The document provides an overview of the biomechanics of bone. It describes the structure and function of bones, including their composition of cells, minerals, and collagen. Bones are classified based on their shape and location in the body. Bones adapt to stresses through remodeling and modeling in response to mechanical loading as described by Wolff's law. The document outlines the different modes of loading bones experience and how stresses are distributed. It discusses the elastic properties and strength of bones in relation to stresses and strains. Factors that weaken bones like stress raisers, osteoporosis, and rickets are also summarized.

1. The Biomechanics of Bone
By:
Tewodros Belay Alemneh (MSc)
Year III, Sem. II
Course code: 3122
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2. Course objectives
After completing this chapter, you will be able to:
• Describe various function and structure of human bones
• Explain basic material components used in bone composition
• Explain the principle of mechanics to evaluate stress and strain in
bones.
• Explain the relationship between different forms of mechanical
loading and common bone injuries.
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3. Introduction
• The word bone typically conjures up a mental image of a dead bone:
dry, brittle chunk of mineral that a dog would enjoy chewing.
However:
3

4. Introduction
• A living bone is an extremely dynamic
tissue that is continually modelled and
remodelled by the forces acting on it.
• Bone is living tissue that makes up the
body's skeleton
• The human skeleton is made up of
between 206 and 210 bones.
• Bones are categorized according to
various shapes:
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5. Introduction
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6. Function of skeleton
• The skeleton consists of approximately 20% of total body weight.
• Bone tissue performs many functions:
o Support
o Attachment sites
o Leverage
o Protection
o Storage
o Blood cell formation.
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7. Function of skeleton
• Support: Structural support and can maintain a posture while
accommodating large external forces.
• Attachment sites: Provide sites of attachment for tendons, muscles,
and ligaments, allowing for the generation of movement.
• Leverage: Provides the levers and axes of rotation about which the
muscular system generates the movements.
• Protection: Provides protection of vital organs.
• Storage: Stores fats and minerals ( *calcium and phosphate)
• Blood cell formation: Haematopoiesis
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8. Anatomical classification of bones
• Skeletal system has two main parts:
o The axial (skull, spine, ribs, and sternum)
oAppendicular (shoulder, pelvic girdles, arm, and legs)
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9. Anatomical classification of bones
oLong: offer the body support, the interconnected set of levers, and linkages that
allow us to move.
e.g. humerus, femur, tibia, radius ulna fibula
o Short: role in shock absorption and the transmission of forces.
e.g. phalanges, metatarsals patella
o Flat: protect internal structures and offer broad surfaces for muscular
attachment.
e.g. scapula, illium, sternum, skull
o Irregular: supporting weight, dissipating loads, protecting the spinal cord,
contributing to movement, and providing sites for muscular attachment.
e.g: carpals, ossicles
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10. Anatomical classification of bones
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11. Cells making up bone
• There are three types of cells which
make up the bone.
• Osteoblasts: (bone-forming cells)
• Osteocytes:(bone-maintaining cells)
• Osteoclasts: (bone-resorbing cells)
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12. Cells making up bone
Osteoblasts: (bone-forming cells)
o Immature, unspecialized cells which give
rise to other bone cells.
oThese are the stem cells that will
differentiate to become one of the
specialized cells.
oThey produce a new bone called osteoid.
oThese cells are also responsible for the
calcification of the bone.
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13. Cells making up bone
Osteocytes:(bone-maintaining cells)
o Mature and specialized cells; these cells form the solid structure of the bone.
oHelps to maintain bones as living tissue
o Some of the osteoblasts are trapped within the formation of the new bone and
develop into osteocytes.
Osteoclasts: (bone-resorbing cells)
o They work to dissolve the bone in areas of micro fracture.
o Scavenger cells which remodel the bone; these cells use enzymes and acid to
break down bone tissue, allowing the shape of the bone to be changed to suit
the needs of the body, such as during healing or during normal growth.
13

14. Composition and structure
of bone tissue
• The major building blocks of bone are calcium carbonate, calcium
phosphate, collagen, and water.
• Calcium carbonate and calcium phosphate: ~ 60–70% of dry bone
weight.
 Give bone its stiffness and are the primary determiners of its
compressive strength.
• Other minerals, including magnesium, sodium, and fluoride, also have
vital structural and metabolic roles in bone growth and development.
• Collagen is a protein that provides bone with flexibility and contributes to
its tensile strength.

15. Structure of the bone
1. Compact/cortical Bone (Outer
Layer):
• Compact mineralized connective tissue
with low porosity that is found in the
shafts of long bones.
• 5–30% of bone volume occupied by
nonmineralized tissue.
• Dense, Smooth, and Solid to naked eye
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16. Structure of the bone
2. Spongy/cancellous/trabecular Bone
(Inner Layer):
• Less compact mineralized connective tissue
with high porosity that is found in the ends of
long bones and in the vertebrae.
• >30% of bone volume occupied by
nonmineralized tissue.
• Open spaces between trabeculae are filled with
red or yellow bone marrow.
• Hole-y (like a honeycomb)
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Both cortical and trabecular bone are anisotropic; that is, they exhibit different strength and
stiffness in response to forces applied from different directions

17. Bone Density
• The amount of bone minerals in bone tissue.
• The density of different bones and their mechanical properties of bone
vary from type to type.
- Cortical bone: has a density of approximately 2g/cm3, with 5 –30%
porosity.
- Spongy bone: has a density of approximately 0.07 –1g/cm3, with
porosity > 30%.
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18. Bone Density
Factors affecting bone density:
• Hormonal diseases (mainly those affecting oestrogen, testosterone and
cortisol)
• Local injuries/inflammation
• Lack of physical activity
• Malnutrition (lack of calcium)
• Genetics: (white vs black)
• Lack of vitamin D.
• Ageing
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19. ASSIGNMENT 2
• Explain briefly about bone formation (Ossification, Modeling, and
Remodeling)
Submission date: Thursday June 7 2023 (class time)
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20. Mechanical properties of bone
• The mechanical properties of bone are as complex and varied as its
composition.
• The mechanical properties also vary with age and gender and with
the location of the bone (humerus versus the tibia).
• Measurement of bone strength, stiffness, and energy depends on
both the material composition and the structural properties of
bone.
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21. Bone response to stress
• Bone responds dynamically to the
presence or absence of different forces
with changes in size, shape, and
density.
• Julius Wolff in 1892 (Wolff ’s Law):
o The bone strength increases and decreases
as the functional forces on the bone
increase and decrease.
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22. Bone response to stress
Bone Modeling and Remodeling:
• The densities, the shapes and sizes (to a much lesser extent) of the
bones of a given human being are a function of the magnitude and
direction of the mechanical stresses that act on the bones.
• Dynamic mechanical loading causes bones to deform or strain, with larger
loads producing higher levels of strain.
• These strains are translated into changes in bone shape and strength
through a process known as remodeling.
* Bone remodeling Vs Bone modeling
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23. Load applied to Bone
• The skeletal system is subject to a variety of applied forces as bone is
loaded in various directions.
• Loads are produced by weight bearing, gravity, muscular forces, and
external forces.
• Six modes of loading
• During activity usually a combination of loading modes takes place.
• Each mode of loading produces a distinctive fracture pattern if the
ultimate strength of the bone is exceeded.
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24. Load applied to Bone
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25. Load applied to Boneloading
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26. Load applied to Bone
• Bone is anisotropic: its strength varies according to the orientation
of the loading.
• In general, bone is stronger in compression than in tension, and
stronger in tension than in shear.
Comparative Loading Strengths
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27. Load applied to Bone
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28. Strength of Bones
Stress Strain behaviour:
• The stress is defined as measure of the force
distribution over a given cross-sectional area:
𝒔𝒕𝒓𝒆𝒔 𝝈 =
𝑭𝒐𝒓𝒄𝒆 (𝑭)
𝑨𝒓𝒆𝒂 (𝑨)
(
𝑵
𝒎𝟐)
• The relative deformation of an object in
response to an applied load is called strain:
𝑺𝒕𝒓𝒂𝒊𝒏 (𝜺) =
𝒄𝒉𝒂𝒏𝒈𝒆 𝒊𝒏 𝒍𝒆𝒏𝒈𝒕𝒉
𝒐𝒓𝒊𝒈𝒊𝒏𝒂𝒍 𝒍𝒆𝒏𝒈𝒕𝒉
=
∆𝜾
𝚤
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29. Strength of Bones
• Youngu’s Modulus
𝑺𝒕𝒓𝒆𝒔𝒔
𝑺𝒕𝒓𝒂𝒊𝒏
=
𝑭
𝑨
∆𝑳
𝑳𝟎
= E
.
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30. Strength of Bones
• The relative deformations created at any point are referred to as the
strains at that point.
𝑆𝑡𝑟𝑎𝑖𝑛 =
𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑙𝑒𝑛𝑔𝑡ℎ
𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ
= 𝜀 =
∆𝜄
𝚤
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31. Strength of bones
Stress-strain curves of typical cortical bone
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32. Strength of bones
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33. Strength of Bones
• 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
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34. Strength of bones
• Bone is visco-elastic, meaning the mechanical properties such as
stiffness change as the strain rate changes.
• Stiffness increases for higher strain rates.
• This improves the bone’s ability to withstand high-impact loading,
such as might occur during running.
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35. Stress raisers in bone
• In engineering terms stress is considered to “flow” in a material.
• Any sudden changes in the material result in a change in the flow
pattern.
• Fractures are more likely to occur at stress concentrated sites if the
bone is loaded.
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36. Stress risers in bone
Particular examples of stress raisers in bone are:
• Holes drilled into bone to secure metal bone plates.
• Pins and screws placed in the bone during surgery.
• Stress fractures and hairline cracks.
• The junction between implants and the bone.
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37. Stress risers in bone
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38. Degenerative processes in bone
Osteoporosis:
• This is a loss of bone density, thereby increasing the bone porosity.
• It is usually hormonally linked, i.e. related to a loss in oestrogen or
testosterone levels.
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39. Degenerative processes in bone
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40. Degenerative processes in bone
Bone resorption:
• This is the excessive resorption of bone when the bone is
insufficiently loaded.
• It may be caused by Orthopedic implants such as artificial joints,
plates and screws, or by a sedentary lifestyle, or physical inactivity.
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41. Degenerative processes in bone
Rickets:
• This is a bone condition
found in growing or
developing bones in
children and adolescents as
a result of vitamin D,
calcium, magnesium and
phosphates deficiencies
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42. Common bone injuries
Fractures:
• A fracture is a disruption in the continuity of a bone.
Types of Bone fracture:
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A B C D E F

43. Common bone injuries
A- Greenstick fracture is incomplete, and the break occurs on the
convex surface of the bend in the bone.
B- Fissured fracture involves an incomplete longitudinal break.
C- Comminuted fracture is complete and fragments the bone.
D- Transverse fracture is complete, and the break occurs at a right angle
to the axis of the bone.
E- An oblique fracture occurs at an angle other than a right angle to the
axis of the bone.
F- Spiral fracture is caused by twisting a bone excessively.
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44. Fractures of the femur
The mode of loading, as well as the
magnitude of the forces applied to the bone,
will influence the type of fractures that can be
expected
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45. THE END