DETERMINATION OF THE DEPTH OF CURE FOR VLC COMPOSITES BY NUCLEAR-MAGNETIC-RESONANCE MICROIMAGING

Objectives. The purpose of this study was to evaluate the potential of nuclear magnetic resonance microimaging (NMRM) for investigating the depth of cure for visible light curing of dental composite materials. Methods. Cylindrical composite specimens were light-cured within the N MRM instrument for predetermined times. Vertical slice, mid-resolution , spin-echo images were acquired using a Bruker AM300WB instrument wit h a Bruker microimaging attachment. Images were digitized and intensit y profiles generated using deuterium oxide/water as a reference intens ity. Separate specimens were made to obtain Vickers microhardness valu es as a function of depth to compare an established method for determi ning depth of cure with NMRM. Results. A difference in NMR image inten sity between uncured and cured composite resin has been detected. Valu es for integrated intensities were obtained at 150 mu m intervals over the mid-fifth of the total image width. An abrupt transition was not seen at the cure front which advances with cumulative exposure time. N MRM produced similar data trends to microhardness measurement. Signifi cance. NMRM produces three-dimensional images of ''mobile'' hydrogen n uclei to a resolution of 10(-5) mm(3). It is non-invasive, non-destruc tive and able to selectively image protons in different chemical envir onments. It can be used to investigate depth of cure for light curing materials, but must be regarded as a research technique and not one fo r routine measurement. Refinement is necessary, possible, and in progr ess to improve resolution from the present 10(-2) mm(3) towards the li mit, and to reduce noise.