Reproducibility and accuracy of machine bent 3- and 6-stranded twistflex lingual fixed retainers—An in-vitro-study

Background

The present study aimed to evaluate the precision with which a retainer bending machine can produce the designed CAD/CAM retainer.

Methods

Canine-to-canine retainers were digitally designed and fabricated ten times each on 20 debonding casts (10 upper, 10 lower) using the Bender II (YOAT Corp., Lynnwood, WA) with either 3- or 6-stranded twistflex wire. A master retainer for each model was created from the CAD/CAM coordinates and saved as a standard tessellation language (STL) file. All bent retainers were subsequently digitized using a laboratory scanner and superimposed with the master retainer (best-fit). Deviations between the machine-bent and master retainers were measured in millimeters across transversal, sagittal, and vertical dimensions. Statistical analysis was performed using a mixed linear model with fixed factors (jaw, wire type, number of retainers) and the number of casts as a random factor.

Results

For the entire sample, mean deviations ± standard deviation between master and bent retainers were 0.22 mm ± 0.64 in the transverse, –0.01 mm ± 0.33 in the sagittal, and 0.48 mm ± 0.23 in the vertical dimension. Lower retainers were found to be significantly more precise than upper retainers across all dimensions (transverse: P < 0.001; sagittal: P < 0.05; vertical: P < 0.001). Additionally, 3-stranded twistflex retainers demonstrated significantly greater accuracy compared to 6-stranded wires (transverse: P < 0.05; sagittal: P < 0.05; vertical: P < 0.001).

Conclusions

The deviations in accuracy between the CAD/CAM planned retainer design and the machine-bent retainers could be clinically relevant, particularly for upper retainers in the transverse and vertical dimensions.