:بيومكانيك رشد و نمو استخوان انسان
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Bone Structural support of the body Connective tissue that has the potential to repair
and regenerate Comprised of a rigid matrix of calcium salts
deposited around protein fibers
Minerals provide rigidity
Proteins provide elasticity and strength
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Bone Tissue Mineral: ~50% of bone weight, provides stiffness and cmpressive strength (primarily calcium compounds) Collagen: ~ 25% of bone weight, provides tensile strength and stiffness Water: ~25% of bone weight, provides compressive strength and helps maintains bone health Ground Substance: ~1% of bone weight, increases elastic capabilities
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There are 2 types of bones: compact bone and spongy bone. 1. Compact Bone: Is a solid bone that at microscopic level has a concentric ring structure. It is made of bone cells called osteoblasts. These cells are growing within a material that is called a bone matrix. The bone matrix is made of 70% inorganic salts such as calcium phosphate and 30% protein called collagen. The calcium salts give the bone strength while the protein gives it flexibility. Blood vessels as well as nerve fibres are within the bone. 2. Spongy Bone: irregular openwork of thin plates of bone. It is also known as trabecular or cancellous bone. The mineral deposits are arranged as a system of struts. Bone marrow fills the spaces between the plates. The marrow cavity is the space within the diaphysis that also contains marrow.
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Inside view of cavities within spongy bone. They are filled with red bone marrow.
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Functions Osseous tissue performs numerous functions including: Directly: Support for muscles, organs, and soft tissues. Leverage and movement. Protection of vital organs, e.g., the heart. (Note: not all vital organs are protected by bones, e.g., the intestines.) Calcium phosphate storage. Indirectly: Hemopoiesis - formation of blood cells by the bone marrow interspersed within the spongy bone.
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سیستم اسکلتی مرکزی یا حموری
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.
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سیستم اسکلتی خارجی یا زائده ای
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).
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Shape Long, short, flat, and irregular
Long bones are cylindrical and “hollow” to achieve
strength and minimize weight www.sirinet.net/ ~jgjohnso/skeleton.html
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1. Long bone
a. Longer than wide
b. Shaft with two ends
c. Mostly compact
d. Bones of limbs
2. Short bone
a. Cube-like
b. Mostly spongy
c. Sesamoid—
bones embedded in tendon
i. Patella
3. Flat bone
a. Spongy bone embedded
within parallel layers
of thin compact bone
4. Irregular bone
a. Vertebrae and hip bones
b. Complicated shapes
c. Mostly spongy with a
thin covering of compact
bone
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Bone Physiology. Courtesy Gray's Anatomy 35th edit Longman Edinburgh 1973
Cancellous Bone
Cortical Bone
Osteon
Periosteum
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Microstructure of the Bone
(a) (b) (c)
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Microstructure of Bone (Cont’d)
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Two architectures (classified by porosity) also relate to function: 1. Cortical (compact) bone is 5-30% porous 2. Cancellous (trabecular or spongy) bone is 30-90% porous Bone strength and stiffness are influenced by bone architecture
Bone Architecture
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Composition of Bone: Cells
Osteocytes
Osteoblasts
Osteoclasts
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Osteocyte: a bone cell Osteoblasts: specialized bone cells that form new bone tissue Osteoclasts: specialized bone cells that resorb existing bone tissue Under normal circumstances, activity of these cells is balanced
Bone Cells
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Controlling Factors
Hormones
Estrogen
Testosterone
Cytokines
Growth factors,
Interleukins (1, 6, and 11),
Transforming growth factor-b
Tumor necrosis factor-a
of osteoclasts and osteoblasts
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Macrophage
Phagocytose invading pathogens
Cell alters shape to surround bacteria or debris
Process: Chemotaxis, adherence, phagosome formation, phagolysosome formation
Secrete Interleukin-1
(IL-1)
Involved in bone resorption
Controlling Factors of osteoclasts and osteoblasts
http://saints.css.edu/bio/schroeder/macrophage.html http://academic.brooklyn.cuny.edu/biology/bio4fv/page/phago.htm http://www.allsciencestuff.com/mbiology/research/osteoporosis
Bacterium
Nuclei
Ingested bacterium
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Composition of Bone: Matrix
Cortical/ Compact
Bone
Cancellous/
Trabecular/ Spongy
Bone
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Cortical Cancellous
Physical Description
Dense protective shell Rigid lattice designed for strength; Interstices are filled with marrow
Location
Around all bones, beneath periosteum; Primarily in the shafts of long bones
In vertebrae, flat bones (e.g. pelvis) and the ends of long bones
% of Skeletal Mass
80% 20%
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Cortical Cancellous First Level Structure
Osteons Trabeculae
Porosity 5-10% 50-90%
Circulation Slow circulation of nutrients and waste
Haversian system allows diffusion of nutrients and waste between blood vessels and cells; Cells are close to the blood supply in lacunae
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Cortical Cancellous Strength Withstand greater stress Withstand greater strain
Direction of Strength
Bending and torsion, e.g. in the middle of
long bones
Compression; Young’s modulus is much greater
in the longitudinal direction
Stiffness Higher Lower
Fracture Point
Strain>2% Strain>75%
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Properties of Cortical and Cancellous Bones
Load Type Elastic modulus (109N/m2)
Ultimate stress (106N/m2)
Bone Type Cortical Cancellous Cortical Cancellous
Tension 11-19 ~0.2-5 107-146 ~3-20
Compression 15-20 0.1-3 156-212 1.5–50
Shear 73-82 6.6+/-1.6
http://www.orthoteers.co.uk/Nrujp~ij33lm/Orthbonemech.htm 40
Bone Remodeling
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Bone deposition
Bone resorption
Bone Remodeling Bone structural integrity is continually maintained by
remodeling
Osteoclasts and osteoblasts assemble into Basic Multicellular Units (BMUs)
Bone is completely remodeled in approximately 3 years
Amount of old bone removed equals new bone formed
http://www.elixirindustry.com/resource/osteoporosis/jilka.htm 43
Basic Multicellular Units “The Basic Multicellular Unit (BMU) is a
wandering team of cells that dissolves a pit in the bone surface and then fills it with new bone.”
http://uwcme.org/site/courses/legacy/bonephys/physiology.php
BMUs are discrete temporary anatomic structures organized as functional unit
Osteoclasts remove old bone, then osteoblasts synthesize new bone
old bone is replaced by new bone in quantized packets
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BMU Remodeling Sequence
Activation
Resorption
Reversal
Quiescence
Formation & Mineralization
www.ifcc.org/ejifcc/ vol13no4/130401004n.htm
Osteocytes
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Schematic diagram of the Davy and Hart model
for bone remodelling
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The cell-biology based model of Davy and Hart expresses functional dependence of bone remodelling on the strain field, based on cell activity. The load applied to the bone together with geometric and material properties determine the local strain. Strain is detected by a transducer which generates the strain remodelling potential (SRP). This signal is modulated by genetic, hormonal and metabolic factors, generating the remodelling potential which regulates the recruitment rate and the activity of osteoblasts and osteoclasts, stimulating bone formation and bone resorption. The balance between bone deposition and bone resorption determines the net bone remodelling. remodelling modifies bone geometric and material properties through a feedback loop.
Mechanical Properties of Biological Tissue
• Mechanical properties of biological tissue can be described using strength and stiffness
• These two properties are shown graphically in this load × deformation curve
• Strength is related to the load, while stiffness (k) is the slope of the load × deformation curve
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The load × deformation curve The elastic region of the curve is between points A and B With initial loading bone can change shape (up to ~3% deformation) When deformation is < 3%, bone is more likely to return to its original shape after the load is removed (elastic deformation)
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The load × deformation curve The plastic region of the curve is between points B and C If loading continues beyond the yield point, plastic deformation is likely to occur The transition from the elastic to the plastic region is called the yield point
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The stress × strain curve
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Load Characteristics of Bone Load characteristics of a bone include:
Direction of the applied force Tension
Compression
Bending
Torsion
Shear
Magnitude of the load
Rate of load application
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Principal Mechanical Stresses (𝜎) • How might your bones be stressed in these ways? • Bones respond differently to different stress • What stress do you think your bone most effectively bears?
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Resistance to Stress Bone bears compressive stress most effectively and shear stress least effectively. How do you suppose we know this?
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Mechanical Testing Device
Note the various measures of strength 57
Sample Problem A bone sample is subjected to a stress of 80 KN. The cross sectional area is 1 cm2 (0.0001 m2) and Young’s Modulus is 70 GPa. What strain can be expected as a result of this stress?
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More stress × strain curves…ductile or brittle?
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Two Additional, Unique Mechanical
Characteristics of Bone
Anisotropic: bone responds differently depending on the direction of applied load. Stress × strain curves differ, depending on load direction. Viscoelastic: bone responds differently depending on rate of load application. Stress × strain curves differ, depending on rate of load application.
Comparison between the mechanical behavior of isotropic and anisotropic materials
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Material Properties Comparison* Material
Compressive Strength (MPa)
Modulus (GPa)
Cortical 10-160 4-27
Trabelcular 7-180 1-11
Concrete ~ 4 30
Steel 400-1500 200
Wood 100 13
Pink: http://www.engineeringtoolbox.com/24_417.html Yellow: http://www.brown.edu/Departments/EEB/EML/background/Background_Bone.htm Green: http://ttb.eng.wayne.edu/%7Egrimm/BME5370/Lect3Out.html#TrabecularBone
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*Variability of Properties Material properties listed may vary widely due to test
methods used to determine them
Variances of the following can effect results:
Orientation of sample
Bone and wood are elastically anistropic; steel is not
Condition of sample
Dry or wet with various liquids
Specifics of sample
Bone: age of donor, particular bone studied
Wood: species of tree
Steel/Concrete: preparation methods, components
http://silver.neep.wisc.edu/~lakes/BoneAniso.html 64
Ultimate strength (MPa) and ultimate strain (%) of cortical bone from the human femur as a function of age
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Cortical bone is an anisotropic material, meaning that its mechanical properties vary according to the direction of load. Cortical bone is often considered an orthotropic material. Orthotropic materials are a class of anisotropic materials characterized by three different Young's moduli E1, E2, E3 according to the direction of load, three shear moduli G12, G13, G23 and six Poisson's ratios ν12, ν13, ν23, ν21, ν31, ν32.
Average elastic constants of mandible bone in corpus and ramus
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Average elastic constants of corpus cortical bone in inferior, lingual and buccal zones
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Anisotropic Behavior of Bone
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Viscoelastic Behavior of Bone
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Three stress × strain curves for cortical bone (tension) at three different loading rates As loading rate increases, the modulus of elasticity and strength increases
قانون ولف
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Wolff’s Law
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Wolff’s Law (1892)
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Bone elements place or displace themselves in the direction of functional forces, and increase or decrease their mass to reflect the magnitude of those functional forces… In other words, bone adapts to increased use (physical activity) or disuse (bed rest) Mechanical properties of bone (strength and stiffness) that depend upon form (size, shape) can be altered in response to load
Increased Bone Mineral Content
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Osteoblast Activity > Osteoclast Activity Degree of increase in bone density
directly proportional to the
magnitude of force application
Bones with increased density are
stronger and more resistant to
fractures
Increased Bone Mineral Content
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External loads, especially high-magnitude
loads, increase bone density:
Weight bearing loads…
Obesity…
Certain athletes: tennis
A tibia that was a fibula (Adrian and Cooper, 1989)
A construction worker (Ross, 1997)
Decreased Bone Mineral Content
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Osteoclast Activity > Osteoblast Activity Reduced loading on bone can lead to substantial demineralization: 17 weeks of bedrest has been shown to lead to 10.5 % reduction in bone density Bone that is less dense is not as strong or resistant to fracture
Trabecular structures in the calcaneus of a 24 year old
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Trabecular structures of vertebrae in a 36 year old woman
Trabecular structures of vertebrae in a 74 year old woman
Bone remodelling: effect of reduction (from A to B) and of intensification of strain (from B to A) on bone trabecules.
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Decreased Bone Mineral Content
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Decreased loading results in decreases in bone
mineral density
• Physical activity levels…
• Certain athletes: swimmers, cyclists…
Anything else?
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Decreased Bone Mineral Content
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Space related bone loss:
Amount of bone loss is
proportional to time spent
away from gravitational
field (~1% per month)
Countermeasures are
now being developed to
delay rate of bone loss
Countermeasures
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A study in the March issue of the Journal of Applied Physiology reported that a person in a “reclining bedrest” position experiences the same weightless environment that an astronaut’s body must adapt to while in space. Without any exercise, Peterson’s body is going through the same muscle atrophy and bone density loss endured during space travel. Astronauts can lose 1% to 2% of their leg’s bone density in just one month in space, said Englehaupt. That is a substantial effect, he added, and is equivalent to the bone density loss a postmenopausal woman has in one year on Earth. According to Englehaupt, astronauts also experience muscle atrophy in their legs from not having to support their own weight.
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Countermeasures
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Bed rest studies at the NASA Flight Analogs Research Unit at the University of Texas Medical Branch in Galveston, Texas utilize 6-degree head down tilt best rest to simulate the physiological deconditioning in muscle, bone and cardiovascular systems associated with spaceflight.
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Function of Bone Mechanical support
Hematopoiesis زنده خونسازي درموجود
Protection of vital structures
Mineral homeostasis
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Mechanical Support Provides strength and stiffness
Hollow cylinder: Strong and light
Have mechanisms for avoiding fatigue fracture
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Hematopoiesis Development of blood cells
• Occurs in the marrow of bone
These regions are mainly composed of trabecular bone
• (e.g. The iliac crest, vertebral body, proximal and distal femur)
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Protection of Vital Structures
Flat bones in the head protect the brain
Protects heart and lungs in chest
Vertebrae in the spine protect the spinal cord
and nerves
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Mineral Homeostasis Primary storehouse of calcium and
phosphorus
Trabecular bone are rapidly formed or
destroyed
In response to shifts in calcium stasis without
serious mechanical consequences
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Fatigue of Bone
Microstructural damage due to repeated loads below the bone’s ultimate strength
Occurs when muscles become fatigued and less able to counter-act loads during continuous strenuous physical activity
Results in Progressive loss of strength and stiffness
Cracks begin at discontinuities within the bone (e.g. haversian canals, lacunae)
Affected by the magnitude of the load, number of cycles, and frequency of loading
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Fatigue of Bone (Cont’) 3 Stages of fatigue fracture
Crack Initiation Discontinuities result in points of increased local stress where
micro cracks form Often bone remodeling repairs these cracks
Crack Growth (Propagation) If micro cracks are not repaired they grow until they encounter a
weaker material surface and change direction Often transverse growth is stopped when the crack turns from
perpendicular to parallel to the load
Final Fracture Occurs only when the fatigue process progresses faster than
the rate of remodeling
http://www.orthoteers.co.uk/Nrujp~ij33lm/Orthbonemech.htm
Simon, SR. Orthopaedic Basic Science. Ohio: American Academy of Orthopaedic Surgeons; 1994. 101
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Fatigue Curve
Probability of Injury
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Process to Fatigue Failure Road to Failure: Region 1
1. Crack initiation
2. Accumulation انباشتگي
3. Growth
Characteristics:
Matrix damage in regions of
High stress concentration
Low strength
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Relatively rapid loss of stiffness
Bear less load
Absorb more energy ( can sustain larger deflections)
Cracks develop rapidly
May stabilize quickly without much
propagation
Process to Fatigue Failure (cont’d)
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Process to Fatigue Failure (Cont’d)
Cracks occur first in regions of high strain
Accumulate with either
Increased number of cycles
Increased strain
Cracks develop perpendicular to the load axis
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Road to Failure: Region 2
1. Crack growth
2. Coalescenceبهم اميختگي ،
3. Delamination اليه اليه شدگيand debonding
Characteristics:
After a crack forms
Interlamellar tensile and shear stresses are generated at its tip
Tend to separate and shear lamellae at the fiber-matrix interface
Process to Fatigue Failure (cont’d)
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Secondary cracks may extend between lamellae in the
load direction
Cracks tend to grow parallel to the load
Delamination along the load axis
Elevated and probably unidirectional strain redistributions
Along the fibers parallel to the load axis
Process to Fatigue Failure (cont’d)
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Process to Fatigue Failure (Cont’d)
Road to Failure: Region 3
Stiffness declines rapidly
End of a material’s fatigue life
Fiber failure
Coalescence of accumulated damage
Crack propagation along interfaces
Rapid process
Ultimate failure of the structure
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Fatigue Theory During repeated efforts (as in running)
Muscles become unable to support during impact
Muscles do not absorb the shock
Load is transferred to the bone
As the loading surpasses the capacity of the bone to adapt
A fracture develops
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The End
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Activation Occurs when bone experiences micro damage or
mechanical stress, or at random
A BMU originates and travels along the bone surface
•Differentiated cells are recruited from stem cell
populations
Pre-osteoclasts merge to form multi-nucleated osteoclasts
http://uwcme.org/site/courses/legacy/bonephys/physiology.php 112
Bone Resorption Newly differentiated osteoclasts are activated and begin to
resorb bone
Minerals are dissolved and the matrix is digested by enzymes and hydrogen ions secreted by the osteoclastic cells
Move longitudinally on bone surface
This process is more rapid than formation, though it may last several days
http://uwcme.org/site/courses/legacy/bonephys/physiology.php http://www.britannica.com/ebc/article?tocId=41887
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Reversal Transition from osteoclastic to osteoblastic activity
Takes several days
Results in a cylindral space (tunnel) between the resorptive
region and the refilling region
Forms the cement line
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Bone Formation Following Resorption, osteoclasts are replaced by osteoblasts
around the periphery of the tunnel
Attracted by cytokines and growth factors
Active osteoblasts secrete and produce layers of osteoid, refilling the
tunnel
Osteoblasts do not completely refill the tunnel
Leaves a Haversian canal
•Contains capillaries to support the metabolism of the
BMU and bone matrix cells
•Carries calcium and phosphorus to and from the bone
http://uwcme.org/site/courses/legacy/bonephys/physiology.php 115
Mineralization When the osteoid is about 6 microns thick, it begins to
mineralize
Formation of the initial mineral deposits at multiple discrete
sites (initiation)
Mineral is deposited within and between the collagen fibers
This process, also, is regulated by the osteoclasts
Mineral maturation
Once the cavity is full the mineral crystals pack together,
increasing the density of the new bone
http://uwcme.org/site/courses/legacy/bonephys/physiology.php 116