Bone Structure & Dev: Readings
– Frankel and Nordin, Chapter 2
– Frost, H.M. (2000) Muscle, bone, and the Utah
paradigm: A 1999 overview. Medicine & Science in
Sports & Exercise, 32:5, pp 911-917.
– Turner, C.H. and Robling, A.G. (2003) Designing
exercise regimens to increase bone strength. Ex & Sp Sci
Rev, 31:1 pp 45-50.
– Modlesky, C.M. and Lewis, R.D. (2002) Ex & Sp Sci
Rev, 30:4 pp 111-176.
– Humphries, B., et al. (2000) Effect of exercise intensity
on bone density, strength, and calcium turnover in older
women. Medicine & Science in Sports & Exercise, 32:6,
pp 1043-1050.
Bone Structure & Dev Outline
• Outline
– Structure and architecture
– Development and growth
• Process – continuous remodeling
• Factors affecting bone density and strength
– Mechanical properties
– Osteoporosis
Bone Gross Structure, Architecture and Development
Long Bone
Structure
Bone
MicroStructure,
cont’d
Projections of osteocytes
are thought to be cite of
strain sensing, which
stimulates bone to form
Bone Composition & Structure
• Material Constituents:
– Calcium carbonate and Calcium phosphate
• 60-70% bone weight
• Adds stiffness
• Primary determinant for compressive strength.
– Collagen
• Adds flexibility
• Contributes to tensile strength
– Material Constituents
– Water
• 25-30% bone weight
• Contributes to bone strength
• Provides transportation for nutrients and wastes.
Bone Composition & Structure
• Structural Organization
– Bone mineralization ratio specific to bone
– Two categories of porous bone:
• Cortical bone(70-95% mineral content)
• Trabecular bone (10-70% mineral content)
– More porous bones have:
• Less calcium phosphate
• More calcium carbonate
• Greater proportion of non-mineralized tissue
Bone Composition & Structure
• Cortical Bone
– Low porosity
– 5-30% bone volume is nonmineralized tissue
– Withstand greater stress but less
strain before fracturing
Bone Composition & Structure
• Trabecular Bone
– High porosity
– 30 - >90% bone volume is non-mineralized
tissue
– Trabeculae filled with marrow and fat
– Withstand more strain (but less stress) before
fracturing
Bone Composition & Structure
• Both cortical and trabecular bone are
anisotropic – stress/strain response is
directional
• Bone function determines structure (Wolff’s
law)
• Strongest at resisting compressive stress
• Weakest at resisting shear stress
Bone Growth & Development
• Longitudinal Growth
– at epiphyses or epiphyseal plates
– Stops at 18 yrs of age (approx.)
• can be seen up to 25 yrs of age
• Circumferential Growth
– Diameter increases throughout lifespan
– Most rapid growth before adulthood
• Periosteum build-up in concentric layers
• Endosteal growth
• Internal remodeling
Bone Growth & Development
• Osteoblasts – bone building cells
• Osteoclasts – bone absorbing cells
• Osteocytes – mature bone cells, embedded in bony
matrix in circular pattern
• Adult Bone Development
– Balance between oseoblast and osetoclast activity
– Increase in age yields progressive decrease in collagen
and increase in bone brittleness.
• Greater in women
lamella
Bone Growth & Development
• Women
– Peak bone mineral content: 25-28 yrs.
– 0.5%-1.0% loss per year following age 50 or menopause
– 6.5% loss per year post-menopause for first 5-8 years.
• Youth – bones are vulnerable during peak growing years
– Bone mineral density (BMD) is least during peak growing
years
– Growth plates are thickest during peak growing years
Bone Growth & Development
• Aging
– Bone density loss as soon as early 20’s
– Decrease in mechanical properties and general
toughness of bone
– Increasing loss of bone substance
– Increasing porosity
– Disconnection and disintegration of trabeculae
leads to weakness
Bone loading modes:
Compression – pushing together
Tension – pulling apart
Torsion – twisting
Shear – cutting across
Cutting across
Stress-strain curve:
Loaddeformation
relationship:
Repetitive vs.
Acute Loads
•
•
•
•
Repetitive loading
Acute loading
Macrotrauma
Microtrauma
I: bone vs glass and metal
II: Anisotropic behavior of bone
Comparison of tendon and
ligament
Bone Response to Stress
• Wolf’s Law
– Indicates that bone strength increases and decreases as
the functional forces on the bone increase and decrease.
• Bone Modeling and Remodeling
– Mechanical loading causes strain
– Bone Modeling
• If Strain > modeling threshold, then bone modeling occurs.
– “conservation mode”: no change in bone mass
– “disuse mode”: net loss of bone mass
• Osteocytes – projections sense strain, or pressure,
beginning remodeling process
Bone Response to Stress
• Bone mineral density generally parallels
body weight
– Body weight provides most constant
mechanical stress
– Determined by stresses that produce strain on
skeleton
– Think: weight gain or loss and its effect on
bone density
Frost’s mechanostat
Theory of bone’s
Response to stress
What factors might
Change threshold
Levels?
Bone Hypertrophy
• An increase in bone mass due to predominance of
osteoblast activity.
• Seen in response to regular physical activity
– Ex: tennis players have muscular and bone hypertrophy
in playing arm.
• The greater the habitual load, the more
mineralization of the bone.
– Also relates to amount of impact of activity/sport
Bone Atrophy
• A decrease in bone mass resulting form a
predominance of osteoclast activity
– Accomplished via remodeling
– Decreases in:
• Bone calcium
• Bone weight and strength
• Seen in bed-ridden patients, sedentary
elderly, and astronauts
Osteoporosis
• Website on osteporosis: http://www.nof.org
National Osteoporosis Foundation
• A disorder involving decreased bone mass and
strength with one or more resulting fractures.
• Found in elderly
– Mostly in postmenopausal and elderly women
– Causes more than 1/2 of fractures in women, and 1/3 in
men.
• Begins as osteopenia
Osteoporosis
• Type I Osteoporosis = Post-menopausal
Osteoporosis
– Affects about 40% of women over 50
– Gender differences
• Men reach higher peak bone mass and strength in
young adulthood
• Type II Osteoporosis = Age-Associated
Osteoporosis
– Affects most women and men over 70
Osteoporosis
• Symptoms:
– Painful, deforming and debilitating crush
fractures of vertebrae
• Usually of lumbar vertebrae from weight bearing
activity, which leads to height loss
– Estimated 26% of women over 50 suffer from these
fractures
Osteoporosis
• Men have an increase in vertebral diameter
with aging
– Reduces compressive stress during weight
bearing activities
– Structural strength not reduced
– Not known why same compensatory changes
do not occur in women
Position Statement of ACSM on
Osteoporosis
• Weightbearing physical activity is essential for developing
and maintaining a healthy skeleton
• Strength exercises may also be beneficial, particularly for
non-weightbearing bones
• An increase in physical activity for sedentary women can
prevent further inactivity-related bone loss and can even
improve bone mass
• Exercise is not an adequate substitute for postmenopausal
hormone replacement
• Ex programs for older women should include activities for
improving strength, flexibility, and coordination, to lessen
the likelihood of falls
Osteoporosis Treatment
•
•
•
•
•
Hormone replacement therapy
Estrogen deficiency damages bone
Increased dietary calcium
Lifestyle factors affect bone mineralization
Risk factors for osteoporosis:
–
–
–
–
Smoking, alcohol
Inactivity
Low body fat
White, female, postmenopausal
Osteoporosis Treatment
• Future use of pharmacologic agents
– May stimulate bone formation
– Low doses of growth factors to stimulate osteoblast
recruitment and promote bone formation.
• Best Bet:
– Engaging in regular physical activity involving weight
bearing and resistive exercise
– Avoiding the lifestyle (risk) factors that negatively
affect bone mass.
Common Bone Injuries
• Stress Fractures
– Begin as small disruption in continuity of outer layers of cortical
bone.
– Occur when there is no time for repair process (osteoblast activity)
• Injuries to articular cartilage (osteoarthritis)
• Epiphyseal injuries
– Injuries to cartilaginous epiphyseal plate
• Acute and repetitive loading can cause
– Premature closing of epiphyseal junction and termination of bone growth
– Osteochondrosis
• Disruption of blood supply to epiphyses
• Associated with tissue necrosis and potential deformation of the epiphyses.
– Injuries to tendon-bone junction, the apophysis
• Apophysitis
– Osteochondrosis of the apophysis
– Associated with traumatic avulsions.