Specific Finite Element Models Research Articles

Determination of the internal loads experienced by proximal phalanx fracture fixations during rehabilitation exercises.

Phalangeal fractures are common, particularly in younger patients, leading to a large economic burden due to higher incident rates among patients of working age. In traumatic cases where the fracture may be unstable, plate fixation has grown in popularity due to its greater construct rigidity. However, these metal plates have increased reoperation rates due to inflammation of the surrounding soft tissue. To overcome these challenges, a novel osteosynthesis platform, AdhFix, has been developed. This method uses a light-curable polymer that can be shaped in situ around traditional metal screws to create a plate-like structure that has been shown to not induce soft tissue adhesions. However, to effectively evaluate any novel osteosynthesis device, the biomechanical environment must first be understood. In this study, the internal loads in a phalangeal plate osteosynthesis were measured under simulated rehabilitation exercises. In a human hand cadaver study, a plastic plate with known biomechanical properties was used to fix a 3mm osteotomy and each finger was fully flexed to mimic traditional rehabilitation exercises. The displacements of the bone fragments were tracked with a stereographic camera system and coupled with specimen specific finite element (FE) models to calculate the internal loads in the osteosynthesis. Following this, AdhFix patches were created and monotonically tested under similar conditions to determine survival of the novel technique. The internal bending moment in the osteosynthesis was 6.78 ± 1.62Nmm and none of the AdhFix patches failed under the monotonic rehabilitation exercises. This study demonstrates a method to calculate the internal loads on an osteosynthesis device during non-load bearing exercises and that the novel AdhFix solution did not fail under traditional rehabilitation protocols in this controlled setting. Further studies are required prior to clinical application.

Critical condition for initiation of dynamic recrystallisation in electron beam powder bed fused Ti-6Al-4 V Alloy

Despite the unique capabilities of Additive Manufacturing (AM) processes for producing Ti components with complex geometries, the desired properties are only achievable if a holistic scheme is devised, considering the synergistic role of post-processing steps. This work aimed to enhance the applicability of metal AM products by improving the process chain in the manufacturing industry. For this purpose, the current work demonstrates a comprehensive investigation into the hot deformation of Ti-6Al-4 V (Ti64) pre-forms produced via the Electron Beam Powder Bed Fusion (EB-PBF) process focusing on the determination of critical conditions for initiating Dynamic Recrystallization (DRX) in these components. Following a sequential evaluation procedure, hot deformation experiments were carried out at temperatures ranging from 1000 to 1200 °C and strain rates of 0.001–1 s−1 to determine the critical stress and strain required for initiating DRX in Ti64 pre-forms and compare them to their wrought counterparts. In addition, a specific Finite Element Model (FEM) was coupled with DRX kinetics equations to predict the volume percentage of DRX grains during the hot deformation of the pre-forms. The analysis of flow stress curves showed a significant peak stress at low strains, which is then followed by a period of flow softening and eventually transitions into a nearly steady-state flow at higher strains. In addition, compared to their wrought counterparts, the EB-PBF samples exhibited a significantly superior flow softening behavior, as evidenced by a more significant volume fraction of DRX and faster recrystallisation rates. It was also revealed that the normalised strain for DRX initiation in the EB-PBF Ti64 and wrought one was 0.55 and 0.67, respectively. FEM results closely matched the experimental finding, confirming its reliability in providing valuable insights into microstructural evolution and offering a time-efficient alternative for process design and property optimisation, lowering dependence on trial-and-error approaches. Through a combination of experiments, numerical analysis, and finite element simulations, this study sheds light on the macroscopic deformation and microstructural transformations occurring during hot working processes.

Influence of stem design parameters on periprosthetic femoral fractures examined by subject specific finite element analyses.

Due to the increasing number of periprosthetic femoral fractures (PFF), the optimisation of implant design gains importance. For the presented research a validated, subject specific finite element model of a human femur with an inlying total hip stem was used to compare the influence of different geometrical implant parameters on the development of PFF. The heterogeneous bone tissue was modelled on the basis of computed tomography scans. A ductile damage model with element deletion was applied to simulate bone fracture in a load case re-enacting a stumbling scenario. The results were compared in terms of fracture load, subsidence and fracture pattern to analyse the influence of friction at the implant-bone interface, implant size and stem length. The results showed that higher friction coefficients lead to an increase of fracture load. Also, the usage of an oversized implant has a negligible effect while an undersized implant reduces the fracture load by 48.9% for the investigated femur. Lastly, a higher fracture load was reached with an elongated stem, but the bending and change in fracture path indicate a more distal force transmission and subsequent stress shielding in the proximal femur.

The role of muscle forces and gait cycle discretization when assessing acetabular cup primary stability: A finite element study

The aim of this study was to investigate the influence of the muscle force contribution and loading cycle discretization on the predicted micromotion and interfacial bone strains in the implanted acetabulum. To this end, a patient specific finite element model of the hemipelvis was developed, based on the CT-scan and gait analysis results, collected as part of the authors’ previous work. Outcomes of this study suggests that the acetabular cup micromotion and interfacial bone strains can be predicted just using the joint contact force. This helps to reduce the complexity of the finite element models by ignoring the contribution of muscle forces and the associated challenges of mapping these forces to the pelvis. However, the gait cycle needs to be adequately discretised to capture the micromotion at the bone-implant interface. Background and ObjectiveThe Dalstra load case, which includes muscle forces, has been widely adopted in the literature for studying the mechanical environment in the intact and implanted acetabulum. To simplify the modelling approach, some researchers ignore the contribution of muscle forces. The Dalstra load case is also divided into eight separate load steps (five in the stance phase and three in the swing phase), however, it is unclear whether this adequately captures the micromotions, for a cementless acetabular cup, during a simulated activity. The aim of this study was to investigate the influence of the muscle force contribution and loading cycle discretization on the predicted micromotion and interfacial bone strains. MethodsIn this work, a patient specific finite element model of the hemipelvis was developed, based on the CT-scan and gait analysis results, collected as part of the authors’ previous work. Finite element simulations were performed using the joint contact and muscle forces derived from two sources. The first approach was used the load case proposed by Dalstra et al. The second approach used joint contact and muscle forces predicted by a musculoskeletal model. Additionally, the musculoskeletal load case was discretised into 50 equal load steps and the results compared with the equivalent Dalstra load steps. ResultsThe results showed that the contribution of the muscle forces resulted in minor differences in both the magnitude and distribution of the predicted acetabular micromotion (up to 4.01% in the mean acetabular micromotion) and interfacial bone strains (up to 10.34% in the mean interfacial bone strains). The degree of gait cycle discretisation had a significant influence on the acetabular micromotion with a difference of 20.89% in the mean acetabular micromotion. ConclusionOutcomes of this study suggests that the acetabular cup micromotion and interfacial bone strains can be predicted just using the joint contact force. This helps to reduce the complexity of the finite element models by ignoring the contribution of muscle forces and the associated challenges of mapping these forces to the pelvis. However, the gait cycle needs to be adequately discretised to capture the micromotion at the bone-implant interface.

Normalization technique to build patient specific muscle model in finite element head neck spine.

Finite element models of the head and neck are widely used in automotive and clinical fields to understand spinal biomechanics. These models are developed based on CT and MRI scans of the subjects, but historically the muscle data are obtained from cadaveric specimen. The cadaver data is often obtained from older specimens which commonly have undergone degenerative changes resulting in reduction in muscle cross section area. The objective of the current study is to compare the muscle cross-section area used by various finite element models of neck muscles used in the literature and to develop a normalization technique to scale the MRI muscle cross-section area with those available in the literature. Four male and seven female healthy asymptomatic young adult volunteers enrolled in the study after obtaining necessary approval from Institutional Review Board. T1 and T2 weighted magnetic resonance imaging was performed in neutral upright sitting position wearing military helmet. Muscle cross sectional area was obtained for multifidus muscles from the MRI images. Data was compared with those in the literature. Based on the literature review of prior studies, the cross-sectional area of cadaver specimens was smaller than the MRI obtained muscle area. Multifidus muscle scaling factor was obtained by ratio of sum of MRI cross section area with that of cadaver data. Based on the analysis, the scaling factor for male data is 1.6 and for female data is 1.3. the cadaver data can be multiplied by the scaling factor to obtain the MRI specific cross-sectional area. A Normalization technique was developed for scaling MRI data into finite element model. This technique can be used in developing subject specific finite element model of spine which has applications in clinical, automotive, and military environment.

The relationship between subchondral bone cysts and cartilage health in the Tibiotalar joint: A finite element analysis

BackgroundSubchondral bone cysts are a common presentation in ankle haemarthropathy. The relationship with ankle joint health has however not previously been investigated. The aim of this study was to assess the influence of subchondral bone cysts of differing shapes, volumes and depths on joint health. MethodsChronologically sequential Magnetic Resonance imaging scans of four hemophilic ankles with subchondral bone cysts present (N = 18) were used to build patient specific finite element models under two cystic conditions to assess their influence on cartilage contact pressures. Variables such as location, volume and depth were considered individually, to investigate whether certain cystic conditions may be more detrimental to cartilage health. FindingsSignificant quantifiable contact redistribution was seen in the presence of subchondral bone cysts and this redistribution reflected the shape and size of the cysts, however, with the presence of cysts in both bones in 10 of the 18 cases a direct relationship to volume could not be correlated. InterpretationThis work demonstrated a redistribution of contact pressures in the presence of subchondral bone cysts. This alteration to loading history could be linked to cartilage degeneration due to the biological response to abnormal loading.

Development of robust finite element models to investigate the stability of osteochondral grafts within porcine femoral condyles

Osteoarthritis (OA) is the most prevalent chronic rheumatic disease worldwide with knee OA having an estimated lifetime risk of approximately 14%. Autologous osteochondral grafting has demonstrated positive outcomes in some patients, however, understanding of the biomechanical function and how treatments can be optimised remains limited. Increased short-term stability of the grafts allows cartilage surfaces to remain congruent prior to graft integration. In this study methods for generating specimen specific finite element (FE) models of osteochondral grafts were developed, using parallel experimental data for calibration and validation. Experimental testing of the force required to displace osteochondral grafts by 2 mm was conducted on three porcine knees, each with four grafts. Specimen specific FE models of the hosts and grafts were created from registered μCT scans captured from each knee (pre- and post-test). Material properties were based on the μCT background with a conversion between μCT voxel brightness and Young's modulus. This conversion was based on the results of the separate testing of eight porcine condyles and optimization of specimen specific FE models. The comparison between the experimental and computational push-in forces gave a strong agreement with a concordance correlation coefficient (CCC) = 0.75, validating the modelling approach. The modelling process showed that homogenous material properties based on whole bone BV/TV calculations are insufficient for accurate modelling and that an intricate description of the density distribution is required. The robust methodology can provide a method of testing different treatment options and can be used to investigate graft stability in full tibiofemoral joints.

Cell membrane tensile strain under cyclic compression: A viscoelastic myoblast finite element model

Cyclic mechanical stimulation could lead to subsequent biomechanical and biological effects on cells. Viscoelastic cells could deform or show energy dissipation with hysteresis behavior in response to external cyclic compression. The aim of this study was to investigate the effect of cyclic compression on a single viscoelastic myoblast through a confocal–based cell–specific finite element model, including cell membrane tensile strain and damaged elements. Sinusoidal compression was applied to the apical surface of the myoblast (cell membrane) with compressive stress of 500 ​± ​500 ​Pa (with stress offset at 500 ​Pa and amplitude of 500 ​Pa) at 0 ​Hz (static compression of 500 ​Pa), 0.25 ​Hz, 0.5 ​Hz, 0.75 ​Hz, 1 ​Hz, 5 ​Hz, and 10 ​Hz. Results showed that the ratio of average tensile strain integral in all cell membrane elements over a certain period of time (T) to that duration (T) (MAS index) decreased under cyclic compression compared to that of static compression in the short term (within 4 ​s). Furthermore, compared to static compression, the percentage of damaged elements of cell membrane under cyclic compression decreased assuming a 3% cell membrane tensile strain damage threshold. The optimal cyclic compression frequency 0.25 ​Hz led to the largest difference of MAS index under cyclic compression and static compression. These results may provide support for the application of cyclic compressive stimulation in the prevention of cell damage.

Farm wide sensitivity assessments of resonant frequencies of integrated offshore wind turbine finite element models

To date, despite advancements in the design of offshore wind turbines, the as-designed and identified as-built natural frequencies of offshore wind turbines still show discrepancies. These discrepancies are partially rooted in modelling uncertainties, as well as uncertain input parameters, related to e.g. aero-, fluid- or soil-structure interaction. The first objective of this article is to present a wind farm wide comparison of the first and second, modelled and identified, fore-aft natural frequencies for turbines in parked conditions for a wind farm located in the Belgian north sea. Secondly, the effect of different model parameters on the computed natural frequencies will be assessed using wind farm wide sensitivity studies, with the aim to describe the potential of each considered parametrization in reducing the discrepancy between modelled and measured resonance frequencies. The in-depth considered parametrizations are aimed at assessing the effect of the linearization of the p-y curves, soil stiffness, local scour as well as the mass of the rotor nacelle assembly, whereas results for wall thickness, marine growth, added mass coefficient and sea water level variations will be presented without further discussion. In order to perform this study, turbine specific finite element models have been prepared and verified based on detailed design documents; subsequently updated best-estimate soil data has been used to model the foundation for two different design scenarios. Furthermore, modal parameters have been identified for each turbine, based on vibration data collected in parked condition and state of the art operational modal analysis tools. The results show that the discrepancies between the modelled and identified first fore-aft natural frequencies could potentially be bridged by adjusting combination of the investigated parameters, whereas the discrepancies observed on the second natural frequency cannot be bridged by making changes to the investigated parameters. As such, future work will entail a more detailed investigation on modelling uncertainties.

Finite element model for a coupled thermo-mechanical system in nonlinear strain-limiting thermoelastic body

We investigate a specific finite element model to study the thermoelastic behavior within the context of nonlinear strain-limiting constitutive relation. As a special subclass of implicit relations, the thermoelastic response of our interest is such that stresses can be arbitrarily large but strains remain small, particularly in the neighborhood of crack-tips. In the present communication, we consider a two-dimensional coupled system — linear and quasi-linear partial differential equations for temperature and displacements, respectively. A standard finite element method of continuous Galerkin is then employed to obtain the numerical solutions for the field variables, where two distinct temperature distribution of the Dirichlet type are considered for boundary condition. From a domain with an edge-crack, we find that the near-tip strain growth in the proposed model is slower than the growth of stress, which is the salient feature compared to the predictions of singular strain based on the classical linearized description of the elastic body. In essence, the model can be inherently consistent with the assumption of linearized elasticity and infinitesimal strain theory. This study can provide a theoretical and computational framework to develop physically meaningful models and examine other coupled multi-physics such as an evolution of complex network of cracks induced by thermal shocks.

Investigation of the Influence of Osteoporosis and Aging on Periprosthetic Femoral Fractures using Finite Element Analyses

With the increasing life expectancy and more active lifestyles of patients, the number of total hip replacements is constantly rising. Consequently, the incidence of periprosthetic femoral fractures (PFF) is also growing and facilitated by osteoporosis as well as advancing age of the patient. One way to prevent PFF is to optimize the treatment of individual patients, who are potentially at risk of osteoporotic fractures, by choosing the most suitable implant design for their subject specific circumstances. Thereby, revision surgeries due to PFF can be avoided. In this context appropriate models are necessary to predict fractures for aging and osteoporotic bone. On the basis of quantitative computed tomography (QCT) data subject specific finite element models with a heterogeneous material definition were created that can simulate PFF via element deletion. Statistical studies on the changes of bone quality with increasing patient age and developing osteoporosis were incorporated into the models. With statistical data, the bone mineral density (BMD) values measured in the QCT scans of the modelled femurs were varied, while the individual heterogeneous density distribution was preserved. The new density values were mapped onto the FE meshes of the femurs. This way, the femur was modelled to mirror the condition of the bone in different age groups and with different severity levels of osteoporosis in high age. The final models incorporate the subject specific geometry and density distribution of a patient's femur as well as the statistical density values resulting from aging and osteoporosis. The FE analyses revealed that a statistical change of bone density with aging leads to a relative small decrease in fracture load for periprosthetic fractures of a femur with a cementless endoprosthesis. However, for osteoporosis, the fracture load can differ by up to 56% with a cementless endoprosthesis, depending on the severity level of the disease.

Subject specific finite element modelling of periprosthetic femoral fractures in different load cases

Periprosthetic femoral fractures (PFF) around total hip replacements are one of the biggest challenges for orthopaedic surgeons. To understand the risk factors and formation of these fractures, the development of a reliable finite element (FE) model incorporating bone failure is essential. Due to the anisotropic and complex hierarchical structure of bone, the mechanical behaviour under large strains is difficult to predict. In this study, a state-of-the-art subject specific FE modelling technique for bone is utilised to generate and investigate PFF. A bilinear constitutive law is applied to bone tissue in subject specific FE models of five human femurs which are virtually implanted with a straight hip stem to numerically analyse PFF. The material parameters of the models are expressed as a function of bone ash density and mapped node wise to the FE mesh. In this way the subject specific, heterogeneous structure of bone is mimicked. For material mapping of the parameters, computed tomography (CT) images of the original fresh-frozen femurs are used. Periprosthetic fractures are generated by deleting elements on the basis of a critical plastic strain failure criterion. The models are analysed under physiological and clinically relevant conditions in two different load cases re-enacting stumbling and a sideways fall on the hip. The results of the analyses are quantified with experimental data from previous work. With regard to fracture pattern, stiffness and failure load the simulations of the load case stumbling delivered the most stable and accurate results. In general, mapping of material properties was found to be an appropriate way to reproduce PFF with finite element models.

Calibration of the numerical model of a track section over a railway bridge based on dynamic tests

Progressive updating strategies are commonly used for the calibration of numerical models such as those of a track-bridge system. These strategies involve, first, updating the track model based on dedicated dynamic tests, and second, the calibration of the bridge model based on ambient vibration tests considering the updated track parameters. This subsequent procedure is suggested as some track parameters are, in relation to the modal responses of the bridge, generally not sufficiently sensitive for a successful parameter estimation. This work describes the calibration of a specific finite element (FE) model of a track section over a bowstring-arch railway bridge. The model includes the rails, rail pads, sleepers, the ballast layer and the interfaces with the bridge and neighbouring track sections. A track receptance test allowed for the identification of natural frequencies and mode shapes of the track by applying two different techniques, an output-only and an input-output. Several modes of vibration related to the movement of the sleepers on the ballast were identified using the SSI-DATA method, at frequencies between 83.8 Hz and 142.2 Hz. The calibration of the track FE model was performed using an iterative methodology by means of a genetic algorithm, which proves efficiency and robustness in estimating several track numerical parameters, besides a significant upgrade in relation to the numerical model before calibration.

Toward Patient Specific Models of Pediatric IVDs: A Parametric Study of IVD Mechanical Properties.

Patient specific finite element (FE) modeling of the pediatric spine is an important challenge which offers to revolutionize the treatment of pediatric spinal pathologies, for example adolescent idiopathic scoliosis (AIS). In particular, modeling of the intervertebral disc (IVD) is a unique challenge due to its structural and mechanical complexity. This is compounded by limited ability to non-invasively interrogate key mechanical parameters of a patient's IVD. In this work, we seek to better understand the link between mechanical properties and mechanical behavior of patient specific FE models of the pediatric lumbar spine. A parametric study of IVD parameter was conducted, coupled with insights from current knowledge of the pediatric IVD. In particular, the combined effects of parameters was investigated. Recommendations are made toward areas of importance in patient specific FE modeling of the pediatric IVD. In particular, collagen fiber bundles of the IVD are found to dominate IVD mechanical behavior and are thus recommended as an area of primary focus for patient specific FE models. In addition, areas requiring further experimental research are identified. This work provides a valuable building block toward the development of patient specific models of the pediatric spine.

Development of a lead foil crown delineation technique for implant rehabilitations to generate patient specific finite element model of occlusal loading points

Understanding the clinical biomechanical basis of dental implant supported functional rehabilitation of edentulous jaws improves precision, longevity and overall success of a planned treatment. Stress distribution pattern around dental implants is an important determinant for rate of bone resorption around them. During planning the treatment for most prosthetic rehabilitations, the surgeon uses a software to virtually plan the dimension, position and angulation of the implants considering only the quantity of available bone in the area of interest but does not usually consider the strain generated around the implants after prosthetically loading them. We hence hypothesise that dental implants not be subjected to abnormal strain they should be positioned and angulated not only based on volume of bone available but also based on the vector of occlusal load. The virtual FEA model to analyse the stress distribution would hence require alveolar bone with future tooth/ teeth in centric relation to be modelled. This paper proposes a simple innovative technique to develop a 3D FE model of occlusal loading surface by using a radio-opaque malleable lead foil to generate a patient specific FE model. This would greatly minimise modelling errors and also help determine the best position of the dental implant based on both the volume of bone in the CT scan and the results of FE analyses.•Functional rehabilitation using dental implant supported prosthesis needs to be biomechanically analysed to know and understand the stress distribution pattern around the implant.•When teeth (Loading points) are missing, patient specific virtual model of occlusal loading points cannot be generated.•‘Lead foil crown delineation technique’ helps to generate patient specific 3D model of occlusal surface for load application.

Preoperative Transcranial Direct Current Stimulation in Glioma Patients: A Proof of Concept Pilot Study.

Background: Transcranial direct current stimulation (tDCS) has been used extensively in patient populations to facilitate motor network plasticity. However, it has not been studied in patients with brain tumors. We aimed to determine the feasibility of a preoperative motor training and tDCS intervention in patients with glioma. In an exploratory manner, we assessed changes in motor network connectivity following this intervention and related these changes to predicted electrical field strength from the stimulated motor cortex.Methods: Patients with left-sided glioma (n=8) were recruited in an open label proof of concept pilot trial and participated in four consecutive days of motor training combined with tDCS. The motor training consisted of a 60-min period where the subject learned to play the piano with their right hand. Concurrently, they received 40 min of 2 mA anodal tDCS of the left motor cortex. Patients underwent task and resting state fMRI before and after this intervention. Changes in both the connectivity of primary motor cortex (M1) and general connectivity across the brain were assessed. Patient specific finite element models were created and the predicted electrical field (EF) resulting from stimulation was computed. The magnitude of the EF was extracted from left M1 and correlated to the observed changes in functional connectivity.Results: There were no adverse events and all subjects successfully completed the study protocol. Left M1 increased both local and global connectivity. Voxel-wide measures, not constrained by a specific region, revealed increased global connectivity of the frontal pole and decreased global connectivity of the supplementary motor area. The magnitude of EF applied to the left M1 correlated with changes in global connectivity of the right M1.Conclusion: In this proof of concept pilot study, we demonstrate for the first time that tDCS appears to be feasible in glioma patients. In our exploratory analysis, we show preoperative motor training combined with tDCS may alter sensorimotor network connectivity. Patient specific modeling of EF in the presence of tumor may contribute to understanding the dose-response relationship of this intervention. Overall, this suggests the possibility of modulating neural networks in glioma patients.

Metaphyseal anchoring short stem hip arthroplasty provides a more physiological load transfer: a comparative finite element analysis study

BackgroundShort stem total hip arthroplasty (SHA) preserves femoral bone stock and is supposed to provide a more natural load transfer compared to standard stem total hip arthroplasty (THA). As comparative biomechanical reference data are rare we used a finite element analysis (FEA) approach to compare cortical load transfer after implantations of a metaphyseal anchoring short and standard stem in native biomechanical femora.MethodsThe subject specific finite element models of biomechanical femora, one native and two with implanted metaphyseal anchoring SHA (Metha, B. Braun Aesculap) and standard THA (CLS, Zimmer-Biomet), were generated from computed tomography datasets. The loading configuration was performed with an axial force of 1400 N. Von Mises stress was used to investigate the change of cortical stress distribution.ResultsCompared to the native femur, a considerable reduction of cortical stress was recorded after implantation of SHA and standard THA. The SHA showed less reduction proximally with a significant higher metaphyseal cortical stress compared to standard THA. Moreover, the highest peak stresses were observed metaphyseal for the SHA stem while for the standard THA high stress pattern was observed more distally.ConclusionsBoth, short and standard THA, cause unloading of the proximal femur. However, the metaphyseal anchoring SHA features a clearly favorable pattern in terms of a lower reduction proximally and improved metaphyseal loading, while standard THA shows a higher proximal unloading and more distal load transfer. These load patterns implicate a reduced stress shielding proximally for metaphyseal anchoring SHA stems and might be able to translate in a better bone preservation.

Development of Subject Specific Finite Element Models of the Mouse Knee Joint for Preclinical Applications.

Osteoarthritis is the most common musculoskeletal disabling disease worldwide. Preclinical studies on mice are commonly performed to test new interventions. Finite element (FE) models can be used to study joint mechanics, but usually simplified geometries are used. The aim of this project was to create a realistic subject specific FE model of the mouse knee joint for the assessment of joint mechanical properties. Four different FE models of a C57Bl/6 female mouse knee joint were created based on micro-computed tomography images of specimens stained with phosphotungstic acid in order to include different features: individual cartilage layers with meniscus, individual cartilage layers without meniscus, homogeneous cartilage layers with two different thickness values, and homogeneous cartilage with same thickness for both condyles. They were all analyzed under compressive displacement and the cartilage contact pressure was compared at 0.3 N reaction force. Peak contact pressure in the femur cartilage was 25% lower in the model with subject specific cartilage compared to the simpler model with homogeneous cartilage. A much more homogeneous pressure distribution across the joint was observed in the model with meniscus, with cartilage peak pressure 5–34% lower in the two condyles compared to that with individual cartilage layers. In conclusion, modeling the meniscus and individual cartilage was found to affect the pressure distribution in the mouse knee joint under compressive load and should be included in realistic models for assessing the effect of interventions preclinically.

Combined Hind Limb Suspension and sRANK-L on Bone Strength in Mice by Finite Element Analysis: Effects of Anatomic Model Height.

With commercial space travel on the horizon, it is important to understand how the microgravity environment of space effects bone strength. The reduction in skeletal loading is known to cause a rapid loss in bone density. How this corresponds to losses of bone strength is not well known, especially when combined with the osteoporotic effects of aging. In this study, a mouse model of hind limb suspension (HLS) was used to simulate the effects of gravitational unloading. This was combined with soluble receptor activator of nuclear factor kappa beta ligand (sRANK-L), which simulates age related osteoporosis. The proximal region of the tibia in mouse legs was scanned in-vivo pre-treatment as well as at the conclusion of the study with high resolution micro computed tomography (µCT). Subject specific finite element (FE) models were constructed from these 3D images to assess bone strength by simulating mechanical loading on these bone microstructures. Parameters indicative of bone strength obtained from the FE models were bone volume, stiffness, structural efficiency, and the 10th and 90th percentile nodal Von-Mises Stresses. Additionally, a model sensitivity analysis was performed to assess how these parameters varied with changes in anatomic model height. In regards to FE stiffness, HLS resulted in a 31% decline, sRANK-L resulted in a 16.8% decline, and HLS combined with sRANK-L (HLS+sRANK-L) resulted in a 38.6% decline. One interesting finding is that HLS caused a reduction in both bone stiffness and bone structural efficiency, while sRANK-L did not cause changes in bone structural efficiency, suggesting the importance of skeletal loading for maintaining bone health. In addition, sRANK-L combined with HLS caused an additional decline in bone stiffness, but did not further alter bone structural efficiency. In conclusion, this study shows that depending on the cause of osteoporosis, bone strength changes are not necessarily proportional to bone density changes. Thus, it is important to develop new clinical bone assessments beyond the current bone density measurement.Clinical Relevance- These parameters are associated with the microstructural mechanics of bone, and understanding how strength is decreased on a structural level may lead to the development of in-vivo bone strength testing clinically.

Previous Damage Accumulation Can Influence Femoral Fracture Strength: A Finite Element Study.

To manage osteoporotic hip fracture risk, it is necessary to understand failure mechanisms of bone at both the material and organ level. The structural response of bone is dependent on load history. Repeated loading causes progressive microstructural cracking, resulting in reduced apparent-level stiffness and, if damage is significant, reductions to peak load bearing capability. However, the effect of previous damage accumulation has not been well explored at the organ level. It was hypothesized that femoral fracture load and fracture pattern may be sensitive to damage accumulation from previous loading events. Six cadaveric specimens were used to develop patient specific finite element (FE) models from quantitative tomographic (qCT) scans. Material properties were assigned from qCT intensity at each element location, and damage evolution was predicted using a previously validated quasi-brittle FE model. Three scenarios were investigated: stumble followed by another stumble (S-S), fall followed by another fall (F-F), and stumble followed by a fall (S-F). Fracture load and pattern were compared to FE predictions for a single stumble (S) or single fall (F) loading event. Most specimens were resilient to accumulated damage, showing little (<5%) change in fracture load from the multiple-load scenarios (S-S, F-F, and S-F) compared to an equivalent single load scenario (S or F). Only one specimen demonstrated moderate (5-15%) reductions in strength from all three multiple-load scenarios. However, two specimens experienced moderate (20-30%) increase in fracture load in some load cases. In these cases, initial damage caused the load to be more evenly distributed upon subsequent loading events. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2197-2203, 2019.