University of Scranton - Department of Physical Therapy
Edmund M. Kosmahl, PT, EdD
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Dr. Kosmahl
copyright 2000 Edmund M. Kosmahl
MECHANICAL PROPERTIES OF MUSCULOSKELETAL TISSUES
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Objectives:
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Define the terms listed under the section labeled
"terminology".
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Explain the material strength properties
of bone, articular cartilage, and collagenous tissues.
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Explain the effects of activity and disuse
on bone, articular cartilage, and collagenous tissues.
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Explain the effect of rate of loading
on bone, articular cartilage, and collagenous tissues.
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Explain the effect of inflammation and repair
on collagenous tissues.
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TERMINOLOGY
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Force - a push or a pull.
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Load - same as force.
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Material strength - The types and amounts of
forces a material (tissue) can withstand before it fails (breaks).
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Stress - same as force.
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Strain - deformation in response to stress.
See
Stress-Strain Curve Picture - 14K
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Deformation - a change in the shape of a material
(tissue).
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Plastic - a permanent change.
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Elastic - a temporary change.
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Tension - a force that
pulls apart.
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Compression - a force that pushes together.
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Shear - The tendency of opposing but cantilevered
forces to tear apart a material (tissue).
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Torsion - twisting forces.
See
Force Types Picture - 29K
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Creep - a slow deformation.
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Fatigue - failure (breaking) of a material (tissue).
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BONE
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Resists compression
- a function of inorganic salts, trabeculae, compact bone
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25,000 lbs/in2 compressive strength (compare
to oak 12,000 lbs/in2)
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resists tension, a function of collagen matrix
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17,000 lbs/in2 tensile strength (compare to granite
15,000 lbs/in2)
See
Bone Compression-Tension Picture - 45K
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resists shear
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somewhat elastic
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Rate of loading
- stress bone can withstand is inversely proportional to time of application
of force (a large force may not cause damage if quickly withdrawn)remodels
according to Wolff's law
See
Rate of Loading Bone Picture - 41K
ARTICULAR CARTILAGE
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Dissipates loads applied to joints
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Minimizes friction
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Tensile strength 5% of bone
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Well suited for compressive loading (as long
as hydrophilic nature is intact)
RATE OF LOADING
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Rapid elastic response (i.e. it "bounces back")
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Slow plastic response (i.e. it deforms)
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Increased thickness
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Increased elasticity
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Repetitive stress = fatigue failure
EFFECTS OF DISUSE (inactivity or immobilization)
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Matrix fibrillation (the intercellular matrix
or "binding factor" falls apart)
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Cleft formation
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Ulceration
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COLLAGENOUS
TISSUES (TENDON, LIGAMENT, SKIN)
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Resist tension (no compressive strength)
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Collagen fiber orientation influences the direction
of tensile strength
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Collagen fiber orientation influenced by loads
(movement) applied during remodeling
See
Fiber vs Tensile Strength Picture - 61K
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FAILURE WITH LOADING
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Fast loading = higher stress, ligament failure
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Slow loading = avulsion fracture
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Prolonged loading = creep
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EFFECTS OF ACTIVITY
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Increased size and amount of collagen fibers
(increased strength)
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Orientation of fibers along lines of tensile
stress
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EFFECTS OF DISUSE (inactivity or immobilization)
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Decreased size and amount of collagen fibers
(decreased strength)
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Random orientation of fibers during remodeling
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EFFECTS
OF INFLAMMATION AND REPAIR
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Rapid turnover of collagen (remodeling)
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Random fiber orientation if no tensile loads
applied during healing
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Inelastic scar
See
Random Orientation Picture - 87K
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