Bone Structure, Growth and Development

Bone Structure, Growth and Development

• Assignment: – Read Ch 4 on bone growth and development– Review stress-strain curves (pp 77-79)– Homework (due Monday, February 21)

• Introductory problems 1,2, 3,8,10

Objectives• Explain how material constituents and structural organization

of bone affect its ability to withstand mechanical loads. • Explain bone and other tissue loading modes• Explain and interpret stress/strain curves as a descriptor of

mechanical properties of bone• Describe the processes involved in the normal growth and

maturation of bone.• Describe the effects of exercise factors on bone mineral

content.• Explain the significance of osteoporosis and discuss current

theories on its prevention.

Bone Structure and Integrity• Architecture and dev - • Microstructure of bone • How bones grow?• Adaptability - Wolff’s law • Mechanical properties of bone

– stress-strain relationship – strength (density, mineral content, or BMC)– elasticity– energy absorption – resistance to fatigue

• Loading modes -– tension, compression, torsion, shear

Bone Gross Structure, Architecture and Development

Long Bone Structure

Bone Micro-Structure

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• Gives bone tensile strength

– 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 non-

mineralized tissue– Withstand greater stress but less

strain before fracturing


• 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


• Both cortical and trabecular bone are anisotropic, meaning the stress/strain response is directional

• Bone function determines structure

• 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


• Osteoblasts

• Osteoclasts

• 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


• 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 vulnerabe during peak growing years– Bone mineral density (BMD) is least during peak growing

years

– Growth plates are thickest during peak growing years


• 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

I: bone vs glass and metal

II: Anisotropic behavior of bone

Comparison of tendon andligament

Mechanical Loading Modes on the Human Body

• Compression

• Tension

• Shear

• Torsion

Bone loading modes: Compression – pushing together Tension – pulling apart Torsion – twisting Shear – cutting across

Cutting across

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 is greater than modeling threshold, then bone

modeling occurs.


• Bone Remodeling– If Strain is less than lower remodeling

threshold, then bone remodeling occurs.– at bone that is close to marrow

– “conservation mode”: no change in bone mass– “disuse mode”: net loss of bone mass

• Osteocytes


• 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

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

lamella

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

http://www.nof.org/advocacy/prevalence/

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

Female Athlete Triad

• 1) Eating Disorders affect 1-10% of all adolescent and college-age women.– Displayed in 62% female athletes

• Mostly in endurance or appearance-related sports

• 2) Amenorrhea is the cessation of the menses.

• 3) Osteoporosis is the decrease in bone mass and strength.

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

• 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– Avoiding the lifestyle (risk) factors that

negatively affect bone mass.

Types of Fractures

Common Bone Injuries

• Bone stronger in resisting compression than tension, so the side loaded with tension will fracture first.– Acute compression fractures (in absence of

osteoporosis) is rare

• Stress Fractures occur when there is no time for repair process (osteoblast activity)– Begin as small disruption in continuity of outer layers

of cortical bone (microtrauma) due to repetitive loading

Epiphyseal Injuries

• Include injuries to:• Cartilaginous epiphyseal plate

• Articular cartilage

• Apophysis

• Either acute or repetitive loading can injure growth plate– Leads to premature closing of epiphyseal

junction and termination of bone growth.

Epiphyseal Injuries

• Osteochondrosis– Disruption of blood supply to epiphyses– Associated with tissue necrosis and potential

deformation of the epiphyses.

• Apophysitis– Osteochondrosis of the apophysis– Associated with traumatic avulsions.

Summary• Bone is an important living tissue that is

continuously being remodeled.• Bone Strength and resistance to fracture depend on

its material composition and organizational structure.• Bones continue to change in density.• Osteoporosis is extremely prevalent among post

menopausal women.• Osteoporosis can be prevented through lifestyle

changes (including weight bearing and resistive exercise) and hormone replacement therapy

Documents

Bone Structure, Growth and Development