New Asymmetric Transition Zone Centration Strategy For Refractive Corrections: Smartsight Nova

To report the use of a novel asymmetric transition zone (aTZ) centration strategy (CenTrax) for 
Keratorefractive Lenticule Extraction (KLEx) procedures and evaluate its potential to 
compensate for fixation-related misalignments.

The study was conducted in a tertiary refractive surgery center equipped with the SmartSight® NOVA platform (SCHWIND eye-tech-solutions) for keratorefractive lenticule extraction (KLEx) procedures. 

The study was designed as a retrospective pilot clinical analysis, using anonymized clinical data from patients undergoing routine KLEx procedures. All examinations and treatments were performed in accordance with institutional guidelines and adhered to the principles of the Declaration of Helsinki. 

The SmartSight NOVA modifies correction profiles to account for both lateral and torsional 
fixation errors by decentering the optical zone (OZ) relative to the laser axis and employing an 
asymmetric crescent-shaped transition zone (aTZ) by defining the target pupil position, 
capturing the actual intraoperative pupil position, calculating deviations, and adapting the OZ 
and TZ geometries accordingly.

 Correction depth and central thickness remain largely unchanged. In aTZ design, the spherical component predominantly 
pushes induced coma and the astigmatic component pushes induced trefoil out of the OZ to 
within the TZ. Applying the method of centre of "mass" of the individual differences between post and 
preoperative maps with a 8.5mm clipping , the lenticule centration and its spatial distance to a 
reference point (aimed centration) determined the decentration. The method has been tested 
on a pilot cohort of clinical data showing 65% and 81% of the treatments within 200 µm of 
decentration for corneal thickness and refractive equivalent power, respectively.

The aTZ strategy provides a theoretical framework for compensating lateral and torsional 
misalignments in KLEx procedures without increasing ablation depth or sacrificing central 
optical quality. This approach builds on the concept of customized transition zones used in 
excimer platforms, extending it to lenticule-based surgery. Clinical implementation and 
validation are warranted to assess its efficacy in real-world practice. 
The aTZ strategy (in its commercial form CenTrax/NOVA) provides a reliable and objective 
way to automatically intraoperatively refine and correct the centration of a lenticule of tissue 
extracted from a cornea, improving postoperative topo- and tomographic derived metrics.