Establishing the Finite Element Model of Mid-Palatal Suture Fracture by Microimplant-Assisted Palatal Expansion / 医用生物力学

Objective To analyze mechanical characteristics for stress accumulation of the maxillary complex during expansion until complete fracture of the mid-palatal suture by using the three-dimensional (3D) finite element method, and verify validity of the model. Methods The finite element maxillary complex model containing the microimplants was established. The yield strength of the mid-palatal suture was set, and the transverse displacement of 0.25 mm was loaded every 5 ms as the load until the suture was completely fractured. The CT images of one clinical patient before and after expansion were compared and verified. Results During 0-17 ms, the stress was mainly concentrated around the microimplants, the middle of the mid-palatal suture and the zygomatico-maxillary sutures. During 18-60 ms, cracks began to occur in the mid-palatal suture, and expanded forward and backward. During 61-71 ms, the rupture path was followed by posterior part of the mid, palatal suture-the front part and the back part. Conclusions The 3D finite element model of microimplant-assisted expansion based on fracture mechanics was effective and the calculated fracture process result were more consistent with clinical practice. The research findings provide the mechanical reference model for more effective research on microimplant-assisted palatal expansion in the future.

Finite element simulation of fracture in a restored premolar tooth using high aspect ratio elements

Abstract Introduction The term cracked tooth syndrome refers to an incomplete fracture of a vital posterior tooth that involves the dentin and occasionally extends into the pulp. There is a very limited number of publications trying to model dentin crack growth using numerical techniques. Therefore, it is essential to numerically model this phenomenon in order to improve the clinical procedures. Methods A 2D finite element model is proposed to simulate crack initiation and propagation in a restored premolar tooth. The geometric model was based on computed tomography data. A special finite element technique, named mesh fragmentation technique, is used to model and analyze the behavior of the tooth. This technique was used to model cracks in quasi-brittle materials based on the use of interface solid finite elements with high aspects ratio. A tension damage constitutive relation between stresses and strains consistent with the continuous strong discontinuity approach is used to describe crack formation and propagation. Results The main aspects of modeling technique and procedures are explained in detail as well as the whole results, including both elastic and fracture analyses of the restored tooth. Conclusion The results of the current fracture analysis show that, under various loading conditions, there is no crack initiation in the restored tooth under typical loading magnitude. However, in the case of tooth with a pre-existing crack, which can be aroused during the restoration process, a crack propagation was observed, while they did not reach a critical fracture state.

Effect of grinding and aging on subcritical crack growth of a Y-TZP ceramic

Abstract This study aimed to investigate slow crack growth (SCG) behavior of a zirconia ceramic after grinding and simulated aging with low-temperature degradation (LTD). Complementary analysis of hardness, surface topography, crystalline phase transformation, and roughness were also measured. Disc-shaped specimens (15 mm Ø × 1.2 mm thick, n = 42) of a full-contour Y-TZP ceramic (Zirlux FC, Amherst) were manufactured according to ISO:6872-2008, and then divided into: Ctrl - as-sintered condition; Ctrl LTD - as-sintered after aging in autoclave (134°C, 2 bar, 20 h); G - ground with coarse diamond bur (grit size 181 μm); G LTD - ground and aged. The SCG parameters were measured by a dynamic biaxial flexural test, which determines the tensile stress versus stress rate under four different rates: 100, 10, 1 and 0.1 MPa/s. LTD led to m-phase content increase, as well as grinding (m-phase content: Ctrl - 0%; G - 12.3%; G LTD - 59.9%; Ctrl LTD - 81%). Surface topography and roughness analyses showed that grinding created an irregular surface (increased roughness) and aging did not promote any relevant surface change. There was no statistical difference on surface hardness among different conditions. The control group presented the lowest strength values in all tested rates. Regarding SCG, ground conditions were less susceptible to SCG, delaying its occurrence. Aging (LTD) caused an increase in SCG susceptibility for the as-sintered condition (i.e. G < G LTD < Ctrl < Ctrl LTD).

Ultra-high cycle fatigue behaviors of implanted Ti-6Al-4V in high frequency after subjection to simulated body fluid / 医用生物力学

Objective To study the ultra-high cycle fatigue behaviors of the Ti-6Al-4V alloy implant after subjection to simulated body fluid (SBF) environment exceeding 107 cycles, and to predict its service life. Method The Ti-6Al-4V alloy specimens were subjected to SBF in two groups for two days and six days respectively; using the ultrasonic fatigue testing technique, the ultra high cycle fatigue properties of Ti-6Al-4V alloy after subjection to SBF in body temperature were studied and the initiation mechanisms of fatigue cracks were investigated and analyzed with scanning electron microscopy (SEM) and energy dispersive atomic X-ray (EDA X ray). Then, a comparison with corresponding behaviors of the normal Ti 6Al 4V alloy was made. Results The S~N curve of the Ti-6Al-4V alloy after subjection had the similar tendency as that of the normal one, showing that they descended continuously during 104 and 108 cycles and the descending tendency was lower in the region of 107～108 cycles; the life of the specimens after subjection dropped before the fatigue life of 107 cycles and was smaller after the fatigue life of 107 cycles than that of the normal ones; the life of the specimens subjected for six days had little difference with that subjected for two days; fracture could still occur beyond 107 cycles, showing that there was no fatigue limit as the traditional fatigue conception described. Fatigue cracks mainly initiated from the surface of specimen before the fatigue life of 107 cycles and some specimens had multiple crack initiation sites; fatigue failure initiated from internal inclusion where mostly the element Al aggregated after the fatigue life of 108 cycles. Conclusions The ultra high cycle fatigue properties of the Ti-6Al-4V alloy decreas a little with SBF subjection; the initiation site of the fatigue crack changes from the surface to the internal defect with the increase of the number of cycle.