Bone Structure and Physiology

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Bone Structure and PhysiologyFatigue
Properties of Bone and Stress Fractures
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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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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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Osteon
Periosteum
Cancellous Bone
Cortical Bone
Bone Physiology. Courtesy Gray's Anatomy 35th
edit Longman Edinburgh 1973
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Microstructure of the Bone
(a)
(b)
(c)
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Microstructure of Bone (Contd)
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Composition of Bone Cells

• Osteocytes
• Osteoblasts
• Osteoclasts

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Controlling Factors
of osteoclasts and osteoblasts

• Hormones
• Estrogen
• Testosterone
• Cytokines
• Growth factors,
• Interleukins (1, 6, and 11),
• Transforming growth factor-b
• Tumor necrosis factor-a

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Controlling Factors
of osteoclasts and osteoblasts

• 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

Bacterium
Nuclei
Ingested bacterium
http//saints.css.edu/bio/schroeder/macrophage.htm
l http//academic.brooklyn.cuny.edu/biology/bio4fv
/page/phago.htm http//www.allsciencestuff.com/mbi
ology/research/osteoporosis
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Composition of Bone Matrix

• Cortical/ Compact
• Bone
• Cancellous/
• Trabecular/ Spongy Bone

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Properties of Cortical and Cancellous Bones
http//www.orthoteers.co.uk/Nrujpij33lm/Orthbonem
ech.htm
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Bone Remodeling
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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/osteoporosi
s/jilka.htm
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BMU Remodeling Sequence
www.ifcc.org/ejifcc/ vol13no4/130401004n.htm
Osteocytes
Activation
Quiescence
Resorption
Formation Mineralization
Reversal
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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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Material Properties Comparison
Pink http//www.engineeringtoolbox.com/24_417
.html Yellow http//www.brown.edu/Departments/E
EB/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
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Function of Bone

• Mechanical support
• Hematopoiesis
• Protection of vital structures
• Mineral homeostasis

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Fatigue of Bone

• Microstructural damage due to repeated loads
  below the bones 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/Nrujpij33lm/Orthbonem
  ech.htm
• Simon, SR. Orthopaedic Basic Science. Ohio
  American Academy of Orthopaedic Surgeons 1994.

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Process to Fatigue Failure

• Road to Failure Region 1
• Crack initiation
• Accumulation
• Growth
• Characteristics
• Matrix damage in regions of
• High stress concentration
• Low strength

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Process to Fatigue Failure (contd)

• Relatively rapid loss of stiffness
• Bear less load
• Absorb more energy ( can sustain larger
  deflections)
• Cracks develop rapidly
• May stabilize quickly without much propagation

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Process to Fatigue Failure (Contd)

• 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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Process to Fatigue Failure (contd)

• Road to Failure Region 2
• Crack growth
• Coalescence
• Delamination and 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

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Process to Fatigue Failure (contd)

• 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

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Process to Fatigue Failure (Contd)

• Road to Failure Region 3
• Stiffness declines rapidly
• End of a materials fatigue life
• Fiber failure
• Coalescence of accumulated damage
• Crack propagation along interfaces
• Rapid process
• Ultimate failure of the structure

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Stress Fractures

• Stress fractures are
• Partial or complete fractures of bone
• Repetitive strain during sub-maximal activity
• There are two main types
• Fatigue fracture
• Insufficiency fracture

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Fatigue Fracture

• A fatigue fracture may be caused by
• Abnormal muscle stress
• Loss of shock absorption
• Strenuous or repeated activity
• Torque
• bone with normal elastic resistance
• Associated with new or different activity
• Abnormal loading
• Abnormal stress distribution

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Fatigue Micro Damage
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Insufficiency Fractures

• Due to normal muscular activity stressing the
  bone
• Seen in post-menopausal and/or amenhorroeic women
  whose bones are
• Deficient in mineral
• Reduced elastic resistance
• Occurs if osteoporosis or some other disease
  weakens the bones

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Signs and Symptoms

• Pain that develops gradually
• Increases with weight-bearing activity
• Diminishes with rest
• Swelling on the top of the foot or the outside
  ankle
• Tenderness to touch at the site of the fracture
• Possible bruising

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Causes of Stress Fractures

• There are two theories about the origin of stress
  fractures
• Fatigue theory
• Overload theory

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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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Overload Theory

• Certain muscle groups contract
• Cause the attached bones to bend
• After repeated contractions and bending
• Bone finally breaks

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Risk Factors for Stress Fractures

• Age
• The risk increases with age
• Bone is less resistant to fatigue in older people
• Training errors
• Sudden, drastic increase in running mileage or
  intensity
• Running with an unequal distribution of weight
  across the foot
• Intense training after an extended period of rest
• Beginning training too great in quantity or
  intensity

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Risk Factors for Stress Fractures (Contd)

• Fitness history
• Sedentary people entering a sports program are
  prone to injury
• Gradual increase in training loads is important
• Footwear
• Only significant factor is the condition of the
  running shoe
• Newer shoes lead to fewer fractures

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Risk Factors for Stress Fractures (Contd)

• Endocrine status
• Women athletes suffering from amenorrhea are at
  especially high risk
• Heavy endurance training may also compromise
  androgen status in men
• Nutritional factors
• Recommended calcium intake in post-puberty is
  800mg/day
• Stress-fracture patients are encouraged to
  consume 1500mg of calcium daily

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Risk Factors for Stress Fractures (Contd)

• Biomechanical factors
• Incidence of stress fractures are due to
• Tibial torsion (twisting/bending)
• Degree of external rotation at the hip
• When neither were present
• Incidence was 17
• When both were present
• Incidence was 45

- Gilati and Abronson (1985)
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Risk Factors for Stress Fractures (Contd)

• Other factors include
• High arched foot
• Excessive pronation of foot (turning inward)
• Excessive supination of foot (turning outward)
• Longer second toe
• Bunion on the great toe

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Prevention of Stress Fractures

• Avoid abrupt increases in overall training load
  and intensity
• Take adequate rest
• Replace running shoes
• Tend to lose their shock-absorbing capacity by
  400 miles
• Bony alignment may be modified to some extent by
  the use of orthotics
• Women athletes should pay careful attention to
• Training
• Hormonal status
• Nutrition and eating disorders

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Treatment of Stress Fractures

• Discontinue the activity
• Rest
• Ice
• Elevate the affected part
• Non-impact aerobic activity (e.g. swimming and
  cycling)
• Cast (if necessary)
• Crutches

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The End
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Osteon
Haversian Canal

• Major structural unit of cortical bone
• Concentric cylinders of bone matrix around
  haversian canals

http//www.nd.edu/humosteo/OsteonModel.gi
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Periosteum

• Capillary-rich, fibrous membrane coating exterior
  bone surface
• Responsible for nourishing bone

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nuclei
The osteoclast is a large cell with multiple
nuclei
cytoplasm
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Osteoclasts

• Located in lacunae
• Derive from pluripotent cells of the bone marrow
• Responsible for bone resorption
• Bind to bone via integrins
• Enzymes digest bone matrix
• Controlled by hormonal and growth factors
• Identifying traits
• Large size
• Mulitple nuclei
• Ruffled edge
• Location of active resorption

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Osteoblasts

• Bone forming cells
• Line the surface of the bone
• Surrounded by unmineralized bone matrix
• Derived from osteoprogenitor cell line
• Produce type I collagen
• Secretion is polarized towards the bone surface
• Attract Ca salts and P to precipitate to
  mineralize the bone

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Osteoblasts (Contd)

• Upon completion of bone formation,
• Remains on the surface of bone
• Covered by non-calcified osteoid
• Identifying traits
• Outer membrane surface coated in alkaline
  phosphates
• Polarized (nucleus away from bone surface)
• Basophilic stains

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Osteocytes

• Osteoblasts surrounded by mineralized bone matrix
• Most numerous bone cell
• Positioned between lamellae in a concentric
  pattern around the central lumen of osteons
• Regulate extracellular concentration of calcium
  and phosphate

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Osteocytes (Contd)

• Mechanosensory cells
• Respond to deformation
• Flow of interstitial fluid through the osteocytic
  canalicular network
• Directed away from regions of high strain
• Initiates electrokinetic and mechanical signals
• Growth Facors (intercellular signal molecules)
• Insulin-like growth factor, IGF-1,
• Prostaglandins G/H synthase
• PGE2 and Nitric oxide

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(a) First Level

• Hydroxyapatite crystals embedded between collagen
  fibril

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(b) Second Level

• Fibrils are arranged into lamellae
• Sheets of collagen fibers with a preferred
  orientation

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(c) Third Level

• Lamellae are arranged into tubular osteons

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Osteoclast
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Osteocytes
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Osteoblast
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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/boneph
  ys/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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Basic Multicellular Units (contd)
A photomicrograph of bone showing osteoblasts and
osteoclasts together in one Bone Metabolic Unit
http//uwcme.org/site/courses/legacy/bonephys/ph
ysiology.php
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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/phys
iology.php
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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/phys
iology.php http//www.britannica.com/ebc/article?t
ocId41887
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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/phys
iology.php
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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/phys
iology.php
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Quiescence

• After the tunneling and refilling
• Some osteoblasts become osteocytes
• Remain in bone, sense mechanical stresses on bone
• Remaining osteoblasts become lining cells
• Calcium release from bones
• Period of relative inactivity
• Secondary osteon and its associated cells carry
  on their mechanical, metabolic and homeostatic
  functions

http//uwcme.org/site/courses/legacy/bonephys/phys
iology.php
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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 Curve
Probability of Injury
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