Anatomy of Bone

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Anatomy of bone


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The skeletal system is

divided into two

functional parts:-

Axial skeleton ---

head , neck & trunk

Appendicular---limbs( including

pectoral and pelvic

girdles)


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The skeleton is made of cartilage and bones. The

five major functions of the skeleton are:-

Support

Protection

Movement

Storage

Blood cell production


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Cartilage is a resilient, semirigid form of

connective tissue that forms parts of the

skeleton where more flexibility is recquired.

Bone is a living tissue which is a hard form ofhighly specialized connective tissue that

makes up most of the skeleton.


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Cartilage

Is an avascular form of connective tissue that

consists of extracellular matrix embedded in a

matrix that contains cells localized in small

cavities. The amount and kind of extracellular

fibers in the matrix varies depending on the

type of cartilage.


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The functions ofcartilage are:-

Support soft

tissues Provide a

smooth,gliding surface

for bonearticulationsat joints

Enable the

developmentand growth oflong bones

Macroscopical joint lesions in sows. a: Cartilage erosion (arrows)

on the medial humeral condyle. b: Cartilage ulceration (arrow)

on the medial femoral condyle. c: Cartilage repair (arrow) of the

medial femoral condyle d: Marginal osteophytes (arrows) on

processus anconeus of ulna.


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There are 3 types of cartilage:-

Hyaline-most common. Its matrix contains amoderate amount of collagen fibers( e,g.articular surfaces of bones)

Elastic-matrix contains collagen fibers along

with a large number of elastic fibers ( e.gexternal ear)

Fibrocartilage-matrix contains a limited

number of cells and ground substance amidsta substantial amount of collagen( e.g.intervertebral disc)


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BoneConsists of an intercellular calcified matrix,

which also contains collagen fibers, andseveral types of cells within the matrix.

Bone Maturity

woven ( immature bone)

Here collagen fibres are aligned randomly and

have no lamellae,making the bone weaker and

more flexible than lamellar bone.


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Lamellar ( mature) bone

Forms the structural component of cortical

and cancellous bone with stress-oriented

collagen fibres contributing to its anisotropic

characteristics.


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Histological cut showing details of lamellar bone concentricallyorganized and woven bone mixed with cartilage and calcifiedcartilage tissues (HE).

Matos et al.Journal of Orthopaedic Surgery and Research 2008


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Two types of bones;compact and spongy

bone( trabecular orcancellous). Compactbone is dense that formsthe outer shell of all

bones and surroundsspongy bone. Spongybone consists of spicules

of bone enclosing cavitiescontaining blood-formingcells( marrow).


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Cortical

(compact

bone)

Comprises of 80 % of adult skeleton forming thenevelope of cuboid bones and the diaphysis of

long bones. Contains lamellae that are laid down

as concentric rings forming trabecular lamellae

systems called osteons or haversian systems.


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Cancellous ( trabecular bone)

Found mainly in the metaphyses and epiphyses of long bones and centrallyin cuboid bones. Its has a 3D lattice of interconnecting trabeculae which arealigned along axes of mechanical stress, enclosing elements of bone marrow.Each of the trabeculae is made up of parallel sheets of lamellae.


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Classification of bones is by shape

Long bones are tubular e.g humerus, femur

Short bones are cuboidal e.g bones of wrist

and ankle

Flat bones consist of two compact bones

plated separated by spongy bone e.g skull

Irregular bones are bones with various shapes

e.g bones of the face

Sesamoid bones are round or oval bones that

develop in tendons


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Bone structure

All bones are derived from mesenchyme (embryonicconnective tissue) by two different processes:

Intramembranous ossification( directly frommesenchyme)-mesenchymal models of bones formduring the embryonic period and direct ossification ofthe mesenchyme begins in the fetal period.

Endochondral ossification( from cartilage derived frommesenchyme) cartilage models of the bones formfrom mesenchyme during the fetal period and bonesubsequently replaces most of the cartilage.


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Mesenchymal cells differentiate into

chondroblasts-forming a cartilaginous bonemodel- midregion of cartilage calcifies-

periosteal capillaries( from fibrous sheath

surrounding the model)- grow into thecalcified cartilage of the bone model and

supply its interior.


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Most secondary ossificationcentres appear in otherparts of the developing

bone after birth; the partsof a bone ossified fromthese centres are epiphyses.

Epiphyseal arteries grow intodeveloping cavities withassociated osteogenic cells.The flared part of thediaphysis nearest theepiphyses is the metaphysis.


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Gross Structures of Bone

The gross structure of a long bone can be

divided into several regions.

Epiphysis

In the long bones, the epiphysis is the regionbetween the growth plate or growth plate scarand the expanded end of bone, covered by

articular cartilage. An epiphysis in a skeletallymature person consists of abundanttrabecular bone and a thin shell of corticalbone.


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Cortical bone is

composed of

haversiansystems

(osteons). Each

osteon has acentral haversian

canal and

peripheral

concentric layers

of lamellae.


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Metaphysis

The metaphysis is thejunctional region

between the growthplate (see the imagebelow) and thediaphysis. Themetaphysis containsabundant trabecularbone, but the corticalbone thins hererelative to thediaphysis. This regionis a common site formany primary bonetumors and similarlesions.


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Growth

plate

The relative predilection of osteosarcoma for the metaphyseal region of long

bones in children has been attributed to the rapid bone turnover due to extensive

bone remodeling during growth spurts.

Di h i


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Diaphysis

The diaphysis is the shaft of long bones and is

located in the region between metaphyses,

composed mainly of compact cortical bone. Themedullary canal contains marrow and a small

amount of trabecular bone.

Physis (epiphyseal plate, growth plate)

The physis is the region that separates the

epiphysis from the metaphysis. It is the zone ofendochondral ossification in an actively growing

bone or the epiphyseal scar in a fully grown bone.


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Blood Supply

Bones are richly supplied with

blood vessels. Most apparent arenutrient arteries that ariseindependent branches of adjacentarteries outside the periosteumand pass obliquely through thecompact bone of the shaft of a

long bone via nutrient foramina.

The nutrient artery divides inthe medullary cavity intolongitudinal branches thatproceed toward each endsupplying the bone marrow,spongy bone and deeper portionsof compact bone.


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The ends of bones are supplied by metaphysial and epiphysial arteries that

arise mainly from the arteries that supply the joints; in the limbs these

arteries are typically part of a periarticular arterial plexus, which

surrounds the joint ensuring blood flow distal to the joint regardless of theposition assumed by the joint.

The skeletal system receives 5-


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The skeletal system receives 510 % of cardiac output.Individual long bones have 3interactive circulatory

systems all of whichcommunicate in an adult.

In children the metaphyseal-epiphyseal system separates

when the ossific nucleus isformed.

1. Nutrient artery system2. Metaphyseal-epiphyseal

system

3. Periosteal system ( low

pressure system)


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Nutrient artery system-high pressure

system A major artery of the systemic

circulation enters the mid-diaphysis through a nutrientforamen. Once in the medullarycanal , it divides into ascendingand descending arteries whichanastomose with metaphysealvessels and directly penetratethe endosteal surface,supplyingthe inner 2/3 rds of the cortex.

In the child these vessels end onthe metaphyseal side of thephysis,contributing to theprocess of endochondralossification.


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Metaphyseal-epiphyseal system

The periarticular vascular complex penetratesthe thin cortex and supplies the metaphysis,physis and epiphysis. The metaphyseal vesselsanastomose with the medullary and

epiphyseal arteries after growth plate fusion.

In epiphyses with large articular surfaces, suchas radial and femoral heads, vessels enter the

bone between the articular cartilage and thephysis, making the blood supply relativelytenuous.


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Periosteal system(low pressure system)

Capillaries enter at the sites of major muscle

attachments, normally supplying the outer

third of the cortex. This is the dominant

system in the child and is responsible forcircumferential growth.


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these 3 systems are interconnected, and each is able to become dominantsupply if another is damaged. The normal direction of flow iscentrifugal ( inside to out) but if endosteal system is damaged theperiosteal system becomes dominant and the flow becomes centripetal( outside to in).


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Large irregular bones, short bones, and flat bones

These bones receive a superficial blood supplyfrom the periosteum, as well as frequently fromlarge nutrient arteries that penetrate directly intothe medullary bone. The 2 systems anastomosefreely.

Venous and Lymphatic Drainage of Bone

Blood is drained from bone through veins thataccompany the arteries and frequently leavesthrough foramina near the articular ends of thebones. Lymph vessels are abundant in theperiosteum.


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Veins accompany arteries through the nutrientforamina. Lymphatic vessels are abundant in theperiosteum.

Nerves accompany blood vessels supplying thebones. The periosteum is richly supplied withsensory nerves-periosteal nerves that carry painfibres. Within bones, vasomotor nerves cause

constriction or dilation of blood vessels,regulating blood flow through bone marrow.

Classification of joints


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Classification of joints


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Blood supply of joints

Joints receive blood from

articular arteries that arise

from the vessels around the

joint. The arteries often

anastomose to form networks( periarticular arterial

anastomoses) to ensure blood

supply to and across the jointin the various positions

assumed by the joints.


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Synovial joints include:-

Plane joints

Hinge joints

Saddle joints

Condyloid joints

Ball and socket joints

Pivot joints

Synovial joints are also associated

with bursae, which are flattenedfibrous sacs lined with synovial

membrane that develop in areas

of friction. Tendon sheaths are

special bursae that wrap around

tendons in areas of friction.

Synovial fluid
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All freely moveable joints have some synovial fluid in them. Synovial fluid originates

from plasma that is filtered by the capillary net and diffuses into the knee along with

hyaluronic acid, which is locally synthesized. Synovial fluid transports nutrients, assists

in the joint's defense, and lubricates the joint.

Blood supply to knee joint


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Blood supply to knee joint

Blood supply to skin around the knee

is random (as opposed to axial)

- intrinsic contributors to skin

overlying knee are perforating branches

of the superior and inferior genicular

systems;

- extrinsic supply: 3 sources;

Descending genicular

Recurrent branch of anterior tibial

artery

Descending branch of the lateralfemorartery;

Pediatric Bone Circulation:


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Pediatric Bone Circulation:

Circulation in pediatric bone:

- differs from adult circulation due torequirements of growth & presence of

epiphyseal plate

- terminal branches of nutrient artery, along

with metaphyseal vessels, approach growthplate in a parallel relationship

- branches are so numerous as they reach

growth plate that there is almost one vessel

for each column of cartilage cells

- in final few mm before terminal

arteriole reaches cartilage, it is encased in a

tube of enchondral bone

as vessel extends to end of cartilage


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as vessel extends to end of cartilagecolumn, it makes abrupt 180 deg turnto enter larger venule

- slowing of the circulation maypermit lodging & proliferation ofbacteria to produce focus ofhematogenous osteomyelitis

- function of metaphyseal blood

supply(metaphyseal side of growthplate) is to provide nutrition for activecells involved in endochondralossification

- epiphyseal vessels are essential for

they supply dividing cells of growthplate & hence are responsible formaintaining longitudinal bone growth


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References

Basic Orthopaedic Sciences ( The Stanmore

guide) by Manoj Ramachandran

Apleys System of orthopaedics and fractures

Clinically oriented anatomy

Grays anatomy for students

Wheeless textbook oforthopaedics www.emedicine.medscape.com

Documents

Anatomy of Bone