Purpose
To develop a novel biocompatible solid fiducial marker that prevents radiopaque imaging artifacts and also maintains high imaging contrast for kilovoltage x‐ray image‐guided radiation ...therapy.
Methods
The fiducial marker was made of pure zinc. An in‐house water‐equivalent phantom was designed to evaluate artifacts and visibility under various simulated treatment scenarios. Image artifacts were quantitatively assessed in terms of the metal artifact index (MAI) on kilovoltage computed tomography (CT) and cone‐beam CT (CBCT) scans. Marker visibility was evaluated on two types of kilovoltage planar x‐ray images in terms of the contrast‐to‐background ratio (CBR). Comparisons with a conventional gold fiducial marker were conducted.
Results
The use of zinc rather than a gold marker mitigates imaging artifacts. The MAI near the zinc marker decreased by 76, 79, and 77 % in CT, and by 77 (81), 74 (80), and 79 (85) % in CBCT full‐fan (half‐fan) scans, when using one‐, two‐, and three‐marker phantom settings, respectively. The high‐contrast part of the zinc marker exhibited CBRs above 2.00 for 28/32 exposures under four (lung, tissue, low‐density bone, and high‐density bone) different simulation scenarios, making its visibility comparable to that of the gold marker (30/32 exposures with CBRs > 2.00).
Conclusions
We developed a biocompatible, artifact‐robust, and highly visible solid zinc fiducial marker. Although further evaluation is needed in clinical settings, our findings suggest its feasibility and benefits for kilovoltage x‐ray image‐guided radiation therapy.
The fracture toughness in extruded Mg–Zn binary alloys with the same grain size and texture increased with higher solute concentration and was proportional to the one-third power of the concentration ...of zinc. This toughening was related to solid-solution strengthening.
Calcium phosphate precipitated on pure magnesium from artificial plasma (modified Hanks’ solution) was varied by anodization and autoclaving, aiming the control of corrosion rate of bioabsorbable ...magnesium. Rough and smooth anodized film was formed depending on anodizing voltage in 1
N NaOH. The amount of calcium phosphate precipitated on the porous film was 2–3 times larger than that on the smooth film. The Ca/P ratio on the porous film was slightly higher than that on the smooth film. The autoclaving did not significantly influence the morphology of anodized film; however, the precipitation of calcium phosphate was restricted. No significant local corrosion occurred after the immersion in modified Hanks’ solution. It is demonstrated that the precipitation of calcium phosphate on magnesium can be controlled by anodization and autoclaving.
The fracture toughness in Mg–Al–Zn alloy was improved by the equal-channel-angular extrusion process. The improvement was the result of a large plastic zone size, which was sensitive to ...elongation-to-failure and a high strain hardening exponent, created ahead of the fracture pre-crack, because of the modified distribution of the basal texture.
Transmission electron microscopic study of crack propagation in a fine-grained Mg-Zn alloy shows a complex interplay of slip and twinning, leading to formation of twin-related nano-sized domains of ...50 nm size related by {1 0
1} and {1 0
2} twin orientations. Subgrain formation with low angle boundaries is followed by formation of {1 0
1} twins and twin multiplication by nucleation and growth. Basal and prismatic slip play an important role in the growth and formation of new domains of twins and boundaries. These basal and prismatic slip occur in the matrix as well as inside the {1 0
1} twins. Steps on these twin boundaries indicate interaction with slip, leading to their growth. Evidence of nucleation of twins on grain boundaries is found. Most interesting is the indication of nucleation of {1 0
1}-type twins. String or stacks of nano-sized domains related to each other by {1 0
1} twinning were found in the matrix, where the matrix contained basal, prismatic as well as pyramidal dislocations. It appears that a localization of slip or formation of low angle boundaries to accommodate stress from surrounding domains leads to nucleation of these twins.
The effect of texture on fracture toughness was investigated for a wrought AZ31 magnesium alloy, which was a commercial rolled plate having strong basal texture. The value of plane-strain fracture ...toughness,
K
IC
=
17.6–20.7
MPam
1/2, was obtained from the stretched zone (SZ) analysis. The value of
K
IC varied with the distribution of basal texture; the sample having a pre-crack normal to rolled direction was the highest value of
K
IC in all present samples. The crack-tip having parallel to the rolled direction was easily able to propagate and/or proceed with applied load.
The effects of microstructural factors, i.e., grain size and texture, on the deformation behavior near the crack-tip region during the fracture toughness test were investigated using wrought Mg–3
...wt.%Al–1
wt.%Zn alloys, which were produced by extrusion or equal-channel-angular extrusion (ECAE). The stress distributions related to deformation twins were predicted by finite element analysis (FEA), and the microstructural evolutions were confirmed by experiment. Deformation twins were formed at the beginning of the fracture toughness test due to the creation of a large stress field near the crack-tip region. The formation region of deformation twins became small with grain refinement due to the changes in the dominant plastic deformation mechanisms. The results from microstructure observations showed similarities to the stress distribution, which used simple stress–strain relations that assumed a mechanical asymmetry following the Hill's potential function. The texture, i.e., basal plane distribution, also affected the formation of deformation twins. The formation region of deformation twins in the ECAE-ed alloy was predicted to be smaller than that in the extruded alloy by FEA, and this tendency was very similar to that in the microstructural observations of the deformed samples. These results can be used to predict the macroscopic deformation features near the crack-tip, and could be helpful for developing magnesium alloys with high performance mechanical properties.
Purpose
The optimal hinge position to prevent hinge fractures in medial closing wedge distal femoral osteotomy (MCWDFO) based on the biomechanical background has not yet been well examined. This ...study aimed to examine the appropriate hinge position in MCWDFO using finite element (FE) analysis to prevent hinge fractures.
Methods
Computer‐aided design (CAD) models were created using composite replicate femurs. FE models of the MCWDFO with a 5° wedge were created with three different hinge positions: (A) 5 mm proximal to the proximal margin of the lateral epicondylar region, (B) proximal margin level and (C) 5 mm distal to the proximal margin level. The maximum and minimum principal strains in the cortical bone were calculated for each model. To validate the FE analysis, biomechanical tests were performed using composite replicate femurs with the same hinge position models as those in the FE analysis.
Results
In the FE analysis, the maximum principal strains were in the order of Models A > B > C. The highest value of maximum principal strain was observed in the area proximal to the hinge. In the biomechanical test, hinge fractures occurred in the area proximal to the hinge in Models A and B, whereas the gap closed completely without hinge fractures in Model C. Fractures occurred in an area similar to where the highest maximal principal strain was observed in the FE analysis.
Conclusion
Distal to the proximal margin of the lateral epicondylar region is an appropriate hinge position in MCWDFO to prevent hinge fractures.
Level of Evidence
Level V.
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