1. 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

2. 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.

3. 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

4. Bone Gross Structure, Architecture and Development

5. Long Bone Structure

6. Bone Micro-Structure
Projections of osteocytes are thought to be cite of strain sensing, which
stimulates bone to form

8. 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.

9. 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

10. Bone Composition & Structure
Cortical Bone
Low porosity
5-30% bone volume is non-mineralized tissue
Withstand greater stress but less strain before fracturing

11. 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

12. Bone Composition & Structure
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

13. 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

14. Bone Growth & Development
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

15. Bone Growth & Development
Women
Peak bone mineral content: 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

16. 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

18. I: bone vs glass and metal
II: Anisotropic behavior of bone

19. Comparison of tendon and
ligament

20. Mechanical Loading Modes on the Human Body
Compression
Tension
Shear
Torsion

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

22. Cutting across

23. 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.

24. Bone Response to Stress
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

25. 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

26. 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

28. 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

29. lamella

31. 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

32. 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

33. 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.

34. 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

35. 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.

36. Types of Fractures

37. 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

38. 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.

39. Epiphyseal Injuries Osteochondrosis Apophysitis
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.

40. 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