spotlight
on technology & technique
Through a Lens, Smartly
Being able to check an ablation ahead of time puts
refractive surgery in a whole new light.
By Christopher Kent, Senior Associate Editor
Advances in technology and increasing surgical experience continue to make refractive surgery more accurate. Nevertheless, ablating corneal tissue is (so far) an irreversible procedure, and despite technology and surgical skill, it's still possible for something to go wrong. Especially when using new technology like wavefront-guided ablation, the possibility of an unexpected outcome is a concern for every surgeon.
Now, a clever use of this technology is making is possible to do a "dry run" and let the patient sample the outcome -- before you perform the surgery.
Visions of Things to Come
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The PreVue lens (centered in a holder, below) is ablated by the STAR S4 laser (above, right), using data from VISX's WaveScan (above, left). The lens lets the patient see how the planned ablation will change his or her vision. |
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The PreVue lens from VISX, designed to be used in conjunction with VISX's WaveScan unit and STAR S4 laser, makes it possible to "try out" a custom ablation before you operate on the patient's eye.
First, the patient's eye is scanned with the WaveScan. This information is then used to create a unique ablation pattern on a plastic lens, acting as a "substitute cornea." When this lens is placed in a trial frame and properly positioned in front of the patient's eye, the patient experiences the same vision that a successful wavefront-driven LASIK or PRK procedure would produce.
The PreVue lens, which is being used in clinical trials as part of VISX's complete custom ablation system, can serve a number of useful purposes:
- It gives the surgeon a way to confirm that the custom ablation is going to produce the desired result.
- Many surgeons are still inexperienced and uncomfortable with the idea of basing treatment on a wavefront refraction rather than a conventional manifest refraction. Being able to try the ablation out ahead of time should go a long way toward alleviating these concerns.
- The PreVue lens will let patients who are still nervous about having laser surgery experience the results of custom ablation before committing to the procedure, increasing the likelihood that they'll choose to proceed.
- The lens makes it possible to try different treatment approaches to see which will produce the most desirable result.
- The lens can help identify which patients will benefit more from a custom treatment than a standard treatment.
- The PreVue lens can be used to screen out patients who aren't good candidates for LASIK or PRK.
- If wavefront and manifest refractions disagree, the PreVue lens will make it clear whether the wavefront refraction is valid. (Differing refractions could indicate lenticular or pathological problems.)
- The lens provides a way to determine whether treatment may successfully address problems such as night vision glare or halos caused by higher-order aberrations.
Creating the Lens
To create a PreVue Lens, the patient's eye is scanned using the WaveScan. The software then generates two versions of the treatment parameters: one for corneal tissue, and a second designed to produce the same correction in the plastic PreVue lens material, which has a different refractive index.
To generate the ablation parameters, you must also input:
- whether the procedure will be LASIK or PRK
- physician nomogram factors
- treatment dimensions
- minimal optical zone
- ablation zone.
The instrument then generates a treatment table for the PreVue lens.
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The PreVue lens must be positioned precisely so the patient can experience the refractive effect of the proposed ablation. PHOTO COURTESY OF: TERRENCE O'BRIEN, M.D. |
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This information is saved onto a floppy disk, which is manually inserted into the STAR S4 laser console. The surgeon or technician places a blank lens on the calibration platform and aligns it; the laser then creates a duplicate of the intended ablation.
In order for the patient to experience the effect this ablation will have, the PreVue lens has to be aligned in front of the eye very precisely (just as the ablation must be accurately centered during surgery).
To accomplish this, the lens is placed in a trial frame, which also covers the other eye. (See photo, right.) The doctor or technician then moves the patient's head gently until the patient is seeing through the correct part of the ablated lens. Getting the lens correctly positioned isn't always easy; it requires training and patience.
Once the patient's vision and lens are aligned, the patient may experience the corrected vision as "snapping into place." At this point, a technician conducts a Snellen acuity test.
Comments from the Field
Colman R. Kraff, M.D., director of refractive surgery at the Kraff Eye Institute in Chicago, has been performing wavefront-driven ablations since April 2001 as principal investigator in the VISX trials.
"To be included in the trial," he explains, "patients had to read 20/20 plus three letters on the 20/16 line, using their PreVue lens simulation of wavefront-guided BCVA. In fact, every patient was able to see clearly through the PreVue lens and passed the qualification test." (He adds that the results of the custom ablations so far have been "outstanding.")
"The PreVue lens is a nice adjunct for validation for both doctor and patient. However, I don't know if all doctors will use it. Once they're comfortable with the VISX system, they may not feel the need."
Terrence P. O'Brien, M.D., director of refractive eye surgery at Johns Hopkins-Wilmer Eye Institute in Baltimore, has used the PreVue lens for the past year and a half.
"With any new technology, there's always some skepticism," he observes. "The PreVue lens helps to ease the transition from traditional refractive surgery to wavefront-guided surgery; it lets the patient provide subjective verification of the wavefront information, so you don't have to accept it on faith. It also builds patient confidence in the procedure. However, the steps required to create it can be time-consuming -- it calls for a lot of hands-on activity, and your staff will need extensive training.
"We've conducted examinations in which the manifest refraction and wavefront refraction differed. By creating a PreVue lens, we were able to validate which refraction was better. So far, the wavefront refraction has always been more accurate.
"Using the PreVue has also uncovered lens pathology, such as posterior lenticonus. In one case of this kind, the lens didn't improve the patient's vision, and as a result we knew that treatment wouldn't either.
"Some patients with currently untreatable problems from prior refractive surgery are depressed, even despondent," he adds. "Doctors have told them they can't do anything to help them. We show them the custom treatment possibilities using the PreVue lens and they see a glimmer of hope. We've seen some patients weep at the realization that they may eventually be able to have better vision."
Dr. O'Brien also points out that the PreVue lens may have practice management advantages: Offering this option could give a practice something unique and desirable (from the prospective patient's point of view) to feature in marketing efforts. And, conservative referring eye doctors are likely to be more willing to refer to a practice, knowing that a subjective element is part of the refractive surgery process.
The Future Looks Bright
Technicians at VISX are working on the possibility of incorporating dynamic adaptive optics into the WaveScan system. This would make it possible to demonstrate potential corrections to the patient in real time. That possibility, however, is still in the research stages.
For more information about the PreVue lens or the WaveScan system, contact VISX at (800) 246-VISX, or visit www.visx.com on the Web.