Registration a cornerstone for customized ablation success

Washington, DC-Registration is a vital link in the refractive surgeon's quest to optimize quality-of-vision outcomes after customized ablation procedures, said Kerry D. Solomon, MD, at World Cornea Congress V.

"Snellen acuity has been our traditional metric for evaluating the success of refractive surgery, but our focus in 2005 and the future needs to go beyond that and concentrate on quality of vision," said Dr. Solomon, professor of ophthalmology, Medical University of South Carolina, and medical director of the university's Magill Laser Center for Vision Correction, Charleston.

"As we continue to raise the bar on refractive surgery outcomes, registration becomes a more critical factor because it affords the precise placement of ablation patterns that will allow us to meet our goals," he said. "Wavefront technology is here to stay, and as we move forward with it and with forthcoming developments in registration, our results will only continue to improve."

Assessing quality of vision

Discussing Snellen acuity and the need for other outcome measures, Dr. Solomon acknowledged that best-corrected visual acuity (BCVA) and uncorrected visual acuity (UCVA) results clearly are important to refractive surgery patients. In addition, they have been shown in various studies to be predictive of night-vision difficulties after surgery. Nevertheless, Snellen acuity by itself is not a good marker of quality of vision.

"Having patients in a darkened room reading black letters on a brightly lit white chart has little to do with real-world vision. Instead, contrast sensitivity that measures quality of vision in daytime and low-light conditions, provides more relevant information," Dr. Solomon said.

He demonstrated the benefits of customized ablation surgery by presenting results achieved at his institution performing customized ablation with CustomCornea (Alcon Laboratories). Relative to historical controls treated with conventional LASIK, the outcomes of the wavefront-guided procedures were improved with respect to proportions of eyes achieving UCVA of 20/15 and 20/20. In addition, the predictability results were superior because about half of the patients in the customized ablation group had a postoperative spherical equivalent within 0.25 D of target.

"This is progress because a few years ago, we were measuring our outcomes in terms of correcting vision within 0.5 D of intended," Dr. Solomon said.

Contrast sensitivity testing showed that across all spatial frequencies, almost all patients had improved contrast sensitivity at 1 and 3 months after surgery compared with preoperatively. The aberrometry results showed a decrease in total higher-order aberrations.

"Conventional LASIK tends to induce higher-order aberrations that perhaps play a role in postoperative glare, halos, and visual aberrations. Now, for the first time, we are able to achieve results characterized by not just a reduction in the magnitude of induced higher-order aberrations but a decrease in pre-existing higher-order aberrations. That is another indication that we are improving quality of vision for our patients," Dr. Solomon said.

However, the ability to achieve such good outcomes consistently will require that the laser energy hits its intended mark. Registration is the key to that process, Dr. Solomon said.

Importance of registration

Registration compensates for inaccurate treatment delivery that can occur secondary to cyclotorsional misalignment caused when patients move from an upright position in front of the aberrometer to lying supine under the laser. Registration ensures that the wavefront map acquired before surgery lines up precisely with that measured during surgery and thereby optimizes the ablation.

While the pupil has been used in the past for achieving alignment, it is a moving target and so not helpful for guiding precise treatment delivery. To address the need for a better method, laser manufacturers have introduced various types of registration technology. In the United States, automatic registration is available for both the CustomCornea (Alcon Laboratories) and CustomVue (VISX/AMO) platforms.

The automated registration system for the CustomCornea system is based on limbus recognition. Under infrared light conditions, limbal landmarks are automatically found while the technician aligns preplaced scleral marks. The information captured at the LADARWave aberrometer (Alcon Laboratories) and laser is automatically registered and matched for the purpose of alignment.