3D Tomography of Crack Interfaces in Dental Composites

Objective: The objective of this study was to quantify the ratio of crack edge area (ca) to total volume (v) of multiaxial confined compression specimens of dental composite. Methods: The dental composite (Renew, Bisco Inc.) was examined using a high resolution microtomography system at beamline 2-BM of the Advanced Photon Source (APS).  The cylindrical multiaxial specimens, 2.4 mm in diameter and 3.7 mm in height, were subjected to 6 and 12% axial strain, two different size confining rings, and cyclic loading at 400N for 100,000 cycles.  The specimens were contained within aluminum (Al) rings, similar to a class I occlusal restoration and loaded in compression via stainless steel plungers on the composite only.  The axial load is known and the constrained load is measured via a strain gage on the exterior of the Al ring. The Al rings varied with respect to the outer diameter versus the inner diameter, the variable l .  The cracks developed during loading were quantified using image analysis of the data sets obtained at APS.  The comparison was the % ca/v. Results:

Testing Conditions                    N           Mean(SD) % ca/v

Control                                      2                 3.7(1.0)

6% strain, l=2                          6                 5.3(2.8)

12% strain, l =2                       6                 6.4(4.1)

12% strain, l =2, load cycled    3               10.2(9.4)

12% stain, l =2.7                      3               36.9(10.3)

Conclusions:  This preliminary 3D image analysis indicates as the loading conditions intensified, from controls to confined compression (6 and 12%), to cyclic loaded, and from l 2 to 2.7, the amount of cracking observed in the composite also increased. This data would indicate that class I occlusal restorations are subject to increased cracking depending on the thickness of the surrounding tooth structure.  Supported by NIDCR grant DE07979 and APS by the U. S. DOE, Office of Science, Basic Energy Sciences, Contract No. W-31-109-ENG-38.