Finite element analysis of the influence of porosity and pore geometry on mechanical properties of orthopaedic scaffolds.

PURPOSE: Scaffolds play a key role in regenerative medicine in the repair of injuries, defects and cancerous changes in long bones. For this reason, scaffolds should meet certain mechanobiological requirements, such as adequate porosity and pore geometry to ensure appropriate osteointegration as well as load transfer. Taking into account the most frequently used cell units, this study attempted to evaluate the porous structures of orthopaedic scaffolds in terms of their strength parameters. MATERIALS AND METHODS: Four pore geometries were selected for analyses: sphere, octagonal prism, cube and triangular prism, all with porosities of 10% up to 60%. Three different material properties were considered: Ti6Al4V alloy, CoCr alloy, 316 L steel. Strength compression simulations were carried out on 144 models, 72 structures of cell units with dimensions of 4 × 4 × 4 mm and 72 structures of scaffolds with a diameter of 16 mm and a height of 15 mm. Effective Young's modulus, as well as 0.2%, offset effective yield strength was estimated. RESULTS: Research has shown that scaffolds with bone-like strength properties should be made of Ti6Al4V alloy. The value of 40% turned out to be of the best porosity. The remaining porosities showed much lower or much higher strength parameters and were significantly different from the properties of the bones. CONCLUSIONS: The obtained data allow to indicate the most functional porous structure with Young's modulus similar to that possesses by core bone, while maintaining mechanical strength, allowing for its appropriate use in orthopaedic regenerative medicine.

Bioactivity of PEEK GRF30 and Ti6Al4V SLM in Simulated Body Fluid and Hank's Balanced Salt Solution.

In recent years, scientists have defined two main paths for orthopedic implant fabrication: searching for new materials with properties closest to natural bone in order to reduce the stress-shielding effect or creating individually adapted geometry of the implant with the use and Rapid Prototyping methods. Therefore, materials such as PEEK GRF30 and Ti6Al4V selective laser melting (SLM) are of interest. They are defined as materials suitable for implants, however, the knowledge of their bioactivity, a feature which is one of the most desirable properties of biomaterials, is still insufficient. Using Simulated Body Fluid and Hank's Balanced Salt Solution, the bioactivity of PEEK GRF30 and Ti6Al4V SLM was assessed, as well as commercial Ti6Al4V as a reference material. Ten cylindrical samples of each material were prepared and immersed in solutions per period from 2 to 28 days at 37 °C. Optical analysis of the changes on the examined surfaces suggested that right after 2-day crystals with different morphologies were formed on each material. Further analysis of the chemical composition of the altered surfaces confirmed the formation of a calcium phosphate layer on them, however, the Ca/P ratio was slightly different from 1.67. On the basis of the obtained results, it can be concluded that both PEEK GRF30 and Ti6Al4V SLM are characterized by appropriate-comparable to Ti6Al4V-bioactivity.

Experimental evaluation of a novel concept of an implant for direct skeletal attachment of limb prosthesis.

PURPOSE: The aim of the study was to experimentally evaluate a proposed two-part implantation system (medullary part made of PEEK composite and percutaneous part made of Ti6Al4V) for bone-anchored prosthesis and to compare it to typical press-fit design (also made of Ti6Al4V) used for the same purpose. METHODS: Simplified prototypes of both implants were prepared for the research. Both implants were evaluated in vitro with the use of porcine femur (6 bones for each implant). ARAMIS vision system was used to measure strains in selected area of bone shaft, generated when putting an axial load on the implants to simulate static load bearing exercises performed during rehabilitation activities in primary stabilisation. RESULTS: Obtained maps revealed high concentrations of strains, located near to distal part of the implant, during using a typical press-fit design with relatively low strain around the implant's shaft. In the case of proposed design, noticeable strains occurred in the entire examined area of bone, with stronger concentration towards the proximal direction. CONCLUSIONS: Presented experimental results suggest that proposed design provides more appropriate implant-bone load transfer than typically used press-fit design. This may result in obtaining more beneficial mechanobiological stimulus which enables the researchers to achieve appropriate primary stability and maintain appropriate bone quality during its long-term use after achieving full osseointegration.

Effect of the material's stiffness on stress-shielding in osseointegrated implants for bone-anchored prostheses: a numerical analysis and initial benchmark data.

PURPOSE: This study attempted to establish the link between design of implants for bone-anchored prostheses and stress-shielding, affecting the stability of the bone-implant coupling using numerical approach. The objectives were to share a numerical model capable to evaluate the long-term stability of implants and to use this model to extract data sets showing how shape and material stiffness of threaded, press-fit and modular press-fit implants affect stress-shielding intensity. METHODS: Three designs were considered: threaded, press-fit and modular press-fit. The effect of shape and material stiffness of each design on stress-shielding intensity was assessed using Young's modulus (10 to 210 GPa). Furthermore, the impact of the diameter of percutaneous part (10 to 18 mm) and thickness of medullar part (5 to 1 mm) was investigated for the modular press-fit implant. RESULTS: The threaded design generated 4% more bone mass loss at the distal femur but an overall loss of bone mass was by 5% lower to press-fit design. The influence of Young's modulus on bone mass changes was noticeable for modular press-fit implant, depending on diameter of percutaneous or medullary part. A 20 GPa change of stiffness caused a bone mass change from 0.65% up to 2.45% and from 0.07% up to 0.32% for percutaneous parts with 18 mm and 10 mm diameter, respectively. CONCLUSIONS: Results suggested that threaded implant provides greater stability despite an increased bone loss at the distal femur. Altogether, this work provided an initial model that could be applied in subsequent studies on the long-term stability of current and upcoming implants.

Numerical Analysis of the Influence of Porosity and Pore Geometry on Functionality of Scaffolds Designated for Orthopedic Regenerative Medicine.

BACKGROUND: Scaffolds are vital for orthopedic regenerative medicine. Therefore, comprehensive studies evaluating their functionality with consideration of variable parameters are needed. The research aim was to evaluate pore geometry and scaffold porosity influence on first, cell culture efficiency in a perfusion bioreactor and second, osteogenic cell diffusion after its implantation. METHODS: For the studies, five pore geometries were selected (triangular prism with a rounded and a flat profile, cube, octagonal prism, sphere) and seven porosities (up to 80%), on the basis of which 70 models were created for finite element analyses. First, scaffolds were placed inside a flow channel to estimate growth medium velocity and wall shear stress. Secondly, scaffolds were placed in a bone to evaluate osteogenic cell diffusion. RESULTS: In terms of fluid minimal velocity (0.005 m/s) and maximal wall shear stress (100 mPa), only cubic and octagonal pores with 30% porosity and spherical pores with 20% porosity fulfilled the requirements. Spherical pores had the highest osteogenic cell diffusion efficiency for porosities up to 30%. For higher porosities, the octagonal prism's pores gave the best results up to 80%, where no differences were noted. CONCLUSIONS: The data obtained allows for the appropriate selection of pore geometry and scaffold porosity for orthopedic regenerative medicine.

Influence of the Surface Roughness of PEEK GRF30 and Ti6Al4V SLM on the Viability of Primary Human Osteoblasts Determined by the MTT Test.

The aim of the study was to clearly determine whether selected modern medical materials and three dimensional printing allow for satisfactory viability of human osteoblasts, which is important from the point of view of the subsequent osseointegration process. Moreover, as implants are produced with various topography, the influence of surface roughness on viability of bone cells was evaluated. To conduct the research, primary human osteoblasts (PromoCell) were used. Cells were seeded on samples of glass-reinforced polyetheretherketone (30% of the filling), Ti6Al4V manufactured with the use of selective laser melting technology and forged Ti6Al4V with appropriately prepared variable surface roughness. To assess the viability of the tested cells the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide was used. Results showed that all evaluated materials do not exhibit cytotoxic properties. Moreover, on their basis it can be concluded that there is a certain surface topography (designated i.a. as roughness equal to approx. Ra = 0.30 µm), which ensures the highest possible viability of human osteoblasts. On the basis of the received data, it can also be concluded that modern glass-reinforced polyetheretherketone or Ti6Al4V produced by rapid prototyping method allow to manufacture implants that should be effectively used in clinical conditions.

The Influence of Geometry of Implants for Direct Skeletal Attachment of Limb Prosthesis on Rehabilitation Program and Stress-Shielding Intensity.

The purpose of the research was to evaluate the influence of selected parameters of the implants for bone anchored prostheses on possibility of conducting static load bearing exercises and stress-shielding intensity. A press-fit implant, a threaded implant, and the proposed design were compared using the finite element method. For the analyses two features were examined: diameter (19.0 - 21.0 mm) and length (75.0 - 130.0 mm). To define the possibility of conducting rehabilitation exercises the micromotion of implants while axial loading with a force up to 1000 N was examined to evaluate the changes at implant-bone interface. The stress-shielding intensity was estimated by bone mass loss over 60 months. The results suggest that, in terms of micromotion generated during rehabilitation exercises, the threaded (max. micromotion of 16.00 µm) and the proposed (max. micromotion of 45.43 µm) implants ensure low and appropriate micromotion. In the case of the press-fit solution the load values should be selected with care, as there is a risk of losing primary stabilisation. The allowed forces (that do not stimulate the organism to generate fibrous tissue) were approx. 140 N in the case of the length of 75 mm, increasing up to 560 N, while using the length of 130 mm. Moreover, obtained stress-shielding intensities suggest that the proposed implant should provide appropriate secondary stability, similar to the threaded solution, due to the low bone mass loss during long-term use (improving at the same time more bone remodelling in distal Gruen zones, by providing lower bone mass loss by approx. 13% to 20% in dependency of the length and diameter used). On this basis it can be concluded that the proposed design can be an appropriate alternative to commercially used implants.

A Comparative Analysis of Standardised Threads for Use in Implants for Direct Skeletal Attachment of Limb Prosthesis: A Finite Element Analysis.

The aim of the research was to determine the optimal thread's shape to be used in implants for direct skeletal attachment of limb prosthesis. In addition, by testing appropriate parameters' modification of the suitable thread, an attempt was made to maximise its effectiveness. The analyses included three thread types described in the ISO standards: shallow, symmetrical, and asymmetrical. The obtained results suggest that shallow thread ensures the lowest equivalent and directional stress peaks generated in the bone as well as favourable stress patterns and profiles during implant loading in relation to symmetrical and asymmetrical threads. Moreover, shallow thread ensured the generation of single equivalent and directional stress peaks, while symmetrical and asymmetrical threads provided additional stress peak for equivalent as well as for each of directional peaks. Subsequently, optimisation of the shallow thread's shape was conducted by changing two relevant thread's parameters (flank angle and rounding arc) which influence the generated stress distribution. The highest reduction of stress peaks was obtained while reducing the rounding arc by 0.2 mm. Therefore, it can be stated that the proposed modification of the HA thread can lead to obtaining a higher biomechanical effectiveness of implants for direct skeletal attachment of limb prosthesis.

A comparative analysis of internal bone remodelling concepts in a novel implant for direct skeletal attachment of limb prosthesis evaluation: A finite element analysis.

Nowadays, numerous internal bone remodelling concepts are under development, in order to estimate long-term functionality of implants by evaluating the intensity of stress-shielding effect. This effect is also analysed for the implants for direct skeletal attachment, considered as a better exoprosthesis fixation method than prosthetic sockets. Most of bone remodelling approaches are based on basic concepts, differing with certain assumptions, which may affect the accuracy of the results. This article compares commonly used internal bone remodelling concepts and evaluates the functionality of the proposed Limb Prosthesis Osseointegrated Fixation System for direct skeletal attachment of limb prosthesis in comparison with two currently available implants: the Intraosseous Transcutaneous Amputation Prosthesis and the Osseointegrated Prostheses for the Rehabilitation of Amputees. Three concepts were chosen: without and with lazy zone and with the use of quadratic formula which considers bone overloading. Therefore, three finite element models were created with identical femur implanted with each of analysed implants. The implants were loaded with loads that refer to two stages of gait cycle (heel strike and toe-off). The analysed concepts have given similar results, allowing to assume that each of them can be successfully used to estimate internal bone remodelling around analysed implants for direct skeletal attachment of limb prosthesis. The results also present higher functionality of the proposed implant for direct skeletal attachment of limb prosthesis due to a significant reduction in stress-shielding in the analysed areas around implant in comparison with the Intraosseous Transcutaneous Amputation Prosthesis and the Osseointegrated Prostheses for the Rehabilitation of Amputees. It suggests that the proposed design is a better alternative to the currently used solutions.

Biomechanical evaluation of a novel Limb Prosthesis Osseointegrated Fixation System designed to combine the advantages of interference-fit and threaded solutions.

PURPOSE: The study was aimed at biomechanical evaluation of a novel Limb Prosthesis Osseointegrated Fixation System (LPOFS) designed to combine the advantages of interference-fit and threaded solutions. METHODS: Three cases, the LPOFS (designed), the OPRA (threaded) and the ITAP (interference-fit) implants were studied. Von-Mises stresses in bone patterns and maximal values generated while axial loading on an implant placed in bone and the force reaction values in contact elements while extracting an implant were analysed. Primary and fully osteointegrated connections were considered. RESULTS: The results obtained for primary connection indicate more effective anchoring of the OPRA, however the LPOFS provides more appropriate stress distribution (lower stress-shielding, no overloading) in bone. In the case of fully osteointegrated connection the LPOFSs kept the most favourable stress distribution in cortical bone which is the most important long-term feature of the implant usage and bone remodelling. Moreover, in fully bound connection its anchoring elements resist extracting attempts more than the ITAP and the OPRA. CONCLUSIONS: The results obtained allow us to conclude that in the case of features under study the LPOFS is a more functional solution to direct skeletal attachment of limb prosthesis than the referential implants during short and long-term use.