A 3-dimensional accuracy analysis of chairside CAD/CAM milling processes 
G.Bosch, A.Ender, A.Mehl,  J Prosthet Dent 2014 – article in press

Statement of problem. Milling is a central and important aspect of computer-aided design and computer-aided manufacturing (CAD/CAM) technology. High milling accuracy reduces the time needed to adapt the workpiece and provides restorations with better longevity and esthetic appeal. The influence of different milling processes on the accuracy of milled restorations has not yet been reviewed.

Purpose. The purpose of this study was to investigate the influence of different milling processes on the accuracy of ceramic restorations.

Material and methods. Four groups of partial crowns were milled (each n1?417): Three groups in a 4-axial milling unit: (1) 1-step mode and Step Bur 12S (12S), (2) 1-step mode and Step Bur 12 (1Step), (3) 2-step mode and Step Bur 12 (2Step), and (4) one group in a 5-axial milling unit (5axis). The milled occlusal and inner surfaces were scanned and superimposed over the digital data sets of calculated restorations with specialized difference analysis software. The trueness of each restoration and each group was measured. One-way ANOVA with a post hoc Tukey test was used to compare the data (a1?4.05).

Results. The highest trueness for the inner surface was achieved in group 5axis (trueness, 41 `15 mm, P<.05). The 4-axial milling unit exhibited trueness at settings ranging from 61 mm (2Step) to 96 mm (12S). For the occlusal surface, the highest trueness was achieved with group 5axis (trueness, 42 `10 mm). The 4-axial milling unit exhibited trueness at settings ranging from 55 mm (1Step) to 76 mm (12S).

Conclusions. Restorations milled with a 5-axial milling unit have a higher trueness than those milled with a 4-axial milling unit. A rotary cutting instrument with a smaller diameter results in a more accurate milling process. The 2-step mode is not significantly better than the 1-step mode.

Clinical Implications

The goal of the milling process is to generate an exact copy of the digitally calculated restoration. The 5-axial milling unit came closer to the digitally calculated restoration than the 4-axial milling unit.