Impact of infill density on morphology and mechanical properties of 3D printed ABS/CF-ABS composites using design of experiments

Metal Extrusion (MEX) is a leading 3D printing technology for polymers, enabling intricate designs and personalized products in various applications. The current study evaluate how infill density affects the tensile, flexural, compressive, Izod impact and fracture behaviour of Acrylonitrile Butadiene Styrene (ABS) and Carbon Fiber Reinforced-Acrylonitrile Butadiene Styrene (CF-ABS) specimens manufactured using the MEX method. Different infill densities of 20, 40, 60 and 80 % are used in the production of honeycomb infill pattern samples for investigating the mechanical as well as fracture behaviour of MEX ABS/CF-ABS components. The experimental runs of fabricated composites were tested using a digital Izod impact tester and servo-controlled hydraulic universal testing machine, following ASTM standard procedures. The experimental findings show that CF-ABS specimens with an 80 % infill density and honeycomb fill pattern showed significant improvements in tensile strength, modulus, yield strength and elongation. The flexural strength (64.74 %), flexural modulus (209.15 %), compressive strength (125.21 %), compressive modulus (108.34 %) and impact strength (38.91 %) of these specimens are comparable to those of 3D printed ABS specimens and other infill densities. The research shows that precise management of processing variables can greatly improve the mechanical properties of 3D-printed ABS samples, providing valuable insights for a range of applications.

Imminent threat of rock-ice avalanches in High Mountain Asia

Upsurge of glacier-related hazards in High Mountain Asia (HMA) has been evident in recent years due to global warming. While many glacial-related hazards are instantaneous, some large landslides were preceded by slow gravitational deformation, which can be predicted to evade catastrophes. Here, we present robust evidence of historical deformation in 2021 Chamoli rock-ice avalanche of Himalaya using space imaging techniques. Multi-temporal satellite data provide evidence of a precursor event in 2016 and expansion of a linear fracture along joint planes, indicating 2021 rock-ice avalanche is a retrogressive wedge failure. The deformation history shows that the fracture propagated at a velocity of ~0.07 m day^-1 until September 2020, and with an accelerated velocity (~0.14 m day^-1 on average) lately. Analysis of recent similar cases in HMA supported our inference on global warming-induced glacier retreat and thermomechanical effects in enhancing the weakening of fractured rock masses in tectonically active mountain belts. Recent advances in Earth observation and seismic monitoring system can offer clues to the location and timing of impending catastrophic failures in high mountain regions.

Is a Complete Anatomical Fit of the Tomofix Plate Biomechanically Favorable? A Parametric Study Using the Finite Element Method

BACKGROUND

The opening wedge high tibial osteotomy (HTO) fixation using the Tomofix system is at the risk of mechanical failure due to unstable fixation, lateral hinge fracture, and hardware breakage. This study aimed to investigate the effect of the level of anatomical fit (LOF) of the plate on the failure mechanisms of fixation.

METHODS

A finite element model of the HTO with a correction angle of 12 degrees was developed. The LOF of the TomoFix plate was changed parametrically by altering the curvature of the plate in the sagittal plane. The effect of the LOF on the fixation performance was studied in terms of the factor of safety (FOS) against failure mechanisms. The FOSs were found by 1) dividing the actual stiffness of the plate-bone construct by the minimum allowable one for unstable fixation, 2) dividing the compressive strength of the cortical bone by the actual maximum pressure at the lateral hinge for the lateral hinge fracture, and 3) the Soderberg criterion for fatigue fracture of the plate and screws.

RESULTS

The increase of the LOF by applying a larger bent to the plate changed the fixation stiffness slightly. However, it reduced the lateral hinge pressure substantially (from 182 MPa to 71 MPa) and increased the maximum equivalent stresses in screws considerably (from 187 MPa to 258 MPa). Based on the FOS-LOF diagram, a gap smaller than 2.3 mm was safe, with the highest biomechanical performance associated with a 0.5 mm gap size.

CONCLUSION

Although a high LOF is necessary for the Tomofix plate fixation to avoid mechanical failure, a gap size of 0.5mm is favored biomechanically over complete anatomical fit.

Developments in Surface/Interface Engineering of Ni-Rich Layered Cathode Materials

Ni-rich layered cathodes with high energy densities reveal an enormous potential for lithium-ion batteries (LIBs), however, their poor stability and reliability have inhibited their application. To ensure their stability over extensive cycles at high voltage, surface/interface modifications are necessary to minimize the adverse reactions at the cathode-electrolyte interface (CEI), which is a critical factor impeding electrode performance. Therefore, this review provides a comprehensive discussion on the surface engineering of Ni-rich cathode materials for enhancing their lithium storage property. Based on the structural characteristics of the Ni-rich cathode, the major failure mechanisms of these structures during synthesis and operation are summarized. Then the existing surface modification techniques are discussed and compared. Recent breakthroughs in various surface coatings and modification strategies are categorized and their unique functionalities in structural protection and performance-enhancing are elaborated. Finally, the challenges and outlook on the Ni-rich cathode materials are also proposed.

Computational homogenisation based extraction of transverse tensile cohesive responses of cortical bone tissue

The numerical assessment of fracture properties of cortical bone is important in providing suggestions on patient-specific clinical treatments. We present a generic finite element modelling framework incorporating computational fracture approaches and computational homogenisation techniques. Finite element computations for statistical volume elements (SVEs) at the microscale are performed for different sizes with random osteon packing with a fixed volume fraction. These SVEs are loaded in the transverse direction under tension. The minimal SVE size in terms of ensuring a representative effective cohesive law is suggested to be 0.6 mm. Since cement lines as weak interfaces play a key role in bone fracture, the effects of their fracture properties on the effective fracture strength and toughness are investigated. The extracted effective fracture properties can be used as homogenised inputs to a discrete crack simulation at macroscopic or structural scale. The extrinsic toughening mechanisms observed in the SVE models are discussed with a comparison against experimental observations from the literature, giving beneficial insights to cortical bone failure.

Microtensile properties and failure mechanisms of cortical bone at the lamellar level

Bone features a remarkable combination of toughness and strength which originates from its complex hierarchical structure and motivates its investigation on multiple length scales. Here, in situ microtensile experiments were performed on dry ovine osteonal bone for the first time at the length scale of a single lamella. The micromechanical response was brittle and revealed larger ultimate tensile strength compared to the macroscale (factor of 2.3). Ultimate tensile strength for axial and transverse specimens was 0.35 ± 0.05 GPa and 0.13 ± 0.02 GPa, respectively. A significantly greater strength anisotropy relative to compression was observed (axial to transverse strength ratio of 2.7:1 for tension, 1.3:1 for compression). Fracture surface and transmission electron microscopic analysis suggested that this may be rationalized by a change in failure mode from fibril-matrix interfacial shearing for axial specimens to fibril-matrix debonding in the transverse direction. An improved version of the classic Hashin's composite failure model was applied to describe lamellar bone strength as a function of fibril orientation. Together with our experimental observations, the model suggests that cortical bone strength at the lamellar level is remarkably tolerant to variations of fibrils orientation of about ±30°. This study highlights the importance of investigating bone's hierarchical organization at several length scales for gaining a deeper understanding of its macroscopic fracture behavior. STATEMENT OF SIGNIFICANCE: Understanding bone deformation and failure behavior at different length scales of its hierarchical structure is fundamental for the improvement of bone fracture prevention, as well as for the development of multifunctional bio-inspired materials combining toughness and strength. The experiments reported in this study shed light on the microtensile properties of dry primary osteonal bone and establish a baseline from which to start further investigations in more physiological conditions. Microtensile specimens were stronger than their macroscopic counterparts by a factor of 2.3. Lamellar bone strength seems remarkably tolerant to variations of the sub-lamellar fibril orientation with respect to the loading direction (±30°). This study underlines the importance of studying bone on all length scales for improving our understanding of bone's macroscopic mechanical response.

Compressive characteristics of radially graded porosity scaffolds architectured with minimal surfaces

Scaffolds with gradient pore characteristics have received a great deal of attention as they can better mimic the structure of the native tissues and concurrently meet both biological and mechanical requirements. In the present study, the effects of porosity geometry and porosity gradient patterns on the deformation mechanism and compressive mechanical properties of the structures were investigated in the context of stretching (I-WP and P surfaces) versus bending dominated (D surface) triply periodic minimal surface (TPMS) based architectures. Different gradient patterns were found to significantly alter the deformation mechanism. Radial gradient patterns (perpendicular to loading direction) provide higher deformability while longitudinally graded scaffolds suffer from low failure strain. In the stretching dominated architectures vertical cracks propagated under compression due to the materials transverse expansion under compression. Deformations in the bending dominated architectures, however, were accompanied by a progressive collapse owing to the shearing of the struts. In general, stretching dominated structures showed the higher mechanical properties and provided more efficiency under mechanical loads. Finite Element simulations also demonstrated a high capability for predicting the deformation as well as mechanical responses (especially for elastic properties) and can be used as a tool for designing multifunctional gradient porous scaffolds.

Mechanical properties of all-suture anchors for rotator cuff repair

Objectives

All-suture anchors are increasingly used in rotator cuff repair procedures. Potential benefits include decreased bone damage. However, there is limited published evidence for the relative strength of fixation for all-suture anchors compared with traditional anchors.

Materials and Methods

A total of four commercially available all-suture anchors, the ‘Y-Knot’ (ConMed), Q-FIX (Smith & Nephew), ICONIX (Stryker) and JuggerKnot (Zimmer Biomet) and a traditional anchor control TWINFIX Ultra PK Suture Anchor (Smith & Nephew) were tested in cadaveric human humeral head rotator cuff repair models (n = 24). This construct underwent cyclic loading applied by a mechanical testing rig (Zwick/Roell). Ultimate load to failure, gap formation at 50, 100, 150 and 200 cycles, and failure mechanism were recorded. Significance was set at p < 0.05.

Results

Overall, mean maximum tensile strength values were significantly higher for the traditional anchor (181.0 N, standard error (se) 17.6) compared with the all-suture anchors (mean 133.1 N se 16.7) (p = 0.04). The JuggerKnot anchor had greatest displacement at 50, 100 and 150 cycles, and at failure, reaching statistical significance over the control at 100 and 150 cycles (22.6 mm se 2.5 versus 12.5 mm se 0.3; and 29.6 mm se 4.8 versus 17.0 mm se 0.7). Every all-suture anchor tested showed substantial (> 5 mm) displacement between 50 and 100 cycles (6.2 to 14.3).

All-suture anchors predominantly failed due to anchor pull-out (95% versus 25% of traditional anchors), whereas a higher proportion of traditional anchors failed secondary to suture breakage.

Conclusion

We demonstrate decreased failure load, increased total displacement, and variable failure mechanisms in all-suture anchors, compared with traditional anchors designed for rotator cuff repair. These findings will aid the surgeon’s choice of implant, in the context of the clinical scenario.

Cite this article: N. S. Nagra, N. Zargar, R. D. J. Smith, A. J. Carr. Mechanical properties of all-suture anchors for rotator cuff repair. Bone Joint Res 2017;6:82–89. DOI: 10.1302/2046-3758.62.BJR-2016-0225.R1

Update on the etiology of revision TKA -- Evident trends in a retrospective survey of 1449 cases

BACKGROUND

A working and complete knowledge of the different causes of dysfunction and pain after TKA is essential for the ability to correctly determine the cause of failure and to address this problem specifically. The purpose of this study was to update the etiology of implant failure. New diagnosis and current trends should be displayed.

METHODS

All TKA revisions performed in our institution between 2001 and 2010 were reviewed retrospectively. Patient demographics and the precise indication for the surgery were documented. Descriptive statistical analyses and association analyses of both the diagnosis and patient demographics were performed.

RESULTS

Within our collective of 1449 revision TKA a total of 40 different pathologies leading to revision surgery were identified and categorized. Overall 68.5% of the revisions were categorized aseptic, 31.5% as septic implant failure. Some recently debated diagnoses like low-grade-infection showed a high increase in incidence whereas classic failure mechanisms like polyethylene wear showed a decrease over the time.

CONCLUSION

We believe that this study successfully updates the current knowledge of different failure mechanisms in revision TKA, which have to be considered on dysfunction or pain after surgery. We were able to evaluate the clinical relevance of each pathology and could shift from implant related problems like wear to more surgical problems like instability and or malalignment over the last years. With a higher alertness to chronic low-grade-infections the incidence of infection is even increasing.

LEVEL OF EVIDENCE

Level II, economic and decision analysis.

Effect of the metastatic defect on the structural response and failure process of human vertebrae: an experimental study

BACKGROUND

Pathologic vertebral fractures are associated with intractable pain, loss of function and high morbidity in patients with metastatic spine disease. However, the failure mechanisms of vertebrae with lytic defects and the failed vertebrae's ability to retain load carrying capacity remain unclear.

METHODS

Eighteen human thoracic and lumbar vertebrae with simulated uncontained bone defects were tested under compression-bending loads to failure. Failure was defined as 50% reduction in vertebral body height. The vertebrae were allowed to recover under load and re-tested to failure using the initial criteria. Repeated measure ANOVA was used to test for changes in strength and stiffness parameters.

FINDINGS

Vertebral failure occurred via buckling and fracture of the cortex around the defect, followed by collapse of the defect region. Compared to the intact vertebrae, the failed vertebrae exhibited a significant loss in compressive strength (59%, p<0.001), stiffness (53%, p<0.05) and flexion (70%, p<0.01) strength. Significant reduction in anterior-posterior shear (strength (63%, p<0.01) and stiffness (67%, p<0.01)) and lateral bending strength (134%, p<0.05) were similarly recorded. In the intact vertebrae, apart from flexion strength (r(2)=0.63), both compressive and anterior-posterior shear strengths were weakly correlated with their stiffness parameters (r(2)=0.24 and r(2)=0.31). By contrast, in the failed vertebrae, these parameters were strongly correlated, (r(2)=0.91, r(2)=0.86, and r(2)=0.92, p<0.001 respectively).

INTERPRETATION

Failure of the vertebral cortex at the defect site dominated the initiation and progression of vertebral failure with the vertebrae failing via a consolidation process of the vertebral bone. Once failed, the vertebrae showed remarkable loss of load carrying capacity.