Bone biology,

healing and

biomechanics

Bone biology

Histology

Woven bone

Predominantly in bone healing

Lamellar bone

Cortical bone

Cancellous bone

Bone structure

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

Bone healing

Basic biomechanics

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.

Basic biomechanics

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

Basic biomechanics

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.

Forces acting on bone

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)

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