The mechanical properties of enamel and dentin were studied using test specimens having the same shape and dimensions because these properties might vary with the experimental conditions and specimen shapes and dimensions. Healthy human teeth were used as specimens for mechanical tests. The stress (MPa), strain (%), and elastic modulus (E, MPa) of the specimens were obtained from compression tests. The maximum stresses of the enamel, dentin, and enamel–dentin specimens were 62.2 ± 23.8, 193.7 ± 30.6, and 126.1 ± 54.6 MPa, respectively. The maximum strains of the enamel, dentin, and enamel–dentin specimens were 4.5 ± 0.8%, 11.9 ± 0.1%, and 8.7 ± 2.7%, respectively. The elastic moduli of the enamel, dentin, and enamel–dentin specimens were 1338.2 ± 307.9, 1653.7 ± 277.9, and 1628.6 ± 482.7 MPa, respectively. The measured hardness value of enamel specimens (HV = 274.8 ± 18.1) was around 4.2 times higher than that of dentin specimens (HV = 65.6 ± 3.9). Judging from the measured values of the stress and strain of enamel specimens, enamel tended to fracture earlier than dentin; therefore, it was considered more brittle than dentin. However, judging from the measured hardness values, enamel was considered harder than dentin. Therefore, enamel has higher wear resistance, making it suitable for grinding and crushing foods, and dentin has higher force resistance, making it suitable for absorbing bite forces. The different mechanical roles of enamel and dentin may arise from their different compositions and internal structures, as revealed through scanning electron micrographs of enamel and dentin.

There are two objectives. One is to show the differences in the mechanical properties of various dental restorative materials compared to those of enamel and dentin. The other is to ascertain which dental restorative materials are more suitable for clinical treatments. Amalgam, dental ceramic, gold alloy, dental resin, zirconia, and titanium alloy were processed as dental restorative material specimens. The specimens (width, height, and length of 1.2, 1.2, and 3.0 mm, respectively) were compressed at a constant loading speed of 0.1 mm/min. The maximum stress (115.0 ± 40.6, 55.0 ± 24.8, 291.2 ± 45.3, 274.6 ± 52.2, 2206.0 ± 522.9, and 953.4 ± 132.1 MPa), maximum strain (7.8% ± 0.5%, 4.0% ± 0.1%, 12.7% ± 0.8%, 32.8% ± 0.5%, 63.5% ± 14.0%, and 45.3% ± 7.4%), and elastic modulus (1437.5 ± 507.2, 1548.4 ± 583.5, 2323.4 ± 322.4, 833.1 ± 92.4, 3895.2 ± 202.9, and 2222.7 ± 277.6 MPa) were evident for amalgam, dental ceramic, gold alloy, dental resin, zirconia, and titanium alloy, respectively. The reference hardness value of amalgam, dental ceramic, gold alloy, dental resin, zirconia, and titanium alloy was 90, 420, 130–135, 86.6–124.2, 1250, and 349, respectively. Since enamel grinds food, its abrasion resistance is important. Therefore, hardness value should be prioritized for enamel. Since dentin absorbs bite forces, mechanical properties should be prioritized for dentin. The results suggest that gold alloy simultaneously has a hardness value lower than enamel (74.8 ± 18.1), which is important in the wear of the opposing natural teeth, and higher maximum stress, maximum strain, and elastic modulus than dentin (193.7 ± 30.6 MPa, 11.9% ± 0.1%, 1653.7 ± 277.9 MPa, respectively), which are important considering the rigidity to absorb bite forces.

No abstract

Osteoporosis is a disease characterized by low bone mass, increased bone fragility, and a greater risk for bone fracture. Currently, pharmacological intervention can generally aid in the prevention and treatment of osteoporosis, but these therapies are often accompanied by undesirable side effects. Therefore, alternative therapies that minimize side effects are necessary. Biophysical stimuli, especially low-intensity ultrasound stimulation (LIUS), may be potential alternatives to drug-based therapies for osteoporosis. Hence, we sought to address whether LIUS therapy can effectively prevent or treat osteoporotic bone loss induced by estrogen deficiency. LIUS (1.5 MHz frequency, 1.0 kHz pulse repetition on frequency, 30 mW/cm 2 intensity, 200 s pulse length) was applied to right tibiae of eight 14-week-old ovariectomized virgin ICR female mice for 20 min per day, 5 days per week, over a 6-week period. Changes in 3D structural bone characteristics were detected using in vivo micro-computed tomography. Left tibiae served as controls. Structural characteristics including bone volume/tissue volume, trabecular number, trabecular bone pattern factor, and mean polar moment inertia were significantly enhanced 6 weeks after LIUS compared to the control, nonstimulated group (p < 0.05). In particular, the bone volume/tissue volume in the region exposed directly to LIUS was significantly higher in the treated group (p < 0.05). These findings indicate that new bone formation may be activated or that bone structure may be maintained by LIUS, and that LIUS may be effective for preventing estrogen deficiency-induced bone loss.

Tendon specimens were repeatedly extended to peak strains of either 2, 3, 4, or 6 percent. During the three 1800 s (30 min.) periods of cyclic extension, the peak loads relaxed with decreases in hysteresis and increases in slack strain. During the two 1800 s wait periods of no extension, the specimens recovered with increases in peak load and hysteresis and decreases in slack strain. However, the recovery during the wait periods was eradicated in the first few subsequent extensions and the relaxation continued as if there were no 1800 s wait periods. Stress-strain responses were well fit with power relations.