Additive Manufacturing of Honeycomb Lattice Structure-From Theoretical Models to Polymer and Metal Products.

The study aims to compare mechanical properties of polymer and metal honeycomb lattice structures between a computational model and an experiment. Specimens with regular honeycomb lattice structures made of Stratasys Vero PureWhite polymer were produced using PolyJet technology while identical specimens from stainless steel 316L and titanium alloy Ti6Al4V were produced by laser powder bed fusion. These structures were tested in tension at quasi-static rates of strain, and their effective Young's modulus was determined. Analytical models and finite element models were used to predict effective Young's modulus of the honeycomb structure from the properties of bulk materials. It was shown, that the stiffness of metal honeycomb lattice structure produced by laser powder bed fusion could be predicted with high accuracy by the finite element model. Analytical models slightly overestimate global stiffness but may be used as the first approximation. However, in the case of polymer material, both analytical and FEM modeling significantly overestimate material stiffness. The results indicate that computer modeling could be used with high accuracy to predict the mechanical properties of lattice structures produced from metal powder by laser melting.

Modeling of load transmission and distribution of deformation energy before and after healing of basal dental implants in the human mandible.

The purpose of this study was to present the amount and distribution of pressure, stress, and deformation energy when basal implants in the mandible are restored with a bridge which is loaded at two different stages of bone healing. The model geometry and material properties of the mandible were gained from CT scans of a human mandible. The material model used in this study defined bone as an inhomogeneous, linear elastic isotropic material. The masseter and temporal muscles were considered as rigid connections between the bones in typical positions and directions. The rotation axis was simulated in the temporomandibular joint. The loading force of 450 N was assumed to be in the middle between the left molar and left canine implant. In freshly operated bone, the total deformation energy is 30% higher than in healed bone, due to the defined energy absorbing soft bone areas. Approximately 90% of the deformation energy is absorbed by the bone, regardless of the healing state of the bone. The immediate rigid implant splinting distributes peak forces. To cope with these energies, the necessity of a reduction of total masticatory forces or the use of additional implants for force distribution should be considered individually.

Implementation of contact definitions calculated by FEA to describe the healing process of basal implants.

AIMS: Bone structure around basal implants shows a dual healing mode: direct contact areas manifest primary osteonal remodeling, in the void osteotomy-induced spaces, the repair begins with woven bone formation. This woven bone is later converted into osteonal bone. The purpose of this study was to develop a model to accurately represent the interface between bone and basal implant throughout the healing process. The model was applied to the biological scenario of changing load distribution in a basal implant system over time. METHODS: Computations were made through finite element analysis using multiple models with changing boneimplant contact definitions which reflected the dynamic nature of the interface throughout the bony healing process. Five stages of bony healing were calculated taking into account the changes in mineral content of bone in the vicinity of the load transmitting implant surfaces. RESULTS: As the bony integration of basal implants proceeds during healing, peak stresses within the metal structure shift geographically. While bony repair may still weaken osteonal bone, woven bone has already matured. This leads to changes in the load distribution between and within the direct contact areas, and bone areas which make later contact with implant. CONCLUSIONS: This study shows that basal implants undergo an intrinsic shift of maximum stress regions during osseointegration. Fatigue testing methods in the case of basal implants must therefore take into account this gradual shift from early healing phase until full osseointegration is achieved.

The use of finite element analysis to model bone-implant contact with basal implants.

OBJECTIVE: The purpose of this study was to develop a model that accurately represents the interface between bone and basal implants throughout the healing process. STUDY DESIGN: The model was applied to the biological scenario of changing load distribution in a basal implant system over time. We did this through finite element analysis (FEA, or finite element method [FEM]), using multiple models with changing bone-implant contact definitions, which reflected the dynamic nature of the interface throughout the bony healing process. RESULTS: In the simple models, peak von Mises stresses decreased as the bone-implant-contact definition was changed from extremely soft contact (i.e., immature bone during early loading) to hard contact (i.e., mature bone). In upgraded models, which more closely approximate the biological scenario with basal dental implant, peak von Mises stresses decreased at the implant interface; however, they increased at the bone interface as a harder contact definition was modeled. Further, we found a shift in peak stress location within the implants during different contact definitions (i.e., different stages of bony healing). In the case of hard contact, the peak stress occurs above the contact surface, whereas in soft contact, the stress peak occurs in the upper part of the contact area between bone and the vertical shaft of the implant. Only in the extreme soft contact definitions were the peak stresses found near the base plate of the implant. CONCLUSION: Future FEM studies evaluating the functional role of dental implants should consider a similar model that takes into account bone tissue adaptations over time.

Bendable vs. angulated dental implants: consideration of elastic and plastic material properties based on experimental implant material data and FEA.

AIM: Bendable and angulated single-piece implants are used alternatively to screwable abutments in two-piece dental implant designs. Though used frequently, data on the stress distribution within such implants are not available and the question whether the bending contributes to fracture resistance has not been addressed. METHODS: We used the method of finite element to identify von Mises stresses and maximum stresses in bent and non-bent but angulated implants. Implants with one (e.g. applicable to screw designs) or two (applicable to basal implants) bending areas were the variables under investigation. RESULTS: For bends up to 13 degrees we discovered that if there is only one bend, the maximum stress is in the bent area. If two bends are made in two different bending areas, the maximum stresses are distributed between the two and, if either one of the bent areas is machined, there are no residual stresses within the implant body in this area. The maximum stresses are always located near the base-plates. The absolute value of the maximum stress is higher because no residual stresses are available to compensate stresses that stem from loading. CONCLUSION: Assuming that all other parameters are equal, bendable (basal) implants show a more even stress distribution along the vertical implant region than identically shaped implants with a machine-angulated area. Bendable basal implants therefore probably resist masticatory forces better than pre-angulated, machined implants, and unbent implants which provide a thin region in the vertical implant area.

Time analysis of hard and soft bolus processing.

OBJECTIVE: Clinical observations and mathematical models show that dental implants are influenced by the magnitude of loading. Therefore, the knowledge of mandible movement during mastication is important to assess occlusal and masticatory force vectors. The purpose of this study was to detect the path of movement of the lower jaw and to distinguish stages of mastication, duration of bolus processing and peak amplitude of mastication. METHOD: Motion analysis was used to record three-dimensional mandible movements. Individualized sensors were rigidly attached to the mandible of 51 study participants. At the beginning of the measurement, all subjects were asked to move the mandible in extreme positions (maximal opening and maximal lateral movements). Then, each subject masticated a bite of hard and soft food. Duration of bolus mastication and peak amplitude of mastication movement in mesio-distal, cranio-caudal and vestibulo-oral axes related to peak amplitude of marginal movements were evaluated for each subject. The chewing record of each subject was divided into three phases (chopping, grinding and swallowing), and the duration of mastication and number of closing movements were evaluated. RESULTS: The findings of this pilot study suggest that masticatory movements vary in individuals. Bolus character influences the process duration, but not the frequency of closing movements. Neither gender nor age had any influence on either the time or frequency of bolus processing. CONCLUSION: Relationships to directions and magnitudes of acting chewing force should be more precisely examined since transversally acted forces during grinding are important factors in tooth/implant overloading.

Determination of elastic coefficients of bone and composite materials by acoustic immersion technique.

Elastic properties of single parts of a human skeleton are necessary to know for modelling bone tissue-implants interactions as well as for diagnostic purposes. This paper contributes to the methodology of the evaluation of elastic properties of bones by the ultrasonic wave inversion. The method was developed on composite structures such as plates and cylindrical shells. Final results are then demonstrated on the bovine cortical bone specimen. Properties are supposed to exhibit an orthotropic or a transversally isotropic symmetry. Quasi-longitudinal and quasi-transversal waves are generated from the wave diffraction on the liquid/specimen interface. Wave velocity fields obtained by the ultrasonic scanning technique are used as an input to the inversion procedure for all elastic constants determination. Experimental results are confronted with the numerical modelling of the wave propagation and the stability of resulting data is evaluated by the statistical method based on the Monte-Carlo simulation. The suggested approach has a potential for the qualify of such measurements performed on fresh bones and also for improvement in-situ ultrasonic techniques.