Trueness of milled prostheses according to number of ball-end mill burs

Statement of problem Making a computer-aided design/computer-aided manufacturing (CAD/CAM) prosthesis with a milling machine often requires 2 (2- and 1-mm diameter) or 3 (2-, 1-, and 0.6-mm diameter) burs; however, using 3 burs can reduce time effectiveness and increase cost. Studies evaluating the trueness of prostheses made with 2 and 3 burs are lacking. Purpose The purpose of this in vitro study was to evaluate the 3-dimensional trueness of crown prostheses made using 2 and 3 ball-end mill burs in the milling process. Material and methods The abutment die of the maxillary right first molar for ceramic crowns was designed with computer-aided design software. After the crown prosthesis design was completed, polyurethane blocks were milled using 2 and 3 burs with a 5-axis milling machine. The outer and inner surfaces of the milled crown prostheses were scanned with a dental scanner. The inner part was separated into a marginal part and an internal part using 3-dimensional evaluation software. The 3-dimensional trueness of the prostheses milled with 2 or 3 burs was compared. Results No significant differences in trueness were found for the inner or internal parts of the prosthesis (P>.05). However, the outer and marginal parts of the prosthesis did show significant differences in trueness (P<.05). Conclusions Milling the marginal part of the inner prosthesis was better with 2 burs, whereas milling the outer part was better with 3 burs.