Machining variability impacts on the strength of a 'chair-side' CAD-CAM ceramic

To develop a novel methodology to generate specimens for bi-axial flexure strength (BFS) determination from a 'chair-side' CAD-CAM feldspathic ceramic with surface defect integrals analogous to the clinical state. The hypotheses tested were: BFS and surface roughness (R(a)) are independent of machining variability introduced by the renewal or deterioration of form-grinding tools and that a post-machining annealing cycle would significantly modify BFS.

Nominally identical disc-shaped specimens (11 mm diameter, 1.1mm thickness) were machined with identical design and operative parameters from Vita Mark II feldspathic ceramic. Six individual bur sets (Groups A-F) generated 14 specimens each. Three groups were annealed between glass transition and softening temperatures. 3D contact profilometry determined surface roughness before and following annealing and prior to BFS determination. Scanning electron microscopy was undertaken to examine machining tools and perform fractographic analyses of ceramic fracture fragments. Statistical analysis included independent and pairwise analyses of R(a)-values (P<0.05), Weibull analysis of BFS data and Pearsons correlations.

Mean R(a) differed significantly between groups (P<0.01) but was unaffected by annealing (P = 0.42). Mean BFS was significantly altered by bur-set (P<0.01). Annealing resulted in no significant modification of the Weibull parameters as the 95% confidence intervals overlapped. No significant correlations between machining order and BFS or R(a) were observed (P>0.05).

Surface roughness and the nature of strength limiting defects appear to be probabilistic with flaw generation dependent on a random selection of a bur and a random machining sequence. The variability in BFS with machining could account for premature clinical failures.