30 July 2019

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Are clinical adjustment of cemented CAD-CAM monolithic indirect restorations a treath for the material propriety over time?

Lorenzo Breschi

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Nowadays, numerous researches are conducted to improve and understand CAD-CAM processes in terms of technology and materials. Indeed, the improvement in the knowledge of the topic is becoming tangible in terms of efficiency, esthetic and precision. There are several monolithic materials indicated for indirect restorations (yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic, lithium disilicate, zirconia-reinforced lithium silicate, polymer-infiltrated ceramic, and nanofilled composite resin) and most of them have reached optimal mechanical and esthetic properties, thus becoming the materials of choice in clinical practice. Ideally, indirect CAD-CAM restorations should not require adjustment during the delivery appointment. However, this is not always possible because the removal of premature contacts or adjustment of the proximal contact areas might be necessary. Clinical adjustments, made by using diamond rotary instruments, are proved to create undesirable consequences such as increase of surface roughness, that facilitate biofilm formation (the threshold smoothness for bacterial adhesion is 0,20µm) and antagonist wear. In addition, surface roughness induces stress concentrations, which compromise porcelain strength. Furthermore, surface corrections are responsible of color modification of certain restorations.  Even if there is great concern about the problems created during restoration adjustments, there is little knowledge on the effects that clinical adjustments and polishing have after aging.            
A remarkable work performed by Francesco Saverio Ludovichetti and his team and recently published on the Journal of  Prosthetic Dentistry attempt to assess the surface roughness and fracture resistance (with or without mechanical aging) of cemented CAD-CAM monolithic materials submitted to grinding and polishing procedures.  

MATERIALS AND METHODS            
98 CAD/CAM blocks (IPS e.max CAD (Ivoclar vivadent AG); Vita Suprinity (Vita Zahnfabrik); Vita Enamic (Vita Zahnfabrik); Lava Ultimate (3M ESPE)) were transformed into cylinders, polished and crystallized. To evaluate the roughness of the cylinder three different measurements were performed with a 3D laser conofocal microscope: the first measurement was collected after polishing or glazing, the second after a 0,3 mm grinding with a 30-µm grit and the last one after the  polishing procedures with a polishing kit and a diamond paste.             Sixty additional 1.8-mm disks and 30 1.5-mm disks from each material were prepared for fracture resistance test. A simplified trilayer model, was used to mimic the tooth structure. The different specimen were divided in three different groups:  control group (without no surface treatment); ground surface group and ground and polished surface group. Half of the specimen of each group were tested immediately and the second part was aged in artificial saliva. All the specimens were loaded in a mechanical testing machine and the load (N) at failure of each specimen was recorded as the fracture resistance.  

RESULTS            
The results showed that in the control groups, no significant differences were found between the IPS e.max CAD and Vita Suprinity or between the Lava Ultimate  and Vita Enamic, which showed higher roughness than the glass-ceramic materials. After grinding, this behavior was maintained, except for the Vita Enamic, whose roughness was similar to that of the IPS e.max CAD. After polishing, the Vita Enamic showed the highest roughness, whereas for the other materials there were no differences. From this results it’s interesting to note that when the polishing was finished with a diamond paste, the smoothness of the Lava Ultimate and Vita Enamic increased notably, evidencing that even though these materials do not require clinical adjustments, they should be polished after the cementation in the clinical practice. Surface treatment and aging did not influence the fracture resistance of the materials. Vita Suprinity showed an increase in fracture resistance after cyclic fatigue, regardless of the surface treatment condition. This unexpected result could be explained, according to Ludovichetti and his team, by the higher resistance against crack propagation and higher flexural strength of the material. This behavior might be attribute to the presence of the zirconia fillers used to reinforce the glassy matrix of Vita Suprinity.                        

CONCLUSIONS
Grinding increased the roughness of the materials, except for the Vita Enamic and that the smoothness of the glazed glass-ceramics was not restored by the polishing procedures. Grinding, followed or not by polishing, did not impair the fracture resistance of the materials and aging did influence the fracture resistance of the materials, even in the ground groups. Finally, the reliability of the materials was not influenced by the material, surface treatment, or aging.  

For additional information:
Effect of grinding and polishing on the roughness and fracture resistance of cemented CAD-CAM monolithic materials submitted to mechanical aging.