Bone graft

It is a viable tissue/organ that after removal from donor site is implanted/transplanted within the host tissue, which is then repaired, restored and remodelled.

Mechanism of Bone Regeneration and Augmentation

The three processes that are associated with successful bone grafting are as follows:

Osteogenesis

This occurs when vital osteoblasts originating from the graft itself contribute to new bone growth. An osteogenic graft, which works by the principle of osteogenesis, is composed of tissue involved in the natural growth or repair of bone. e.g., autogenous chin or iliac graft.

Osteoinduction

This is the process of stimulating osteogenesis. Osteoinductive grafts can be used to enhance bone regeneration. e.g., DFDBA (demineralized freezedried bone allograft). These grafts stimulate the osteoprogenitor cells to form new bone.

Osteoconduction

Osteoconductive grafts provide a physical matrix or scaffolding suitable for the deposition of new bone. They require the presence of existing bone or differentiated mesenchymal cells, e.g., synthetic hydroxyapatite (HA), b-tricalcium phosphate (b-TCP), HTR (hard tissue replacement) polymers. Osteoconduction occurs when the bone graft material serves as a scaffold for new bone growth that is perpetuated by the native bone. Osteoblasts from the margin of the defect that is being grafted utilize the bone graft material as a framework upon which to spread and generate new bone.

Types of Graft Materials

Autograft/Autogenous bone graft

Tissues transferred from one position to another within the same individual are called autograft. They form new bone by osteogenesis, osteoinduction, and osteoconduction. The common areas from which autogenous bone can be harvested are as follows:

1. Intraoral: Mandibular symphysis, maxillary tuberosity, ramus, exostoses
2. Extraoral: Iliac crest, tibial plateau.

• Advantage: Autografts are highly osteogenic.
• Disadvantages: Include need for a second operative site, patient morbidity, and diffculty in obtaining a sufficient amount of graft material, more so in intraoral sites.

Allografts

Allografts refer to a graft between genetically dissimilar members of the same species. The allografts, most commonly used for restoring osseous defects, are mineralized or demineralized freeze-dried bone allografts (FDBA). Bone allografts that are obtained from cadavers may be cortical or trabecular. They are osteoinductive and osteoconductive but not osteogenic. They are available from tissue banks which process and store the allografts under complete sterility.

• Advantages: They are readily available, eliminate the morbidity associated with a second surgical site, reduce anesthesia and surgical time, decrease blood loss.
• Disadvantages: antigenicity of tissues and graft rejection.

The most commonly used forms of allografts are frozen, freeze-dried (lyophilized), demineralized freeze-dried, and irradiated. Examples: MTF allograft (Musculoskeletal Transplant Foundation; an allogeneic freeze-dried bone available in both mineralized and demineralized forms), Puros, Osteofil, Grafton (demineralized bone matrix), Rocky mountain, Dembone, Osseograft, DynaGraft (demineralized bone matrix in a reverse phase medium), Lambone(membrane-like sheet of demineralized, freezedried human laminar cortical bone),

Xenograft or heterograft

Xenograft or heterograft is a graft taken from a donor of another species. Examples: BioGen, Bio-Oss (bovine bone grafts), OsteoGraf/N, Interpore 200 (coralline material).

Isograft

Isograft refers to a graft between genetically identical individuals (twins).

Alloplasts

Alloplasts are the most commonly used form of bone augmentation materials. They are composed of natural or synthetic material and are typically only osteoconductive. The most commonly used bone substitutes are ceramic materials, synthetic calcium phosphate ceramics (e.g., HA, TCP), calcium carbonate (coralline), HTR polymers, and bioactive glass ceramics. Alloplastic and xenograft materials are available in a variety of textures, sizes, and shapes.

Classification of alloplasts

1. Based on porosity: Dense, macroporous, microporous
2. Based on crystallinity: Crystalline, amorphous, granular, molded

Hydroxyapatite

This primary, inorganic, natural component of bone is highly biocompatible and bonds well with the adjacent hard and soft tissues. Its applications are as follows:

1. Periodontal osseous defects
2. Ridge augmentation/implant placement procedures
3. Restoration of cystic defects
4. Postapicoectomy bone regeneration
5. Sinus elevation surgeries
6. Postextraction for ridge maintenance
7. Treatment of peri-implantitis
8. Post third molar extraction, to prevent distal pocket formation on the second molar

Examples: The commercially available HA are BioGraft-HA, OsteoGen, G-bone (HA granules and blocks), DentaPlug-HA (bioactive resorbable graft with bovine collagen, HA, and chlorhexidine).

Tricalcium phosphate (TCP)

Although tricalcium phosphate (TCP) is similar to HA, it is not a natural component of bone material. In the body, TCP is converted in part to crystalline HA. It is osteoconductive and provides a scaffold for deposition of new bone. It is used with osteogenic or osteoinductive materials to improve the handling characteristics of the graft during placement. It is resorbed completely within 3–24 months. During the process, collagen and blood vessels are incorporated with the granules. Although highly porous, it is stable and highly resistant to abrasion. Its particle size ranges from 10 nm to 63 nm. Examples: Commercially available TCP are Cerasorb (b-TCP), Bioresorb (pure b-TCP ceramic), Syn Oss b-TCP. Combinations of HA and b-TCP: BioGraft-HT (HA + b-TCP), Ossifi (b-TCP + HA).

Coralline

It is a ceramic graft material synthesized from the calcium carbonate skeleton of coral. It has a three-dimensional structure similar to bone. It is mostly used in ridge preservation of posterior maxilla.

Osteoinductal

It is an oily calcium hydroxide suspension with enhanced osteopromotive action.

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