lecture 2 bone

8/7/2019 Lecture 2 Bone

1/22

Bone

ME251: Biomechanics


2/22

Suggested Reading for Class Nordin, M & Frankel, V.H. Basic biomechanics of the musculoskeletal

system good book and covers most material well. Wainwright, S. A., Mechanical Design in Organisms, Princeton

University Press, Princeton, New Jersey (1982). A fantastic book anda must read for anyone interested in biomaterials. Good sectionsfrom experts for bone, elastic proteins etc.

Currey, JD, The mechanical adaptation of bones, PrincetonUniversity Press, Princeton, NJ (1984). Another big expert in study ofbones and a must have book for those interested in hard tissue

mechanics. Vincent, JFV, Structural Biomaterials, Princeton University Press,

Princeton, NJ (1990). Another excellent read written by an expert inthe field. Has a materials and fracture perspective of naturallyoccurring biomaterials like bone, teeth, rubber proteins, etc.

Hayes, W. C., Mow, V. C., Basic Orthopaedic Mechanics, RavenPress, New York, NY (1997). A must read for those interested inbone and joint mechanics. Good all round text. Spine mechanics,joint statics, gait dynamics, total joint replacement etc. Muscle notcovered. Written by many authors so style not consistent but writtenby many experts in the field.


3/22

Orthopaedics and Biomechanics Areas: fractures and fracture fixation devices,

congenital deformities (scoliosis), osteopetrosis (brittlebones in children), osteoporosis and arthritis in elderly requires joint replacement

First How can we estimate bone strength & stiffness?

Second Mechanobiology perspective. Can we dosomething about it?

Both relate to mechanical bone quality and load-

induced bone metabolism


4/22

bone living material Adaptive material designed for different functionalrequirements

Mechanical response of bones to loads mustunderstand structure-property relationships Bone, passive soft tissues (tendons, ligaments,

cartilage, meniscus, joint capsules), muscles andnerves 206 bones in adult human (270 in children)

o Long bones: Femur, tibia and humerus.o Short bones: Metacarpals and vertebral bodieso Flat bones: ilium, cranium and scapulao Irregular bones: those that do not fit into the other three

categories and include wrist bones (carpals) and theposterior vertebral elements


5/22

Functions of Bone Support and Motion: Most important function. The relatively rigid bones

articulate at the synovial joint and enable the body to move quickly andin an agile manner.

Protection of internal organs: The skeleton absorbs large amounts ofenergy and yet remains lightweight.

Mineral Storage: Mineral bank for storage of calcium and phosphorous.Approximately 99% of the calcium in the body is stored in the bone.

One of the ways that the body regulates the level of these minerals inthe bloodstream is by a continuous process of remodeling (resorptionand formation of bone tissue).

Hematopoiesis: Spongy trabecular bone, found at the ends of long

bones, vertebrae and several other locations (e.g. skull, pelvis andsternum) provides sites for the formation of red blood cells by aprocess known as hematopoiesis which occurs in the red bonemarrow. Yellow bone marrow, found in the middle (or diaphysis) ofmost long bones serves as a storage area for fat cells.


6/22

bone as composite material Main constituents of bone: collagen (90% 15mlength, 50-70 nm diameter)

Small amount of other proteins, protein-polysaccharides & glycoproteins (10%) Mineral 50% bone volume & 75% weight.

Calcium phosphate and calcium carbonate main;small amounts of sodium, magnesium & fluoride

Hydroxyapatite Ca10 (PO4)6 (OH)2. Generallyaccepted hydroxyapatite is in dahllite (carbonateapatite) or francolite (carbonate fluorapatite).Mineral crystals exist like plates and are about

200 long, 2-3 nm thickness. Also amorphaouscalcium phosphate present Collagen (type I present) initiates formation of

crystals (not all collagens will do with equalfacility) with stable intermolecular cross-links


7/22

Cells in bone Osteoblasts (bone-forming cells generally appear in clusters)

Osteoclasts (bone-resorbing cells byproduction of proteolytic enzymes aremotile)

Osteocytes (bone maintaining cells ) &

Bone lining cells reside permanently inthe bone & are interconnected by a systemof canaliculi. Osteocytes are osteoblaststrapped in extracellular matrix.

During bone formation uncalcifiedosteoid secreted by osteoblasts.Hydroxyapatite crystals precipitate around

collagen fibers in osteoid.


8/22

long bone structure Diaphysis (hollow shaft) with

metaphysis at end Growing animals, metaphysis covered

with epiphysis joined to metaphysis by

cartilagenous growth plate Periosteum covers bone all bones

except joint surfaces that havecartilage. Inner osteogenic layer cellsproducing enlargement of bone(osteoblasts). Outer layer fibrous &permeated by blood vessels, nerves

In long bones, the endosteum (thinnermembrane) lines the central medullary

canal filled with yellow fatty marrow.Endosteum has osteoblasts andosteoclasts remodeling and resorptionof bone


9/22

Bone tissue: Osteon structure Fundamental unit of bone osteon or haversian system;about 200 mm in diameter

At center of osteon haversiancanal containing blood vesseland nerve fibers

Osteon contains concentric

series of layers (lamellae) ofmineralized matrix surroundingcentral canal like tree rings

Along boundary of each layer small cavities called lacunae

exist that contain bone cell Osteon boundary cement line

of GAG mainly. Deflect cracksand source of toughness

http://www.cytochemistry.net/microanatomy/bone/compact_bone_histology.htm

Vincent, JFV


10/22

Osteon Small channels (cannaliculi)

radiate from lacuna andconnect adlacent lamellae until

haversian canal. Cell processes extend fromosteocyte into canaliculi transport of nutrients

Intertwining of collagen inosteon source of strength inbone

Space between osteons filledwith interstitial lamellae. Samematerial but changedorientation

http://www.cytochemistry.net/microanatomy/bone/compact_bone_histology.htm


11/22

Macroscopic organization Cortical bone in diaphysis of

long bones & thin shellsurrounding metaphyses

Trabecular bone in metaphysesand epiphyses continuous withinner surface of shell andexhibits 3D interconnectednetwork of trabecular rods andplates

Interstices between trabelulaefilled with red marrow.Trabecular bone arranged in

concentric lamellae but doesnot contain haversian canal.Nutrients through marrow.

Arola, D et al, Intl J Fatigue (2010)


12/22

hierarchical structure of bone Layout of fibrils in bone: woven (immature) - random orientation. Found in

embryo, newborn, fracture callus & metaphysial region.Also in tumors, osteogenesis imperfecta lamellar bone (mature) - preferred fibril orientation. 1

month after birth & actively replaces woven bone

All types of bone structure found in compact bone;open more-structured bone in center and ends oflong bones

Ratio of mineral to collagen phases important formechanical properties


13/22

material properties of bone

MPa50Strength of collagen

MPa1Stress in Collagen at strain of 0.001

10 -3Ultimate strain of hydroxyapatite

GPa130Stiffness of hydroxyapatite

GPa0.1Strength of hydroxyapatite

Efluorapatite=165 GPa (steel=200 GPa, Al 6061 alloy 70 GPa).

Ecollagen=1.24GPa (collagen does not obey Hookes law)

Ebone=18GPa tension

1. Bone is a biphasic composite material with mineral as one phaseand collagen (ground substance) as the other.

2. Strength & stiffness are important properties


14/22

Mechanical properties Cortical bone stiffer than

cancellous bone,withstands greater stress

but less strain (1.5-2%)before failure

Cancellous bone may

sustain upto 50% strainbefore yielding Porous structure of

cancellous bone largecapacity for energystorage


15/22

Anisotropic mechanical response Yielding in bone (tension) caused

by debonding of osteons alongcement lines. Yielding in

compression cracking ofosteons Different properties in transverse

& longitudinal direction

Values highest for longitudinaldirection & lowest for transversedirection

Behavior affected by mechancialproperties, geometriccharacteristics, loading mode,direction of loading, rate andfrequency of loading


16/22

Strains during walking & jogging

Bone is usually subjected to complex loading.


17/22

Influence of muscle activity onstress distribution in bones

Contraction of muscles altersthe stress distribution in thebone

Can decrease/ eliminate tensilestress on bone by producing

compressive stress Adult bone is good incompression but not immaturebone. Muscle force can be used

to eliminate high tensilestresses in bone duringcomplex loading such as 1)loads on tibia when falling downand 2) loads in hip joint duringlocomotion

http://nusxcountry.wordpress.com/injuries/


18/22

stress strain response of bone

Bone is viscoelastic material Must be kept in fluid (HBSS: Hanks balanced

salt solution for about 2 days before test)


19/22

Strain rate dependency Bone is viscoelastic Stiffer and

sustains higher load to failurewhen loads applied at high strainrates (physiological)

In vivo strains slow walking is0.001 per second. Slow running 0.03 per second

Brittle fractures at very high strainrates

Loading rate clinically important also affects soft tissue damage

Stored energy is released when

boen fails. Low energy fracture(torsional ski fracture); highenergy automobile accidents;very high energy gunshotwounds


20/22

Bone remodeling Alters its shape, size and structureto meet mechanical requirements

Wolffs law: remodeling of bone isinfluenced and modulated bymechanical stresses (Wolff, 1892)

Load gravity or muscle activity Greater body weight more bone

mass. Less body weight (space travel)

decreased bone mass with fastloss of calcium & bone loss. These

changes are not completelyreversible Disease or inactivity reduces

bone mass. Bed rest 1%

decrease per week.


21/22

Age related changes in bone Progressive decrease in cancellous bone

& thinning of cortical bone Age related loss depends on gender, age,

post menopause, endocrine factors,inactivity, disuse, calcium deficiency

http://www.bushmanfitness.com/articles/articles/bone-fix/328/


22/22

Ref:

www.hughston.com

Coefficients of Friction for various joints and commonbearing materials

(Mow, V. C. et al, Biomaterials, V13 N2, 67-97 (1992).

0.010.1IceonIceat0C

0.60.8SteelonSteel

2.8Goldongold

L.L.Malcolm(1976)

0.0020.03BovineShoulder

A.Unsworth

(1975)

0.010.04Humanhip

J.Charnley (1960)0.0050.02Humanknee

InvestigatorCoefficientofFriction

Joint/Materials

Ref:

www.health.allrefer.com

lecture 2 bone

Documents