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Bone Structure and Physiology & Fatigue Properties of Bone and Stress Fractures.

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1 Bone Structure and Physiology & Fatigue Properties of Bone and Stress Fractures

2 Bone Structural support of the body Structural support of the body Connective tissue that has the potential to repair and regenerate Connective tissue that has the potential to repair and regenerate Comprised of a rigid matrix of calcium salts deposited around protein fibers Comprised of a rigid matrix of calcium salts deposited around protein fibers Minerals provide rigidityMinerals provide rigidity Proteins provide elasticity and strengthProteins provide elasticity and strength

3 Shape ~jgjohnso/skeleton.html Long, short, flat, and irregular Long, short, flat, and irregular Long bones are cylindrical and “hollow” to achieve strength and minimize weightLong bones are cylindrical and “hollow” to achieve strength and minimize weight

4 Bone Physiology. Courtesy Gray's Anatomy 35th edit Longman Edinburgh 1973 Cancellous Bone Cortical Bone Osteon Periosteum

5 Microstructure of the Bone (a)(b)(c)

6 Microstructure of Bone (Cont’d)

7 Composition of Bone: Cells Osteocytes Osteocytes Osteocytes Osteoblasts Osteoblasts Osteoblasts Osteoclasts Osteoclasts Osteoclasts

8 Controlling Factors Controlling Factors Hormones Hormones EstrogenEstrogen TestosteroneTestosterone CytokinesCytokines  Growth factors,  Interleukins (1, 6, and 11),  Transforming growth factor-b  Tumor necrosis factor-a of osteoclasts and osteoblasts

9 Macrophage Macrophage Phagocytose invading pathogensPhagocytose invading pathogens Cell alters shape to surround bacteria or debris Cell alters shape to surround bacteria or debris Process: Chemotaxis, adherence, phagosome formation, phagolysosome formation Process: Chemotaxis, adherence, phagosome formation, phagolysosome formation Secrete Interleukin-1Secrete Interleukin-1 (IL-1) (IL-1) Involved in boneInvolved in boneresorption Controlling Factors Controlling Factors of osteoclasts and osteoblasts Bacterium Nuclei Ingested bacterium

10 Composition of Bone: Matrix Cortical/ Compact Cortical/ CompactBone Cancellous/ Cancellous/ Trabecular/ Spongy Bone

11 CorticalCancellous Physical Description Dense protective shell Rigid lattice designed for strength; Interstices are filled with marrow Location Around all bones, beneath periosteum; Primarily in the shafts of long bones In vertebrae, flat bones (e.g. pelvis) and the ends of long bones % of Skeletal Mass 80%20%

12 CorticalCancellous First Level Structure OsteonsTrabeculae Porosity5-10%50-90% Circulation Slow circulation of nutrients and waste Haversian system allows diffusion of nutrients and waste between blood vessels and cells; Cells are close to the blood supply in lacunae

13 CorticalCancellous Strength Withstand greater stress Withstand greater strain Direction of Strength Bending and torsion, e.g. in the middle of long bones Compression; Young’s modulus is much greater in the longitudinal direction StiffnessHigherLower Fracture Point Strain>2%Strain>75%

14 Properties of Cortical and Cancellous Bones Load Type Elastic modulus (10 9 N/m 2 ) Ultimate stress (10 6 N/m 2 ) Bone Type CorticalCancellousCorticalCancellous Tension11-19~ ~3-20 Compression –50 Shear /-1.6

15 Bone Remodeling

16 Bone structural integrity is continually maintained by remodeling Bone structural integrity is continually maintained by remodeling Osteoclasts and osteoblasts assemble into Basic Multicellular Units (BMUs)Osteoclasts and osteoblasts assemble into Basic Multicellular Units (BMUs)Basic Multicellular Units (BMUs)Basic Multicellular Units (BMUs) Bone is completely remodeled in approximately 3 yearsBone is completely remodeled in approximately 3 years Amount of old bone removed equals new bone formedAmount of old bone removed equals new bone formed

17 BMU Remodeling Sequence Activation Resorption Reversal Quiescence Formation & Mineralization vol13no4/ n.htm Osteocytes

18 Load Characteristics of Bone Load characteristics of a bone include: Load characteristics of a bone include: Direction of the applied force Direction of the applied force Direction of the applied force Direction of the applied force TensionTension CompressionCompression BendingBending TorsionTorsion ShearShear Magnitude of the load Magnitude of the load Rate of load application Rate of load application

19 Material Properties Comparison* Material Compressive Strength (MPa) Modulus (GPa) Cortical Trabelcular Concrete ~ 4 30 Steel Wood10013 Pink: Yellow: Green:http://ttb.eng.wayne.edu/%7Egrimm/BME5370/Lect3Out.html#TrabecularBonehttp://ttb.eng.wayne.edu/%7Egrimm/BME5370/Lect3Out.html#TrabecularBone

20 *Variability of Properties   Material properties listed may vary widely due to test methods used to determine them   Variances of the following can effect results:   Orientation of sample   Bone and wood are elastically anistropic; steel is not   Condition of sample   Dry or wet with various liquids   Specifics of sample   Bone: age of donor, particular bone studied   Wood: species of tree   Steel/Concrete: preparation methods, components

21 Function of Bone Mechanical support Mechanical support Mechanical support Mechanical support Hematopoiesis Hematopoiesis Hematopoiesis Protection of vital structures Protection of vital structures Protection of vital structures Protection of vital structures Mineral homeostasis Mineral homeostasis Mineral homeostasis Mineral homeostasis

22 Fatigue of Bone Microstructural damage due to repeated loads below the bone’s ultimate strength Microstructural damage due to repeated loads below the bone’s ultimate strength Occurs when muscles become fatigued and less able to counter-act loads during continuous strenuous physical activityOccurs when muscles become fatigued and less able to counter-act loads during continuous strenuous physical activity Results in Progressive loss of strength and stiffnessResults in Progressive loss of strength and stiffness Cracks begin at discontinuities within the bone (e.g. haversian canals, lacunae) Cracks begin at discontinuities within the bone (e.g. haversian canals, lacunae) Affected by the magnitude of the load, number of cycles, and frequency of loadingAffected by the magnitude of the load, number of cycles, and frequency of loading

23 Fatigue of Bone (Cont’) 3 Stages of fatigue fracture 3 Stages of fatigue fracture Crack InitiationCrack Initiation Discontinuities result in points of increased local stress where micro cracks form Discontinuities result in points of increased local stress where micro cracks form Often bone remodeling repairs these cracksOften bone remodeling repairs these cracks Crack Growth (Propagation)Crack Growth (Propagation) If micro cracks are not repaired they grow until they encounter a weaker material surface and change direction If micro cracks are not repaired they grow until they encounter a weaker material surface and change direction Often transverse growth is stopped when the crack turns from perpendicular to parallel to the loadOften transverse growth is stopped when the crack turns from perpendicular to parallel to the load Final FractureFinal Fracture Occurs only when the fatigue process progresses faster than the rate of remodeling Occurs only when the fatigue process progresses faster than the rate of remodeling Simon, SR. Orthopaedic Basic Science. Ohio: American Academy of Orthopaedic Surgeons; 1994.

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25 Process to Fatigue Failure Road to Failure: Region 1 1.Crack initiation 2.Accumulation 3.Growth Characteristics: Matrix damage in regions ofMatrix damage in regions of High stress concentration High stress concentration Low strength Low strength

26 Relatively rapid loss of stiffnessRelatively rapid loss of stiffness Bear less loadBear less load Absorb more energy ( can sustain larger deflections)Absorb more energy ( can sustain larger deflections) Cracks develop rapidlyCracks develop rapidly May stabilize quickly without much propagation May stabilize quickly without much propagation Process to Fatigue Failure (cont’d)

27 Process to Fatigue Failure (Cont’d) Cracks occur first i n regions of high strainCracks occur first i n regions of high strain Accumulate with either Accumulate with either  Increased number of cycles  Increased strain Cracks develop perpendicular to the load axisCracks develop perpendicular to the load axis

28 Road to Failure: Region 2 1.Crack growth 2.Coalescence 3.Delamination and debonding Characteristics: After a crack formsAfter a crack forms Interlamellar tensile and shear stresses are generated at its tip Interlamellar tensile and shear stresses are generated at its tip Tend to separate and shear lamellae at the fiber-matrix interface Tend to separate and shear lamellae at the fiber-matrix interface Process to Fatigue Failure (cont’d)

29 Secondary cracks may extend between lamellae in the load directionSecondary cracks may extend between lamellae in the load direction Cracks tend to grow parallel to the loadCracks tend to grow parallel to the load Delamination along the load axisDelamination along the load axis Elevated and probably unidirectional strain redistributions Elevated and probably unidirectional strain redistributions  Along the fibers parallel to the load axis Process to Fatigue Failure (cont’d)

30 Process to Fatigue Failure (Cont’d) Road to Failure: Region 3 Stiffness declines rapidlyStiffness declines rapidly End of a material’s fatigue lifeEnd of a material’s fatigue life Fiber failureFiber failure Coalescence of accumulated damage Coalescence of accumulated damage Crack propagation along interfaces Crack propagation along interfaces Rapid processRapid process Ultimate failure of the structureUltimate failure of the structure

31 Stress Fractures Stress fractures are Stress fractures are Partial or complete fractures of bonePartial or complete fractures of bone Repetitive strain during sub-maximal activityRepetitive strain during sub-maximal activity There are two main types: There are two main types: 1.Fatigue fracture 2.Insufficiency fracture

32 Fatigue Fracture A fatigue fracture may be caused by: A fatigue fracture may be caused by: Abnormal muscle stressAbnormal muscle stress Loss of shock absorption Loss of shock absorption Strenuous or repeated activity Strenuous or repeated activity Strenuous or repeated activity Strenuous or repeated activity TorqueTorque bone with normal elastic resistance bone with normal elastic resistance Associated with new or different activityAssociated with new or different activity Abnormal loading Abnormal loading Abnormal stress distribution Abnormal stress distribution

33 Fatigue Micro Damage

34 Insufficiency Fractures Due to normal muscular activity stressing the bone Due to normal muscular activity stressing the bone Seen in post-menopausal and/or amenhorroeic women whose bones are Seen in post-menopausal and/or amenhorroeic women whose bones are Deficient in mineralDeficient in mineral Reduced elastic resistanceReduced elastic resistance Occurs if osteoporosis or some other disease weakens the bones Occurs if osteoporosis or some other disease weakens the bones

35 Signs and Symptoms  Pain that develops gradually  Increases with weight-bearing activity  Diminishes with rest  Swelling on the top of the foot or the outside ankle  Tenderness to touch at the site of the fracture  Possible bruising

36 Causes of Stress Fractures There are two theories about the origin of stress fractures: 1. Fatigue theory 2. Overload theory

37 Fatigue Theory During repeated efforts (as in running)During repeated efforts (as in running) Muscles become unable to support during impact Muscles become unable to support during impact Muscles do not absorb the shock Muscles do not absorb the shock Load is transferred to the bone Load is transferred to the bone As the loading surpasses the capacity of the bone to adapt As the loading surpasses the capacity of the bone to adapt A fracture develops A fracture develops

38 Overload Theory Certain muscle groups contract Certain muscle groups contract Cause the attached bones to bendCause the attached bones to bend After repeated contractions and bending After repeated contractions and bending  Bone finally breaks

39 Risk Factors for Stress Fractures  Age: The risk increases with ageThe risk increases with age Bone is less resistant to fatigue in older peopleBone is less resistant to fatigue in older people  Training errors: Sudden, drastic increase in running mileage or intensitySudden, drastic increase in running mileage or intensity Running with an unequal distribution of weight across the footRunning with an unequal distribution of weight across the foot Intense training after an extended period of restIntense training after an extended period of rest Beginning training too great in quantity or intensityBeginning training too great in quantity or intensity

40 Fitness history: Fitness history: Sedentary people entering a sports program are prone to injurySedentary people entering a sports program are prone to injury Gradual increase in training loads is importantGradual increase in training loads is important Footwear: Footwear: Only significant factor is the condition of the running shoeOnly significant factor is the condition of the running shoe Newer shoes lead to fewer fracturesNewer shoes lead to fewer fractures Risk Factors for Stress Fractures (Cont’d)

41 Endocrine status: Endocrine status: Women athletes suffering from amenorrhea are at especially high riskWomen athletes suffering from amenorrhea are at especially high risk Heavy endurance training may also compromise androgen status in menHeavy endurance training may also compromise androgen status in men Nutritional factors: Nutritional factors: Recommended calcium intake in post-puberty is 800mg/dayRecommended calcium intake in post-puberty is 800mg/day Stress-fracture patients are encouraged to consume 1500mg of calcium dailyStress-fracture patients are encouraged to consume 1500mg of calcium daily Risk Factors for Stress Fractures (Cont’d)

42 Biomechanical factors: Biomechanical factors: Incidence of stress fractures* are due toIncidence of stress fractures* are due to  Tibial torsion (twisting/bending)  Degree of external rotation at the hip When neither were presentWhen neither were present  Incidence was 17% When both were presentWhen both were present  Incidence was 45% Risk Factors for Stress Fractures (Cont’d) * - Gilati and Abronson (1985)

43 Other factors include: Other factors include: High arched footHigh arched foot Excessive pronation of foot (turning inward)Excessive pronation of foot (turning inward) Excessive supination of foot (turning outward)Excessive supination of foot (turning outward) Longer second toeLonger second toe Bunion on the great toeBunion on the great toe Risk Factors for Stress Fractures (Cont’d)

44 Prevention of Stress Fractures Avoid abrupt increases in overall training load and intensity Avoid abrupt increases in overall training load and intensity Take adequate rest Take adequate rest Replace running shoes Replace running shoes Tend to lose their shock-absorbing capacity by 400 miles Tend to lose their shock-absorbing capacity by 400 miles Bony alignment may be modified to some extent by the use of orthotics Bony alignment may be modified to some extent by the use of orthotics Women athletes should pay careful attention to Women athletes should pay careful attention to TrainingTraining Hormonal statusHormonal status Nutrition and eating disordersNutrition and eating disorders

45 Treatment of Stress Fractures Discontinue the activity Discontinue the activity Rest Rest Ice Ice Elevate the affected part Elevate the affected part Non-impact aerobic activity (e.g. swimming and cycling) Non-impact aerobic activity (e.g. swimming and cycling) Cast (if necessary) Cast (if necessary) Crutches Crutches

46 The End

47 Osteon Major structural unit of cortical bone Major structural unit of cortical bone Concentric cylinders of bone matrix around haversian canalsConcentric cylinders of bone matrix around haversian canals Haversian Canal

48 Periosteum Capillary-rich, fibrous membrane coating exterior bone surface Capillary-rich, fibrous membrane coating exterior bone surface Responsible for nourishing boneResponsible for nourishing bone

49 The osteoclast is a large cell with multiple nuclei nuclei cytoplasm

50 Osteoclasts Located in lacunae Located in lacunae Derive from pluripotent cells of the bone marrow Derive from pluripotent cells of the bone marrow Responsible for bone resorption Responsible for bone resorption Bind to bone via integrinsBind to bone via integrins Enzymes digest bone matrixEnzymes digest bone matrix Controlled by hormonal and growth factorsControlled by hormonal and growth factors Identifying traits Identifying traits Large sizeLarge size Mulitple nucleiMulitple nuclei Ruffled edgeRuffled edge Location of active resorption Location of active resorption

51 Osteoblasts Bone forming cells Bone forming cells Line the surface of the boneLine the surface of the bone Surrounded by unmineralized bone matrixSurrounded by unmineralized bone matrix Derived from osteoprogenitor cell lineDerived from osteoprogenitor cell line Produce type I collagen Produce type I collagen Secretion is polarized towards the bone surfaceSecretion is polarized towards the bone surface Attract Ca salts and P to precipitate to mineralize the bone Attract Ca salts and P to precipitate to mineralize the bone

52 Osteoblasts (Cont’d) Upon completion of bone formation, Upon completion of bone formation, Remains on the surface of boneRemains on the surface of bone Covered by non-calcified osteoidCovered by non-calcified osteoid Identifying traits: Identifying traits: Outer membrane surface coated in alkaline phosphatesOuter membrane surface coated in alkaline phosphates Polarized (nucleus away from bone surface)Polarized (nucleus away from bone surface) Basophilic stainsBasophilic stains

53 Osteocytes Osteoblasts surrounded by mineralized bone matrix Osteoblasts surrounded by mineralized bone matrix Most numerous bone cellMost numerous bone cell Positioned between lamellae in a concentric pattern around the central lumen of osteons Positioned between lamellae in a concentric pattern around the central lumen of osteons Regulate extracellular concentration of calcium and phosphate Regulate extracellular concentration of calcium and phosphate

54 Osteocytes (Cont’d) Mechanosensory cells Mechanosensory cells Respond to deformationRespond to deformation Flow of interstitial fluid through the osteocytic canalicular networkFlow of interstitial fluid through the osteocytic canalicular network Directed away from regions of high strain Directed away from regions of high strain Initiates electrokinetic and mechanical signals Initiates electrokinetic and mechanical signals Growth Facors (intercellular signal molecules) Growth Facors (intercellular signal molecules) Insulin-like growth factor, IGF-1,Insulin-like growth factor, IGF-1, Prostaglandins G/H synthaseProstaglandins G/H synthase PGE2 and Nitric oxidePGE2 and Nitric oxide

55 (a) First Level Hydroxyapatite crystals embedded between collagen fibril Hydroxyapatite crystals embedded between collagen fibril

56 (b) Second Level Fibrils are arranged into lamellae Fibrils are arranged into lamellae Sheets of collagen fibers with a preferred orientationSheets of collagen fibers with a preferred orientation

57 (c) Third Level Lamellae are arranged into tubular osteons Lamellae are arranged into tubular osteons

58

59 Osteoclast

60 Osteocytes

61 Osteoblast

62 Basic Multicellular Units “The Basic Multicellular Unit (BMU) is a wandering team of cells that dissolves a pit in the bone surface and then fills it with new bone.” “The Basic Multicellular Unit (BMU) is a wandering team of cells that dissolves a pit in the bone surface and then fills it with new bone.” BMUs are discrete temporary anatomic structures organized as functional unitBMUs are discrete temporary anatomic structures organized as functional unit Osteoclasts remove old bone, then osteoblasts synthesize new bone Osteoclasts remove old bone, then osteoblasts synthesize new bone old bone is replaced by new bone in quantized packetsold bone is replaced by new bone in quantized packets

63 Basic Multicellular Units (cont’d) A photomicrograph of bone showing osteoblasts and osteoclasts together in one Bone Metabolic Unit

64 Activation  Occurs when bone experiences micro damage or mechanical stress, or at random  A BMU originates and travels along the bone surface Differentiated cells are recruited from stem cell populationsDifferentiated cells are recruited from stem cell populations Pre-osteoclasts merge to form multi-nucleated osteoclastsPre-osteoclasts merge to form multi-nucleated osteoclasts

65 Bone Resorption  Newly differentiated osteoclasts are activated and begin to resorb bone Minerals are dissolved and the matrix is digested by enzymes and hydrogen ions secreted by the osteoclastic cellsMinerals are dissolved and the matrix is digested by enzymes and hydrogen ions secreted by the osteoclastic cells Move longitudinally on bone surfaceMove longitudinally on bone surface  This process is more rapid than formation, though it may last several days

66 Reversal  Transition from osteoclastic to osteoblastic activity  Takes several days  Results in a cylindral space (tunnel) between the resorptive region and the refilling region  Forms the cement line

67 Bone Formation  Following Resorption, osteoclasts are replaced by osteoblasts around the periphery of the tunnel  Attracted by cytokines and growth factors  Active osteoblasts secrete and produce layers of osteoid, refilling the tunnel  Osteoblasts do not completely refill the tunnel Leaves a Haversian canalLeaves a Haversian canal Contains capillaries to support the metabolism of the BMU and bone matrix cells Contains capillaries to support the metabolism of the BMU and bone matrix cells Carries calcium and phosphorus to and from the bone Carries calcium and phosphorus to and from the bone

68 Mineralization  When the osteoid is about 6 microns thick, it begins to mineralize  Formation of the initial mineral deposits at multiple discrete sites (initiation) Mineral is deposited within and between the collagen fibersMineral is deposited within and between the collagen fibers This process, also, is regulated by the osteoclastsThis process, also, is regulated by the osteoclasts  Mineral maturation Once the cavity is full the mineral crystals pack together, increasing the density of the new boneOnce the cavity is full the mineral crystals pack together, increasing the density of the new bone

69 Quiescence  After the tunneling and refilling Some osteoblasts become osteocytesSome osteoblasts become osteocytes Remain in bone, sense mechanical stresses on bone Remain in bone, sense mechanical stresses on bone Remaining osteoblasts become lining cellsRemaining osteoblasts become lining cells Calcium release from bones Calcium release from bones  Period of relative inactivity Secondary osteon and its associated cells carry on their mechanical, metabolic and homeostatic functionsSecondary osteon and its associated cells carry on their mechanical, metabolic and homeostatic functions

70 Mechanical Support Provides strength and stiffness Provides strength and stiffness Hollow cylinder: Strong and light Hollow cylinder: Strong and light Have mechanisms for avoiding fatigue fracture Have mechanisms for avoiding fatigue fracture

71 Hematopoiesis  Development of blood cells Occurs in the marrow of boneOccurs in the marrow of bone  These regions are mainly composed of trabecular bone (e.g. The iliac crest, vertebral body, proximal and distal femur)(e.g. The iliac crest, vertebral body, proximal and distal femur)

72 Protection of Vital Structures Flat bones in the head protect the brain Flat bones in the head protect the brain Protects heart and lungs in chest Protects heart and lungs in chest Vertebrae in the spine protect the spinal cord and nerves Vertebrae in the spine protect the spinal cord and nerves

73 Mineral Homeostasis Primary storehouse of calcium and phosphorus Primary storehouse of calcium and phosphorus Trabecular bone are rapidly formed or destroyed Trabecular bone are rapidly formed or destroyed In response to shifts in calcium stasis without serious mechanical consequencesIn response to shifts in calcium stasis without serious mechanical consequences

74 Fatigue Curve Probability of Injury

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