Tissue mechanics and static examinations of bones

Mechanics

Dynamics & Kinematics

Dynamics

Statics & Kinetics

Statics

Examination of elastic bodies
Examination of stresses and deformations
Dimensioning, checking

Loading models

Static
Dynamic
-Pulsing
-Alternating
-Irregular

Stress

Normal stress (tension, compression, bending) - σ

Tangential stress (shear, torsion) – τ

Deformation

We examine the deformation in 3 perpendicular directions

Mechanical loads

Compression
Tension
Bending
Torsion
Shear

Combined loads

Combination of simple loads

BENDING

•Compressive and tensile stress at the same time

• Torque generates bending

TORSION

•The rotational axis is the longitudinal axis of the body
• Generates only tangential stress
• Deformation to a spiral

SHEARING

• Generates only tangential stress
• The loading force lies in the cross section

Material properties of tissue

– Tensile properties (strength, elongation)
– Compressive properties (strenght, strain)
– Torsional properties (strength, rotation)
– Hardness
– Density
– Thermal properties (melting/freezing point)
– Colour, etc.

Isotropic materials

Forces propagates in the material in a uniform way in all directions

Anisotropic materials

The propagation of forces in the material is different in certain directions

Composites

The body is formed from more, easily separatable parts from different material

Brinell-Hardness

Hardness: resistance against a penetrating body

Tensile test: Elastic Materials

After the deforming external loads stop, the inner forces inside the material try to reset the original shape of the body.

– Material properties • Tensile strength • Tensile strain at break • Tensile extension at break • Young modulus: Slope of the proportional section of the curv

Tensile test: Rigid materials

After the proportional limit the body suddenly breaks

Material properties:
• Tensile strength
• Tensile strain at break
• Tensile extension at break
• Young modulus: Slope of the proportional section of the curve

Charpy impact test

Impact energy: KV = mg(h-h1)

Bearing capacity

– Spongious bone tensile strength: 7 MPa
– Cortical bone tensile strength : 140 MPa
– Steel tensile strength : from 200 MPa

Both cortical and spongious bone has a much more strength against longitudinal (tensile or compression) than transversal (torque and shear) loads