Does CAD/CAM Machining Protocol Affect the Quality of Crown Margins?

Objectives: Objective: The objective of the present study was to apply a newly developed hoop-strength test in sphero-cylindrical shapes using a variety of dental materials to be machined by CAD/CAM with diamond burs of different grits. Also, we aim to establish the susceptibility to cervical damage and define favorable machining conditions for different CAD/CAM materials.
Methods: Materials and methods: Five CAD/CAM materials were evaluated: two pre-polymerized resin composite (Lava Ultimate, 3M ESPE [Lava Group]; Grandio Blocs, Voco [GB Group]) and three ceramics (IPS e.max CAD, Ivoclar-Vivadent; Suprinity, Vita Zahnfabrik [VS Group] and Enamic, Vita). A half-sphere model crown with elongated cylindrical margins and uniform loading contact around the internal axial wall was scanned in CAD/CAM machine (Sirona CEREC Machine, Dentisply Sirona, USA). The groups were machined (n = 20) according to two machining protocols available in the system regarding surface finish: rough and fine. The rough option used two coarser bur (Step Bur 12S, Dentisply Sirona, USA) and the fine option used two thinner bur (Cylinder Bur 12 EF, Dentisply Sirona, USA). Two samples of each group were analysed in scanning electron microscopy (SEM), after that, all samples were loaded with a metallic conical piston until fracture, a test termed “hoop-strength test”.
Results: Results: SEM images showed that rough protocol produced more damages than fine protocol in all groups; also, ceramic materials had the most clearly defined damages areas, whereas the resin composite materials had very low damages, however, this difference was not reflected in the force at fracture. The hoop-strength results showed that IPS e.maxCAD presented higher resistance than VS group when comparing ceramics. Compering resin composite, GB presented higher resistance and Enamic lower.
Conclusions: Conclusion: Fine and rough protocol machining showed no significant difference in hoop strength test, however, SEM images showed that the fine option produce less damages in outer margin.