Penetration of radiopaque dental restorative materials using a novel ultrasound imaging system.

PURPOSE: To determine whether a novel ultrasound imaging system could detect 25 microm thick cracks beneath gold, silver amalgam, and porcelain restorations on tooth phantoms.
METHODS: Tooth phantoms were constructed using acoustically-matched composite to simulate dentin, with 25 microm thick water-filled cracks located approximately 1 mm inside the simulated dentin. Porcelain and gold restorations were bonded using resinous cement, and an amalgam restoration was attached using mechanical retention. A portion of the gold restoration was left unattached to simulate cement washout. A novel monostatic pulse-echo ultrasound system with a 19 MHz single-element PLZT transducer, custom transmit/receive electronics and signal processing with a Ga-In alloy couplant was used to measure the relative return echo amplitude from restoration surfaces, simulated dentin/restoration interfaces, and cracks. Extracted teeth were also used to demonstrate that the system was capable of detecting cracks in real teeth.
RESULTS: Cracks were detected beneath porcelain and amalgam, and within a human molar. Cracks were not detected in simulated dentin beneath gold; however, simulated cement washout directly beneath gold was identified. The ability to detect defects in the phantom was dependent on the acoustic reflection coefficient between interfaces, the attenuation of each material, acoustic clutter within the phantom, and geometry. Gold restorations transmitted minimal acoustic energy due to their large acoustic impedance; however, the ability to distinguish the gold/cement interface from the gold/washout interface indicated that ultrasound can detect density changes immediately below restorations, such as caries, fractures, or debonding. The ability to penetrate resin-composite was also demonstrated.