The dentin-enamel junction and the fracture of human teeth.

The dentin-enamel junction (DEJ), which is the interfacial region between the dentin and outer enamel coating in teeth, is known for its unique biomechanical properties that provide a crack-arrest barrier for flaws formed in the brittle enamel1. In this work, we re-examine how cracks propagate in the proximity of the DEJ, and specifically quantify, using interfacial fracture mechanics, the fracture toughness of the DEJ region. Careful observation of crack penetration through the interface and the new estimate of the DEJ toughness ( approximately 5 to 10 times higher than enamel but approximately 75% lower than dentin) shed new light on the mechanism of crack arrest. We conclude that the critical role of this region, in preventing cracks formed in enamel from traversing the interface and causing catastrophic tooth fractures, is not associated with the crack-arrest capabilities of the interface itself; rather, cracks tend to penetrate the (optical) DEJ and arrest when they enter the tougher mantle dentin adjacent to the interface due to the development of crack-tip shielding from uncracked-ligament bridging.

Crystallographic texture for tube and plate of the superelastic/shape-memory alloy Nitinol used for endovascular stents.

The superelastic/shape-memory material, Nitinol, an approximately equiatomic alloy of Ni and Ti, is rapidly becoming one of the most important metallic implant materials in the biomedical industry, in particular for the manufacture of endovascular stents. As such stents are invariably laser-machined from Nitinol tubes or sheets rolled into tubes, it is important to fully understand the physical phenomena that may affect the mechanical behavior of this material. With tubing and plate, one major issue is crystallographic texture, which can play a key role in influencing the mechanical properties of Nitinol. In this article, we present a study on how geometry and heat treatment can affect the texture of Nitinol, with specific quantification of the texture of Nitinol tube used for the production of endovascular stents.

In vitro fracture toughness of human dentin.

The in vitro fracture toughness of human dentin has been reported to be of the order of 3 MPa (square root)m. This result, however, is based on a single study for a single orientation, and furthermore involves notched, rather than fatigue precracked, test samples. The present study seeks to obtain an improved, lower-bound, value of the toughness, and to show that previously reported values may be erroneously high because of the absence of a sharp crack as the stress concentrator. Specifically, the average measured critical stress intensity, K(c), for the onset of unstable fracture along an orientation perpendicular to the long axis of the tubules in dentin is found to be 1.8 MPa (square root)m in simulated body fluid (Hanks' balanced salt solution), when tested in a three-point bending specimen containing a (nominally) atomically sharp precrack generated during prior fatigue cycling. This is to be compared with a value of 2.7 MPa (square root)m measured under identical experimental conditions except that the bend specimen contained a sharp machined notch (of root radius 30-50 microm). The effect of acuity of the precrack on the fracture toughness of human dentin is discussed in the context of these data.

In vitro fatigue behavior of human dentin with implications for life prediction.

Although human dentin is known to be susceptible to failure under repetitive cyclic fatigue loading, there are few reports in the literature that reliably quantify this phenomenon. This study seeks to address the paucity of fatigue data through a systematic investigation of the effects of prolonged cyclical loading on human dentin in an environment of ambient temperature Hank's balanced salt solution (HBSS) at cyclic frequencies of 2 and 20 Hz. The "stress-life" (S/N) data thus obtained are discussed in the context of possible mechanisms of fatigue damage and failure in this material. In addition, stiffness loss data collected in situ during the S/N tests are used to deduce crack velocities and the thresholds for such cracking. These results are presented in a fracture mechanics context as plots of fatigue-crack propagation rates (da/dN) as a function of the stress-intensity range (Delta K). Such S/N and da/dN-Delta K data are discussed in light of the development of a framework for a fracture-mechanics-based methodology for the prediction of the fatigue life of teeth. It is concluded that the presence of small (on the order of 250 microm) incipient flaws in human teeth will not radically affect their useful life.