1. 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
Turner, C.H. and Robling, A.G. (2003) Designing exercise regimens to increase bone strength. Ex & Sp Sci Rev, 31:1 pp
Modlesky, C.M. and Lewis, R.D. (2002) Ex & Sp Sci Rev, 30:4 pp
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

2. Bone Structure & Dev Outline
Structure and architecture
Development and growth
Process – continuous remodeling
Factors affecting bone density and strength
Mechanical properties
Osteoporosis

3. Bone Gross Structure, Architecture and Development

4. Long Bone Structure

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

7. 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.

8. 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

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

10. 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

11. 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

12. 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

13. 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

14. lamella

16. 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 vulnerable during peak growing years
Bone mineral density (BMD) is least during peak growing years
Growth plates are thickest during peak growing years

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

19. Cutting across

20. Load-deformation relationship:
Stress-strain curve:
Load-deformation relationship:

21. Repetitive vs. Acute Loads
Repetitive loading
Acute loading
Macrotrauma
Microtrauma

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

23. Comparison of tendon and
ligament

25. 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

26. 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

27. Frost’s mechanostat
Theory of bone’s
Response to stress
What factors might
Change threshold
Levels?

28. 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

29. 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

30. 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

31. 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

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. 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

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
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

36. 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.

37. 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.