# Mechanical Properties and in Vivo Assessment of Electron Beam Melted Porous Structures for Orthopedic Applications

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## Abstract

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## 1. Introduction

## 2. Materials and Methods

#### 2.1. Materials

#### 2.2. Design and Fabrication of Porous Scaffolds

#### 2.3. Measurement and Characterization

#### 2.4. Mechanical Testing

^{2}. Specimens were abraded repeatedly and cleaned ultrasonically for a set number of rotational cycles. The specimens were weighed after each cleaning, and the mass loss was the measure of abrasive wear to the specimen. Three replications were tested for each design.

#### 2.5. In Vivo Assessment

## 3. Results

#### 3.1. Morphological Characterization

#### 3.2. Mechanical Properties

#### 3.3. In Vivo Assessment of Porous Structures

## 4. Discussion

#### 4.1. The Effect of Size and Boundary Conditions on the Mechanical Properties

#### 4.2. The Effect of Porous Geometry on the In Vivo Performances

- (a)
- Radially oriented pore geometry. A radially oriented pore can integrate the surrounding tissue better than a random pore. Additionally, at the same time, cells can migrate deeper into the porous structures, so capillaries can grow deeper with less barrier. Moreover, a mixed tissue of fiber and fibrocartilage can be formed in porous structures with radially oriented pores, while only fibrous tissue can be formed in porous structures with irregular pores. Generally, the radially oriented pores can facilitate cell ingrowth, longitudinal alignment of cells, and integration with the surrounding tissue, and may be suitable for in vivo applications [41]. Similar findings were also discussed in the research of Matsugaki [42] and Ishimoto [43]. They designed a honeycomb tree structure with through-pores and a grooved substrate for the spinal cages. Such a grooved through-pore structure was similar to the radially oriented pore geometry in our study. It was found in their research that such a through-pore honeycomb tree structure provided a direct scaffold that guided the bone matrix in its collagen and apatite orientation. Besides, such a design also exhibited greater strength at the bone interface compared with that of conventional and gold-standard box-type designs with autologous iliac bone grafts [42,43]. These findings can also prove the advantages of radially oriented pore geometry on bone ingrowth.
- (b)
- Homogenous pore size distribution. Porous structures with a high number of homogenous pore sizes allow faster colonization. Heterogeneous pore size distribution may also allow cell colonization, although the higher proportion of low pore size may reduce the diffusion of nutrients, oxygen, and cellular waste [44].

## 5. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 2.**Schematic image of stochastic porous structure modeling of (

**a**) the Voronoi structure and (

**b**) the randomized structure; and the calculation method of the pore size for the porous structures (

**c**).

**Figure 3.**Fabricated porous samples for mechanical properties and the corresponding testing devices: (

**a**) tensile testing; (

**b**) shear testing; (

**c**) abrasion resistance testing. The porous implant preparation (

**d**–

**f**) and the implantation of the porous implant (

**g**–

**i**): (

**d**) acetabular cup with the designed porous geometry; (

**e**) acetabular cup after cutting; (

**f**) the sterilized porous implant; (

**g**) the femur model of beagle dog; (

**h**) schematic image of the implantation; (

**i**) the porous implant in the dog femur.

**Figure 4.**μCT feathers of porous structures: porosity calculation (

**a**), pore size calculation (

**b**), and strut thickness (

**c**) of the Voronoi structure; porosity calculation (

**d**), pore size calculation (

**e**), and strut thickness (

**f**) of the randomized structure.

**Figure 5.**Comparisons of the main feathers of porous structures: (

**a**) strut thickness of two structures; (

**b**) pore size of two structures; (

**c**) porosity of two structures.

**Figure 6.**Gross samples at each follow-up time point for the two porous designs (the black arrow indicates the implant location).

**Figure 7.**Micro-CT of the Voronoi structure group and the randomized structure group at each follow-up time point.

**Figure 8.**Pathological observation of the Voronoi structure group and the randomized structure group at each follow-up time point (the yellow box indicates the porous area of the implant).

**Figure 9.**(

**a**) Loading conditions of porous structures; (

**b**) FEA results for the Voronoi structure; (

**c**) FEA results for the randomized structure. (The red circles indicate the maximum stress locations in porous structures).

**Figure 10.**(

**a**) Fabricated porous samples with different heights; (

**b**) loading conditions during the compressive test; (

**c**) strain–stress curves for Voronoi structures; (

**d**) strain–stress curves for randomized structures.

**Figure 11.**Relationships between the mechanical properties and the sample height: (

**a**) compressive strength; (

**b**) Young’s modulus.

**Figure 12.**Designed and fabricated pore geometry of the two porous designs: (

**a**) fabricated Voronoi structure; (

**b**) designed Voronoi structure; (

**c**) bone growth direction of Voronoi structure; (

**d**) fabricated randomized structure; (

**e**) designed randomized structure; (

**f**) bone growth direction of randomized structure.

Al | Fe | V | C | N | H | O | Ti |
---|---|---|---|---|---|---|---|

6.08 | 0.10 | 3.98 | 0.012 | 0.0069 | 0.0018 | 0.105 | Bal. |

Design | Abrasive Mass (mg) | Tensile Strength (MPa) | Shear Strength (MPa) |
---|---|---|---|

Voronoi structure | 3.83 ± 2.89 | 169.29 ± 7.06 | 86.08 ± 2.65 |

Randomized dodecahedron | 23.23 ± 3.46 | 62.72 ± 6.87 | 53.32 ± 3.87 |

Design | Month 1 | Month 3 | Month 6 | ||||||
---|---|---|---|---|---|---|---|---|---|

Dog #1 | Dog #2 | Dog #3 | Dog #4 | Dog #5 | Dog #6 | Dog #7 | Dog #8 | Dog #9 | |

Voronoi structure | 10.54 | 19.31 | 2.47 | 14.65 | 11.3 | 12.85 | 8.38 | 6.03 | 7.56 |

Randomized structure | 5.58 | 7.93 | 2.17 | 6.97 | 8.3 | 7.2 | 0.05 | 5.94 | 9.81 |

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## Share and Cite

**MDPI and ACS Style**

Wu, Y.; Wang, Y.; Liu, M.; Shi, D.; Hu, N.; Feng, W.
Mechanical Properties and in Vivo Assessment of Electron Beam Melted Porous Structures for Orthopedic Applications. *Metals* **2023**, *13*, 1034.
https://doi.org/10.3390/met13061034

**AMA Style**

Wu Y, Wang Y, Liu M, Shi D, Hu N, Feng W.
Mechanical Properties and in Vivo Assessment of Electron Beam Melted Porous Structures for Orthopedic Applications. *Metals*. 2023; 13(6):1034.
https://doi.org/10.3390/met13061034

**Chicago/Turabian Style**

Wu, Yan, Yudong Wang, Mengxing Liu, Dufang Shi, Nan Hu, and Wei Feng.
2023. "Mechanical Properties and in Vivo Assessment of Electron Beam Melted Porous Structures for Orthopedic Applications" *Metals* 13, no. 6: 1034.
https://doi.org/10.3390/met13061034