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The Biomechanics of
Human Bone Growth and
Development
Explain how the material constituents and structural organization
of bone affect its ability to withstand mechanical loads.
Describe the effects of exercise and of weightlessness on bone
mineralization.
Explain the relationship between different forms of mechanical
loading and common bone injuries.
BONES
Two Important Mechanical Functions of
Bone For Human Being
Provides a rigid skeletal framework that
supports and protects other body tissues.
It forms a system of rigid levers that can be
moved by forces from the attaching
muscles.
BONE CONSTITUENTS
Material constituents influence bone
responds to mechanical loading.
Bone major building blocks:
 Calcium Carbonate
60–70% of dry bone weight
 Calcium Phosphate
 Collagen – protein.
 Water – 25 to 30% of bone weight
STRENGTH AND STIFFNESS
DETERMINES MATERIALS BEHAVIOR
UNDER LOADING
Stiffness?
Stress/strain in a loaded material (slope), represent material resistant
to load as the structure deform.
Steep slope represent higher stiffness. If the same amount of force
applied to two different material, the lower stiffness will deforms
more.
Compressive strength?
Ability to resist the stress due to compression. How do we compare
strength? Strength indicated by the area under the stress-strain curve.
STRENGTH AND STIFFNESS OF
VARIOUS MATERIALS
Bone – flexible
and weak
(Shipman, P.,
Walker, A., and
Bichell, D.
[1985]
BONE STRESS- STRAIN
CURVE
Bone has both
brittle and
ductile
properties.
It deforms
slightly
before
failure or
fracture.
Ductile Material
Brittle material
Bone
Strain
COMPOSITION AND STRUCTURE OF
BONE
What contributes to stiffness and
compressive strength in bone?
MINERALS
• Calcium carbonate
• Calcium phosphate 60-70% of dry
bone weight
COMPOSITION AND STRUCTURE OF
BONE
What contributes to flexibility and
tensile strength in bone?
COLLAGEN.
What is the effect of aging on collagen
in bone? Collagen is progressively lost
and bone brittleness increases with
aging.
COMPOSITION AND STRUCTURE OF
BONE
What else affects bone strength?
• water content of bone, which
comprises 25%-30% of bone
weight. Bone specimens keep wet.
• bone porosity, or the amount of bone
volume filled with pores or cavities.
COMPOSITION AND STRUCTURE OF
BONE
Endosteum
Proximal epiphysis
Cortical bone
Epiphyseal plate
Marrow
Trabecular bone
Periosteum
Diaphysis
Trabecular bone
Nutrient artery
Cortical bone
Medullary cavity
Distal epiphysis
Epiphyseal
plate
Structures of cortical (compact) and
trabecular (spongy) bone.
COMPOSITION AND STRUCTURE OF
BONE
Categories of bone based on porosity:
• Cortical bone:
Low porosity (5-30%), higher mineral
content, therefore higher stiffness; found
in the shafts of long bones
• Trabecular (or cancellous) bone- (3090%): Higher porosity (, less mineral
content, therefore lower stiffness compare
to cortical bone, found in the ends of long
bones and the vertebrae.
CORTICAL AND TRABECULAR
CHARACTERISTICS
What else does bone porosity affect?
• Cortical bone is stiffer than trabecular
bone, it can withstand greater stress but
less strain.
• Trabecular bone is spongier than
cortical bone, it can undergo more strain
before fracturing. Shock Absorbing
capability.
TYPICAL STRESS STRAIN BEHAVIOUR
FOR HUMAN CORTICAL BONE
The bone is
stiffer in the
longitudinal
direction.
It is stronger in
compression
than tension
BONE STRENGTH CHARACTERISTICS
Bone is strongest in resisting compression and weakest in resisting
shear.
Approximate strength
of cortical bone
Compression:
200X106 N/m2
Tension:
100X106 N/m2
SHEAR
TENSION
COMPRESSION
Stress to Fracture
Bone is anisotropic, it has different strength and stiffness depending
on the direction of the load
Shear:
50X106 N/m2
TYPES OF BONES
• Axial skeleton (bones that form the
axis of the body) - skull, vertebrae,
sternum and ribs.
• Appendicular skeleton - bones
composing the body appendages.
TYPES OF BONES (SHAPES AND
FUNCTIONS)
• Short bones (carpals and tarsals)approximately cubical; include the carpals and
tarsals. Spongy bone covered with thin layer of
compact bone. Shock absorption and force
transmissions.
• Flat bones (scapula,sternum,ribs,
patellae, some bones of the skull)- protect
organs & provide surfaces for muscle
attachments. Protect internal structures and
offer broad surfaces for muscular attachment.
Types of bones (shapes and functions)
• Irregular bones (the vertebrae, sacrum, coccyx,
maxilla): have different shapes to serve different
functions (supporting weight, contribution to
movement, processes for muscle and ligament
attachment, protection).
• Long bones: Offer body support and form the
framework of the appendicular skeleton; include
humerus, radius, ulna, femur, tibia and fibula. Tibia &
Femur – large and massive to support the weight of
the body. The upper extremity long bones promotes
ease of movement.
TYPES OF LOADCOMPRESSION FORCE



Press the end of the bone together and
produce by muscles, weight bearing,
gravity and etc.
It shorts and widen the bone.
If the load applied surpass the stress
limits of the structure, a compression
fracture will occur.
COMPRESSION AT HIP JOINTS
The hip joint absorb compressive forces
of approximately 3 to 7 times body
weight during walking.
15 to 20 times body weight in jumping.
THE EFFECTS OF EXERCISE AND OF
WEIGHTLESSNESS ON BONE MINERALIZATION
How do bones grow in length?
The epiphyses, or epiphyseal plates, are
growth centers where new bone cells are
produced until the epiphysis closes during late
adolescence or early adulthood.
Epiphysis is a cartilageous disc which can be
found near the end of a long bone.
Most epiphyses close around age 18 although
some may be present until about age 25.
Bone Growth and Development
How do bones grow in circumference?
• The inner layer of the periosteum, a doublelayered membrane covering bone, builds concentric
layers of new bone on top of existing ones.
• Specialized cells called osteoblasts build new bone
tissue and osteoclasts resorb bone tissue
WOLFF’S LAW
The densities, the sizes and shapes of
bones are determined by the magnitude
and direction of the acting forces.
A bone in a healthy person or animal will adapt
to the loads it is placed under. If loading on a
particular bone increases, the bone will remodel
itself over time to become stronger to resist
that sort of loading. The external cortical
portion of the bone becomes thicker as a result.
The converse is true as well: if the loading on a
bone decreases, the bone will become weaker
due to turnover as it is less metabolically costly
to maintain and there is no stimulus for
continued remodeling that is required to
maintain bone mass.
BONE RESPONSE TO STRESS
• Dynamics loading will cause the bone to
deform or strain. When strain exceed certain
threshold, a new bone is laid down at the
strain sites.
•Increased or decreased mechanical stress
leads to osteoblast or osteoclast activity,
respectively.
• Osteoblasts and osteoclasts are continually
building and resorbing bone, respectively.
BONE REMODELING
Bone remodeling involves resorption or
reabsorption (the process of losing substance) of
fatigue damaged older bone and subsequent
formation of new bone. It involves:
A balance of osteoblast and osteoclast activity or
A predominance of osteoclast activity with
associated maintenance of or loss of bone mass.
It maintain or reduce bone mass.
An activity such as walking is sufficient enough to
provoke bone turnover and new bone formation.
BONE MODELING
Bone modeling involves formation of new
bone that is not preceded by
resorption (immature bone growth).
REMODELING AND MODELING
Osteocytes cell embedded in bone are sensitive to
the change in the flow of interstitial fluid through
the pores resulting from strain on the bone.
In response to the motion of the fluid within the
bone matrix, osteocytes trigger the action of
osteoblasts and osteoclasts which form and
remove bone, respectively.
The process is not the same in all bones or even in
a single bone.
BONE DENSITY
Body weight provides constant mechanical
stress to the bone and related to bone
density.
Heavier people having more massive bones.
Few factors affect bone density including
regular exercises (weight bearing activities
exert more influence compare to body
weight, height and race)
HOW DO BONES RESPOND TO TRAINING?
Bone hypertrophy
Increase in bone mass/density resulting from
osteoblast activity (through modeling) in
response to regular physical activity.
What kinds of activity tend to promote bone
density?
Weight bearing exercise, since the larger the
forces the skeletal system sustains, the greater
the osteoblast response.
Bone hypertrophy is stimulated more by the
magnitude of the skeletal loading than by the
frequency of loading.
HOW DO BONES RESPOND TO TRAINING?
Older women, both yard work and
weight training good for hypertropy.
However, jogging, swimming and
calisthenics are less effective. [ Prince
R: Calcium controversy revisited:
implications of new data, Med J Aust
159:404,1993.
Swimmers may have bone mineral
densities lower than sedentary
individuals.
WHAT TENDS TO DIMINISH BONE
DENSITY (ATROPHY)?
Bone atrophy
Decrease in bone mass results from osteoclast
activity (through remodeling). The amount of
Calcium contained in the bone diminishes, both the
weight and the strength of the bone decrease.
Bedridden patients, sedentary senior citizen,
astronauts.
Why Atrophy?
• Lack of weight bearing exercise
• Spending time in the water, (since the
buoyant force counteracts gravitational
force)
• Bed rest – 4 to 6 weeks of bed rest can
results in significant decrements in
bone mineral density and are not fully
reversed after 6 months of normal
weight bearing activities.
• Traveling in space outside of the
earth’s gravitational field – one month
in space, astronauts lost 1-3% of bone
mass.
SAMPLE PROBLEM 1
Tibia – major weight bearing bone
in lower extremity. Sample
weight 600N. If 88% of body
mass proximal to knee joint
(about 88% of body mass acting
on the knee joint), how much
compressive forces acting on
each Tibia if the person holds a
20N sack of groceries?
OSTEOPOROSIS
A disorder involving decreased bone
mass and strength with one or
more resulting fractures. It starts
with Ostopenia (reduce bone mass
w/o fracture) which often finally
results in bone pain and fracturing.
Who?
Majority – postmenopausal and elderly women. Half of
all women, 1/3 of men develop fractures due to
osteoporosis.
Type 1 – postmenopausal osteoporosis, 40% of
women after age 50. Fracture often occurs 15 years
after menopause.
Type 2 – age associated osteoporosis affects most
women and men after age 70.
PREVENT AND TREATING
OSTEOPOROSIS
Regular Exercise
Has been shown to be effective to some extent
in mediating age related bone loss.
Weight bearing exercise – walking, stair
climbing , osteogenic impact forces activities
such as jumping – effective in increasing
bone mass in children.
3 times/ week with 10-20 minutes exercise.
Rest intervals between impact exercise double
the effects of mechanical loading on bone
building.
PREVENT AND TREATING
OSTEOPOROSIS
Hormonal factors
Low level of estrogen in women and low level of
testosterone in men promote bone loss.
Diet
More calcium in diets, absorption of Ca promotes
with cacitriol ( active form of vit D) and influenced
negatively with dietary fibers.
Lifestyles
Physical inactivities, excessive thinnes or weight
loss, smoking, excessive consumptions of protein
and caffeine.
Genetic factors
Less influence.
Mechanical loading and common
bone injuries
Fractures
Depend on magnitude of loading,
direction, loading rate, duration load
sustained, health and maturity of bone
at the time of injury.
Avulsion
Fractures caused by tensile loading.
Explosive throwing and jumping
movements – avulsion of humerus and
calcaneus.
Mechanical loading and common
bone injuries
Excessive bending and torsional loads
produce spiral fractures of the long
bones. Bending moment cause bending
and fracture of bone.
When bending occurs, one side of the
bone is in tension, the other side is in
compression. Since bone resist
compression better than tension, the
side with tension will fracture first.
Mechanical loading and common
bone injuries

Torque cause torsion (twisting of
the structure). When skiers rotate
with respect to one boot, torsional
load can cause a spiral fracture of
the tibia.
Mechanical loading and common bone injuries

Stress fracture (fatigue fracture)
due to repeating low magnitude
loads. Runners, muscle
fatigue,abrupt change in the
running surface or direction.
Mechanical loading and common bone injuries

Children contain larger amount of
collagen than adults, therefore
their bone is more flexible and
more resistant to fracture.
EPIPHYSIAL INJURIES
Injuries to cartilaginous epiphysial
plate, the articular cartilage, and
the apophysis.
 Both acute and repetitive loading
can injure the plate and can result
in growth termination.

TYPES OF LOADS
Loads
Various directions - different types of
loads.
Compression,Tension, Shear,
Bending,Torsion
Compression Force and
Injuries
Bone a stronger in resisting
compression as compare to other
loads. Acute compression fracture
is rare.
Compressive force are responsible for
patellar pain, softening and
destruction of cartilage underneath
the patella.
Chondromalacia
Patellae
Chondromalacia patellae
- literally means
"softening of the
cartilage", and Patellae
means "the knee-cap.
The compressive
patellofemoral force is
approximately at 50º of
flexion.
Injury may be due to
direct trauma, such as a
fall on a flexed knee.
Weight gain, or other
increased load on the
knee
Vertebrae
Fractures to
cervical area due
to sport activity
such as water
sports,
gymnastics,
wrestling, rugby,
ice hockey and
etc.
Cervical spine –
anteriorly
convex.
Cervical Vertebrae
If head is lowered,
cervical spine is
almost flatten. If
forces is applied in
this position,
cervical spine will
have a compression
force effect. This can
result in dislocation
or fracturedislocation of the
facets of the
vertebrae.
When spearing and
butting was
outlawed in football,
Femoral Neck
The hip joints must absorb compressive
forces of approximately 3 to 7 times of body
weight during walking, 15 to 20 times body
weight during jumping.
Large compressive forces on the inferior
portion of the femoral neck, and a large
tensile force on the superior portion of the
neck.
During stance, the hip abductors contracts,
reducing the tension force on superior,
which prevent fracture of the neck.
TENSION FORCE INJURIES
The source of the tensile force is
usually the pull of contracting
muscle tendon.
Failure usually occurs at the site
of muscle insertion.
Avulsion fracture
Avulsion fractures
when ligaments
pull a small chip
of bone away
from the rest of
the bone.
It occurs more in
frequently in
children than
adults.
SPRAIN AND STRAIN
Ankle sprain - ankle
sprains happen when
the foot turns inward
as a person runs,
turns, falls, or lands on
the ankle after a jump.
The most frequently
injured ligaments are
the anterior talofibular
followed by the
calcaneofibular
ligament.
SHEAR FORCES
INJURIES
Vertebral disc
problem
Spondylolisthesis the vertebrae slip
anteriorly over one
another.
SHEAR FORCE FRACTURE
Fractures due to shear forces are
commonly found in Femoral
Condyle and tibial plateau.
Epiphyseal fracture in childs.
BENDING FORCES
Injuries caused by bending when
multiple loads applied at different
point s on the bone. Generally
called, 3 or 4 point force
applications.
The bone will break at the middle
froce application. Bone will
normally fracture at the side
where the tensile force is
applied.
TORSIONAL FORCES
Humerus when poor
throwing technique
creates a twist on the
arm.
A spiral fracture is a
result of torsional
forces.