Fatigue Behavior of Coronal Dentin after Bur Preparation

The strength of human coronal dentin under cyclic loading is an important factor in discussions of the durability of dental restorations and the resistance to tooth fracture.  Indeed, the fatigue strength of dentin is considerably less than that assessed by static loading. Flaws introduced during the preparation of a cavity (i.e. with the use of an abrasive diamond or tungsten carbide bur) may reduce the fatigue strength of this tissue. 

Objective:  The objective of this investigation was to evaluate if the instruments most commonly used for cavity preparations reduce the fatigue strength of dentin.

Method:  Beams of human coronal dentin were obtained from unrestored 3^rd molars and subjected to either quasi-static or cyclic flexural four-point loading to failure.  Control beams were prepared with diamond abrasive slicing wheels.  Treated beams were further processed using a conventional, straight-sided bur under ultra-high speed and with copious coolant.

Result:  There was no difference in the average strength or Weibull parameters obtained for the control or treated dentin beams resulting from quasi-static loading. However, the fatigue life diagrams distinguished that coronal dentin treated with burs underwent a significant (p≤0.01) reduction in fatigue strength over the entire stress-life regime.  Treated dentin beams exhibited a large degree of variation in the fatigue life received when assessed at a specific cyclic stress, which was indicative of the relative contribution of flaws that were introduced during the cutting process. In evaluation of the apparent endurance limit, there was nearly a 50% reduction in the fatigue strength of bur-treated dentin with respect to the control. Microscopic evaluations of the fracture surface did not reveal the origin of failure.  

Conclusion:  The introduction of cavity preparations with burs significantly reduces the fatigue strength of dentin.