (Figure 4) enhancements were performed.
With the Allegretto Wave's wavefront-optimized (standard) program as seen in Figure 2, the resultant optical zone sizes meet the predicted ones.

Wavefront-Optimization and Optical Zones

The Allegretto Wave system's large true physiological optical zone, results from its optimized approach. Using builtin wavefront principles anda proprietary algorithum to minimize induction of C 12 and mantain the balnce of the natural wavefront. The Allegretto Wave delivers a larger number of pulses to the peripheral cornea, compensating for the more tangential energy delivered in this area. This 30% to 40% increase in peripheral corneal ablation helps to preserve the pre-operative aspheristy, which results in less postoperative shrinkage of the optical zone.
By maintaining the natural corneal shape during LASIK, transition zones are minimized and a wide, uniform central optical zone is created. This leaves room for the pupil to expand in mesopic conditions, without the patient having to visually deal with zones of competing powers. This can result in better quality-of-vision at night when the pupil enlarges and help to prevent troubling night vision symptoms.
It is also possible, if you have a case with a thin cornea in which you are conhyperopia cerned about tissue preservation, of course to electively create a smaller true optical zone.
However, it is the increased optical zone sizes, available with the Allegretto Wave, for the treatment of and hyperopic astigmatism in postoperative radial keratotomy cases that make it possible for surgeons to safely treat complicated cases.

Figure 1: Corneal topography after Visx Star S3 Standard Treatment with an
8mm blended OZ.



Figure 2: Large, true optical zone after a wavefront optimized treatment.

Figure 3: Patient who underwent a Visx Star S4 CustomVue wavefront-guided treatment. Note that the resulting optical zone size was ignificantly smaller than the indended optical zone size (6.00x5.00mm)


Figure 4: Post op topography after a wavefront-optimized enhancement. The retreatment created an optical zone of 6.62mm as compared to 3.82mm with her original wavefront guided Visx S4 CustomVue treatment.

A Case of Multiple Refractive Procedures

It is even now possible to expand the optical zone in patients who have been previously treated with a conventional laser - even in patients who have been treated many times before. We recently had a case involving a patient who had initially undergone RK to correct myopia back in 1993. After RK enhancements her vision ultimately was 20/50 in the right eye and 20/25 in the left eye.
A decade after her initial surgery, in 2003, the patient, who's UCVA had slipped to 20/60 with monocular diplopia underwent a wavefront- guided procedure with the Visx Star CustomVue. Following her wavefrontguided custom treatment with the Visx Star S4, her UCVA was at the 20/25 level, but the patient was unhappy with her quality-of-vision. The reason appeared to be a too small optical zone, which measured just 3.85 mm instead of the intended 6.0 x 5.0 mms (Figure 2). The patient's UCVA in her right eye had regressed to 20/100 and she had a best spectaclecorrected visual acuity (BSCVA) of 20/40-2.
However, just one month after undergoing an optimized retreatment with the Allegretto Wave in 2005, her UCVA was 20/25.
By the three-month postoperative mark, the patient was telling us how pleased she was by her improved quality-of-vision. Her binocular UCVA was now at 20/16 and her uncorrected monocular acuity in her right eye was at 20/25-1. Meanwhile, her predicted optical zone size of 6.5 mm was actually exceeded by her postoperative measured optical zone size at 6.62 mm. (Figure 4). By increasing her optical zone diameter and smoothing her ocular surface, we were able to vastly improve her quality-ofvision. This is typical of what we have been able to accomplish with the Allegretto Wave.

Conclusion

Overall, in my experience with the Allegretto Wave, I have found that treatment with this laser has resulted in better quality-of-vision post- LASIK for many patients than what they attained preoperatively with spectacles or contact lenses. No matter what the pupil size, my patients do not report glare or halos postoperatively. With the help of the Allegretto Wave, which I have found to be the best in terms of achieving an enhanced, uniform optical zone, I have had first-rate outcomes and very satisfied patients.





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