Computer-Aided Tissue Engineering In Whole Bone Replacement Treatment, 06/2005


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Presentation given to the American Society of Mechanical Engineers, June 2005

Tissue engineering is developing into a less speculative field involving the careful interplay of numerous design parameters and multi-disciplinary professionals. Problem solving abilities and state of the art research tools are required to develop solutions for a wide variety of clinical issues. One area of particular interest is orthopaedic biomechanics, a field that is responsible for the treatment of over 700,000 vertebral fractures in the U.S alone last year. Engineers are currently lacking the technology and knowledge required to govern the subsistence of cells in vivo, let alone the knowledge to create a functional tissue replacement for a whole organ. Despite this, advances in Computer Aided Tissue Engineering (CATE) are continually growing. Using a combinatory approach to scaffold design, patient-specific implants may be constructed. Computer aided design (CAD), optimization of geometry using voxel finite element models or other optimization routines, creation of a library of architectures with specific material properties, rapid prototyping, and determination of a defect site using imaging modalities highlight the current availability of design resources.
Our study represents a patient specific approach for constructing a complete vertebral body via building blocks. Though some of the methods described cannot be realized with current technology, namely complete construction of the vertebral body via FDM, the necessary advances are not far off. Computing power and CAD programs need to improve slightly to allow the rapid generation of complex models that would ease the fabrication of an appropriate number of building blocks. The main bottleneck of the process described in this study is the general lack of knowledge of human mechanobiology and the role of cellular interactions on artificial substrates including immune responses, and foreign body reactions. Assuming these biological parameters can be identified, a scaffold may be designed with a proper pore size and interconnectivity, microstructure, degradation rate, and surface chemistry. The advantage of the outlined process lies in adjustment of the vertebral compliance first, to ensure adequate load transfer, an important property for vertebral replacement. Subsequently, net biological properties can be fine tuned by simply scaling the final construct. Mixing and matching of geometries may be utilized to design asymmetric scaffolds, or scaffolds that exhibit a discontinuous microstructural stiffness with the goal of accentuating fluid flow. Finally, while these techniques lend themselves to the formulation of bone constructs, they can be used for other parts of the body as well that do not require load-bearing support.

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Computer-Aided Tissue Engineering In Whole Bone Replacement Treatment, 06/2005

  1. 1. Computer-aided Tissue Engineering In Whole Bone Replacement Treatment 1 Wettergreen MA, 1 Bucklen BS, 2 Sun W, 1 Liebschner MAK 1 Department of Bioengineering, Rice University, Houston, TX 2 Department of Mechanical Engineering, Drexel University, Philadelphia, PA
  2. 2. Overview <ul><li>Motivation / Design Goal </li></ul><ul><li>Tissue Imaging </li></ul><ul><li>Tissue Modeling </li></ul><ul><li>Implant Fabrication </li></ul>
  3. 3. Motivation / Design Goal <ul><li>700,000 vertebral fractures per year (as of 2003) </li></ul><ul><li>Research standard for defect repair is a randomly porous solid lacking defect demands in design or fabrication </li></ul><ul><ul><li>Poor mechanical strength matching to patient defect demands </li></ul></ul><ul><li>Computer Aided Tissue Engineering (CATE) employs a combinatory approach to scaffold design for patient-specific implants </li></ul><ul><li>Utilizes tissue modeling, tissue informatics, and </li></ul><ul><li>scaffold design / manufacturing </li></ul><ul><li>Design goal: Enact patient specific approach for constructing a complete vertebral body </li></ul>
  4. 4. Computer Aided Tissue Engineering Density~Material property assignment
  5. 5. <ul><li>QCT powerful for obtaining information about bone </li></ul><ul><ul><li>Spatial distribution of bone mineral density </li></ul></ul><ul><ul><li>Attainable resolution of 1.0 mm </li></ul></ul><ul><ul><li>Image density related to BMD via phantoms </li></ul></ul><ul><li>Extraction to geometric three-dimensional model is routine via numerous programs (i.e. Analyze, IDL) </li></ul><ul><ul><li>Reconstruction: raw projection data converted to 3-D data </li></ul></ul><ul><ul><li>Segmentation: surface geometry generated </li></ul></ul><ul><ul><li>Volume creation: volume data created from 3-D profiles and data </li></ul></ul>Tissue Imaging
  6. 6. Tissue Modeling <ul><li>Library generation via CAD </li></ul><ul><ul><li>Architecture generation </li></ul></ul><ul><ul><li>3x3x3 mm 3 volume </li></ul></ul><ul><ul><li>Feature size > 200 um </li></ul></ul><ul><li>Finite element analysis (FEA) of architecture </li></ul><ul><ul><li>Characterization for internal and apparent material properties </li></ul></ul><ul><ul><li>Force, stiffness, and stress-strain relations </li></ul></ul><ul><ul><li>Quantification of structural organization is material independent </li></ul></ul>
  7. 7. Implant Design and Manufacturing <ul><li>Library assembly requires marriage of mechanical properties and biological considerations </li></ul><ul><li>Assembly progresses in parallel and series based on regional properties </li></ul><ul><li>Shape matching and load transfer requires interface between parts </li></ul><ul><li>Boolean processes completes global contours </li></ul><ul><li>Conversion of final file for rapid prototyping processes </li></ul>
  8. 8. Implant Assembly <ul><li>Scaffold assembled based upon stiffness values derived from density </li></ul><ul><li>Four characterized architectures used in assembly and printed using Patternmaster, 3D Printer </li></ul>
  9. 9. Conclusions <ul><li>CAD library created for assembled scaffold with unique properties </li></ul><ul><li>Defect geometry and density values defined using QCT scan </li></ul><ul><li>Scaffold assembled based on mechanical and biological requirements </li></ul><ul><li>Implant printed with rapid prototyping based on CATE principles for human vertebral body </li></ul><ul><li>Demonstrated feasibility of creating patient-specific implant </li></ul><ul><li>Current limitations related to scale and computing capabilities </li></ul>
  10. 10. Future Directions <ul><li>Optimize architecture based on mechanotransduction principles </li></ul><ul><li>Build entire vertebral body replacement with biomaterial molding processes </li></ul><ul><li>Combine tissue derived primitives in scaffold design for added tissue property matching </li></ul>
  11. 11. Acknowledgements <ul><li>Kay Sun for QCT density </li></ul><ul><li>NSF-ITR as funding source </li></ul>