Biology of Bone Grafting

Periodontal Regeneration

• Definition: The reproduction or reconstitution of lost or injured part so that the form and function of lost structures are restored.
• Includes regeneration of alveolar bone, cementum, and periodontal ligament.

Periodontal Repair and Reattachment

• Periodontal repair: Healing that does not completely restore the architecture or function of the part.
• Healing by a long junctional epithelium.
• Reattachment: The reunion of epithelial and connective tissue with a root surface that has been mechanically or surgically separated.
• New attachment may be epithelial adhesion and/or connective tissue adaptation or attachment.

Classification of Periodontal Pockets

• A- 3 wall defect
• B- 2-wall defect
• C- combination defect
• D- 1-wall defect
• Periodontal pockets can involve one, two, or more tooth surfaces, and can be of different depths and types on different surfaces of the same tooth and on approximal surfaces of the same interdental space.
• Pockets can also be spiral (i.e., originating on one tooth surface and twisting around the tooth to involve one or more additional surfaces).

Horizontal and Vertical Bone Loss

• Horizontal bone loss: When the bone loss occurs on a plane that is parallel to a line drawn from the cementoenamel junction (CEJ) of a tooth to that of an adjacent tooth.
• Supra bony pockets: When the bottom of the pocket is coronal to the underlying alveolar bone.
• Intra bony pockets: When the bottom of the pocket is apical to the level of the adjacent alveolar bone.
• Horizontal bone loss indicates an inflammatory process that has existed over a long period of time.
• Vertical bone loss indicates more rapid bone resorption.

Regenerative Techniques

• Three wall intrabony defects provide the best prognosis for regenerative technique.
• Availability of regenerative resources is greatly decreased in one wall intrabony defects.
• Wound healing of a deep narrow three-wall intrabony defect is similar to healing of an extraction socket.
• Root surface conditioning is essential for formation of new connective tissue attachment.

Periodontal Regeneration Procedures

• Grafting procedures
• Guided tissue regeneration (GTR)
• Root conditioning and biomodification
• Biologic mediators for periodontal regeneration (e.g., Emdogain)
• Combination of any of the previous procedures

Grafting Materials

• Osteoinductive: Bone formation is induced in the surrounding soft tissues immediately adjacent to the grafted material.
• Osteoconductive: The grafted material serves as a scaffold or frame for new bone formation originating from adjacent host bone.
• Osteoneutral: Serve only as space fillers.
• Osteogentic: Means that the new bone is formed by bone cells contained in the grafted material itself.

Classification of Grafts

• Autogenous grafts: Obtained from the same individual.
• Allogenic grafts: Obtained from another individual of the same species.
• Xenogenic grafts: Obtained from another species.
• Bone substitutes: Bovine derived hydroxyapatite, coralline calcium carbonate, etc.
• Alloplastic grafts: Bioceramics, polymers, bioactive glasses, etc.

Selection of Graft Material

• An ideal graft material should have osteoinductive potential, osteoconductive, accessibility, availability, and safety.

Guided Tissue Regeneration (GTR)

• Aims to regenerate lost periodontal structures through selective cell and tissue repopulation of periodontal wound.
• Uses barriers to exclude undesirable cell types and to create a space for regenerative cells to enter the defect.
• Indications: Furcations, intra bony defects, gingival recession, alveolar ridge augmentation.
• Technique: Use of biocompatible and non-resorbable or bioabsorbable barrier membranes, including shaping and placement of the membrane.

Causes of Failure and Contra-Indications for GTR

• Causes of failure: Surgical technique, post-surgical factors, barrier dependent factors, and patient-related factors.
• Contra-indications: Class II furcations on the mesial and distal maxillary molars and premolar furcations, horizontal bone loss, patient's health, compliance, and tooth mobility, and any medication, condition, or disease that may interfere with patient's healing.### Non-Absorbable Barrier Membranes
• Guidor: a periodontal mesh made of a copolymer of glycolide and lactide, degrades over a period of 3-12 weeks.

Absorbable Barrier Membranes

• Atrisorb: a polymer of lactic acid, poly (D, L lactide), dissolved in N-methyl-2-pyrolidone, sets to a firm consistency on contact with water or aqueous solutions, and degrades over a period of up to 3 months.
• Resolut: a copolymer of PGA and PLA, degrades over a period of 4 weeks to 8 months, with results similar to e-PTFE due to its prolonged resorption time.
• Epi-Guide: a hydrophilic membrane formed from PLA, contains a flexible open cell structure.

Root Surface Conditioning Materials

• Tetracycline: used for root conditioning, acts by exposing the dentin and cementum collagen matrix, removes endotoxin, and has substantivity, collagenase inhibition, and antibacterial effects.
• Citric acid: used as an adjunct in periodontal therapy, removes endotoxin from root surface, exposes dentin and cementum collagen matrix, and provides anchorage for new fibrin clot and collagen fibrils.

Biologic Mediators for Periodontal Regeneration

• Growth factors: stimulate a wide variety of cellular events such as proliferation, chemotaxis, differentiation, and production of extracellular matrix proteins.
  • Platelet-derived growth factor (PDGF)
  • Insulin-like growth factor (IGF)
  • Fibroblast growth factor (FGF)
• Mediators of bone metabolism: bone morphogenetic proteins (BMPs)
• Attachment factors: fibronectin
• Extracellular matrix proteins: enamel matrix proteins (enamel matrix derivative)

Bone Regeneration in Implant Dentistry

• Bone regeneration procedures are essential to create an adequate foundation for the placement and long-term success of dental implants.
• Factors affecting clinical outcomes of bone regeneration in implant dentistry include underlying biology, patient-related considerations, and surgery-related factors.

The Biology of Bone Regeneration

• Successful bone regeneration requires the recruitment of osteoprogenitor cells, matrix deposition, and signaling pathways.
• The deposition of a provisional extracellular matrix provides a scaffold for the migration and proliferation of recruited cells.

Submerged vs. Non-Submerged Healing

• Choice of submerged and non-submerged healing protocols can impact the stability and predictability of regenerative outcomes.
• Submerged healing may increase the risk of contamination of inserted biomaterials due to the inability to attain complete primary closure.

Surgical Complications

• Complications such as soft tissue dehiscence, membrane exposure, and infection can negatively impact bone regeneration.
• Minimizing the impact of such complications is crucial for predictable regenerative outcomes.

The Impact of Implant Surface Characteristics

• Implant surface properties, such as topography, wettability, and chemistry, can directly influence osseointegration and bone regeneration.
• Hydrophilic surfaces can enhance early bone formation and promote a more favorable immune response.

Impact of Implant Surface on Osseointegration

• Titanium surface topography and chemistry can influence the proteomic profile released by platelets, which can subsequently influence macrophage pro-inflammatory cytokine expression.
• Hydrophilic surfaces can elicit a macrophage phenotype associated with reduced inflammation and enhanced pro-osteogenic signaling.### Guided Bone Regeneration
• Guided bone regeneration is based on the use of an occlusive barrier membrane to create a secluded space around a bone defect
• The membrane facilitates the recruitment and proliferation of osteoprogenitor cells from the marrow spaces directly into the defect while preventing the downgrowth of neighboring soft tissues

Properties of Ideal Barrier Membranes

• Occlusiveness: enables the passage of nutrients, fluids, oxygen, and bioactive substances for cell growth while preventing the passage of unwanted cells
• Porosity: influences the regenerative outcomes, with moderate porosity allowing the passage of bacteria, cells, and tissue integration/migration
• Resorption pattern: should gradually resorb over time, with degradation products not jeopardizing the regeneration process

Stabilization

• Stabilization of the blood clot and membrane is crucial for bone regeneration to take place
• Micromovements between bone and implanted material prevent bone formation and result in fibrous tissue development
• Stabilization methods include fixation screws, non-resorbable pins, or titanium pins, with membrane tucked under the flaps

Resorption Pattern

• Resorption rate depends on the physical and chemical properties of the grafting material
• Autologous bone grafts are incorporated into surrounding bone through "creeping substitution"
• Allograft incorporation follows a similar sequence of events, but with slower vascular penetration, bone formation, and remodeling

Bioactivity

• Membranes not only prevent migration of undesired cells but also promote and direct biological events underpinning bone formation
• Both resorbable and non-resorbable membranes can host cells that express and secrete pro-osteogenic and bone-promoting factors

One-Layer vs. Two-Layer Membranes

• Double-layer membrane technique can enhance the membrane barrier effect and increase the stability of the underlying graft
• One layer of collagen membrane is often sufficient to promote bone regeneration

Bone Grafts

• Bone grafts or substitutes enhance bone healing by bridging small to large defects, preventing membrane collapse, and stabilizing the blood clot
• Bone augmentation materials are classified according to their origin into autologous grafts, allogenic grafts, xenogenic grafts, or alloplastic grafts

Properties of Ideal Bone Replacement Grafts

• Biocompatibility, porosity, osteoinductivity, osteoconductivity, surface properties, biodegradability, mechanical properties, angiogenicity, and ease of handling and manufacturing processes

Micro- and Macro-Architecture of Grafting Materials

• Microarchitecture influences the resorption rate and degree of angiogenesis, which can affect volume maintenance over time