Consideration of multiple load cases is critical in modelling orthotropic bone adaptation in the femur

Mechanobiochemical bone remodelling around an uncemented acetabular component: influence of bone orthotropy

Mechanical loosening of an implant is often caused by bone resorption, owing to stress/strain shielding. Adaptive bone remodelling elucidates the response of bone tissue to alterations in mechanical and biochemical environments. This study aims to propose a novel framework of bone remodelling based on the combined effects of bone orthotropy and mechanobiochemical stimulus. The proposed remodelling framework was employed in the finite element model of an implanted hemipelvis to predict evolutionary changes in bone density and associated orthotropic bone material properties. In order to account for variations in load transfer during common daily activities, several musculoskeletal loading conditions of hip joint corresponding to sitting down/up, stairs ascend/descend and normal walking were considered. The bone remodelling predictions were compared with those of isotropic strain energy density (SED)-based, isotropic mechanobiochemical and orthotropic strain-based bone remodelling formulations. Although similar trends of bone resorption were predicted by orthotropic mechanobiochemical (MBC) and orthotropic strain-based models across implanted acetabulum, more volume (10-20%) of bone elements was subjected to bone resorption for the orthotropic MBC model. Higher bone resorption (75-85%) was predicted by the orthotropic strain-based and orthotropic MBC models compared to the isotropic MBC and SED-based models. Higher bone apposition (35-160%) across the implanted acetabulum was predicted by the isotropic MBC model, compared to the SED-based model. The remodelling predictions indicated that a reduction in estrogen level might lead to an increase in bone resorption. The study highlighted the importance of including mechanobiochemical stimulus and bone anisotropy to predict bone remodelling adequately.

Multiscale topology optimization of pelvic bone for combined walking and running gait cycles

We propose a multiscale topology optimization procedure of pelvic bone using weighted compliance minimization. In macroscale optimization, a level set-based method is used, which gives a binary structure. In microscale optimization, cubic lattice-based homogenization is done while keeping the global geometry fixed. For the macroscale, a volume constraint equal to the volume of the pelvic bone is imposed, whereas, for the microscale, a mass constraint equal to the mass of the pelvic bone is imposed. The optimal geometries are compared with pelvic bone using different metrics and show good similarity with the same. Designed geometries are additively manufactured and experimentally tested for stiffness.

Depth camera-based model for studying the effects of muscle loading on distal radius fracture healing

BACKGROUND

Distal radius fractures (DRFs) treated with volar locking plates (VLPs) allows early rehabilitation exercises favourable to fracture recovery. However, the role of rehabilitation exercises induced muscle forces on the biomechanical microenvironment at the fracture site remains to be fully explored. The purpose of this study is to investigate the effects of muscle forces on DRF healing by developing a depth camera-based fracture healing model.

METHOD

First, the rehabilitation-related hand motions were captured by a depth camera system. A macro-musculoskeletal model is then developed to analyse the data captured by the system for estimating hand muscle and joint reaction forces which are used as inputs for our previously developed DRF model to predict the tissue differentiation patterns at the fracture site. Finally, the effect of different wrist motions (e.g., from 60° of extension to 60° of flexion) on the DRF healing outcomes will be studied.

RESULTS

Muscle and joint reaction forces in hands which are highly dependent on hand motions could significantly affect DRF healing through imposed compressive and bending forces at the fracture site. There is an optimal range of wrist motion (i.e., between 40° of extension and 40° of flexion) which could promote mechanical stimuli governed healing while mitigating the risk of bony non-union due to excessive movement at the fracture site.

CONCLUSION

The developed depth camera-based fracture healing model can accurately predict the influence of muscle loading induced by rehabilitation exercises in distal radius fracture healing outcomes. The outcomes from this study could potentially assist osteopathic surgeons in designing effective post-operative rehabilitation strategies for DRF patients.

Exploration of the synergistic role of cortical thickness asymmetry ("Trabecular Eccentricity" concept) in reducing fracture risk in the human femoral neck and a control bone (Artiodactyl Calcaneus)

The mechanobiology of the human femoral neck is a focus of research for many reasons including studies that aim to curb age-related bone loss that contributes to a near-exponential rate of hip fractures. Many believe that the femoral neck is often loaded in rather simple bending, which causes net tension stress in the upper (superior) femoral neck and net compression stress in its inferior aspect ("T/C paradigm"). This T/C loading regime lacks in vivo proof. The "C/C paradigm" is a plausible alternative simplified load history that is characterized by a gradient of net compression across the entire femoral neck; action of the gluteus medius and external rotators of the hip are important in this context. It is unclear which paradigm is at play in natural loading due to lack of in vivo bone strain data and deficiencies in understanding mechanisms and manifestations of bone adaptation in tension vs. compression. For these reasons, studies of the femoral neck would benefit from being compared to a 'control bone' that has been proven, by strain data, to be habitually loaded in bending. The artiodactyl (sheep and deer) calcaneus model has been shown to be a very suitable control in this context. However, the application of this control in understanding the load history of the femoral neck has only been attempted in two prior studies, which did not examine the interplay between cortical and trabecular bone, or potential load-sharing influences of tendons and ligaments. Our first goal is to compare fracture risk factors of the femoral neck in both paradigms. Our second goal is to compare and contrast the deer calcaneus to the human femoral neck in terms of fracture risk factors in the T/C paradigm (the C/C paradigm is not applicable in the artiodactyl calcaneus due to its highly constrained loading). Our third goal explores interplay between dorsal/compression and plantar/tension regions of the deer calcaneus and the load-sharing roles of a nearby ligament and tendon, with insights for translation to the femoral neck. These goals were achieved by employing the analytical model of Fox and Keaveny (J. Theoretical Biology 2001, 2003) that estimates fracture risk factors of the femoral neck. This model focuses on biomechanical advantages of the asymmetric distribution of cortical bone in the direction of habitual loading. The cortical thickness asymmetry of the femoral neck (thin superior cortex, thick inferior cortex) reflects the superior-inferior placement of trabecular bone (i.e., "trabecular eccentricity," TE). TE helps the femoral neck adapt to typical stresses and strains through load-sharing between superior and inferior cortices. Our goals were evaluated in the context of TE. Results showed the C/C paradigm has lower risk factors for the superior cortex and for the overall femoral neck, which is clinically relevant. TE analyses of the deer calcaneus revealed important synergism in load-sharing between the plantar/tension cortex and adjacent ligament/tendon, which challenges conventional understanding of how this control bone achieves functional adaptation. Comparisons with the control bone also exposed important deficiencies in current understanding of human femoral neck loading and its potential histocompositional adaptations.

The impact of the parameters of the constitutive model on the distribution of strain in the femoral head

The rapid spread of the finite element method has caused that it has become, among other methods, the standard tool for pre-clinical estimates of bone properties. This paper presents an application of this method for the calculation and prediction of strain and stress fields in the femoral head. The aim of the work is to study the influence of the considered anisotropy and heterogeneity of the modeled bone on the mechanical fields during a typical gait cycle. Three material models were tested with different properties of porous bone carried out in literature: a homogeneous isotropic model, a heterogeneous isotropic model, and a heterogeneous anisotropic model. In three cases studied, the elastic properties of the bone were determined basing on the Zysset-Curnier approach. The tensor of elastic constants defining the local properties of porous bone is correlated with a local porosity and a second order fabric tensor describing the bone microstructure. In the calculations, a model of the femoral head generated from high-resolution tomographic scans was used. Experimental data were drawn from publicly available database "Osteoporotic Virtual Physiological Human Project." To realistically reflect the load on the femoral head, main muscles were considered, and their contraction forces were determined based on inverse kinematics. For this purpose, the results from OpenSim packet were used. The simulations demonstrated that differences between the results predicted by these material models are significant. Only the anisotropic model allowed for the plausible distribution of stresses along the main trabecular groups. The outcomes also showed that the precise evaluation of the mechanical fields is critical in the context of bone tissue remodeling under mechanical stimulations.

Topology Optimisation for Compliant Hip Implant Design and Reduced Strain Shielding

Stiff total hip arthroplasty implants can lead to strain shielding, bone loss and complex revision surgery. The aim of this study was to develop topology optimisation techniques for more compliant hip implant design. The Solid Isotropic Material with Penalisation (SIMP) method was adapted, and two hip stems were designed and additive manufactured: (1) a stem based on a stochastic porous structure, and (2) a selectively hollowed approach. Finite element analyses and experimental measurements were conducted to measure stem stiffness and predict the reduction in stress shielding. The selectively hollowed implant increased peri-implanted femur surface strains by up to 25 percentage points compared to a solid implant without compromising predicted strength. Despite the stark differences in design, the experimentally measured stiffness results were near identical for the two optimised stems, with 39% and 40% reductions in the equivalent stiffness for the porous and selectively hollowed implants, respectively, compared to the solid implant. The selectively hollowed implant's internal structure had a striking resemblance to the trabecular bone structures found in the femur, hinting at intrinsic congruency between nature's design process and topology optimisation. The developed topology optimisation process enables compliant hip implant design for more natural load transfer, reduced strain shielding and improved implant survivorship.

Bone remodelling in implanted proximal femur using topology optimization and parameterized cellular model

The clinical relevance of bone remodelling predictions calls for accurate finite element (FE) modelling of implant-bone structure and musculoskeletal loading conditions. However, simplifications in muscle loading, material properties, has often been used in FE simulations. Bone adaptation induces changes in bone apparent density and its microstructure. Multiscale simulations, involving optimization methods and biomimetic microstructural models, have proven to be promising for predicting changes in bone morphology. The objective of the study is to develop a novel computational framework to predict bone remodelling around an uncemented femoral implant, using multiscale topology optimization and a parameterized cellular model. The efficacy of the scheme was evaluated by comparing the remodelling predictions with those of isotropic strain energy density (SED) and orthotropy based formulations. The characteristic functional groups and low-density regions of Ward's triangle, predicted by the optimization scheme, were comparable to micro-CT images of the proximal femur. Although the optimization scheme predicted well comparable material distribution in the 2D femur models, the obscured material orientations in some planes of the 3D model indicate the need for a more robust modelling of the boundary conditions. Regression analysis revealed a higher correlation (0.6472) between the topology optimization and SED models than the orthotropic predictions (0.4219). Despite higher bone apposition of 10-20% around the distal tip of the implant, the bone density distributions were well comparable to clinical observations towards the proximal femur. The proposed computational scheme appears to be a viable method for including bone anisotropy in the remodelling formulation.

Architecture of the cancellous bone in human proximal tibia based on P45 sectional plastinated specimens

PURPOSE

To reveal differences in the pattern of trabecular architecture in the epiphysis and metaphysis of the proximal tibia.

METHODS

The trabecular architecture of the proximal tibia was observed in 27 P45 plastinated knee specimens.

RESULTS

In the medial and lateral condyles, under the articular cartilage surrounded by the medial or lateral meniscus, the cancellous bone is formed by thick and dense trabecular bands, which run longitudinally in the epiphysis and then pass through the epiphyseal line to terminate on the slanted cortex of the metaphysis. In the intercondylar eminence, the trabeculae are arranged basically in a network. In the central portion of the tibial metaphysis, cancellous bone consists of fine arcuate trabeculae, which extend to the anterior and posterior cortices, respectively. These trabeculae are intersected sparsely and form trusses over the medullary cavity. Near the areas of attachment of the iliotibial tract, tibial collateral ligament, anterior and posterior cruciate ligaments, and patellar ligament, the cancellous bone is locally reinforced with patchy trabeculae, dense radiating trabeculae, or two orthotropic trabecular bands.

CONCLUSION

This study provides further accurate anatomical information on the trabeculae of the proximal tibia. The soft structures of knee joint, including the articular cartilage, menisci, and ligaments, and the slanted cortices of the metaphysis are important landmarks for the location of different arrangements of the cancellous architecture. The present results are beneficial for clinical diagnosis and treatment of pathologies of the knee joint, or the establishment of a finite element analysis model of the knee joint.

Finite-element analysis of the influence of tibial implant fixation design of total ankle replacement on bone-implant interfacial biomechanical performance

PURPOSE

Implant loosening in tibia after primary total ankle replacement (TAR) is one of the common postoperative problems in TAR. Innovations in implant structure design may ideally reduce micromotion at the bone-implant interface and enhance the bone-implant fixation and initial stability, thus eventually prevents long-term implant loosening. This study aimed to investigate (1) biomechanical characteristics at the bone-implant interface and (2) the influence of design features, such as radius, height, and length.

METHODS

A total of 101 finite-element models were created based on four commercially available implants. The models predicted micromotion at the bone-implant interface, and we investigated the impact of structural parameters, such as radius, length, and height.

RESULTS

Our results suggested that stem-type implants generally required the highest volume of bone resection before implantation, while peg-type implants required the lowest. Compared with central fixation features (stem and keel), peripherally distributed geometries (bar and peg) were associated with lower initial micromotions. The initial stability of all types of implant design can be optimized by decreasing fixation size, such as reducing the radius of the bars and pegs and lowering the height.

CONCLUSION

Peg-type tibial implant design may be a promising fixation method, which is required with a minimum bone resection volume and yielded minimum micromotion under an extreme axial loading scenario. Present models can serve as a useful platform to build upon to help physicians or engineers when making incremental improvements related to implant design.

Orthotropic bone remodelling around uncemented femoral implant: a comparison with isotropic formulation

Peri-prosthetic bone adaptation has usually been predicted using subject-specific finite element analysis in combination with remodelling algorithms and assuming isotropic bone material property. The objective of the study is to develop an orthotropic bone remodelling algorithm for evaluation of peri-prosthetic bone adaptation in the uncemented implanted femur. The simulations considered loading conditions from a variety of daily activities. The orthotropic algorithm was tested on 2D and 3D models of the intact femur for verification of predicted results. The predicted orthotropic directionality, based on principal stress directions, was in agreement with the trabecular orientation in a micro-CT data of proximal femur. The validity of the proposed strain-based algorithm was assessed by comparing the predicted results of the orthotropic model with those of the strain-energy-density-based isotropic formulation. Despite agreement in cortical densities [Formula: see text], the isotropic remodelling algorithm tends to predict relatively higher values around the distal tip of the implant as compared to the orthotropic model. Both formulations predicted 4-8% bone resorption in the proximal femur. A linear regression analysis revealed a significant correlation [Formula: see text] between the stresses and strains on the cortex of the proximal femur, predicted by the isotropic and orthotropic formulations. Despite reasonable agreement in peri-prosthetic bone density distributions, the quantitative differences with isotropic model predictions highlight the combined influences of bone orthotropy and mechanical stimulus in the adaptation process.

The influence of loading configurations on numerical evaluation of failure mechanisms in an uncemented femoral prosthesis

The clinical relevance of numerical predictions of failure mechanisms in femoral prosthesis could be impaired due to simplification of musculoskeletal loading. This study investigated the extent to which loading configurations affect the preclinical analysis of an uncemented femoral implant. Patient-specific, CT-scan based FE models of intact and implanted femurs were developed and analysed using three loading configurations, which comprised of load cases representing daily activities. First loading configuration consisted of two load cases, each of walking and stair climbing. The second consisted of more number of load cases for each of these activities. The third included load cases of additional activities of standing up and sitting down. Failure criteria included maximum principal strains, interface debonding, implant-bone relative displacement and adaptive bone remodelling. Simplified loading configurations led to a reduction (100-1500 με) around cortical principal strains. The area prone to interface debonding were observed in the proximo-medial part of implant and was maximum when all activities were considered. This area was reduced by 35%, when simplified loading configurations were chosen. Interfacial area of 88%-96% experienced implant-bone relative displacements below 40 μm; however maximum of 110 μm was observed at the calcar region. Lack of consideration of variety of activities overestimated (30%-50%) bone resorption around the lateral part of the implant; hence, these bone remodelling results were less clinically relevant. Considering a variety daily activities along with an adequate number of load cases for each activity seemed necessary for pre-clinical evaluations of reconstructed femur.

BONE ORTHOTROPIC REMODELING AS A THERMODYNAMICALLY-DRIVEN EVOLUTION

In this paper, we present and discuss a model of bone remodeling set up in the framework of the theory of generalized continuum mechanics which was first introduced by DiCarlo et al. [Sur le remodelage des tissus osseux anisotropes, Comptes Rendus Mécanique 334(11):651–661, 2006]. Bone is described as an orthotropic body experiencing remodeling as a rotation of its microstructure. Thus, the complete kinematic description of a material point is provided by its position in space and a rotation tensor describing the orientation of its microstructure. Material motion is driven by energetic considerations, namely by the application of the Clausius–Duhem inequality to the microstructured material. Within this framework of orthotropic remodeling, some key features of the remodeling equilibrium configurations are deduced in the case of homogeneous strain or stress loading conditions. First, it is shown that remodeling equilibrium configurations correspond to energy extrema. Second, stability of the remodeling equilibrium configurations is assessed in terms of the local convexity of the strain and complementary energy functionals hence recovering some classical energy theorems. Eventually, it is shown that the remodeling equilibrium configurations are not only highly dependent on the loading conditions, but also on the material properties.

Adaptation of Proximal Femur to Mechanical Loading in Young Adults: Standard Vs Localized Regions Evaluated by DXA

Regions of the proximal femur with less adaptive protection by mechanical loading may be at increased risk of structural failure. Since the size and location of these regions diverge from those defined by the dual-energy X-ray absorptiometry manufacturers the purpose of this study was to compare areal bone mineral density (aBMD) of different regions of the proximal femur considering impact loads from physical activity (PA). The participants were 134 young adults divided into 2 groups according to the impact of PA performed in the last 12 mo: high-impact PA and low-impact PA. The aBMD of the proximal femur was assessed by dual-energy X-ray absorptiometry at the standard femoral neck, intertrochanter, and trochanter, and at specific locations of the superolateral femoral neck and intertrochanteric region. The bone-specific physical activity questionnaire was used to estimate the impact load of PA. Comparisons between groups were adjusted for body height and body lean mass. Interaction analysis between sex and PA groups were conducted with analysis of variance. Comparisons of aBMD between bone regions were analyzed separately for men and women with repeated measures analysis of variance. In the high-impact PA group, men benefit more than women at all bone regions, except the aBMD at intertrochanteric region. Analyses of repeated measures did not reveal any significant interaction effect between bone regions (standard vs specific) and PA groups (low vs high-impact). In conclusion, aBMD differences due to mechanical loading were more pronounced in men than in women; the magnitude of the aBMD differences as a result of different levels of PA was similar between standard and localized regions.

Dynamic finite element analysis of implants for femoral neck fractures simulating walking

BACKGROUND

To examine postoperative complications for osteosynthesizing femoral neck fractures (Pauwels III), biomechanical analysis should be conducted under dynamic conditions simulating for walking, not static conditions. Among the two main aims of this study, one is to pioneer the technique of dynamic finite element (FE) analysis, and the other is to compare stress distribution between two implants during walking.

MATERIALS AND METHODS

First, we performed an inverse dynamic analysis with optimization method using a musculoskeletal model to calculate the inter-segmental and muscular forces during walking. Second, three FE models were prepared: (I) intact hip joint, (II) fractures treated with two Hansson pins (HP), and (III) fractures with Dual SC Screws (DSCS) maintaining an angular stability. The direction and magnitude of the loadings varied continuously. Stress distribution during the walking was evaluated by using a dynamic explicit method. We examined the time-dependent von Mises stresses at two representative spots: medial cortex at the femoral neck fracture site and lateral pin (presumed) insertion holes.

RESULTS

In general, stress values are always changing during walking cycle. Regarding medial femoral neck cortex at the fracture line, intact model showed almost consistent value. Both HP model and DSCS model amounted the highest around 30 MPa. At lateral holes, highest values were 18.8, 104.0, and 63.1 MPa of intact, HP, and DSCS models, respectively.

CONCLUSION

Thus, our analysis simulating the real walking will be useful in evaluating time-varying stress distribution to assess postoperative complication.

CLINICAL RELEVANCE

DSCS is expected to be paramount for treatment of unstable femoral neck fractures.

Numerical predictions of hip joint and muscle forces during daily activities: A comparison of musculoskeletal models

Musculoskeletal loading plays an important role in pre-clinical evaluations of hip implants, in particular, bone ingrowth and bone remodelling. Joint force estimation using musculoskeletal models evolved as a viable alternative to in vivo measurement owing to the development of computational resources. This study investigated the efficiencies of four eminent open-source musculoskeletal models in order to determine the model that predicts the most accurate values of hip joint reaction and muscle forces during daily activities. Seven daily living activities of slow walking, normal walking, fast walking, sitting down, standing up, stair down and stair up were simulated in OpenSim using inverse dynamics method. Model predictions of joint kinematics, kinetics and muscle activation patterns were compared with published results. The estimated values of hip joint reaction force were found to corroborate well with in vivo measurements for each activity. Although the estimated values of hip joint reaction force were within a satisfactory range, overestimation of hip joint reaction force (75% BW of measured value) was observed during the late stance phase of walking cycles for all the models. In case of stair up, stair down, standing up and sitting down activities, the error in estimated values of hip joint reaction force were within ~20% BW of the measured value. Based on the results of our study, the London Lower Extremity Model predicted the most accurate value of hip joint reaction force and therefore can be used for applied musculoskeletal loading conditions for numerical investigations on hip implants.

A computationally efficient strategy to estimate muscle forces in a finite element musculoskeletal model of the lower limb

Concurrent multiscale simulation strategies are required in computational biomechanics to study the interdependence between body scales. However, detailed finite element models rarely include muscle recruitment due to the computational burden of both the finite element method and the optimization strategies widely used to estimate muscle forces. The aim of this study was twofold: first, to develop a computationally efficient muscle force prediction strategy based on proportional-integral-derivative (PID) controllers to track gait and chair rise experimental joint motion with a finite element musculoskeletal model of the lower limb, including a deformable knee representation with 12 degrees of freedom; and, second, to demonstrate that the inclusion of joint-level deformability affects muscle force estimation by using two different knee models and comparing muscle forces between the two solutions. The PID control strategy tracked experimental hip, knee, and ankle flexion/extension with root mean square errors below 1°, and estimated muscle, contact and ligament forces in good agreement with previous results and electromyography signals. Differences up to 11% and 20% in the vasti and biceps femoris forces, respectively, were observed between the two knee models, which might be attributed to a combination of differing joint contact geometry, ligament behavior, joint kinematics, and muscle moment arms. The tracking strategy developed in this study addressed the inevitable tradeoff between computational cost and model detail in musculoskeletal simulations and can be used with finite element musculoskeletal models to efficiently estimate the interdependence between muscle forces and tissue deformation.

Efficacy and efficiency of multivariate linear regression for rapid prediction of femoral strain fields during activity

Multivariate Linear Regression-based (MLR) surrogate models were explored to reduce the computational cost of predicting femoral strains during normal activity in comparison with finite element analysis. The musculoskeletal model of one individual, the finite-element model of the right femur, and experimental force and motion data for normal walking, fast walking, stair ascent, stair descent, and rising from a chair were obtained from a previous study. Equivalent Von Mises strain was calculated for 1000 frames uniformly distributed across activities. MLR surrogate models were generated using training sets of 50, 100, 200 and 300 samples. The finite-element and MLR analyses were compared using linear regression. The Root Mean Square Error (RMSE) and the 95th percentile of the strain error distribution were used as indicators of average and peak error. The MLR model trained using 200 samples (RMSE < 108 µε; peak error < 228 µε) was used as a reference. The finite-element method required 66 s per frame on a standard desktop computer. The MLR model required 0.1 s per frame plus 1848 s of training time. RMSE ranged from 1.2% to 1.3% while peak error ranged from 2.2% to 3.6% of the maximum micro-strain (5020 µε). Performance within an activity was lower during early and late stance, with RMSE of 4.1% and peak error of 8.6% of the maximum computed micro-strain. These results show that MLR surrogate models may be used to rapidly and accurately estimate strain fields in long bones during daily physical activity.

Cancellous bone and theropod dinosaur locomotion. Part II-a new approach to inferring posture and locomotor biomechanics in extinct tetrapod vertebrates

This paper is the second of a three-part series that investigates the architecture of cancellous bone in the main hindlimb bones of theropod dinosaurs, and uses cancellous bone architectural patterns to infer locomotor biomechanics in extinct non-avian species. Cancellous bone is widely known to be highly sensitive to its mechanical environment, and therefore has the potential to provide insight into locomotor biomechanics in extinct tetrapod vertebrates such as dinosaurs. Here in Part II, a new biomechanical modelling approach is outlined, one which mechanistically links cancellous bone architectural patterns with three-dimensional musculoskeletal and finite element modelling of the hindlimb. In particular, the architecture of cancellous bone is used to derive a single 'characteristic posture' for a given species-one in which bone continuum-level principal stresses best align with cancellous bone fabric-and thereby clarify hindlimb locomotor biomechanics. The quasi-static approach was validated for an extant theropod, the chicken, and is shown to provide a good estimate of limb posture at around mid-stance. It also provides reasonable predictions of bone loading mechanics, especially for the proximal hindlimb, and also provides a broadly accurate assessment of muscle recruitment insofar as limb stabilization is concerned. In addition to being useful for better understanding locomotor biomechanics in extant species, the approach hence provides a new avenue by which to analyse, test and refine palaeobiomechanical hypotheses, not just for extinct theropods, but potentially many other extinct tetrapod groups as well.

A subject-specific integrative biomechanical framework of the pelvis for gait analysis

Analysis of the human locomotor system using rigid-body musculoskeletal models has increased in the biomechanical community with the objective of studying muscle activations of different movements. Simultaneously, the finite element method has emerged as a complementary approach for analyzing the mechanical behavior of tissues. This study presents an integrative biomechanical framework for gait analysis by linking a musculoskeletal model and a subject-specific finite element model of the pelvis. To investigate its performance, a convergence study was performed and its sensitivity to the use of non-subject-specific material properties was studied. The total hip joint force estimated by the rigid musculoskeletal model and by the finite element model showed good agreement, suggesting that the integrative approach estimates adequately (in shape and magnitude) the hip total contact force. Previous studies found movements of up to 1.4 mm in the anterior-posterior direction, for single leg stance. These results are comparable with the displacement values found in this study: 0-0.5 mm in the sagittal axis. Maximum von Mises stress values of approximately 17 MPa were found in the pelvic bone. Comparing this results with a previous study of our group, the new findings show that the introduction of muscular boundary conditions and the flexion-extension movement of the hip reduce the regions of high stress and distributes more uniformly the stress across the pelvic bone. Thus, it is thought that muscle force has a relevant impact in reducing stresses in pelvic bone during walking of the finite element model proposed in this study. Future work will focus on including other deformable structures, such as the femur and the tibia, and subject-specific material properties.

Altered biomechanical stimulation of the developing hip joint in presence of hip dysplasia risk factors

Fetal kicking and movements generate biomechanical stimulation in the fetal skeleton, which is important for prenatal musculoskeletal development, particularly joint shape. Developmental dysplasia of the hip (DDH) is the most common joint shape abnormality at birth, with many risk factors for the condition being associated with restricted fetal movement. In this study, we investigate the biomechanics of fetal movements in such situations, namely fetal breech position, oligohydramnios and primiparity (firstborn pregnancy). We also investigate twin pregnancies, which are not at greater risk of DDH incidence, despite the more restricted intra-uterine environment. We track fetal movements for each of these situations using cine-MRI technology, quantify the kick and muscle forces, and characterise the resulting stress and strain in the hip joint, testing the hypothesis that altered biomechanical stimuli may explain the link between certain intra-uterine conditions and risk of DDH. Kick force, stress and strain were found to be significantly lower in cases of breech position and oligohydramnios. Similarly, firstborn fetuses were found to generate significantly lower kick forces than non-firstborns. Interestingly, no significant difference was observed in twins compared to singletons. This research represents the first evidence of a link between the biomechanics of fetal movements and the risk of DDH, potentially informing the development of future preventative measures and enhanced diagnosis. Our results emphasise the importance of ultrasound screening for breech position and oligohydramnios, particularly later in pregnancy, and suggest that earlier intervention to correct breech position through external cephalic version could reduce the risk of hip dysplasia.

Bone Strength and Exercise During Youth-The Year That Was 2017

The positive effects of physical activity on bone strength are certain. However, researchers have yet to precisely quantify the contribution of specific characteristics of physical activity that affect bone strength in children and adolescents. This commentary highlights 2 noteworthy 2017 publications that addressed osteogenic physical activity dose-response issues. Both papers moved the field forward by providing new insights on physical activity exposures beyond high-impact loading. Koedijk et al's paper was selected because, to the best of our knowledge, it is the first systematic review to solely examine associations between sedentary behavior and indicators of bone strength. The second selected paper, Gabel et al, used novel approaches in accelerometer processing and statistical modeling to separate the osteogenic effects of frequency of short bouts of physical activity from total volume of physical activity. As such, the authors of this paper begin to explore in youth what animal models have shown for some time, that is, optimal bone adaptation requires the correct combination of intensity, frequency, duration, nonrepetitive movement, and rest. Together, these papers signal new and important approaches for the conceptualization, measurement, and interpretation of osteogenic physical activity.

Bioinspired Technologies to Connect Musculoskeletal Mechanobiology to the Person for Training and Rehabilitation

Musculoskeletal tissues respond to optimal mechanical signals (e.g., strains) through anabolic adaptations, while mechanical signals above and below optimal levels cause tissue catabolism. If an individual's physical behavior could be altered to generate optimal mechanical signaling to musculoskeletal tissues, then targeted strengthening and/or repair would be possible. We propose new bioinspired technologies to provide real-time biofeedback of relevant mechanical signals to guide training and rehabilitation. In this review we provide a description of how wearable devices may be used in conjunction with computational rigid-body and continuum models of musculoskeletal tissues to produce real-time estimates of localized tissue stresses and strains. It is proposed that these bioinspired technologies will facilitate a new approach to physical training that promotes tissue strengthening and/or repair through optimal tissue loading.

Computational modelling of motion at the bone-implant interface after total knee arthroplasty: The role of implant design and surgical fit

BACKGROUND

Aseptic loosening, osteolysis, and infection are the most commonly reported reasons for revision total knee arthroplasty (TKA). This study examined the role of implant design features (e.g. condylar box, pegs) and stems in resisting loosening, and also explored the sensitivity of the implants to a loose surgical fit due to saw blade oscillation.

METHODS

Finite element models of the distal femur implanted with four different implant types: cruciate retaining (CR), posterior stabilising (PS), total stabilising (TS) with short stem (12mm×50mm), and a total stabilising (TS) with long stem (19mm×150mm) were developed and analysed in this study. Two different fit conditions were considered: a normal fit, where the resections on the bone exactly match the internal profile of the implant, and a loose fit due to saw blade oscillation, characterised by removal of one millimetre of bone from the anterior and posterior surfaces of the distal femur. Frictional interfaces were employed at the bone-implant interfaces to allow relative motions to be recorded.

RESULTS

The results showed that interface motions increased with increasing flexion angle and loose fit. Implant design features were found to greatly influence the surface area under increased motion, while only slightly influencing the values of peak motion. Short uncemented stems behaved similarly to PS implants, while long canal filling stems exhibited the least amount of motion at the interface under any fit condition.

CONCLUSION

In conclusion, long stemmed prostheses appeared less susceptible to surgical cut errors than short stemmed and stemless implants.

Microscale poroelastic metamodel for efficient mesoscale bone remodelling simulations

Bone functional tissue adaptation is a multiaspect physiological process driven by interrelated mechanical and biological stimuli which requires the combined activity of osteoclasts and osteoblasts. In previous work, the authors developed a phenomenological mesoscale structural modelling approach capable of predicting internal structure of the femur based on daily activity loading, which relied on the iterative update of the cross-sectional areas of truss and shell elements representative of trabecular and cortical bones, respectively. The objective of this study was to introduce trabecular reorientation in the phenomenological model at limited computational cost. To this aim, a metamodel derived from poroelastic microscale continuum simulations was used to predict the functional adaptation of a simplified proximal structural femur model. Clear smooth trabecular tracts are predicted to form in the regions corresponding to the main trabecular groups identified in literature, at minimal computational cost.

Parametric analysis of glenoid implant design and fixation type

Common post-operative problems in shoulder arthroplasty such as glenoid loosening and joint instability may be reduced by improvements in glenoid design, shape, material choice, and fixation method. A framework for parametric analysis of different implant fixation configurations was developed in order to efficiently sift through potential glenoid component designs. We investigated the influence of design factors such as fixation type, component thickness, and peg position, number, diameter, and length in a multi-factorial design investigation. The proposed method allowed for simultaneous comparison of the mechanical performance of 344 different parametric variations of 10 different reference geometries with either large central fixation features or small peripheral pegs, undergoing four different worst-case scenario loading conditions, and averaging 64.7 s per model. The impact of design parameters were assessed for different factors responsible for post-operative problems in shoulder arthroplasty, such as bone volume preservation, stresses in the implant, central displacement or fixation stability, and the worst performing geometries all relied on conventional central fixation. Of the remaining geometries, four peripheral fixation configurations produced von Mises stresses comfortably below the material's yield strength. We show that the developed method allows for simple, direct, rapid, and repeatable comparison of different design features, material choices, or fixation methods by analyzing how they influence the bone-implant mechanical environment. The proposed method can provide valuable insight in implant design optimization by screening through multiple potential design modifications at an early design evaluation stage and highlighting the best performing combinations according to the failure mechanism to mitigate. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:775-784, 2017.

Informing phenomenological structural bone remodelling with a mechanistic poroelastic model

Studies suggest that fluid motion in the extracellular space may be involved in the cellular mechanosensitivity at play in the bone tissue adaptation process. Previously, the authors developed a mesoscale predictive structural model of the femur using truss elements to represent trabecular bone, relying on a phenomenological strain-based bone adaptation algorithm. In order to introduce a response to bending and shear, the authors considered the use of beam elements, requiring a new formulation of the bone adaptation drivers. The primary goal of the study presented here was to isolate phenomenological drivers based on the results of a mechanistic approach to be used with a beam element representation of trabecular bone in mesoscale structural modelling. A single-beam model and a microscale poroelastic model of a single trabecula were developed. A mechanistic iterative adaptation algorithm was implemented based on fluid motion velocity through the bone matrix pores to predict the remodelled geometries of the poroelastic trabecula under 42 different loading scenarios. Regression analyses were used to correlate the changes in poroelastic trabecula thickness and orientation to the initial strain outputs of the beam model. Linear (R(2) > 0.998) and third-order polynomial (R(2) > 0.98) relationships were found between change in cross section and axial strain at the central axis, and between beam reorientation and ratio of bending strain to axial strain, respectively. Implementing these relationships into the phenomenological predictive algorithm for the mesoscale structural femur has the potential to produce a model combining biofidelic structure and mechanical behaviour with computational efficiency.