Bone Structure, Growth and Development

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: 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

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 • 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