Mechanical stability of custom-made implants: Numerical study of anatomical device and low elastic Young's modulus alloy

PEEK stress-shielding with artificial bone for dental implants

Dental titanium implants and their surface modifications markedly improve implant biocompatibility. However, studies evaluating the mechanical biocompatibility of implants are scarce. In particular, the analysis of mechanical biocompatibility deficiencies leading to stress shield-induced bone resorption. Recently, we focused on using PEEK as a dental material. This study explored the hypothesis that PEEK implants improve the stress shielding of titanium. In this study, artificial bone surfaces were examined to measure strains on the artificial bone surface under compressive loading with the implants in place. Additionally, 3D image analysis of the fracture state inside the bone tissue was performed using micro-CT (µCT). This hypothesis was supported by µCT imaging analysis of bone tissue changes under stress, which revealed that PEEK implants transfer greater loads than titanium implants. µCT imaging and statistical analysis showed that bone porosity had little effect on stress shielding.

Stability Analysis of Plate-Screw Fixation for Femoral Midshaft Fractures

An understanding of the biomechanical characteristics and configuration of flexible and locked plating in order to provide balance stability and flexibility of implant fixation will help to construct and promote fast bone healing. The relationship between applied loading and implantation configuration for best bone healing is still under debate. This study aims to investigate the relationship between implant strength, working length, and interfragmentary strain (εIFM) on implant stability for femoral midshaft transverse fractures. The transverse fracture was fixed with a fragment locking compression plate (LCP) system. Finite element analysis was performed and subsequently characterised based on compression loading (600 N up to 900 N) and screw designs (conventional and locking) with different penetration depths (unicortical and bicortical). Strain theory was used to evaluate the stability of the model. The correlation of screw configuration with screw type shows a unicortical depth for both types (p < 0.01) for 700 N and 800 N loads and (p < 0.05) for configurations 134 and 124. Interfragmentary strain affected only the 600 N load (p < 0.01) for the bicortical conventional type (group BC), and the screw configurations that were influenced were 1234 and 123 (p < 0.05). The low steepness of the slope indicates the least εIFM for the corresponding biomechanical characteristic in good-quality stability. A strain value of ≤2% promotes callus formation and is classified as absolute stability, which is the minimum required value for the induction of callus and the maximum value that allows bony bridging. The outcomes have provided the correlation of screw configuration in femoral midshaft transverse fracture implantation which is important to promote essential primary stability.

Optimization of stress distribution of bone-implant interface (BII)

Many studies have found that the threshold of occlusal force tolerated by titanium-based implants is significantly lower than that of natural teeth due to differences in biomechanical mechanisms. Therefore, implants are considered to be susceptible to occlusal trauma. In clinical practice, many implants have shown satisfactory biocompatibility, but the balance between biomechanics and biofunction remains a huge clinical challenge. This paper comprehensively analyzes and summarizes various stress distribution optimization methods to explore strategies for improving the resistance of the implants to adverse stress. Improving stress resistance reduces occlusal trauma and shortens the gap between implants and natural teeth in occlusal function. The study found that: 1) specific implant-abutment connection design can change the force transfer efficiency and force conduction direction of the load at the BII; 2) reasonable implant surface structure and morphological character design can promote osseointegration, maintain alveolar bone height, and reduce the maximum effective stress at the BII; and 3) the elastic modulus of implants matched to surrounding bone tissue can reduce the stress shielding, resulting in a more uniform stress distribution at the BII. This study concluded that the core BII stress distribution optimization lies in increasing the stress distribution area and reducing the local stress peak value at the BII. This improves the biomechanical adaptability of the implants, increasing their long-term survival rate.

Numerical investigation of the effective mechanical properties and local stress distributions of TPMS-based and strut-based lattices for biomedical applications

Porous structures, including those with lattice geometries, have been shown to mimic the mechanical properties of the human bone. Apart from the widely known strut-based lattices, the Triply Periodic Minimal Surfaces (TPMS) concept has been introduced recently to create surface-based lattices and to tailor their mechanical behaviors. In this study, the numerical investigation of the effective elastic properties, the anisotropic behavior, and the local stress distributions of a broad range of topologies provide us with a complete numerical tool to assist bone implant design. The comparison database of the lattices includes TPMS-based lattices, both sheet, and skeletal, as well as strut-based lattices. The lattices are subjected to periodic boundary conditions and also, a homogenization method is deployed to simulate the response of the lattice unit cells determining their apparent equivalent stiffness. A correlation among the lattice topologies, their effective mechanical properties, and the local Von Mises stress concentrations in them is observed. The stress distribution of various topologies with the same elastic modulus is examined to combine all the investigations. Finally, a large variety of numerical results are presented to allow the comparison of the lattice structures and the selection of the optimal configuration that mimics the elastic properties of the bone.

Enhanced biomechanical performance of additively manufactured Ti-6Al-4V bone plates

As the global trauma fixation devices market expands rapidly, it is imperative to improve the production of fixation devices through enhanced design accuracy and fit for best performance and maximum patient comfort. Selective laser melting (SLM) is one of the mature additive manufacturing methods, which provides a viable route for the rapid production of such devices. In this work, the ability of SLM to produce near-net-shape parts, as desired for medical implants, was utilized for the fabrication of bone plates from Ti-6Al-4V alloy powder. Martensitic microstructure obtained after the printing of alloy resulted in poor ductility, limiting its application in the field of orthopedics. A specially designed repeated cyclic heating and cooling close to but below the β-transus was used to transform from acicular to a bimodal microstructure without the need for plastic deformation prior to heat treatment for improving the ductility. Bone plates subjected to this heat treatment were mechanically tested by means of tensile and 3-point bend tests and demonstrated large improvement in ductility, and the values were comparable to those similar plates prepared from wrought alloy. Other important properties required for implants were assessed, such as corrosion resistance in simulated body fluid and cytocompatibility in vitro using MC3T3-E1 cells. These results for the bone plate after heat treatment were excellent and similar to those of the additively manufactured and wrought plates. Taken together, the performance of the additively manufactured bone plates after subjecting to heat treatment was similar to those of bone plate manufactured using wrought alloy. These results have important implications for the fabrication of patient-specific metallic orthopedic devices using SLM without compromising their biomechanical performance by subjecting them to a tailored heat treatment.

Mechanical behaviour of femoral prosthesis with various cross-section shape, implant configurations and material properties

The maximum Von-Mises stress analysis introduced by dynamic body load on hip implant was considered in this research. The different hip stem shapes including elliptical, circular and oval cross sections with various configurations and material properties including that of Ti-6Al-4V, Co-Cr-Mo and SS316L alloys were simulated by 3D computational analysis to evaluate the "stress-shielding effect" on the implant surface. In order to categorise the various shapes and materials based on "stress-shielding effect", a factor called "stress distribution" (SD) was developed. Results showed that the highest Von-Mises stress levels are attributed to the elliptical cross-section, whereas oval cross-sections comprised the least figures with certain stem profiles. In terms of SD factor, titanium alloy with elliptical cross section indicated the lowest level, while the better results specified to the oval shape.

Selective Laser Melting and Electron Beam Melting of Ti6Al4V for Orthopedic Applications: A Comparative Study on the Applied Building Direction

The 3D printing process offers several advantages to the medical industry by producing complex and bespoke devices that accurately reproduce customized patient geometries. Despite the recent developments that strongly enhanced the dominance of additive manufacturing (AM) techniques over conventional methods, processes need to be continually optimized and controlled to obtain implants that can fulfill all the requirements of the surgical procedure and the anatomical district of interest. The best outcomes of an implant derive from optimal compromise and balance between a good interaction with the surrounding tissue through cell attachment and reduced inflammatory response mainly caused by a weak interface with the native tissue or bacteria colonization of the implant surface. For these reasons, the chemical, morphological, and mechanical properties of a device need to be designed in order to assure the best performances considering the in vivo environment components. In particular, complex 3D geometries can be produced with high dimensional accuracy but inadequate surface properties due to the layer manufacturing process that always entails the use of post-processing techniques to improve the surface quality, increasing the lead times of the whole process despite the reduction of the supply chain. The goal of this work was to provide a comparison between Ti6Al4V samples fabricated by selective laser melting (SLM) and electron beam melting (EBM) with different building directions in relation to the building plate. The results highlighted the influence of the process technique on osteoblast attachment and mineralization compared with the building orientation that showed a limited effect in promoting a proper osseointegration over a long-term period.

In silico evaluation of the stress fields on the cortical bone surrounding dental implants: Comparing root-analogue and screwed implants

Tooth loss is a problem that affects both old and young people. It may be caused by several conditions, such as poor oral hygiene, lifestyle choices or even diseases like periodontal disease, tooth grinding or diabetes. Nowadays, replacing a missing tooth by an implant is a very common process. However, many limitations regarding the actual strategies can be enumerated. Conventional screwed implants tend to induce high levels of stress in the peri-implant bone area, leading to bone loss, bacterial bio-film formation, and subsequent implant failure. In this sense, root-analogue dental implants are becoming promising solutions for immediate implantation due to their minimally invasive nature, improved bone stress distribution and because they do not require bone drilling, sinus lift, bone augmentation nor other traumatic procedures. The aim of this study was to analyse and compare, by means of FEA, the stress fields of peri-implant bone around root-analogue and screwed conventional zirconia implants. For that purpose, one root-analogue implant, one root-analogue implant with flaps, two conventional implants (with different threads) and a replica of a natural tooth were modelled. COMSOL was used to perform the analysis and implants were subjected to two simultaneous loads: 100 N axially and 100 N oblique (45°). RESULTS: revealed that root-analogue implants, namely with flaps, should be considered as promising alternatives for dental implant solutions since they promote a better stress distribution in the cortical bone when compared with conventional implants.

Icariin-loaded porous scaffolds for bone regeneration through the regulation of the coupling process of osteogenesis and osteoclastic activity

Objective

Icariin (IC) promotes osteogenic differentiation, and it may be a potential small molecule drug for local application in bone regeneration. Icariin-loaded hydroxyapatite/alginate (IC/HAA) porous composite scaffolds were designed in this study for the potential application of the sustainable release of icariin and subsequent bone regeneration.

Methods

An icariin-loaded hydroxyapatite/alginate porous composite scaffold was prepared and characterized by SEM and HPLC for morphology and release behavior, respectively. The mechanical properties, degradation in PBS and cytotoxicity on BMSCs were also evaluated by MTT assay, compression strength and calculation of weight remaining ratio, respectively. Rabbit BMSCs were cocultured with IC/HAA scaffolds, and ALP activity and Alizarin Red staining were performed to evaluate osteogenic differentiation induction. The mRNA and protein expression level of an osteogenic gene was detected by RT-PCR and Western blotting, respectively. In vivo animal models of critical bone defects in the radius of rabbit were used. Four and 12 weeks after the implantation of IC/HAA scaffolds in the bone defect, radiographic images of the radius were obtained and scored by using the Lane and Sandhu X-ray scoring system. Tissue samples were also evaluated using H&E and Masson staining, and an osteogenic gene and Wnt signaling pathway genes were detected.

Results

A hydroxyapatite/alginate (HAA) porous composite scaffold-loaded icariin was fabricated using a freeze-drying method. Our data indicated that the icariin was loaded in alginate scaffold without compromising the macro/microstructure or mechanical properties of the scaffold. Notably, the IC/HAA promoted the proliferation of rBMSCs without exerting cytotoxicity on rBMSCs. In vivo, rabbit radius bone defect experiments demonstrated that the IC/HAA scaffold exhibited better capacity for bone regeneration than HAA, and IC/HAA upregulated the relative expression levels of an osteogenic gene and the Wnt signaling pathway genes. Most notably, the IC/HAA scaffold also inhibited osteoclast activity in vivo.

Conclusion

Our data suggests a promising application for the use of HAA scaffolds to load icariin and promote bone regeneration in situ through mediation of the coupling processes of osteogenesis induction and osteoclast activity inhibition.