Chapter 5
The Biomechanics
of Human Bone
Growth and
Development
Composition and Structure of Bone
What is stiffness?
(stress/strain in a loaded material;
stress divided by the relative amount
of change in shape)
What is compressive strength?
(ability to resist compression)
Composition and Structure of Bone
What contributes to stiffness and
compressive strength in bone?
• calcium carbonate
• calcium phosphate
Composition and Structure of Bone
What contributes to flexibility and tensile
strength (ability to resist tension) in bone?
(collagen)
What is the effect of aging on collagen in
bone?
(collagen is progressively lost and bone
brittleness increases with aging)
collagen
Collagen is a group of naturally occurring proteins found in animals, especially in
the flesh and connective tissues of vertebrates. It is the main component of
connective tissue, and is the most abundant protein in mammals, making up about
25% to 35% of the whole-body protein content. Collagen, in the form of elongated
fibrils, is mostly found in fibrous tissues such as tendon, ligament and skin, and is
also abundant in cornea, cartilage, bone, blood vessels, the gut, and intervertebral
disc. The fibroblast is the most common cell which creates collagen.
Composition and Structure of Bone
What else affects bone strength?
•
water content of bone,
which comprises 25%-30%
of bone weight
•
bone porosity, or the amount of
bone volume filled with pores or
cavities
Composition and Structure of Bone
Categories of bone based on porosity:
•cortical bone: compact mineralized bone with low
porosity; found in the shafts of long bones
•trabecular (or cancellous) bone: less compact bone
with high porosity; found in the ends of long bones and
the vertebrae
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
What else does bone porosity affect?
•
because cortical
bone is
stiffer than trabecular bone, it
can withstand greater stress but
less strain
•
because trabecular bone is
spongier than cortical bone, it
can undergo more strain before
fracturing
Composition and Structure of Bone
How does the structure of bone affect
its strength?
(bone is anisotropic, it has different
strength and stiffness depending
on the direction of the load)
(Bone is strongest in resisting
compression and weakest in
SHEAR
TENSION
resisting shear.)
COMPRESSION
How does
the structure
of bone
affect its
strength?
Stress to Fracture
Composition and Structure of Bone
Bone has relatively high compressive strength, of about 170 Mpa but poor tensile
strength of 104– 121 MPa and very low shear stress strength (51.6 MPa).
Example 1
In the human femur, bone tissue is strongest in resisting compressive
force, approximately half as strong in resisting tensile force, and only
about one-fifth as strong in resisting shear force.
If a tensile force
of 8000 N is sufficient to produce a fracture, how much compressive
force will produce a fracture? How much shear force will produce a
fracture?
Solution
Compressive = 1.0; Tensile = 0.5; Shear = 0.2
8000 NTensile (Compressive/Tensile) = Fracture FCompressive
8000 NTensile (1.0/0.5) = Fracture FCompressive
16000 N = Fracture FCompressive
16000 N (0.2) = Fracture FShear
3200 N = Fracture FShear
Types of bones
• axial skeleton: skull, vertebrae, sternum, ribs
• appendicular skeleton: bones composing the
body appendages
Axial skeleton diagram
Appendicular skeleton diagram
Axial skeleton
The axial skeleton consists of the 80 bones along the central axis of the human
body. It is composed of six parts; the human skull, the ossicles of the middle ear, the
hyoid bone of the throat, the rib cage, sternum and the vertebral column. The axial
skeleton and the appendicular skeleton together form the complete skeleton.
Flat bones house the brain, spinal cord, and other vital organs. This article
mainly deals with the axial skeletons of humans; however, it is important to
understand the evolutionary lineage of the axial skeleton. The human axial
skeleton consists of 80 different bones. It is the midial core of the body and
connects the pelvis to the body. where the appendix skeleton attaches. As
the skeleton grows older the bones get weaker with the exception of the
skull. The skull remains strong to protect the brain from injury.
Introduction to Biomechanics, Chapter5
BMT 228- The Biomechanics of Human
Bone Growth and Development
Appendicular skeleton
The appendicular skeleton is composed of 126 bones in the human body. The word appendicular is the adjective of the noun
appendage, which itself means a part that is joined to something larger. Functionally it is involved in locomotion (Lower limbs) of the axial
skeleton and manipulation of objects in the environment (Upper limbs).
The appendicular skeleton forms during development from cartlilage, by the process of endochondral ossification.
The appendicular skeleton is divided into six major regions:
1) Pectoral Girdles (4 bones) - Left and right Clavicle (2) and Scapula (2).
2) Arm and Forearm (6 bones) - Left and right Humerus (2) (Arm), Ulna (2) and Radius (2) (Fore Arm).
3) Hands (54 bones) - Left and right Carpal (16) (wrist), Metacarpal (10), Proximal phalanges (10),
Middle phalanges (8), distal phalanges (10).
4) Pelvis (2 bones) - Left and right os coxae (2) (ilium).
5) Thigh and leg (8 bones) - Femur (2) (thigh), Tibia (2), Patella (2) (knee), and Fibula (2) (leg).
6) Feet and ankles (52 bones) - Tarsals (14) (ankle), Metatarsals (10), Proximal phalanges (10),
middle phalanges (8), distal phalanges (10).
It is important to realize that through anatomical variation it is common for the skeleton to have many
extra bones (sutural bones in the skull, cervical ribs, lumbar ribs and even extra lumbar vertebrae)
The appendicular skeleton of 126 bones and the axial skeleton of 80 bones together form the
complete skeleton of 206 bones in the human body.
Unlike the axial skeleton, the appendicular skeleton is unfused. T
his allows for a much greater range of motion.
Types of bones
•
short bones: approximately
cubical; include the carpals and
tarsals
•
flat bones: protect organs &
provide surfaces for muscle
attachments; include the
scapulae, sternum, ribs,
patellae, some bones of the skull
short bones
flat bones
Types of bones
•
irregular bones: have
different
shapes to serve different
functions; include vertebrae,
sacrum, coccyx, maxilla
•
long bones: form the framework
of the appendicular skeleton;
include humerus, radius, ulna,
femur, tibia, fibula
Bone Growth and Development
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)
Bone Growth and Development
How do bones grow in circumference?
•
•
the inner layer of the periosteum, a
double- layered 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
Periosteum, dense fibrous membrane covering the surfaces of bones, consisting of an
outer fibrous layer and an inner cellular layer (cambium). The outer layer is composed
mostly of collagen and contains nerve fibres that cause pain when the tissue is damaged. It
also contains many blood vessels, branches of which penetrate the bone to supply the
osteocytes, or bone cells. These perpendicular branches pass into the bone along channels
known as Volkmann canals to the vessels in the haversian canals, which run the length of
the bone.
Osteocyte, a cell that lies within the substance of fully formed bone. It occupies a small
chamber called a lacuna, which is contained in the calcified matrix of bone. Osteocytes
derive from osteoblasts, or bone-forming cells, and are essentially osteoblasts surrounded
by the products they secreted. Cytoplasmic processes of the osteocyte extend away from the
cell toward other osteocytes in small channels called canaliculi. By means of these
canaliculi, nutrients and waste products are exchanged to maintain the viability of the
osteocyte.
Osteoblasts:
Osteoblast, large cell responsible for the synthesis and mineralization of bone during both
initial bone formation and later bone remodeling. Osteoblasts form a closely packed sheet on
the surface of the bone, from which cellular processes extend through the developing bone.
They arise from the differentiation of osteogenic cells in the periosteum, the tissue that covers
the outer surface of the bone, and in the endosteum of the marrow cavity. This cell
differentiation requires a regular supply of blood, without which cartilage-forming
chondroblasts, rather than osteoblasts, are formed. The osteoblasts produce many cell
products, including the enzymes alkaline phosphatase and collagenase.
Osteoclasts:
Osteoclast, large multinucleated cell responsible for the dissolution and absorption of bone.
Bone is a dynamic tissue that is continuously being broken down and restructured in
response to such influences as structural stress and the body’s requirement for calcium. The
osteoclasts are the mediators of the continuous destruction of bone. Osteoclasts occupy
small depressions on the bone’s surface, called Howship lacunae; the lacunae are thought to
be caused by erosion of the bone by the osteoclasts’enzymes. Osteoclasts are formed by the
fusion of many cells derived from circulating monocytes in the blood. Osteoclasts were
discovered by Kolliker in 1873. Osteoclasts and osteoblasts are instrumental in controlling
the amount of bone tissue: osteoblasts form bone, osteoclasts resorb bone.
bone remodeling:
bone remodeling, continuing process of synthesis and destruction that gives bone its
mature structure and maintains normal calcium levels in the body. Destruction, or
resorption, of bone by large cells called osteoclasts releases calcium into the bloodstream
to meet the body’s metabolic needs and simultaneously allows the bone—which is
inhibited by its inorganic component from growing by cell division like other tissues—to
alter size and shape as it grows to adult proportions. While the osteoclasts resorb bone at
various sites, other cells called osteoblasts make new bone to maintain the skeletal
structure. During childhood, bone formation outpaces destruction as growth proceeds.
Osteoblasts actively
synthesizing osteoid
containing two osteocytes.
Osteoclast, with bone below
it, showing typical
distinguishing characteristics:
a large cell with multiple
nuclei and a "foamy" cytosol.
Osteoblasts and Osteoclasts
http://www.youtube.com/watch?v=78RBpWSOl08
Introduction to Bone Biology
Bone Remodeling and Modeling
Bone Response to Stress
How do bones respond to training?
•
just like muscle, bones
respond to certain kinds of
training by hypertrophying
•
according to Wolff’s law, the
densities, and to a lesser extent,
the sizes and shapes of bones are
determined by the magnitude and
direction of the acting forces
Bone Response to Stress
How is Wolff’s law carried out?
•
Osteoblasts and osteoclasts are
continually building and
resorbing bone, respectively.
•
Increased or decreased
mechanical stress leads to a
predominance of osteoblast or
osteoclast activity, respectively.
Bone Response to Stress
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 Response to Stress
What tends to diminish bone density?
•
lack of weight bearing exercise
• spending time in the water, (since the
buoyant force counteracts
gravitational force)
• bed rest
•
traveling in space outside of the
earth’s gravitational field
Osteoporosis
Osteoporosis is a disorder involving decreased
bone mass and strength with pain and one or
more fractures resulting from daily activity.
Osteoporosis
Who is affected by osteoporosis?
•
•
Type
I
(postmenopausal)
osteoporosis affects about 40% of
women after age 50
Type II (age-associated)
osteoporosis affects most women
and men after age 70
Osteoporosis
Are younger people ever affected by
osteoporosis?
• The female athlete triad includes:
Female Athlete Triad is a syndrome in which eating disorders (or low energy availability),
amenorrhoea/oligomenorrhoea and decreased bone mineral density (osteoporosis and
osteopenia) are present. Also known simply as the Triad, this condition is seen in females
participating in sports that emphasize leanness or low body weight. The Triad is a serious
illness with lifelong health consequences and can potentially be fatal
• disordered eating
• amenorrhea, and
is the absence of a menstrual period in a woman of reproductive age
• osteoporosis
Osteoporosis
How can osteoporosis be prevented
and treated?
(regular weight bearing exercise is
the key to prevention and treatment)
•Walking
•Jogging
•Stair climbing
•Dancing
•Hiking
•Volleyball
•Tennis
•Certain types of weight lifting/resistance exercises
Osteoporosis
How can osteoporosis be prevented and
treated?
•
postmenopausal hormone
replacement
• adequate dietary calcium and vitamin D
•
avoiding smoking and excessive
consumption of protein, caffeine,
and alcohol
Anatomy of a Fracture as a Result of Systemic Bone Loss
Postmenopausal Osteoporosis