Offset system yields new approach to alignment control

July 15, 2007

A new offset system (Torsion Error Detection, Nidek) for alignment control measures the distance between the line of sight and the visual axis as well as the difference between photopic and mesopic fixation images. The amount of translation needed to match these images can be an indicator of poor fixation. The offset system allows detection and definition of line of sight and visual axis positions.

Key Points

San Diego-A new offset system for alignment control (Torsion Error Detection, Nidek) measures the distance between the line of sight and the visual axis as well as the difference between photopic and mesopic fixation images. The amount of translation needed to match these images can be an indicator of poor fixation, according to Fabrizio Camesasca, MD, who described the system here at the annual meeting of the American Society of Cata-ract and Refractive Surgery.

The offset system allows detection and definition of line of sight and visual axis positions.

"The evolution of the alignment system began with manual alignment; progressed to the suction ring, eye-tracking that identified the center of the pupil, and to the tracking of ocular movements; and finally, cyclotorsional movement was tracked by following iris patterns," said Dr. Camesasca, vice chairman, Department of Ophthalmology, Istituto Clinico Humanitas, Milan.

In many cases, the pupil is decentered and the geometric corneal center does not correspond with the entrance pupil center, the visual axis, and the coaxially sighted corneal reflex, according to Dr. Camesasca.

With all this in mind, the question of proper centration arises.

"The optical path difference instrument (OPD, Nidek) measures everything on the visual axis, on fixation, and the eye-tracking system works on the center of the pupil, the line of sight," Dr. Camesasca said. "There is a difference between the line of sight, the visual axis, and the angle-kappa distance between the pupil center and the visual axis, which in 20% of patients is 3 mm."

Pupil dilation is another factor. When the pupil dilates or constricts, the center of the pupil changes. He demonstrated with a mesopic pupil and a photopic pupil that the distances between the centers of the pupils differ. Therefore, there is a difference between the pupil center, the line of sight, and the visual axis based on the illumination, which is defined as offset.

"It measures the distance from the center of the pupil under mesopic and photopic conditions, calculates an average (referred to as offsets)," he added. Surgeons should be aware that the pupil diameter and center change when patients are fixating at near and at distance, he said. During refractive treatment under the laser, the patient is fixating at a distant target.

Dr. Camesasca also discussed whether a conflict exists between line of sight and visual axis-what is measured and what is the tracking system.

"We think there is. We think that an improper alignment might possibly induce coma in patients if we do not center on the visual axis," he said. "We need to analyze the distance between the centers of the mesopic and photopic pupils as well."

The advantage of centration is that the physiologic formation of the retinal image is followed and currently avoided through the use of wide optical zones, which is an effective way to reduce aberrations, he said. The prolate cornea, when not decentered, is believed to reduce coma, improve contrast sensitivity, and provide better vision. Factors that are available for alignment, according to Dr. Camesasca, include the x-y axis, the tilt control, the torsion error detector on the system, and the eye-tracker movement.

"We currently have an eye tracker that tells us the tilting position of the eye at the beginning of surgery, and the position of the head of the patient can then be reset as needed," Dr. Camesasca said. "In the future, a constant torsion error detector will be available to track the iris position, which can be very useful when treating patients with high astigmatism, because it induces aberrations and imprecision in the axis and power of the cylinder.

"Thus, we believe that the platform improves cylinder correction accuracy by ensuring that the excimer laser ablates with precision during customized and conventional refractive procedures and that accurate laser alignment is necessary during laser surgery," he concluded.