Effect of High-Speed Handpiece Vibration on the Fractal Dimension of Y-TZP Ceramic

Objectives: Fracture surfaces of ceramic restorations can be characterized using fractal geometry to determine conditions that were present at the time of failure. Failed zirconia-based restorations can be clinically retrieved from the tooth preparation using a coarse diamond bur with a high-speed handpiece. This process could induce vibration and changes in the fracture surface because of phase transformation. The aim was to test the hypothesis that there is no effect of the handpiece vibration on fractal dimension (D) values of yttria-tetragonal polycrystalline zirconia (Y-TZP) dental ceramic.
Methods: Fracture surfaces of Y-TZP specimens (IPS e.max ZirCAD, Ivoclar Vivadent) (n=19) that had been tested in four-point bending were analyzed. An impression was taken of each fracture surface using light body polyvinylsiloxane (Extrude, Kerr), and epoxy replicas were produced (EpoxySet, Allied High Tech). Then, a 5µm x 5µm area on each of the replicas was scanned using an atomic force microscope (Bioscope Catalyst, Bruker). The scan was exported as a .txt file and converted to a .sur file using custom script in MathCAD. The FRACTALS software (Fractal Surfaces, Russ) with Minkowski Cover algorithm was used to calculate D values. The same fracture surfaces evaluated previously were cut off from the broken Y-TZP beams using coarse diamond burs attached to a high-speed handpiece. After cutting and sonicating, measurement process was repeated.
Results: One-way repeated measures ANOVA showed no statistical difference between D values before and after cutting (p=0.52). The mean D values and standard deviation before and after cutting were 2.150±0.021 and 2.146±0.015, respectively. The study had a power of 81.5% to detect a difference of 0.02.
Conclusions: The vibration did not affect the fractal dimension, and the hypothesis was accepted. This means that a broken restoration could be removed by a high-speed handpiece without interfering with the subsequent use of fractal geometry as a failure analysis tool.