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Bone cells
Osteoblasts – bone formation
Mesenchymal precursor
Osteoclasts – bone resorption
Monocytic precursor
Osteocytes – ‘spent’ osteoblasts
Obscure function - ?mechanosensory
Maybe facilitate bone resorp and ca+ transport
Acts as connectors between all bone cells
Mechanical properties of bone
8
Relatively hard;
Lightweight;
Composite material;
high compressive strength of about
170 MPa (1800 kgf/cm²);
poor tensile strength of 104–121 MPa;
very low shear stress strength (51.6 MPa).
Basic Biomechanics
Material Properties
Elastic-Plastic
Yield point
Brittle-Ductile
Toughness
Independent of
Shape!
Structural Properties
Bending Stiffness
Torsional Stiffness
Axial Stiffness
Depends on Shape
and Material!
Force
Displacement
Slope Stiffness =
Force/Displacement
Force, Displacement & Stiffness
Basic Biomechanics
Basic Biomechanics
Stress = Force/Area Strain = Change Height (L)
/ Original Height(L0)
Force
Area L
Stress =
Force/Area
Strain =
Change in Length/Original Length (L/ L0)
Elastic Modulus =
Stress/Strain
Stress-Strain & Elastic Modulus
Basic Biomechanics
Elastic Modulus (GPa) of Common
Materials in Orthopaedics
Stainless Steel 200
Titanium 100
Cortical Bone 7-21
Bone Cement 2.5-3.5
Cancellous Bone 0.7-4.9
UHMW-PE 1.4-4.2
Basic Biomechanics
Basic Biomechanics
Anisotropic
Mechanical properties
dependent upon
direction of loading
Viscoelastic
Stress-Strain character
dependent upon rate of
applied strain (time
dependent).
Material properties of bones:
Wolff’s Law
“Each change in the form and function of a
bone or only its function is followed by
certain definitive changes in its internal
architecture, and secondary changes
equally definitive in its external compliance,
in accordance to the mathematics law”
Wolff’s Law (simplified)
The principle that every change in the for
m and the function of a bone or in the fun
ction of the bone alone, leads to changes
in its internal architecture and in its extern
al form
Wolff’s Law (further simplified)
Bone in a healthy person or
animal will adapt to the loads
under which it is placed
Anistropic properties
the bone tissue can bear higher loads in
the longitudinal direction
lesser quantity of load when applied over
the bone surface
The bone is strong to support loads in the
longitudinal direction because it is used to
receive loads in this direction. (Holtrop,
1975)
Bone Biomechanics
Bone is anisotropic - its modulus is dependent upon the direction of loading.
Bone is weakest in shear, then tension, then compression.
Ultimate Stress at Failure Cortical Bone
Compression < 212 N/m2
Tension < 146 N/m2
Shear < 82 N/m2
Bone Biomechanics
Bone is viscoelastic: its force-
deformation characteristics are
dependent upon the rate of
loading.
Trabecular bone becomes stiffer
in compression the faster it is
loaded.
Elastic Deformation
Plastic Deformation
Energy
Energy
Absorbed
Force
Displacement
PlasticElastic
Basic Biomechanics
• Stiffness-Flexibility
• Yield Point
• Failure Point
• Brittle-Ductile
• Toughness-Weakness
Force
Displacement
PlasticElastic
FailureYield
Stiffness
Basic Biomechanics
Flexible
Ductile
Tough
Strong
Flexible
Brittle, Strong
Flexible
Ductile
Weak
Flexible
Brittle
Weak
Strain
Stress
Bone Mechanics
Bone Density
Subtle density changes greatly changes strength and elastic modulus
Density changes
Normal aging
Disease
Use
Disuse
Cortical Bone
Trabecular Bone
Figure from: Browner et al: Skeletal Trauma
2nd Ed. Saunders, 1998.
Basic biomechanics of bone
healing
In vitro:
High strain = unstable = non-union (fibrous)
Low strain = better union
Static strain = no mobility = non-union
Dynamic strain = frequent, minimal mobility =
good union
Basic biomechanics of bone
healing
Clinical setting for good union:
Small periods of daily axial strain (initial phase
of healing)
Fracture gap <2mm
Amplitude of movement = 0.2 – 1mm
Strain as stated before
Load distributed evenly across fracture site
(load sharing)
Recommended