Scaffold Optimization For Load-Bearing Applications In Orthopaedics, 4/2003

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Information about Scaffold Optimization For Load-Bearing Applications In Orthopaedics, 4/2003

Published on September 3, 2008

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Poster presented at the Houston Society for Engineering Medicine and Biology, Houston TX, April 2003

Scaffold Optimization For Load-Bearing Applications In Orthopaedics
Matthew Wettergreen, Michael A.K. Liebschner
Department of Bioengineering, Rice University, Houston, TX

INTRODUCTION: Design of a scaffold that provides mechanical strength while housing cells and growth factors is an important step to creating a physiologically sound construct. While characteristics such as elastic modulus and degradation time are material properties, architecture is a structural property and reflects the internal distribution and orientation of the scaffold material, which may play a large role in scaffold viability. It has been shown previously that cells plated on a three-dimensionally modified plate had a higher proliferative capability and gene expression compared to cells plated on a two-dimensional cell plate [1], indicating the importance of a regulated architecture in overall cellular health. Past attempts at the creation of specific micro-architecture have been relatively unsuccessful due to technological limitations. With recent advents in rapid prototyping technology, scaffolds with a regular hierarchal structure with unit sizes approximating trabecular bone pores are easily fabricated in a wide array of materials. The purpose of this study was to investigate scaffold architectures that could be fabricated with rapid prototyping and its local mechanical environment evaluated using finite element modeling.

MATERIALS AND METHODS: We designed several scaffold architectures consisting of single repeated shapes based on Archmidean and Platonic solids, including the truncated hexahedron, truncated octahedron, and rhombitruncated cuboctahedron. To evaluate the mechanical properties of these designs, we assumed a linear relationship between the properties of a single repeated unit and the entire construct. Wireframe models of the solids were generated using IronCAD (IronCAD, Atlanta, Georgia) CAD software. The shapes were modified to obtain porosity similar to normal trabecular bone (~91.5% porosity) [2]. The files were imported as .igs files into ALGOR (ALGOR Inc., Pittsburgh, PA), which was used to generate the volume mesh. All finite element analysis was completed using ALGOR for the geometric shapes. Peak stress, apparent modulus, and construct strength were evaluated.

RESULTS AND CONCLUSION: Finite element models of basic geometric constructs were successfully created using basic geometric models as foundation for the models. Three models were created: truncated octahedron, truncated hexahedron, rhombitruncated cubocatahedron. The porosity of the architecture was adjusted to match that of the average value for trabecular bone, (~91.5% porosity) obtained from the literature [2]. The shapes were chosen as they represent a micro-architecture with relatively high surface to volume ratio and a strut thickness close to the thickness of trabeculae. Current believe is that those factors most influence tissue growth on a 3-D scaffold.
The design of regulated architecture allows for the tailoring of scaffold mechanical properties, indicating a use for these scaffolds for replacing pathologic or osteoporotic bone. Current advances in rapid prototyping have paved the way to the creation of these scaffolds for growing cells inside them. Future work will include the growth of osteogenic cells in these scaffolds to evaluate tissue growth and architecture to modify mechanical properties and biological factors. The culturing of these cells will increase our knowledge of cell-environment interactions, which are extremely important in bone adaptation.

ACKNOWLEDGEMENT: Funding provided by the Texas ATP Grant Program

REFERENCE: [1]Snyder, JD and Desai, TA. “Microscale three-dimensional polymeric platforms for in vitro cell culture systems.” J Biomater Sci Polym Ed, 2001; 12 (8), pp. 921-32. [2] Cowin, Stephen C. Ed. Bo

INTRODUCTION The design of a scaffold that provides mechanical strength while housing cells and growth factors is an important step to repairing damaged tissue. While material properties are important, architecture, a structural property, reflects the internal distribution and orientation of the scaffold material. This geometric arrangement may directly relate to physiological function. Previous studies (1) have indicated the importance of a patterned micro-architecture in three-dimensional surfaces for cell growth; however, past attempts at the creation of specific micro-architectures have been unsuccessful due to technological limitations. With rapid prototyping technology, scaffolds of hierarchical structure with unit sizes approximating trabecular bone pores can be easily fabricated. Here we illustrate a design process to generate scaffolds with complex internal architecture. Using a combinatory approach of imaging techniques, computer simulation and rapid prototyping, a scaffold that is optimized on several hierarchical levels can be built. CONCLUSIONS It is possible to tailor the mechanical properties, such as stiffness and strength for the purpose of replacing trabecular bone at various anatomical sites. Strength, is greatly improved in all engineered architectures, providing greater mechanical integrity to the bone structure as a whole. Stiffness is significantly higher in all engineered architectures compared to that of trabecular bone, raising the fracture probability of trabecular bone at adjacent sites. Through engineered architecture, it is possible to increase the overall strength of the construct with a minimal increase in the stiffness as seen in the rhombitruncated cuboctahedron. Using a combinatory approach to scaffold design can result in optimized structural properties while maintaining physiologically important parameters. Table 1. Results of Finite Element Analysis of CAD generated polyhedron to mimic bone Figure 2. HVTB was scanned with uCT 80 at resolution of 10um. Slices were reconstructed using Analyze (Analyze Direct, Lenexa, KS) and imported into Rhino 3D CAD software (McNeel, Seattle, WA). Polyhedra approximating the architecture of trabecular bone were generated and evaluated for apparent properties using ALGOR Finite Element Software. Human Vertebral Trabecular Bone built with FDM Figure 5. A. Scaffold of the TMJ disc with complex architecture generated using FEA and CAD, mimicking apparent mechanical properties of in vivo disc. B. scaffold generated using FDM and destructive injection molding. A Material Reservoir Fused Deposition Modeling Figure 4. Examples of Rapid prototyping for use in scaffold generation and building replicas using using Fused Deposition Modeling (FDM). Examples of novel architectures Piezo ACKNOWLEDGEMENTS The authors thank Mark Timmer for his generous help in the fabrication of scaffolds. This project was supported by the Texas ATP Grant Program. REFERENCES (1) Snyder JD, Desai TA. J. Biomater. Sci. Polym. Ed., 2001; 12(8): pp. 921-32. Human Vertebral Trabecular bone core μ CT Assembly Figure 3. Architectures generated with CAD processes to mimic trabecular bone architecture Truncated Hexahedron Porosity – 91.5% Surface Area – 23.185 #Elements – 22651 Rhombitruncated Cuboctahedron Porosity – 91.4% Surface Area – 64.465 #Elements – 35326 Truncated Octahedron Porosity – 91.55% Surface Area – 32.2327 #Elements – 30399 Cancellous Bone Porosity – 91.5% Surface Area – 182.3 #Elements – 246720 Figure 1. Scaffold Design Process Human Vertebral Trabecular Bone μ Computed Tomography Computed Aided Design Finite Element Analysis Rapid Prototyping Scaffold Generation Bioreactor Culturing Figure 6. Bioreactor design for producing a mechanical load in cultured scaffolds. Piston at top of culture chamber is able to deliver a regulated high frequency load. The system is also capable of generating high levels of fluid shear stress. Piston driven compression Perfusion Pump Culture media Gas supply Reservoir Heater In-Line Heater Culture Chamber Waste  DAQ 1.5 mm B Z y Scaffold Optimization for Load Bearing Applications in Orthopaedics Wettergreen MA, White JT, Bucklen B, Lemoine JJ, Mikos AG, Liebschner MAK Department of Bioengineering, Rice University; Houston, TX

CONCLUSIONS

It is possible to tailor the mechanical properties, such as stiffness and strength for the purpose of replacing trabecular bone at various anatomical sites.

Strength, is greatly improved in all engineered architectures, providing greater mechanical integrity to the bone structure as a whole.

Stiffness is significantly higher in all engineered architectures compared to that of trabecular bone, raising the fracture probability of trabecular bone at adjacent sites.

Through engineered architecture, it is possible to increase the overall strength of the construct with a minimal increase in the stiffness as seen in the rhombitruncated cuboctahedron.

Using a combinatory approach to scaffold design can result in optimized structural properties while maintaining physiologically important parameters.

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