Clinical comparison confirms benefits of torsion error correction system

October 15, 2008

An excimer laser platform features technology for automated cyclotorsional compensation. A study comparing outcomes of patients undergoing LASIK for myopic astigmatism treated with and without the system demonstrates that its use increases the predictability of astigmatism correction.

Key Points

New Delhi-Active cyclotorsional compensation using the torsion error correction (TEC) system of an excimer laser platform (EC-5000 CXIII, Nidek) increases predictability of astigmatic correction in patients undergoing LASIK or surface ablation procedures, said Sudhank Bharti, MD, medical director and chief consultant, Bharti Eye Foundation, New Delhi.

Dr. Bharti investigated the performance of the automated method for cyclotorsional correction in a study comparing outcomes for two groups of patients who underwent LASIK for treatment of myopic astigmatism with or without TEC.

At 3 months after surgery, no significant differences existed between the TEC group and the controls in the uncorrected visual acuity (UCVA), manifest refraction spherical equivalent (MRSE), and safety results. Significantly better correction of cylinder was present, however, in the TEC group based on comparisons of mean residual cylinder, standard deviation, and vector-based analysis of percent cylinder reduction.

"Various manual and automated methods can be used to compensate for cyclotorsion," he said. "The iris registration-based TEC system of the . . . platform is a fully automated method that eliminates the opportunity for human error, and its performance and benefits are confirmed in our study. This technology may also increase the predictability of wavefront correction and ultimately may reduce enhancement rates."

The TEC system uses an infrared iris image that is captured during aberrometer examination with a corneal analyzer (OPD-Scan II, Nidek). Iris landmarks of the image are compared with the live image while the patient is lying under the laser.

"The system operates with physiologic pupil sizes ranging from 1.50 to 6.0 mm, so there is no need to administer dilating drops, and the system works well with varying light conditions and pupil shapes," Dr. Bharti said.

The system can detect 15° of cyclotorsion in either direction and works rapidly with a detection time of less than 1 second. It has an active correction range of ±6° during surgery.

When the patient is lying under the laser and before the flap is lifted, the patient's head must first be adjusted so that there is less than 5° of cyclotorsion to take a second reference image. Then the flap is lifted and the TEC system is activated by pressing the start switch. Once the laser ablation starts, the position of the laser optics automatically is adjusted to precisely deliver the ablation based on intraoperative monitoring by the eye-tracking mechanism and TEC system.

Lateral movements constantly are corrected during the ablation. Torsional movements that exceed an adjustable limit (typically ±2°) automatically are corrected by lifting and re-pressing the laser foot switch.

"TEC has shown to improve the predictability of astigmatism treatments and is vital for wavefront ablations, which treat higher-order aberrations," Dr. Bharti said.

Comparison results

The comparative study included 26 patients who underwent bilateral surgery. Fifteen patients were operated on with the TEC system; 11 patients were in the control group. Dr. Bharti performed all of the surgeries.

In statistical analysis, the TEC and control groups were similar at baseline with respect to mean MRSE (–4.53 and –5.13 D), manifest sphere (–3.30 and –4.55 D), and manifest cylinder (–2.46 and –1.16 D). Mean residual manifest cylinder (absolute value) at 3 months after surgery significantly was greater in the controls compared with the TEC group, 0.555 versus 0.210 D.

Cylinder data dispersion in each group was evaluated based on calculation of standard deviation, and the results showed a significant difference favoring better predictability in the TEC group compared with the controls, 0.057 versus 0.190 D. In addition, by vector analysis, the TEC group had almost 90% reduction in cylinder compared with only 46% reduction in the controls. The mean axis error was –0.60° in the TEC group and –8.22° in the control group.

"Although the UCVA results were not significantly different in statistical analysis, the better cylinder outcomes in the TEC group were associated with better visual outcomes. The proportion of patients achieving UCVA of 20/20 or better was 90% in the TEC group compared with 78% of the controls," Dr. Bharti concluded.

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