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Both wavefront and topography needed for optimum customization
Wavefront measures the error in the optical system of the whole eye, while topography isolates the error in the patient’s cornea.
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For the best results with customized refractive surgery, a combination of wavefront and topographical analysis of the eye’s optical errors is needed, according to one optics expert.
“Taking into account both wavefront and topography, you can truly provide customized ablation that is tailored to the patient’s needs,” said Jack T. Holladay, MD, MSEE, FACS. “Those systems that include both will be the way we do it in the future.”
No systems using both wavefront and topography are currently commercially available, but according to Dr. Holladay integrated systems using both diagnostic modalities are “on the horizon” for surgeons in the United States, coming to market in the not-too-distant future.
Incremental improvements
Dr. Holladay said significant improvements have been seen in excimer laser systems for refractive surgery since their introduction in the United States in 1995 and earlier in the rest of the world.
These improvements have included an improved radial compensation function, so that the energy delivered by the laser in the periphery of the cornea is sufficient to achieve the intended tissue removal. (For more on this concept, see part 7 of the Quality of Vision series, “Modern excimer improvements driven by more than wavefront,” October 1 issue, page 1.)
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“This improvement has allowed us to achieve prolate corneas following myopic refractive surgery,” Dr. Holladay said.
A second improvement is related to the size of the optical zone in astigmatic treatments, he said.
“Any time we put an astigmatic treatment on the cornea, the ablation zone by definition must be elliptical,” Dr. Holladay said. “In 2002, we convinced the Food and Drug Administration to specify the smaller axis of the oval to be the optical zone size rather than larger diameter.”
Before this change, if a plano patient with –4 D of astigmatism was treated with a 6-mm optical zone, the dimensions of the actual treatment on the cornea would have been 4.5 mm by 6 mm.
“This would induce haloes and glare in the shorter axis in a patient with a 6-mm pupil,” Dr. Holladay said. “And because the treatment is not symmetrical, it’s even more bothersome than if it was just circular.”
Today, he said, a 6-mm treatment for the same patient would be 6 mm by 7.5 mm, so that for the patient with the 6-mm pupil, the entire pupil is covered by the optical zone. (This concept is also discussed in the October 1 issue, which is referenced above.)
A third improvement has to do with the quality of the surface in the center of the optic zone, Dr. Holladay said.
“Refinements of both broad-beam and flying spot lasers have allowed us to make a much smoother central optical zone area,” he said, “which ultimately relates to the quality of vision the patients have in the long term.”
Eye trackers have also improved since their introduction, Dr. Holladay noted.
“Our original trackers following the pupil at 60 Hz have been improved, and now we use iris registration and recognition, which takes into account both torsional and saccadic movements of the eye,” he said. “The enhanced tracking means we are putting laser energy at exactly the right spot on the cornea.”
The final ingredient in this series of incremental improvements has been the introduction of wavefront aberrometry measurements, Dr. Holladay said. Use of wavefront-guided treatments has been shown to improve outcomes in people with higher-than-average preoperative aberrations, he said.
“All of these ingredients, taken together, are the explanation for why we have improved outcomes,” he said.
Topography is next
The next improvement, Dr. Holladay said, will be the introduction of topography as a second diagnostic modality for guiding customized ablations.
“In some parts of the world, topography-guided custom ablation systems have had very successful outcomes,” he said. “Topography helps us relate our treatment directly to the problem the patient is having. If a patient has a decentered ablation zone or other abnormality clearly is limited to the cornea that is a consequence of an earlier laser treatment, the topographic measurements of that problem are much more precise, have many more points and higher resolution, than wavefront analysis. That allows you to design a treatment that restores the cornea to a more symmetrical outcome because you are directly measuring the surface in which the problem exists.”
An even greater advantage will be realized when both topography and wavefront can be used in tandem, he said.
Wavefront measurements provide important information about the aberrations of the entire eye’s optical system, Dr. Holladay said. Wavefront measurement can help assure that a custom laser treatment leaves the cornea with the right amount of spherical aberration, he said.
“We know that the cornea has positive spherical aberration, and we know that when we’re young the lens has negative spherical aberration. So, in an individual their 20s an ablation that preserves the spherical aberration of their cornea would be ideal because they have no total ocular spherical aberration. Matching the postop result to their preop shape would maintain no spherical aberration in their eye,” he said.
“So that ocular wavefront, describing the amount of spherical aberration in the entire eye, can then be coupled with topography measurements that tell us the Q-value, the asphericity quotient, that is the power and shape of the cornea. With those two measurements together, we can create a new cornea that’s correct in power to achieve emmetropia and is correct in shape to reduce spherical aberration if there is some present in the eye or maintain the spherical aberration of the cornea. It basically allows us to choose the right shape of the cornea, in addition to the power that is best for that patient,” he continued.
Dr. Holladay said that although the amount of spherical aberration in the eye is related to age, it is not always the case that a 20-year-old will have no spherical aberration and a 50-year-old will.
“It’s like anything else,” he said. “There are some 50-year-olds who have no spherical aberration and some 20-year-olds that have significant spherical aberration. So we need the wavefront measurement to be sure which is which.”
So, he said, by using both wavefront and topography measurements, “We can truly provide a customized ablation that is delivered to the cornea based on the topographic measurements and the wavefront aberrometry measurements. And that will be the way we do it in the future.”
Next issue
The December 15, 2005, issue will run the 10th and final installment of the Quality of Vision 2005 series. The topic is neural adaption.
For Your Information:
- Jack T. Holladay, MD, MSEE, FACS, can be reached at Holladay LASIK Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200, Bellaire, TX 77401; 713-668-7337; 713-668-7336; e-mail:docholladay@docholladay.com.
- Tim Donald, OSN Copy Chief, is writing the QOV series.