Predictive structural models resulting in a trabecular bone topology closely resembling real bone would be a step toward 3D printing of sympathetic prosthetics. This study modifies an established trabecular bone structural adaptation approach, with the objective of achieving an improved adapted topology, specifically connectivity, compared to CT imaging studies; whilst retaining continuum level mechanical properties consistent with those reported in experimental studies. Strain driven structural adaptation models successfully identify trabecular trajectories, although tend to overpredict connectivity and skew trabecular radii distribution towards the smallest radius included in the adaptation. Radius adaptation of each trabecula is driven by a mechanostat approach with a target strain (1250 µɛ) below which radius is decreased (resorption), and above which radius is increased (apposition). Simulations include a lazy zone, in which neither resorption nor apposition takes place (1000 to 1500 µɛ); and a dead zone (<250 µɛ) in which complete resorption of trabeculae with the smallest included radius takes place. This study assesses the impact of increasing the dead zone threshold from <250 µɛ to <1000 µɛ, the lower limit of the lazy zone. In-silico structural models with an initial connectivity (number of trabeculae connecting at each joint) of 14 were generated using a nearest neighbour approach applied to a random cloud of points. Trabeculae were modelled using circular beams whose radii were adapted in response to normal strains caused by the axial force and bending moments due to a vertical pressure of 1 MPa applied to the top of the lattice, with the bottom of the lattice fixed in the vertical direction. Lattices in which nodes are either able (rigid jointed) or unable (pin jointed) to transmit bending moments were considered. Five virtual samples of each lattice type were used, and each simulation repeated twice: with a dead zone of either <250 µɛ or <1000 µɛ.Abstract
1.0 Objectives
2.0 Methods
Several experimental studies derived relationships between density and macroscale material properties of trabecular bone, taking the form E=αρβ, where E is Young's modulus, ρ is density, and α and β are constants. Classical structural mechanics demonstrates β can vary between 1 (behaviour of the trabecular lattice is dominated by the axial stiffness of individual trabeculae) and 3 (behaviour is dominated by the bending stiffness of individual trabeculae). The ratio between rods (round trabeculae characterised by radius) and plates (flat trabeculae characterised by thickness) is also believed to govern the macroscale material properties of trabecular bone. To assess feasible ranges of α and β for trabecular bone, and their dependence on rod to plate ratio, 25 virtual samples of trabecular bone were generated as Voronoi lattices. Each 8×8×8mm sample was composed of 320 randomly generated Voronoi cells forming a foam like structure. Edges formed the rod network. Faces formed the plate network. Tissue level Young's modulus was set to 18,000MPa. Relative density was varied: 0.05, 0.1, 0.15, 0.2, 0.25. Rod to plate ratio was varied: 100:0, 75:25, 50:50, 25:75, 0:100. Macroscale Young's modulus was averaged in three orthotropic directions and used to find α and β. Around 14,000 3-noded quadratic beam elements represented rods, with average length of 0.63mm, and around 42,000 8-noded quadratic shell elements represented plates, with average area of 0.10mm2. Results for α and β were 3274 and 1.463 for 100% rods, 3646 and 1.067 for 50:50 rods and plates, and 4981 and 1.062 for 100% plates, showing the presence of plates improves the stiffness characteristics of trabecular bone. Work investigating the impact of element based geometry optimisation is ongoing. The work has important implications for the onset of conditions including osteoporosis and osteoarthritis, as well as those designing 3D printed scaffolds and implants.Abstract
Ceramic on Ceramic bearings in Total Hip Arthroplasty (THA) afford a low friction coefficient, low wear rates and extreme hardness. Significant complications include hip squeak, ceramic fracture and poor polyethylene performance in revision procedures due to imbedding of abrasive microscopic ceramic fragments. We report on the results of this bearing at a minimum of 10 years. A single-centre retrospective review of 449 THAs was performed. Primary outcome measures included aseptic revision and all-cause revision rates at a minimum of 10 years post operatively. Evaluation of functionality was performed with WOMAC and SF-36 scores which were performed pre-operatively and at intervals of 6 months, one year, 2 years, 5 years and 10 years post operatively.Introduction and Objective
Materials and Methods
Bone shape and internal architecture are accepted as optimised to resist joint contact and muscle forces the skeleton is subjected to through daily living and more demanding activities. Finite element studies to predict bone architecture, either using continuum or structural approaches have made assumptions common in structural optimisation, that lead to trabecular bone effectively being modelled as a truss-type structure, with compressive or tensile strains, present due to axial forces driving adaptation. These models are successful in predicting bone fracture, and trends in bone degradation associated with disuse or unloading osteopenia but tend to overpredict bone mineral density reduction compared to clinical observations. A new structural model of bone adaptation, including both trabeculae (element) cross-section adaptation in response to axial force and biaxial bending moments, and alteration of joint (node) positions within the trabecular network, was developed using a Voronoi space partition to define the initial network. This was compared to results from a structural bone adaptation using a truss-type network generated by connecting each node to its nearest 16 neighbours [1].Abstract
Objectives
Methods
Osteoporosis of the pelvis and femur is diagnosed in a high proportion of lower-limb amputees which carries an increased fracture risk and subsequently serious implications on mobility, physical dependency and morbidity. Through the development of biofidelic musculoskeletal and finite element (FE) models, we aim to determine the effect of lower-limb amputation on long-term bone remodelling in the hip and to understand the potential underpinning mechanisms for bone degradation in the younger amputee population. Our models are patient specific and anatomically accurate. Geometries are derived from MRI-scans of one bilateral, above-knee, amputee and one body-matched control subject. Musculoskeletal modelling enables comparison of muscle and joint reaction-forces throughout gait. This provides the loading scenario implemented in FE. FE modelling demonstrates the effect of loading on the amputated limb via a prosthetic socket by comparing bone mechanical stimulation in amputee and control cases.Abstract
Objectives
Methods
Bone health deterioration is a major public health issue. General guidelines for the limitation of bone loss prescribe a healthy lifestyle and a minimum level of physical activity. However, there is no specific recommendation regarding targeted activities that can effectively maintain lumbar spine bone health. To provide a better understanding of such influencing activities, a new predictive modelling framework was developed to study bone remodelling under various loading conditions. The approach is based on a full-body subject-specific musculoskeletal model [1] combined with structural finite element models of the lumbar vertebrae. Using activities recorded with the subject, musculoskeletal simulations provide physiological loading conditions to the finite element models which simulate bone remodelling using a strain-driven optimisation algorithm [2]. With a combination of daily living activities representative of a healthy lifestyle including locomotion activities (walking, stair ascent and descent, sitting down and standing up) and spine-focused activities involving twisting and reaching, this modelling framework generates a healthy bone architecture in the lumbar vertebrae. The influence of spine-focused tasks was studied by adapting healthy vertebrae to an altered loading scenario where only locomotion activities were performed.Abstract
OBJECTIVES
METHODS
Finite element (FE) modelling has been widely used to create and assess musculoskeletal models. However to achieve a high degree of resolution in describing the structure, significant computational power and time are required. The objective of this study was to introduce a complimentary approach to FE modelling using structural beam theory. This requires far less computational power and models can be analyzed in a fraction of a second, offering quick, intuitive results for engineers and surgeons. Beam theory was first introduced as a method for analyzing the stresses in long bones in 1917. It was used as the de facto method for several decades. The introduction of FE modelling offered great advances; beam theory calculations were considered laborious and less accurate. However with the advances in computational power so too comes the ability to create modern automated beam theory models. A study was conducted using the commercially available general structural analysis software Oasys GSA. A synthetic biomechanical femur was CT scanned and the solid model constructed. This model was sectioned into approximately seventy sections in the regions of the shaft and condyles, thirty in the neck and thirty in the head. Line plots of the shape of each of the sections, for both cortical and trabecular parts, were then imported into Oasys GSA. The centroid, area, second moments of area and torsion constant were calculated for each section. The sections were plotted at the position of the cortical centroid and parallel axis theorem was used to plot the trabecular section in the same position. A force representing the hip joint reaction force was applied to a node corresponding to the centre of the femoral head. Muscular forces were applied to stiff radial elements according to those active at the point of peak joint contact force during gait. Oasys GSA produced instant results showing moment and deflection characteristics of the femur. This data was then used to predict strain plots, which were directly compared to FE results. Initial results compare favourably. This study has demonstrated an updated fast, efficient and intuitive alternative to finite element modelling.
Recently finite element studies have incorporated bone remodelling algorithms in an attempt to simulate bone's mechano-adaptation to loading conditions. In order to simplify these analyses, bone is usually considered to be isotropic, which does not explain the directionality of its internal structures; neither the orthotropic properties measured at the continuum level. Furthermore, simplified loading is usually applied to the bone models, which can result in an unrealistic remodelling stimulus. However, free boundary condition modelling of the femoral and pelvic constructs has been shown to produce more physiological stress and strain distributions. This paper describes the application of a 3D remodelling algorithm (with bone modelled as a strain-adaptive continuum with local orthotropic material properties) to a free boundary model of the femoral construct, where the hip and knee joints, as well as muscles and ligaments crossing the joints were included explicitly. Two load cases were analysed: single leg stance and standing up. Material properties and directionality distributions were produced for the whole femur, showing good agreement with observed structures from clinical studies. This indicates that the loading conditions modelled correspond to those experienced in vivo. In addition, the impact of the different load cases in bone structure modelling could be compared. Observations of the material properties distribution and orientation for standing up indicate that it promotes changes in bone stiffness in the anterior regions of the femoral neck and cortical shaft and the posterior side of the condyles. Development of this approach to modelling and bone structure prediction can lead to a better understanding of bone's mechanical behaviour and to the development and public release of orthotropic heterogeneous models for different constructs. These can be applied in many areas of interest in orthopaedic biomechanics, such as the study of bone-implant interfaces, improvement of the currently used surgical tools and techniques and the influence of certain activities in affecting local bone strength and mineralisation.
Finite element models of the musculoskeletal system have the possibility of describing the in vivo situation to a greater extent than a single in vitro experimental study ever could. However these models and the assumptions made must be validated before they can be considered truly useful. The object of this study was to validate, using digital image correlation (DIC) and strain gauging, a novel free boundary condition finite element model of the femur. The femur was treated as a complete musculoskeletal construct without specific fixed restraint acting on the bone. Spring elements with defined force-displacement relationships were used to characterize all muscles and ligaments crossing the hip and knee joints. This model was subjected to a loading condition representing single leg stance. From the developed model muscle, ligament and joint reaction forces were extracted as well as displacement and strain plots. The muscles with the most influence were selected to be represented in the simplified experimental setup. To validate the finite element model a balanced in vitro experimental set up was designed. The femur was loaded proximally through a construct representative of the pelvis and balanced distally on a construct representing the tibio-femoral joint. Muscles were represented using a cabling system with glued attachments. Strains were recorded using DIC and strain gauging. DIC is an image analysis technique that enables non-contact measurement of strains across surfaces. The resulting strain distributions were compared to the finite element model. The finite element model produced hip and knee joint reaction forces comparable to in vivo data from instrumented implants. The experimental models produced strain data from both DIC and strain gauging; these were in good agreement with the finite element models. The DIC process was also shown to be a viable method for measuring strain on the surface of the specimen. In conclusion a novel approach to finite element modeling of the femur was validated, allowing greater confidence for the model to be further developed and used in clinical settings.
Metal on metal press-fit acetabular cups are the worst performing acetabular cup type with severe failure consequences compared to cups made from more inert materials such as polyethylene or ceramic. The cause of failure of these cup types is widely acknowledged to be multi-factorial, therefore creating a complex scenario for analysis through clinical studies. A factorial analysis has been carried out using an experimentally validated finite element analysis to investigate the relative influence of four input factors associated with acetabular cup implantation on output parameters indicating potential failure of the implantation. These input factors were: cup material stiffness; cup inclination; cup version; cup seating; and level of press-fit. The output parameter failure indicators were: wear; tensile strains in the underlying bone; bone remodelling; and cup-bone micromotions. The factorial analysis concluded that the most significant influence was that of cup inclination on wear, and the second most significant was the influence of the level of press-fit on bone remodelling at the acetabular rim. Significant influence was also observed between version angle and wear, and cup-seating and micro-motion. The results demonstrated the clear multi-factorial nature of implant failure and highlighted the importance of correct implant positioning and fit.
