Ultrasound Microscopy-Based Identification of Enamel and Restorative Materials: An Ex Vivo Acoustic Impedance Study

Differentiating restorative materials from enamel during dental examinations is challenging because of their similar appearances. Even with ultrasound microscopy, the acoustic properties of restorative materials remain unassessed. This study investigated the potential of ultrasound microscopy to differentiate between enamel, composite resin, and glass ionomer.

Extracted third molars served as the tooth model: a 1.2 mm-diameter cylindrical cavity drilled into the enamel and restored with either composite resin (flowable bulk-fill or paste) or glass-ionomer cement (conventional, high-filler, or multi-ion). To evaluate the restorative materials, a second model was prepared by milling a 2.0 mm-diameter, 3.0 mm-deep cavity into a PMMA block and filling it with the same materials. Both models were imaged with ultrasound microscopy to obtain acoustic-impedance maps, and the PMMA specimens subsequently underwent Vickers hardness testing to explore the correlation between hardness and acoustic impedance.

Acoustic impedance was measured with an accuracy of 16 µm × 16 µm per pixel over an area of 4.8 mm × 4.8 mm, allowing for the construction of 2-dimensional colour images that effectively differentiated between enamel and restorative materials. The colour distribution for CR was homogeneous, while GIC exhibited a heterogeneous distribution across all samples. The mean acoustic impedance of enamel (15.6 ± 4.37 kg/m²s) was significantly greater than that of CR (Type Flow 5.36 ± 0.264 kg/m²s, Type Paste 5.49 ± 0.323 kg/m²s) and GIC (Type high-filler 4.80 ± 0.360 kg/m²s, Multiple ion 3.80 ± 0.360 kg/m²s, Conventional 3.74 ± 0.353 kg/m²s) (P < .01). A distinct threshold was established based on the combined standard deviations (σ₁ + σ₂). Pairwise comparisons confirming the distinguishability of enamel, CR, and GIC.

Ultrasound microscopy effectively distinguishes between enamel and restorative materials, as well as between restorative materials (CR and GIC) through acoustic impedance measurement.

These findings suggest that ultrasound microscopy may assist in identifying restoration margins and assessing materials in clinical settings.