Edge Chip Resistance of Dental Restorative Materials

The edge chip resistance (ECR), determined by the force necessary to create a chip at a specified distance from the edge of a specimen, has been strongly correlated with fracture toughness.  Since the initiation and propagation of the crack are critical to the edge chipping event, the type of indenter and the indenter tip angle are significant factors.

Objective: Measure the ECR of restorative materials.  Identify key experimental variables that affect results such as indenter type and angle, and recommend best practices for edge chip testing.

Method:  An edge chipping machine (Engineering Systems, Model CK 10, Nottingham, UK) was utilized to measure the ECR of a feldspathic dental porcelain using a rounded Rockwell C indenter, or alternatively, sharp conical indenters with varying tip angles (90^o–140^o).    Edge toughness (Te) is equal to the slope of the line on a chipping force versus edge distance graph.  Several other materials (three zirconias and a filled resin-matrix composite) were chipped in order to identify other factors that can have an effect on Te.

Result: The edge toughness for the dental porcelain was lower using the Rockwell C indenter (77.2±4.2 N/mm, one std. dev.) compared with the 120^o conical indenter (137.5±4.9 N/mm).  The Te values (N/mm) varied with indenter tip angle: 88.5±5.7 (90°), 116.2±8.4 (100^o), 137.5±4.9 (120^o), and 222.6±14.0 (140°).  Te values for the three zirconias were similar to each other and ranged from 414-446 N/mm using a 120^o sharp conical indenter. 

Conclusion: Te can vary with the type of indenter, the indenter tip angle, and the edge distance range used.  Other variables such as: specimen mounting, quality of edge, distance measurement method, and loading rate are also factors that can contribute to variation in Te results. It is essential that a well-defined testing methodology be used.  Support: ADAF, NIST, and NIDCR Grant DE17983.