Iris registration system adjusts treatment for ocular rotation

April 1, 2005

With the push of a button, refractive surgeons now have the capability to align wavefront-guided customized ablations to the correct corneal site. Iris Registration (IR) technology from VISX for use with CustomVue treatment is the first and only FDA-approved, fully automated, non-contact method available to CustomVue surgeons in the United States. The technology is being regarded for its potential to enhance the further efficacy and safety of custom ablations with the VISX system.

With the push of a button, refractive surgeons now have the capability to align wavefront-guided customized ablations to the correct corneal site. Iris Registration (IR) technology from VISX for use with CustomVue treatment is the first and only FDA-approved, fully automated, non-contact method available to CustomVue surgeons in the United States. The technology is being regarded for its potential to enhance the further efficacy and safety of custom ablations with the VISX system.

Colman Kraff, MD, has been using the IR technology prior to its FDA approval in the context of a clinical trial. Since clearance for marketing was received in early March, he has begun to use it in treating his private practice patients.

"The technology is easy to use," Dr. Kraff said. "All the surgeon has to do is push a button on the laser to engage the IR, and the software and hardware have been performing very reliably.

Principles Cyclorotational and cyclotorsional registration using the IR technology are designed to align the wavefront treatment map precisely as measured at the Wave-Scan aberrometer with the STAR S4 laser to optimize efficacy and safety. Alignment is important due to the potential for cyclo-misalignment to occur between the WaveScan measurement and laser treatment as a result of the patient's change in position from sitting upright in front of the aberrometer to lying supine under the laser.

Intraoperative ocular movement can also lead to cyclotorsion, and the STAR S4 IR excimer laser system allows surgeons to re-register the treatment easily as needed so that the laser will always place the pulses at exactly the correct location. If ocular rotation is detected through intraoperative monitoring, surgeons simply need to release the footpedal, re-register the patient eye by pushing the IR button, and then step on the pedal again to continue the treatment.

"Eyes may rotate due to positional changes either clockwise or counterclockwise and to varying degrees," Dr. Kraff continued. "Depending on the amount of rotation that occurs and the amount of cylinder that is present, the accuracy of the spherocylindrical ablation can be significantly affected. With a wavefront-guided treatment for higher-order aberrations, even a small degree of cyclorotation can have a marked effect on outcomes."

Cyclorotation is real Results of a VISX-sponsored study of 51 eyes defined the problem of cyclotorsional movement that can occur when patients move from the seated position at the WaveScan to lying supine under the laser. Its results showed there was a mean rotation of 2.2° in the clockwise direction for the left eye and 2.0° counterclockwise for the right eye. However, in some individuals, there was up to 10° of ocular rotation.

An earlier larger study from Swami and Steinert evaluating 240 eyes found an average rotational malposition of 4.1° with more than 10° of deviation occurring in 8% of eyes. Those authors noted failure to correct for the average misalignment could translate into 14% undercorrection of astigmatism, while the error increased to 35% in the case of 10° of rotation. Similarly, another study from Smith et al. measuring position-induced ocular torsion in 50 eyes found about one-fourth of their sample manifested a 25% change in axis varying between 7° and 16°.

Quality of vision impact The impact of cyclotorsional movement on visual outcomes has been demonstrated with computer modeling that uses software to cyclorotate wavefront images and then to measure the RMS error induced by the cyclorotation.

In an eye with 1 D of cylinder, it was found that 15° of cyclotorsional motion resulted in an RMS error >0.5 µm while 30° of cyclorotation resulted in an error of about 1 µm. Analyses of residual point spread function and simulated eye chart letters indicated even a 1° rotational shift had a measurable impact on outcome in that case.