basic concept in bone and its biomechanics

1. Bone biology,
healing and
biomechanics

2. Bone biology

3. Histology
 Woven bone
 Predominantly in bone healing
 Lamellar bone
 Cortical bone
 Cancellous bone

4. Bone structure

5. 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

6. Bone healing

7. Basic biomechanics

8. 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).

9. 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!

10. Force
Displacement
Slope Stiffness =
Force/Displacement
Force, Displacement & Stiffness
Basic Biomechanics

11. Basic Biomechanics
Stress = Force/Area Strain = Change Height (L)
/ Original Height(L0)
Force
Area L

12. Stress =
Force/Area
Strain =
Change in Length/Original Length (L/ L0)
Elastic Modulus =
Stress/Strain
Stress-Strain & Elastic Modulus
Basic Biomechanics

13.  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

14. 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:

15. 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”

16. 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

17. Wolff’s Law (further simplified)
Bone in a healthy person or
animal will adapt to the loads
under which it is placed

18. 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)

19. 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

20. 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.

21. Basic biomechanics

22.  Elastic Deformation
 Plastic Deformation
 Energy
Energy
Absorbed
Force
Displacement
PlasticElastic
Basic Biomechanics

23. • Stiffness-Flexibility
• Yield Point
• Failure Point
• Brittle-Ductile
• Toughness-Weakness
Force
Displacement
PlasticElastic
Failure
Yield
Stiffness
Basic Biomechanics

24. Flexible
Ductile
Tough
Strong
Flexible
Brittle, Strong
Flexible
Ductile
Weak
Flexible
Brittle
Weak
Strain
Stress

25. Basic biomechanics

26. 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.

27. Forces acting on bone

28. 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

29. 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)

30. THANK YOU