Zirconia restorations and the tool diameter compensation
Aim:
The aim of the present article is to describe a new method to reduce the undesirable loss of material thickness that results from overmilling due to the tool diameter compensation correction of common CAD/CAM software.
Materials And Methods:
Today's CAD/CAM software (eg, 3Shape or Exocad) specifies the same tool diameter compensation for different ceramics. In the case of zirconia ceramics milled in the raw state, this leads to excessive milling of the inner surfaces of crowns, which results in unnecessarily large cementation gaps and a restoration that is thinned out from the inside. By manually reducing the preset correction in the digital design process by the volumetric sintering shrinkage factor specified by the manufacturer, excessive thinning of the zirconia can be avoided.
Results:
The inner geometry of the restorations changes only slightly after manually reducing the preset tool diameter compensation correction. Consequently, a design of the restoration with the required minimum interocclusal thickness yet with accurate passive seating and marginal fit is possible without any further interventions.
Conclusions:
Understanding the specifics of the subtractive fabrication process as well as the properties of the restorative materials is a key factor in achieving optimal clinical outcomes with all-ceramic restorations fabricated with CAD/CAM technology. The use of monolithic zirconia combined with a calculated reduction in the preset tool diameter compensation correction might be beneficial in cases with thin or uneven geometry.
