Objectives: To investigate the impact of chairside CAD-CAM machining tool variability and tool deterioration on the bi-axial flexure strength and surface roughness of a fine grain feldspathic ceramic.
Methods: 84 disc-shaped ceramic specimens (11.0 mm diameter and 0.7 mm thickness) were milled from presintered Vita Mark II ceramic blocks (14.0 mm x 7.0 mm) in a CEREC MC XL unit using a single CAD template. Six nominally identical new bur sets were used to create six groups (n=14) (Groups A-F) and each specimen was identified according to the machining sequence. The roughness (Ra-value) of the surface of the ceramic discs was quantified by contact stylus profilometry. Specimens from Groups C-F were annealed and the surface roughness re-quantified. Bi-axial flexure strength (BFS) was determined in a ball-on-ring geometry and the resultant data analysed using factorial analyses of variance (P<0.05) and Weibull statistics. Scanning electron micrographs of the machining tools were obtained prior to and following milling.
Results: The selection of machining tool set resulted in significant differences in the mean Ra-values (P<0.05) which ranged from 5.76±0.88 μm (Group A) to 3.82±0.71 μm (Group C). Similarly a significant difference in mean BFS (P<0.01) was observed for Groups A, B and C (156.9±8.2, 137.2±15.8 and 148.8±12.5 MPa respectively). No correlation was observed between the machining sequence and BFS for each of the groups investigated.
Conclusion: The BFS of a chairside CAD-CAM fine grain feldspathic dental ceramic is dependent on the machining tools used during the milling procedure. Machining tool variability impacts on the surface roughness of the machined ceramic but the quantified ‘roughness' fails to fully correlate with BFS and therefore was not a reliable indicator of the nature of the critical ceramic surface defect.