- Senior Design Spring 2004
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Stevens Institute of Technology
- Mechanical and Biomedical Engineering Departments
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The composition of bone is roughly estimated to be one-third apatite (Ap), a crystalline carbonate substance similar to naturally occurring minerals, one-third collagen and other organic substances, and one-third water. Although the bone is a viscoelastic material, which means that it has viscous as well as elastic properties, experimental analysis of bone using quasi-static mechanical testing that assumes the bone to have anisotropic properties has enabled reasonable estimates for quantifying the mechanical properties of the composite. A series of mechanical tests have been performed on trabecular bone and have revealed its Young’s Modulus, which will be pertinent information for the design of the distraction device. Through the process of nanoindentation, the Young’s modulus of bone has been found to be 19.4 GPa longitudinally and 15.0 GPa transversely.[1]
When a bone heals, collagen fibers first grow in the blood clot between the broken ends of the bone, forming a web. Then, mineralization, or ossification, of the bone occurs, causing hardening of the collagen web. Distraction osteogenesis is possible because as the collagen fibers begin to harden, the fibrous layer is able to stretch and widen, so as older fibers covert into bone, the newer fibers are able to stretch if tension is gradually applied to the broken bone.[2]
Normal Bone[3] Distraction Regenerate[4]
As a rough representation of the strain, placed on the expanding tissue, the strain on one of the tissue’s key components, collagen, will be analyzed. The strain can be determined from the initial length of the distraction gap (the space needed for the distraction device), L0, and the change in length, ΔL, this value is 1mm. Young’s Modulus for collagen is found to be approximately 1010 dynes/cm2, or 1000 MPa, and after about 3-4 % strain, the collagen no longer acts elastically; it only deforms plastically.[5] Because the strain acting on the soft tissue in the distraction gap, during distraction osteogenesis, is much greater than this value (it will be estimated to be greater than 50 percent), the collagen will plastically deform at the projected value of approximately 1 mm per day. It can be proven mechanically that this soft tissue containing collagen will also stiffen, as the Young’s Modulus for bone (~10 – 20 GPa) is greater than the Young’s Modulus for collagen.
[1] Katz, J.L. Ed. Joseph D. Bronzino. “Mechanics of Hard Tissue.” The Biomedical Engineering Handbook: Second Edition. Boca Raton: CRC Press LLC, 2000.
[2] Sokolow, Stanley. “Distraction Osteogenesis: A New Dimension in Orthodontics and Dentofacial Orthopedics.” http://home.earthlink.net/~overbyte/Distraction.htm
[3] “Biological Foundation: Distraction Regenerate” http://www.globalmednet.com/do-cdrom/Biol/Histomor/rh07.htm
[4] “Biological Foundation: Distraction Regenerate” http://www.globalmednet.com/do-cdrom/Biol/Histomor/rh07.htm
[5] “Lecture 7: Cells under stress, strain, pressure, and flow fields, Part 1,” http://www.bme.jhu.edu/courses/580.637/lectures/lecture7.pdf
| Limb lengthening |
Distraction osteogenesis is used for many cases where patients have a limb-length discrepancy, or an abnormally short stature.
| Craniofacial deformities |
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