Tandem-scanning microscopy of slow-speed enamel cutting interactions

The aim of this study was to observe the subsurface microstructure of dental enamel during cutting procedures. Real-time confocal scanning optical microscopy, with a tandem-scanning reflected-light microscope, is suitable for making high-resolution images of enamel prisms in intact teeth under near-normal conditions. Blocks of enamel were held on a micro-positioning stage and advanced toward the tip of a diamond knife. The orientation of the samples was varied to simulate common clinical cutting conditions. Oil immersion objectives were used with the knife rigidly mounted in a specially made carrier. This was attached to the objective below its end-lens. The images were recorded with a sensitive video camera and cassette recorder for analysis of the cutting interactions at a slow playback speed. The presence of decussating enamel prisms was particularly effective at slowing crack propagation when loading was applied in an appropriate direction. Residual subsurface cracking was seen when the blade had finished cutting. This was most pronounced in regions where the enamel prisms were cut transverse to their long axis; this was equivalent to the preparation of the cervical margin of an approximately cavity. Where there is surface adhesion to enamel (e.g., glass-ionomer cements), residual subsurface cracking will produce a weakened substrate for bonding that could have important implications for restoration longevity.