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Stress shielding secondary to bone resorption is one of the main causes of aseptic loosening, which limits the lifespan of hip prostheses and exacerbates revision surgery rates. In order to minimise post-hip replacement stress variations, this investigation proposes a low-stiffness, porous Ti6Al4V hip prosthesis, developed through selective laser melting (SLM). The stress shielding effect and potential bone resorption properties of the porous hip implant were investigated through both in vitro quasi-physiological experimental assays, together with finite element analysis. A solid hip implant was incorporated in this investigation for contrast, as a control group. The stiffness and fatigue properties of both the solid and the porous hip implants were measured through compression tests. The safety factor of the porous hip stem under both static and dynamic loading patterns was obtained through simulation. The porous hip implant was inserted into Sawbone/PMMA cement and was loaded to 2,300 N (compression). The proposed porous hip implant demonstrated a more natural stress distribution, with reduced stress shielding (by 70%) and loss in bone mass (by 60%), when compared to a fully solid hip implant. Solid and porous hip stems had a stiffness of 2.76 kN/mm and 2.15 kN/mm respectively. Considering all daily activities, the porous hip stem had a factor of safety greater than 2. At the 2,300 N load, maximum von Mises stresses on the hip stem were observed as 112 MPa on the medial neck and 290 MPa on the distal restriction point, whereby such values remained below the endurance limit of 3D printed Ti6Al4V (375 MPa). Overall, through the strut thickness optimisation process for a Ti6Al4V porous hip stem, stress shielding and bone resorption can be reduced, therefore proposing a potential replacement for the generic solid implant., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Naghavi, Tamaddon, Garcia-Souto, Moazen, Taylor, Hua and Liu.)

Stress shielding secondary to bone resorption is one of the main causes of aseptic loosening, which limits the lifespan of the hip prostheses and increases the rates of revision surgery. This study proposes a low stiffness polyether-ether-ketone (PEEK) hip prostheses, produced by fused deposition modelling to minimize the stress difference after the hip replacement. The stress shielding effect and the potential bone resorption of the PEEK implant was investigated through both experimental tests and FE simulation. A generic Ti6Al4V implant was incorporated in this study to allow fair comparison as control group. Attributed to the low stiffness, the proposed PEEK implant showed a more natural stress distribution, less stress shielding (by 104%), and loss in bone mass (by 72%) compared with the Ti6Al4V implant. The stiffness of the Ti6Al4V and the PEEK implant were measured through compression tests to be 2.76 kN/mm and 0.276 kN/mm. The factor of safety for the PEEK implant in both static and dynamic loading scenarios were obtained through simulation. Most of the regions in the PEEK implant were tested to be safe (FoS larger than 1) in terms of representing daily activities (2300 N), while the medial neck and distal restriction point of the implant attracts large von Mises stress 82 MPa and 76 MPa, respectively, and, thus, may possibly fail during intensive activities by yield and fatigue. Overall, considering the reduction in stress shielding and bone resorption in cortical bone, PEEK could be a promising material for the patient-specific femoral implants.

Additive manufacturing has been used to develop a variety of scaffold designs for clinical and industrial applications. Mechanical properties (i.e., compression, tension, bending, and torsion response) of these scaffolds are significantly important for load-bearing orthopaedic implants. In this study, we designed and additively manufactured porous metallic biomaterials based on two different types of triply periodic minimal surface structures (i.e., gyroid and diamond) that mimic the mechanical properties of bone, such as porosity, stiffness, and strength. Physical and mechanical properties, including compressive, tensile, bending, and torsional stiffness and strength of the developed scaffolds, were then characterised experimentally and numerically using finite element method. Sheet thickness was constant at 300 μm, and the unit cell size was varied to generate different pore sizes and porosities. Gyroid scaffolds had a pore size in the range of 600-1200 μm and a porosity in the range of 54-72%, respectively. Corresponding values for the diamond were 900-1500 μm and 56-70%. Both structure types were validated experimentally, and a wide range of mechanical properties (including stiffness and yield strength) were predicted using the finite element method. The stiffness and strength of both structures are comparable to that of cortical bone, hence reducing the risks of scaffold failure. The results demonstrate that the developed scaffolds mimic the physical and mechanical properties of cortical bone and can be suitable for bone replacement and orthopaedic implants. However, an optimal design should be chosen based on specific performance requirements.

Additively manufactured Ti scaffolds have been used for bone replacement and orthopaedic applications. In these applications, both morphological and mechanical properties are important for their in vivo performance. Additively manufactured Ti6Al4V triply periodic minimal surface (TPMS) scaffolds with diamond and gyroid structures are known to have high stiffness and high osseointegration properties, respectively. However, morphological deviations between the as-designed and as-built types of these scaffolds have not been studied before. In this study, the morphological and mechanical properties of diamond and gyroid scaffolds at macro and microscales were examined. The results demonstrated that the mean printed strut thickness was greater than the designed target value. For diamond scaffolds, the deviation increased from 7.5 μm (2.5% excess) for vertical struts to 105.4 μm (35.1% excess) for horizontal struts. For the gyroid design, the corresponding deviations were larger, ranging from 12.6 μm (4.2% excess) to 198.6 μm (66.2% excess). The mean printed pore size was less than the designed target value. For diamonds, the deviation of the mean pore size from the designed value increased from 33.1 μm (-3.0% excess) for vertical struts to 92.8 μm (-8.4% excess) for horizontal struts. The corresponding deviation for gyroids was larger, ranging from 23.8 μm (-3.0% excess) to 168.7 μm (-21.1% excess). Compressive Young's modulus of the bulk sample, gyroid and diamond scaffolds was calculated to be 35.8 GPa, 6.81 GPa and 7.59 GPa, respectively, via the global compression method. The corresponding yield strength of the samples was measured to be 1012, 108 and 134 MPa. Average microhardness and Young's modulus from α and β phases of Ti6Al4V from scaffold struts were calculated to be 4.1 GPa and 131 GPa, respectively. The extracted morphology and mechanical properties in this study could help understand the deviation between the as-design and as-built matrices, which could help develop a design compensation strategy before the fabrication of the scaffolds.

Polyether-ether-ketone (PEEK) is widely used in producing prosthesis and have gained great attention for repair of large bone defect in recent years with the development of additive manufacturing. This is due to its excellent biocompatibility, good heat and chemical stability and similar mechanical properties which mimics natural bone. In this study, three replicates of rectilinear scaffolds were designed for compression, tension, three-point bending and torsion test with unit cell size of 0.8 mm, a pore size of 0.4 mm, strut thickness of 0.4 mm and nominal porosity of 50%. Stress-strain graphs were developed from experimental and finite element analysis models. Experimental Young's modulus and yield strength of the scaffolds were measured from the slop of the stress-strain graph to be 395 and 19.50 MPa respectively for compression, 427 and 6.96 MPa respectively for tension, 257 and 25.30 MPa respectively for three-point bending and 231 and 12.83 MPa respectively for torsion test. The finite element model was found to be in good agreement with the experimental results. Ductile fracture of the struct subjected to tensile strain was the main failure mode of the PEEK scaffold, which stems from the low crystallinity of additive manufacturing PEEK. The mechanical properties of porous PEEK are close to those of cancellous bone and thus are expected to be used in additive manufacturing PEEK bone implants in the future, but the lower yield strength poses a design challenge.

An antibubble is an unusual object: a submerged water drop encapsulated in a thin shell of air that is stable underwater for 10-100 s. They are often thought of as the inverse of a soap bubble because they are a spherical shell of air in water in contrast to a shell of water in air. Antibubbles may be formed when water droplets impact the surface of surfactant-covered water, within a limited range of drop radius and drop impact velocity. In this paper, the range of drop size and impact velocity over which large antibubbles (radius 1-3 mm) are generated by the impact of falling drops is characterised, and the relationship of these parameters to the size of the antibubble formed is shown. Measurements of the two acoustical signals that may be produced as an antibubble is formed by drop impact are reported, and their relationship to the antibubble radius and shell thickness is established. Acoustical measurements taken are interpreted in the context of a modified Rayleigh-Plesset equation that provides a good fit to the frequency data for air shells greater than 100 μm in thickness. However, these results highlight the need for future work on the damping mechanisms associated with these larger antibubbles.