Chapter 4 The Biomechanics of Human Bone Growth

Chapter 4 The Biomechanics of Human Bone Growth

Chapter 4 The Biomechanics of Human Bone Growth and Development Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. McGraw-Hill/Irwin 2012 The McGraw-Hill Companies, Inc. All rights reserved. Composition and Structure of Bone What is stiffness? (stress/strain in a loaded material; stress divided by the relative amount of change in shape)

What is compressive strength? (ability to resist compression) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-2 Composition and Structure of Bone Calcium carbonate and calcium phosphate give bone stiffness Calcium compounds make up 6070% of dry bone weight primary determiners of compressive strength Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D.

4-3 Composition and Structure of Bone Collagen contributes to flexibility and tensile strength (ability to resist tension) in bone? Collagen is progressively lost and bone brittleness increases with aging Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-4 Composition and Structure of Bone What else affects bone

strength? water content of bone, which comprises 25%-30% of bone weight bone porosity, or the amount of bone volume filled with pores or cavities Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-5 Composition and Structure of Bone Categories of bone based on porosity: cortical bone: compact mineralized

bone with low porosity; found in the shafts of long bones trabecular (or cancellous) bone: less compact bone with high porosity; found in the ends of long bones and the vertebrae Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-6 Composition and Structure of Bone Endosteum Proximal epiphysis

Cortical bone Epiphyseal plate Marrow Trabecular bone Periosteum Diaphysis Trabecular bone Nutrient artery Cortical bone Medullary cavity Distal epiphysis

Epiphyseal plate Structures of cortical (compact) and trabecular (spongy) bone. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-7 Composition and Structure of Bone What else does bone porosity affect? because cortical bone is stiffer than trabecular bone, it can withstand greater stress but less strain because trabecular bone is

spongier than cortical bone, it can undergo more strain before fracturing Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-8 Composition and Structure of Bone How does the structure of bone affect its strength? (bone is anisotropic, it has different strength and stiffness depending on the direction of

the load) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-9 Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. SHEAR (Bone is strongest in resisting compression and weakest in resisting shear.)

TENSION COMPRESSION How does the structure of bone affect its strength? Stress to Fracture Composition and Structure of Bone 4-10

Types of bones axial skeleton: skull, vertebrae, sternum, ribs appendicular skeleton: bones composing the body appendages Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-11 Types of bones short bones: approximately cubical; include the carpals and

tarsals flat bones: protect organs & provide surfaces for muscle attachments; include the scapulae, sternum, ribs, patellae, some bones of the skull Basic Biomechanics, 6 edition th By Susan J. Hall, Ph.D. 4-12 Types of bones

irregular bones: have different shapes to serve different functions; include vertebrae, sacrum, coccyx, maxilla long bones: form the framework of the appendicular skeleton; include humerus, radius, ulna, femur, tibia, fibula Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-13 Bone Growth and Development How do bones grow in length?

(the epiphyses, or epiphyseal plates, are growth centers where new bone cells are produced until the epiphysis closes during late adolescence or early adulthood) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-14 Bone Growth and Development How do bones grow in circumference? the inner layer of the periosteum, a

double- layered membrane covering bone, builds concentric layers of new bone on top of existing ones specialized cells called osteoblasts build new bone tissue and osteoclasts resorb bone tissue Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-15 Bone Response to Stress

How do bones respond to training? just like muscle, bones respond to certain kinds of training by hypertrophying according to Wolffs law, the densities, and to a lesser extent, the sizes and shapes of bones are determined by the magnitude and direction of the acting forces Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-16

Bone Response to Stress How is Wolffs law carried out? Osteoblasts and osteoclasts are continually building and resorbing bone, respectively. Increased or decreased mechanical stress leads to a predominance of osteoblast or osteoclast activity, respectively. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-17 Bone Response to Stress What kinds of activity tend to

promote bone density? (weight bearing exercise, since the larger the forces the skeletal system sustains, the greater the osteoblast response) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-18 Bone Response to Stress What tends to diminish bone density? lack of weight bearing exercise spending time in the water, (since

the buoyant force counteracts gravitational force) bed rest traveling in space outside of the earths gravitational field Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-19 Osteoporosis Osteoporosis is a disorder involving decreased bone mass and strength with pain and one or more fractures resulting from daily activity.

Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-20 Osteoporosis Who is affected by osteoporosis? Type I (postmenopausal) osteoporosis affects about 40% of women after age 50 Type II (age-associated) osteoporosis affects most women and men after age 70 Basic Biomechanics, 6th edition

By Susan J. Hall, Ph.D. 4-21 Osteoporosis Are younger people ever affected by osteoporosis? The female athlete triad includes: disordered eating amenorrhea, and osteoporosis Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-22

Osteoporosis How can osteoporosis be prevented and treated? (regular weight bearing exercise is the key to prevention and treatment) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-23 Osteoporosis How can osteoporosis be prevented and treated? postmenopausal hormone

replacement adequate dietary calcium and vitamin D avoiding smoking and excessive consumption of protein, caffeine, and alcohol Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 4-24

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