Abstract
Statement of problem
Tools will gradually deteriorate with repetitive milling. However, tool lifetime can
vary depending on the type of milling machine, the hardness of the ceramic material,
and the size of the restoration. Studies evaluating the effect of tool deterioration
on the trueness of milled restorations are lacking.
Purpose
The purpose of this study was to evaluate the effect of tool deterioration on the
trueness of milled restorations.
Material and methods
A patient requiring a ceramic crown was recruited. Repetitive milling of zirconia
crowns (inCoris TZI; Dentsply Sirona) with tungsten carbide rotary instruments and
glass-ceramic crowns (VITABLOCS mark II; Vita Zahnfabrik) with diamond rotary instruments
was performed by using a 5-axis milling machine (inLab MC X5; Dentsply Sirona) until
the machine software program indicated replacing the largest tool. The percentage
of deterioration for the largest tool was recorded before each milling. The trueness
for different crown areas was evaluated by using a 3-dimensional evaluation software
program (Geomagic Control X; 3D systems), and the crowns were clinically evaluated.
The crowns were grouped into 4 quadrants as per the deterioration percentage. The
Pearson correlation (r) between the deterioration percentage and trueness and linear
contrast analysis (partial eta squared [η]) among the groups were performed.
Results
Glass-ceramic crown areas had low correlation except for mesial (high, r=0.63, high,
η=0.37) and distal (medium, r=0.42, high, η= 0.34) areas, whereas zirconia crowns
had low correlation for inner and internal areas and medium to high correlation for
mesial (r= -0.31, η=0.33), distal (r= -0.53, η=0.36), occlusal (r= -0.32, η=0.164),
external (r= 0.69, η=0.52), and marginal (r= -0.44, η=0.19) areas. Zirconia crowns
had excellent marginal fit and occlusal contact and light to excellent proximal contact,
whereas glass-ceramic crowns had excellent marginal fit, minor to major occlusal correction,
and heavy proximal contact.
Conclusions
The effect of tool deterioration on trueness depended on material type and crown area.
Only proximal areas were affected in the glass-ceramic crowns, whereas external, proximal,
occlusal, and marginal areas were affected in the zirconia crowns. Trueness did not
necessarily reflect the clinical quality of the crown, as both crown types were clinically
satisfactory. Although zirconia crowns were overmilled in comparison with the reference
design and glass-ceramic crowns, they had better clinical quality.
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Article info
Publication history
Published online: December 17, 2020
Footnotes
Supported by the Jordan University of Science and Technology (Grant# 20190394).
Identification
Copyright
© 2020 by the Editorial Council for the Journal of Prosthetic Dentistry.
