The effect of functionally graded materials on bone remodeling around osseointegrated trans-femoral prostheses

An FE model investigating the bone-implant interface of Osseointegrated prosthetics to better understand how forces are transferred under loading

BACKGROUND

Osseointegrated prostheses (OIP) use is increasing for above-knee amputees who have difficulties with sockets. This study aims to simulate the bone-implant interface under loading using a 3D finite element (FE) model and quantify force distribution to produce hypotheses on bone remodelling and implant failure, informing implant and surgical design, and rehabilitation protocols.

METHODS

Ten customised 3D femur FE models (5 female, 5 male) were generated from CT scans and bone-implant assemblies created. The bone was subdivided into seven Gruen Zones and four proximal femur regions. Boundary conditions were taken from the literature.

RESULTS

The highest stresses were found in the implant (Max: 113.9 MPa), whilst highest strains were seen in the bone (Max: 4.89 %). Stress and strain were unevenly distributed, with distal regions experiencing stress shielding effects and areas around the implant tip experiencing significantly higher stresses and strains (p < .001). Maximum stresses were higher in female bones (p < .01), whilst shorter residuum lengths saw significantly lower stresses (p < .05).

CONCLUSION

Sex, size and limb length are all important factors and these need to be accounted for when designing and implanting OIPs.

Biomechanical Evaluation of a Newly Developed Functional-Grade Composite Material for Pedicle Screws

OBJECTIVE

Pedicle screw and rod systems are widely employed in spine surgeries and loosening due to insufficient mechanical stimulation on the bone is frequently encountered in pedicle screws. This mechanical stimulation problem also arises due to the high rigidity of the implant material. This study aimed to develop new pedicle screws with composite material to solve the pedicle screw loosening problem.

METHODS

The vertebrae and vertebral disk were modeled in three dimension using computerized tomography images obtained from a patient. A commercially available pedicle screw was modeled using Fusion software, and all models were assembled in accordance with the surgical procedure. Pedicle screw models were also divided radially and longitudinally to resemble functionally graded materials, which are composite materials. The load was applied to the top of the T12 vertebra and the screw-vertebral system was fixed to the bottom of the L1 vertebra.

RESULTS

The strain results on the vertebrae were examined according to the mechanostat theorem. According to the results, functionally graded material (FGM) pedicle screw decreased the strain on the vertebral bones, and the positive effects on the bone were determined when using the radially functionally graded screws. The maximum stress values were also examined to determine the strengths of all the models.

CONCLUSION

In conclusion, FGM pedicle screw decreased the strain on the bone which is an important parameter for the loosening failure according to the study. The other important conclusion is that FGM pedicle screw can be the solution to the loosening of the screw but not in all vertebrae.

Optimizing functionally graded tibial components for total knee replacements: a finite element analysis and multi-objective optimization study

The optimal design of complex engineering systems requires tracing precise mathematical modeling of the system's behavior as a function of a set of design variables to achieve the desired design. Despite the success of current tibial components of knee implants, the limited lifespan remains the main concern of these complex systems. The mismatch between the properties of engineered biomaterials and those of biological materials leads to inadequate bonding with bone and the stress-shielding effect. Exploiting a functionally graded material for the stem of the tibial component of knee implants is attractive because the properties can be designed to vary in a certain pattern, meeting the desired requirements at different regions of the knee joint system. Therefore, in this study, a Ti6Al4V/Hydroxyapatite functionally graded stem with a laminated structure underwent simulation-based multi-objective design optimization for a tibial component of the knee implant. Employing finite element analysis and response surface methodology, three material design variables (stem's central diameter, gradient factor, and number of layers) were optimized for seven objective functions related to stress-shielding and micro-motion (including Maximum stress on the cancellous bone, maximum and mean stresses on predefined paths, the standard deviation of mean stress on paths, maximum and mean micro-motions at the bone-implant interface and the standard deviation of mean micro-motion). Then, the optimized functionally graded stem with 6 layers, a central diameter of 5.59 mm, and a gradient factor of 1.31, was compared with a Ti6Al4V stem for various responses. In stress analysis, the optimal stem demonstrated a 1.92% improvement in cancellous bone stress while it had no considerable influence on the maximum, mean, and standard deviation of stresses on paths. In micro-motion analysis, the maximum, mean, and standard deviation of mean micro-motion at the interface were enhanced by 24.31%, 39.53%, and 19.77%, respectively.

Bone remodelling prediction using mechanical stimulus with bone connectivity theory in porous implants

Strain energy density (SED) is considered to be the primary remodelling stimulus influencing the process of bone growth into porous implants. A bone remodelling algorithm incorporating the concept of bone connectivity, that newly formed bone should only grow from existing bone, was developed to provide a more biologically realistic simulation of bone growth. Results showed that the new algorithm prevented the occurrence of unconnected mature bone within porous implants, an unrealistic phenomenon observed using conventional adaptive elasticity theories. The bone connectivity algorithm had minimal effect (0.67% difference) on the final bone density distribution for standard bending and torsional moment cases. For a porous implant model, both algorithms, with and without bone connectivity implementation, reached the same final stiffness, with a difference of less than 0.01%. The bone connectivity algorithm predicted a slower and more gradual bone remodelling process, requiring at least 50% additional time for full remodelling compared to the conventional adaptive elasticity algorithm, which should be accounted for in the planning of rehabilitation strategies. The developed modelling can be employed to improve porous implant designs to achieve better clinical outcomes.

Mechano-driven intervertebral bone bridging via oriented mechanical stimulus in a twist metamaterial cage: An in silico study

Stiffer cages provide sufficient mechanical support but fail to promote bone ingrowth due to stress shielding. It remains challenging for fusion cage to satisfy both bone bridging and mechanical stability. Here we designed a fusion cage based on twist metamaterial for improved bone ingrowth, and proved its superiority to the conventional diagonal-based cage in silico. The fusion process was numerically reproduced via an injury-induced osteogenesis model and the mechano-driven bone remodeling algorithm, and the outcomes fusion effects were evaluated by the morphological features of the newly-formed bone and the biomechanical behaviors of the bone-cage composite. The twist-based cages exhibited oriented bone formation in the depth direction, in comparison to the diagonal-based cages. The axial stiffness of the bone-cage composites with twist-based cages was notably higher than that with diagonal-based cages; meanwhile, the ranges of motion of the twist-based fusion segment were lower. It was concluded that the twist metamaterial cages led to oriented bone ingrowth, superior mechanical stability of the bone-cage composite, and less detrimental impacts on the adjacent bones. More generally, metamaterials with a tunable displacement mode of struts might provide more design freedom in implant designs to offer customized mechanical stimulus for osseointegration.

Mandibular bone remodeling around osseointegrated functionally graded biomaterial implant using three dimensional finite element model

Dental implantation surgery has been progressed as one of the most efficient prosthetic technologies, however, it still fails very often and one of the main causes is the large difference between implant mechanical properties and those in welcoming bony tissues, making it problematical in osseointegration and bone remodeling. Biomaterial and tissue engineering research shows that there is a requirement in developing implants with Functionally Graded Materials (FGM). Indeed, the great potential of FGM lies not only in the field of bone tissue engineering but also in dentistry. To improve the acceptance of dental implants inside the living bone, FGM were proposed to step up the challenge of ensuring a better match of mechanical properties between biologically and mechanically compatible biomaterials. The aim of the present work is to investigate mandibular bone remodeling induced by FGM dental implant. Three-dimensional (3D) mandibular bone structure around an osseointegrated dental implant has been created to analyze the biomechanical behavior of the bone-implant system depending on implant material composition. In order to implement the numerical algorithm into ABAQUS software, UMAT subroutines and user-defined material were employed. Finite element analysis have been conducted to determine the stress distributions in implant and bony system, and to evaluate bone remodeling induced by the use of various FGM and pure titanium dental implants over the period of 48 months.

Biomechanical analysis of printable functionally graded material (FGM) dental implants for different bone densities

The long-term success of a dental implant is related to the material and design of the implant, and bone density. Conventional implants cause stress-shielding due to a mismatch between the implant and bone stiffness. Functionally graded porous materials and designs are a great choice for the design of implants to control the local stiffness at a certain location to meet the biomechanical requirements. The purpose of this study is to analyze five designs of axial and radial functionally graded materials (FGM) implants besides the conventional implant and conical and cylindrical shapes that were simulated with five different bone densities. The results showed that strain in bone increased with a decrease in cancellous bone density. The shape of the implant did not play an important role in strain/stress distribution. Conventional implants showed optimal strain (1000-2240 με) in low-density (0.7-0.8 g/cm3) bone, however, FGM implants produced optimal strain (990-1280 με) in the high-density bone (0.9-1 g/cm3) as compared to conventional implants. The proposed designs of FGM implants have the potential to address the complications of conventional implants in high-density bone.

Application of finite element analysis to tissue differentiation and bone remodelling approaches and their use in design optimization of orthopaedic implants: A review

Post-operative bone growth and long-term bone adaptation around the orthopaedic implants are simulated using the mechanoregulation based tissue-differentiation and adaptive bone remodelling algorithms, respectively. The primary objective of these algorithms was to assess biomechanical feasibility and reliability of orthopaedic implants. This article aims to offer a comprehensive review of the developments in mathematical models of tissue-differentiation and bone adaptation and their applications in studies involving design optimization of orthopaedic implants over three decades. Despite the different mechanoregulatory models developed, existing literature confirm that none of the models can be highly regarded or completely disregarded over each other. Not much development in mathematical formulations has been observed from the current state of knowledge due to the lack of in vivo studies involving clinically relevant animal models, which further retarded the development of such models to use in translational research at a fast pace. Future investigations involving artificial intelligence (AI), soft-computing techniques and combined tissue-differentiation and bone-adaptation studies involving animal subjects for model verification are needed to formulate more sophisticated mathematical models to enhance the accuracy of pre-clinical testing of orthopaedic implants.

Biomechanical Comparison Between Porous Ti6Al4V Block and Tumor Prosthesis UHMWPE Block for the Treatment of Distal Femur Bone Defects

Purpose: The management of bone defects is a crucial content of total knee revision. This study compared the biomechanical performance of porous Ti6Al4V block and tumor prosthesis UHMWPE block in treating distal femoral bone defects.

Methods: The finite element models of AORI type 3 distal femoral bone defect treated with porous Ti6Al4V block and UHMWPE block were established. Sensitivity analysis was performed to obtain the appropriate mesh size. The biomechanical performance of treatment methods in bone defects were evaluated according to the peak stress, the Von Mises stress distribution, and the average stresses of regions of interest under the condition of standing on one foot and flexion of the knee. Statistical analysis was conducted by independent samples t-test in SPSS (p < 0.05).

Results: In the standing on one-foot state, the peak stress of the porous Ti6Al4V block was 12.42 MPa and that of the UHMWPE block was 19.97 MPa, which is close to its yield stress (21 MPa). Meanwhile, the stress distribution of the UHMWPE block was uneven. In the flexion state, the peak stress of the porous Ti6Al4V block was 16.28 MPa, while that of the UHMWPE block was 14.82 MPa. Compared with the porous Ti6Al4V block group, the average stress of the region of interest in UHMWPE block group was higher in the standing on one foot state and lower in the flexion state (p < 0.05).

Conclusion: More uniform stress distribution was identified in the porous Ti6Al4V block application which could reserve more bone. On the contrary, uneven stress distribution and a larger high-stress concentration area were found in the UHMWPE block. Hence, the porous Ti6Al4V block is recommended for the treatment of AORI type 3 distal femoral bone defect.

Using a radial point interpolation meshless method and the finite element method for application of a bio-inspired remodelling algorithm in the design of optimized bone scaffold

The design of bone scaffold involves the analysis of stress shielding, which can occur when the Young’s modulus of the implant is higher than the Young’s modulus of the bone it is replacing, leading to bone decay in the surrounding tissue. It is therefore very important that the material is adequately designed to match the properties of the surrounding tissue, allowing an appropriate load transfer. While some approaches exist in the literature exploring functional gradients of material density, there are much less solutions based on biological laws. A homogenized model of gyroid infill obtained with PLA (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$E = 3145$$\end{document}E=3145 MPa) was obtained through mechanical tests of 3D printed specimens, namely tensile and compression, and the obtained model was implemented in a bone remodelling algorithm. The homogenized law was compared to the results obtained with a bone tissue law to assess the equivalence of density distribution and mechanical properties. Through a radial point interpolation method, it was found that similar density fields were obtained for the gyroid infill and for bone tissue when subject to the same boundary conditions. The finite element method was also used for comparison and validation. With the density field results, the gyroid mechanical behaviour was extrapolated to other materials, and similar stiffness values were obtained for bone tissue and titanium alloy (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$E = 110$$\end{document}E=110 GPa) scaffold, which justify this proposal of gyroid scaffolds for mimicking bone properties.