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Bone Graft and its substitutes


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Introduction

Need for bone grafting: To replace skeletal defects

To augment bony recontruction

Bridge joints in arthrodesis

To provide bone blocks to limit joint motion [arthroereisis]

Establish union in pseudoarthrosis

To promote union : in fresh #s, delayed unions, non

unions and osteotomies.


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Bone Graft Terminology

Based on anatomic placement: Orthotopic: Transplantation to similar anatomic site

Heterotopic: Grafting to anatomically dissimilar site

Based on tissue architecture: Cortical, cancellous,corticocancellous, Osteochondral

Based on source: Autograft: same individual,different sitea] non vascularised b]

vascularised

Allograft: same species but genetically different individual

Xenograft: different species

Isograft: monozygotic twin


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Bone Graft Function

Osteogenesis

Mechanical support


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Biology of Bone Grafts

Bone regeneration: Osteoinduction

Osteoconduction

Osteoprogenetor cells

Osteoinduction: stimulation of host bed to synthesise new bone.-Graft derived Low molecular wgt protiens stimulate mesenchymal stem cellsto differentiate into bone forming cells.

This induction of stem cells does not require viable graft cells because it is aproperty of bone matrix.

Osteoconduction:The Matrix/ scaffolding that permits cellinfiltration and ingrowth of new host bone, this process is reffered asosteoconduction. A three dimentional configuration for ingrowth into graft ofhost capillaries, perivascular tissues, and osteoproginator cells from therecepient. [creeping sunstitution]


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Incorporation of the graft

Haematoma formation Inflamatory response releaseof cytokines and growth factors.

Osteoinduction drives chemotaxis, mitosis, anddifferentiation of osteoproginetor cells

Day5

chondrocyts form

Day 10- osteoblasts

Host blood vessels infiltrate through harvesian andvolkmanns canals and bring with them osteoclasts forresorption.

Remodelling occurs along the lines of biomechanicalforces.

Hence succesful incorporation needs: good bloodsupply, osteogenesis, grafts response to load applied.


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Cancellous autografts

Larger surface area

Faster incorporation [ 4 weeks- healing 6 months complete remodelling]

Good osteoinductive, osteoconductive

and osteogenetic properties.

No ability to confer structural strength


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Cortical Autografts

Due to density- revascularisation andremodelling are slower

Unlike cancellous grafts these are

incorporated by appositional bone growthover a necrotic area.

Good structural support

Minimal osteogenetic, moderateosteoinductive and conductive properties

Types: Vascularised , non vascularised.


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Disadvantages of autografts

A separate donor site is required

Increased surgical time

Another potential site for infection created

Additional amount of blood loss Weaknening of normal structure

Local complications

Limited availability

relative paucity of osteogenic cells in the graft inolder individuals


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Vascularised auto graft

Incorporates independent of host bed andfunctions even in biologically defecient hostenvironment

Advantages: Compromised vascularity in the host bed causing poor osteogenic

cells, poor surrounding tissues

If infection at surgical site

poor soft tissue coverage

Systemic chemotherapy

Disadvantages: Time consuming

Technically very demanding

Limited donor sites [ proximal fibula, ribs]


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Allografts

Graft taken from two genetically different

individuals of the same species.

Mode of availability: Frozen

Freeze dried [morcellised]

Cancellous allografts: mainly to fill the

cavitory defects Cortical allografts: for structural support


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How are they Preserved

Freezing

Freeze drying

[ helps in better incorporation as it reducesimmunogenecity]

Decalcification and demineralisation

[ these grafts have not much mechanicalstrength]


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Sterilisation techniques

Irradiation : destroys osteoinductive

property

Chemosterilisation

Autolysed

Antigen extraction

[ last three has good osteoinduction but poormechanical strength]


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Advantages

Abundantly available in amount, shapesand sizes

No donor site morbidity

Good mechanical strength

Good Osteoconductive property


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Disadvantages

Immunogenic

Higher failure rate notes

Graft incorporation very much delayed

[ more than 2 yrs]

Risk of disease transmission

Low osteoinductive property If these grafts fracture unlikely to heal


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Bone Banking

Tissues recovered under sterile conditions fromyoung donors with strong bone after carefulscreening for neoplasm and infection

If cartilage present graft is first treated by

dimethylsulfoxide and later freezed to preservethe viability of chondrocytes

The muscular, teninous, and ligamentousattachments are preserved

After biplanar radiographs taken, specimens arecultured, rapidly frozen, and maintained at -80*cuntil they are used


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Factors for success of a bone

grafting Clean, well vascularised host bed is critical

Wide excision of scar tissue

Treatment of infection

Protection of blood supply Satisfactory soft tissue coverage

Appropriate selection of graft material as per thedesired clinical function

Firmly/ tight apposition of graft bone and hostbone

Stable fixation of the graft


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Bone Substitutes


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Types

Naturally Derived Substitutes: Demineralised Bone Matrix [DBM]

Bone Marrow and Bone Marrow Products

Synthetically derived substitutes Calcium sulphates

Tricalcium Phosphates

Calcium phosphate cements

Corralline based hydroxyapatites

Bone Morphogenetic Proteins


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Synthetically derived substitutes

Less chances of viral transmission

compared to natural derived substitutes

Less variability

They are tested rigorously for safety and

efficacy

It mainly provides a scaffold forosteoproginetor cells to proliferate and

differentiate.


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Calcium sulphates [POP]

First used in in 1892 complete healing of nine

cavitory lesions

Has no weight bearing ability

Resorbs very quickly [6 weeks] [i.e: dissappearsfrom implantation site whether bone formation

has taken place or not]

A promising carrier for antibiotics,DBMs,BMPs,or any other molecules being

delivered to defect site.


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Tricalcium Phosphates

TCPs Less crystalline

Two types: Alpha and beta

Alpha soluble and resorbs fast

Beta less soluble and resorbs by

osteoclastic activity only : hence it will

disappear only after new bone formation

Has only osteoconductive property


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Calcium Phosphate Cements

No weight bearing capability

Powder and solvent is mixed and the resultantceremic paste injected/ moulded into non weight

bearing defect. Setting time 10 to 30 minutes depending on

formulations

After curing they form a apatitic compound

similar to bone mineral These cements do not degrade during a patients

lifetime.


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Coral based hydroxyapatite

Calcium carbonate exoskeleton of reef buildingmarine corals processed to form calciumphosphates.

Corals of scleractinian genus used

Goniopora species- first used- its pores aresimilar to cancellous bone pores- 500 to600microm

Porites species similar to cortical bone pore

diameter- 200microm. Useful to fill defects after tumour excision with

wide defects.


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Collagen Based technology

Type 1 collagen extracted from animal

source, mostly from bovine source

Good osteoconductive property

Disadv:

Hypersensitivity to bovine collagen

Uses: Mainly as a carrier along with other

materials.


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Naturally Derived

Available clinically from 1990s.

Disadv:

High tissue variability

Viral transmission


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