Lecture 6 THE SKELETAL SYSTEM –  Bones and Cartilage
Figure 6.1:  The bones and cartilages of the human skeleton, p. 177. Epiglottis Larynx Trachea Lung Respiratory tube carti...
Figure 6.2:  Classification of bones on the basis of shape, p. 178. (a) (b) (d) (c) Long bone (humerus) Short bone (trique...
2 Types of Bone Tissue Spongy bone Compact bone
2 types of bone tissue <ul><li>Compact bone tissue  - composed of  OSTEONS  =  structural units of compact bone.  </li></u...
Structure of a long bone <ul><li>Epiphyses  = expanded ends of long bones </li></ul><ul><li>spongy bone surrounded by a th...
Figure 6.3:  The structure of a long bone (humerus of arm), p. 180. (b) (c) (a) Proximal epiphysis Articular cartilage Yel...
Figure 6.3a:  The structure of a long bone (humerus  of arm), p. 180. (a) Proximal epiphysis Articular cartilage Epiphysea...
Figure 6.6:  Microscopic anatomy of compact bone, p. 183. (a) (b) (c) Perforating (Sharpe’s) fibers Compact bone Periostea...
Figure 6.5:  A single osteon, p. 182. Lamellae Collagen fibers Twisting force Nerve fiber Vein Artery with capillaries Str...
Figure 6.3c:  The structure of a long bone (humerus of arm), p. 180. (c) Yellow bone marrow Endosteum Compact bone Periost...
The 2  membranes <ul><li>Endosteum:  covers the internal surfaces of bone such as the canals. It  contains osteoblasts and...
The bone cells <ul><li>Osteoblasts : bone-forming cells – secrete bone tissue </li></ul><ul><li>Osteogenic cells:  give ri...
Microscopic structure of compact bone <ul><li>Composed of osteons= structural units of compact bone </li></ul><ul><li>Each...
Figure 6.6a:  Microscopic anatomy of compact bone, p. 183. (a) Perforating (Sharpey’s) fibers Compact bone Periosteal bloo...
Figure 6.15:  Fetal primary  ossification  centers at 12 weeks, p. 198. Parietal bone Radius Ulna Humerus Femur Occipital ...
OSSIFICATION ( Osteogenesis)  -  Development of the bony skeleton from the embryonic skeleton <ul><li>2 forms: PRENATAL AN...
Figure 6.7 :  Intramembranous ossification,  p. 184. Mesenchymal cell Collagen fiber Ossification center Osteoid Osteoblas...
Figure 6.7 :  Intramembranous ossification  (continued), p. 184. Mesenchyme Condensing to form the periosteum Blood vessel...
The Embryonic Skeleton
Figure 6.8:  Endochondral ossification in a long bone,  p. 185. Formation of bone collar around hyaline cartilage model. H...
<ul><li>Derived from  HYALINE CARTILAGE  produced by chondroblasts;  (chondroblasts are derived from mesenchyme )  </li></...
Endochondral Ossification <ul><li>Hyaline cartilage is ossified into bone. </li></ul><ul><li>After endochondral ossificati...
Postnatal Bone Growth <ul><li>Ossification that occurs after birth </li></ul><ul><li>2 types : Longitudinal bone growth an...
LONGITUDINAL  Bone Growth <ul><li>Involves the EPIPHYSEAL PLATES </li></ul><ul><li>New hyaline cartilage is added on at th...
Figure 6.9:  Growth in length of a long bone – zones of the Epiphyseal plate Calcified cartilage spicule Osseous tissue  (...
Figure 6.10:  Long bone growth and remodeling during youth, p. 187. Growth Bone grows in length because: Cartilage grows h...
APPOSITIONAL BONE GROWTH <ul><li>All bones widen and  increase in diameter/thickness  via appositional bone growth </li></...
Hormonal Control of Postnatal Bone Growth <ul><li>Growth hormone  – stimulates hepatocytes to produce Insulin-like growth ...
Bone Remodeling <ul><li>Adult bones constantly undergo bone formation on the periosteal surface and bone resorption on the...
Normal and osteoporotic bone Normal bone Osteoporotic bone www.mayoclinic.com/health/ osteoporosis /DS00128
Figure 6.11 :  Hormonal control of blood calcium levels , p. 189. PTH; calcitonin secreted Calcitonin stimulates calcium s...
Factors that Control Bone Remodeling <ul><li>2 factors: Hormonal control and mechanical stress </li></ul><ul><li>Hormonal ...
Forms of evidence in support of Wolff’s Law <ul><li>Bone attachment sites for active skeletal muscles appear thicker – pro...
Figure 6.12:  Bone anatomy and stress, p. 190 Load here (body weight) Head of femur Compression here Point of no stress Te...
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Skeletal system

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  • Started on slide 7 Moved back to slide 6 Slide 7
  • Structure of a long bone Diaphysis Epiphysis – expanded ends of a long bone consists of mostly spongy bone tissue surrounded by thin layers of compact bone Spongy bone tissue in epiphyses contains red bone marrow in between the trabeculae  hematopoiesis (process by which blood cells and platelets are produced) The ends of the epiphyses are covered by thin layers of hyaline cartilage called articular cartilages The surface of a long bone is covered by a membrane called PERIOSTEUM Periosteum double layered outer fibrous layer – highly vascularized because it is composed of dense irregular connective tissue blood vessels extend from the fibrous layer into the bone to provide nutrients and remove metabolic waste inner osteogenic layer – composed of two cell types osteoblasts – secrete bone tissue (derived from mesenchyme) osteoclasts - modified type of white blood cells; destroy bone tissue…. They cause BONE RESORPTION (break down of bone) -------------------------------- After slide 12, 13… moved on to 16? 17
  • Periosteum is attached to the surface of bone by collagenous fibers called the SHARPEY fibers (also called the PERFORATING fibers) (back to slide 8 to review what we’ve covered so far)
  • 2 nd type of membrane in bones Endosteum – single layered membrane that covers the surfaces of trabeculae in spongy bone end lines the internal surface of the medullary cavity, Haversian canals, and the canaliculi (back to slide 12)
  • Ossification – Process by which embryonic skeleton is turned into the bony skeleton 2 forms prenatal (before birth, in utero) postnatal (after birth) 2 types of prenatal bone development intramembranous ossification – starting material is fibrous connective tissue membrane (derived from mesenchyme) bones formed via intramembranous ossification are the cranial bones (paired temporal bones, paired parietal bone, single frontal bone, single occipital bone, single ethrnoid bone, single sphenoid bone) AND CLAVICLES the above bones formed via intramembranous ossification are referred to as membrane bones- all membrane bones are flat bones NOT ALL FLAT BONES ARE MEMBRANE BONES HOWEVER, ALL MEMBRANE BONES ARE FLAT BONES. A flat bone is a structural class of bones with flattened, slightly curved appearance consists of 2 thin plates of periosteum-covered compact bones with endosteum covered spongy bone in between the plates (SLIDE 4) endochondral ossification
  • Prenatal ossification  Endochondral ossification starting material is hyaline cartilage secreted by chondroblasts, which are derived from mesenchyme basically, endochondral ossification is the conversion of hyaline cartilage into bone tissue. How? osteoblasts infiltrate the hyaline cartilage and mineralization of the matrix from semi-solid to solid matrix bones form via endochondral ossification are called endochondral bones or cartilage bone (everything minus the 8 cranial bones and 2 clavicles) 126 named bones in the adult skeleton (10 membrane bones vs. 116 endochondral bones) Then she wrote out what she had on slide 23
  • Now, slide 24
  • Postnatal bone growth – after birth 2 types longitudinal (linear) bone growth appositional bone growth
  • Recall that epiphyseal plates are located at the junctions of the diaphysis and the 2 epiphyses of a long bone http://www.bbc.co.uk/schools/gcsebitesize/pe/images/bone_anatomy.gif
  • Hormonal Control of Bone Remodeling Calcium levels are maintained strictly between 9-11mg/100cc of blood Total Calcium in the adult is about 1.2kg… 99% is stored in bone tissue as the hydroxyapatites When blood levels of calcium fall below 9mg/100cc of blood, it is called HYPOCALCEMIA When blood levels of calcium rise above 11mg/100cc of blood, it is called HYPERCALCEMIA HYPOCALCEMIA AND HYPERCALCEMIA ARE HOMEOSTATIC IMBALANCES… hormones released to maintain normocalcemia (9-11mg/100cc) Homeostatic imbalance – Hypercalcemia stimulates the release of the hormone called calcitonin biological action of calcitonin: stimulates osteoblasts to produce bone tissue  mineralized using calcium from blood  blood calcium level drops into the normal range also inhibits osteoclasts from causing bone resorption Hypocalcemia stimulates the release of the hornone called PARATHYROID HORMONE (PTH) biological action: stimulates osteoclasts to cause bone resorption  release calcium from bones into the blood also stimulates calcium absorption from the small intestine. However PTH action in the Small intestine is INDIRECT b/c PTH first stimulates the synthesis of another hormone called 1,25 dihydroxyvitamin D (vitamin D)  directly stimulates calcium absorption from the SI PTH directly stimulates calcium reabsorption from tubular fluid inside the kidneys SUMMARY – PTH increases calcium levels in blood to 9-11mg/100cc of blood *refer to previous slide as necessary*
  • Skeletal system

    1. 1. Lecture 6 THE SKELETAL SYSTEM – Bones and Cartilage
    2. 2. Figure 6.1: The bones and cartilages of the human skeleton, p. 177. Epiglottis Larynx Trachea Lung Respiratory tube cartilages in neck and thorax = Hyaline cartilages Key: = Fibrocartilages = Elastic cartilages = Bones of axial skeleton = Bones of appendicular skeleton Cartilage in external ear Cartilages in nose Articular cartilage of a joint Costal cartilage Cartilage in intervertebral disc Pubic symphysis Articular cartilage of a joint Meniscus (padlike cartilage in knee joint)
    3. 3. Figure 6.2: Classification of bones on the basis of shape, p. 178. (a) (b) (d) (c) Long bone (humerus) Short bone (triquetral) Irregular bone (vertebra), left lateral view Flat bone (sternum)
    4. 4. 2 Types of Bone Tissue Spongy bone Compact bone
    5. 5. 2 types of bone tissue <ul><li>Compact bone tissue - composed of OSTEONS = structural units of compact bone. </li></ul><ul><li>Spongy bone tissue – like a honeycomb – composed of needle-like structures called TRABECULAE = structural units of spongy bone </li></ul>Compact bone Spongy bone
    6. 6. Structure of a long bone <ul><li>Epiphyses = expanded ends of long bones </li></ul><ul><li>spongy bone surrounded by a thin layer of compact bone </li></ul><ul><li>Diaphysis = shaft = long axis of a long bone </li></ul><ul><li>composed of a thick collar of compact bone which surrounds a Medullary Cavity – contains red bone marrow in childhood and yellow bone marrow in adulthood </li></ul><ul><li>Hematopoiesis, the process by which blood cells and platelets are formed, occurs only in red bone marrow </li></ul><ul><li>Membranes: Endosteum and Periosteum </li></ul>
    7. 7. Figure 6.3: The structure of a long bone (humerus of arm), p. 180. (b) (c) (a) Proximal epiphysis Articular cartilage Yellow bone marrow Endosteum Epiphyseal line Spongy bone Periosteum Compact bone Medullary cavity Spongy bone Compact bone Articular cartilage Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries Diaphysis Distal epiphysis
    8. 8. Figure 6.3a: The structure of a long bone (humerus of arm), p. 180. (a) Proximal epiphysis Articular cartilage Epiphyseal line Spongy bone Periosteum Compact bone Medullary cavity Diaphysis Distal epiphysis Fat
    9. 9. Figure 6.6: Microscopic anatomy of compact bone, p. 183. (a) (b) (c) Perforating (Sharpe’s) fibers Compact bone Periosteal blood vessel Periosteum Lacuna Blood vessel Endosteum lining bony canals and covering trabeculae Central (Haversian) canal Spongy bone Blood vessel continues into medullary cavity containing marrow Central (Haversian) canal Canaliculus Lacuna Lamella Osteocyte Osteon (Haversian system) Circumferential lamellae Lamellae Osteon Interstitial lamellae Central canal Perforating (Volkmann’s) canal
    10. 10. Figure 6.5: A single osteon, p. 182. Lamellae Collagen fibers Twisting force Nerve fiber Vein Artery with capillaries Structures in the central canal
    11. 11. Figure 6.3c: The structure of a long bone (humerus of arm), p. 180. (c) Yellow bone marrow Endosteum Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries
    12. 12. The 2 membranes <ul><li>Endosteum: covers the internal surfaces of bone such as the canals. It contains osteoblasts and osteoclasts </li></ul><ul><li>Periosteum: is double-layered – composed of the outer fibrous layer and the inner osteogenic layer. </li></ul><ul><li>The fibrous layer is composed of dense irregular connective tissue </li></ul><ul><li>The osteogenic layer contains of osteoblasts and osteoclasts . </li></ul><ul><li>The periosteum is attached to compact bone by tough collagenous fibers called the </li></ul><ul><li>SHARPEY”S(perforating) FIBERS </li></ul>
    13. 13. The bone cells <ul><li>Osteoblasts : bone-forming cells – secrete bone tissue </li></ul><ul><li>Osteogenic cells: give rise to osteoblasts </li></ul><ul><li>Osteocytes: matured osteoblasts </li></ul><ul><li>Osteoclasts: bone-resorbing cells – destroy bone tissue </li></ul>
    14. 14. Microscopic structure of compact bone <ul><li>Composed of osteons= structural units of compact bone </li></ul><ul><li>Each osteon is an elongated cylinder consisting of concentric tubes called LAMELLAE hence, compact bone is also known as Lamellar bone. The collagen fibers in adjacent lamellae run in opposite directions to resist twisting </li></ul><ul><li>HAVERSIAN CANAL = Central canal – runs in the core of each osteon contains blood vessels and nerves </li></ul><ul><li>Perforating or Volkmann’s canals – connect blood vessels and nerves between the periosteum and the Haversian canals </li></ul><ul><li>LACUNAE – shallow cavities in the solid bone matrix that house the osteocytes. </li></ul><ul><li>CANALICULI – tiny canals that connect lacunae to each other and to the Haversian canal to allow for transfer of substances from the blood vessel in the Haversian canal </li></ul>
    15. 15. Figure 6.6a: Microscopic anatomy of compact bone, p. 183. (a) Perforating (Sharpey’s) fibers Compact bone Periosteal blood vessel Periosteum Blood vessel Endosteum lining bony canals and covering trabeculae Central (Haversian) canal Spongy bone Blood vessel continues into medullary cavity containing marrow Osteon (Haversian system) Circumferential lamellae Lamellae Perforating (Volkmann’s) canal
    16. 16. Figure 6.15: Fetal primary ossification centers at 12 weeks, p. 198. Parietal bone Radius Ulna Humerus Femur Occipital bone Clavicle Scapula Ribs Vertebra Ilium Tibia Frontal bone of skull Mandible
    17. 17. OSSIFICATION ( Osteogenesis) - Development of the bony skeleton from the embryonic skeleton <ul><li>2 forms: PRENATAL AND POSTNATAL </li></ul><ul><li>Prenatal bone development – occurs before birth; 2 types </li></ul><ul><li>i) INTRAMEMBRANOUS OSSIFICATION ii)ENDOCHONDRAL OSSIFICATION </li></ul><ul><li>Intramembranous ossification : </li></ul><ul><li>develops from FIBROUS CONNECTIVE TISSUE MEMBRANE ( derived directly from mesenchyme) and results in the formation of MEMBRANE BONES = cranial bones and clavicles </li></ul><ul><li>Note : all membrane bones are flat bones. </li></ul>
    18. 18. Figure 6.7 : Intramembranous ossification, p. 184. Mesenchymal cell Collagen fiber Ossification center Osteoid Osteoblast Osteoid Osteocyte Newly calcified bone matrix Osteoblast An ossification center appears in the fibrous connective tissue membrane . • Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center. Bone matrix (osteoid) is secreted within the fibrous membrane. • Osteoblasts begin to secrete osteoid, which is mineralized within a few days. • Trapped osteoblasts become osteocytes. 1 2
    19. 19. Figure 6.7 : Intramembranous ossification (continued), p. 184. Mesenchyme Condensing to form the periosteum Blood vessel Trabecula of woven bone Fibrous periosteum Osteoblast Plate of compact bone Diploë (spongy bone) cavities contain red marrow Woven bone and periosteum form. • Accumulating osteoid is laid down between embryonic blood vessels, which form a random network. The result is a network (instead of lamellae) of trabeculae. • Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum. Bone collar of compact bone forms and red marrow appears. • Trabeculae just deep to the periosteum thicken, forming a woven bone collar that is later replaced with mature lamellar bone. • Spongy bone (diploë), consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow. 3 4 Intramembranous ossification : develops from FIBROUS CT MEMBRANE ( derived directly from mesenchyme) and results in the formation of MEMBRANE BONES = cranial bones and clavicles Note : all membrane bones are flat bones.
    20. 20. The Embryonic Skeleton
    21. 21. Figure 6.8: Endochondral ossification in a long bone, p. 185. Formation of bone collar around hyaline cartilage model. Hyaline cartilage Cavitation of the hyaline carti- lage within the cartilage model. Invasion of internal cavities by the periosteal bud and spongy bone formation. Formation of the medullary cavity as ossification continues; appearance of sec- ondary ossification centers in the epiphy- ses in preparation for stage 5. Ossification of the epiphyses; when completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. Deteriorating cartilage matrix Epiphyseal blood vessel Spongy bone formation Epiphyseal plate cartilage Secondary ossificaton center Blood vessel of periosteal bud Medullary cavity Articular cartilage Spongy bone Primary ossification center Bone collar 1 2 3 4 5
    22. 22. <ul><li>Derived from HYALINE CARTILAGE produced by chondroblasts; (chondroblasts are derived from mesenchyme ) </li></ul><ul><li>The bones formed from endochondral ossification are called endochondral or cartilage bones = all bones in the body except the cranial bones and the clavicles </li></ul>Endochondral Ossification
    23. 23. Endochondral Ossification <ul><li>Hyaline cartilage is ossified into bone. </li></ul><ul><li>After endochondral ossification, hyaline cartilage still persists in two areas in the long bones as the: </li></ul><ul><li> 1. Articular cartilage – capping the ends of the epiphyses of long bones </li></ul><ul><li>2. Epiphyseal plates = at the junctions of the epiphyses and the diaphysis of a long bone </li></ul>
    24. 24. Postnatal Bone Growth <ul><li>Ossification that occurs after birth </li></ul><ul><li>2 types : Longitudinal bone growth and Appositional bone growth </li></ul><ul><li>Longitudinal bone growth = Linear bone growth increases the length of long bones = height </li></ul><ul><li>Appositional bone growth = increases the width/diameter of all bones </li></ul>
    25. 25. LONGITUDINAL Bone Growth <ul><li>Involves the EPIPHYSEAL PLATES </li></ul><ul><li>New hyaline cartilage is added on at the epiphyseal faces of the epihyseal plates </li></ul><ul><li>New bone tissue is added on at the diaphyseal faces of the epiphyseal plates </li></ul><ul><li>Results in lengthening of the diaphysis of the long bone = lengthening of the long bone </li></ul><ul><li>The amount of new hyaline cartilage added on the epiphyseal face = the amount of bone tissue formed on the diaphyseal face hence, the width ( thickness) of the epiphyseal plates does NOT change </li></ul>
    26. 26. Figure 6.9: Growth in length of a long bone – zones of the Epiphyseal plate Calcified cartilage spicule Osseous tissue (bone) covering cartilage spicules Growth ( proliferation) zone Cartilage cells undergo mitosis Resting (quiescent) zone Hypertrophic zone Older cartilage cells enlarge Ossification (osteogenic) zone New bone formation is occurring Resorption zone Calcification zone Matrix becomes calcified; cartilage cells die; matrix begins deteriorating Osteoblast depositing bone matrix Diaphyseal face Epiphyseal fac e
    27. 27. Figure 6.10: Long bone growth and remodeling during youth, p. 187. Growth Bone grows in length because: Cartilage grows here Cartilage grows here Cartilage replaced by bone here Cartilage replaced by bone here Remodeling Growing shaft is remodeled by: Articular cartilage Bone resorbed here Bone added by appositional growth here Bone resorbed here Epiphyseal plate 1 2 3 4 1 2 3
    28. 28. APPOSITIONAL BONE GROWTH <ul><li>All bones widen and increase in diameter/thickness via appositional bone growth </li></ul><ul><li>Sequence of events: </li></ul><ul><li> Osteoblasts in the osteogenic layer of the periosteum secrete new bone tissue onto the external surface of the bone </li></ul><ul><li>Osteoclasts in the endosteum slightly resorb bone tissue in the internal surface of the bone </li></ul><ul><li>Overall, more new bone tissue is added onto the external surface and old bone tissue is slightly resorbed from the internal surface resulting in a thicker but lighter bone. </li></ul>
    29. 29. Hormonal Control of Postnatal Bone Growth <ul><li>Growth hormone – stimulates hepatocytes to produce Insulin-like growth factors ( IGFs) </li></ul><ul><li>IGFs stimulate chondroblasts to produce hyaline cartilage on the epiphyseal faces of the epiphyseal plates and bone formation on the diaphyseal faces </li></ul><ul><li>Sex steroid hormones ( testosterone in the male and the estrogens in the female) synergize with growth hormone to cause “ growth spurt” </li></ul><ul><li>Towards the end of adolescence, the sex steroid hormones antagonize the actions of growth hormone and the epiphyseal plates become ossified = EPIPHYSEAL PLATE CLOSURE – height determined </li></ul>
    30. 30. Bone Remodeling <ul><li>Adult bones constantly undergo bone formation on the periosteal surface and bone resorption on the endosteal surface = Bone Remodeling </li></ul><ul><li>In healthy adults, the bone density remains constant because </li></ul><ul><li>Rate of Bone formation = rate of bone resorption </li></ul><ul><li>If the rate of resorption outpaces the rate of formation = OSTEOPOROSIS </li></ul><ul><li>Functions of Bone Remodeling: </li></ul><ul><li>i) To maintain calcium homeostasis </li></ul><ul><li>ii) To allow for bone repair after fractures </li></ul>
    31. 31. Normal and osteoporotic bone Normal bone Osteoporotic bone www.mayoclinic.com/health/ osteoporosis /DS00128
    32. 32. Figure 6.11 : Hormonal control of blood calcium levels , p. 189. PTH; calcitonin secreted Calcitonin stimulates calcium salt deposit in bone Parathyroid glands release parathyroid hormone (PTH) Thyroid gland Thyroid gland Parathyroid glands Osteoclasts degrade bone matrix and release Ca 2+ into blood Falling blood Ca 2+ levels Rising blood Ca 2+ levels Calcium homeostasis of blood: 9–11 mg/100 ml PTH Imbalance Imbalance
    33. 33. Factors that Control Bone Remodeling <ul><li>2 factors: Hormonal control and mechanical stress </li></ul><ul><li>Hormonal Control : </li></ul><ul><li>Under hypercalcemic conditions, CALCITONIN is released to stimulate osteoblasts to produce bone tissue and stimulate mineralization – uses calcium from blood </li></ul><ul><li>Under hypocalcemic conditions, PARATHYROID HORMONE (PTH) is released to stimulate osteoclasts to cause bone resorption to release calcium from bones into blood </li></ul><ul><li>I,25 dihydroxyvitamin D stimulates calcium absorption from the small intestine </li></ul><ul><li>Mechanical Stress : </li></ul><ul><li>Bones remodel/grow in response to mechanical stresses placed on the bones = WOLFF’S LAW </li></ul>
    34. 34. Forms of evidence in support of Wolff’s Law <ul><li>Bone attachment sites for active skeletal muscles appear thicker – projections such as trochanters, spines, </li></ul><ul><li>Bones of the upper limb often used are thicker than the less used limb – bones in the right arm of a right-handed individual are thicker than bones in the left arm and vice versa </li></ul><ul><li>Long bones are thickest in the middle region of the diaphysis where bending stresses are greatest </li></ul><ul><li>Bedridden individual not subjected to the stresses of walking or exercises lose bone density </li></ul><ul><li>Astronauts who spend appreciably amount of time in space (where there’s no gravity and they cannot walk), lose bone density </li></ul>
    35. 35. Figure 6.12: Bone anatomy and stress, p. 190 Load here (body weight) Head of femur Compression here Point of no stress Tension here
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