Fracture behavior of human cortical bone with high glycation content under dynamic loading.

The present study simulates the fracture behavior of diabetic cortical bone with high levels of advanced glycation end-products (AGEs) under dynamic loading. We consider that the increased AGEs in diabetic cortical bone degrade the materials heterogeneity of cortical bone through a reduction in critical energy release rates of the microstructural features. To simulate the initiation and propagation of cracks, we implement a phase field fracture framework on 2D models of human tibia cortical microstructure. The simulations show that the mismatch between the fracture properties (e.g., critical energy release rate) of osteons and interstitial tissue due to high AGEs contents can change crack growth trajectories. The results show crack branching in the cortical microstructure under dynamic loading is affected by the mismatches related to AGEs. In addition, we observe cortical features such as osteons and cement lines can prevent multiple cracking under dynamic loading even with changing the mismatches due to high AGEs. Furthermore, under dynamic loading, some toughening mechanisms can be activated and deactivated with different AGEs contents. In conclusion, the current findings present that the combination of the loading type and materials heterogeneity of microstructural features can change the fracture response of diabetic cortical bone and its fragility.

Microstructural fatigue fracture behavior of glycated cortical bone.

The current study aims to simulate fatigue microdamage accumulation in glycated cortical bone with increased advanced glycation end-products (AGEs) using a phase field fatigue framework. We link the material degradation in the fracture toughness of cortical bone to the high levels of AGEs in this tissue. We simulate fatigue fracture in 2D models of cortical bone microstructure extracted from human tibias. The results present that the mismatch between the critical energy release rate of microstructural features (e.g., osteons and interstitial tissue) can alter crack initiation and propagation patterns. Moreover, the high AGEs content through the increased mismatch ratio can cause the activation or deactivation of bone toughening mechanisms under cyclic loading. The fatigue fracture simulations also show that the lifetime of diabetic cortical bone samples can be dependent on the geometry of microstructural features and the mismatch ratio between the features. Additionally, the results indicate that the trapped cracks in cement lines in the diabetic cortical microstructure can prevent further crack growth under cyclic loading. The present findings show that alterations in the materials heterogeneity of microstructural features can change the fatigue fracture response, lifetime, and fragility of cortical bone with high AGEs contents. Cortical bone models are created from microscopy images taken from the cortical cross-section of human tibias. Increased glycation contents in the cortical bone sample can change the crack growth trajectories.

Morphology and mechanical performance of dental crown designed by 3D-DCGAN.

OBJECTIVES: This study utilised an Artificial Intelligence (AI) method, namely 3D-Deep Convolutional Generative Adversarial Network (3D-DCGAN), which is one of the true 3D machine learning methods, as an automatic algorithm to design a dental crown. METHODS: Six hundred sets of digital casts containing mandibular second premolars and their adjacent and antagonist teeth obtained from healthy personnel were machine-learned using 3D-DCGAN. Additional 12 sets of data were used as the test dataset, whereas the natural second premolars in the test dataset were compared with the designs in (1) 3D-DCGAN, (2) CEREC Biogeneric, and (3) CAD for morphological parameters of 3D similarity, cusp angle, occlusal contact point number and area, and in silico fatigue simulations with finite element (FE) using lithium disilicate material. RESULTS: The 3D-DCGAN design and natural teeth had the lowest discrepancy in morphology compared with the other groups (root mean square value = 0.3611). The Biogeneric design showed a significantly (p < 0.05) higher cusp angle (67.11°) than that of the 3D-DCGAN design (49.43°) and natural tooth (54.05°). No significant difference was observed in the number and area of occlusal contact points among the four groups. FE analysis showed that the 3D-DCGAN design had the best match to the natural tooth regarding the stress distribution in the crown. The 3D-DCGAN design was subjected to 26.73 MPa and the natural tooth was subjected to 23.97 MPa stress at the central fossa area under physiological occlusal force (300 N); the two groups showed similar fatigue lifetimes (F-N curve) under simulated cyclic loading of 100-400 N. Designs with Biogeneric or technician would yield respectively higher or lower fatigue lifetime than natural teeth. SIGNIFICANCE: This study demonstrated that 3D-DCGAN could be utilised to design personalised dental crowns with high accuracy that can mimic both the morphology and biomechanics of natural teeth.

Which dentine analogue material can replace human dentine for crown fatigue test?

OBJECTIVE: To seek dentine analogue materials in combined experimental, analytical, and numerical approaches on the mechanical properties and fatigue behaviours that could replace human dentine in a crown fatigue laboratory test. METHODS: A woven glass fibre-filled epoxy (NEMA grade G10; G10) and a glass fibre-reinforced polyamide-nylon (30% glass fibre reinforced polyamide-nylon 6,6; RPN) were investigated and compared with human dentine (HD). Flexural strength and elastic modulus (n = 10) were tested on beam-shaped specimens via three-point bending, while indentation hardness (n = 3) was tested after fracture. Abutment substrates of G10, RPN and HD were prepared and resin-bonded with monolithic lithium disilicate crowns (n = 10), then subjected to wet cyclic loading in a step-stress manner (500 N initial load, 100 N step size, 100,000 cycles per step, 20 Hz frequency). Data were statistically analysed using Kruskal-Wallis one-way ANOVA followed by post-hoc comparisons (α = 0.05). Survival probability estimation was performed by Mantel-Cox Log-Rank test with 95% confidence intervals. The fatigue failure load (FFL) and the number of cycles until failure (NCF) were evaluated with Weibull statistics. Finite Element Models of the fatigue test were established for stress distribution analysis and lifetime prediction. Fractographic observations were qualitatively analysed. RESULTS: The flexural strength of HD (164.27 ± 14.24 MPa), G10 (116.48 ± 5.93 MPa), and RPN (86.73 ± 3.56 MPa) were significantly different (p < 0.001), while no significant difference was observed in their flexural moduli (p = 0.377) and the indentation hardness between HD and RPN (p = 0.749). The wet cyclic fatigue test revealed comparable mean FFL and NCF of G10 and RPN to HD (p = 0.237 and 0.294, respectively) and similar survival probabilities for the three groups (p = 0.055). However, RPN promotes higher stability and lower deviation of fatigue test results than G10 in Weibull analysis and FEA. SIGNIFICANCE: Even though dentine analogue materials might exhibit similar elastic properties and fatigue performance to human dentine, different reliabilities of fatigue on crown-dentine analogues were shown. RPN seems to be a better substrate that could provide higher reliability and predictability of laboratory study results.

Influence of age-related changes on crack growth trajectories and toughening mechanisms in human dentin.

OBJECTIVES: Dentin microstructure undergoes changes with age and its materials properties degrade over time. In the present study, we investigate the coupled influence of increased filled tubules and decreased materials properties on the fracture behavior of human dentin. METHODS: We assume degraded materials properties are linked with increased advanced glycation end-products (AGEs) crosslinks in dentin tissue. We use morphological data of human molars to create 2D and 3D models of dentin microstructure, and utilize a phase field fracture framework to study crack growth trajectories. We construct aged dentin samples (i.e., filled tubules and degraded properties) and compare the fracture results with the samples without age-related changes. RESULTS: The simulations show an increase in the number of filled tubules can deactivate the toughening mechanisms such as crack deflection and microcracking. In addition, filled tubules have adverse impacts on the ability of peritubular dentin to shield microcracking. We further show how the dentinal tubules' orientations affect the crack surface growth. We also investigate that an increase in the AGEs level can result in increased brittleness. SIGNIFICANCE: The developed model and findings of the present study provide region-dependent information on crack growth trajectories as well as more understanding of crack surface growth at the presence of filled tubules.

Damage analysis of human cortical bone under compressive and tensile loadings.

Developing advanced fracture tools can increase the understanding of crack growth trajectories in human cortical bone. The present study investigates fracture micromechanics of human cortical bone under compressive and tensile loadings utilizing a phase field method. We construct two-dimensional finite element models from cortical microstructure of a human tibia cross section. We apply compression on the cortical bone models to create compressive microcracks. Then, we simulate the fracture of these models under tension to discover influential parameters on microcracks formation and post-yielding behavior. The results show that cement lines are susceptible sites to damage nucleation under compression rather than tension. The findings of this study also indicate a higher accumulation of initial damage (induced by compression) can lead to a lower microscopic stiffness as well as a less resistant material to damage initiation under tension. The simulations further indicate that the post-yielding properties (e.g., toughness) can be dependent on different variables such as morphological information of the osteons, the initial accumulation of microcracks, and the total length of cement lines.

Computational study of the mechanical influence of lacunae and perilacunar zones in cortical bone microcracking.

The mechanical behavior of cortical bone is influenced by microstructural components such as osteons, Haversian canals, and osteocyte lacunae that arise from biological remodeling processes. This study takes a computational approach to investigate the role of the perilacunar zones formed by the local remodeling processes of lacunar-dwelling osteocytes by utilizing phase-field finite element models based on histological imaging of human bone. The models simulated the microdamage accumulation that occurs in cortical bone under transverse compression in bone without lacunae, with lacunae, and with a perilacunar zone surrounding lacunae in order to investigate the role of these features. The results of the simulations found that while lacunae create stress concentration which initiate further damage, perilacunar regions can delay or prevent the emergence and growth of microcracks.

Dental students' preference and perception on intraoral scanning and impression making.

BACKGROUND: To investigate the preference and perception on intraoral scanning and impression making among dental students. METHODS: Final-year dental students from the 2019 and 2020 cohorts were invited to complete an online questionnaire via Google-Form. Their preference on the intraoral-scanning/impression making techniques and their perception on these techniques including the ease of defect identification, ease of infection control, need of chairside support, ease to master the technique as a beginner, efficiency in their hands and ease to handle the scanner software (yes/no) were collected. The results were analysed using McNemar tests and binary logistic regression test. All tests were performed at significance level α = 0.05. RESULTS: Ninety-seven students participated in this study with a response rate of 96.0 %. Eighty-one students (83.5 %) have tried intraoral scanning on peers. Fifty-three (54.6 %) students preferred intraoral-scanning and were categorized as Pro-scanning group. Forty-four (45.4 %) students either preferred impression-making (n = 21) or not sure (n = 23) were categorized as Others. More than half of students in both groups felt that intraoral-scanning is easier to identify defect, easier in infection control and require less chairside support. Higher proportion of students in the Pro-scanning group felt that intraoral-scanning requires less chairside support, easier to master as a beginner, more efficient in their hands and they can deal well with the scanner software than that in Others (P < 0.05). Regression shown that students preferred a technique that they perceived is more efficient (P = 0.000). CONCLUSIONS: While intraoral scanning has perceived advantages, many students still prefer impression making that works more efficient to them.

Fracture behavior of human cortical bone: Role of advanced glycation end-products and microstructural features.

Diabetes is associated with increased fracture risk in human bone, especially in the elderly population. In the present study, we investigate how simulated advanced glycation end-products (AGEs) and materials heterogeneity affect crack growth trajectory in human cortical bone. We used a phase field fracture framework on 2D models of cortical microstructure created from human tibias to analyze crack propagation. The increased AGEs level results in a higher rate of crack formation. The simulations also indicate that the mismatch between the fracture properties (e.g., critical energy release rate) of osteons and interstitial tissue can alter the post-yielding behavior. The results show that if the critical energy release rate of cement lines is lower than that of osteons and the surrounding interstitial matrix, cracks can be arrested by cement lines. Additionally, activation of toughening mechanisms such as crack merging and branching depends on bone microstructural morphology (i.e., osteons geometrical parameters, canals, and lacunae porosities). In conclusion, the present findings suggest that materials heterogeneity of microstructural features and the crack-microstructure interactions can play important roles in bone fragility.

Fracture micromechanics of human dentin: A microscale numerical model.

In the present study, we investigate the effects of microstructural morphology and heterogeneity on the initiation and propagation of microcracks in dentin. We create 2D pre-cracked models of human dentin at the microscale level and use a brittle fracture framework of the phase-field method to analyze the crack growth. We discuss the influence of the microstructural features on crack deflection, microcracking, and uncracked ligament bridging through various regions in dentin. The results demonstrate that the difference between the critical energy release rates of peritubular (PTD) and intertubular dentin (ITD) has considerable impacts on microcracking. Our simulations reveal that tubules surrounded by PTDs play an important role in the crack deflection. Our results also indicate that the toughness of dentin increases from the inner to outer dentin. In conclusion, the findings in our study provide valuable insights into the fracture behavior in various regions of dentin.

Effect of preparation design for all-ceramic restoration on maxillary premolar: a 3D finite element study.

PURPOSE: The aim of this study was to investigate and quantify the effect of preparation design parameters on a premolar restored with two different CAD/CAM ceramic crowns by three-dimensional finite element analysis (FEA). METHODS: A restored human first premolar was digitized by a micro-CT scanner and a 3D model was created by a medical image processing software (Mimics). Following segmentation, dentine and ceramic were extracted by a surface meshing software (3-matic). Models with different preparation designs with three convergence angles (6°, 12° and 20°) and two preparation heights (3.1mm and 4.1mm) were produced. Mesh generation for models was performed in IA-FEMesh software with a lithium disilicate glass ceramic (LD, E=95.9GPa) and a polymer-infiltrated ceramic (PIC, E=30.1GPa) as the restorative materials. A 5-mm diameter stainless steel hemisphere was employed as an indenter. Twelve models were analyzed numerically in Abaqus™. RESULTS: The results indicated that preparation height was found to be a major factor affecting stress distribution in different components. In all models, the maximum principal stress of the ceramic crowns was found in contact area against the indenter. This stress was lesser in the longer abutment than the shorter one and it was greater for LD ceramic. Convergence angle had limited effect on stress distribution of ceramic crown in all models. CONCLUSIONS: The preparation height appeared to play a more important role in the stress distribution of ceramic crown than the convergence angle.