Tissue Mechanics – Bone
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Bone is the primary structural element of the body
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Self-repairing
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Alters properties and geometry in response to mechanical
demands
Simpler structure (whole bone properties) makes it better
known in relation to its (bone tissue) material properties
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Long (major bones of arms and legs)
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Short (phalanges, metacarpals and metatarsals)
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Both long & short bones are characterized by tubular shafts
and articular surfaces at each end
Flat (ribs, sternum)
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Thin bones with a broad surface
Irregular (vertebral column and patella)
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Generally compact, variable shape and size
Microscopic Bone : Woven vs. Lamellar
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Woven = immature
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Primary, coarse collagen fibers with non-uniform orientation
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Actively resorbed by 1 year
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Isotropic Mechanical Characteristics
Lamellar results from remodeling of woven bone
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Highly organized stress-oriented collagen, anisotropic
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Organized into layers called lamellae
Bone Tissue : Cortical
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Cortical = dense, compact
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4x mass of trabecular bone, lower
turnover
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Subjected to bending; exhibits
torsion & compressive strength
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Type I compact bone (layers of
lamellae, small animals)
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Type II plexiform bone (layers of
lamellar & woven, large animals)
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Type III Haversian (vascular
channels surrounded by lamellae)
Haversian System is comprised of a
Haversian canal, concentric lamellae,
lacunae with osteocytes and canaliculi.
Separate Haversian systems are
joined to each other by means of
interstitial lamellae.
Bone Tissue: Trabecular
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Trabecular = spongy, cancellous
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Internal beams form 3D lattice aligned along applications of
stress, exhibits mostly compressive strength
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Lamellae are not arranged in layers, interstitial
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Spaces between trabeculae contain marrow and blood vessels
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Lacunae house osteocytes
Bone Cells
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Osteoblasts – produces type I collagen
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Lie in initial, nonmineralized bone
Osteocytes – mature osteoblasts in lacunae surrounded by
mineralized matrix
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Most numerous
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Communicate strain/stress signals
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Regulate overall metabolism of bone
Osteoclasts – major resorptive cells
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Located on bone surface
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Bind to bone surface, lower pH of local environment thru H2 ion
increases solubility of local HA
Bone Structure
Biomechanical Behavior of Bone
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Cortical Bone
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Elastic Behavior – typically treated as transversely isotropic
(compression > tension)
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Viscoelastic Behavior – strain rate sensitivity
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Typical daily activities, strain rate < 0.03%/sec
Trauma = 0.10%/sec
Trabecular Bone
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Elastic Behavior – depends on anatomic site
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Age, connectivity
Viscoelastic Behavior – strain rate sensitive, age effects
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Modulus and strength weakly dependent upon strain rate