Relationship between K_IC and annual failure-rate of all-ceramic crowns.

Objectives: Many seem to be concluding that fracture toughness is somehow predictive of clinical behavior. However, this does not appear to have been quantitatively evaluated.
Methods: Failure data (except for Buxir, obtained from the manufacturer) was taken from a 2008 special issue of the Journal of the American Dental Association, normalized per year and included Empress 2 data from a 2019 paper by Malament et al. Fracture toughness data was obtained from published literature for DICOR (mica-filled glass ceramic, Dentsply-Corning), Empress 1 (leucite-filled, now IPS Empress, Ivoclar), Empress 2 (lithium disilicate-filled, now IPS e.maxCAD, Ivoclar), InCeram Alumina (Vita), Procera All-ceram (NobelBiocare). Failure data for Bruxir (Glidewell Laboratory) was obtained from the manufacturer and its toughness from the ADA Laboratory (Spiro Megremis, personal communication). Plot was done in Sigma Plot 13 and curve-fitting in TableCurve 2D v. 5.01 (Systat Software).
Results: Three remarkable insights can be drawn from this analysis. First is that fracture toughness does provide a significant correlation with clinical behavior (y = a + b/x², r² = 0.76). Since this data contains a mix of veneered and full-thickness crowns, K_IC of the substructure dictates clinical success; this makes sense since failures are seen to originate from the intaglio surface. Second is the observation that the annual failure rate has plateaued with respect to increasing toughness by around Empress 2 (K_IC = 2.6 ± 2.6 MPam^1/2); implying that manufacturers may not be advised to spend research time and funds on higher toughness ceramics. Third, note the better than predicted clinical success of Empress 2 (now IPS e.maxCAD and Press), with actual failures much less than the regression predicts for that toughness. This is likely the “bonding effect” since this is a very etchable ceramic and the profession was universally bonding by that time.
Conclusions: K_IC of the substructure ceramicis clearly related to clinical performance.