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TECHNICAL PAPERS: Cell

Cyclic Mechanical Compression Increases Mineralization of Cell-Seeded Polymer Scaffolds In Vivo

[+] Author and Article Information
Angel O. Duty

Biomedical Engineering Department, Georgia Institute of Technology, IBB Room 2414, 315 Ferst Drive NW, Atlanta, GA 30332abcduty@yahoo.com

Megan E. Oest

Biomedical Engineering Department, Georgia Institute of Technology, IBB Room 2414, 315 Ferst Drive NW, Atlanta, GA 30332gtg996c@prism.gatech.edu

Robert E. Guldberg1

Woodruff School of Mechanical Engineering, Georgia Institute of Technology, IBB Room 2311, 315 Ferst Drive NW, Atlanta, GA 30332-0405robert.guldberg@me.gatech.edu

1

Corresponding author.

J Biomech Eng 129(4), 531-539 (Jan 07, 2007) (9 pages) doi:10.1115/1.2746375 History: Received August 13, 2006; Revised January 07, 2007

Despite considerable documentation of the ability of normal bone to adapt to its mechanical environment, very little is known about the response of bone grafts or their substitutes to mechanical loading even though many bone defects are located in load-bearing sites. The goal of this research was to quantify the effects of controlled in vivo mechanical stimulation on the mineralization of a tissue-engineered bone replacement and identify the tissue level stresses and strains associated with the applied loading. A novel subcutaneous implant system was designed capable of intermittent cyclic compression of tissue-engineered constructs in vivo. Mesenchymal stem cell-seeded polymeric scaffolds with 8 weeks of in vitro preculture were placed within the loading system and implanted subcutaneously in male Fisher rats. Constructs were subjected to 2 weeks of loading (3 treatments per week for 30min each, 13.3N at 1Hz) and harvested after 6 weeks of in vivo growth for histological examination and quantification of mineral content. Mineralization significantly increased by approximately threefold in the loaded constructs. The finite element method was used to predict tissue level stresses and strains within the construct resulting from the applied in vivo load. The largest principal strains in the polymer were distributed about a modal value of 0.24% with strains in the interstitial space being about five times greater. Von Mises stresses in the polymer were distributed about a modal value of 1.6MPa, while stresses in the interstitial tissue were about three orders of magnitude smaller. This research demonstrates the ability of controlled in vivo mechanical stimulation to enhance mineralized matrix production on a polymeric scaffold seeded with osteogenic cells and suggests that interactions with the local mechanical environment should be considered in the design of constructs for functional bone repair.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Subcutaneous mechanical loading device consisting of two sample chambers on either end and a central manifold with an angled barbed connector to accept polyurethane tubing from the external loading system

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Figure 2

Exploded view of chamber assembly. The platen rests against the distal end of the chamber with the sample centered with respect to the vascularization ports. Actuation of the piston and O-ring sealing the medial end of the chamber results in sample deformation.

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Figure 3

Von Kossa staining of PLDL cultured in osteogenic media for 8 weeks with MSCs; 10×(A) and 40×(B)

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Figure 4

Cuboidal shaped cells lining the scaffold pore in a precultured construct after 6 weeks in vivo without mechanical loading; H&E 40×

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Figure 5

Examples of positive Von Kossa staining (black) both on a polymer surface and within the pore space for a precultured construct after 6 weeks in vivo with mechanical loading; 10×(A) and 40×(B)

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Figure 6

Mineralization after 6 weeks in vivo with or without 2 weeks loading of PLDL constructs; samples were prepared with 8 weeks preculture or scaffold-only (mean + standard error, 5≤n≤10); * (p<0.05) denotes that mineral volume was significantly affected by both the implantation of cells and application of mechanical load as determined by ANOVA

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Figure 7

Representative micro-CT images of mineral formed on PLDL in the scaffold-only (A & B) and 8 week preculture group (C & D) without in vivo loading (A & C) and with in vivo loading (B & D)

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Figure 8

Histogram of the largest principal strain within the polymeric structure for Each of the three PLDL samples modeled; counts represents the number of finite elements having that value

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Figure 9

Histogram of the Von Mises stress (MPa) within the polymeric structure for each of the three PLDL samples modeled; counts represents the number of finite elements having that value

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Figure 10

Longitudinal cross section (central 300 microns) of the finite element model coded for the tissue level (A) largest principal strain and (B) von mises stress within the polymeric structure

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