Tissue Mechanics Bone Bone is the primary structural element of the body

Self-repairing 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 Long (major bones of arms and legs) Short (phalanges, metacarpals and metatarsals)

Both long & short bones are characterized by tubular shafts and articular surfaces at each end

Flat (ribs, sternum) Thin bones with a broad surface

Irregular (vertebral column and patella) Generally compact, variable shape and size

Microscopic Bone : Woven vs. Lamellar Woven = immature

Primary, coarse collagen fibers with non-uniform orientation Actively resorbed by 1 year Isotropic Mechanical Characteristics

Lamellar results from remodeling of woven bone Highly organized stress-oriented collagen, anisotropic Organized into layers called lamellae

Bone Tissue : Cortical Cortical = dense, compact

4x mass of trabecular bone, lower turnover

Subjected to bending; exhibits torsion & compressive strength

Type I compact bone (layers of lamellae, small animals)

Type II plexiform bone (layers of lamellar & woven, large animals)

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 Trabecular = spongy, cancellous

Internal beams form 3D lattice aligned along applications of stress, exhibits mostly compressive strength

Lamellae are not arranged in layers, interstitial Spaces between trabeculae contain marrow and blood vessels Lacunae house osteocytes

Bone Cells Osteoblasts produces type I collagen

Lie in initial, nonmineralized bone Osteocytes mature osteoblasts in lacunae surrounded by

mineralized matrix Most numerous Communicate strain/stress signals Regulate overall metabolism of bone

Osteoclasts major resorptive cells Located on bone surface Bind to bone surface, lower pH of local environment thru H2 ion

increases solubility of local HA

Bone Structure

Biomechanical Behavior of Bone Cortical Bone

Elastic Behavior typically treated as transversely isotropic (compression > tension)

Viscoelastic Behavior strain rate sensitivity Typical daily activities, strain rate < 0.03%/sec Trauma = 0.10%/sec

Trabecular Bone Elastic Behavior depends on anatomic site

Age, connectivity Viscoelastic Behavior strain rate sensitive, age effects

Modulus and strength weakly dependent upon strain rate